START-INFO-DIR-ENTRY * Libc: (libc). C library. END-INFO-DIR-ENTRY This file documents the GNU C library. This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library Reference Manual', for Version 2.2.x. Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software Needs Free Documentation" and "GNU Lesser General Public License", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development. Main Menu ********* This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library Reference Manual', for Version 2.2.x of the GNU C Library. Introduction ************ The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard "library", which you compile and link with your programs. The GNU C library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system. The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability. Getting Started =============== This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (*note ISO C::), rather than "traditional" pre-ISO C dialects, is assumed. The GNU C library includes several "header files", each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file `stdio.h' declares facilities for performing input and output, and the header file `string.h' declares string processing utilities. The organization of this manual generally follows the same division as the header files. If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a _lot_ of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly _how_ to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize _when_ to make use of library functions, and _where_ in this manual you can find more specific information about them. Standards and Portability ========================= This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations. The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual. *Note Library Summary::, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from. ISO C ----- The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): `American National Standard X3.159-1989--"ANSI C"' and later by the International Standardization Organization (ISO): `ISO/IEC 9899:1990, "Programming languages--C"'. We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard. If you are concerned about strict adherence to the ISO C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define _only_ ISO standard features from the library header files, unless you explicitly ask for additional features. *Note Feature Test Macros::, for information on how to do this. Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. *Note Reserved Names::, for more information about these restrictions. This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness. POSIX (The Portable Operating System Interface) ----------------------------------------------- The GNU library is also compatible with the ISO "POSIX" family of standards, known more formally as the "Portable Operating System Interface for Computer Environments" (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system. The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments. The GNU C library implements all of the functions specified in `ISO/IEC 9945-1:1996, the POSIX System Application Program Interface', commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (*note File System Interface::), device-specific terminal control functions (*note Low-Level Terminal Interface::), and process control functions (*note Processes::). Some facilities from `ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard' (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (*note Pattern Matching::). Berkeley Unix ------------- The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as "Berkeley Unix") and from "SunOS" (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all. The BSD facilities include symbolic links (*note Symbolic Links::), the `select' function (*note Waiting for I/O::), the BSD signal functions (*note BSD Signal Handling::), and sockets (*note Sockets::). SVID (The System V Interface Description) ----------------------------------------- The "System V Interface Description" (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (*note POSIX::). The GNU C library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.) The supported facilities from System V include the methods for inter-process communication and shared memory, the `hsearch' and `drand48' families of functions, `fmtmsg' and several of the mathematical functions. XPG (The X/Open Portability Guide) ---------------------------------- The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system. The GNU C library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions. The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems. Using the Library ================= This section describes some of the practical issues involved in using the GNU C library. Header Files ------------ Libraries for use by C programs really consist of two parts: "header files" that define types and macros and declare variables and functions; and the actual library or "archive" that contains the definitions of the variables and functions. (Recall that in C, a "declaration" merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A "definition", on the other hand, actually allocates storage for a variable or says what a function does.) In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file. Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first, #include "HEADER" is typically used to include a header file HEADER that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast, #include is typically used to include a header file `file.h' that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files. Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (*note Feature Test Macros::). For more information about the use of header files and `#include' directives, *note Header Files: (cpp.info)Header Files.. The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using. Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter. *Compatibility Note:* Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations. Strictly speaking, you don't _have to_ include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration. Macro Definitions of Functions ------------------------------ If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs--the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call. Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call. You might occasionally want to avoid using the macro definition of a function--perhaps to make your program easier to debug. There are two ways you can do this: * You can avoid a macro definition in a specific use by enclosing the name of the function in parentheses. This works because the name of the function doesn't appear in a syntactic context where it is recognizable as a macro call. * You can suppress any macro definition for a whole source file by using the `#undef' preprocessor directive, unless otherwise stated explicitly in the description of that facility. For example, suppose the header file `stdlib.h' declares a function named `abs' with extern int abs (int); and also provides a macro definition for `abs'. Then, in: #include int f (int *i) { return abs (++*i); } the reference to `abs' might refer to either a macro or a function. On the other hand, in each of the following examples the reference is to a function and not a macro. #include int g (int *i) { return (abs) (++*i); } #undef abs int h (int *i) { return abs (++*i); } Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower. Reserved Names -------------- The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program *may not* redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions: * Other people reading your code could get very confused if you were using a function named `exit' to do something completely different from what the standard `exit' function does, for example. Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability. * It avoids the possibility of a user accidentally redefining a library function that is called by other library functions. If redefinition were allowed, those other functions would not work properly. * It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user. Some library facilities, such as those for dealing with variadic arguments (*note Variadic Functions::) and non-local exits (*note Non-Local Exits::), actually require a considerable amount of cooperation on the part of the C compiler, and with respect to the implementation, it might be easier for the compiler to treat these as built-in parts of the language. In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs. Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names. * Names beginning with a capital `E' followed a digit or uppercase letter may be used for additional error code names. *Note Error Reporting::. * Names that begin with either `is' or `to' followed by a lowercase letter may be used for additional character testing and conversion functions. *Note Character Handling::. * Names that begin with `LC_' followed by an uppercase letter may be used for additional macros specifying locale attributes. *Note Locales::. * Names of all existing mathematics functions (*note Mathematics::) suffixed with `f' or `l' are reserved for corresponding functions that operate on `float' and `long double' arguments, respectively. * Names that begin with `SIG' followed by an uppercase letter are reserved for additional signal names. *Note Standard Signals::. * Names that begin with `SIG_' followed by an uppercase letter are reserved for additional signal actions. *Note Basic Signal Handling::. * Names beginning with `str', `mem', or `wcs' followed by a lowercase letter are reserved for additional string and array functions. *Note String and Array Utilities::. * Names that end with `_t' are reserved for additional type names. In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file. * The header file `dirent.h' reserves names prefixed with `d_'. * The header file `fcntl.h' reserves names prefixed with `l_', `F_', `O_', and `S_'. * The header file `grp.h' reserves names prefixed with `gr_'. * The header file `limits.h' reserves names suffixed with `_MAX'. * The header file `pwd.h' reserves names prefixed with `pw_'. * The header file `signal.h' reserves names prefixed with `sa_' and `SA_'. * The header file `sys/stat.h' reserves names prefixed with `st_' and `S_'. * The header file `sys/times.h' reserves names prefixed with `tms_'. * The header file `termios.h' reserves names prefixed with `c_', `V', `I', `O', and `TC'; and names prefixed with `B' followed by a digit. Feature Test Macros ------------------- The exact set of features available when you compile a source file is controlled by which "feature test macros" you define. If you compile your programs using `gcc -ansi', you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. *Note GNU CC Command Options: (gcc.info)Invoking GCC, for more information about GCC options. You should define these macros by using `#define' preprocessor directives at the top of your source code files. These directives _must_ come before any `#include' of a system header file. It is best to make them the very first thing in the file, preceded only by comments. You could also use the `-D' option to GCC, but it's better if you make the source files indicate their own meaning in a self-contained way. This system exists to allow the library to conform to multiple standards. Although the different standards are often described as supersets of each other, they are usually incompatible because larger standards require functions with names that smaller ones reserve to the user program. This is not mere pedantry -- it has been a problem in practice. For instance, some non-GNU programs define functions named `getline' that have nothing to do with this library's `getline'. They would not be compilable if all features were enabled indiscriminately. This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard. - Macro: _POSIX_SOURCE If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities. The state of `_POSIX_SOURCE' is irrelevant if you define the macro `_POSIX_C_SOURCE' to a positive integer. - Macro: _POSIX_C_SOURCE Define this macro to a positive integer to control which POSIX functionality is made available. The greater the value of this macro, the more functionality is made available. If you define this macro to a value greater than or equal to `1', then the functionality from the 1990 edition of the POSIX.1 standard (IEEE Standard 1003.1-1990) is made available. If you define this macro to a value greater than or equal to `2', then the functionality from the 1992 edition of the POSIX.2 standard (IEEE Standard 1003.2-1992) is made available. If you define this macro to a value greater than or equal to `199309L', then the functionality from the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available. Greater values for `_POSIX_C_SOURCE' will enable future extensions. The POSIX standards process will define these values as necessary, and the GNU C Library should support them some time after they become standardized. The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that if you define `_POSIX_C_SOURCE' to a value greater than or equal to `199506L', then the functionality from the 1996 edition is made available. - Macro: _BSD_SOURCE If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material. Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions. Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special "BSD compatibility library" when linking programs compiled for BSD compatibility. This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library. If your program defines `_BSD_SOURCE', you must give the option `-lbsd-compat' to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library. - Macro: _SVID_SOURCE If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material. - Macro: _XOPEN_SOURCE - Macro: _XOPEN_SOURCE_EXTENDED If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact `_POSIX_SOURCE' and `_POSIX_C_SOURCE' are automatically defined. As the unification of all Unices, functionality only available in BSD and SVID is also included. If the macro `_XOPEN_SOURCE_EXTENDED' is also defined, even more functionality is available. The extra functions will make all functions available which are necessary for the X/Open Unix brand. If the macro `_XOPEN_SOURCE' has the value 500 this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2. - Macro: _LARGEFILE_SOURCE If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions `fseeko' and `ftello' are available. Without these functions the difference between the ISO C interface (`fseek', `ftell') and the low-level POSIX interface (`lseek') would lead to problems. This macro was introduced as part of the Large File Support extension (LFS). - Macro: _LARGEFILE64_SOURCE If you define this macro an additional set of functions is made available which enables 32 bit systems to use files of sizes beyond the usual limit of 2GB. This interface is not available if the system does not support files that large. On systems where the natural file size limit is greater than 2GB (i.e., on 64 bit systems) the new functions are identical to the replaced functions. The new functionality is made available by a new set of types and functions which replace the existing ones. The names of these new objects contain `64' to indicate the intention, e.g., `off_t' vs. `off64_t' and `fseeko' vs. `fseeko64'. This macro was introduced as part of the Large File Support extension (LFS). It is a transition interface for the period when 64 bit offsets are not generally used (see `_FILE_OFFSET_BITS'). - Macro: _FILE_OFFSET_BITS This macro determines which file system interface shall be used, one replacing the other. Whereas `_LARGEFILE64_SOURCE' makes the 64 bit interface available as an additional interface, `_FILE_OFFSET_BITS' allows the 64 bit interface to replace the old interface. If `_FILE_OFFSET_BITS' is undefined, or if it is defined to the value `32', nothing changes. The 32 bit interface is used and types like `off_t' have a size of 32 bits on 32 bit systems. If the macro is defined to the value `64', the large file interface replaces the old interface. I.e., the functions are not made available under different names (as they are with `_LARGEFILE64_SOURCE'). Instead the old function names now reference the new functions, e.g., a call to `fseeko' now indeed calls `fseeko64'. This macro should only be selected if the system provides mechanisms for handling large files. On 64 bit systems this macro has no effect since the `*64' functions are identical to the normal functions. This macro was introduced as part of the Large File Support extension (LFS). - Macro: _ISOC99_SOURCE Until the revised ISO C standard is widely adopted the new features are not automatically enabled. The GNU libc nevertheless has a complete implementation of the new standard and to enable the new features the macro `_ISOC99_SOURCE' should be defined. - Macro: _GNU_SOURCE If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence. If you want to get the full effect of `_GNU_SOURCE' but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions: #define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. *Note:* If you forget to do this, you may get very strange errors at run time. - Macro: _REENTRANT - Macro: _THREAD_SAFE If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface. Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe. We recommend you use `_GNU_SOURCE' in new programs. If you don't specify the `-ansi' option to GCC and don't define any of these macros explicitly, the effect is the same as defining `_POSIX_C_SOURCE' to 2 and `_POSIX_SOURCE', `_SVID_SOURCE', and `_BSD_SOURCE' to 1. When you define a feature test macro to request a larger class of features, it is harmless to define in addition a feature test macro for a subset of those features. For example, if you define `_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no effect. Likewise, if you define `_GNU_SOURCE', then defining either `_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no effect. Note, however, that the features of `_BSD_SOURCE' are not a subset of any of the other feature test macros supported. This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros. For this reason, defining `_BSD_SOURCE' in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features. Roadmap to the Manual ===================== Here is an overview of the contents of the remaining chapters of this manual. * *Note Error Reporting::, describes how errors detected by the library are reported. * *Note Language Features::, contains information about library support for standard parts of the C language, including things like the `sizeof' operator and the symbolic constant `NULL', how to write functions accepting variable numbers of arguments, and constants describing the ranges and other properties of the numerical types. There is also a simple debugging mechanism which allows you to put assertions in your code, and have diagnostic messages printed if the tests fail. * *Note Memory::, describes the GNU library's facilities for managing and using virtual and real memory, including dynamic allocation of virtual memory. If you do not know in advance how much memory your program needs, you can allocate it dynamically instead, and manipulate it via pointers. * *Note Character Handling::, contains information about character classification functions (such as `isspace') and functions for performing case conversion. * *Note String and Array Utilities::, has descriptions of functions for manipulating strings (null-terminated character arrays) and general byte arrays, including operations such as copying and comparison. * *Note I/O Overview::, gives an overall look at the input and output facilities in the library, and contains information about basic concepts such as file names. * *Note I/O on Streams::, describes I/O operations involving streams (or `FILE *' objects). These are the normal C library functions from `stdio.h'. * *Note Low-Level I/O::, contains information about I/O operations on file descriptors. File descriptors are a lower-level mechanism specific to the Unix family of operating systems. * *Note File System Interface::, has descriptions of operations on entire files, such as functions for deleting and renaming them and for creating new directories. This chapter also contains information about how you can access the attributes of a file, such as its owner and file protection modes. * *Note Pipes and FIFOs::, contains information about simple interprocess communication mechanisms. Pipes allow communication between two related processes (such as between a parent and child), while FIFOs allow communication between processes sharing a common file system on the same machine. * *Note Sockets::, describes a more complicated interprocess communication mechanism that allows processes running on different machines to communicate over a network. This chapter also contains information about Internet host addressing and how to use the system network databases. * *Note Low-Level Terminal Interface::, describes how you can change the attributes of a terminal device. If you want to disable echo of characters typed by the user, for example, read this chapter. * *Note Mathematics::, contains information about the math library functions. These include things like random-number generators and remainder functions on integers as well as the usual trigonometric and exponential functions on floating-point numbers. * *Note Low-Level Arithmetic Functions: Arithmetic, describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings. * *Note Searching and Sorting::, contains information about functions for searching and sorting arrays. You can use these functions on any kind of array by providing an appropriate comparison function. * *Note Pattern Matching::, presents functions for matching regular expressions and shell file name patterns, and for expanding words as the shell does. * *Note Date and Time::, describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers. * *Note Character Set Handling::, contains information about manipulating characters and strings using character sets larger than will fit in the usual `char' data type. * *Note Locales::, describes how selecting a particular country or language affects the behavior of the library. For example, the locale affects collation sequences for strings and how monetary values are formatted. * *Note Non-Local Exits::, contains descriptions of the `setjmp' and `longjmp' functions. These functions provide a facility for `goto'-like jumps which can jump from one function to another. * *Note Signal Handling::, tells you all about signals--what they are, how to establish a handler that is called when a particular kind of signal is delivered, and how to prevent signals from arriving during critical sections of your program. * *Note Program Basics::, tells how your programs can access their command-line arguments and environment variables. * *Note Processes::, contains information about how to start new processes and run programs. * *Note Job Control::, describes functions for manipulating process groups and the controlling terminal. This material is probably only of interest if you are writing a shell or other program which handles job control specially. * *Note Name Service Switch::, describes the services which are available for looking up names in the system databases, how to determine which service is used for which database, and how these services are implemented so that contributors can design their own services. * *Note User Database::, and *Note Group Database::, tell you how to access the system user and group databases. * *Note System Management::, describes functions for controlling and getting information about the hardware and software configuration your program is executing under. * *Note System Configuration::, tells you how you can get information about various operating system limits. Most of these parameters are provided for compatibility with POSIX. * *Note Library Summary::, gives a summary of all the functions, variables, and macros in the library, with complete data types and function prototypes, and says what standard or system each is derived from. * *Note Maintenance::, explains how to build and install the GNU C library on your system, how to report any bugs you might find, and how to add new functions or port the library to a new system. If you already know the name of the facility you are interested in, you can look it up in *Note Library Summary::. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from. Error Reporting *************** Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed. This chapter describes how the error reporting facility works. Your program should include the header file `errno.h' to use this facility. Checking for Errors =================== Most library functions return a special value to indicate that they have failed. The special value is typically `-1', a null pointer, or a constant such as `EOF' that is defined for that purpose. But this return value tells you only that an error has occurred. To find out what kind of error it was, you need to look at the error code stored in the variable `errno'. This variable is declared in the header file `errno.h'. - Variable: volatile int errno The variable `errno' contains the system error number. You can change the value of `errno'. Since `errno' is declared `volatile', it might be changed asynchronously by a signal handler; see *Note Defining Handlers::. However, a properly written signal handler saves and restores the value of `errno', so you generally do not need to worry about this possibility except when writing signal handlers. The initial value of `errno' at program startup is zero. Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors. These error conditions are listed for each function. These functions do not change `errno' when they succeed; thus, the value of `errno' after a successful call is not necessarily zero, and you should not use `errno' to determine _whether_ a call failed. The proper way to do that is documented for each function. _If_ the call failed, you can examine `errno'. Many library functions can set `errno' to a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter `errno' when the function returns an error. *Portability Note:* ISO C specifies `errno' as a "modifiable lvalue" rather than as a variable, permitting it to be implemented as a macro. For example, its expansion might involve a function call, like `*_errno ()'. In fact, that is what it is on the GNU system itself. The GNU library, on non-GNU systems, does whatever is right for the particular system. There are a few library functions, like `sqrt' and `atan', that return a perfectly legitimate value in case of an error, but also set `errno'. For these functions, if you want to check to see whether an error occurred, the recommended method is to set `errno' to zero before calling the function, and then check its value afterward. All the error codes have symbolic names; they are macros defined in `errno.h'. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. *Note Reserved Names::. The error code values are all positive integers and are all distinct, with one exception: `EWOULDBLOCK' and `EAGAIN' are the same. Since the values are distinct, you can use them as labels in a `switch' statement; just don't use both `EWOULDBLOCK' and `EAGAIN'. Your program should not make any other assumptions about the specific values of these symbolic constants. The value of `errno' doesn't necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations. The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function. On non-GNU systems, almost any system call can return `EFAULT' if it is given an invalid pointer as an argument. Since this could only happen as a result of a bug in your program, and since it will not happen on the GNU system, we have saved space by not mentioning `EFAULT' in the descriptions of individual functions. In some Unix systems, many system calls can also return `EFAULT' if given as an argument a pointer into the stack, and the kernel for some obscure reason fails in its attempt to extend the stack. If this ever happens, you should probably try using statically or dynamically allocated memory instead of stack memory on that system. Error Codes =========== The error code macros are defined in the header file `errno.h'. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems. - Macro: int EPERM Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation. - Macro: int ENOENT No such file or directory. This is a "file doesn't exist" error for ordinary files that are referenced in contexts where they are expected to already exist. - Macro: int ESRCH No process matches the specified process ID. - Macro: int EINTR Interrupted function call; an asynchronous signal occurred and prevented completion of the call. When this happens, you should try the call again. You can choose to have functions resume after a signal that is handled, rather than failing with `EINTR'; see *Note Interrupted Primitives::. - Macro: int EIO Input/output error; usually used for physical read or write errors. - Macro: int ENXIO No such device or address. The system tried to use the device represented by a file you specified, and it couldn't find the device. This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer. - Macro: int E2BIG Argument list too long; used when the arguments passed to a new program being executed with one of the `exec' functions (*note Executing a File::) occupy too much memory space. This condition never arises in the GNU system. - Macro: int ENOEXEC Invalid executable file format. This condition is detected by the `exec' functions; see *Note Executing a File::. - Macro: int EBADF Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa). - Macro: int ECHILD There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate. - Macro: int EDEADLK Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. The system does not guarantee that it will notice all such situations. This error means you got lucky and the system noticed; it might just hang. *Note File Locks::, for an example. - Macro: int ENOMEM No memory available. The system cannot allocate more virtual memory because its capacity is full. - Macro: int EACCES Permission denied; the file permissions do not allow the attempted operation. - Macro: int EFAULT Bad address; an invalid pointer was detected. In the GNU system, this error never happens; you get a signal instead. - Macro: int ENOTBLK A file that isn't a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error. - Macro: int EBUSY Resource busy; a system resource that can't be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error. - Macro: int EEXIST File exists; an existing file was specified in a context where it only makes sense to specify a new file. - Macro: int EXDEV An attempt to make an improper link across file systems was detected. This happens not only when you use `link' (*note Hard Links::) but also when you rename a file with `rename' (*note Renaming Files::). - Macro: int ENODEV The wrong type of device was given to a function that expects a particular sort of device. - Macro: int ENOTDIR A file that isn't a directory was specified when a directory is required. - Macro: int EISDIR File is a directory; you cannot open a directory for writing, or create or remove hard links to it. - Macro: int EINVAL Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function. - Macro: int EMFILE The current process has too many files open and can't open any more. Duplicate descriptors do count toward this limit. In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased. If you get this error, you might want to increase the `RLIMIT_NOFILE' limit or make it unlimited; *note Limits on Resources::. - Macro: int ENFILE There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see *Note Linked Channels::. This error never occurs in the GNU system. - Macro: int ENOTTY Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file. - Macro: int ETXTBSY An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. Often using a debugger to run a program is considered having it open for writing and will cause this error. (The name stands for "text file busy".) This is not an error in the GNU system; the text is copied as necessary. - Macro: int EFBIG File too big; the size of a file would be larger than allowed by the system. - Macro: int ENOSPC No space left on device; write operation on a file failed because the disk is full. - Macro: int ESPIPE Invalid seek operation (such as on a pipe). - Macro: int EROFS An attempt was made to modify something on a read-only file system. - Macro: int EMLINK Too many links; the link count of a single file would become too large. `rename' can cause this error if the file being renamed already has as many links as it can take (*note Renaming Files::). - Macro: int EPIPE Broken pipe; there is no process reading from the other end of a pipe. Every library function that returns this error code also generates a `SIGPIPE' signal; this signal terminates the program if not handled or blocked. Thus, your program will never actually see `EPIPE' unless it has handled or blocked `SIGPIPE'. - Macro: int EDOM Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined. - Macro: int ERANGE Range error; used by mathematical functions when the result value is not representable because of overflow or underflow. - Macro: int EAGAIN Resource temporarily unavailable; the call might work if you try again later. The macro `EWOULDBLOCK' is another name for `EAGAIN'; they are always the same in the GNU C library. This error can happen in a few different situations: * An operation that would block was attempted on an object that has non-blocking mode selected. Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation). You can use `select' to find out when the operation will be possible; *note Waiting for I/O::. *Portability Note:* In many older Unix systems, this condition was indicated by `EWOULDBLOCK', which was a distinct error code different from `EAGAIN'. To make your program portable, you should check for both codes and treat them the same. * A temporary resource shortage made an operation impossible. `fork' can return this error. It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop. - Macro: int EWOULDBLOCK In the GNU C library, this is another name for `EAGAIN' (above). The values are always the same, on every operating system. C libraries in many older Unix systems have `EWOULDBLOCK' as a separate error code. - Macro: int EINPROGRESS An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected. Some functions that must always block (such as `connect'; *note Connecting::) never return `EAGAIN'. Instead, they return `EINPROGRESS' to indicate that the operation has begun and will take some time. Attempts to manipulate the object before the call completes return `EALREADY'. You can use the `select' function to find out when the pending operation has completed; *note Waiting for I/O::. - Macro: int EALREADY An operation is already in progress on an object that has non-blocking mode selected. - Macro: int ENOTSOCK A file that isn't a socket was specified when a socket is required. - Macro: int EMSGSIZE The size of a message sent on a socket was larger than the supported maximum size. - Macro: int EPROTOTYPE The socket type does not support the requested communications protocol. - Macro: int ENOPROTOOPT You specified a socket option that doesn't make sense for the particular protocol being used by the socket. *Note Socket Options::. - Macro: int EPROTONOSUPPORT The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). *Note Creating a Socket::. - Macro: int ESOCKTNOSUPPORT The socket type is not supported. - Macro: int EOPNOTSUPP The operation you requested is not supported. Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols. In the GNU system, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call. - Macro: int EPFNOSUPPORT The socket communications protocol family you requested is not supported. - Macro: int EAFNOSUPPORT The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. *Note Sockets::. - Macro: int EADDRINUSE The requested socket address is already in use. *Note Socket Addresses::. - Macro: int EADDRNOTAVAIL The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. *Note Socket Addresses::. - Macro: int ENETDOWN A socket operation failed because the network was down. - Macro: int ENETUNREACH A socket operation failed because the subnet containing the remote host was unreachable. - Macro: int ENETRESET A network connection was reset because the remote host crashed. - Macro: int ECONNABORTED A network connection was aborted locally. - Macro: int ECONNRESET A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation. - Macro: int ENOBUFS The kernel's buffers for I/O operations are all in use. In GNU, this error is always synonymous with `ENOMEM'; you may get one or the other from network operations. - Macro: int EISCONN You tried to connect a socket that is already connected. *Note Connecting::. - Macro: int ENOTCONN The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data. For a connectionless socket (for datagram protocols, such as UDP), you get `EDESTADDRREQ' instead. - Macro: int EDESTADDRREQ No default destination address was set for the socket. You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with `connect'. - Macro: int ESHUTDOWN The socket has already been shut down. - Macro: int ETOOMANYREFS ??? - Macro: int ETIMEDOUT A socket operation with a specified timeout received no response during the timeout period. - Macro: int ECONNREFUSED A remote host refused to allow the network connection (typically because it is not running the requested service). - Macro: int ELOOP Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links. - Macro: int ENAMETOOLONG Filename too long (longer than `PATH_MAX'; *note Limits for Files::) or host name too long (in `gethostname' or `sethostname'; *note Host Identification::). - Macro: int EHOSTDOWN The remote host for a requested network connection is down. - Macro: int EHOSTUNREACH The remote host for a requested network connection is not reachable. - Macro: int ENOTEMPTY Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory. - Macro: int EPROCLIM This means that the per-user limit on new process would be exceeded by an attempted `fork'. *Note Limits on Resources::, for details on the `RLIMIT_NPROC' limit. - Macro: int EUSERS The file quota system is confused because there are too many users. - Macro: int EDQUOT The user's disk quota was exceeded. - Macro: int ESTALE Stale NFS file handle. This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host. Repairing this condition usually requires unmounting and remounting the NFS file system on the local host. - Macro: int EREMOTE An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.) - Macro: int EBADRPC ??? - Macro: int ERPCMISMATCH ??? - Macro: int EPROGUNAVAIL ??? - Macro: int EPROGMISMATCH ??? - Macro: int EPROCUNAVAIL ??? - Macro: int ENOLCK No locks available. This is used by the file locking facilities; see *Note File Locks::. This error is never generated by the GNU system, but it can result from an operation to an NFS server running another operating system. - Macro: int EFTYPE Inappropriate file type or format. The file was the wrong type for the operation, or a data file had the wrong format. On some systems `chmod' returns this error if you try to set the sticky bit on a non-directory file; *note Setting Permissions::. - Macro: int EAUTH ??? - Macro: int ENEEDAUTH ??? - Macro: int ENOSYS Function not implemented. This indicates that the function called is not implemented at all, either in the C library itself or in the operating system. When you get this error, you can be sure that this particular function will always fail with `ENOSYS' unless you install a new version of the C library or the operating system. - Macro: int ENOTSUP Not supported. A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all. For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only _that specific object_ (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values. If the entire function is not available at all in the implementation, it returns `ENOSYS' instead. - Macro: int EILSEQ While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid. - Macro: int EBACKGROUND In the GNU system, servers supporting the `term' protocol return this error for certain operations when the caller is not in the foreground process group of the terminal. Users do not usually see this error because functions such as `read' and `write' translate it into a `SIGTTIN' or `SIGTTOU' signal. *Note Job Control::, for information on process groups and these signals. - Macro: int EDIED In the GNU system, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file. - Macro: int ED The experienced user will know what is wrong. - Macro: int EGREGIOUS You did *what*? - Macro: int EIEIO Go home and have a glass of warm, dairy-fresh milk. - Macro: int EGRATUITOUS This error code has no purpose. - Macro: int EBADMSG - Macro: int EIDRM - Macro: int EMULTIHOP - Macro: int ENODATA - Macro: int ENOLINK - Macro: int ENOMSG - Macro: int ENOSR - Macro: int ENOSTR - Macro: int EOVERFLOW - Macro: int EPROTO - Macro: int ETIME _The following error codes are defined by the Linux/i386 kernel. They are not yet documented._ - Macro: int ERESTART - Macro: int ECHRNG - Macro: int EL2NSYNC - Macro: int EL3HLT - Macro: int EL3RST - Macro: int ELNRNG - Macro: int EUNATCH - Macro: int ENOCSI - Macro: int EL2HLT - Macro: int EBADE - Macro: int EBADR - Macro: int EXFULL - Macro: int ENOANO - Macro: int EBADRQC - Macro: int EBADSLT - Macro: int EDEADLOCK - Macro: int EBFONT - Macro: int ENONET - Macro: int ENOPKG - Macro: int EADV - Macro: int ESRMNT - Macro: int ECOMM - Macro: int EDOTDOT - Macro: int ENOTUNIQ - Macro: int EBADFD - Macro: int EREMCHG - Macro: int ELIBACC - Macro: int ELIBBAD - Macro: int ELIBSCN - Macro: int ELIBMAX - Macro: int ELIBEXEC - Macro: int ESTRPIPE - Macro: int EUCLEAN - Macro: int ENOTNAM - Macro: int ENAVAIL - Macro: int EISNAM - Macro: int EREMOTEIO - Macro: int ENOMEDIUM - Macro: int EMEDIUMTYPE Error Messages ============== The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call. The functions `strerror' and `perror' give you the standard error message for a given error code; the variable `program_invocation_short_name' gives you convenient access to the name of the program that encountered the error. - Function: char * strerror (int ERRNUM) The `strerror' function maps the error code (*note Checking for Errors::) specified by the ERRNUM argument to a descriptive error message string. The return value is a pointer to this string. The value ERRNUM normally comes from the variable `errno'. You should not modify the string returned by `strerror'. Also, if you make subsequent calls to `strerror', the string might be overwritten. (But it's guaranteed that no library function ever calls `strerror' behind your back.) The function `strerror' is declared in `string.h'. - Function: char * strerror_r (int ERRNUM, char *BUF, size_t N) The `strerror_r' function works like `strerror' but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user supplied buffer starting at BUF with the length of N bytes. At most N characters are written (including the NUL byte) so it is up to the user to select the buffer large enough. This function should always be used in multi-threaded programs since there is no way to guarantee the string returned by `strerror' really belongs to the last call of the current thread. This function `strerror_r' is a GNU extension and it is declared in `string.h'. - Function: void perror (const char *MESSAGE) This function prints an error message to the stream `stderr'; see *Note Standard Streams::. The orientation of `stderr' is not changed. If you call `perror' with a MESSAGE that is either a null pointer or an empty string, `perror' just prints the error message corresponding to `errno', adding a trailing newline. If you supply a non-null MESSAGE argument, then `perror' prefixes its output with this string. It adds a colon and a space character to separate the MESSAGE from the error string corresponding to `errno'. The function `perror' is declared in `stdio.h'. `strerror' and `perror' produce the exact same message for any given error code; the precise text varies from system to system. On the GNU system, the messages are fairly short; there are no multi-line messages or embedded newlines. Each error message begins with a capital letter and does not include any terminating punctuation. *Compatibility Note:* The `strerror' function was introduced in ISO C89. Many older C systems do not support this function yet. Many programs that don't read input from the terminal are designed to exit if any system call fails. By convention, the error message from such a program should start with the program's name, sans directories. You can find that name in the variable `program_invocation_short_name'; the full file name is stored the variable `program_invocation_name'. - Variable: char * program_invocation_name This variable's value is the name that was used to invoke the program running in the current process. It is the same as `argv[0]'. Note that this is not necessarily a useful file name; often it contains no directory names. *Note Program Arguments::. - Variable: char * program_invocation_short_name This variable's value is the name that was used to invoke the program running in the current process, with directory names removed. (That is to say, it is the same as `program_invocation_name' minus everything up to the last slash, if any.) The library initialization code sets up both of these variables before calling `main'. *Portability Note:* These two variables are GNU extensions. If you want your program to work with non-GNU libraries, you must save the value of `argv[0]' in `main', and then strip off the directory names yourself. We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from `main'. Here is an example showing how to handle failure to open a file correctly. The function `open_sesame' tries to open the named file for reading and returns a stream if successful. The `fopen' library function returns a null pointer if it couldn't open the file for some reason. In that situation, `open_sesame' constructs an appropriate error message using the `strerror' function, and terminates the program. If we were going to make some other library calls before passing the error code to `strerror', we'd have to save it in a local variable instead, because those other library functions might overwrite `errno' in the meantime. #include #include #include #include FILE * open_sesame (char *name) { FILE *stream; errno = 0; stream = fopen (name, "r"); if (stream == NULL) { fprintf (stderr, "%s: Couldn't open file %s; %s\n", program_invocation_short_name, name, strerror (errno)); exit (EXIT_FAILURE); } else return stream; } Using `perror' has the advantage that the function is portable and available on all systems implementing ISO C. But often the text `perror' generates is not what is wanted and there is no way to extend or change what `perror' does. The GNU coding standard, for instance, requires error messages to be preceded by the program name and programs which read some input files should should provide information about the input file name and the line number in case an error is encountered while reading the file. For these occasions there are two functions available which are widely used throughout the GNU project. These functions are declared in `error.h'. - Function: void error (int STATUS, int ERRNUM, const char *FORMAT, ...) The `error' function can be used to report general problems during program execution. The FORMAT argument is a format string just like those given to the `printf' family of functions. The arguments required for the format can follow the FORMAT parameter. Just like `perror', `error' also can report an error code in textual form. But unlike `perror' the error value is explicitly passed to the function in the ERRNUM parameter. This elimintates the problem mentioned above that the error reporting function must be called immediately after the function causing the error since otherwise `errno' might have a different value. The `error' prints first the program name. If the application defined a global variable `error_print_progname' and points it to a function this function will be called to print the program name. Otherwise the string from the global variable `program_name' is used. The program name is followed by a colon and a space which in turn is followed by the output produced by the format string. If the ERRNUM parameter is non-zero the format string output is followed by a colon and a space, followed by the error message for the error code ERRNUM. In any case is the output terminated with a newline. The output is directed to the `stderr' stream. If the `stderr' wasn't oriented before the call it will be narrow-oriented afterwards. The function will return unless the STATUS parameter has a non-zero value. In this case the function will call `exit' with the STATUS value for its parameter and therefore never return. If `error' returns the global variable `error_message_count' is incremented by one to keep track of the number of errors reported. - Function: void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...) The `error_at_line' function is very similar to the `error' function. The only difference are the additional parameters FNAME and LINENO. The handling of the other parameters is identical to that of `error' except that between the program name and the string generated by the format string additional text is inserted. Directly following the program name a colon, followed by the file name pointer to by FNAME, another colon, and a value of LINENO is printed. This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file etc). If the global variable `error_one_per_line' is set to a non-zero value `error_at_line' will avoid printing consecutive messages for the same file anem line. Repetition which are not directly following each other are not caught. Just like `error' this function only returned if STATUS is zero. Otherwise `exit' is called with the non-zero value. If `error' returns the global variable `error_message_count' is incremented by one to keep track of the number of errors reported. As mentioned above the `error' and `error_at_line' functions can be customized by defining a variable named `error_print_progname'. - Variable: void (* error_print_progname ) (void) If the `error_print_progname' variable is defined to a non-zero value the function pointed to is called by `error' or `error_at_line'. It is expected to print the program name or do something similarly useful. The function is expected to be print to the `stderr' stream and must be able to handle whatever orientation the stream has. The variable is global and shared by all threads. - Variable: unsigned int error_message_count The `error_message_count' variable is incremented whenever one of the functions `error' or `error_at_line' returns. The variable is global and shared by all threads. - Variable: int error_one_per_line The `error_one_per_line' variable influences only `error_at_line'. Normally the `error_at_line' function creates output for every invocation. If `error_one_per_line' is set to a non-zero value `error_at_line' keeps track of the last file name and line number for which an error was reported and avoid directly following messages for the same file and line. This variable is global and shared by all threads. A program which read some input file and reports errors in it could look like this: { char *line = NULL; size_t len = 0; unsigned int lineno = 0; error_message_count = 0; while (! feof_unlocked (fp)) { ssize_t n = getline (&line, &len, fp); if (n <= 0) /* End of file or error. */ break; ++lineno; /* Process the line. */ ... if (Detect error in line) error_at_line (0, errval, filename, lineno, "some error text %s", some_variable); } if (error_message_count != 0) error (EXIT_FAILURE, 0, "%u errors found", error_message_count); } `error' and `error_at_line' are clearly the functions of choice and enable the programmer to write applications which follow the GNU coding standard. The GNU libc additionally contains functions which are used in BSD for the same purpose. These functions are declared in `err.h'. It is generally advised to not use these functions. They are included only for compatibility. - Function: void warn (const char *FORMAT, ...) The `warn' function is roughly equivalent to a call like error (0, errno, format, the parameters) except that the global variables `error' respects and modifies are not used. - Function: void vwarn (const char *FORMAT, va_list) The `vwarn' function is just like `warn' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void warnx (const char *FORMAT, ...) The `warnx' function is roughly equivalent to a call like error (0, 0, format, the parameters) except that the global variables `error' respects and modifies are not used. The difference to `warn' is that no error number string is printed. - Function: void vwarnx (const char *FORMAT, va_list) The `vwarnx' function is just like `warnx' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void err (int STATUS, const char *FORMAT, ...) The `err' function is roughly equivalent to a call like error (status, errno, format, the parameters) except that the global variables `error' respects and modifies are not used and that the program is exited even if STATUS is zero. - Function: void verr (int STATUS, const char *FORMAT, va_list) The `verr' function is just like `err' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void errx (int STATUS, const char *FORMAT, ...) The `errx' function is roughly equivalent to a call like error (status, 0, format, the parameters) except that the global variables `error' respects and modifies are not used and that the program is exited even if STATUS is zero. The difference to `err' is that no error number string is printed. - Function: void verrx (int STATUS, const char *FORMAT, va_list) The `verrx' function is just like `errx' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. Virtual Memory Allocation And Paging ************************************ This chapter describes how processes manage and use memory in a system that uses the GNU C library. The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory. Memory mapped I/O is not discussed in this chapter. *Note Memory-mapped I/O::. Process Memory Concepts ======================= One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e. not all of these addresses actually can be used to store data. The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a "frame") or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it - there's just a flag saying it is all zeroes. The same frame of real memory or backing store can back multiple virtual pages belonging to multiple processes. This is normally the case, for example, with virtual memory occupied by GNU C library code. The same real memory frame containing the `printf' function backs a virtual memory page in each of the existing processes that has a `printf' call in its program. In order for a program to access any part of a virtual page, the page must at that moment be backed by ("connected to") a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called "paging". When a program attempts to access a page which is not at that moment backed by real memory, this is known as a "page fault". When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called "paging in" or "faulting in"), then resumes the process so that from the process' point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in *Note Locking Pages:: can control it. Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things. Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it's not very interesting. *Note Creating a Process::. Exec is the operation of creating a virtual address space for a process, loading its basic program into it, and executing the program. It is done by the "exec" family of functions (e.g. `execl'). The operation takes a program file (an executable), it allocates space to load all the data in the executable, loads it, and transfers control to it. That data is most notably the instructions of the program (the "text"), but also literals and constants in the program and even some variables: C variables with the static storage class (*note Memory Allocation and C::). Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C library, there are two kinds of programmatic allocation: automatic and dynamic. *Note Memory Allocation and C::. Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. *Note Memory-mapped I/O::. Just as it programmatically allocates memory, the program can programmatically deallocate ("free") it. You can't free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. *Note Program Termination::. A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are: * The "text segment" contains a program's instructions and literals and static constants. It is allocated by exec and stays the same size for the life of the virtual address space. * The "data segment" is working storage for the program. It can be preallocated and preloaded by exec and the process can extend or shrink it by calling functions as described in *Note Resizing the Data Segment::. Its lower end is fixed. * The "stack segment" contains a program stack. It grows as the stack grows, but doesn't shrink when the stack shrinks. Allocating Storage For Program Data =================================== This section covers how ordinary programs manage storage for their data, including the famous `malloc' function and some fancier facilities special the GNU C library and GNU Compiler. Memory Allocation in C Programs ------------------------------- The C language supports two kinds of memory allocation through the variables in C programs: * "Static allocation" is what happens when you declare a static or global variable. Each static or global variable defines one block of space, of a fixed size. The space is allocated once, when your program is started (part of the exec operation), and is never freed. * "Automatic allocation" happens when you declare an automatic variable, such as a function argument or a local variable. The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited. In GNU C, the size of the automatic storage can be an expression that varies. In other C implementations, it must be a constant. A third important kind of memory allocation, "dynamic allocation", is not supported by C variables but is available via GNU C library functions. Dynamic Memory Allocation ......................... "Dynamic memory allocation" is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs. For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line. Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it. When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want. Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve. For example, if you want to allocate dynamically some space to hold a `struct foobar', you cannot declare a variable of type `struct foobar' whose contents are the dynamically allocated space. But you can declare a variable of pointer type `struct foobar *' and assign it the address of the space. Then you can use the operators `*' and `->' on this pointer variable to refer to the contents of the space: { struct foobar *ptr = (struct foobar *) malloc (sizeof (struct foobar)); ptr->name = x; ptr->next = current_foobar; current_foobar = ptr; } Unconstrained Allocation ------------------------ The most general dynamic allocation facility is `malloc'. It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never). Basic Memory Allocation ....................... To allocate a block of memory, call `malloc'. The prototype for this function is in `stdlib.h'. - Function: void * malloc (size_t SIZE) This function returns a pointer to a newly allocated block SIZE bytes long, or a null pointer if the block could not be allocated. The contents of the block are undefined; you must initialize it yourself (or use `calloc' instead; *note Allocating Cleared Space::). Normally you would cast the value as a pointer to the kind of object that you want to store in the block. Here we show an example of doing so, and of initializing the space with zeros using the library function `memset' (*note Copying and Concatenation::): struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo)); You can store the result of `malloc' into any pointer variable without a cast, because ISO C automatically converts the type `void *' to another type of pointer when necessary. But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C. Remember that when allocating space for a string, the argument to `malloc' must be one plus the length of the string. This is because a string is terminated with a null character that doesn't count in the "length" of the string but does need space. For example: char *ptr; ... ptr = (char *) malloc (length + 1); *Note Representation of Strings::, for more information about this. Examples of `malloc' .................... If no more space is available, `malloc' returns a null pointer. You should check the value of _every_ call to `malloc'. It is useful to write a subroutine that calls `malloc' and reports an error if the value is a null pointer, returning only if the value is nonzero. This function is conventionally called `xmalloc'. Here it is: void * xmalloc (size_t size) { register void *value = malloc (size); if (value == 0) fatal ("virtual memory exhausted"); return value; } Here is a real example of using `malloc' (by way of `xmalloc'). The function `savestring' will copy a sequence of characters into a newly allocated null-terminated string: char * savestring (const char *ptr, size_t len) { register char *value = (char *) xmalloc (len + 1); value[len] = '\0'; return (char *) memcpy (value, ptr, len); } The block that `malloc' gives you is guaranteed to be aligned so that it can hold any type of data. In the GNU system, the address is always a multiple of eight on most systems, and a multiple of 16 on 64-bit systems. Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use `memalign', `posix_memalign' or `valloc' (*note Aligned Memory Blocks::). Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to `malloc'. If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that `malloc' uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use `realloc' (*note Changing Block Size::). Freeing Memory Allocated with `malloc' ...................................... When you no longer need a block that you got with `malloc', use the function `free' to make the block available to be allocated again. The prototype for this function is in `stdlib.h'. - Function: void free (void *PTR) The `free' function deallocates the block of memory pointed at by PTR. - Function: void cfree (void *PTR) This function does the same thing as `free'. It's provided for backward compatibility with SunOS; you should use `free' instead. Freeing a block alters the contents of the block. *Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.* Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to: struct chain { struct chain *next; char *name; } void free_chain (struct chain *chain) { while (chain != 0) { struct chain *next = chain->next; free (chain->name); free (chain); chain = next; } } Occasionally, `free' can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to `malloc' to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by `malloc'. There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates. Changing the Size of a Block ............................ Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer. You can make the block longer by calling `realloc'. This function is declared in `stdlib.h'. - Function: void * realloc (void *PTR, size_t NEWSIZE) The `realloc' function changes the size of the block whose address is PTR to be NEWSIZE. Since the space after the end of the block may be in use, `realloc' may find it necessary to copy the block to a new address where more free space is available. The value of `realloc' is the new address of the block. If the block needs to be moved, `realloc' copies the old contents. If you pass a null pointer for PTR, `realloc' behaves just like `malloc (NEWSIZE)'. This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when `realloc' is passed a null pointer. Like `malloc', `realloc' may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated. In most cases it makes no difference what happens to the original block when `realloc' fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use a subroutine, conventionally called `xrealloc', that takes care of the error message as `xmalloc' does for `malloc': void * xrealloc (void *ptr, size_t size) { register void *value = realloc (ptr, size); if (value == 0) fatal ("Virtual memory exhausted"); return value; } You can also use `realloc' to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available. If the new size you specify is the same as the old size, `realloc' is guaranteed to change nothing and return the same address that you gave. Allocating Cleared Space ........................ The function `calloc' allocates memory and clears it to zero. It is declared in `stdlib.h'. - Function: void * calloc (size_t COUNT, size_t ELTSIZE) This function allocates a block long enough to contain a vector of COUNT elements, each of size ELTSIZE. Its contents are cleared to zero before `calloc' returns. You could define `calloc' as follows: void * calloc (size_t count, size_t eltsize) { size_t size = count * eltsize; void *value = malloc (size); if (value != 0) memset (value, 0, size); return value; } But in general, it is not guaranteed that `calloc' calls `malloc' internally. Therefore, if an application provides its own `malloc'/`realloc'/`free' outside the C library, it should always define `calloc', too. Efficiency Considerations for `malloc' ...................................... As opposed to other versions, the `malloc' in the GNU C Library does not round up block sizes to powers of two, neither for large nor for small sizes. Neighboring chunks can be coalesced on a `free' no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation. Very large blocks (much larger than a page) are allocated with `mmap' (anonymous or via `/dev/zero') by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed. Therefore, it cannot happen that a large chunk becomes "locked" in between smaller ones and even after calling `free' wastes memory. The size threshold for `mmap' to be used can be adjusted with `mallopt'. The use of `mmap' can also be disabled completely. Allocating Aligned Memory Blocks ................................ The address of a block returned by `malloc' or `realloc' in the GNU system is always a multiple of eight (or sixteen on 64-bit systems). If you need a block whose address is a multiple of a higher power of two than that, use `memalign', `posix_memalign', or `valloc'. These functions are declared in `stdlib.h'. With the GNU library, you can use `free' to free the blocks that `memalign', `posix_memalign', and `valloc' return. That does not work in BSD, however--BSD does not provide any way to free such blocks. - Function: void * memalign (size_t BOUNDARY, size_t SIZE) The `memalign' function allocates a block of SIZE bytes whose address is a multiple of BOUNDARY. The BOUNDARY must be a power of two! The function `memalign' works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary. - Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE) The `posix_memalign' function is similar to the `memalign' function in that it returns a buffer of SIZE bytes aligned to a multiple of ALIGNMENT. But it adds one requirement to the parameter ALIGNMENT: the value must be a power of two multiple of `sizeof (void *)'. If the function succeeds in allocation memory a pointer to the allocated memory is returned in `*MEMPTR' and the return value is zero. Otherwise the function returns an error value indicating the problem. This function was introduced in POSIX 1003.1d. - Function: void * valloc (size_t SIZE) Using `valloc' is like using `memalign' and passing the page size as the value of the second argument. It is implemented like this: void * valloc (size_t size) { return memalign (getpagesize (), size); } *Note Query Memory Parameters:: for more information about the memory subsystem. Malloc Tunable Parameters ......................... You can adjust some parameters for dynamic memory allocation with the `mallopt' function. This function is the general SVID/XPG interface, defined in `malloc.h'. - Function: int mallopt (int PARAM, int VALUE) When calling `mallopt', the PARAM argument specifies the parameter to be set, and VALUE the new value to be set. Possible choices for PARAM, as defined in `malloc.h', are: `M_TRIM_THRESHOLD' This is the minimum size (in bytes) of the top-most, releasable chunk that will cause `sbrk' to be called with a negative argument in order to return memory to the system. `M_TOP_PAD' This parameter determines the amount of extra memory to obtain from the system when a call to `sbrk' is required. It also specifies the number of bytes to retain when shrinking the heap by calling `sbrk' with a negative argument. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided. `M_MMAP_THRESHOLD' All chunks larger than this value are allocated outside the normal heap, using the `mmap' system call. This way it is guaranteed that the memory for these chunks can be returned to the system on `free'. `M_MMAP_MAX' The maximum number of chunks to allocate with `mmap'. Setting this to zero disables all use of `mmap'. Heap Consistency Checking ......................... You can ask `malloc' to check the consistency of dynamic memory by using the `mcheck' function. This function is a GNU extension, declared in `mcheck.h'. - Function: int mcheck (void (*ABORTFN) (enum mcheck_status STATUS)) Calling `mcheck' tells `malloc' to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with `malloc'. The ABORTFN argument is the function to call when an inconsistency is found. If you supply a null pointer, then `mcheck' uses a default function which prints a message and calls `abort' (*note Aborting a Program::). The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below. It is too late to begin allocation checking once you have allocated anything with `malloc'. So `mcheck' does nothing in that case. The function returns `-1' if you call it too late, and `0' otherwise (when it is successful). The easiest way to arrange to call `mcheck' early enough is to use the option `-lmcheck' when you link your program; then you don't need to modify your program source at all. Alternatively you might use a debugger to insert a call to `mcheck' whenever the program is started, for example these gdb commands will automatically call `mcheck' whenever the program starts: (gdb) break main Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10 (gdb) command 1 Type commands for when breakpoint 1 is hit, one per line. End with a line saying just "end". >call mcheck(0) >continue >end (gdb) ... This will however only work if no initialization function of any object involved calls any of the `malloc' functions since `mcheck' must be called before the first such function. - Function: enum mcheck_status mprobe (void *POINTER) The `mprobe' function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called `mcheck' at the beginning of the program, to do its occasional checks; calling `mprobe' requests an additional consistency check to be done at the time of the call. The argument POINTER must be a pointer returned by `malloc' or `realloc'. `mprobe' returns a value that says what inconsistency, if any, was found. The values are described below. - Data Type: enum mcheck_status This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values: `MCHECK_DISABLED' `mcheck' was not called before the first allocation. No consistency checking can be done. `MCHECK_OK' No inconsistency detected. `MCHECK_HEAD' The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far. `MCHECK_TAIL' The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far. `MCHECK_FREE' The block was already freed. Another possibility to check for and guard against bugs in the use of `malloc', `realloc' and `free' is to set the environment variable `MALLOC_CHECK_'. When `MALLOC_CHECK_' is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of `free' with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If `MALLOC_CHECK_' is set to `0', any detected heap corruption is silently ignored; if set to `1', a diagnostic is printed on `stderr'; if set to `2', `abort' is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down. There is one problem with `MALLOC_CHECK_': in SUID or SGID binaries it could possibly be exploited since diverging from the normal programs behavior it now writes something to the standard error descriptor. Therefore the use of `MALLOC_CHECK_' is disabled by default for SUID and SGID binaries. It can be enabled again by the system administrator by adding a file `/etc/suid-debug' (the content is not important it could be empty). So, what's the difference between using `MALLOC_CHECK_' and linking with `-lmcheck'? `MALLOC_CHECK_' is orthogonal with respect to `-lmcheck'. `-lmcheck' has been added for backward compatibility. Both `MALLOC_CHECK_' and `-lmcheck' should uncover the same bugs - but using `MALLOC_CHECK_' you don't need to recompile your application. Memory Allocation Hooks ....................... The GNU C library lets you modify the behavior of `malloc', `realloc', and `free' by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example. The hook variables are declared in `malloc.h'. - Variable: __malloc_hook The value of this variable is a pointer to the function that `malloc' uses whenever it is called. You should define this function to look like `malloc'; that is, like: void *FUNCTION (size_t SIZE, const void *CALLER) The value of CALLER is the return address found on the stack when the `malloc' function was called. This value allows you to trace the memory consumption of the program. - Variable: __realloc_hook The value of this variable is a pointer to function that `realloc' uses whenever it is called. You should define this function to look like `realloc'; that is, like: void *FUNCTION (void *PTR, size_t SIZE, const void *CALLER) The value of CALLER is the return address found on the stack when the `realloc' function was called. This value allows you to trace the memory consumption of the program. - Variable: __free_hook The value of this variable is a pointer to function that `free' uses whenever it is called. You should define this function to look like `free'; that is, like: void FUNCTION (void *PTR, const void *CALLER) The value of CALLER is the return address found on the stack when the `free' function was called. This value allows you to trace the memory consumption of the program. - Variable: __memalign_hook The value of this variable is a pointer to function that `memalign' uses whenever it is called. You should define this function to look like `memalign'; that is, like: void *FUNCTION (size_t SIZE, size_t ALIGNMENT, const void *CALLER) The value of CALLER is the return address found on the stack when the `memalign' function was called. This value allows you to trace the memory consumption of the program. You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself. - Variable: __malloc_initialize_hook The value of this variable is a pointer to a function that is called once when the malloc implementation is initialized. This is a weak variable, so it can be overridden in the application with a definition like the following: void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook; An issue to look out for is the time at which the malloc hook functions can be safely installed. If the hook functions call the malloc-related functions recursively, it is necessary that malloc has already properly initialized itself at the time when `__malloc_hook' etc. is assigned to. On the other hand, if the hook functions provide a complete malloc implementation of their own, it is vital that the hooks are assigned to _before_ the very first `malloc' call has completed, because otherwise a chunk obtained from the ordinary, un-hooked malloc may later be handed to `__free_hook', for example. In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by `__malloc_initialize_hook'--then the hooks will be set up safely at the right time. Here is an example showing how to use `__malloc_hook' and `__free_hook' properly. It installs a function that prints out information every time `malloc' or `free' is called. We just assume here that `realloc' and `memalign' are not used in our program. /* Prototypes for __malloc_hook, __free_hook */ #include /* Prototypes for our hooks. */ static void *my_init_hook (void); static void *my_malloc_hook (size_t, const void *); static void my_free_hook (void*, const void *); /* Override initializing hook from the C library. */ void (*__malloc_initialize_hook) (void) = my_init_hook; static void my_init_hook (void) { old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; } static void * my_malloc_hook (size_t size, const void *caller) { void *result; /* Restore all old hooks */ __malloc_hook = old_malloc_hook; __free_hook = old_free_hook; /* Call recursively */ result = malloc (size); /* Save underlying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `malloc', so protect it too. */ printf ("malloc (%u) returns %p\n", (unsigned int) size, result); /* Restore our own hooks */ __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; return result; } static void * my_free_hook (void *ptr, const void *caller) { /* Restore all old hooks */ __malloc_hook = old_malloc_hook; __free_hook = old_free_hook; /* Call recursively */ free (ptr); /* Save underlying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `free', so protect it too. */ printf ("freed pointer %p\n", ptr); /* Restore our own hooks */ __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; } main () { ... } The `mcheck' function (*note Heap Consistency Checking::) works by installing such hooks. Statistics for Memory Allocation with `malloc' .............................................. You can get information about dynamic memory allocation by calling the `mallinfo' function. This function and its associated data type are declared in `malloc.h'; they are an extension of the standard SVID/XPG version. - Data Type: struct mallinfo This structure type is used to return information about the dynamic memory allocator. It contains the following members: `int arena' This is the total size of memory allocated with `sbrk' by `malloc', in bytes. `int ordblks' This is the number of chunks not in use. (The memory allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual `malloc' requests; see *Note Efficiency and Malloc::.) `int smblks' This field is unused. `int hblks' This is the total number of chunks allocated with `mmap'. `int hblkhd' This is the total size of memory allocated with `mmap', in bytes. `int usmblks' This field is unused. `int fsmblks' This field is unused. `int uordblks' This is the total size of memory occupied by chunks handed out by `malloc'. `int fordblks' This is the total size of memory occupied by free (not in use) chunks. `int keepcost' This is the size of the top-most releasable chunk that normally borders the end of the heap (i.e. the high end of the virtual address space's data segment). - Function: struct mallinfo mallinfo (void) This function returns information about the current dynamic memory usage in a structure of type `struct mallinfo'. Summary of `malloc'-Related Functions ..................................... Here is a summary of the functions that work with `malloc': `void *malloc (size_t SIZE)' Allocate a block of SIZE bytes. *Note Basic Allocation::. `void free (void *ADDR)' Free a block previously allocated by `malloc'. *Note Freeing after Malloc::. `void *realloc (void *ADDR, size_t SIZE)' Make a block previously allocated by `malloc' larger or smaller, possibly by copying it to a new location. *Note Changing Block Size::. `void *calloc (size_t COUNT, size_t ELTSIZE)' Allocate a block of COUNT * ELTSIZE bytes using `malloc', and set its contents to zero. *Note Allocating Cleared Space::. `void *valloc (size_t SIZE)' Allocate a block of SIZE bytes, starting on a page boundary. *Note Aligned Memory Blocks::. `void *memalign (size_t SIZE, size_t BOUNDARY)' Allocate a block of SIZE bytes, starting on an address that is a multiple of BOUNDARY. *Note Aligned Memory Blocks::. `int mallopt (int PARAM, int VALUE)' Adjust a tunable parameter. *Note Malloc Tunable Parameters::. `int mcheck (void (*ABORTFN) (void))' Tell `malloc' to perform occasional consistency checks on dynamically allocated memory, and to call ABORTFN when an inconsistency is found. *Note Heap Consistency Checking::. `void *(*__malloc_hook) (size_t SIZE, const void *CALLER)' A pointer to a function that `malloc' uses whenever it is called. `void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)' A pointer to a function that `realloc' uses whenever it is called. `void (*__free_hook) (void *PTR, const void *CALLER)' A pointer to a function that `free' uses whenever it is called. `void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)' A pointer to a function that `memalign' uses whenever it is called. `struct mallinfo mallinfo (void)' Return information about the current dynamic memory usage. *Note Statistics of Malloc::. Allocation Debugging -------------------- A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later. The `malloc' implementation in the GNU C library provides some simple means to detect such leaks and obtain some information to find the location. To do this the application must be started in a special mode which is enabled by an environment variable. There are no speed penalties for the program if the debugging mode is not enabled. How to install the tracing functionality ........................................ - Function: void mtrace (void) When the `mtrace' function is called it looks for an environment variable named `MALLOC_TRACE'. This variable is supposed to contain a valid file name. The user must have write access. If the file already exists it is truncated. If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done. The behavior of `malloc' etc. is not changed. For obvious reasons this also happens if the application is installed with the SUID or SGID bit set. If the named file is successfully opened, `mtrace' installs special handlers for the functions `malloc', `realloc', and `free' (*note Hooks for Malloc::). From then on, all uses of these functions are traced and protocolled into the file. There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during normal use. This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'. - Function: void muntrace (void) The `muntrace' function can be called after `mtrace' was used to enable tracing the `malloc' calls. If no (successful) call of `mtrace' was made `muntrace' does nothing. Otherwise it deinstalls the handlers for `malloc', `realloc', and `free' and then closes the protocol file. No calls are protocolled anymore and the program runs again at full speed. This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'. Example program excerpts ........................ Even though the tracing functionality does not influence the runtime behavior of the program it is not a good idea to call `mtrace' in all programs. Just imagine that you debug a program using `mtrace' and all other programs used in the debugging session also trace their `malloc' calls. The output file would be the same for all programs and thus is unusable. Therefore one should call `mtrace' only if compiled for debugging. A program could therefore start like this: #include int main (int argc, char *argv[]) { #ifdef DEBUGGING mtrace (); #endif ... } This is all what is needed if you want to trace the calls during the whole runtime of the program. Alternatively you can stop the tracing at any time with a call to `muntrace'. It is even possible to restart the tracing again with a new call to `mtrace'. But this can cause unreliable results since there may be calls of the functions which are not called. Please note that not only the application uses the traced functions, also libraries (including the C library itself) use these functions. This last point is also why it is no good idea to call `muntrace' before the program terminated. The libraries are informed about the termination of the program only after the program returns from `main' or calls `exit' and so cannot free the memory they use before this time. So the best thing one can do is to call `mtrace' as the very first function in the program and never call `muntrace'. So the program traces almost all uses of the `malloc' functions (except those calls which are executed by constructors of the program or used libraries). Some more or less clever ideas .............................. You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program: #include #include static void enable (int sig) { mtrace (); signal (SIGUSR1, enable); } static void disable (int sig) { muntrace (); signal (SIGUSR2, disable); } int main (int argc, char *argv[]) { ... signal (SIGUSR1, enable); signal (SIGUSR2, disable); ... } I.e., the user can start the memory debugger any time s/he wants if the program was started with `MALLOC_TRACE' set in the environment. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless. Interpreting the traces ....................... If you take a look at the output it will look similar to this: = Start [0x8048209] - 0x8064cc8 [0x8048209] - 0x8064ce0 [0x8048209] - 0x8064cf8 [0x80481eb] + 0x8064c48 0x14 [0x80481eb] + 0x8064c60 0x14 [0x80481eb] + 0x8064c78 0x14 [0x80481eb] + 0x8064c90 0x14 = End What this all means is not really important since the trace file is not meant to be read by a human. Therefore no attention is given to readability. Instead there is a program which comes with the GNU C library which interprets the traces and outputs a summary in an user-friendly way. The program is called `mtrace' (it is in fact a Perl script) and it takes one or two arguments. In any case the name of the file with the trace output must be specified. If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace. drepper$ mtrace tst-mtrace log No memory leaks. In this case the program `tst-mtrace' was run and it produced a trace file `log'. The message printed by `mtrace' shows there are no problems with the code, all allocated memory was freed afterwards. If we call `mtrace' on the example trace given above we would get a different outout: drepper$ mtrace errlog - 0x08064cc8 Free 2 was never alloc'd 0x8048209 - 0x08064ce0 Free 3 was never alloc'd 0x8048209 - 0x08064cf8 Free 4 was never alloc'd 0x8048209 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at 0x80481eb 0x08064c60 0x14 at 0x80481eb 0x08064c78 0x14 at 0x80481eb 0x08064c90 0x14 at 0x80481eb We have called `mtrace' with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace. We can do better: drepper$ mtrace tst errlog - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39 - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39 - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at /home/drepper/tst.c:33 0x08064c60 0x14 at /home/drepper/tst.c:33 0x08064c78 0x14 at /home/drepper/tst.c:33 0x08064c90 0x14 at /home/drepper/tst.c:33 Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found. Interpreting this output is not complicated. There are at most two different situations being detected. First, `free' was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output. Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes. The other situation which is much harder to detect are memory leaks. As you can see in the output the `mtrace' function collects all this information and so can say that the program calls an allocation function from line 33 in the source file `/home/drepper/tst-mtrace.c' four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated. Obstacks -------- An "obstack" is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other. Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary. Creating Obstacks ................. The utilities for manipulating obstacks are declared in the header file `obstack.h'. - Data Type: struct obstack An obstack is represented by a data structure of type `struct obstack'. This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves. You should not try to access the contents of the structure directly; use only the functions described in this chapter. You can declare variables of type `struct obstack' and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object. Dynamic allocation of obstacks allows your program to have a variable number of different stacks. (You can even allocate an obstack structure in another obstack, but this is rarely useful.) All the functions that work with obstacks require you to specify which obstack to use. You do this with a pointer of type `struct obstack *'. In the following, we often say "an obstack" when strictly speaking the object at hand is such a pointer. The objects in the obstack are packed into large blocks called "chunks". The `struct obstack' structure points to a chain of the chunks currently in use. The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk. Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk. Usually you supply a function which uses `malloc' directly or indirectly. You must also supply a function to free a chunk. These matters are described in the following section. Preparing for Using Obstacks ............................ Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this: #include Also, if the source file uses the macro `obstack_init', it must declare or define two functions or macros that will be called by the obstack library. One, `obstack_chunk_alloc', is used to allocate the chunks of memory into which objects are packed. The other, `obstack_chunk_free', is used to return chunks when the objects in them are freed. These macros should appear before any use of obstacks in the source file. Usually these are defined to use `malloc' via the intermediary `xmalloc' (*note Unconstrained Allocation::). This is done with the following pair of macro definitions: #define obstack_chunk_alloc xmalloc #define obstack_chunk_free free Though the memory you get using obstacks really comes from `malloc', using obstacks is faster because `malloc' is called less often, for larger blocks of memory. *Note Obstack Chunks::, for full details. At run time, before the program can use a `struct obstack' object as an obstack, it must initialize the obstack by calling `obstack_init'. - Function: int obstack_init (struct obstack *OBSTACK-PTR) Initialize obstack OBSTACK-PTR for allocation of objects. This function calls the obstack's `obstack_chunk_alloc' function. If allocation of memory fails, the function pointed to by `obstack_alloc_failed_handler' is called. The `obstack_init' function always returns 1 (Compatibility notice: Former versions of obstack returned 0 if allocation failed). Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable: static struct obstack myobstack; ... obstack_init (&myobstack); Second, an obstack that is itself dynamically allocated: struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr); - Variable: obstack_alloc_failed_handler The value of this variable is a pointer to a function that `obstack' uses when `obstack_chunk_alloc' fails to allocate memory. The default action is to print a message and abort. You should supply a function that either calls `exit' (*note Program Termination::) or `longjmp' (*note Non-Local Exits::) and doesn't return. void my_obstack_alloc_failed (void) ... obstack_alloc_failed_handler = &my_obstack_alloc_failed; Allocation in an Obstack ........................ The most direct way to allocate an object in an obstack is with `obstack_alloc', which is invoked almost like `malloc'. - Function: void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE) This allocates an uninitialized block of SIZE bytes in an obstack and returns its address. Here OBSTACK-PTR specifies which obstack to allocate the block in; it is the address of the `struct obstack' object which represents the obstack. Each obstack function or macro requires you to specify an OBSTACK-PTR as the first argument. This function calls the obstack's `obstack_chunk_alloc' function if it needs to allocate a new chunk of memory; it calls `obstack_alloc_failed_handler' if allocation of memory by `obstack_chunk_alloc' failed. For example, here is a function that allocates a copy of a string STR in a specific obstack, which is in the variable `string_obstack': struct obstack string_obstack; char * copystring (char *string) { size_t len = strlen (string) + 1; char *s = (char *) obstack_alloc (&string_obstack, len); memcpy (s, string, len); return s; } To allocate a block with specified contents, use the function `obstack_copy', declared like this: - Function: void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE) This allocates a block and initializes it by copying SIZE bytes of data starting at ADDRESS. It calls `obstack_alloc_failed_handler' if allocation of memory by `obstack_chunk_alloc' failed. - Function: void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE) Like `obstack_copy', but appends an extra byte containing a null character. This extra byte is not counted in the argument SIZE. The `obstack_copy0' function is convenient for copying a sequence of characters into an obstack as a null-terminated string. Here is an example of its use: char * obstack_savestring (char *addr, int size) { return obstack_copy0 (&myobstack, addr, size); } Contrast this with the previous example of `savestring' using `malloc' (*note Basic Allocation::). Freeing Objects in an Obstack ............................. To free an object allocated in an obstack, use the function `obstack_free'. Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack. - Function: void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT) If OBJECT is a null pointer, everything allocated in the obstack is freed. Otherwise, OBJECT must be the address of an object allocated in the obstack. Then OBJECT is freed, along with everything allocated in OBSTACK since OBJECT. Note that if OBJECT is a null pointer, the result is an uninitialized obstack. To free all memory in an obstack but leave it valid for further allocation, call `obstack_free' with the address of the first object allocated on the obstack: obstack_free (obstack_ptr, first_object_allocated_ptr); Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (*note Preparing for Obstacks::). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk. Obstack Functions and Macros ............................ The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C. If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address). Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this: obstack_alloc (get_obstack (), 4); you will find that `get_obstack' may be called several times. If you use `*obstack_list_ptr++' as the obstack pointer argument, you will get very strange results since the incrementation may occur several times. In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here: char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc; This is the same situation that exists in ISO C for the standard library functions. *Note Macro Definitions::. *Warning:* When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C. If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once. Growing Objects ............... Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of "growing objects". The special functions for adding data to the growing object are described in this section. You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it. However, it is necessary to say explicitly when the object is finished. This is done with the function `obstack_finish'. The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk. While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object. - Function: void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE) The most basic function for adding to a growing object is `obstack_blank', which adds space without initializing it. - Function: void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE) To add a block of initialized space, use `obstack_grow', which is the growing-object analogue of `obstack_copy'. It adds SIZE bytes of data to the growing object, copying the contents from DATA. - Function: void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE) This is the growing-object analogue of `obstack_copy0'. It adds SIZE bytes copied from DATA, followed by an additional null character. - Function: void obstack_1grow (struct obstack *OBSTACK-PTR, char C) To add one character at a time, use the function `obstack_1grow'. It adds a single byte containing C to the growing object. - Function: void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA) Adding the value of a pointer one can use the function `obstack_ptr_grow'. It adds `sizeof (void *)' bytes containing the value of DATA. - Function: void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA) A single value of type `int' can be added by using the `obstack_int_grow' function. It adds `sizeof (int)' bytes to the growing object and initializes them with the value of DATA. - Function: void * obstack_finish (struct obstack *OBSTACK-PTR) When you are finished growing the object, use the function `obstack_finish' to close it off and return its final address. Once you have finished the object, the obstack is available for ordinary allocation or for growing another object. This function can return a null pointer under the same conditions as `obstack_alloc' (*note Allocation in an Obstack::). When you build an object by growing it, you will probably need to know afterward how long it became. You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function `obstack_object_size', declared as follows: - Function: int obstack_object_size (struct obstack *OBSTACK-PTR) This function returns the current size of the growing object, in bytes. Remember to call this function _before_ finishing the object. After it is finished, `obstack_object_size' will return zero. If you have started growing an object and wish to cancel it, you should finish it and then free it, like this: obstack_free (obstack_ptr, obstack_finish (obstack_ptr)); This has no effect if no object was growing. You can use `obstack_blank' with a negative size argument to make the current object smaller. Just don't try to shrink it beyond zero length--there's no telling what will happen if you do that. Extra Fast Growing Objects .......................... The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant. You can reduce the overhead by using special "fast growth" functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster. The function `obstack_room' returns the amount of room available in the current chunk. It is declared as follows: - Function: int obstack_room (struct obstack *OBSTACK-PTR) This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack OBSTACK using the fast growth functions. While you know there is room, you can use these fast growth functions for adding data to a growing object: - Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C) The function `obstack_1grow_fast' adds one byte containing the character C to the growing object in obstack OBSTACK-PTR. - Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA) The function `obstack_ptr_grow_fast' adds `sizeof (void *)' bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR. - Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA) The function `obstack_int_grow_fast' adds `sizeof (int)' bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR. - Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE) The function `obstack_blank_fast' adds SIZE bytes to the growing object in obstack OBSTACK-PTR without initializing them. When you check for space using `obstack_room' and there is not enough room for what you want to add, the fast growth functions are not safe. In this case, simply use the corresponding ordinary growth function instead. Very soon this will copy the object to a new chunk; then there will be lots of room available again. So, each time you use an ordinary growth function, check afterward for sufficient space using `obstack_room'. Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again. Here is an example: void add_string (struct obstack *obstack, const char *ptr, int len) { while (len > 0) { int room = obstack_room (obstack); if (room == 0) { /* Not enough room. Add one character slowly, which may copy to a new chunk and make room. */ obstack_1grow (obstack, *ptr++); len--; } else { if (room > len) room = len; /* Add fast as much as we have room for. */ len -= room; while (room-- > 0) obstack_1grow_fast (obstack, *ptr++); } } } Status of an Obstack .................... Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it. - Function: void * obstack_base (struct obstack *OBSTACK-PTR) This function returns the tentative address of the beginning of the currently growing object in OBSTACK-PTR. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk--then its address will change! If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk). - Function: void * obstack_next_free (struct obstack *OBSTACK-PTR) This function returns the address of the first free byte in the current chunk of obstack OBSTACK-PTR. This is the end of the currently growing object. If no object is growing, `obstack_next_free' returns the same value as `obstack_base'. - Function: int obstack_object_size (struct obstack *OBSTACK-PTR) This function returns the size in bytes of the currently growing object. This is equivalent to obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR) Alignment of Data in Obstacks ............................. Each obstack has an "alignment boundary"; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is 4 bytes. To access an obstack's alignment boundary, use the macro `obstack_alignment_mask', whose function prototype looks like this: - Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR) The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0. The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2. The default value of the mask is 3, so that addresses are multiples of 4. A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required). The expansion of the macro `obstack_alignment_mask' is an lvalue, so you can alter the mask by assignment. For example, this statement: obstack_alignment_mask (obstack_ptr) = 0; has the effect of turning off alignment processing in the specified obstack. Note that a change in alignment mask does not take effect until _after_ the next time an object is allocated or finished in the obstack. If you are not growing an object, you can make the new alignment mask take effect immediately by calling `obstack_finish'. This will finish a zero-length object and then do proper alignment for the next object. Obstack Chunks .............. Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects. The obstack library allocates chunks by calling the function `obstack_chunk_alloc', which you must define. When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling `obstack_chunk_free', which you must also define. These two must be defined (as macros) or declared (as functions) in each source file that uses `obstack_init' (*note Creating Obstacks::). Most often they are defined as macros like this: #define obstack_chunk_alloc malloc #define obstack_chunk_free free Note that these are simple macros (no arguments). Macro definitions with arguments will not work! It is necessary that `obstack_chunk_alloc' or `obstack_chunk_free', alone, expand into a function name if it is not itself a function name. If you allocate chunks with `malloc', the chunk size should be a power of 2. The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used. - Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR) This returns the chunk size of the given obstack. Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly: if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE) obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE; Summary of Obstack Functions ............................ Here is a summary of all the functions associated with obstacks. Each takes the address of an obstack (`struct obstack *') as its first argument. `void obstack_init (struct obstack *OBSTACK-PTR)' Initialize use of an obstack. *Note Creating Obstacks::. `void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)' Allocate an object of SIZE uninitialized bytes. *Note Allocation in an Obstack::. `void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE bytes, with contents copied from ADDRESS. *Note Allocation in an Obstack::. `void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE+1 bytes, with SIZE of them copied from ADDRESS, followed by a null character at the end. *Note Allocation in an Obstack::. `void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)' Free OBJECT (and everything allocated in the specified obstack more recently than OBJECT). *Note Freeing Obstack Objects::. `void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object. *Note Growing Objects::. `void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object. *Note Growing Objects::. `void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object, and then add another byte containing a null character. *Note Growing Objects::. `void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object. *Note Growing Objects::. `void *obstack_finish (struct obstack *OBSTACK-PTR)' Finalize the object that is growing and return its permanent address. *Note Growing Objects::. `int obstack_object_size (struct obstack *OBSTACK-PTR)' Get the current size of the currently growing object. *Note Growing Objects::. `void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `int obstack_room (struct obstack *OBSTACK-PTR)' Get the amount of room now available for growing the current object. *Note Extra Fast Growing::. `int obstack_alignment_mask (struct obstack *OBSTACK-PTR)' The mask used for aligning the beginning of an object. This is an lvalue. *Note Obstacks Data Alignment::. `int obstack_chunk_size (struct obstack *OBSTACK-PTR)' The size for allocating chunks. This is an lvalue. *Note Obstack Chunks::. `void *obstack_base (struct obstack *OBSTACK-PTR)' Tentative starting address of the currently growing object. *Note Status of an Obstack::. `void *obstack_next_free (struct obstack *OBSTACK-PTR)' Address just after the end of the currently growing object. *Note Status of an Obstack::. Automatic Storage with Variable Size ------------------------------------ The function `alloca' supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically. Allocating a block with `alloca' is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time. But all the blocks are freed when you exit the function that `alloca' was called from, just as if they were automatic variables declared in that function. There is no way to free the space explicitly. The prototype for `alloca' is in `stdlib.h'. This function is a BSD extension. - Function: void * alloca (size_t SIZE); The return value of `alloca' is the address of a block of SIZE bytes of memory, allocated in the stack frame of the calling function. Do not use `alloca' inside the arguments of a function call--you will get unpredictable results, because the stack space for the `alloca' would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is `foo (x, alloca (4), y)'. `alloca' Example ................ As an example of the use of `alloca', here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure: int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1); stpcpy (stpcpy (name, str1), str2); return open (name, flags, mode); } Here is how you would get the same results with `malloc' and `free': int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1); int desc; if (name == 0) fatal ("virtual memory exceeded"); stpcpy (stpcpy (name, str1), str2); desc = open (name, flags, mode); free (name); return desc; } As you can see, it is simpler with `alloca'. But `alloca' has other, more important advantages, and some disadvantages. Advantages of `alloca' ...................... Here are the reasons why `alloca' may be preferable to `malloc': * Using `alloca' wastes very little space and is very fast. (It is open-coded by the GNU C compiler.) * Since `alloca' does not have separate pools for different sizes of block, space used for any size block can be reused for any other size. `alloca' does not cause memory fragmentation. * Nonlocal exits done with `longjmp' (*note Non-Local Exits::) automatically free the space allocated with `alloca' when they exit through the function that called `alloca'. This is the most important reason to use `alloca'. To illustrate this, suppose you have a function `open_or_report_error' which returns a descriptor, like `open', if it succeeds, but does not return to its caller if it fails. If the file cannot be opened, it prints an error message and jumps out to the command level of your program using `longjmp'. Let's change `open2' (*note Alloca Example::) to use this subroutine: int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1); stpcpy (stpcpy (name, str1), str2); return open_or_report_error (name, flags, mode); } Because of the way `alloca' works, the memory it allocates is freed even when an error occurs, with no special effort required. By contrast, the previous definition of `open2' (which uses `malloc' and `free') would develop a memory leak if it were changed in this way. Even if you are willing to make more changes to fix it, there is no easy way to do so. Disadvantages of `alloca' ......................... These are the disadvantages of `alloca' in comparison with `malloc': * If you try to allocate more memory than the machine can provide, you don't get a clean error message. Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation (*note Program Error Signals::). * Some non-GNU systems fail to support `alloca', so it is less portable. However, a slower emulation of `alloca' written in C is available for use on systems with this deficiency. GNU C Variable-Size Arrays .......................... In GNU C, you can replace most uses of `alloca' with an array of variable size. Here is how `open2' would look then: int open2 (char *str1, char *str2, int flags, int mode) { char name[strlen (str1) + strlen (str2) + 1]; stpcpy (stpcpy (name, str1), str2); return open (name, flags, mode); } But `alloca' is not always equivalent to a variable-sized array, for several reasons: * A variable size array's space is freed at the end of the scope of the name of the array. The space allocated with `alloca' remains until the end of the function. * It is possible to use `alloca' within a loop, allocating an additional block on each iteration. This is impossible with variable-sized arrays. *Note:* If you mix use of `alloca' and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with `alloca' during the execution of that scope. Resizing the Data Segment ========================= The symbols in this section are declared in `unistd.h'. You will not normally use the functions in this section, because the functions described in *Note Memory Allocation:: are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls. - Function: int brk (void *ADDR) `brk' sets the high end of the calling process' data segment to ADDR. The address of the end of a segment is defined to be the address of the last byte in the segment plus 1. The function has no effect if ADDR is lower than the low end of the data segment. (This is considered success, by the way). The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit (*note Limits on Resources::). The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the "break". The return value is zero on success. On failure, the return value is `-1' and `errno' is set accordingly. The following `errno' values are specific to this function: `ENOMEM' The request would cause the data segment to overlap another segment or exceed the process' data storage limit. - Function: int sbrk (ptrdiff_t DELTA) This function is the same as `brk' except that you specify the new end of the data segment as an offset DELTA from the current end and on success the return value is the address of the resulting end of the data segment instead of zero. This means you can use `sbrk(0)' to find out what the current end of the data segment is. Locking Pages ============= You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way -- i.e. cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault. This is called "locking" a page. The functions in this chapter lock and unlock the calling process' pages. Why Lock Pages -------------- Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are: * Speed. A page fault is transparent only insofar as the process is not sensitive to how long it takes to do a simple memory access. Time-critical processes, especially realtime processes, may not be able to wait or may not be able to tolerate variance in execution speed. A process that needs to lock pages for this reason probably also needs priority among other processes for use of the CPU. *Note Priority::. In some cases, the programmer knows better than the system's demand paging allocator which pages should remain in real memory to optimize system performance. In this case, locking pages can help. * Privacy. If you keep secrets in virtual memory and that virtual memory gets paged out, that increases the chance that the secrets will get out. If a password gets written out to disk swap space, for example, it might still be there long after virtual and real memory have been wiped clean. Be aware that when you lock a page, that's one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory. Locked Memory Details --------------------- A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don't page it out. Memory locks do not stack. I.e. you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn't. A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more). Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). *Note Creating a Process::. Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page. The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. *Note Limits on Resources::. In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked. But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data. This is known as a "copy-on-write page fault". It takes a small amount of time and in a pathological case, getting that frame may require I/O. To make sure this doesn't happen to your program, don't just lock the pages. Write to them as well, unless you know you won't write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope. Functions To Lock And Unlock Pages ---------------------------------- The symbols in this section are declared in `sys/mman.h'. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They _are_ available with a Linux kernel. *Portability Note:* POSIX.1b requires that when the `mlock' and `munlock' functions are available, the file `unistd.h' define the macro `_POSIX_MEMLOCK_RANGE' and the file `limits.h' define the macro `PAGESIZE' to be the size of a memory page in bytes. It requires that when the `mlockall' and `munlockall' functions are available, the `unistd.h' file define the macro `_POSIX_MEMLOCK'. The GNU C library conforms to this requirement. - Function: int mlock (const void *ADDR, size_t LEN) `mlock' locks a range of the calling process' virtual pages. The range of memory starts at address ADDR and is LEN bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range. When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them. When the function fails, it does not affect the lock status of any pages. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. `errno' values specific to this function are: `ENOMEM' * At least some of the specified address range does not exist in the calling process' virtual address space. * The locking would cause the process to exceed its locked page limit. `EPERM' The calling process is not superuser. `EINVAL' LEN is not positive. `ENOSYS' The kernel does not provide `mlock' capability. You can lock _all_ a process' memory with `mlockall'. You unlock memory with `munlock' or `munlockall'. To avoid all page faults in a C program, you have to use `mlockall', because some of the memory a program uses is hidden from the C code, e.g. the stack and automatic variables, and you wouldn't know what address to tell `mlock'. - Function: int munlock (const void *ADDR, size_t LEN) `mlock' unlocks a range of the calling process' virtual pages. `munlock' is the inverse of `mlock' and functions completely analogously to `mlock', except that there is no `EPERM' failure. - Function: int mlockall (int FLAGS) `mlockall' locks all the pages in a process' virtual memory address space, and/or any that are added to it in the future. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory, and memory mapped files. FLAGS is a string of single bit flags represented by the following macros. They tell `mlockall' which of its functions you want. All other bits must be zero. `MCL_CURRENT' Lock all pages which currently exist in the calling process' virtual address space. `MCL_FUTURE' Set a mode such that any pages added to the process' virtual address space in the future will be locked from birth. This mode does not affect future address spaces owned by the same process so exec, which replaces a process' address space, wipes out `MCL_FUTURE'. *Note Executing a File::. When the function returns successfully, and you specified `MCL_CURRENT', all of the process' pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them. When the process is in `MCL_FUTURE' mode because it successfully executed this function and specified `MCL_CURRENT', any system call by the process that requires space be added to its virtual address space fails with `errno' = `ENOMEM' if locking the additional space would cause the process to exceed its locked page limit. In the case that the address space addition that can't be accommodated is stack expansion, the stack expansion fails and the kernel sends a `SIGSEGV' signal to the process. When the function fails, it does not affect the lock status of any pages or the future locking mode. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. `errno' values specific to this function are: `ENOMEM' * At least some of the specified address range does not exist in the calling process' virtual address space. * The locking would cause the process to exceed its locked page limit. `EPERM' The calling process is not superuser. `EINVAL' Undefined bits in FLAGS are not zero. `ENOSYS' The kernel does not provide `mlockall' capability. You can lock just specific pages with `mlock'. You unlock pages with `munlockall' and `munlock'. - Function: int munlockall (void) `munlockall' unlocks every page in the calling process' virtual address space and turn off `MCL_FUTURE' future locking mode. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific `errno' values. Character Handling ****************** Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose. Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale. (More precisely, they are affected by the locale currently selected for character classification--the `LC_CTYPE' category; see *Note Locale Categories::.) The ISO C standard specifies two different sets of functions. The one set works on `char' type characters, the other one on `wchar_t' wide characters (*note Extended Char Intro::). Classification of Characters ============================ This section explains the library functions for classifying characters. For example, `isalpha' is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this: if (isalpha (c)) printf ("The character `%c' is alphabetic.\n", c); Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with `is'. Each of them takes one argument, which is a character to test, and returns an `int' which is treated as a boolean value. The character argument is passed as an `int', and it may be the constant value `EOF' instead of a real character. The attributes of any given character can vary between locales. *Note Locales::, for more information on locales. These functions are declared in the header file `ctype.h'. - Function: int islower (int C) Returns true if C is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. - Function: int isupper (int C) Returns true if C is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. - Function: int isalpha (int C) Returns true if C is an alphabetic character (a letter). If `islower' or `isupper' is true of a character, then `isalpha' is also true. In some locales, there may be additional characters for which `isalpha' is true--letters which are neither upper case nor lower case. But in the standard `"C"' locale, there are no such additional characters. - Function: int isdigit (int C) Returns true if C is a decimal digit (`0' through `9'). - Function: int isalnum (int C) Returns true if C is an alphanumeric character (a letter or number); in other words, if either `isalpha' or `isdigit' is true of a character, then `isalnum' is also true. - Function: int isxdigit (int C) Returns true if C is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'. - Function: int ispunct (int C) Returns true if C is a punctuation character. This means any printing character that is not alphanumeric or a space character. - Function: int isspace (int C) Returns true if C is a "whitespace" character. In the standard `"C"' locale, `isspace' returns true for only the standard whitespace characters: `' '' space `'\f'' formfeed `'\n'' newline `'\r'' carriage return `'\t'' horizontal tab `'\v'' vertical tab - Function: int isblank (int C) Returns true if C is a blank character; that is, a space or a tab. This function is a GNU extension. - Function: int isgraph (int C) Returns true if C is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic. - Function: int isprint (int C) Returns true if C is a printing character. Printing characters include all the graphic characters, plus the space (` ') character. - Function: int iscntrl (int C) Returns true if C is a control character (that is, a character that is not a printing character). - Function: int isascii (int C) Returns true if C is a 7-bit `unsigned char' value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension. Case Conversion =============== This section explains the library functions for performing conversions such as case mappings on characters. For example, `toupper' converts any character to upper case if possible. If the character can't be converted, `toupper' returns it unchanged. These functions take one argument of type `int', which is the character to convert, and return the converted character as an `int'. If the conversion is not applicable to the argument given, the argument is returned unchanged. *Compatibility Note:* In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion. Thus for portability, you may need to write `islower(c) ? toupper(c) : c' rather than just `toupper(c)'. These functions are declared in the header file `ctype.h'. - Function: int tolower (int C) If C is an upper-case letter, `tolower' returns the corresponding lower-case letter. If C is not an upper-case letter, C is returned unchanged. - Function: int toupper (int C) If C is a lower-case letter, `toupper' returns the corresponding upper-case letter. Otherwise C is returned unchanged. - Function: int toascii (int C) This function converts C to a 7-bit `unsigned char' value that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension. - Function: int _tolower (int C) This is identical to `tolower', and is provided for compatibility with the SVID. *Note SVID::. - Function: int _toupper (int C) This is identical to `toupper', and is provided for compatibility with the SVID. Character class determination for wide characters ================================================= Amendment 1 to ISO C90 defines functions to classify wide characters. Although the original ISO C90 standard already defined the type `wchar_t', no functions operating on them were defined. The general design of the classification functions for wide characters is more general. It allows extensions to the set of available classifications, beyond those which are always available. The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C library implementation of the `localedef' program. The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class. For the wide character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value. On top of this the normal character classification functions as used for `char' objects can be defined. - Data type: wctype_t The `wctype_t' can hold a value which represents a character class. The only defined way to generate such a value is by using the `wctype' function. This type is defined in `wctype.h'. - Function: wctype_t wctype (const char *PROPERTY) The `wctype' returns a value representing a class of wide characters which is identified by the string PROPERTY. Beside some standard properties each locale can define its own ones. In case no property with the given name is known for the current locale selected for the `LC_CTYPE' category, the function returns zero. The properties known in every locale are: `"alnum"' `"alpha"' `"cntrl"' `"digit"' `"graph"' `"lower"' `"print"' `"punct"' `"space"' `"upper"' `"xdigit"' This function is declared in `wctype.h'. To test the membership of a character to one of the non-standard classes the ISO C standard defines a completely new function. - Function: int iswctype (wint_t WC, wctype_t DESC) This function returns a nonzero value if WC is in the character class specified by DESC. DESC must previously be returned by a successful call to `wctype'. This function is declared in `wctype.h'. To make it easier to use the commonly-used classification functions, they are defined in the C library. There is no need to use `wctype' if the property string is one of the known character classes. In some situations it is desirable to construct the property strings, and then it is important that `wctype' can also handle the standard classes. - Function: int iswalnum (wint_t WC) This function returns a nonzero value if WC is an alphanumeric character (a letter or number); in other words, if either `iswalpha' or `iswdigit' is true of a character, then `iswalnum' is also true. This function can be implemented using iswctype (wc, wctype ("alnum")) It is declared in `wctype.h'. - Function: int iswalpha (wint_t WC) Returns true if WC is an alphabetic character (a letter). If `iswlower' or `iswupper' is true of a character, then `iswalpha' is also true. In some locales, there may be additional characters for which `iswalpha' is true--letters which are neither upper case nor lower case. But in the standard `"C"' locale, there are no such additional characters. This function can be implemented using iswctype (wc, wctype ("alpha")) It is declared in `wctype.h'. - Function: int iswcntrl (wint_t WC) Returns true if WC is a control character (that is, a character that is not a printing character). This function can be implemented using iswctype (wc, wctype ("cntrl")) It is declared in `wctype.h'. - Function: int iswdigit (wint_t WC) Returns true if WC is a digit (e.g., `0' through `9'). Please note that this function does not only return a nonzero value for _decimal_ digits, but for all kinds of digits. A consequence is that code like the following will *not* work unconditionally for wide characters: n = 0; while (iswdigit (*wc)) { n *= 10; n += *wc++ - L'0'; } This function can be implemented using iswctype (wc, wctype ("digit")) It is declared in `wctype.h'. - Function: int iswgraph (wint_t WC) Returns true if WC is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic. This function can be implemented using iswctype (wc, wctype ("graph")) It is declared in `wctype.h'. - Function: int iswlower (wint_t WC) Returns true if WC is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. This function can be implemented using iswctype (wc, wctype ("lower")) It is declared in `wctype.h'. - Function: int iswprint (wint_t WC) Returns true if WC is a printing character. Printing characters include all the graphic characters, plus the space (` ') character. This function can be implemented using iswctype (wc, wctype ("print")) It is declared in `wctype.h'. - Function: int iswpunct (wint_t WC) Returns true if WC is a punctuation character. This means any printing character that is not alphanumeric or a space character. This function can be implemented using iswctype (wc, wctype ("punct")) It is declared in `wctype.h'. - Function: int iswspace (wint_t WC) Returns true if WC is a "whitespace" character. In the standard `"C"' locale, `iswspace' returns true for only the standard whitespace characters: `L' '' space `L'\f'' formfeed `L'\n'' newline `L'\r'' carriage return `L'\t'' horizontal tab `L'\v'' vertical tab This function can be implemented using iswctype (wc, wctype ("space")) It is declared in `wctype.h'. - Function: int iswupper (wint_t WC) Returns true if WC is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. This function can be implemented using iswctype (wc, wctype ("upper")) It is declared in `wctype.h'. - Function: int iswxdigit (wint_t WC) Returns true if WC is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'. This function can be implemented using iswctype (wc, wctype ("xdigit")) It is declared in `wctype.h'. The GNU C library also provides a function which is not defined in the ISO C standard but which is available as a version for single byte characters as well. - Function: int iswblank (wint_t WC) Returns true if WC is a blank character; that is, a space or a tab. This function is a GNU extension. It is declared in `wchar.h'. Notes on using the wide character classes ========================================= The first note is probably not astonishing but still occasionally a cause of problems. The `iswXXX' functions can be implemented using macros and in fact, the GNU C library does this. They are still available as real functions but when the `wctype.h' header is included the macros will be used. This is the same as the `char' type versions of these functions. The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear. int is_in_class (int c, const char *class) { if (strcmp (class, "alnum") == 0) return isalnum (c); if (strcmp (class, "alpha") == 0) return isalpha (c); if (strcmp (class, "cntrl") == 0) return iscntrl (c); ... return 0; } Now, with the `wctype' and `iswctype' you can avoid the `if' cascades, but rewriting the code as follows is wrong: int is_in_class (int c, const char *class) { wctype_t desc = wctype (class); return desc ? iswctype ((wint_t) c, desc) : 0; } The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows: int is_in_class (int c, const char *class) { wctype_t desc = wctype (class); return desc ? iswctype (btowc (c), desc) : 0; } *Note Converting a Character::, for more information on `btowc'. Note that this change probably does not improve the performance of the program a lot since the `wctype' function still has to make the string comparisons. It gets really interesting if the `is_in_class' function is called more than once for the same class name. In this case the variable DESC could be computed once and reused for all the calls. Therefore the above form of the function is probably not the final one. Mapping of wide characters. =========================== The classification functions are also generalized by the ISO C standard. Instead of just allowing the two standard mappings, a locale can contain others. Again, the `localedef' program already supports generating such locale data files. - Data Type: wctrans_t This data type is defined as a scalar type which can hold a value representing the locale-dependent character mapping. There is no way to construct such a value apart from using the return value of the `wctrans' function. This type is defined in `wctype.h'. - Function: wctrans_t wctrans (const char *PROPERTY) The `wctrans' function has to be used to find out whether a named mapping is defined in the current locale selected for the `LC_CTYPE' category. If the returned value is non-zero, you can use it afterwards in calls to `towctrans'. If the return value is zero no such mapping is known in the current locale. Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale: `"tolower"' `"toupper"' These functions are declared in `wctype.h'. - Function: wint_t towctrans (wint_t WC, wctrans_t DESC) `towctrans' maps the input character WC according to the rules of the mapping for which DESC is a descriptor, and returns the value it finds. DESC must be obtained by a successful call to `wctrans'. This function is declared in `wctype.h'. For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call `wctrans' for them. - Function: wint_t towlower (wint_t WC) If WC is an upper-case letter, `towlower' returns the corresponding lower-case letter. If WC is not an upper-case letter, WC is returned unchanged. `towlower' can be implemented using towctrans (wc, wctrans ("tolower")) This function is declared in `wctype.h'. - Function: wint_t towupper (wint_t WC) If WC is a lower-case letter, `towupper' returns the corresponding upper-case letter. Otherwise WC is returned unchanged. `towupper' can be implemented using towctrans (wc, wctrans ("toupper")) This function is declared in `wctype.h'. The same warnings given in the last section for the use of the wide character classification functions apply here. It is not possible to simply cast a `char' type value to a `wint_t' and use it as an argument to `towctrans' calls. String and Array Utilities ************************** Operations on strings (or arrays of characters) are an important part of many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the `memcpy' function can be used to copy the contents of any kind of array. It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability. For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in `strcmp' function, you're less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too. Representation of Strings ========================= This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section. A "string" is an array of `char' objects. But string-valued variables are usually declared to be pointers of type `char *'. Such variables do not include space for the text of a string; that has to be stored somewhere else--in an array variable, a string constant, or dynamically allocated memory (*note Memory Allocation::). It's up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a "null pointer" in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error. "string" normally refers to multibyte character strings as opposed to wide character strings. Wide character strings are arrays of type `wchar_t' and as for multibyte character strings usually pointers of type `wchar_t *' are used. By convention, a "null character", `'\0'', marks the end of a multibyte character string and the "null wide character", `L'\0'', marks the end of a wide character string. For example, in testing to see whether the `char *' variable P points to a null character marking the end of a string, you can write `!*P' or `*P == '\0''. A null character is quite different conceptually from a null pointer, although both are represented by the integer `0'. "String literals" appear in C program source as strings of characters between double-quote characters (`"') where the initial double-quote character is immediately preceded by a capital `L' (ell) character (as in `L"foo"'). In ISO C, string literals can also be formed by "string concatenation": `"a" "b"' is the same as `"ab"'. For wide character strings one can either use `L"a" L"b"' or `L"a" "b"'. Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage. Character arrays that are declared `const' cannot be modified either. It's generally good style to declare non-modifiable string pointers to be of type `const char *', since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string. The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term "allocated size" is always used to refer to the total amount of memory allocated for the string, while the term "length" refers to the number of characters up to (but not including) the terminating null character. A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions _do not_ do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string. Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see *Note Extended Char Intro::). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly. But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes a call to `strncpy' could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte sequence in the target buffer. To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on "wide characters" (*note Extended Char Intro::). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied. The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available. String and Array Conventions ============================ This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters. Functions that operate on arbitrary blocks of memory have names beginning with `mem' and `wmem' (such as `memcpy' and `wmemcpy') and invariably take an argument which specifies the size (in bytes and wide characters respectively) of the block of memory to operate on. The array arguments and return values for these functions have type `void *' or `wchar_t'. As a matter of style, the elements of the arrays used with the `mem' functions are referred to as "bytes". You can pass any kind of pointer to these functions, and the `sizeof' operator is useful in computing the value for the size argument. Parameters to the `wmem' functions must be of type `wchar_t *'. These functions are not really usable with anything but arrays of this type. In contrast, functions that operate specifically on strings and wide character strings have names beginning with `str' and `wcs' respectively (such as `strcpy' and `wcscpy') and look for a null character to terminate the string instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.) The array arguments and return values for these functions have type `char *' and `wchar_t *' respectively, and the array elements are referred to as "characters" and "wide characters". In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size. Some of the memory and string functions take single characters as arguments. Since a value of type `char' is automatically promoted into an value of type `int' when used as a parameter, the functions are declared with `int' as the type of the parameter in question. In case of the wide character function the situation is similarly: the parameter type for a single wide character is `wint_t' and not `wchar_t'. This would for many implementations not be necessary since the `wchar_t' is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types the `wint_t' type is used. String Length ============= You can get the length of a string using the `strlen' function. This function is declared in the header file `string.h'. - Function: size_t strlen (const char *S) The `strlen' function returns the length of the null-terminated string S in bytes. (In other words, it returns the offset of the terminating null character within the array.) For example, strlen ("hello, world") => 12 When applied to a character array, the `strlen' function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character array that holds a string using the `sizeof' operator: char string[32] = "hello, world"; sizeof (string) => 32 strlen (string) => 12 But beware, this will not work unless STRING is the character array itself, not a pointer to it. For example: char string[32] = "hello, world"; char *ptr = string; sizeof (string) => 32 sizeof (ptr) => 4 /* (on a machine with 4 byte pointers) */ This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays. It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and `wcslen' can be used or something like the following code can be used: /* The input is in `string'. The length is expected in `n'. */ { mbstate_t t; char *scopy = string; /* In initial state. */ memset (&t, '\0', sizeof (t)); /* Determine number of characters. */ n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t); } This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters. The wide character equivalent is declared in `wchar.h'. - Function: size_t wcslen (const wchar_t *WS) The `wcslen' function is the wide character equivalent to `strlen'. The return value is the number of wide characters in the wide character string pointed to by WS (this is also the offset of the terminating null wide character of WS). Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters. This function was introduced in Amendment 1 to ISO C90. - Function: size_t strnlen (const char *S, size_t MAXLEN) The `strnlen' function returns the length of the string S in bytes if this length is smaller than MAXLEN bytes. Otherwise it returns MAXLEN. Therefore this function is equivalent to `(strlen (S) < n ? strlen (S) : MAXLEN)' but it is more efficient and works even if the string S is not null-terminated. char string[32] = "hello, world"; strnlen (string, 32) => 12 strnlen (string, 5) => 5 This function is a GNU extension and is declared in `string.h'. - Function: size_t wcsnlen (const wchar_t *WS, size_t MAXLEN) `wcsnlen' is the wide character equivalent to `strnlen'. The MAXLEN parameter specifies the maximum number of wide characters. This function is a GNU extension and is declared in `wchar.h'. Copying and Concatenation ========================= You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `wchar.h'. A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array. Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program. All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like `sprintf' (*note Formatted Output Functions::) and `scanf' (*note Formatted Input Functions::). - Function: void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE) The `memcpy' function copies SIZE bytes from the object beginning at FROM into the object beginning at TO. The behavior of this function is undefined if the two arrays TO and FROM overlap; use `memmove' instead if overlapping is possible. The value returned by `memcpy' is the value of TO. Here is an example of how you might use `memcpy' to copy the contents of an array: struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo)); - Function: wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t *restruct WFROM, size_t SIZE) The `wmemcpy' function copies SIZE wide characters from the object beginning at WFROM into the object beginning at WTO. The behavior of this function is undefined if the two arrays WTO and WFROM overlap; use `wmemmove' instead if overlapping is possible. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by `wmemcpy' is the value of WTO. This function was introduced in Amendment 1 to ISO C90. - Function: void * mempcpy (void *restrict TO, const void *restrict FROM, size_t SIZE) The `mempcpy' function is nearly identical to the `memcpy' function. It copies SIZE bytes from the object beginning at `from' into the object pointed to by TO. But instead of returning the value of TO it returns a pointer to the byte following the last written byte in the object beginning at TO. I.e., the value is `((void *) ((char *) TO + SIZE))'. This function is useful in situations where a number of objects shall be copied to consecutive memory positions. void * combine (void *o1, size_t s1, void *o2, size_t s2) { void *result = malloc (s1 + s2); if (result != NULL) mempcpy (mempcpy (result, o1, s1), o2, s2); return result; } This function is a GNU extension. - Function: wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) The `wmempcpy' function is nearly identical to the `wmemcpy' function. It copies SIZE wide characters from the object beginning at `wfrom' into the object pointed to by WTO. But instead of returning the value of WTO it returns a pointer to the wide character following the last written wide character in the object beginning at WTO. I.e., the value is `WTO + SIZE'. This function is useful in situations where a number of objects shall be copied to consecutive memory positions. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } This function is a GNU extension. - Function: void * memmove (void *TO, const void *FROM, size_t SIZE) `memmove' copies the SIZE bytes at FROM into the SIZE bytes at TO, even if those two blocks of space overlap. In the case of overlap, `memmove' is careful to copy the original values of the bytes in the block at FROM, including those bytes which also belong to the block at TO. The value returned by `memmove' is the value of TO. - Function: wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM, size_t SIZE) `wmemmove' copies the SIZE wide characters at WFROM into the SIZE wide characters at WTO, even if those two blocks of space overlap. In the case of overlap, `memmove' is careful to copy the original values of the wide characters in the block at WFROM, including those wide characters which also belong to the block at WTO. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by `wmemmove' is the value of WTO. This function is a GNU extension. - Function: void * memccpy (void *restrict TO, const void *restrict FROM, int C, size_t SIZE) This function copies no more than SIZE bytes from FROM to TO, stopping if a byte matching C is found. The return value is a pointer into TO one byte past where C was copied, or a null pointer if no byte matching C appeared in the first SIZE bytes of FROM. - Function: void * memset (void *BLOCK, int C, size_t SIZE) This function copies the value of C (converted to an `unsigned char') into each of the first SIZE bytes of the object beginning at BLOCK. It returns the value of BLOCK. - Function: wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t SIZE) This function copies the value of WC into each of the first SIZE wide characters of the object beginning at BLOCK. It returns the value of BLOCK. - Function: char * strcpy (char *restrict TO, const char *restrict FROM) This copies characters from the string FROM (up to and including the terminating null character) into the string TO. Like `memcpy', this function has undefined results if the strings overlap. The return value is the value of TO. - Function: wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM) This copies wide characters from the string WFROM (up to and including the terminating null wide character) into the string WTO. Like `wmemcpy', this function has undefined results if the strings overlap. The return value is the value of WTO. - Function: char * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `strcpy' but always copies exactly SIZE characters into TO. If the length of FROM is more than SIZE, then `strncpy' copies just the first SIZE characters. Note that in this case there is no null terminator written into TO. If the length of FROM is less than SIZE, then `strncpy' copies all of FROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is specified by the ISO C standard. The behavior of `strncpy' is undefined if the strings overlap. Using `strncpy' as opposed to `strcpy' is a way to avoid bugs relating to writing past the end of the allocated space for TO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `strncpy' will waste a considerable amount of time copying null characters. - Function: wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcscpy' but always copies exactly SIZE wide characters into WTO. If the length of WFROM is more than SIZE, then `wcsncpy' copies just the first SIZE wide characters. Note that in this case there is no null terminator written into WTO. If the length of WFROM is less than SIZE, then `wcsncpy' copies all of WFROM, followed by enough null wide characters to add up to SIZE wide characters in all. This behavior is rarely useful, but it is specified by the ISO C standard. The behavior of `wcsncpy' is undefined if the strings overlap. Using `wcsncpy' as opposed to `wcscpy' is a way to avoid bugs relating to writing past the end of the allocated space for WTO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `wcsncpy' will waste a considerable amount of time copying null wide characters. - Function: char * strdup (const char *S) This function copies the null-terminated string S into a newly allocated string. The string is allocated using `malloc'; see *Note Unconstrained Allocation::. If `malloc' cannot allocate space for the new string, `strdup' returns a null pointer. Otherwise it returns a pointer to the new string. - Function: wchar_t * wcsdup (const wchar_t *WS) This function copies the null-terminated wide character string WS into a newly allocated string. The string is allocated using `malloc'; see *Note Unconstrained Allocation::. If `malloc' cannot allocate space for the new string, `wcsdup' returns a null pointer. Otherwise it returns a pointer to the new wide character string. This function is a GNU extension. - Function: char * strndup (const char *S, size_t SIZE) This function is similar to `strdup' but always copies at most SIZE characters into the newly allocated string. If the length of S is more than SIZE, then `strndup' copies just the first SIZE characters and adds a closing null terminator. Otherwise all characters are copied and the string is terminated. This function is different to `strncpy' in that it always terminates the destination string. `strndup' is a GNU extension. - Function: char * stpcpy (char *restrict TO, const char *restrict FROM) This function is like `strcpy', except that it returns a pointer to the end of the string TO (that is, the address of the terminating null character `to + strlen (from)') rather than the beginning. For example, this program uses `stpcpy' to concatenate `foo' and `bar' to produce `foobar', which it then prints. #include #include int main (void) { char buffer[10]; char *to = buffer; to = stpcpy (to, "foo"); to = stpcpy (to, "bar"); puts (buffer); return 0; } This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG. Its behavior is undefined if the strings overlap. The function is declared in `string.h'. - Function: wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM) This function is like `wcscpy', except that it returns a pointer to the end of the string WTO (that is, the address of the terminating null character `wto + strlen (wfrom)') rather than the beginning. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. The behavior of `wcpcpy' is undefined if the strings overlap. `wcpcpy' is a GNU extension and is declared in `wchar.h'. - Function: char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `stpcpy' but copies always exactly SIZE characters into TO. If the length of FROM is more then SIZE, then `stpncpy' copies just the first SIZE characters and returns a pointer to the character directly following the one which was copied last. Note that in this case there is no null terminator written into TO. If the length of FROM is less than SIZE, then `stpncpy' copies all of FROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the `strncpy' is used. `stpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap. The function is declared in `string.h'. - Function: wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcpcpy' but copies always exactly WSIZE characters into WTO. If the length of WFROM is more then SIZE, then `wcpncpy' copies just the first SIZE wide characters and returns a pointer to the wide character directly following the one which was copied last. Note that in this case there is no null terminator written into WTO. If the length of WFROM is less than SIZE, then `wcpncpy' copies all of WFROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the `wcsncpy' is used. `wcpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap. `wcpncpy' is a GNU extension and is declared in `wchar.h'. - Macro: char * strdupa (const char *S) This macro is similar to `strdup' but allocates the new string using `alloca' instead of `malloc' (*note Variable Size Automatic::). This means of course the returned string has the same limitations as any block of memory allocated using `alloca'. For obvious reasons `strdupa' is implemented only as a macro; you cannot get the address of this function. Despite this limitation it is a useful function. The following code shows a situation where using `malloc' would be a lot more expensive. #include #include #include const char path[] = _PATH_STDPATH; int main (void) { char *wr_path = strdupa (path); char *cp = strtok (wr_path, ":"); while (cp != NULL) { puts (cp); cp = strtok (NULL, ":"); } return 0; } Please note that calling `strtok' using PATH directly is invalid. It is also not allowed to call `strdupa' in the argument list of `strtok' since `strdupa' uses `alloca' (*note Variable Size Automatic::) can interfere with the parameter passing. This function is only available if GNU CC is used. - Macro: char * strndupa (const char *S, size_t SIZE) This function is similar to `strndup' but like `strdupa' it allocates the new string using `alloca' *note Variable Size Automatic::. The same advantages and limitations of `strdupa' are valid for `strndupa', too. This function is implemented only as a macro, just like `strdupa'. Just as `strdupa' this macro also must not be used inside the parameter list in a function call. `strndupa' is only available if GNU CC is used. - Function: char * strcat (char *restrict TO, const char *restrict FROM) The `strcat' function is similar to `strcpy', except that the characters from FROM are concatenated or appended to the end of TO, instead of overwriting it. That is, the first character from FROM overwrites the null character marking the end of TO. An equivalent definition for `strcat' would be: char * strcat (char *restrict to, const char *restrict from) { strcpy (to + strlen (to), from); return to; } This function has undefined results if the strings overlap. - Function: wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM) The `wcscat' function is similar to `wcscpy', except that the characters from WFROM are concatenated or appended to the end of WTO, instead of overwriting it. That is, the first character from WFROM overwrites the null character marking the end of WTO. An equivalent definition for `wcscat' would be: wchar_t * wcscat (wchar_t *wto, const wchar_t *wfrom) { wcscpy (wto + wcslen (wto), wfrom); return wto; } This function has undefined results if the strings overlap. Programmers using the `strcat' or `wcscat' function (or the following `strncat' or `wcsncar' functions for that matter) can easily be recognized as lazy and reckless. In almost all situations the lengths of the participating strings are known (it better should be since how can one otherwise ensure the allocated size of the buffer is sufficient?) Or at least, one could know them if one keeps track of the results of the various function calls. But then it is very inefficient to use `strcat'/`wcscat'. A lot of time is wasted finding the end of the destination string so that the actual copying can start. This is a common example: /* This function concatenates arbitrarily many strings. The last parameter must be `NULL'. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actually `va_copy', but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ for (s = str; s != NULL; s = va_arg (ap, const char *)) total += strlen (s); va_end (ap); result = (char *) malloc (total); if (result != NULL) { result[0] = '\0'; /* Copy the strings. */ for (s = str; s != NULL; s = va_arg (ap2, const char *)) strcat (result, s); } va_end (ap2); return result; } This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient: char * concat (const char *str, ...) { va_list ap; size_t allocated = 100; char *result = (char *) malloc (allocated); char *wp; if (allocated != NULL) { char *newp; va_start (ap, atr); wp = result; for (s = str; s != NULL; s = va_arg (ap, const char *)) { size_t len = strlen (s); /* Resize the allocated memory if necessary. */ if (wp + len + 1 > result + allocated) { allocated = (allocated + len) * 2; newp = (char *) realloc (result, allocated); if (newp == NULL) { free (result); return NULL; } wp = newp + (wp - result); result = newp; } wp = mempcpy (wp, s, len); } /* Terminate the result string. */ *wp++ = '\0'; /* Resize memory to the optimal size. */ newp = realloc (result, wp - result); if (newp != NULL) result = newp; va_end (ap); } return result; } With a bit more knowledge about the input strings one could fine-tune the memory allocation. The difference we are pointing to here is that we don't use `strcat' anymore. We always keep track of the length of the current intermediate result so we can safe us the search for the end of the string and use `mempcpy'. Please note that we also don't use `stpcpy' which might seem more natural since we handle with strings. But this is not necessary since we already know the length of the string and therefore can use the faster memory copying function. The example would work for wide characters the same way. Whenever a programmer feels the need to use `strcat' she or he should think twice and look through the program whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use `strcat'. - Function: char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is like `strcat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 1' bytes longer than its initial length. The `strncat' function could be implemented like this: char * strncat (char *to, const char *from, size_t size) { to[strlen (to) + size] = '\0'; strncpy (to + strlen (to), from, size); return to; } The behavior of `strncat' is undefined if the strings overlap. - Function: wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is like `wcscat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 1' bytes longer than its initial length. The `wcsncat' function could be implemented like this: wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { wto[wcslen (to) + size] = L'\0'; wcsncpy (wto + wcslen (wto), wfrom, size); return wto; } The behavior of `wcsncat' is undefined if the strings overlap. Here is an example showing the use of `strncpy' and `strncat' (the wide character version is equivalent). Notice how, in the call to `strncat', the SIZE parameter is computed to avoid overflowing the character array `buffer'. #include #include #define SIZE 10 static char buffer[SIZE]; main () { strncpy (buffer, "hello", SIZE); puts (buffer); strncat (buffer, ", world", SIZE - strlen (buffer) - 1); puts (buffer); } The output produced by this program looks like: hello hello, wo - Function: void bcopy (const void *FROM, void *TO, size_t SIZE) This is a partially obsolete alternative for `memmove', derived from BSD. Note that it is not quite equivalent to `memmove', because the arguments are not in the same order and there is no return value. - Function: void bzero (void *BLOCK, size_t SIZE) This is a partially obsolete alternative for `memset', derived from BSD. Note that it is not as general as `memset', because the only value it can store is zero. String/Array Comparison ======================= You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. *Note Searching and Sorting::, for an example of this. Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are _not_ equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater". The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'. All of these functions are declared in the header file `string.h'. - Function: int memcmp (const void *A1, const void *A2, size_t SIZE) The function `memcmp' compares the SIZE bytes of memory beginning at A1 against the SIZE bytes of memory beginning at A2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as `unsigned char' objects, then promoted to `int'). If the contents of the two blocks are equal, `memcmp' returns `0'. - Function: int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE) The function `wmemcmp' compares the SIZE wide characters beginning at A1 against the SIZE wide characters beginning at A2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is A1 is smaller or larger than the corresponding character in A2. If the contents of the two blocks are equal, `wmemcmp' returns `0'. On arbitrary arrays, the `memcmp' function is mostly useful for testing equality. It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers. `wmemcmp' is really only useful to compare arrays of type `wchar_t' since the function looks at `sizeof (wchar_t)' bytes at a time and this number of bytes is system dependent. You should also be careful about using `memcmp' to compare objects that can contain "holes", such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size. The contents of these "holes" are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison. For example, given a structure type definition like: struct foo { unsigned char tag; union { double f; long i; char *p; } value; }; you are better off writing a specialized comparison function to compare `struct foo' objects instead of comparing them with `memcmp'. - Function: int strcmp (const char *S1, const char *S2) The `strcmp' function compares the string S1 against S2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as `unsigned char' objects, then promoted to `int'). If the two strings are equal, `strcmp' returns `0'. A consequence of the ordering used by `strcmp' is that if S1 is an initial substring of S2, then S1 is considered to be "less than" S2. `strcmp' does not take sorting conventions of the language the strings are written in into account. To get that one has to use `strcoll'. - Function: int wcscmp (const wchar_t *WS1, const wchar_t *WS2) The `wcscmp' function compares the wide character string WS1 against WS2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is WS1 is smaller or larger than the corresponding character in WS2. If the two strings are equal, `wcscmp' returns `0'. A consequence of the ordering used by `wcscmp' is that if WS1 is an initial substring of WS2, then WS1 is considered to be "less than" WS2. `wcscmp' does not take sorting conventions of the language the strings are written in into account. To get that one has to use `wcscoll'. - Function: int strcasecmp (const char *S1, const char *S2) This function is like `strcmp', except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard `"C"' locale the characters A" and a" do not match but in a locale which regards these characters as parts of the alphabet they do match. `strcasecmp' is derived from BSD. - Function: int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2) This function is like `wcscmp', except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard `"C"' locale the characters A" and a" do not match but in a locale which regards these characters as parts of the alphabet they do match. `wcscasecmp' is a GNU extension. - Function: int strncmp (const char *S1, const char *S2, size_t SIZE) This function is the similar to `strcmp', except that no more than SIZE wide characters are compared. In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero. - Function: int wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE) This function is the similar to `wcscmp', except that no more than SIZE wide characters are compared. In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero. - Function: int strncasecmp (const char *S1, const char *S2, size_t N) This function is like `strncmp', except that differences in case are ignored. Like `strcasecmp', it is locale dependent how uppercase and lowercase characters are related. `strncasecmp' is a GNU extension. - Function: int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2, size_t N) This function is like `wcsncmp', except that differences in case are ignored. Like `wcscasecmp', it is locale dependent how uppercase and lowercase characters are related. `wcsncasecmp' is a GNU extension. Here are some examples showing the use of `strcmp' and `strncmp' (equivalent examples can be constructed for the wide character functions). These examples assume the use of the ASCII character set. (If some other character set--say, EBCDIC--is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.) strcmp ("hello", "hello") => 0 /* These two strings are the same. */ strcmp ("hello", "Hello") => 32 /* Comparisons are case-sensitive. */ strcmp ("hello", "world") => -15 /* The character `'h'' comes before `'w''. */ strcmp ("hello", "hello, world") => -44 /* Comparing a null character against a comma. */ strncmp ("hello", "hello, world", 5) => 0 /* The initial 5 characters are the same. */ strncmp ("hello, world", "hello, stupid world!!!", 5) => 0 /* The initial 5 characters are the same. */ - Function: int strverscmp (const char *S1, const char *S2) The `strverscmp' function compares the string S1 against S2, considering them as holding indices/version numbers. Return value follows the same conventions as found in the `strverscmp' function. In fact, if S1 and S2 contain no digits, `strverscmp' behaves like `strcmp'. Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them: * integral/integral: we compare values as you would expect. * fractional/integral: the fractional part is less than the integral one. Again, no surprise. * fractional/fractional: the things become a bit more complex. If the common prefix contains only leading zeroes, the longest part is less than the other one; else the comparison behaves normally. strverscmp ("no digit", "no digit") => 0 /* same behavior as strcmp. */ strverscmp ("item#99", "item#100") => <0 /* same prefix, but 99 < 100. */ strverscmp ("alpha1", "alpha001") => >0 /* fractional part inferior to integral one. */ strverscmp ("part1_f012", "part1_f01") => >0 /* two fractional parts. */ strverscmp ("foo.009", "foo.0") => <0 /* idem, but with leading zeroes only. */ This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers. `strverscmp' is a GNU extension. - Function: int bcmp (const void *A1, const void *A2, size_t SIZE) This is an obsolete alias for `memcmp', derived from BSD. Collation Functions =================== In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'. You can use the functions `strcoll' and `strxfrm' (declared in the headers file `string.h') and `wcscoll' and `wcsxfrm' (declared in the headers file `wchar') to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the `LC_COLLATE' category; see *Note Locales::. In the standard C locale, the collation sequence for `strcoll' is the same as that for `strcmp'. Similarly, `wcscoll' and `wcscmp' are the same in this situation. Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence. The functions `strcoll' and `wcscoll' perform this translation implicitly, in order to do one comparison. By contrast, `strxfrm' and `wcsxfrm' perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use `strxfrm' or `wcsxfrm' to transform all the strings just once, and subsequently compare the transformed strings with `strcmp' or `wcscmp'. - Function: int strcoll (const char *S1, const char *S2) The `strcoll' function is similar to `strcmp' but uses the collating sequence of the current locale for collation (the `LC_COLLATE' locale). - Function: int wcscoll (const wchar_t *WS1, const wchar_t *WS2) The `wcscoll' function is similar to `wcscmp' but uses the collating sequence of the current locale for collation (the `LC_COLLATE' locale). Here is an example of sorting an array of strings, using `strcoll' to compare them. The actual sort algorithm is not written here; it comes from `qsort' (*note Array Sort Function::). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using `strxfrm'.) /* This is the comparison function used with `qsort'. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sort `temp_array' by comparing the strings. */ qsort (array, nstrings, sizeof (char *), compare_elements); } - Function: size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE) The function `strxfrm' transforms the string FROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array TO. Up to SIZE characters (including a terminating null character) are stored. The behavior is undefined if the strings TO and FROM overlap; see *Note Copying and Concatenation::. The return value is the length of the entire transformed string. This value is not affected by the value of SIZE, but if it is greater or equal than SIZE, it means that the transformed string did not entirely fit in the array TO. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call `strxfrm' again with a bigger output array. The transformed string may be longer than the original string, and it may also be shorter. If SIZE is zero, no characters are stored in TO. In this case, `strxfrm' simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what TO is if SIZE is zero; TO may even be a null pointer. - Function: size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t *WFROM, size_t SIZE) The function `wcsxfrm' transforms wide character string WFROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array WTO. Up to SIZE wide characters (including a terminating null character) are stored. The behavior is undefined if the strings WTO and WFROM overlap; see *Note Copying and Concatenation::. The return value is the length of the entire transformed wide character string. This value is not affected by the value of SIZE, but if it is greater or equal than SIZE, it means that the transformed wide character string did not entirely fit in the array WTO. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call `wcsxfrm' again with a bigger output array. The transformed wide character string may be longer than the original wide character string, and it may also be shorter. If SIZE is zero, no characters are stored in TO. In this case, `wcsxfrm' simply returns the number of wide characters that would be the length of the transformed wide character string. This is useful for determining what size the allocated array should be (remember to multiply with `sizeof (wchar_t)'). It does not matter what WTO is if SIZE is zero; WTO may even be a null pointer. Here is an example of how you can use `strxfrm' when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings. struct sorter { char *input; char *transformed; }; /* This is the comparison function used with `qsort' to sort an array of `struct sorter'. */ int compare_elements (struct sorter *p1, struct sorter *p2) { return strcmp (p1->transformed, p2->transformed); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings_fast (char **array, int nstrings) { struct sorter temp_array[nstrings]; int i; /* Set up `temp_array'. Each element contains one input string and its transformed string. */ for (i = 0; i < nstrings; i++) { size_t length = strlen (array[i]) * 2; char *transformed; size_t transformed_length; temp_array[i].input = array[i]; /* First try a buffer perhaps big enough. */ transformed = (char *) xmalloc (length); /* Transform `array[i]'. */ transformed_length = strxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (char *) xrealloc (transformed, transformed_length + 1); /* The return value is not interesting because we know how long the transformed string is. */ (void) strxfrm (transformed, array[i], transformed_length + 1); } temp_array[i].transformed = transformed; } /* Sort `temp_array' by comparing transformed strings. */ qsort (temp_array, sizeof (struct sorter), nstrings, compare_elements); /* Put the elements back in the permanent array in their sorted order. */ for (i = 0; i < nstrings; i++) array[i] = temp_array[i].input; /* Free the strings we allocated. */ for (i = 0; i < nstrings; i++) free (temp_array[i].transformed); } The interesting part of this code for the wide character version would look like this: void sort_strings_fast (wchar_t **array, int nstrings) { ... /* Transform `array[i]'. */ transformed_length = wcsxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (wchar_t *) xrealloc (transformed, (transformed_length + 1) * sizeof (wchar_t)); /* The return value is not interesting because we know how long the transformed string is. */ (void) wcsxfrm (transformed, array[i], transformed_length + 1); } ... Note the additional multiplication with `sizeof (wchar_t)' in the `realloc' call. *Compatibility Note:* The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90. Search Functions ================ This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'. - Function: void * memchr (const void *BLOCK, int C, size_t SIZE) This function finds the first occurrence of the byte C (converted to an `unsigned char') in the initial SIZE bytes of the object beginning at BLOCK. The return value is a pointer to the located byte, or a null pointer if no match was found. - Function: wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC, size_t SIZE) This function finds the first occurrence of the wide character WC in the initial SIZE wide characters of the object beginning at BLOCK. The return value is a pointer to the located wide character, or a null pointer if no match was found. - Function: void * rawmemchr (const void *BLOCK, int C) Often the `memchr' function is used with the knowledge that the byte C is available in the memory block specified by the parameters. But this means that the SIZE parameter is not really needed and that the tests performed with it at runtime (to check whether the end of the block is reached) are not needed. The `rawmemchr' function exists for just this situation which is surprisingly frequent. The interface is similar to `memchr' except that the SIZE parameter is missing. The function will look beyond the end of the block pointed to by BLOCK in case the programmer made an error in assuming that the byte C is present in the block. In this case the result is unspecified. Otherwise the return value is a pointer to the located byte. This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like rawmemchr (str, '\0') will never go beyond the end of the string. This function is a GNU extension. - Function: void * memrchr (const void *BLOCK, int C, size_t SIZE) The function `memrchr' is like `memchr', except that it searches backwards from the end of the block defined by BLOCK and SIZE (instead of forwards from the front). - Function: char * strchr (const char *STRING, int C) The `strchr' function finds the first occurrence of the character C (converted to a `char') in the null-terminated string beginning at STRING. The return value is a pointer to the located character, or a null pointer if no match was found. For example, strchr ("hello, world", 'l') => "llo, world" strchr ("hello, world", '?') => NULL The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the C argument. It would be better (but less portable) to use `strchrnul' in this case, though. - Function: wchar_t * wcschr (const wchar_t *WSTRING, int WC) The `wcschr' function finds the first occurrence of the wide character WC in the null-terminated wide character string beginning at WSTRING. The return value is a pointer to the located wide character, or a null pointer if no match was found. The terminating null character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide character string by specifying a null wude character as the value of the WC argument. It would be better (but less portable) to use `wcschrnul' in this case, though. - Function: char * strchrnul (const char *STRING, int C) `strchrnul' is the same as `strchr' except that if it does not find the character, it returns a pointer to string's terminating null character rather than a null pointer. This function is a GNU extension. - Function: wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC) `wcschrnul' is the same as `wcschr' except that if it does not find the wide character, it returns a pointer to wide character string's terminating null wide character rather than a null pointer. This function is a GNU extension. One useful, but unusual, use of the `strchr' function is when one wants to have a pointer pointing to the NUL byte terminating a string. This is often written in this way: s += strlen (s); This is almost optimal but the addition operation duplicated a bit of the work already done in the `strlen' function. A better solution is this: s = strchr (s, '\0'); There is no restriction on the second parameter of `strchr' so it could very well also be the NUL character. Those readers thinking very hard about this might now point out that the `strchr' function is more expensive than the `strlen' function since we have two abort criteria. This is right. But in the GNU C library the implementation of `strchr' is optimized in a special way so that `strchr' actually is faster. - Function: char * strrchr (const char *STRING, int C) The function `strrchr' is like `strchr', except that it searches backwards from the end of the string STRING (instead of forwards from the front). For example, strrchr ("hello, world", 'l') => "ld" - Function: wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C) The function `wcsrchr' is like `wcschr', except that it searches backwards from the end of the string WSTRING (instead of forwards from the front). - Function: char * strstr (const char *HAYSTACK, const char *NEEDLE) This is like `strchr', except that it searches HAYSTACK for a substring NEEDLE rather than just a single character. It returns a pointer into the string HAYSTACK that is the first character of the substring, or a null pointer if no match was found. If NEEDLE is an empty string, the function returns HAYSTACK. For example, strstr ("hello, world", "l") => "llo, world" strstr ("hello, world", "wo") => "world" - Function: wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t *NEEDLE) This is like `wcschr', except that it searches HAYSTACK for a substring NEEDLE rather than just a single wide character. It returns a pointer into the string HAYSTACK that is the first wide character of the substring, or a null pointer if no match was found. If NEEDLE is an empty string, the function returns HAYSTACK. - Function: wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t *NEEDLE) `wcsstr' is an depricated alias for `wcsstr'. This is the name originally used in the X/Open Portability Guide before the Amendment 1 to ISO C90 was published. - Function: char * strcasestr (const char *HAYSTACK, const char *NEEDLE) This is like `strstr', except that it ignores case in searching for the substring. Like `strcasecmp', it is locale dependent how uppercase and lowercase characters are related. For example, strstr ("hello, world", "L") => "llo, world" strstr ("hello, World", "wo") => "World" - Function: void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN, const void *NEEDLE, size_t NEEDLE-LEN) This is like `strstr', but NEEDLE and HAYSTACK are byte arrays rather than null-terminated strings. NEEDLE-LEN is the length of NEEDLE and HAYSTACK-LEN is the length of HAYSTACK. This function is a GNU extension. - Function: size_t strspn (const char *STRING, const char *SKIPSET) The `strspn' ("string span") function returns the length of the initial substring of STRING that consists entirely of characters that are members of the set specified by the string SKIPSET. The order of the characters in SKIPSET is not important. For example, strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET) The `wcsspn' ("wide character string span") function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are members of the set specified by the string SKIPSET. The order of the wide characters in SKIPSET is not important. - Function: size_t strcspn (const char *STRING, const char *STOPSET) The `strcspn' ("string complement span") function returns the length of the initial substring of STRING that consists entirely of characters that are _not_ members of the set specified by the string STOPSET. (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.) For example, strcspn ("hello, world", " \t\n,.;!?") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET) The `wcscspn' ("wide character string complement span") function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are _not_ members of the set specified by the string STOPSET. (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.) - Function: char * strpbrk (const char *STRING, const char *STOPSET) The `strpbrk' ("string pointer break") function is related to `strcspn', except that it returns a pointer to the first character in STRING that is a member of the set STOPSET instead of the length of the initial substring. It returns a null pointer if no such character from STOPSET is found. For example, strpbrk ("hello, world", " \t\n,.;!?") => ", world" Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET) The `wcspbrk' ("wide character string pointer break") function is related to `wcscspn', except that it returns a pointer to the first wide character in WSTRING that is a member of the set STOPSET instead of the length of the initial substring. It returns a null pointer if no such character from STOPSET is found. Compatibility String Search Functions ------------------------------------- - Function: char * index (const char *STRING, int C) `index' is another name for `strchr'; they are exactly the same. New code should always use `strchr' since this name is defined in ISO C while `index' is a BSD invention which never was available on System V derived systems. - Function: char * rindex (const char *STRING, int C) `rindex' is another name for `strrchr'; they are exactly the same. New code should always use `strrchr' since this name is defined in ISO C while `rindex' is a BSD invention which never was available on System V derived systems. Finding Tokens in a String ========================== It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the `strtok' function, declared in the header file `string.h'. - Function: char * strtok (char *restrict NEWSTRING, const char *restrict DELIMITERS) A string can be split into tokens by making a series of calls to the function `strtok'. The string to be split up is passed as the NEWSTRING argument on the first call only. The `strtok' function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the NEWSTRING argument. Calling `strtok' with another non-null NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls `strtok' behind your back (which would mess up this internal state information). The DELIMITERS argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is _not_ a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string NEWSTRING is overwritten by a null character, and the pointer to the beginning of the token in NEWSTRING is returned. On the next call to `strtok', the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to `strtok'. If the end of the string NEWSTRING is reached, or if the remainder of string consists only of delimiter characters, `strtok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS) A string can be split into tokens by making a series of calls to the function `wcstok'. The string to be split up is passed as the NEWSTRING argument on the first call only. The `wcstok' function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same wide character string are indicated by passing a null pointer as the NEWSTRING argument. Calling `wcstok' with another non-null NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls `wcstok' behind your back (which would mess up this internal state information). The DELIMITERS argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is _not_ a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string NEWSTRING is overwritten by a null wide character, and the pointer to the beginning of the token in NEWSTRING is returned. On the next call to `wcstok', the searching begins at the next wide character beyond the one that marked the end of the previous token. Note that the set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to `wcstok'. If the end of the wide character string NEWSTRING is reached, or if the remainder of string consists only of delimiter wide characters, `wcstok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. *Warning:* Since `strtok' and `wcstok' alter the string they is parsing, you should always copy the string to a temporary buffer before parsing it with `strtok'/`wcstok' (*note Copying and Concatenation::). If you allow `strtok' or `wcstok' to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after `strtok' or `wcstok' has modified it, and it would not have the expected value. The string that you are operating on might even be a constant. Then when `strtok' or `wcstok' tries to modify it, your program will get a fatal signal for writing in read-only memory. *Note Program Error Signals::. Even if the operation of `strtok' or `wcstok' would not require a modification of the string (e.g., if there is exactly one token) the string can (and in the GNU libc case will) be modified. This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily. The functions `strtok' and `wcstok' are not reentrant. *Note Nonreentrancy::, for a discussion of where and why reentrancy is important. Here is a simple example showing the use of `strtok'. #include #include ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token, *cp; ... cp = strdupa (string); /* Make writable copy. */ token = strtok (cp, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */ The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings. - Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR) Just like `strtok', this function splits the string into several tokens which can be accessed by successive calls to `strtok_r'. The difference is that the information about the next token is stored in the space pointed to by the third argument, SAVE_PTR, which is a pointer to a string pointer. Calling `strtok_r' with a null pointer for NEWSTRING and leaving SAVE_PTR between the calls unchanged does the job without hindering reentrancy. This function is defined in POSIX.1 and can be found on many systems which support multi-threading. - Function: char * strsep (char **STRING_PTR, const char *DELIMITER) This function has a similar functionality as `strtok_r' with the NEWSTRING argument replaced by the SAVE_PTR argument. The initialization of the moving pointer has to be done by the user. Successive calls to `strsep' move the pointer along the tokens separated by DELIMITER, returning the address of the next token and updating STRING_PTR to point to the beginning of the next token. One difference between `strsep' and `strtok_r' is that if the input string contains more than one character from DELIMITER in a row `strsep' returns an empty string for each pair of characters from DELIMITER. This means that a program normally should test for `strsep' returning an empty string before processing it. This function was introduced in 4.3BSD and therefore is widely available. Here is how the above example looks like when `strsep' is used. #include #include ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */ - Function: char * basename (const char *FILENAME) The GNU version of the `basename' function returns the last component of the path in FILENAME. This function is the preferred usage, since it does not modify the argument, FILENAME, and respects trailing slashes. The prototype for `basename' can be found in `string.h'. Note, this function is overriden by the XPG version, if `libgen.h' is included. Example of using GNU `basename': #include int main (int argc, char *argv[]) { char *prog = basename (argv[0]); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } *Portability Note:* This function may produce different results on different systems. - Function: char * basename (char *PATH) This is the standard XPG defined `basename'. It is similar in spirit to the GNU version, but may modify the PATH by removing trailing '/' characters. If the PATH is made up entirely of '/' characters, then "/" will be returned. Also, if PATH is `NULL' or an empty string, then "." is returned. The prototype for the XPG version can be found in `libgen.h'. Example of using XPG `basename': #include int main (int argc, char *argv[]) { char *prog; char *path = strdupa (argv[0]); prog = basename (path); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } - Function: char * dirname (char *PATH) The `dirname' function is the compliment to the XPG version of `basename'. It returns the parent directory of the file specified by PATH. If PATH is `NULL', an empty string, or contains no '/' characters, then "." is returned. The prototype for this function can be found in `libgen.h'. strfry ====== The function below addresses the perennial programming quandary: "How do I take good data in string form and painlessly turn it into garbage?" This is actually a fairly simple task for C programmers who do not use the GNU C library string functions, but for programs based on the GNU C library, the `strfry' function is the preferred method for destroying string data. The prototype for this function is in `string.h'. - Function: char * strfry (char *STRING) `strfry' creates a pseudorandom anagram of a string, replacing the input with the anagram in place. For each position in the string, `strfry' swaps it with a position in the string selected at random (from a uniform distribution). The two positions may be the same. The return value of `strfry' is always STRING. *Portability Note:* This function is unique to the GNU C library. Trivial Encryption ================== The `memfrob' function converts an array of data to something unrecognizable and back again. It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text. The transformation is analogous to Usenet's "Rot13" encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, `memfrob' works on arbitrary binary data, not just text. For true encryption, *Note Cryptographic Functions::. This function is declared in `string.h'. - Function: void * memfrob (void *MEM, size_t LENGTH) `memfrob' transforms (frobnicates) each byte of the data structure at MEM, which is LENGTH bytes long, by bitwise exclusive oring it with binary 00101010. It does the transformation in place and its return value is always MEM. Note that `memfrob' a second time on the same data structure returns it to its original state. This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in *Note Cryptographic Functions::. *Portability Note:* This function is unique to the GNU C library. Encode Binary Data ================== To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task. - Function: char * l64a (long int N) This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 6 character buffer which contains an encoded version of N. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if N is zero, which is somewhat bizarre but mandated by the standard. *Warning:* Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library. *Compatibility Note:* The XPG standard states that the return value of `l64a' is undefined if N is negative. In the GNU implementation, `l64a' treats its argument as unsigned, so it will return a sensible encoding for any nonzero N; however, portable programs should not rely on this. To encode a large buffer `l64a' must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this: char * encode (const void *buf, size_t len) { /* We know in advance how long the buffer has to be. */ unsigned char *in = (unsigned char *) buf; char *out = malloc (6 + ((len + 3) / 4) * 6 + 1); char *cp = out; /* Encode the length. */ /* Using `htonl' is necessary so that the data can be decoded even on machines with different byte order. */ cp = mempcpy (cp, l64a (htonl (len)), 6); while (len > 3) { unsigned long int n = *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; len -= 4; if (n) cp = mempcpy (cp, l64a (htonl (n)), 6); else /* `l64a' returns the empty string for n==0, so we must generate its encoding ("......") by hand. */ cp = stpcpy (cp, "......"); } if (len > 0) { unsigned long int n = *in++; if (--len > 0) { n = (n << 8) | *in++; if (--len > 0) n = (n << 8) | *in; } memcpy (cp, l64a (htonl (n)), 6); cp += 6; } *cp = '\0'; return out; } It is strange that the library does not provide the complete functionality needed but so be it. To decode data produced with `l64a' the following function should be used. - Function: long int a64l (const char *STRING) The parameter STRING should contain a string which was produced by a call to `l64a'. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character not in the table, rather like `atoi'; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line characters. The decoded number is returned as a `long int' value. The `l64a' and `a64l' functions use a base 64 encoding, in which each character of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits: 0 1 2 3 4 5 6 7 0 `.' `/' `0' `1' `2' `3' `4' `5' 8 `6' `7' `8' `9' `A' `B' `C' `D' 16 `E' `F' `G' `H' `I' `J' `K' `L' 24 `M' `N' `O' `P' `Q' `R' `S' `T' 32 `U' `V' `W' `X' `Y' `Z' `a' `b' 40 `c' `d' `e' `f' `g' `h' `i' `j' 48 `k' `l' `m' `n' `o' `p' `q' `r' 56 `s' `t' `u' `v' `w' `x' `y' `z' This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings. Argz and Envz Vectors ===================== "argz vectors" are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters (`'\0''). "Envz vectors" are an extension of argz vectors where each element is a name-value pair, separated by a `'='' character (as in a Unix environment). Argz Functions -------------- Each argz vector is represented by a pointer to the first element, of type `char *', and a size, of type `size_t', both of which can be initialized to `0' to represent an empty argz vector. All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified. The argz functions use `malloc'/`realloc' to allocate/grow argz vectors, and so any argz vector creating using these functions may be freed by using `free'; conversely, any argz function that may grow a string expects that string to have been allocated using `malloc' (those argz functions that only examine their arguments or modify them in place will work on any sort of memory). *Note Unconstrained Allocation::. All argz functions that do memory allocation have a return type of `error_t', and return `0' for success, and `ENOMEM' if an allocation error occurs. These functions are declared in the standard include file `argz.h'. - Function: error_t argz_create (char *const ARGV[], char **ARGZ, size_t *ARGZ_LEN) The `argz_create' function converts the Unix-style argument vector ARGV (a vector of pointers to normal C strings, terminated by `(char *)0'; *note Program Arguments::) into an argz vector with the same elements, which is returned in ARGZ and ARGZ_LEN. - Function: error_t argz_create_sep (const char *STRING, int SEP, char **ARGZ, size_t *ARGZ_LEN) The `argz_create_sep' function converts the null-terminated string STRING into an argz vector (returned in ARGZ and ARGZ_LEN) by splitting it into elements at every occurrence of the character SEP. - Function: size_t argz_count (const char *ARGZ, size_t ARG_LEN) Returns the number of elements in the argz vector ARGZ and ARGZ_LEN. - Function: void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV) The `argz_extract' function converts the argz vector ARGZ and ARGZ_LEN into a Unix-style argument vector stored in ARGV, by putting pointers to every element in ARGZ into successive positions in ARGV, followed by a terminator of `0'. ARGV must be pre-allocated with enough space to hold all the elements in ARGZ plus the terminating `(char *)0' (`(argz_count (ARGZ, ARGZ_LEN) + 1) * sizeof (char *)' bytes should be enough). Note that the string pointers stored into ARGV point into ARGZ--they are not copies--and so ARGZ must be copied if it will be changed while ARGV is still active. This function is useful for passing the elements in ARGZ to an exec function (*note Executing a File::). - Function: void argz_stringify (char *ARGZ, size_t LEN, int SEP) The `argz_stringify' converts ARGZ into a normal string with the elements separated by the character SEP, by replacing each `'\0'' inside ARGZ (except the last one, which terminates the string) with SEP. This is handy for printing ARGZ in a readable manner. - Function: error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const char *STR) The `argz_add' function adds the string STR to the end of the argz vector `*ARGZ', and updates `*ARGZ' and `*ARGZ_LEN' accordingly. - Function: error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM) The `argz_add_sep' function is similar to `argz_add', but STR is split into separate elements in the result at occurrences of the character DELIM. This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of `':'' for DELIM. - Function: error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const char *BUF, size_t BUF_LEN) The `argz_append' function appends BUF_LEN bytes starting at BUF to the argz vector `*ARGZ', reallocating `*ARGZ' to accommodate it, and adding BUF_LEN to `*ARGZ_LEN'. - Function: error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY) If ENTRY points to the beginning of one of the elements in the argz vector `*ARGZ', the `argz_delete' function will remove this entry and reallocate `*ARGZ', modifying `*ARGZ' and `*ARGZ_LEN' accordingly. Note that as destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as ENTRY will then become invalid. - Function: error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char *BEFORE, const char *ENTRY) The `argz_insert' function inserts the string ENTRY into the argz vector `*ARGZ' at a point just before the existing element pointed to by BEFORE, reallocating `*ARGZ' and updating `*ARGZ' and `*ARGZ_LEN'. If BEFORE is `0', ENTRY is added to the end instead (as if by `argz_add'). Since the first element is in fact the same as `*ARGZ', passing in `*ARGZ' as the value of BEFORE will result in ENTRY being inserted at the beginning. - Function: char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY) The `argz_next' function provides a convenient way of iterating over the elements in the argz vector ARGZ. It returns a pointer to the next element in ARGZ after the element ENTRY, or `0' if there are no elements following ENTRY. If ENTRY is `0', the first element of ARGZ is returned. This behavior suggests two styles of iteration: char *entry = 0; while ((entry = argz_next (ARGZ, ARGZ_LEN, entry))) ACTION; (the double parentheses are necessary to make some C compilers shut up about what they consider a questionable `while'-test) and: char *entry; for (entry = ARGZ; entry; entry = argz_next (ARGZ, ARGZ_LEN, entry)) ACTION; Note that the latter depends on ARGZ having a value of `0' if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here. - Function: error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT) Replace any occurrences of the string STR in ARGZ with WITH, reallocating ARGZ as necessary. If REPLACE_COUNT is non-zero, `*REPLACE_COUNT' will be incremented by number of replacements performed. Envz Functions -------------- Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense. Each element in an envz vector is a name-value pair, separated by a `'='' character; if multiple `'='' characters are present in an element, those after the first are considered part of the value, and treated like all other non-`'\0'' characters. If _no_ `'='' characters are present in an element, that element is considered the name of a "null" entry, as distinct from an entry with an empty value: `envz_get' will return `0' if given the name of null entry, whereas an entry with an empty value would result in a value of `""'; `envz_entry' will still find such entries, however. Null entries can be removed with `envz_strip' function. As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of `error_t', and return either `0' or `ENOMEM'. These functions are declared in the standard include file `envz.h'. - Function: char * envz_entry (const char *ENVZ, size_t ENVZ_LEN, const char *NAME) The `envz_entry' function finds the entry in ENVZ with the name NAME, and returns a pointer to the whole entry--that is, the argz element which begins with NAME followed by a `'='' character. If there is no entry with that name, `0' is returned. - Function: char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME) The `envz_get' function finds the entry in ENVZ with the name NAME (like `envz_entry'), and returns a pointer to the value portion of that entry (following the `'=''). If there is no entry with that name (or only a null entry), `0' is returned. - Function: error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE) The `envz_add' function adds an entry to `*ENVZ' (updating `*ENVZ' and `*ENVZ_LEN') with the name NAME, and value VALUE. If an entry with the same name already exists in ENVZ, it is removed first. If VALUE is `0', then the new entry will the special null type of entry (mentioned above). - Function: error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE) The `envz_merge' function adds each entry in ENVZ2 to ENVZ, as if with `envz_add', updating `*ENVZ' and `*ENVZ_LEN'. If OVERRIDE is true, then values in ENVZ2 will supersede those with the same name in ENVZ, otherwise not. Null entries are treated just like other entries in this respect, so a null entry in ENVZ can prevent an entry of the same name in ENVZ2 from being added to ENVZ, if OVERRIDE is false. - Function: void envz_strip (char **ENVZ, size_t *ENVZ_LEN) The `envz_strip' function removes any null entries from ENVZ, updating `*ENVZ' and `*ENVZ_LEN'. Character Set Handling ********************** Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by 2^8 choices. This chapter shows the functionality that was added to the C library to support multiple character sets. Introduction to Extended Characters =================================== A variety of solutions is available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library. A distinction we have to make right away is between internal and external representation. "Internal representation" means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed. Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets. One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format. For such a common format (= character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: "wide characters" will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary). As shown in some other part of this manual, a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, ISO 10646 was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first `0x10000' code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to `0x10ffff'. To represent wide characters the `char' type is not suitable. For this reason the ISO C standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to `int' for those functions that take a single wide character. - Data type: wchar_t This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of `char[]' for multibyte character strings. The type is defined in `stddef.h'. The ISO C90 standard, where `wchar_t' was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define `wchar_t' as `char', which might make sense for embedded systems. But for GNU systems `wchar_t' is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of ISO 10646. Some Unix systems define `wchar_t' as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646 one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the `wchar_t' type. - Data type: wint_t `wint_t' is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of `int' when using the normal `char' strings. The types `wchar_t' and `wint_t' often have the same representation if their size is 32 bits wide but if `wchar_t' is defined as `char' the type `wint_t' must be defined as `int' due to the parameter promotion. This type is defined in `wchar.h' and was introduced in Amendment 1 to ISO C90. As there are for the `char' data type macros are available for specifying the minimum and maximum value representable in an object of type `wchar_t'. - Macro: wint_t WCHAR_MIN The macro `WCHAR_MIN' evaluates to the minimum value representable by an object of type `wint_t'. This macro was introduced in Amendment 1 to ISO C90. - Macro: wint_t WCHAR_MAX The macro `WCHAR_MAX' evaluates to the maximum value representable by an object of type `wint_t'. This macro was introduced in Amendment 1 to ISO C90. Another special wide character value is the equivalent to `EOF'. - Macro: wint_t WEOF The macro `WEOF' evaluates to a constant expression of type `wint_t' whose value is different from any member of the extended character set. `WEOF' need not be the same value as `EOF' and unlike `EOF' it also need _not_ be negative. In other words, sloppy code like { int c; ... while ((c = getc (fp)) < 0) ... } has to be rewritten to use `WEOF' explicitly when wide characters are used: { wint_t c; ... while ((c = wgetc (fp)) != WEOF) ... } This macro was introduced in Amendment 1 to ISO C90 and is defined in `wchar.h'. These internal representations present problems when it comes to storing and transmittal. Because each single wide character consists of more than one byte, they are effected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and, thereforet require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set. For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here-instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character `'/'' is used in the encoding _only_ to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operation system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow. * The simplest character sets are single-byte character sets. There can be only up to 256 characters (for 8 bit character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text. Handling of a 8 bit character sets is simple. This is not true for other kinds presented later, and therefore, the application one uses might require the use of 8 bit character sets. * The ISO 2022 standard defines a mechanism for extended character sets where one character _can_ be represented by more than one byte. This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text. Each byte in the text might have a different interpretation in each state. The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes. In most uses of ISO 2022 the defined character sets do not allow state changes that cover more than the next character. This has the big advantage that whenever one can identify the beginning of the byte sequence of a character one can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun's operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding). But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN. * Early attempts to fix 8 bit character sets for other languages using the Roman alphabet lead to character sets like ISO 6937. Here bytes representing characters like the acute accent do not produce output themselves: one has to combine them with other characters to get the desired result. For example, the byte sequence `0xc2 0x61' (non-spacing acute accent, followed by lower-case `a') to get the "small a with acute" character. To get the acute accent character on its own, one has to write `0xc2 0x20' (the non-spacing acute followed by a space). Character sets like ISO 6937 are used in some embedded systems such as teletex. * Instead of converting the Unicode or ISO 10646 text used internally, it is often also sufficient to simply use an encoding different than UCS-2/UCS-4. The Unicode and ISO 10646 standards even specify such an encoding: UTF-8. This encoding is able to represent all of ISO 10646 31 bits in a byte string of length one to six. There were a few other attempts to encode ISO 10646 such as UTF-7, but UTF-8 is today the only encoding that should be used. In fact, with any luck UTF-8 will soon be the only external encoding that has to be supported. It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts. Methods like the Unicode compression scheme can alleviate these problems. The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate. The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past. One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or ISO 10646. This is not required, but at least encouraged, by the ISO C standard. The standard defines at least a macro `__STDC_ISO_10646__' that is only defined on systems where the `wchar_t' type encodes ISO 10646 characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems. Overview about Character Handling Functions =========================================== A Unix C library contains three different sets of functions in two families to handle character set conversion. One of the function families (the most commonly used) is specified in the ISO C90 standard and, therefore, is portable even beyond the Unix world. Unfortunately this family is the least useful one. These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications). The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90. Restartable Multibyte Conversion Functions ========================================== The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities: * The character set assumed for the multibyte encoding is not specified as an argument to the functions. Instead the character set specified by the `LC_CTYPE' category of the current locale is used; see *Note Locale Categories::. * The functions handling more than one character at a time require NUL terminated strings as the argument (i.e., converting blocks of text does not work unless one can add a NUL byte at an appropriate place). The GNU C library contains some extensions to the standard that allow specifying a size, but basically they also expect terminated strings. Despite these limitations the ISO C functions can be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide character string if the text is not simple ASCII. The text itself might come from a file with translations and the user should decide about the current locale, which determines the translation and therefore also the external encoding used. In such a situation (and many others) the functions described here are perfect. If more freedom while performing the conversion is necessary take a look at the `iconv' functions (*note Generic Charset Conversion::). Selecting the conversion and its properties ------------------------------------------- We already said above that the currently selected locale for the `LC_CTYPE' category decides about the conversion that is performed by the functions we are about to describe. Each locale uses its own character set (given as an argument to `localedef') and this is the one assumed as the external multibyte encoding. The wide character character set always is UCS-4, at least on GNU systems. A characteristic of each multibyte character set is the maximum number of bytes that can be necessary to represent one character. This information is quite important when writing code that uses the conversion functions (as shown in the examples below). The ISO C standard defines two macros that provide this information. - Macro: int MB_LEN_MAX `MB_LEN_MAX' specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales. It is a compile-time constant and is defined in `limits.h'. - Macro: int MB_CUR_MAX `MB_CUR_MAX' expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than `MB_LEN_MAX'. Unlike `MB_LEN_MAX' this macro need not be a compile-time constant, and in the GNU C library it is not. `MB_CUR_MAX' is defined in `stdlib.h'. Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions, but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem: { char buf[MB_LEN_MAX]; ssize_t len = 0; while (! feof (fp)) { fread (&buf[len], 1, MB_CUR_MAX - len, fp); /* ... process buf */ len -= used; } } The code in the inner loop is expected to have always enough bytes in the array BUF to convert one multibyte character. The array BUF has to be sized statically since many compilers do not allow a variable size. The `fread' call makes sure that `MB_CUR_MAX' bytes are always available in BUF. Note that it isn't a problem if `MB_CUR_MAX' is not a compile-time constant. Representing the state of the conversion ---------------------------------------- In the introduction of this chapter it was said that certain character sets use a "stateful" encoding. That is, the encoded values depend in some way on the previous bytes in the text. Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another. - Data type: mbstate_t A variable of type `mbstate_t' can contain all the information about the "shift state" needed from one call to a conversion function to another. `mbstate_t' is defined in `wchar.h'. It was introduced in Amendment 1 to ISO C90. To use objects of type `mbstate_t' the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful. There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as follows: { mbstate_t state; memset (&state, '\0', sizeof (state)); /* from now on STATE can be used. */ ... } When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this. - Function: int mbsinit (const mbstate_t *PS) The `mbsinit' function determines whether the state object pointed to by PS is in the initial state. If PS is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero. `mbsinit' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Code using `mbsinit' often looks similar to this: { mbstate_t state; memset (&state, '\0', sizeof (state)); /* Use STATE. */ ... if (! mbsinit (&state)) { /* Emit code to return to initial state. */ const wchar_t empty[] = L""; const wchar_t *srcp = empty; wcsrtombs (outbuf, &srcp, outbuflen, &state); } ... } The code to emit the escape sequence to get back to the initial state is interesting. The `wcsrtombs' function can be used to determine the necessary output code (*note Converting Strings::). Please note that on GNU systems it is not necessary to perform this extra action for the conversion from multibyte text to wide character text since the wide character encoding is not stateful. But there is nothing mentioned in any standard that prohibits making `wchar_t' using a stateful encoding. Converting Single Characters ---------------------------- The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subpart of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127. - Function: wint_t btowc (int C) The `btowc' function ("byte to wide character") converts a valid single byte character C in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the `LC_CTYPE' category. If `(unsigned char) C' is no valid single byte multibyte character or if C is `EOF', the function returns `WEOF'. Please note the restriction of C being tested for validity only in the initial shift state. No `mbstate_t' object is used from which the state information is taken, and the function also does not use any static state. The `btowc' function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Despite the limitation that the single byte value always is interpreted in the initial state this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful): wchar_t * itow (unsigned long int val) { static wchar_t buf[30]; wchar_t *wcp = &buf[29]; *wcp = L'\0'; while (val != 0) { *--wcp = btowc ('0' + val % 10); val /= 10; } if (wcp == &buf[29]) *--wcp = L'0'; return wcp; } Why is it necessary to use such a complicated implementation and not simply cast `'0' + val % 10' to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for `wchar_t' representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this it is necessary `btowc'. There also is a function for the conversion in the other direction. - Function: int wctob (wint_t C) The `wctob' function ("wide character to byte") takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is `EOF'. `wctob' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state. - Function: size_t mbrtowc (wchar_t *restrict PWC, const char *restrict S, size_t N, mbstate_t *restrict PS) The `mbrtowc' function ("multibyte restartable to wide character") converts the next multibyte character in the string pointed to by S into a wide character and stores it in the wide character string pointed to by PWC. The conversion is performed according to the locale currently selected for the `LC_CTYPE' category. If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by PS. If PS is a null pointer, a static, internal state variable used only by the `mbrtowc' function is used. If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state. If the next N or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from S that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the `L'\0'' or the one found in the conversion) is stored in the string pointed to by PWC if PWC is not null. If the first N bytes of the multibyte string possibly form a valid multibyte character but there are more than N bytes needed to complete it, the return value of the function is `(size_t) -2' and no value is stored. Please note that this can happen even if N has a value greater than or equal to `MB_CUR_MAX' since the input might contain redundant shift sequences. If the first `n' bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable `errno' is set to the value `EILSEQ', and the function returns `(size_t) -1'. The conversion state is afterwards undefined. `mbrtowc' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Use of `mbrtowc' is straightforward. A function that copies a multibyte string into a wide character string while at the same time converting all lowercase characters into uppercase could look like this (this is not the final version, just an example; it has no error checking, and sometimes leaks memory): wchar_t * mbstouwcs (const char *s) { size_t len = strlen (s); wchar_t *result = malloc ((len + 1) * sizeof (wchar_t)); wchar_t *wcp = result; wchar_t tmp[1]; mbstate_t state; size_t nbytes; memset (&state, '\0', sizeof (state)); while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0) { if (nbytes >= (size_t) -2) /* Invalid input string. */ return NULL; *result++ = towupper (tmp[0]); len -= nbytes; s += nbytes; } return result; } The use of `mbrtowc' should be clear. A single wide character is stored in `TMP[0]', and the number of consumed bytes is stored in the variable NBYTES. If the conversion is successful, the uppercase variant of the wide character is stored in the RESULT array and the pointer to the input string and the number of available bytes is adjusted. The only non-obvious thing about `mbrtowc' might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string. This method yields a pessimistic guess about the size of the result, and if many wide character strings have to be constructed this way or if the strings are long, the extra memory required to be allocated because the input string contains multibyte characters might be significant. The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. There is, however, a function that does part of the work. - Function: size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS) The `mbrlen' function ("multibyte restartable length") computes the number of at most N bytes starting at S, which form the next valid and complete multibyte character. If the next multibyte character corresponds to the NUL wide character, the return value is 0. If the next N bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned. If the the first N bytes possibly form a valid multibyte character but the character is incomplete, the return value is `(size_t) -2'. Otherwise the multibyte character sequence is invalid and the return value is `(size_t) -1'. The multibyte sequence is interpreted in the state represented by the object pointed to by PS. If PS is a null pointer, a state object local to `mbrlen' is used. `mbrlen' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The attentive reader now will note that `mbrlen' can be implemented as mbrtowc (NULL, s, n, ps != NULL ? ps : &internal) This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function `mbslen' using it: size_t mbslen (const char *s) { mbstate_t state; size_t result = 0; size_t nbytes; memset (&state, '\0', sizeof (state)); while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0) { if (nbytes >= (size_t) -2) /* Something is wrong. */ return (size_t) -1; s += nbytes; ++result; } return result; } This function simply calls `mbrlen' for each multibyte character in the string and counts the number of function calls. Please note that we here use `MB_LEN_MAX' as the size argument in the `mbrlen' call. This is acceptable since a) this value is larger then the length of the longest multibyte character sequence and b) we know that the string S ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the `mbrlen' function will never read invalid memory. Now that this function is available (just to make this clear, this function is _not_ part of the GNU C library) we can compute the number of wide character required to store the converted multibyte character string S using wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t); Please note that the `mbslen' function is quite inefficient. The implementation of `mbstouwcs' with `mbslen' would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice. - Function: size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t *restrict PS) The `wcrtomb' function ("wide character restartable to multibyte") converts a single wide character into a multibyte string corresponding to that wide character. If S is a null pointer, the function resets the state stored in the objects pointed to by PS (or the internal `mbstate_t' object) to the initial state. This can also be achieved by a call like this: wcrtombs (temp_buf, L'\0', ps) since, if S is a null pointer, `wcrtomb' performs as if it writes into an internal buffer, which is guaranteed to be large enough. If WC is the NUL wide character, `wcrtomb' emits, if necessary, a shift sequence to get the state PS into the initial state followed by a single NUL byte, which is stored in the string S. Otherwise a byte sequence (possibly including shift sequences) is written into the string S. This only happens if WC is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the `LC_CTYPE' category). If WC is no valid wide character, nothing is stored in the strings S, `errno' is set to `EILSEQ', the conversion state in PS is undefined and the return value is `(size_t) -1'. If no error occurred the function returns the number of bytes stored in the string S. This includes all bytes representing shift sequences. One word about the interface of the function: there is no parameter specifying the length of the array S. Instead the function assumes that there are at least `MB_CUR_MAX' bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur. `wcrtomb' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Using `wcrtomb' is as easy as using `mbrtowc'. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems. char * mbscatwcs (char *s, size_t len, const wchar_t *ws) { mbstate_t state; /* Find the end of the existing string. */ char *wp = strchr (s, '\0'); len -= wp - s; memset (&state, '\0', sizeof (state)); do { size_t nbytes; if (len < MB_CUR_LEN) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } nbytes = wcrtomb (wp, *ws, &state); if (nbytes == (size_t) -1) /* Error in the conversion. */ return NULL; len -= nbytes; wp += nbytes; } while (*ws++ != L'\0'); return s; } First the function has to find the end of the string currently in the array S. The `strchr' call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string). After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array S. We abort if there are not at least `MB_CUR_LEN' bytes available. This is not always optimal but we have no other choice. We might have less than `MB_CUR_LEN' bytes available but the next multibyte character might also be only one byte long. At the time the `wcrtomb' call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution. ... if (len < MB_CUR_LEN) { mbstate_t temp_state; memcpy (&temp_state, &state, sizeof (state)); if (wcrtomb (NULL, *ws, &temp_state) > len) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } } ... Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the `NULL' argument for the destination buffer in the new `wcrtomb' call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information. There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes. Converting Multibyte and Wide Character Strings ----------------------------------------------- The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, the GNU C library contains a few extensions that can help in some important situations. - Function: size_t mbsrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t LEN, mbstate_t *restrict PS) The `mbsrtowcs' function ("multibyte string restartable to wide character string") converts an NUL-terminated multibyte character string at `*SRC' into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by PS or from an internal object of `mbsrtowcs' if PS is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted. If DST is not a null pointer, the result is stored in the array pointed to by DST; otherwise, the conversion result is not available since it is stored in an internal buffer. If LEN wide characters are stored in the array DST before reaching the end of the input string, the conversion stops and LEN is returned. If DST is a null pointer, LEN is never checked. Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable `errno' is set to `EILSEQ' and the function returns `(size_t) -1'. In all other cases the function returns the number of wide characters converted during this call. If DST is not null, `mbsrtowcs' stores in the pointer pointed to by SRC either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character. `mbsrtowcs' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The definition of the `mbsrtowcs' function has one important limitation. The requirement that DST has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is normally no collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the `mbsrtowcs' function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter LEN to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess. There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the `mbsrtowcs' function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state-but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code. The generic conversion interface (*note Generic Charset Conversion::) does not have this limitation (it simply works on buffers, not strings), and the GNU C library contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of `mbsrtowcs''s example above could be solved by determining the line length and passing this length to the function. - Function: size_t wcsrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t LEN, mbstate_t *restrict PS) The `wcsrtombs' function ("wide character string restartable to multibyte string") converts the NUL-terminated wide character string at `*SRC' into an equivalent multibyte character string and stores the result in the array pointed to by DST. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by PS or by a state object locally to `wcsrtombs' in case PS is a null pointer. If DST is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if DST is not a null pointer, the pointer pointed to by SRC gets assigned a null pointer. If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable `errno' to `EILSEQ', and returns `(size_t) -1'. Another reason for a premature stop is if DST is not a null pointer and the next converted character would require more than LEN bytes in total to the array DST. In this case (and if DEST is not a null pointer) the pointer pointed to by SRC is assigned a value pointing to the wide character right after the last one successfully converted. Except in the case of an encoding error the return value of the `wcsrtombs' function is the number of bytes in all the multibyte character sequences stored in DST. Before returning the state in the object pointed to by PS (or the internal object in case PS is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted. The `wcsrtombs' function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The restriction mentioned above for the `mbsrtowcs' function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the LEN parameter). - Function: size_t mbsnrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict PS) The `mbsnrtowcs' function is very similar to the `mbsrtowcs' function. All the parameters are the same except for NMC, which is new. The return value is the same as for `mbsrtowcs'. This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string `*SRC' need not be NUL-terminated. But if a NUL byte is found within the NMC first bytes of the string, the conversion stops here. This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state. A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example): void showmbs (const char *src, FILE *fp) { mbstate_t state; int cnt = 0; memset (&state, '\0', sizeof (state)); while (1) { wchar_t linebuf[100]; const char *endp = strchr (src, '\n'); size_t n; /* Exit if there is no more line. */ if (endp == NULL) break; n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state); linebuf[n] = L'\0'; fprintf (fp, "line %d: \"%S\"\n", linebuf); } } There is no problem with the state after a call to `mbsnrtowcs'. Since we don't insert characters in the strings that were not in there right from the beginning and we use STATE only for the conversion of the given buffer, there is no problem with altering the state. - Function: size_t wcsnrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict PS) The `wcsnrtombs' function implements the conversion from wide character strings to multibyte character strings. It is similar to `wcsrtombs' but, just like `mbsnrtowcs', it takes an extra parameter, which specifies the length of the input string. No more than NWC wide characters from the input string `*SRC' are converted. If the input string contains a NUL wide character in the first NWC characters, the conversion stops at this place. The `wcsnrtombs' function is a GNU extension and just like `mbsnrtowcs' helps in situations where no NUL-terminated input strings are available. A Complete Multibyte Conversion Example --------------------------------------- The example programs given in the last sections are only brief and do not contain all the error checking, etc. Presented here is a complete and documented example. It features the `mbrtowc' function but it should be easy to derive versions using the other functions. int file_mbsrtowcs (int input, int output) { /* Note the use of `MB_LEN_MAX'. `MB_CUR_MAX' cannot portably be used here. */ char buffer[BUFSIZ + MB_LEN_MAX]; mbstate_t state; int filled = 0; int eof = 0; /* Initialize the state. */ memset (&state, '\0', sizeof (state)); while (!eof) { ssize_t nread; ssize_t nwrite; char *inp = buffer; wchar_t outbuf[BUFSIZ]; wchar_t *outp = outbuf; /* Fill up the buffer from the input file. */ nread = read (input, buffer + filled, BUFSIZ); if (nread < 0) { perror ("read"); return 0; } /* If we reach end of file, make a note to read no more. */ if (nread == 0) eof = 1; /* `filled' is now the number of bytes in `buffer'. */ filled += nread; /* Convert those bytes to wide characters-as many as we can. */ while (1) { size_t thislen = mbrtowc (outp, inp, filled, &state); /* Stop converting at invalid character; this can mean we have read just the first part of a valid character. */ if (thislen == (size_t) -1) break; /* We want to handle embedded NUL bytes but the return value is 0. Correct this. */ if (thislen == 0) thislen = 1; /* Advance past this character. */ inp += thislen; filled -= thislen; ++outp; } /* Write the wide characters we just made. */ nwrite = write (output, outbuf, (outp - outbuf) * sizeof (wchar_t)); if (nwrite < 0) { perror ("write"); return 0; } /* See if we have a _real_ invalid character. */ if ((eof && filled > 0) || filled >= MB_CUR_MAX) { error (0, 0, "invalid multibyte character"); return 0; } /* If any characters must be carried forward, put them at the beginning of `buffer'. */ if (filled > 0) memmove (inp, buffer, filled); } return 1; } Non-reentrant Conversion Function ================================= The functions described in the previous chapter are defined in Amendment 1 to ISO C90, but the original ISO C90 standard also contained functions for character set conversion. The reason that these original functions are not described first is that they are almost entirely useless. The problem is that all the conversion functions described in the original ISO C90 use a local state. Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use. These original functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one, and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). *For the above reasons it is highly requested that the functions described in the previous section be used in place of non-reentrant conversion functions.* Non-reentrant Conversion of Single Characters --------------------------------------------- - Function: int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE) The `mbtowc' ("multibyte to wide character") function when called with non-null STRING converts the first multibyte character beginning at STRING to its corresponding wide character code. It stores the result in `*RESULT'. `mbtowc' never examines more than SIZE bytes. (The idea is to supply for SIZE the number of bytes of data you have in hand.) `mbtowc' with non-null STRING distinguishes three possibilities: the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or STRING points to an empty string (a null character). For a valid multibyte character, `mbtowc' converts it to a wide character and stores that in `*RESULT', and returns the number of bytes in that character (always at least 1 and never more than SIZE). For an invalid byte sequence, `mbtowc' returns -1. For an empty string, it returns 0, also storing `'\0'' in `*RESULT'. If the multibyte character code uses shift characters, then `mbtowc' maintains and updates a shift state as it scans. If you call `mbtowc' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. *Note Shift State::. - Function: int wctomb (char *STRING, wchar_t WCHAR) The `wctomb' ("wide character to multibyte") function converts the wide character code WCHAR to its corresponding multibyte character sequence, and stores the result in bytes starting at STRING. At most `MB_CUR_MAX' characters are stored. `wctomb' with non-null STRING distinguishes three possibilities for WCHAR: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and `L'\0''. Given a valid code, `wctomb' converts it to a multibyte character, storing the bytes starting at STRING. Then it returns the number of bytes in that character (always at least 1 and never more than `MB_CUR_MAX'). If WCHAR is an invalid wide character code, `wctomb' returns -1. If WCHAR is `L'\0'', it returns `0', also storing `'\0'' in `*STRING'. If the multibyte character code uses shift characters, then `wctomb' maintains and updates a shift state as it scans. If you call `wctomb' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. *Note Shift State::. Calling this function with a WCHAR argument of zero when STRING is not null has the side-effect of reinitializing the stored shift state _as well as_ storing the multibyte character `'\0'' and returning 0. Similar to `mbrlen' there is also a non-reentrant function that computes the length of a multibyte character. It can be defined in terms of `mbtowc'. - Function: int mblen (const char *STRING, size_t SIZE) The `mblen' function with a non-null STRING argument returns the number of bytes that make up the multibyte character beginning at STRING, never examining more than SIZE bytes. (The idea is to supply for SIZE the number of bytes of data you have in hand.) The return value of `mblen' distinguishes three possibilities: the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or STRING points to an empty string (a null character). For a valid multibyte character, `mblen' returns the number of bytes in that character (always at least `1' and never more than SIZE). For an invalid byte sequence, `mblen' returns -1. For an empty string, it returns 0. If the multibyte character code uses shift characters, then `mblen' maintains and updates a shift state as it scans. If you call `mblen' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns a nonzero value if the multibyte character code in use actually has a shift state. *Note Shift State::. The function `mblen' is declared in `stdlib.h'. Non-reentrant Conversion of Strings ----------------------------------- For convenience the ISO C90 standard also defines functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see *Note Converting Strings::. - Function: size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE) The `mbstowcs' ("multibyte string to wide character string") function converts the null-terminated string of multibyte characters STRING to an array of wide character codes, storing not more than SIZE wide characters into the array beginning at WSTRING. The terminating null character counts towards the size, so if SIZE is less than the actual number of wide characters resulting from STRING, no terminating null character is stored. The conversion of characters from STRING begins in the initial shift state. If an invalid multibyte character sequence is found, the `mbstowcs' function returns a value of -1. Otherwise, it returns the number of wide characters stored in the array WSTRING. This number does not include the terminating null character, which is present if the number is less than SIZE. Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result. wchar_t * mbstowcs_alloc (const char *string) { size_t size = strlen (string) + 1; wchar_t *buf = xmalloc (size * sizeof (wchar_t)); size = mbstowcs (buf, string, size); if (size == (size_t) -1) return NULL; buf = xrealloc (buf, (size + 1) * sizeof (wchar_t)); return buf; } - Function: size_t wcstombs (char *STRING, const wchar_t *WSTRING, size_t SIZE) The `wcstombs' ("wide character string to multibyte string") function converts the null-terminated wide character array WSTRING into a string containing multibyte characters, storing not more than SIZE bytes starting at STRING, followed by a terminating null character if there is room. The conversion of characters begins in the initial shift state. The terminating null character counts towards the size, so if SIZE is less than or equal to the number of bytes needed in WSTRING, no terminating null character is stored. If a code that does not correspond to a valid multibyte character is found, the `wcstombs' function returns a value of -1. Otherwise, the return value is the number of bytes stored in the array STRING. This number does not include the terminating null character, which is present if the number is less than SIZE. States in Non-reentrant Functions --------------------------------- In some multibyte character codes, the _meaning_ of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called "shift sequences" and we say that they set the "shift state" for other sequences that follow. To illustrate shift state and shift sequences, suppose we decide that the sequence `0200' (just one byte) enters Japanese mode, in which pairs of bytes in the range from `0240' to `0377' are single characters, while `0201' enters Latin-1 mode, in which single bytes in the range from `0240' to `0377' are characters, and interpreted according to the ISO Latin-1 character set. This is a multibyte code that has two alternative shift states ("Japanese mode" and "Latin-1 mode"), and two shift sequences that specify particular shift states. When the multibyte character code in use has shift states, then `mblen', `mbtowc', and `wctomb' must maintain and update the current shift state as they scan the string. To make this work properly, you must follow these rules: * Before starting to scan a string, call the function with a null pointer for the multibyte character address--for example, `mblen (NULL, 0)'. This initializes the shift state to its standard initial value. * Scan the string one character at a time, in order. Do not "back up" and rescan characters already scanned, and do not intersperse the processing of different strings. Here is an example of using `mblen' following these rules: void scan_string (char *s) { int length = strlen (s); /* Initialize shift state. */ mblen (NULL, 0); while (1) { int thischar = mblen (s, length); /* Deal with end of string and invalid characters. */ if (thischar == 0) break; if (thischar == -1) { error ("invalid multibyte character"); break; } /* Advance past this character. */ s += thischar; length -= thischar; } } The functions `mblen', `mbtowc' and `wctomb' are not reentrant when using a multibyte code that uses a shift state. However, no other library functions call these functions, so you don't have to worry that the shift state will be changed mysteriously. Generic Charset Conversion ========================== The conversion functions mentioned so far in this chapter all had in common that they operate on character sets that are not directly specified by the functions. The multibyte encoding used is specified by the currently selected locale for the `LC_CTYPE' category. The wide character set is fixed by the implementation (in the case of GNU C library it is always UCS-4 encoded ISO 10646. This has of course several problems when it comes to general character conversion: * For every conversion where neither the source nor the destination character set is the character set of the locale for the `LC_CTYPE' category, one has to change the `LC_CTYPE' locale using `setlocale'. Changing the `LC_TYPE' locale introduces major problems for the rest of the programs since several more functions (e.g., the character classification functions, *note Classification of Characters::) use the `LC_CTYPE' category. * Parallel conversions to and from different character sets are not possible since the `LC_CTYPE' selection is global and shared by all threads. * If neither the source nor the destination character set is the character set used for `wchar_t' representation, there is at least a two-step process necessary to convert a text using the functions above. One would have to select the source character set as the multibyte encoding, convert the text into a `wchar_t' text, select the destination character set as the multibyte encoding, and convert the wide character text to the multibyte (= destination) character set. Even if this is possible (which is not guaranteed) it is a very tiring work. Plus it suffers from the other two raised points even more due to the steady changing of the locale. The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation. In the following text first the interface to `iconv' and then the conversion function, will be described. Comparisons with other implementations will show what obstacles stand in the way of portable applications. Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities. Generic Character Set Conversion Interface ------------------------------------------ This set of functions follows the traditional cycle of using a resource: open-use-close. The interface consists of three functions, each of which implements one step. Before the interfaces are described it is necessary to introduce a data type. Just like other open-use-close interfaces the functions introduced here work using handles and the `iconv.h' header defines a special type for the handles used. - Data Type: iconv_t This data type is an abstract type defined in `iconv.h'. The user must not assume anything about the definition of this type; it must be completely opaque. Objects of this type can get assigned handles for the conversions using the `iconv' functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to. The first step is the function to create a handle. - Function: iconv_t iconv_open (const char *TOCODE, const char *FROMCODE) The `iconv_open' function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle. If the wanted conversion is not available, the `iconv_open' function returns `(iconv_t) -1'. In this case the global variable `errno' can have the following values: `EMFILE' The process already has `OPEN_MAX' file descriptors open. `ENFILE' The system limit of open file is reached. `ENOMEM' Not enough memory to carry out the operation. `EINVAL' The conversion from FROMCODE to TOCODE is not supported. It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions. An `iconv' descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from FROMSET to TOSET. The GNU C library implementation of `iconv_open' has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (*note glibc iconv Implementation::). Here it is only important to say that all directories mentioned in the `GCONV_PATH' environment variable are considered only if they contain a file `gconv-modules'. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is `PREFIX/lib/gconv'. The `GCONV_PATH' environment variable is examined exactly once at the first call of the `iconv_open' function. Later modifications of the variable have no effect. The `iconv_open' function was introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The `iconv_open' function is declared in `iconv.h'. The `iconv' implementation can associate large data structure with the handle returned by `iconv_open'. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore. - Function: int iconv_close (iconv_t CD) The `iconv_close' function frees all resources associated with the handle CD, which must have been returned by a successful call to the `iconv_open' function. If the function call was successful the return value is 0. Otherwise it is -1 and `errno' is set appropriately. Defined error are: `EBADF' The conversion descriptor is invalid. The `iconv_close' function was introduced together with the rest of the `iconv' functions in XPG2 and is declared in `iconv.h'. The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it. - Function: size_t iconv (iconv_t CD, char **INBUF, size_t *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT) The `iconv' function converts the text in the input buffer according to the rules associated with the descriptor CD and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor. The input buffer is specified by `*INBUF' and it contains `*INBYTESLEFT' bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type `char' and the length is measured in bytes even if the input text is encoded in wide characters. The output buffer is specified in a similar way. `*OUTBUF' points to the beginning of the buffer with at least `*OUTBYTESLEFT' bytes room for the result. The buffer pointer again is of type `char' and the length is measured in bytes. If OUTBUF or `*OUTBUF' is a null pointer, the conversion is performed but no output is available. If INBUF is a null pointer, the `iconv' function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with INBUF not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for the GNU C library as it is described below. Therefore an `iconv' call to reset the state should always be performed if some protocol requires this for the output text. The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters. In all of these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in INBUF and OUTBUF, and the available room in each buffer is stored in INBYTESLEFT and OUTBYTESLEFT. Since the character sets selected in the `iconv_open' call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The _current_ behavior of the GNU C library in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished. If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is `(size_t) -1' and `errno' is set appropriately. In such cases the value pointed to by INBYTESLEFT is nonzero. `EILSEQ' The conversion stopped because of an invalid byte sequence in the input. After the call, `*INBUF' points at the first byte of the invalid byte sequence. `E2BIG' The conversion stopped because it ran out of space in the output buffer. `EINVAL' The conversion stopped because of an incomplete byte sequence at the end of the input buffer. `EBADF' The CD argument is invalid. The `iconv' function was introduced in the XPG2 standard and is declared in the `iconv.h' header. The definition of the `iconv' function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below. A complete `iconv' example -------------------------- The example below features a solution for a common problem. Given that one knows the internal encoding used by the system for `wchar_t' strings, one often is in the position to read text from a file and store it in wide character buffers. One can do this using `mbsrtowcs', but then we run into the problems discussed above. int file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail) { char inbuf[BUFSIZ]; size_t insize = 0; char *wrptr = (char *) outbuf; int result = 0; iconv_t cd; cd = iconv_open ("WCHAR_T", charset); if (cd == (iconv_t) -1) { /* Something went wrong. */ if (errno == EINVAL) error (0, 0, "conversion from '%s' to wchar_t not available", charset); else perror ("iconv_open"); /* Terminate the output string. */ *outbuf = L'\0'; return -1; } while (avail > 0) { size_t nread; size_t nconv; char *inptr = inbuf; /* Read more input. */ nread = read (fd, inbuf + insize, sizeof (inbuf) - insize); if (nread == 0) { /* When we come here the file is completely read. This still could mean there are some unused characters in the `inbuf'. Put them back. */ if (lseek (fd, -insize, SEEK_CUR) == -1) result = -1; /* Now write out the byte sequence to get into the initial state if this is necessary. */ iconv (cd, NULL, NULL, &wrptr, &avail); break; } insize += nread; /* Do the conversion. */ nconv = iconv (cd, &inptr, &insize, &wrptr, &avail); if (nconv == (size_t) -1) { /* Not everything went right. It might only be an unfinished byte sequence at the end of the buffer. Or it is a real problem. */ if (errno == EINVAL) /* This is harmless. Simply move the unused bytes to the beginning of the buffer so that they can be used in the next round. */ memmove (inbuf, inptr, insize); else { /* It is a real problem. Maybe we ran out of space in the output buffer or we have invalid input. In any case back the file pointer to the position of the last processed byte. */ lseek (fd, -insize, SEEK_CUR); result = -1; break; } } } /* Terminate the output string. */ if (avail >= sizeof (wchar_t)) *((wchar_t *) wrptr) = L'\0'; if (iconv_close (cd) != 0) perror ("iconv_close"); return (wchar_t *) wrptr - outbuf; } This example shows the most important aspects of using the `iconv' functions. It shows how successive calls to `iconv' can be used to convert large amounts of text. The user does not have to care about stateful encodings as the functions take care of everything. An interesting point is the case where `iconv' returns an error and `errno' is set to `EINVAL'. This is not really an error in the transformation. It can happen whenever the input character set contains byte sequences of more than one byte for some character and texts are not processed in one piece. In this case there is a chance that a multibyte sequence is cut. The caller can then simply read the remainder of the takes and feed the offending bytes together with new character from the input to `iconv' and continue the work. The internal state kept in the descriptor is _not_ unspecified after such an event as is the case with the conversion functions from the ISO C standard. The example also shows the problem of using wide character strings with `iconv'. As explained in the description of the `iconv' function above, the function always takes a pointer to a `char' array and the available space is measured in bytes. In the example, the output buffer is a wide character buffer; therefore, we use a local variable WRPTR of type `char *', which is used in the `iconv' calls. This looks rather innocent but can lead to problems on platforms that have tight restriction on alignment. Therefore the caller of `iconv' has to make sure that the pointers passed are suitable for access of characters from the appropriate character set. Since, in the above case, the input parameter to the function is a `wchar_t' pointer, this is the case (unless the user violates alignment when computing the parameter). But in other situations, especially when writing generic functions where one does not know what type of character set one uses and, therefore, treats text as a sequence of bytes, it might become tricky. Some Details about other `iconv' Implementations ------------------------------------------------ This is not really the place to discuss the `iconv' implementation of other systems but it is necessary to know a bit about them to write portable programs. The above mentioned problems with the specification of the `iconv' functions can lead to portability issues. The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library. Therefore, the conversion information must come from files outside the C library. This is usually done in one or both of the following ways: * The C library contains a set of generic conversion functions that can read the needed conversion tables and other information from data files. These files get loaded when necessary. This solution is problematic as it requires a great deal of effort to apply to all character sets (potentially an infinite set). The differences in the structure of the different character sets is so large that many different variants of the table-processing functions must be developed. In addition, the generic nature of these functions make them slower than specifically implemented functions. * The C library only contains a framework that can dynamically load object files and execute the conversion functions contained therein. This solution provides much more flexibility. The C library itself contains only very little code and therefore reduces the general memory footprint. Also, with a documented interface between the C library and the loadable modules it is possible for third parties to extend the set of available conversion modules. A drawback of this solution is that dynamic loading must be available. Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without this capability it is therefore not possible to use this interface in statically linked programs. The GNU C library has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot. The danger is that one gets acquainted with this situation and forgets about the restrictions on other systems. A second thing to know about other `iconv' implementations is that the number of available conversions is often very limited. Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities. This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of 10 others might count as 10 conversions. Together with the other direction this makes 20 conversion possibilities used up by one character set. One can imagine the thin coverage these platform provide. Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses. This directly leads to a third and probably the most problematic point. The way the `iconv' conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set A to B and the conversion from B to C does _not_ imply that the conversion from A to C is available. This might not seem unreasonable and problematic at first, but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program that has to convert from A to C. A call like cd = iconv_open ("C", "A"); fails according to the assumption above. But what does the program do now? The conversion is necessary; therefore, simply giving up is not an option. This is a nuisance. The `iconv' function should take care of this. But how should the program proceed from here on? If it tries to convert to character set B, first the two `iconv_open' calls cd1 = iconv_open ("B", "A"); and cd2 = iconv_open ("C", "B"); will succeed, but how to find B? Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one _could_ try to find them and determine from the available file which conversions are available and whether there is an indirect route from A to C. This example shows one of the design errors of `iconv' mentioned above. It should at least be possible to determine the list of available conversion programmatically so that if `iconv_open' says there is no such conversion, one could make sure this also is true for indirect routes. The `iconv' Implementation in the GNU C library ----------------------------------------------- After reading about the problems of `iconv' implementations in the last section it is certainly good to note that the implementation in the GNU C library has none of the problems mentioned above. What follows is a step-by-step analysis of the points raised above. The evaluation is based on the current state of the development (as of January 1999). The development of the `iconv' functions is not complete, but basic functionality has solidified. The GNU C library's `iconv' implementation uses shared loadable modules to implement the conversions. A very small number of conversions are built into the library itself but these are only rather trivial conversions. All the benefits of loadable modules are available in the GNU C library implementation. This is especially appealing since the interface is well documented (see below), and it, therefore, is easy to write new conversion modules. The drawback of using loadable objects is not a problem in the GNU C library, at least on ELF systems. Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use `iconv'. The second mentioned problem is the number of supported conversions. Currently, the GNU C library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (150 times 149). If any conversion from or to a character set is missing, it can be added easily. Particularly impressive as it may be, this high number is due to the fact that the GNU C library implementation of `iconv' does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set A to B and from B to C it is always possible to convert from A to C directly). If the `iconv_open' returns an error and sets `errno' to `EINVAL', there is no known way, directly or indirectly, to perform the wanted conversion. Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to "triangulate" (i.e., convert with an intermediate representation). There is no inherent requirement to provide a conversion to ISO 10646 for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The existing set of conversions is simply meant to cover all conversions that might be of interest. All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646. In such a situation one easily can write a new conversion and provide it as a better alternative. The GNU C library `iconv' implementation would automatically use the module implementing the conversion if it is specified to be more efficient. Format of `gconv-modules' files ............................... All information about the available conversions comes from a file named `gconv-modules', which can be found in any of the directories along the `GCONV_PATH'. The `gconv-modules' files are line-oriented text files, where each of the lines has one of the following formats: * If the first non-whitespace character is a `#' the line contains only comments and is ignored. * Lines starting with `alias' define an alias name for a character set. Two more words are expected on the line. The first word defines the alias name, and the second defines the original name of the character set. The effect is that it is possible to use the alias name in the FROMSET or TOSET parameters of `iconv_open' and achieve the same result as when using the real character set name. This is quite important as a character set has often many different names. There is normally an official name but this need not correspond to the most popular name. Beside this many character sets have special names that are somehow constructed. For example, all character sets specified by the ISO have an alias of the form `ISO-IR-NNN' where NNN is the registration number. This allows programs that know about the registration number to construct character set names and use them in `iconv_open' calls. More on the available names and aliases follows below. * Lines starting with `module' introduce an available conversion module. These lines must contain three or four more words. The first word specifies the source character set, the second word the destination character set of conversion implemented in this module, and the third word is the name of the loadable module. The filename is constructed by appending the usual shared object suffix (normally `.so') and this file is then supposed to be found in the same directory the `gconv-modules' file is in. The last word on the line, which is optional, is a numeric value representing the cost of the conversion. If this word is missing, a cost of 1 is assumed. The numeric value itself does not matter that much; what counts are the relative values of the sums of costs for all possible conversion paths. Below is a more precise description of the use of the cost value. Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All that has to be done is to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory and add a file `gconv-modules' with the following content in the same directory: module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1 module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1 To see why this is sufficient, it is necessary to understand how the conversion used by `iconv' (and described in the descriptor) is selected. The approach to this problem is quite simple. At the first call of the `iconv_open' function the program reads all available `gconv-modules' files and builds up two tables: one containing all the known aliases and another that contains the information about the conversions and which shared object implements them. Finding the conversion path in `iconv' ...................................... The set of available conversions form a directed graph with weighted edges. The weights on the edges are the costs specified in the `gconv-modules' files. The `iconv_open' function uses an algorithm suitable for search for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set. Explaining why the above `gconv-modules' files allows the `iconv' implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward. Since the latter conversion takes two steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost is 1+1 = 2. The above `gconv-modules' file, however, specifies that the new conversion modules can perform this conversion with only the cost of 1. A mysterious item about the `gconv-modules' file above (and also the file coming with the GNU C library) are the names of the character sets specified in the `module' lines. Why do almost all the names end in `//'? And this is not all: the names can actually be regular expressions. At this point in time this mystery should not be revealed, unless you have the relevant spell-casting materials: ashes from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed by St. Emacs, assorted herbal roots from Central America, sand from Cebu, etc. Sorry! *The part of the implementation where this is used is not yet finished. For now please simply follow the existing examples. It'll become clearer once it is. -drepper* A last remark about the `gconv-modules' is about the names not ending with `//'. A character set named `INTERNAL' is often mentioned. From the discussion above and the chosen name it should have become clear that this is the name for the representation used in the intermediate step of the triangulation. We have said that this is UCS-4 but actually that is not quite right. The UCS-4 specification also includes the specification of the byte ordering used. Since a UCS-4 value consists of four bytes, a stored value is effected by byte ordering. The internal representation is _not_ the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4. This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4. To avoid trouble with endianess, the internal representation consistently is named `INTERNAL' even on big-endian systems where the representations are identical. `iconv' module data structures .............................. So far this section has described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change a bit in the future but, with luck, only in an upwardly compatible way. The definitions necessary to write new modules are publicly available in the non-standard header `gconv.h'. The following text, therefore, describes the definitions from this header file. First, however, it is necessary to get an overview. From the perspective of the user of `iconv' the interface is quite simple: the `iconv_open' function returns a handle that can be used in calls to `iconv', and finally the handle is freed with a call to `iconv_close'. The problem is that the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion since the handle is all that is passed to the `iconv' function. Therefore, the data structures are really the elements necessary to understanding the implementation. We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in `gconv.h'. - Data type: struct __gconv_step This data structure describes one conversion a module can perform. For each function in a loaded module with conversion functions there is exactly one object of this type. This object is shared by all users of the conversion (i.e., this object does not contain any information corresponding to an actual conversion; it only describes the conversion itself). `struct __gconv_loaded_object *__shlib_handle' `const char *__modname' `int __counter' All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared. One must not expect any of the other elements to be available or initialized. `const char *__from_name' `const char *__to_name' `__from_name' and `__to_name' contain the names of the source and destination character sets. They can be used to identify the actual conversion to be carried out since one module might implement conversions for more than one character set and/or direction. `gconv_fct __fct' `gconv_init_fct __init_fct' `gconv_end_fct __end_fct' These elements contain pointers to the functions in the loadable module. The interface will be explained below. `int __min_needed_from' `int __max_needed_from' `int __min_needed_to' `int __max_needed_to;' These values have to be supplied in the init function of the module. The `__min_needed_from' value specifies how many bytes a character of the source character set at least needs. The `__max_needed_from' specifies the maximum value that also includes possible shift sequences. The `__min_needed_to' and `__max_needed_to' values serve the same purpose as `__min_needed_from' and `__max_needed_from' but this time for the destination character set. It is crucial that these values be accurate since otherwise the conversion functions will have problems or not work at all. `int __stateful' This element must also be initialized by the init function. `int __stateful' is nonzero if the source character set is stateful. Otherwise it is zero. `void *__data' This element can be used freely by the conversion functions in the module. `void *__data' can be used to communicate extra information from one call to another. `void *__data' need not be initialized if not needed at all. If `void *__data' element is assigned a pointer to dynamically allocated memory (presumably in the init function) it has to be made sure that the end function deallocates the memory. Otherwise the application will leak memory. It is important to be aware that this data structure is shared by all users of this specification conversion and therefore the `__data' element must not contain data specific to one specific use of the conversion function. - Data type: struct __gconv_step_data This is the data structure that contains the information specific to each use of the conversion functions. `char *__outbuf' `char *__outbufend' These elements specify the output buffer for the conversion step. The `__outbuf' element points to the beginning of the buffer, and `__outbufend' points to the byte following the last byte in the buffer. The conversion function must not assume anything about the size of the buffer but it can be safely assumed the there is room for at least one complete character in the output buffer. Once the conversion is finished, if the conversion is the last step, the `__outbuf' element must be modified to point after the last byte written into the buffer to signal how much output is available. If this conversion step is not the last one, the element must not be modified. The `__outbufend' element must not be modified. `int __is_last' This element is nonzero if this conversion step is the last one. This information is necessary for the recursion. See the description of the conversion function internals below. This element must never be modified. `int __invocation_counter' The conversion function can use this element to see how many calls of the conversion function already happened. Some character sets require a certain prolog when generating output, and by comparing this value with zero, one can find out whether it is the first call and whether, therefore, the prolog should be emitted. This element must never be modified. `int __internal_use' This element is another one rarely used but needed in certain situations. It is assigned a nonzero value in case the conversion functions are used to implement `mbsrtowcs' et.al. (i.e., the function is not used directly through the `iconv' interface). This sometimes makes a difference as it is expected that the `iconv' functions are used to translate entire texts while the `mbsrtowcs' functions are normally used only to convert single strings and might be used multiple times to convert entire texts. But in this situation we would have problem complying with some rules of the character set specification. Some character sets require a prolog, which must appear exactly once for an entire text. If a number of `mbsrtowcs' calls are used to convert the text, only the first call must add the prolog. However, because there is no communication between the different calls of `mbsrtowcs', the conversion functions have no possibility to find this out. The situation is different for sequences of `iconv' calls since the handle allows access to the needed information. The `int __internal_use' element is mostly used together with `__invocation_counter' as follows: if (!data->__internal_use && data->__invocation_counter == 0) /* Emit prolog. */ ... This element must never be modified. `mbstate_t *__statep' The `__statep' element points to an object of type `mbstate_t' (*note Keeping the state::). The conversion of a stateful character set must use the object pointed to by `__statep' to store information about the conversion state. The `__statep' element itself must never be modified. `mbstate_t __state' This element must _never_ be used directly. It is only part of this structure to have the needed space allocated. `iconv' module interfaces ......................... With the knowledge about the data structures we now can describe the conversion function itself. To understand the interface a bit of knowledge is necessary about the functionality in the C library that loads the objects with the conversions. It is often the case that one conversion is used more than once (i.e., there are several `iconv_open' calls for the same set of character sets during one program run). The `mbsrtowcs' et.al. functions in the GNU C library also use the `iconv' functionality, which increases the number of uses of the same functions even more. Because of this multiple use of conversions, the modules do not get loaded exclusively for one conversion. Instead a module once loaded can be used by an arbitrary number of `iconv' or `mbsrtowcs' calls at the same time. The splitting of the information between conversion- function-specific information and conversion data makes this possible. The last section showed the two data structures used to do this. This is of course also reflected in the interface and semantics of the functions that the modules must provide. There are three functions that must have the following names: `gconv_init' The `gconv_init' function initializes the conversion function specific data structure. This very same object is shared by all conversions that use this conversion and, therefore, no state information about the conversion itself must be stored in here. If a module implements more than one conversion, the `gconv_init' function will be called multiple times. `gconv_end' The `gconv_end' function is responsible for freeing all resources allocated by the `gconv_init' function. If there is nothing to do, this function can be missing. Special care must be taken if the module implements more than one conversion and the `gconv_init' function does not allocate the same resources for all conversions. `gconv' This is the actual conversion function. It is called to convert one block of text. It gets passed the conversion step information initialized by `gconv_init' and the conversion data, specific to this use of the conversion functions. There are three data types defined for the three module interface functions and these define the interface. - Data type: int (*__gconv_init_fct) (struct __gconv_step *) This specifies the interface of the initialization function of the module. It is called exactly once for each conversion the module implements. As explained in the description of the `struct __gconv_step' data structure above the initialization function has to initialize parts of it. `__min_needed_from' `__max_needed_from' `__min_needed_to' `__max_needed_to' These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character sets, respectively. If the characters all have the same size, the minimum and maximum values are the same. `__stateful' This element must be initialized to an nonzero value if the source character set is stateful. Otherwise it must be zero. If the initialization function needs to communicate some information to the conversion function, this communication can happen using the `__data' element of the `__gconv_step' structure. But since this data is shared by all the conversions, it must not be modified by the conversion function. The example below shows how this can be used. #define MIN_NEEDED_FROM 1 #define MAX_NEEDED_FROM 4 #define MIN_NEEDED_TO 4 #define MAX_NEEDED_TO 4 int gconv_init (struct __gconv_step *step) { /* Determine which direction. */ struct iso2022jp_data *new_data; enum direction dir = illegal_dir; enum variant var = illegal_var; int result; if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0) { dir = from_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0) { dir = to_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0) { dir = from_iso2022jp; var = iso2022jp2; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0) { dir = to_iso2022jp; var = iso2022jp2; } result = __GCONV_NOCONV; if (dir != illegal_dir) { new_data = (struct iso2022jp_data *) malloc (sizeof (struct iso2022jp_data)); result = __GCONV_NOMEM; if (new_data != NULL) { new_data->dir = dir; new_data->var = var; step->__data = new_data; if (dir == from_iso2022jp) { step->__min_needed_from = MIN_NEEDED_FROM; step->__max_needed_from = MAX_NEEDED_FROM; step->__min_needed_to = MIN_NEEDED_TO; step->__max_needed_to = MAX_NEEDED_TO; } else { step->__min_needed_from = MIN_NEEDED_TO; step->__max_needed_from = MAX_NEEDED_TO; step->__min_needed_to = MIN_NEEDED_FROM; step->__max_needed_to = MAX_NEEDED_FROM + 2; } /* Yes, this is a stateful encoding. */ step->__stateful = 1; result = __GCONV_OK; } } return result; } The function first checks which conversion is wanted. The module from which this function is taken implements four different conversions; which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case. Next, a data structure, which contains the necessary information about which conversion is selected, is allocated. The data structure `struct iso2022jp_data' is locally defined since, outside the module, this data is not used at all. Please note that if all four conversions this modules supports are requested there are four data blocks. One interesting thing is the initialization of the `__min_' and `__max_' elements of the step data object. A single ISO-2022-JP character can consist of one to four bytes. Therefore the `MIN_NEEDED_FROM' and `MAX_NEEDED_FROM' macros are defined this way. The output is always the `INTERNAL' character set (aka UCS-4) and therefore each character consists of exactly four bytes. For the conversion from `INTERNAL' to ISO-2022-JP we have to take into account that escape sequences might be necessary to switch the character sets. Therefore the `__max_needed_to' element for this direction gets assigned `MAX_NEEDED_FROM + 2'. This takes into account the two bytes needed for the escape sequences to single the switching. The asymmetry in the maximum values for the two directions can be explained easily: when reading ISO-2022-JP text, escape sequences can be handled alone (i.e., it is not necessary to process a real character since the effect of the escape sequence can be recorded in the state information). The situation is different for the other direction. Since it is in general not known which character comes next, one cannot emit escape sequences to change the state in advance. This means the escape sequences that have to be emitted together with the next character. Therefore one needs more room than only for the character itself. The possible return values of the initialization function are: `__GCONV_OK' The initialization succeeded `__GCONV_NOCONV' The requested conversion is not supported in the module. This can happen if the `gconv-modules' file has errors. `__GCONV_NOMEM' Memory required to store additional information could not be allocated. The function called before the module is unloaded is significantly easier. It often has nothing at all to do; in which case it can be left out completely. - Data type: void (*__gconv_end_fct) (struct gconv_step *) The task of this function is to free all resources allocated in the initialization function. Therefore only the `__data' element of the object pointed to by the argument is of interest. Continuing the example from the initialization function, the finalization function looks like this: void gconv_end (struct __gconv_step *data) { free (data->__data); } The most important function is the conversion function itself, which can get quite complicated for complex character sets. But since this is not of interest here, we will only describe a possible skeleton for the conversion function. - Data type: int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int) The conversion function can be called for two basic reason: to convert text or to reset the state. From the description of the `iconv' function it can be seen why the flushing mode is necessary. What mode is selected is determined by the sixth argument, an integer. This argument being nonzero means that flushing is selected. Common to both modes is where the output buffer can be found. The information about this buffer is stored in the conversion step data. A pointer to this information is passed as the second argument to this function. The description of the `struct __gconv_step_data' structure has more information on the conversion step data. What has to be done for flushing depends on the source character set. If the source character set is not stateful, nothing has to be done. Otherwise the function has to emit a byte sequence to bring the state object into the initial state. Once this all happened the other conversion modules in the chain of conversions have to get the same chance. Whether another step follows can be determined from the `__is_last' element of the step data structure to which the first parameter points. The more interesting mode is when actual text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument, which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer. The conversion has to be performed according to the current state if the character set is stateful. The state is stored in an object pointed to by the `__statep' element of the step data (second argument). Once either the input buffer is empty or the output buffer is full the conversion stops. At this point, the pointer variable referenced by the third parameter must point to the byte following the last processed byte (i.e., if all of the input is consumed, this pointer and the fourth parameter have the same value). What now happens depends on whether this step is the last one. If it is the last step, the only thing that has to be done is to update the `__outbuf' element of the step data structure to point after the last written byte. This update gives the caller the information on how much text is available in the output buffer. In addition, the variable pointed to by the fifth parameter, which is of type `size_t', must be incremented by the number of characters (_not bytes_) that were converted in a non-reversible way. Then, the function can return. In case the step is not the last one, the later conversion functions have to get a chance to do their work. Therefore, the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays, so the next element in both cases can be found by simple pointer arithmetic: int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) { struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; ... The `next_step' pointer references the next step information and `next_data' the next data record. The call of the next function therefore will look similar to this: next_step->__fct (next_step, next_data, &outerr, outbuf, written, 0) But this is not yet all. Once the function call returns the conversion function might have some more to do. If the return value of the function is `__GCONV_EMPTY_INPUT', more room is available in the output buffer. Unless the input buffer is empty the conversion, functions start all over again and process the rest of the input buffer. If the return value is not `__GCONV_EMPTY_INPUT', something went wrong and we have to recover from this. A requirement for the conversion function is that the input buffer pointer (the third argument) always point to the last character that was put in converted form into the output buffer. This is trivially true after the conversion performed in the current step, but if the conversion functions deeper downstream stop prematurely, not all characters from the output buffer are consumed and, therefore, the input buffer pointers must be backed off to the right position. Correcting the input buffers is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and, therefore, can correct the input buffer pointer appropriately with a similar computation. Things are getting tricky if either character set has characters represented with variable length byte sequences, and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion (i.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again). The difference now is that it is known how much input must be created, and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return. One final thing should be mentioned. If it is necessary for the conversion to know whether it is the first invocation (in case a prolog has to be emitted), the conversion function should increment the `__invocation_counter' element of the step data structure just before returning to the caller. See the description of the `struct __gconv_step_data' structure above for more information on how this can be used. The return value must be one of the following values: `__GCONV_EMPTY_INPUT' All input was consumed and there is room left in the output buffer. `__GCONV_FULL_OUTPUT' No more room in the output buffer. In case this is not the last step this value is propagated down from the call of the next conversion function in the chain. `__GCONV_INCOMPLETE_INPUT' The input buffer is not entirely empty since it contains an incomplete character sequence. The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it. int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) { struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; gconv_fct fct = next_step->__fct; int status; /* If the function is called with no input this means we have to reset to the initial state. The possibly partly converted input is dropped. */ if (do_flush) { status = __GCONV_OK; /* Possible emit a byte sequence which put the state object into the initial state. */ /* Call the steps down the chain if there are any but only if we successfully emitted the escape sequence. */ if (status == __GCONV_OK && ! data->__is_last) status = fct (next_step, next_data, NULL, NULL, written, 1); } else { /* We preserve the initial values of the pointer variables. */ const char *inptr = *inbuf; char *outbuf = data->__outbuf; char *outend = data->__outbufend; char *outptr; do { /* Remember the start value for this round. */ inptr = *inbuf; /* The outbuf buffer is empty. */ outptr = outbuf; /* For stateful encodings the state must be safe here. */ /* Run the conversion loop. `status' is set appropriately afterwards. */ /* If this is the last step, leave the loop. There is nothing we can do. */ if (data->__is_last) { /* Store information about how many bytes are available. */ data->__outbuf = outbuf; /* If any non-reversible conversions were performed, add the number to `*written'. */ break; } /* Write out all output that was produced. */ if (outbuf > outptr) { const char *outerr = data->__outbuf; int result; result = fct (next_step, next_data, &outerr, outbuf, written, 0); if (result != __GCONV_EMPTY_INPUT) { if (outerr != outbuf) { /* Reset the input buffer pointer. We document here the complex case. */ size_t nstatus; /* Reload the pointers. */ *inbuf = inptr; outbuf = outptr; /* Possibly reset the state. */ /* Redo the conversion, but this time the end of the output buffer is at `outerr'. */ } /* Change the status. */ status = result; } else /* All the output is consumed, we can make another run if everything was ok. */ if (status == __GCONV_FULL_OUTPUT) status = __GCONV_OK; } } while (status == __GCONV_OK); /* We finished one use of this step. */ ++data->__invocation_counter; } return status; } This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the GNU C library sources. It contains many examples of working and optimized modules. Locales and Internationalization ******************************** Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken. "Internationalization" of software means programming it to be able to adapt to the user's favorite conventions. In ISO C, internationalization works by means of "locales". Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables). All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user. What Effects a Locale Has ========================= Each locale specifies conventions for several purposes, including the following: * What multibyte character sequences are valid, and how they are interpreted (*note Character Set Handling::). * Classification of which characters in the local character set are considered alphabetic, and upper- and lower-case conversion conventions (*note Character Handling::). * The collating sequence for the local language and character set (*note Collation Functions::). * Formatting of numbers and currency amounts (*note General Numeric::). * Formatting of dates and times (*note Formatting Calendar Time::). * What language to use for output, including error messages (*note Message Translation::). * What language to use for user answers to yes-or-no questions (*note Yes-or-No Questions::). * What language to use for more complex user input. (The C library doesn't yet help you implement this.) Some aspects of adapting to the specified locale are handled automatically by the library subroutines. For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use `strcoll' or `strxfrm' to compare strings. Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. The C library provides functions to handle translations for multiple languages easily. This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions. Choosing a Locale ================= The simplest way for the user to choose a locale is to set the environment variable `LANG'. This specifies a single locale to use for all purposes. For example, a user could specify a hypothetical locale named `espana-castellano' to use the standard conventions of most of Spain. The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'. Later we will describe how to construct locales. A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of multiple locales. For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars. Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes. Categories of Activities that Locales Affect ============================================ The purposes that locales serve are grouped into "categories", so that a user or a program can choose the locale for each category independently. Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to `setlocale'. `LC_COLLATE' This category applies to collation of strings (functions `strcoll' and `strxfrm'); see *Note Collation Functions::. `LC_CTYPE' This category applies to classification and conversion of characters, and to multibyte and wide characters; see *Note Character Handling::, and *Note Character Set Handling::. `LC_MONETARY' This category applies to formatting monetary values; see *Note General Numeric::. `LC_NUMERIC' This category applies to formatting numeric values that are not monetary; see *Note General Numeric::. `LC_TIME' This category applies to formatting date and time values; see *Note Formatting Calendar Time::. `LC_MESSAGES' This category applies to selecting the language used in the user interface for message translation (*note The Uniforum approach::; *note Message catalogs a la X/Open::) and contains regular expressions for affirmative and negative responses. `LC_ALL' This is not an environment variable; it is only a macro that you can use with `setlocale' to set a single locale for all purposes. Setting this environment variable overwrites all selections by the other `LC_*' variables or `LANG'. `LANG' If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above. When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient. For example, it should be possible to specify more than one locale name. Take a Swedish user who better speaks German than English, and a program whose messages are output in English by default. It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails to use English. This is possible with the variable `LANGUAGE'. For further description of this GNU extension see *Note Using gettextized software::. How Programs Set the Locale =========================== A C program inherits its locale environment variables when it starts up. This happens automatically. However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard `C' locale. To use the locales specified by the environment, you must call `setlocale'. Call it as follows: setlocale (LC_ALL, ""); to select a locale based on the user choice of the appropriate environment variables. You can also use `setlocale' to specify a particular locale, for general use or for a specific category. The symbols in this section are defined in the header file `locale.h'. - Function: char * setlocale (int CATEGORY, const char *LOCALE) The function `setlocale' sets the current locale for category CATEGORY to LOCALE. A list of all the locales the system provides can be created by running locale -a If CATEGORY is `LC_ALL', this specifies the locale for all purposes. The other possible values of CATEGORY specify an single purpose (*note Locale Categories::). You can also use this function to find out the current locale by passing a null pointer as the LOCALE argument. In this case, `setlocale' returns a string that is the name of the locale currently selected for category CATEGORY. The string returned by `setlocale' can be overwritten by subsequent calls, so you should make a copy of the string (*note Copying and Concatenation::) if you want to save it past any further calls to `setlocale'. (The standard library is guaranteed never to call `setlocale' itself.) You should not modify the string returned by `setlocale'. It might be the same string that was passed as an argument in a previous call to `setlocale'. One requirement is that the CATEGORY must be the same in the call the string was returned and the one when the string is passed in as LOCALE parameter. When you read the current locale for category `LC_ALL', the value encodes the entire combination of selected locales for all categories. In this case, the value is not just a single locale name. In fact, we don't make any promises about what it looks like. But if you specify the same "locale name" with `LC_ALL' in a subsequent call to `setlocale', it restores the same combination of locale selections. To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string. It is not guaranteed that the returned pointer remains valid over time. When the LOCALE argument is not a null pointer, the string returned by `setlocale' reflects the newly-modified locale. If you specify an empty string for LOCALE, this means to read the appropriate environment variable and use its value to select the locale for CATEGORY. If a nonempty string is given for LOCALE, then the locale of that name is used if possible. If you specify an invalid locale name, `setlocale' returns a null pointer and leaves the current locale unchanged. Here is an example showing how you might use `setlocale' to temporarily switch to a new locale. #include #include #include #include void with_other_locale (char *new_locale, void (*subroutine) (int), int argument) { char *old_locale, *saved_locale; /* Get the name of the current locale. */ old_locale = setlocale (LC_ALL, NULL); /* Copy the name so it won't be clobbered by `setlocale'. */ saved_locale = strdup (old_locale); if (saved_locale == NULL) fatal ("Out of memory"); /* Now change the locale and do some stuff with it. */ setlocale (LC_ALL, new_locale); (*subroutine) (argument); /* Restore the original locale. */ setlocale (LC_ALL, saved_locale); free (saved_locale); } *Portability Note:* Some ISO C systems may define additional locale categories, and future versions of the library will do so. For portability, assume that any symbol beginning with `LC_' might be defined in `locale.h'. Standard Locales ================ The only locale names you can count on finding on all operating systems are these three standard ones: `"C"' This is the standard C locale. The attributes and behavior it provides are specified in the ISO C standard. When your program starts up, it initially uses this locale by default. `"POSIX"' This is the standard POSIX locale. Currently, it is an alias for the standard C locale. `""' The empty name says to select a locale based on environment variables. *Note Locale Categories::. Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). It is also possible for the user to create private locales. All this will be discussed later when describing the tool to do so. If your program needs to use something other than the `C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed. Accessing Locale Information ============================ There are several ways to access locale information. The simplest way is to let the C library itself do the work. Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale. This is how the locale model is meant to work normally. As an example take the `strftime' function, which is meant to nicely format date and time information (*note Formatting Calendar Time::). Part of the standard information contained in the `LC_TIME' category is the names of the months. Instead of requiring the programmer to take care of providing the translations the `strftime' function does this all by itself. `%A' in the format string is replaced by the appropriate weekday name of the locale currently selected by `LC_TIME'. This is an easy example, and wherever possible functions do things automatically in this way. But there are quite often situations when there is simply no function to perform the task, or it is simply not possible to do the work automatically. For these cases it is necessary to access the information in the locale directly. To do this the C library provides two functions: `localeconv' and `nl_langinfo'. The former is part of ISO C and therefore portable, but has a brain-damaged interface. The second is part of the Unix interface and is portable in as far as the system follows the Unix standards. `localeconv': It is portable but ... ------------------------------------ Together with the `setlocale' function the ISO C people invented the `localeconv' function. It is a masterpiece of poor design. It is expensive to use, not extendable, and not generally usable as it provides access to only `LC_MONETARY' and `LC_NUMERIC' related information. Nevertheless, if it is applicable to a given situation it should be used since it is very portable. The function `strfmon' formats monetary amounts according to the selected locale using this information. - Function: struct lconv * localeconv (void) The `localeconv' function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale. You should not modify the structure or its contents. The structure might be overwritten by subsequent calls to `localeconv', or by calls to `setlocale', but no other function in the library overwrites this value. - Data Type: struct lconv `localeconv''s return value is of this data type. Its elements are described in the following subsections. If a member of the structure `struct lconv' has type `char', and the value is `CHAR_MAX', it means that the current locale has no value for that parameter. Generic Numeric Formatting Parameters ..................................... These are the standard members of `struct lconv'; there may be others. `char *decimal_point' `char *mon_decimal_point' These are the decimal-point separators used in formatting non-monetary and monetary quantities, respectively. In the `C' locale, the value of `decimal_point' is `"."', and the value of `mon_decimal_point' is `""'. `char *thousands_sep' `char *mon_thousands_sep' These are the separators used to delimit groups of digits to the left of the decimal point in formatting non-monetary and monetary quantities, respectively. In the `C' locale, both members have a value of `""' (the empty string). `char *grouping' `char *mon_grouping' These are strings that specify how to group the digits to the left of the decimal point. `grouping' applies to non-monetary quantities and `mon_grouping' applies to monetary quantities. Use either `thousands_sep' or `mon_thousands_sep' to separate the digit groups. Each member of these strings is to be interpreted as an integer value of type `char'. Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point.) The last member is either `0', in which case the previous member is used over and over again for all the remaining groups, or `CHAR_MAX', in which case there is no more grouping--or, put another way, any remaining digits form one large group without separators. For example, if `grouping' is `"\04\03\02"', the correct grouping for the number `123456787654321' is `12', `34', `56', `78', `765', `4321'. This uses a group of 4 digits at the end, preceded by a group of 3 digits, preceded by groups of 2 digits (as many as needed). With a separator of `,', the number would be printed as `12,34,56,78,765,4321'. A value of `"\03"' indicates repeated groups of three digits, as normally used in the U.S. In the standard `C' locale, both `grouping' and `mon_grouping' have a value of `""'. This value specifies no grouping at all. `char int_frac_digits' `char frac_digits' These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively. (Most often, both members have the same value.) In the standard `C' locale, both of these members have the value `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend printing no fractional digits. (This locale also specifies the empty string for `mon_decimal_point', so printing any fractional digits would be confusing!) Printing the Currency Symbol ............................ These members of the `struct lconv' structure specify how to print the symbol to identify a monetary value--the international analog of `$' for US dollars. Each country has two standard currency symbols. The "local currency symbol" is used commonly within the country, while the "international currency symbol" is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously. For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars. `char *currency_symbol' The local currency symbol for the selected locale. In the standard `C' locale, this member has a value of `""' (the empty string), meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string pointed to by this variable. `char *int_curr_symbol' The international currency symbol for the selected locale. The value of `int_curr_symbol' should normally consist of a three-letter abbreviation determined by the international standard `ISO 4217 Codes for the Representation of Currency and Funds', followed by a one-character separator (often a space). In the standard `C' locale, this member has a value of `""' (the empty string), meaning "unspecified". We recommend you simply print the empty string as you would print any other string pointed to by this variable. `char p_cs_precedes' `char n_cs_precedes' `char int_p_cs_precedes' `char int_n_cs_precedes' These members are `1' if the `currency_symbol' or `int_curr_symbol' strings should precede the value of a monetary amount, or `0' if the strings should follow the value. The `p_cs_precedes' and `int_p_cs_precedes' members apply to positive amounts (or zero), and the `n_cs_precedes' and `int_n_cs_precedes' members apply to negative amounts. In the standard `C' locale, all of these members have a value of `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what to do when you find this value. We recommend printing the currency symbol before the amount, which is right for most countries. In other words, treat all nonzero values alike in these members. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. `char p_sep_by_space' `char n_sep_by_space' `char int_p_sep_by_space' `char int_n_sep_by_space' These members are `1' if a space should appear between the `currency_symbol' or `int_curr_symbol' strings and the amount, or `0' if no space should appear. The `p_sep_by_space' and `int_p_sep_by_space' members apply to positive amounts (or zero), and the `n_sep_by_space' and `int_n_sep_by_space' members apply to negative amounts. In the standard `C' locale, all of these members have a value of `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what you should do when you find this value; we suggest you treat it as 1 (print a space). In other words, treat all nonzero values alike in these members. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. There is one specialty with the `int_curr_symbol', though. Since all legal values contain a space at the end the string one either printf this space (if the currency symbol must appear in front and must be separated) or one has to avoid printing this character at all (especially when at the end of the string). Printing the Sign of a Monetary Amount ...................................... These members of the `struct lconv' structure specify how to print the sign (if any) of a monetary value. `char *positive_sign' `char *negative_sign' These are strings used to indicate positive (or zero) and negative monetary quantities, respectively. In the standard `C' locale, both of these members have a value of `""' (the empty string), meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend printing `positive_sign' as you find it, even if it is empty. For a negative value, print `negative_sign' as you find it unless both it and `positive_sign' are empty, in which case print `-' instead. (Failing to indicate the sign at all seems rather unreasonable.) `char p_sign_posn' `char n_sign_posn' `char int_p_sign_posn' `char int_n_sign_posn' These members are small integers that indicate how to position the sign for nonnegative and negative monetary quantities, respectively. (The string used by the sign is what was specified with `positive_sign' or `negative_sign'.) The possible values are as follows: `0' The currency symbol and quantity should be surrounded by parentheses. `1' Print the sign string before the quantity and currency symbol. `2' Print the sign string after the quantity and currency symbol. `3' Print the sign string right before the currency symbol. `4' Print the sign string right after the currency symbol. `CHAR_MAX' "Unspecified". Both members have this value in the standard `C' locale. The ISO standard doesn't say what you should do when the value is `CHAR_MAX'. We recommend you print the sign after the currency symbol. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. Pinpoint Access to Locale Data ------------------------------ When writing the X/Open Portability Guide the authors realized that the `localeconv' function is not enough to provide reasonable access to locale information. The information which was meant to be available in the locale (as later specified in the POSIX.1 standard) requires more ways to access it. Therefore the `nl_langinfo' function was introduced. - Function: char * nl_langinfo (nl_item ITEM) The `nl_langinfo' function can be used to access individual elements of the locale categories. Unlike the `localeconv' function, which returns all the information, `nl_langinfo' lets the caller select what information it requires. This is very fast and it is not a problem to call this function multiple times. A second advantage is that in addition to the numeric and monetary formatting information, information from the `LC_TIME' and `LC_MESSAGES' categories is available. The type `nl_type' is defined in `nl_types.h'. The argument ITEM is a numeric value defined in the header `langinfo.h'. The X/Open standard defines the following values: `CODESET' `nl_langinfo' returns a string with the name of the coded character set used in the selected locale. `ABDAY_1' `ABDAY_2' `ABDAY_3' `ABDAY_4' `ABDAY_5' `ABDAY_6' `ABDAY_7' `nl_langinfo' returns the abbreviated weekday name. `ABDAY_1' corresponds to Sunday. `DAY_1' `DAY_2' `DAY_3' `DAY_4' `DAY_5' `DAY_6' `DAY_7' Similar to `ABDAY_1' etc., but here the return value is the unabbreviated weekday name. `ABMON_1' `ABMON_2' `ABMON_3' `ABMON_4' `ABMON_5' `ABMON_6' `ABMON_7' `ABMON_8' `ABMON_9' `ABMON_10' `ABMON_11' `ABMON_12' The return value is abbreviated name of the month. `ABMON_1' corresponds to January. `MON_1' `MON_2' `MON_3' `MON_4' `MON_5' `MON_6' `MON_7' `MON_8' `MON_9' `MON_10' `MON_11' `MON_12' Similar to `ABMON_1' etc., but here the month names are not abbreviated. Here the first value `MON_1' also corresponds to January. `AM_STR' `PM_STR' The return values are strings which can be used in the representation of time as an hour from 1 to 12 plus an am/pm specifier. Note that in locales which do not use this time representation these strings might be empty, in which case the am/pm format cannot be used at all. `D_T_FMT' The return value can be used as a format string for `strftime' to represent time and date in a locale-specific way. `D_FMT' The return value can be used as a format string for `strftime' to represent a date in a locale-specific way. `T_FMT' The return value can be used as a format string for `strftime' to represent time in a locale-specific way. `T_FMT_AMPM' The return value can be used as a format string for `strftime' to represent time in the am/pm format. Note that if the am/pm format does not make any sense for the selected locale, the return value might be the same as the one for `T_FMT'. `ERA' The return value represents the era used in the current locale. Most locales do not define this value. An example of a locale which does define this value is the Japanese one. In Japan, the traditional representation of dates includes the name of the era corresponding to the then-emperor's reign. Normally it should not be necessary to use this value directly. Specifying the `E' modifier in their format strings causes the `strftime' functions to use this information. The format of the returned string is not specified, and therefore you should not assume knowledge of it on different systems. `ERA_YEAR' The return value gives the year in the relevant era of the locale. As for `ERA' it should not be necessary to use this value directly. `ERA_D_T_FMT' This return value can be used as a format string for `strftime' to represent dates and times in a locale-specific era-based way. `ERA_D_FMT' This return value can be used as a format string for `strftime' to represent a date in a locale-specific era-based way. `ERA_T_FMT' This return value can be used as a format string for `strftime' to represent time in a locale-specific era-based way. `ALT_DIGITS' The return value is a representation of up to 100 values used to represent the values 0 to 99. As for `ERA' this value is not intended to be used directly, but instead indirectly through the `strftime' function. When the modifier `O' is used in a format which would otherwise use numerals to represent hours, minutes, seconds, weekdays, months, or weeks, the appropriate value for the locale is used instead. `INT_CURR_SYMBOL' The same as the value returned by `localeconv' in the `int_curr_symbol' element of the `struct lconv'. `CURRENCY_SYMBOL' `CRNCYSTR' The same as the value returned by `localeconv' in the `currency_symbol' element of the `struct lconv'. `CRNCYSTR' is a deprecated alias still required by Unix98. `MON_DECIMAL_POINT' The same as the value returned by `localeconv' in the `mon_decimal_point' element of the `struct lconv'. `MON_THOUSANDS_SEP' The same as the value returned by `localeconv' in the `mon_thousands_sep' element of the `struct lconv'. `MON_GROUPING' The same as the value returned by `localeconv' in the `mon_grouping' element of the `struct lconv'. `POSITIVE_SIGN' The same as the value returned by `localeconv' in the `positive_sign' element of the `struct lconv'. `NEGATIVE_SIGN' The same as the value returned by `localeconv' in the `negative_sign' element of the `struct lconv'. `INT_FRAC_DIGITS' The same as the value returned by `localeconv' in the `int_frac_digits' element of the `struct lconv'. `FRAC_DIGITS' The same as the value returned by `localeconv' in the `frac_digits' element of the `struct lconv'. `P_CS_PRECEDES' The same as the value returned by `localeconv' in the `p_cs_precedes' element of the `struct lconv'. `P_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `p_sep_by_space' element of the `struct lconv'. `N_CS_PRECEDES' The same as the value returned by `localeconv' in the `n_cs_precedes' element of the `struct lconv'. `N_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `n_sep_by_space' element of the `struct lconv'. `P_SIGN_POSN' The same as the value returned by `localeconv' in the `p_sign_posn' element of the `struct lconv'. `N_SIGN_POSN' The same as the value returned by `localeconv' in the `n_sign_posn' element of the `struct lconv'. `INT_P_CS_PRECEDES' The same as the value returned by `localeconv' in the `int_p_cs_precedes' element of the `struct lconv'. `INT_P_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `int_p_sep_by_space' element of the `struct lconv'. `INT_N_CS_PRECEDES' The same as the value returned by `localeconv' in the `int_n_cs_precedes' element of the `struct lconv'. `INT_N_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `int_n_sep_by_space' element of the `struct lconv'. `INT_P_SIGN_POSN' The same as the value returned by `localeconv' in the `int_p_sign_posn' element of the `struct lconv'. `INT_N_SIGN_POSN' The same as the value returned by `localeconv' in the `int_n_sign_posn' element of the `struct lconv'. `DECIMAL_POINT' `RADIXCHAR' The same as the value returned by `localeconv' in the `decimal_point' element of the `struct lconv'. The name `RADIXCHAR' is a deprecated alias still used in Unix98. `THOUSANDS_SEP' `THOUSEP' The same as the value returned by `localeconv' in the `thousands_sep' element of the `struct lconv'. The name `THOUSEP' is a deprecated alias still used in Unix98. `GROUPING' The same as the value returned by `localeconv' in the `grouping' element of the `struct lconv'. `YESEXPR' The return value is a regular expression which can be used with the `regex' function to recognize a positive response to a yes/no question. The GNU C library provides the `rpmatch' function for easier handling in applications. `NOEXPR' The return value is a regular expression which can be used with the `regex' function to recognize a negative response to a yes/no question. `YESSTR' The return value is a locale-specific translation of the positive response to a yes/no question. Using this value is deprecated since it is a very special case of message translation, and is better handled by the message translation functions (*note Message Translation::). The use of this symbol is deprecated. Instead message translation should be used. `NOSTR' The return value is a locale-specific translation of the negative response to a yes/no question. What is said for `YESSTR' is also true here. The use of this symbol is deprecated. Instead message translation should be used. The file `langinfo.h' defines a lot more symbols but none of them is official. Using them is not portable, and the format of the return values might change. Therefore we recommended you not use them. Note that the return value for any valid argument can be used for in all situations (with the possible exception of the am/pm time formatting codes). If the user has not selected any locale for the appropriate category, `nl_langinfo' returns the information from the `"C"' locale. It is therefore possible to use this function as shown in the example below. If the argument ITEM is not valid, a pointer to an empty string is returned. An example of `nl_langinfo' usage is a function which has to print a given date and time in a locale-specific way. At first one might think that, since `strftime' internally uses the locale information, writing something like the following is enough: size_t i18n_time_n_data (char *s, size_t len, const struct tm *tp) { return strftime (s, len, "%X %D", tp); } The format contains no weekday or month names and therefore is internationally usable. Wrong! The output produced is something like `"hh:mm:ss MM/DD/YY"'. This format is only recognizable in the USA. Other countries use different formats. Therefore the function should be rewritten like this: size_t i18n_time_n_data (char *s, size_t len, const struct tm *tp) { return strftime (s, len, nl_langinfo (D_T_FMT), tp); } Now it uses the date and time format of the locale selected when the program runs. If the user selects the locale correctly there should never be a misunderstanding over the time and date format. A dedicated function to format numbers ====================================== We have seen that the structure returned by `localeconv' as well as the values given to `nl_langinfo' allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts. We have also seen that the underlying rules are quite complex. Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules. - Function: ssize_t strfmon (char *S, size_t MAXSIZE, const char *FORMAT, ...) The `strfmon' function is similar to the `strftime' function in that it takes a buffer, its size, a format string, and values to write into the buffer as text in a form specified by the format string. Like `strftime', the function also returns the number of bytes written into the buffer. There are two differences: `strfmon' can take more than one argument, and, of course, the format specification is different. Like `strftime', the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a `%'. Immediately after the `%', you can optionally specify various flags and formatting information before the main formatting character, in a similar way to `printf': * Immediately following the `%' there can be one or more of the following flags: `=F' The single byte character F is used for this field as the numeric fill character. By default this character is a space character. Filling with this character is only performed if a left precision is specified. It is not just to fill to the given field width. `^' The number is printed without grouping the digits according to the rules of the current locale. By default grouping is enabled. `+', `(' At most one of these flags can be used. They select which format to represent the sign of a currency amount. By default, and if `+' is given, the locale equivalent of +/- is used. If `(' is given, negative amounts are enclosed in parentheses. The exact format is determined by the values of the `LC_MONETARY' category of the locale selected at program runtime. `!' The output will not contain the currency symbol. `-' The output will be formatted left-justified instead of right-justified if it does not fill the entire field width. The next part of a specification is an optional field width. If no width is specified 0 is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the `-' flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa. So far the format looks familiar, being similar to the `printf' and `strftime' formats. However, the next two optional fields introduce something new. The first one is a `#' character followed by a decimal digit string. The value of the digit string specifies the number of _digit_ positions to the left of the decimal point (or equivalent). This does _not_ include the grouping character when the `^' flag is not given. If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the `=' flag and by default is a space. For example, if the field width is selected as 6 and the number is 123, the fill character is `*' the result will be `***123'. The second optional field starts with a `.' (period) and consists of another decimal digit string. Its value describes the number of characters printed after the decimal point. The default is selected from the current locale (`frac_digits', `int_frac_digits', see *note General Numeric::). If the exact representation needs more digits than given by the field width, the displayed value is rounded. If the number of fractional digits is selected to be zero, no decimal point is printed. As a GNU extension, the `strfmon' implementation in the GNU libc allows an optional `L' next as a format modifier. If this modifier is given, the argument is expected to be a `long double' instead of a `double' value. Finally, the last component is a format specifier. There are three specifiers defined: `i' Use the locale's rules for formatting an international currency value. `n' Use the locale's rules for formatting a national currency value. `%' Place a `%' in the output. There must be no flag, width specifier or modifier given, only `%%' is allowed. As for `printf', the function reads the format string from left to right and uses the values passed to the function following the format string. The values are expected to be either of type `double' or `long double', depending on the presence of the modifier `L'. The result is stored in the buffer pointed to by S. At most MAXSIZE characters are stored. The return value of the function is the number of characters stored in S, including the terminating `NULL' byte. If the number of characters stored would exceed MAXSIZE, the function returns -1 and the content of the buffer S is unspecified. In this case `errno' is set to `E2BIG'. A few examples should make clear how the function works. It is assumed that all the following pieces of code are executed in a program which uses the USA locale (`en_US'). The simplest form of the format is this: strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678); The output produced is "@$123.45@-$567.89@$12,345.68@" We can notice several things here. First, the widths of the output numbers are different. We have not specified a width in the format string, and so this is no wonder. Second, the third number is printed using thousands separators. The thousands separator for the `en_US' locale is a comma. The number is also rounded. .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2. Finally, note that the national currency symbol is printed since `%n' was used, not `i'. The next example shows how we can align the output. strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678); The output this time is: "@ $123.45@ -$567.89@ $12,345.68@" Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a `#' modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification. strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@", 123.45, -567.89, 12345.678); The output is "@ $***123.45@-$***567.89@ $12,456.68@" Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be 5 and 123.45 has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality. strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@", 123.45, -567.89, 12345.678); This rather complex format string produces the following output: "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @" The most noticeable change is the alternative way of representing negative numbers. In financial circles this is often done using parentheses, and this is what the `(' flag selected. The fill character is now `0'. Note that this `0' character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator. Since we used the format specifier `i' instead of `n', the international form of the currency symbol is used. This is a four letter string, in this case `"USD "'. The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding. Yes-or-No Questions =================== Some non GUI programs ask a yes-or-no question. If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too. It would be very bad habit to ask a question in one language and request the answer in another, often English. The GNU C library contains `rpmatch' to give applications easy access to the corresponding locale definitions. - Function: int rpmatch (const char *RESPONSE) The function `rpmatch' checks the string in RESPONSE whether or not it is a correct yes-or-no answer and if yes, which one. The check uses the `YESEXPR' and `NOEXPR' data in the `LC_MESSAGES' category of the currently selected locale. The return value is as follows: `1' The user entered an affirmative answer. `0' The user entered a negative answer. `-1' The answer matched neither the `YESEXPR' nor the `NOEXPR' regular expression. This function is not standardized but available beside in GNU libc at least also in the IBM AIX library. This function would normally be used like this: ... /* Use a safe default. */ _Bool doit = false; fputs (gettext ("Do you really want to do this? "), stdout); fflush (stdout); /* Prepare the `getline' call. */ line = NULL; len = 0; while (getline (&line, &len, stdout) >= 0) { /* Check the response. */ int res = rpmatch (line); if (res >= 0) { /* We got a definitive answer. */ if (res > 0) doit = true; break; } } /* Free what `getline' allocated. */ free (line); Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read. Message Translation ******************* The program's interface with the human should be designed in a way to ease the human the task. One of the possibilities is to use messages in whatever language the user prefers. Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and add among the variants every time a message has to be printed. This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets. A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user. The GNU C Library provides two different sets of functions to support message translation. The problem is that neither of the interfaces is officially defined by the POSIX standard. The `catgets' family of functions is defined in the X/Open standard but this is derived from industry decisions and therefore not necessarily based on reasonable decisions. As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are * locate the external data file with the appropriate translations. * load the data and make it possible to address the messages * map a given key to the translated message The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest. X/Open Message Catalog Handling =============================== The `catgets' functions are based on the simple scheme: Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used. This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same. Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind. All the types, constants and functions for the `catgets' functions are defined/declared in the `nl_types.h' header file. The `catgets' function family ----------------------------- - Function: nl_catd catopen (const char *CAT_NAME, int FLAG) The `catgets' function tries to locate the message data file names CAT_NAME and loads it when found. The return value is of an opaque type and can be used in calls to the other functions to refer to this loaded catalog. The return value is `(nl_catd) -1' in case the function failed and no catalog was loaded. The global variable ERRNO contains a code for the error causing the failure. But even if the function call succeeded this does not mean that all messages can be translated. Locating the catalog file must happen in a way which lets the user of the program influence the decision. It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files. All this can be specified by the user by setting some environment variables. The first problem is to find out where all the message catalogs are stored. Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place. To tell the `catopen' function where the catalog for the program can be found the user can set the environment variable `NLSPATH' to a value which describes her/his choice. Since this value must be usable for different languages and locales it cannot be a simple string. Instead it is a format string (similar to `printf''s). An example is /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N First one can see that more than one directory can be specified (with the usual syntax of separating them by colons). The next things to observe are the format string, `%L' and `%N' in this case. The `catopen' function knows about several of them and the replacement for all of them is of course different. `%N' This format element is substituted with the name of the catalog file. This is the value of the CAT_NAME argument given to `catgets'. `%L' This format element is substituted with the name of the currently selected locale for translating messages. How this is determined is explained below. `%l' (This is the lowercase ell.) This format element is substituted with the language element of the locale name. The string describing the selected locale is expected to have the form `LANG[_TERR[.CODESET]]' and this format uses the first part LANG. `%t' This format element is substituted by the territory part TERR of the name of the currently selected locale. See the explanation of the format above. `%c' This format element is substituted by the codeset part CODESET of the name of the currently selected locale. See the explanation of the format above. `%%' Since `%' is used in a meta character there must be a way to express the `%' character in the result itself. Using `%%' does this just like it works for `printf'. Using `NLSPATH' allows arbitrary directories to be searched for message catalogs while still allowing different languages to be used. If the `NLSPATH' environment variable is not set, the default value is PREFIX/share/locale/%L/%N:PREFIX/share/locale/%L/LC_MESSAGES/%N where PREFIX is given to `configure' while installing the GNU C Library (this value is in many cases `/usr' or the empty string). The remaining problem is to decide which must be used. The value decides about the substitution of the format elements mentioned above. First of all the user can specify a path in the message catalog name (i.e., the name contains a slash character). In this situation the `NLSPATH' environment variable is not used. The catalog must exist as specified in the program, perhaps relative to the current working directory. This situation in not desirable and catalogs names never should be written this way. Beside this, this behavior is not portable to all other platforms providing the `catgets' interface. Otherwise the values of environment variables from the standard environment are examined (*note Standard Environment::). Which variables are examined is decided by the FLAG parameter of `catopen'. If the value is `NL_CAT_LOCALE' (which is defined in `nl_types.h') then the `catopen' function use the name of the locale currently selected for the `LC_MESSAGES' category. If FLAG is zero the `LANG' environment variable is examined. This is a left-over from the early days where the concept of the locales had not even reached the level of POSIX locales. The environment variable and the locale name should have a value of the form `LANG[_TERR[.CODESET]]' as explained above. If no environment variable is set the `"C"' locale is used which prevents any translation. The return value of the function is in any case a valid string. Either it is a translation from a message catalog or it is the same as the STRING parameter. So a piece of code to decide whether a translation actually happened must look like this: { char *trans = catgets (desc, set, msg, input_string); if (trans == input_string) { /* Something went wrong. */ } } When an error occurred the global variable ERRNO is set to EBADF The catalog does not exist. ENOMSG The set/message tuple does not name an existing element in the message catalog. While it sometimes can be useful to test for errors programs normally will avoid any test. If the translation is not available it is no big problem if the original, untranslated message is printed. Either the user understands this as well or s/he will look for the reason why the messages are not translated. Please note that the currently selected locale does not depend on a call to the `setlocale' function. It is not necessary that the locale data files for this locale exist and calling `setlocale' succeeds. The `catopen' function directly reads the values of the environment variables. - Function: char * catgets (nl_catd CATALOG_DESC, int SET, int MESSAGE, const char *STRING) The function `catgets' has to be used to access the massage catalog previously opened using the `catopen' function. The CATALOG_DESC parameter must be a value previously returned by `catopen'. The next two parameters, SET and MESSAGE, reflect the internal organization of the message catalog files. This will be explained in detail below. For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers. Neither the set number nor the message number must be consecutive. They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number. Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter STRING helps to handle this case gracefully. If no matching string can be found STRING is returned. This means for the programmer that * the STRING parameters should contain reasonable text (this also helps to understand the program seems otherwise there would be no hint on the string which is expected to be returned. * all STRING arguments should be written in the same language. It is somewhat uncomfortable to write a program using the `catgets' functions if no supporting functionality is available. Since each set/message number tuple must be unique the programmer must keep lists of the messages at the same time the code is written. And the work between several people working on the same project must be coordinated. We will see some how these problems can be relaxed a bit (*note Common Usage::). - Function: int catclose (nl_catd CATALOG_DESC) The `catclose' function can be used to free the resources associated with a message catalog which previously was opened by a call to `catopen'. If the resources can be successfully freed the function returns `0'. Otherwise it return `-1' and the global variable ERRNO is set. Errors can occur if the catalog descriptor CATALOG_DESC is not valid in which case ERRNO is set to `EBADF'. Format of the message catalog files ----------------------------------- The only reasonable way the translate all the messages of a function and store the result in a message catalog file which can be read by the `catopen' function is to write all the message text to the translator and let her/him translate them all. I.e., we must have a file with entries which associate the set/message tuple with a specific translation. This file format is specified in the X/Open standard and is as follows: * Lines containing only whitespace characters or empty lines are ignored. * Lines which contain as the first non-whitespace character a `$' followed by a whitespace character are comment and are also ignored. * If a line contains as the first non-whitespace characters the sequence `$set' followed by a whitespace character an additional argument is required to follow. This argument can either be: - a number. In this case the value of this number determines the set to which the following messages are added. - an identifier consisting of alphanumeric characters plus the underscore character. In this case the set get automatically a number assigned. This value is one added to the largest set number which so far appeared. How to use the symbolic names is explained in section *Note Common Usage::. It is an error if a symbol name appears more than once. All following messages are placed in a set with this number. * If a line contains as the first non-whitespace characters the sequence `$delset' followed by a whitespace character an additional argument is required to follow. This argument can either be: - a number. In this case the value of this number determines the set which will be deleted. - an identifier consisting of alphanumeric characters plus the underscore character. This symbolic identifier must match a name for a set which previously was defined. It is an error if the name is unknown. In both cases all messages in the specified set will be removed. They will not appear in the output. But if this set is later again selected with a `$set' command again messages could be added and these messages will appear in the output. * If a line contains after leading whitespaces the sequence `$quote', the quoting character used for this input file is changed to the first non-whitespace character following the `$quote'. If no non-whitespace character is present before the line ends quoting is disable. By default no quoting character is used. In this mode strings are terminated with the first unescaped line break. If there is a `$quote' sequence present newline need not be escaped. Instead a string is terminated with the first unescaped appearance of the quote character. A common usage of this feature would be to set the quote character to `"'. Then any appearance of the `"' in the strings must be escaped using the backslash (i.e., `\"' must be written). * Any other line must start with a number or an alphanumeric identifier (with the underscore character included). The following characters (starting after the first whitespace character) will form the string which gets associated with the currently selected set and the message number represented by the number and identifier respectively. If the start of the line is a number the message number is obvious. It is an error if the same message number already appeared for this set. If the leading token was an identifier the message number gets automatically assigned. The value is the current maximum messages number for this set plus one. It is an error if the identifier was already used for a message in this set. It is OK to reuse the identifier for a message in another thread. How to use the symbolic identifiers will be explained below (*note Common Usage::). There is one limitation with the identifier: it must not be `Set'. The reason will be explained below. The text of the messages can contain escape characters. The usual bunch of characters known from the ISO C language are recognized (`\n', `\t', `\v', `\b', `\r', `\f', `\\', and `\NNN', where NNN is the octal coding of a character code). *Important:* The handling of identifiers instead of numbers for the set and messages is a GNU extension. Systems strictly following the X/Open specification do not have this feature. An example for a message catalog file is this: $ This is a leading comment. $quote " $set SetOne 1 Message with ID 1. two " Message with ID \"two\", which gets the value 2 assigned" $set SetTwo $ Since the last set got the number 1 assigned this set has number 2. 4000 "The numbers can be arbitrary, they need not start at one." This small example shows various aspects: * Lines 1 and 9 are comments since they start with `$' followed by a whitespace. * The quoting character is set to `"'. Otherwise the quotes in the message definition would have to be left away and in this case the message with the identifier `two' would loose its leading whitespace. * Mixing numbered messages with message having symbolic names is no problem and the numbering happens automatically. While this file format is pretty easy it is not the best possible for use in a running program. The `catopen' function would have to parser the file and handle syntactic errors gracefully. This is not so easy and the whole process is pretty slow. Therefore the `catgets' functions expect the data in another more compact and ready-to-use file format. There is a special program `gencat' which is explained in detail in the next section. Files in this other format are not human readable. To be easy to use by programs it is a binary file. But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library). Details about the binary file format are not important to know since these files are always created by the `gencat' program. The sources of the GNU C Library also provide the sources for the `gencat' program and so the interested reader can look through these source files to learn about the file format. Generate Message Catalogs files ------------------------------- The `gencat' program is specified in the X/Open standard and the GNU implementation follows this specification and so processes all correctly formed input files. Additionally some extension are implemented which help to work in a more reasonable way with the `catgets' functions. The `gencat' program can be invoked in two ways: `gencat [OPTION]... [OUTPUT-FILE [INPUT-FILE]...]` This is the interface defined in the X/Open standard. If no INPUT-FILE parameter is given input will be read from standard input. Multiple input files will be read as if they are concatenated. If OUTPUT-FILE is also missing, the output will be written to standard output. To provide the interface one is used to from other programs a second interface is provided. `gencat [OPTION]... -o OUTPUT-FILE [INPUT-FILE]...` The option `-o' is used to specify the output file and all file arguments are used as input files. Beside this one can use `-' or `/dev/stdin' for INPUT-FILE to denote the standard input. Corresponding one can use `-' and `/dev/stdout' for OUTPUT-FILE to denote standard output. Using `-' as a file name is allowed in X/Open while using the device names is a GNU extension. The `gencat' program works by concatenating all input files and then *merge* the resulting collection of message sets with a possibly existing output file. This is done by removing all messages with set/message number tuples matching any of the generated messages from the output file and then adding all the new messages. To regenerate a catalog file while ignoring the old contents therefore requires to remove the output file if it exists. If the output is written to standard output no merging takes place. The following table shows the options understood by the `gencat' program. The X/Open standard does not specify any option for the program so all of these are GNU extensions. `-V' `--version' Print the version information and exit. `-h' `--help' Print a usage message listing all available options, then exit successfully. `--new' Do never merge the new messages from the input files with the old content of the output files. The old content of the output file is discarded. `-H' `--header=name' This option is used to emit the symbolic names given to sets and messages in the input files for use in the program. Details about how to use this are given in the next section. The NAME parameter to this option specifies the name of the output file. It will contain a number of C preprocessor `#define's to associate a name with a number. Please note that the generated file only contains the symbols from the input files. If the output is merged with the previous content of the output file the possibly existing symbols from the file(s) which generated the old output files are not in the generated header file. How to use the `catgets' interface ---------------------------------- The `catgets' functions can be used in two different ways. By following slavishly the X/Open specs and not relying on the extension and by using the GNU extensions. We will take a look at the former method first to understand the benefits of extensions. Not using symbolic names ........................ Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like catgets (catdesc, set, msg, "string") CATGETS is retrieved from a call to `catopen' which is normally done once at the program start. The `"string"' is the string we want to translate. The problems start with the set and message numbers. In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts). The allocation process can be relaxed a bit by different set numbers for different parts of the program. So the number of developers who have to coordinate the allocation can be reduced. But still lists must be keep track of the allocation and errors can easily happen. These errors cannot be discovered by the compiler or the `catgets' functions. Only the user of the program might see wrong messages printed. In the worst cases the messages are so irritating that they cannot be recognized as wrong. Think about the translations for `"true"' and `"false"' being exchanged. This could result in a disaster. Using symbolic names .................... The problems mentioned in the last section derive from the fact that: 1. the numbers are allocated once and due to the possibly frequent use of them it is difficult to change a number later. 2. the numbers do not allow to guess anything about the string and therefore collisions can easily happen. By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself. This is necessary since the symbolic names must be mapped to numbers before the program sources can be compiled. In the last section it was described how to generate a header containing the mapping of the names. E.g., for the example message file given in the last section we could call the `gencat' program as follow (assume `ex.msg' contains the sources). gencat -H ex.h -o ex.cat ex.msg This generates a header file with the following content: #define SetTwoSet 0x2 /* ex.msg:8 */ #define SetOneSet 0x1 /* ex.msg:4 */ #define SetOnetwo 0x2 /* ex.msg:6 */ As can be seen the various symbols given in the source file are mangled to generate unique identifiers and these identifiers get numbers assigned. Reading the source file and knowing about the rules will allow to predict the content of the header file (it is deterministic) but this is not necessary. The `gencat' program can take care for everything. All the programmer has to do is to put the generated header file in the dependency list of the source files of her/his project and to add a rules to regenerate the header of any of the input files change. One word about the symbol mangling. Every symbol consists of two parts: the name of the message set plus the name of the message or the special string `Set'. So `SetOnetwo' means this macro can be used to access the translation with identifier `two' in the message set `SetOne'. The other names denote the names of the message sets. The special string `Set' is used in the place of the message identifier. If in the code the second string of the set `SetOne' is used the C code should look like this: catgets (catdesc, SetOneSet, SetOnetwo, " Message with ID \"two\", which gets the value 2 assigned") Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.) How does to this allow to develop ................................. To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual: #include int main (void) { printf ("Hello, world!\n"); return 0; } Now we want to internationalize the message and therefore replace the message with whatever the user wants. #include #include #include "msgnrs.h" int main (void) { nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE); printf (catgets (catdesc, SetMainSet, SetMainHello, "Hello, world!\n")); catclose (catdesc); return 0; } We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation. What remains unspecified here are the constants `SetMainSet' and `SetMainHello'. These are the symbolic names describing the message. To get the actual definitions which match the information in the catalog file we have to create the message catalog source file and process it using the `gencat' program. $ Messages for the famous greeting program. $quote " $set Main Hello "Hallo, Welt!\n" Now we can start building the program (assume the message catalog source file is named `hello.msg' and the program source file `hello.c'): % gencat -H msgnrs.h -o hello.cat hello.msg % cat msgnrs.h #define MainSet 0x1 /* hello.msg:4 */ #define MainHello 0x1 /* hello.msg:5 */ % gcc -o hello hello.c -I. % cp hello.cat /usr/share/locale/de/LC_MESSAGES % echo $LC_ALL de % ./hello Hallo, Welt! % The call of the `gencat' program creates the missing header file `msgnrs.h' as well as the message catalog binary. The former is used in the compilation of `hello.c' while the later is placed in a directory in which the `catopen' function will try to locate it. Please check the `LC_ALL' environment variable and the default path for `catopen' presented in the description above. The Uniforum approach to Message Translation ============================================ Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group. There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU project and the GNU `gettext' package provides support for this outside the GNU C Library. The code of the `libintl' from GNU `gettext' is the same as the code in the GNU C Library. So the documentation in the GNU `gettext' manual is also valid for the functionality here. The following text will describe the library functions in detail. But the numerous helper programs are not described in this manual. Instead people should read the GNU `gettext' manual (*note GNU gettext utilities: (gettext)Top.). We will only give a short overview. Though the `catgets' functions are available by default on more systems the `gettext' interface is at least as portable as the former. The GNU `gettext' package can be used wherever the functions are not available. The `gettext' family of functions --------------------------------- The paradigms underlying the `gettext' approach to message translations is different from that of the `catgets' functions the basic functionally is equivalent. There are functions of the following categories: What has to be done to translate a message? ........................................... The `gettext' functions have a very simple interface. The most basic function just takes the string which shall be translated as the argument and it returns the translation. This is fundamentally different from the `catgets' approach where an extra key is necessary and the original string is only used for the error case. If the string which has to be translated is the only argument this of course means the string itself is the key. I.e., the translation will be selected based on the original string. The message catalogs must therefore contain the original strings plus one translation for any such string. The task of the `gettext' function is it to compare the argument string with the available strings in the catalog and return the appropriate translation. Of course this process is optimized so that this process is not more expensive than an access using an atomic key like in `catgets'. The `gettext' approach has some advantages but also some disadvantages. Please see the GNU `gettext' manual for a detailed discussion of the pros and cons. All the definitions and declarations for `gettext' can be found in the `libintl.h' header file. On systems where these functions are not part of the C library they can be found in a separate library named `libintl.a' (or accordingly different for shared libraries). - Function: char * gettext (const char *MSGID) The `gettext' function searches the currently selected message catalogs for a string which is equal to MSGID. If there is such a string available it is returned. Otherwise the argument string MSGID is returned. Please note that all though the return value is `char *' the returned string must not be changed. This broken type results from the history of the function and does not reflect the way the function should be used. Please note that above we wrote "message catalogs" (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (*note Locating gettext catalog::). The `gettext' function does not modify the value of the global ERRNO variable. This is necessary to make it possible to write something like printf (gettext ("Operation failed: %m\n")); Here the ERRNO value is used in the `printf' function while processing the `%m' format element and if the `gettext' function would change this value (it is called before `printf' is called) we would get a wrong message. So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation. The remaining two functions to access the message catalog add some functionality to select a message catalog which is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time. The C library itself is an example: internally it uses the `gettext' functions but since it must not depend on a currently selected default message catalog it must specify all ambiguous information. - Function: char * dgettext (const char *DOMAINNAME, const char *MSGID) The `dgettext' functions acts just like the `gettext' function. It only takes an additional first argument DOMAINNAME which guides the selection of the message catalogs which are searched for the translation. If the DOMAINNAME parameter is the null pointer the `dgettext' function is exactly equivalent to `gettext' since the default value for the domain name is used. As for `gettext' the return value type is `char *' which is an anachronism. The returned string must never be modified. - Function: char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY) The `dcgettext' adds another argument to those which `dgettext' takes. This argument CATEGORY specifies the last piece of information needed to localize the message catalog. I.e., the domain name and the locale category exactly specify which message catalog has to be used (relative to a given directory, see below). The `dgettext' function can be expressed in terms of `dcgettext' by using dcgettext (domain, string, LC_MESSAGES) instead of dgettext (domain, string) This also shows which values are expected for the third parameter. One has to use the available selectors for the categories available in `locale.h'. Normally the available values are `LC_CTYPE', `LC_COLLATE', `LC_MESSAGES', `LC_MONETARY', `LC_NUMERIC', and `LC_TIME'. Please note that `LC_ALL' must not be used and even though the names might suggest this, there is no relation to the environments variables of this name. The `dcgettext' function is only implemented for compatibility with other systems which have `gettext' functions. There is not really any situation where it is necessary (or useful) to use a different value but `LC_MESSAGES' in for the CATEGORY parameter. We are dealing with messages here and any other choice can only be irritating. As for `gettext' the return value type is `char *' which is an anachronism. The returned string must never be modified. When using the three functions above in a program it is a frequent case that the MSGID argument is a constant string. So it is worth to optimize this case. Thinking shortly about this one will realize that as long as no new message catalog is loaded the translation of a message will not change. This optimization is actually implemented by the `gettext', `dgettext' and `dcgettext' functions. How to determine which catalog to be used ......................................... The functions to retrieve the translations for a given message have a remarkable simple interface. But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this. The code is complicated the use is easy. Basically we have two different tasks to perform which can also be performed by the `catgets' functions: 1. Locate the set of message catalogs. There are a number of files for different languages and which all belong to the package. Usually they are all stored in the filesystem below a certain directory. There can be arbitrary many packages installed and they can follow different guidelines for the placement of their files. 2. Relative to the location specified by the package the actual translation files must be searched, based on the wishes of the user. I.e., for each language the user selects the program should be able to locate the appropriate file. This is the functionality required by the specifications for `gettext' and this is also what the `catgets' functions are able to do. But there are some problems unresolved: * The language to be used can be specified in several different ways. There is no generally accepted standard for this and the user always expects the program understand what s/he means. E.g., to select the German translation one could write `de', `german', or `deutsch' and the program should always react the same. * Sometimes the specification of the user is too detailed. If s/he, e.g., specifies `de_DE.ISO-8859-1' which means German, spoken in Germany, coded using the ISO 8859-1 character set there is the possibility that a message catalog matching this exactly is not available. But there could be a catalog matching `de' and if the character set used on the machine is always ISO 8859-1 there is no reason why this later message catalog should not be used. (We call this "message inheritance".) * If a catalog for a wanted language is not available it is not always the second best choice to fall back on the language of the developer and simply not translate any message. Instead a user might be better able to read the messages in another language and so the user of the program should be able to define an precedence order of languages. We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user. We will start with the functions the programmer can use since the user configuration will be based on this. As the functions described in the last sections already mention separate sets of messages can be selected by a "domain name". This is a simple string which should be unique for each program part with uses a separate domain. It is possible to use in one program arbitrary many domains at the same time. E.g., the GNU C Library itself uses a domain named `libc' while the program using the C Library could use a domain named `foo'. The important point is that at any time exactly one domain is active. This is controlled with the following function. - Function: char * textdomain (const char *DOMAINNAME) The `textdomain' function sets the default domain, which is used in all future `gettext' calls, to DOMAINNAME. Please note that `dgettext' and `dcgettext' calls are not influenced if the DOMAINNAME parameter of these functions is not the null pointer. Before the first call to `textdomain' the default domain is `messages'. This is the name specified in the specification of the `gettext' API. This name is as good as any other name. No program should ever really use a domain with this name since this can only lead to problems. The function returns the value which is from now on taken as the default domain. If the system went out of memory the returned value is `NULL' and the global variable ERRNO is set to `ENOMEM'. Despite the return value type being `char *' the return string must not be changed. It is allocated internally by the `textdomain' function. If the DOMAINNAME parameter is the null pointer no new default domain is set. Instead the currently selected default domain is returned. If the DOMAINNAME parameter is the empty string the default domain is reset to its initial value, the domain with the name `messages'. This possibility is questionable to use since the domain `messages' really never should be used. - Function: char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME) The `bindtextdomain' function can be used to specify the directory which contains the message catalogs for domain DOMAINNAME for the different languages. To be correct, this is the directory where the hierarchy of directories is expected. Details are explained below. For the programmer it is important to note that the translations which come with the program have be placed in a directory hierarchy starting at, say, `/foo/bar'. Then the program should make a `bindtextdomain' call to bind the domain for the current program to this directory. So it is made sure the catalogs are found. A correctly running program does not depend on the user setting an environment variable. The `bindtextdomain' function can be used several times and if the DOMAINNAME argument is different the previously bound domains will not be overwritten. If the program which wish to use `bindtextdomain' at some point of time use the `chdir' function to change the current working directory it is important that the DIRNAME strings ought to be an absolute pathname. Otherwise the addressed directory might vary with the time. If the DIRNAME parameter is the null pointer `bindtextdomain' returns the currently selected directory for the domain with the name DOMAINNAME. The `bindtextdomain' function returns a pointer to a string containing the name of the selected directory name. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of `bindtextdomain' the return value is `NULL' and the global variable ERRNO is set accordingly. Additional functions for more complicated situations .................................................... The functions of the `gettext' family described so far (and all the `catgets' functions as well) have one problem in the real world which have been neglected completely in all existing approaches. What is meant here is the handling of plural forms. Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following: printf ("%d file%s deleted", n, n == 1 ? "" : "s"); After the first complaints from people internationalizing the code people either completely avoided formulations like this or used strings like `"file(s)"'. Both look unnatural and should be avoided. First tries to solve the problem correctly looked like this: if (n == 1) printf ("%d file deleted", n); else printf ("%d files deleted", n); But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. There are two things we can differ between (and even inside language families); * The form how plural forms are build differs. This is a problem with language which have many irregularities. German, for instance, is a drastic case. Though English and German are part of the same language family (Germanic), the almost regular forming of plural noun forms (appending an `s') is hardly found in German. * The number of plural forms differ. This is somewhat surprising for those who only have experiences with Romanic and Germanic languages since here the number is the same (there are two). But other language families have only one form or many forms. More information on this in an extra section. The consequence of this is that application writers should not try to solve the problem in their code. This would be localization since it is only usable for certain, hardcoded language environments. Instead the extended `gettext' interface should be used. These extra functions are taking instead of the one key string two strings and an numerical argument. The idea behind this is that using the numerical argument and the first string as a key, the implementation can select using rules specified by the translator the right plural form. The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal `gettext' behavior). In this case the rules for Germanic language is used and it is assumed that the first string argument is the singular form, the second the plural form. This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language. This is a limitation but since the GNU C library (as well as the GNU `gettext' package) are written as part of the GNU package and the coding standards for the GNU project require program being written in English, this solution nevertheless fulfills its purpose. - Function: char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N) The `ngettext' function is similar to the `gettext' function as it finds the message catalogs in the same way. But it takes two extra arguments. The MSGID1 parameter must contain the singular form of the string to be converted. It is also used as the key for the search in the catalog. The MSGID2 parameter is the plural form. The parameter N is used to determine the plural form. If no message catalog is found MSGID1 is returned if `n == 1', otherwise `msgid2'. An example for the us of this function is: printf (ngettext ("%d file removed", "%d files removed", n), n); Please note that the numeric value N has to be passed to the `printf' function as well. It is not sufficient to pass it only to `ngettext'. - Function: char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N) The `dngettext' is similar to the `dgettext' function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way `ngettext' handles them. - Function: char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY) The `dcngettext' is similar to the `dcgettext' function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way `ngettext' handles them. The problem of plural forms ........................... A description of the problem can be found at the beginning of the last section. Now there is the question how to solve it. Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language. Therefore the solution implemented is to allow the translator to specify the rules of how to select the plural form. Since the formula varies with every language this is the only viable solution except for hardcoding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages). The details are explained in the GNU `gettext' manual. Here only a a bit of information is provided. The information about the plural form selection has to be stored in the header entry (the one with the empty (`msgid' string). It looks like this: Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1; The `nplurals' value must be a decimal number which specifies how many different plural forms exist for this language. The string following `plural' is an expression which is using the C language syntax. Exceptions are that no negative number are allowed, numbers must be decimal, and the only variable allowed is `n'. This expression will be evaluated whenever one of the functions `ngettext', `dngettext', or `dcngettext' is called. The numeric value passed to these functions is then substituted for all uses of the variable `n' in the expression. The resulting value then must be greater or equal to zero and smaller than the value given as the value of `nplurals'. The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).(1) Only one form: Some languages only require one single form. There is no distinction between the singular and plural form. An appropriate header entry would look like this: Plural-Forms: nplurals=1; plural=0; Languages with this property include: Finno-Ugric family Hungarian Asian family Japanese Turkic/Altaic family Turkish Two forms, singular used for one only This is the form used in most existing programs since it is what English is using. A header entry would look like this: Plural-Forms: nplurals=2; plural=n != 1; (Note: this uses the feature of C expressions that boolean expressions have to value zero or one.) Languages with this property include: Germanic family Danish, Dutch, English, German, Norwegian, Swedish Finno-Ugric family Estonian, Finnish Latin/Greek family Greek Semitic family Hebrew Romance family Italian, Spanish Artificial Esperanto Two forms, singular used for zero and one Exceptional case in the language family. The header entry would be: Plural-Forms: nplurals=2; plural=n>1; Languages with this property include: Romanic family French Three forms, special cases for one and two The header entry would be: Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2; Languages with this property include: Celtic Gaeilge Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4] The header entry would look like this: Plural-Forms: nplurals=3; \ plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1; Languages with this property include: Slavic family Czech, Russian Three forms, special cases for 1 and 2, 3, 4 The header entry would look like this: Plural-Forms: nplurals=3; \ plural=(n==1) ? 1 : (n>=2 && n<=4) ? 2 : 0; Languages with this property include: Slavic family Slovak Three forms, special case for one and some numbers ending in 2, 3, or 4 The header entry would look like this: Plural-Forms: nplurals=3; \ plural=n==1 ? 0 : \ n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2; (Continuation in the next line is possible.) Languages with this property include: Slavic family Polish Four forms, special case for one and all numbers ending in 2, 3, or 4 The header entry would look like this: Plural-Forms: nplurals=4; \ plural=n==1 ? 0 : n%10==2 ? 1 : n%10==3 || n%10==4 ? 2 : 3; Languages with this property include: Slavic family Slovenian ---------- Footnotes ---------- (1) Additions are welcome. Send appropriate information to . How to specify the output character set `gettext' uses ...................................................... `gettext' not only looks up a translation in a message catalog. It also converts the translation on the fly to the desired output character set. This is useful if the user is working in a different character set than the translator who created the message catalog, because it avoids distributing variants of message catalogs which differ only in the character set. The output character set is, by default, the value of `nl_langinfo (CODESET)', which depends on the `LC_CTYPE' part of the current locale. But programs which store strings in a locale independent way (e.g. UTF-8) can request that `gettext' and related functions return the translations in that encoding, by use of the `bind_textdomain_codeset' function. Note that the MSGID argument to `gettext' is not subject to character set conversion. Also, when `gettext' does not find a translation for MSGID, it returns MSGID unchanged - independently of the current output character set. It is therefore recommended that all MSGIDs be US-ASCII strings. - Function: char * bind_textdomain_codeset (const char *DOMAINNAME, const char *CODESET) The `bind_textdomain_codeset' function can be used to specify the output character set for message catalogs for domain DOMAINNAME. The CODESET argument must be a valid codeset name which can be used for the `iconv_open' function, or a null pointer. If the CODESET parameter is the null pointer, `bind_textdomain_codeset' returns the currently selected codeset for the domain with the name DOMAINNAME. It returns `NULL' if no codeset has yet been selected. The `bind_textdomain_codeset' function can be used several times. If used multiple times with the same DOMAINNAME argument, the later call overrides the settings made by the earlier one. The `bind_textdomain_codeset' function returns a pointer to a string containing the name of the selected codeset. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of `bind_textdomain_codeset', the return value is `NULL' and the global variable ERRNO is set accordingly. How to use `gettext' in GUI programs .................................... One place where the `gettext' functions, if used normally, have big problems is within programs with graphical user interfaces (GUIs). The problem is that many of the strings which have to be translated are very short. They have to appear in pull-down menus which restricts the length. But strings which are not containing entire sentences or at least large fragments of a sentence may appear in more than one situation in the program but might have different translations. This is especially true for the one-word strings which are frequently used in GUI programs. As a consequence many people say that the `gettext' approach is wrong and instead `catgets' should be used which indeed does not have this problem. But there is a very simple and powerful method to handle these kind of problems with the `gettext' functions. As as example consider the following fictional situation. A GUI program has a menu bar with the following entries: +------------+------------+--------------------------------------+ | File | Printer | | +------------+------------+--------------------------------------+ | Open | | Select | | New | | Open | +----------+ | Connect | +----------+ To have the strings `File', `Printer', `Open', `New', `Select', and `Connect' translated there has to be at some point in the code a call to a function of the `gettext' family. But in two places the string passed into the function would be `Open'. The translations might not be the same and therefore we are in the dilemma described above. One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions. To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as Menu|File Menu|Printer Menu|File|Open Menu|File|New Menu|Printer|Select Menu|Printer|Open Menu|Printer|Connect Now all the strings are different and if now instead of `gettext' the following little wrapper function is used, everything works just fine: char * sgettext (const char *msgid) { char *msgval = gettext (msgid); if (msgval == msgid) msgval = strrchr (msgid, '|') + 1; return msgval; } What this little function does is to recognize the case when no translation is available. This can be done very efficiently by a pointer comparison since the return value is the input value. If there is no translation we know that the input string is in the format we used for the Menu entries and therefore contains a `|' character. We simply search for the last occurrence of this character and return a pointer to the character following it. That's it! If one now consistently uses the enlengthened string form and replaces the `gettext' calls with calls to `sgettext' (this is normally limited to very few places in the GUI implementation) then it is possible to produce a program which can be internationalized. With advanced compilers (such as GNU C) one can write the `sgettext' functions as an inline function or as a macro like this: #define sgettext(msgid) \ ({ const char *__msgid = (msgid); \ char *__msgstr = gettext (__msgid); \ if (__msgval == __msgid) \ __msgval = strrchr (__msgid, '|') + 1; \ __msgval; }) The other `gettext' functions (`dgettext', `dcgettext' and the `ngettext' equivalents) can and should have corresponding functions as well which look almost identical, except for the parameters and the call to the underlying function. Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question. * They are easy to write and therefore can be provided by the project they are used in. This is not an answer by itself and must be seen together with the second part which is: * There is no way the C library can contain a version which can work everywhere. The problem is the selection of the character to separate the prefix from the actual string in the enlenghtened string. The examples above used `|' which is a quite good choice because it resembles a notation frequently used in this context and it also is a character not often used in message strings. But what if the character is used in message strings. Or if the chose character is not available in the character set on the machine one compiles (e.g., `|' is not required to exist for ISO C; this is why the `iso646.h' file exists in ISO C programming environments). There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this. User influence on `gettext' ........................... The last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them. The POSIX locale model uses the environment variables `LC_COLLATE', `LC_CTYPE', `LC_MESSAGES', `LC_MONETARY', `NUMERIC', and `LC_TIME' to select the locale which is to be used. This way the user can influence lots of functions. As we mentioned above the `gettext' functions also take advantage of this. To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows: DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo The default value for DIR_NAME is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is `/usr' or `/'. For the former the complete DIR_NAME is: /usr/share/locale We can use `/usr/share' since the `.mo' files containing the message catalogs are system independent, so all systems can use the same files. If the program executed the `bindtextdomain' function for the message domain that is currently handled, the `dir_name' component is exactly the value which was given to the function as the second parameter. I.e., `bindtextdomain' allows overwriting the only system dependent and fixed value to make it possible to address files anywhere in the filesystem. The CATEGORY is the name of the locale category which was selected in the program code. For `gettext' and `dgettext' this is always `LC_MESSAGES', for `dcgettext' this is selected by the value of the third parameter. As said above it should be avoided to ever use a category other than `LC_MESSAGES'. The LOCALE component is computed based on the category used. Just like for the `setlocale' function here comes the user selection into the play. Some environment variables are examined in a fixed order and the first environment variable set determines the return value of the lookup process. In detail, for the category `LC_xxx' the following variables in this order are examined: `LANGUAGE' `LC_ALL' `LC_xxx' `LANG' This looks very familiar. With the exception of the `LANGUAGE' environment variable this is exactly the lookup order the `setlocale' function uses. But why introducing the `LANGUAGE' variable? The reason is that the syntax of the values these variables can have is different to what is expected by the `setlocale' function. If we would set `LC_ALL' to a value following the extended syntax that would mean the `setlocale' function will never be able to use the value of this variable as well. An additional variable removes this problem plus we can select the language independently of the locale setting which sometimes is useful. While for the `LC_xxx' variables the value should consist of exactly one specification of a locale the `LANGUAGE' variable's value can consist of a colon separated list of locale names. The attentive reader will realize that this is the way we manage to implement one of our additional demands above: we want to be able to specify an ordered list of language. Back to the constructed filename we have only one component missing. The DOMAIN_NAME part is the name which was either registered using the `textdomain' function or which was given to `dgettext' or `dcgettext' as the first parameter. Now it becomes obvious that a good choice for the domain name in the program code is a string which is closely related to the program/package name. E.g., for the GNU C Library the domain name is `libc'. A limit piece of example code should show how the programmer is supposed to work: { setlocale (LC_ALL, ""); textdomain ("test-package"); bindtextdomain ("test-package", "/usr/local/share/locale"); puts (gettext ("Hello, world!")); } At the program start the default domain is `messages', and the default locale is "C". The `setlocale' call sets the locale according to the user's environment variables; remember that correct functioning of `gettext' relies on the correct setting of the `LC_MESSAGES' locale (for looking up the message catalog) and of the `LC_CTYPE' locale (for the character set conversion). The `textdomain' call changes the default domain to `test-package'. The `bindtextdomain' call specifies that the message catalogs for the domain `test-package' can be found below the directory `/usr/local/share/locale'. If now the user set in her/his environment the variable `LANGUAGE' to `de' the `gettext' function will try to use the translations from the file /usr/local/share/locale/de/LC_MESSAGES/test-package.mo From the above descriptions it should be clear which component of this filename is determined by which source. In the above example we assumed that the `LANGUAGE' environment variable to `de'. This might be an appropriate selection but what happens if the user wants to use `LC_ALL' because of the wider usability and here the required value is `de_DE.ISO-8859-1'? We already mentioned above that a situation like this is not infrequent. E.g., a person might prefer reading a dialect and if this is not available fall back on the standard language. The `gettext' functions know about situations like this and can handle them gracefully. The functions recognize the format of the value of the environment variable. It can split the value is different pieces and by leaving out the only or the other part it can construct new values. This happens of course in a predictable way. To understand this one must know the format of the environment variable value. There are two more or less standardized forms: _X/Open Format_ `language[_territory[.codeset]][@modifier]' _CEN Format (European Community Standard)_ `language[_territory][+audience][+special][,[sponsor][_revision]]' The functions will automatically recognize which format is used. Less specific locale names will be stripped of in the order of the following list: 1. `revision' 2. `sponsor' 3. `special' 4. `codeset' 5. `normalized codeset' 6. `territory' 7. `audience'/`modifier' From the last entry one can see that the meaning of the `modifier' field in the X/Open format and the `audience' format have the same meaning. Beside one can see that the `language' field for obvious reasons never will be dropped. The only new thing is the `normalized codeset' entry. This is another goodie which is introduced to help reducing the chaos which derives from the inability of the people to standardize the names of character sets. Instead of ISO-8859-1 one can often see 8859-1, 88591, iso8859-1, or iso_8859-1. The `normalized codeset' value is generated from the user-provided character set name by applying the following rules: 1. Remove all characters beside numbers and letters. 2. Fold letters to lowercase. 3. If the same only contains digits prepend the string `"iso"'. So all of the above name will be normalized to `iso88591'. This allows the program user much more freely choosing the locale name. Even this extended functionality still does not help to solve the problem that completely different names can be used to denote the same locale (e.g., `de' and `german'). To be of help in this situation the locale implementation and also the `gettext' functions know about aliases. The file `/usr/share/locale/locale.alias' (replace `/usr' with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling. In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful. Programs to handle message catalogs for `gettext' ------------------------------------------------- The GNU C Library does not contain the source code for the programs to handle message catalogs for the `gettext' functions. As part of the GNU project the GNU gettext package contains everything the developer needs. The functionality provided by the tools in this package by far exceeds the abilities of the `gencat' program described above for the `catgets' functions. There is a program `msgfmt' which is the equivalent program to the `gencat' program. It generates from the human-readable and -editable form of the message catalog a binary file which can be used by the `gettext' functions. But there are several more programs available. The `xgettext' program can be used to automatically extract the translatable messages from a source file. I.e., the programmer need not take care for the translations and the list of messages which have to be translated. S/He will simply wrap the translatable string in calls to `gettext' et.al and the rest will be done by `xgettext'. This program has a lot of option which help to customize the output or do help to understand the input better. Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to _completely_ automize the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves. Searching and Sorting ********************* This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements. Defining the Comparison Function ================================ In order to use the sorted array library functions, you have to describe how to compare the elements of the array. To do this, you supply a comparison function to compare two elements of the array. The library will call this function, passing as arguments pointers to two array elements to be compared. Your comparison function should return a value the way `strcmp' (*note String/Array Comparison::) does: negative if the first argument is "less" than the second, zero if they are "equal", and positive if the first argument is "greater". Here is an example of a comparison function which works with an array of numbers of type `double': int compare_doubles (const void *a, const void *b) { const double *da = (const double *) a; const double *db = (const double *) b; return (*da > *db) - (*da < *db); } The header file `stdlib.h' defines a name for the data type of comparison functions. This type is a GNU extension. int comparison_fn_t (const void *, const void *); Array Search Function ===================== Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in `search.h'. - Function: void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lfind' function searches in the array with `*NMEMB' elements of SIZE bytes pointed to by BASE for an element which matches the one pointed to by KEY. The function pointed to by COMPAR is used decide whether two elements match. The return value is a pointer to the matching element in the array starting at BASE if it is found. If no matching element is available `NULL' is returned. The mean runtime of this function is `*NMEMB'/2. This function should only be used elements often get added to or deleted from the array in which case it might not be useful to sort the array before searching. - Function: void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lsearch' function is similar to the `lfind' function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found the `lsearch' function adds the object pointed to by KEY (with a size of SIZE bytes) at the end of the array and it increments the value of `*NMEMB' to reflect this addition. This means for the caller that if it is not sure that the array contains the element one is searching for the memory allocated for the array starting at BASE must have room for at least SIZE more bytes. If one is sure the element is in the array it is better to use `lfind' so having more room in the array is always necessary when calling `lsearch'. To search a sorted array for an element matching the key, use the `bsearch' function. The prototype for this function is in the header file `stdlib.h'. - Function: void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The `bsearch' function searches the sorted array ARRAY for an object that is equivalent to KEY. The array contains COUNT elements, each of which is of size SIZE bytes. The COMPARE function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the ARRAY must already be sorted in ascending order according to this comparison function. The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified. This function derives its name from the fact that it is implemented using the binary search algorithm. Array Sort Function =================== To sort an array using an arbitrary comparison function, use the `qsort' function. The prototype for this function is in `stdlib.h'. - Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The QSORT function sorts the array ARRAY. The array contains COUNT elements, each of which is of size SIZE. The COMPARE function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. *Warning:* If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects. If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary. Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (*note Comparison Functions::): { double *array; int size; ... qsort (array, size, sizeof (double), compare_doubles); } The `qsort' function derives its name from the fact that it was originally implemented using the "quick sort" algorithm. The implementation of `qsort' in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array. Searching and Sorting Example ============================= Here is an example showing the use of `qsort' and `bsearch' with an array of structures. The objects in the array are sorted by comparing their `name' fields with the `strcmp' function. Then, we can look up individual objects based on their names. #include #include #include /* Define an array of critters to sort. */ struct critter { const char *name; const char *species; }; struct critter muppets[] = { {"Kermit", "frog"}, {"Piggy", "pig"}, {"Gonzo", "whatever"}, {"Fozzie", "bear"}, {"Sam", "eagle"}, {"Robin", "frog"}, {"Animal", "animal"}, {"Camilla", "chicken"}, {"Sweetums", "monster"}, {"Dr. Strangepork", "pig"}, {"Link Hogthrob", "pig"}, {"Zoot", "human"}, {"Dr. Bunsen Honeydew", "human"}, {"Beaker", "human"}, {"Swedish Chef", "human"} }; int count = sizeof (muppets) / sizeof (struct critter); /* This is the comparison function used for sorting and searching. */ int critter_cmp (const struct critter *c1, const struct critter *c2) { return strcmp (c1->name, c2->name); } /* Print information about a critter. */ void print_critter (const struct critter *c) { printf ("%s, the %s\n", c->name, c->species); } /* Do the lookup into the sorted array. */ void find_critter (const char *name) { struct critter target, *result; target.name = name; result = bsearch (&target, muppets, count, sizeof (struct critter), critter_cmp); if (result) print_critter (result); else printf ("Couldn't find %s.\n", name); } /* Main program. */ int main (void) { int i; for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); qsort (muppets, count, sizeof (struct critter), critter_cmp); for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); find_critter ("Kermit"); find_critter ("Gonzo"); find_critter ("Janice"); return 0; } The output from this program looks like: Kermit, the frog Piggy, the pig Gonzo, the whatever Fozzie, the bear Sam, the eagle Robin, the frog Animal, the animal Camilla, the chicken Sweetums, the monster Dr. Strangepork, the pig Link Hogthrob, the pig Zoot, the human Dr. Bunsen Honeydew, the human Beaker, the human Swedish Chef, the human Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice. The `hsearch' function. ======================= The functions mentioned so far in this chapter are searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables. - Function: int hcreate (size_t NEL) The `hcreate' function creates a hashing table which can contain at least NEL elements. There is no possibility to grow this table so it is necessary to choose the value for NEL wisely. The used methods to implement this function might make it necessary to make the number of elements in the hashing table larger than the expected maximal number of elements. Hashing tables usually work inefficient if they are filled 80% or more. The constant access time guaranteed by hashing can only be achieved if few collisions exist. See Knuth's "The Art of Computer Programming, Part 3: Searching and Sorting" for more information. The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables. It is possible to use more than one hashing table in the program run if the former table is first destroyed by a call to `hdestroy'. The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory. - Function: void hdestroy (void) The `hdestroy' function can be used to free all the resources allocated in a previous call of `hcreate'. After a call to this function it is again possible to call `hcreate' and allocate a new table with possibly different size. It is important to remember that the elements contained in the hashing table at the time `hdestroy' is called are _not_ freed by this function. It is the responsibility of the program code to free those strings (if necessary at all). Freeing all the element memory is not possible without extra, separately kept information since there is no function to iterate through all available elements in the hashing table. If it is really necessary to free a table and all elements the programmer has to keep a list of all table elements and before calling `hdestroy' s/he has to free all element's data using this list. This is a very unpleasant mechanism and it also shows that this kind of hashing tables is mainly meant for tables which are created once and used until the end of the program run. Entries of the hashing table and keys for the search are defined using this type: - Data type: struct ENTRY Both elements of this structure are pointers to zero-terminated strings. This is a limiting restriction of the functionality of the `hsearch' functions. They can only be used for data sets which use the NUL character always and solely to terminate the records. It is not possible to handle general binary data. `char *key' Pointer to a zero-terminated string of characters describing the key for the search or the element in the hashing table. `char *data' Pointer to a zero-terminated string of characters describing the data. If the functions will be called only for searching an existing entry this element might stay undefined since it is not used. - Function: ENTRY * hsearch (ENTRY ITEM, ACTION ACTION) To search in a hashing table created using `hcreate' the `hsearch' function must be used. This function can perform simple search for an element (if ACTION has the `FIND') or it can alternatively insert the key element into the hashing table, possibly replacing a previous value (if ACTION is `ENTER'). The key is denoted by a pointer to an object of type `ENTRY'. For locating the corresponding position in the hashing table only the `key' element of the structure is used. The return value depends on the ACTION parameter value. If it is `FIND' the value is a pointer to the matching element in the hashing table or `NULL' if no matching element exists. If ACTION is `ENTER' the return value is only `NULL' if the programs runs out of memory while adding the new element to the table. Otherwise the return value is a pointer to the element in the hashing table which contains newly added element based on the data in KEY. As mentioned before the hashing table used by the functions described so far is global and there can be at any time at most one hashing table in the program. A solution is to use the following functions which are a GNU extension. All have in common that they operate on a hashing table which is described by the content of an object of the type `struct hsearch_data'. This type should be treated as opaque, none of its members should be changed directly. - Function: int hcreate_r (size_t NEL, struct hsearch_data *HTAB) The `hcreate_r' function initializes the object pointed to by HTAB to contain a hashing table with at least NEL elements. So this function is equivalent to the `hcreate' function except that the initialized data structure is controlled by the user. This allows having more than one hashing table at one time. The memory necessary for the `struct hsearch_data' object can be allocated dynamically. The return value is non-zero if the operation were successful. if the return value is zero something went wrong which probably means the programs runs out of memory. - Function: void hdestroy_r (struct hsearch_data *HTAB) The `hdestroy_r' function frees all resources allocated by the `hcreate_r' function for this very same object HTAB. As for `hdestroy' it is the programs responsibility to free the strings for the elements of the table. - Function: int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB) The `hsearch_r' function is equivalent to `hsearch'. The meaning of the first two arguments is identical. But instead of operating on a single global hashing table the function works on the table described by the object pointed to by HTAB (which is initialized by a call to `hcreate_r'). Another difference to `hcreate' is that the pointer to the found entry in the table is not the return value of the functions. It is returned by storing it in a pointer variables pointed to by the RETVAL parameter. The return value of the function is an integer value indicating success if it is non-zero and failure if it is zero. In the latter case the global variable ERRNO signals the reason for the failure. `ENOMEM' The table is filled and `hsearch_r' was called with an so far unknown key and ACTION set to `ENTER'. `ESRCH' The ACTION parameter is `FIND' and no corresponding element is found in the table. The `tsearch' function. ======================= Another common form to organize data for efficient search is to use trees. The `tsearch' function family provides a nice interface to functions to organize possibly large amounts of data by providing a mean access time proportional to the logarithm of the number of elements. The GNU C library implementation even guarantees that this bound is never exceeded even for input data which cause problems for simple binary tree implementations. The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable. In contrast to the `hsearch' functions the `tsearch' functions can be used with arbitrary data and not only zero-terminated strings. The `tsearch' functions have the advantage that no function to initialize data structures is necessary. A simple pointer of type `void *' initialized to `NULL' is a valid tree and can be extended or searched. - Function: void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) The `tsearch' function searches in the tree pointed to by `*ROOTP' for an element matching KEY. The function pointed to by COMPAR is used to determine whether two elements match. *Note Comparison Functions::, for a specification of the functions which can be used for the COMPAR parameter. If the tree does not contain a matching entry the KEY value will be added to the tree. `tsearch' does not make a copy of the object pointed to by KEY (how could it since the size is unknown). Instead it adds a reference to this object which means the object must be available as long as the tree data structure is used. The tree is represented by a pointer to a pointer since it is sometimes necessary to change the root node of the tree. So it must not be assumed that the variable pointed to by ROOTP has the same value after the call. This also shows that it is not safe to call the `tsearch' function more than once at the same time using the same tree. It is no problem to run it more than once at a time on different trees. The return value is a pointer to the matching element in the tree. If a new element was created the pointer points to the new data (which is in fact KEY). If an entry had to be created and the program ran out of space `NULL' is returned. - Function: void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR) The `tfind' function is similar to the `tsearch' function. It locates an element matching the one pointed to by KEY and returns a pointer to this element. But if no matching element is available no new element is entered (note that the ROOTP parameter points to a constant pointer). Instead the function returns `NULL'. Another advantage of the `tsearch' function in contrast to the `hsearch' functions is that there is an easy way to remove elements. - Function: void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) To remove a specific element matching KEY from the tree `tdelete' can be used. It locates the matching element using the same method as `tfind'. The corresponding element is then removed and a pointer to the parent of the deleted node is returned by the function. If there is no matching entry in the tree nothing can be deleted and the function returns `NULL'. If the root of the tree is deleted `tdelete' returns some unspecified value not equal to `NULL'. - Function: void tdestroy (void *VROOT, __free_fn_t FREEFCT) If the complete search tree has to be removed one can use `tdestroy'. It frees all resources allocated by the `tsearch' function to generate the tree pointed to by VROOT. For the data in each tree node the function FREEFCT is called. The pointer to the data is passed as the argument to the function. If no such work is necessary FREEFCT must point to a function doing nothing. It is called in any case. This function is a GNU extension and not covered by the System V or X/Open specifications. In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type: void __action_fn_t (const void *nodep, VISIT value, int level); The NODEP is the data value of the current node (once given as the KEY argument to `tsearch'). LEVEL is a numeric value which corresponds to the depth of the current node in the tree. The root node has the depth 0 and its children have a depth of 1 and so on. The `VISIT' type is an enumeration type. - Data Type: VISIT The `VISIT' value indicates the status of the current node in the tree and how the function is called. The status of a node is either `leaf' or `internal node'. For each leaf node the function is called exactly once, for each internal node it is called three times: before the first child is processed, after the first child is processed and after both children are processed. This makes it possible to handle all three methods of tree traversal (or even a combination of them). `preorder' The current node is an internal node and the function is called before the first child was processed. `postorder' The current node is an internal node and the function is called after the first child was processed. `endorder' The current node is an internal node and the function is called after the second child was processed. `leaf' The current node is a leaf. - Function: void twalk (const void *ROOT, __action_fn_t ACTION) For each node in the tree with a node pointed to by ROOT, the `twalk' function calls the function provided by the parameter ACTION. For leaf nodes the function is called exactly once with VALUE set to `leaf'. For internal nodes the function is called three times, setting the VALUE parameter or ACTION to the appropriate value. The LEVEL argument for the ACTION function is computed while descending the tree with increasing the value by one for the descend to a child, starting with the value 0 for the root node. Since the functions used for the ACTION parameter to `twalk' must not modify the tree data, it is safe to run `twalk' in more than one thread at the same time, working on the same tree. It is also safe to call `tfind' in parallel. Functions which modify the tree must not be used, otherwise the behavior is undefined. Pattern Matching **************** The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does. Wildcard Matching ================= This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'. - Function: int fnmatch (const char *PATTERN, const char *STRING, int FLAGS) This function tests whether the string STRING matches the pattern PATTERN. It returns `0' if they do match; otherwise, it returns the nonzero value `FNM_NOMATCH'. The arguments PATTERN and STRING are both strings. The argument FLAGS is a combination of flag bits that alter the details of matching. See below for a list of the defined flags. In the GNU C Library, `fnmatch' cannot experience an "error"--it always returns an answer for whether the match succeeds. However, other implementations of `fnmatch' might sometimes report "errors". They would do so by returning nonzero values that are not equal to `FNM_NOMATCH'. These are the available flags for the FLAGS argument: `FNM_FILE_NAME' Treat the `/' character specially, for matching file names. If this flag is set, wildcard constructs in PATTERN cannot match `/' in STRING. Thus, the only way to match `/' is with an explicit `/' in PATTERN. `FNM_PATHNAME' This is an alias for `FNM_FILE_NAME'; it comes from POSIX.2. We don't recommend this name because we don't use the term "pathname" for file names. `FNM_PERIOD' Treat the `.' character specially if it appears at the beginning of STRING. If this flag is set, wildcard constructs in PATTERN cannot match `.' as the first character of STRING. If you set both `FNM_PERIOD' and `FNM_FILE_NAME', then the special treatment applies to `.' following `/' as well as to `.' at the beginning of STRING. (The shell uses the `FNM_PERIOD' and `FNM_FILE_NAME' flags together for matching file names.) `FNM_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `FNM_NOESCAPE', then `\' is an ordinary character. `FNM_LEADING_DIR' Ignore a trailing sequence of characters starting with a `/' in STRING; that is to say, test whether STRING starts with a directory name that PATTERN matches. If this flag is set, either `foo*' or `foobar' as a pattern would match the string `foobar/frobozz'. `FNM_CASEFOLD' Ignore case in comparing STRING to PATTERN. `FNM_EXTMATCH' Recognize beside the normal patterns also the extended patterns introduced in `ksh'. The patterns are written in the form explained in the following table where PATTERN-LIST is a `|' separated list of patterns. `?(PATTERN-LIST)' The pattern matches if zero or one occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `*(PATTERN-LIST)' The pattern matches if zero or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `+(PATTERN-LIST)' The pattern matches if one or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `@(PATTERN-LIST)' The pattern matches if exactly one occurrence of any of the patterns in the PATTERN-LIST allows matching the input string. `!(PATTERN-LIST)' The pattern matches if the input string cannot be matched with any of the patterns in the PATTERN-LIST. Globbing ======== The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called "globbing". You could do this using `fnmatch', by reading the directory entries one by one and testing each one with `fnmatch'. But that would be slow (and complex, since you would have to handle subdirectories by hand). The library provides a function `glob' to make this particular use of wildcards convenient. `glob' and the other symbols in this section are declared in `glob.h'. Calling `glob' -------------- The result of globbing is a vector of file names (strings). To return this vector, `glob' uses a special data type, `glob_t', which is a structure. You pass `glob' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: glob_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. For use in the `glob64' function `glob.h' contains another definition for a very similar type. `glob64_t' differs from `glob_t' only in the types of the members `gl_readdir', `gl_stat', and `gl_lstat'. - Data Type: glob64_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir64' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent64 *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat64' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat64' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. - Function: int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR) The function `glob' does globbing using the pattern PATTERN in the current directory. It puts the result in a newly allocated vector, and stores the size and address of this vector into `*VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Globbing::, for details of the flags. The result of globbing is a sequence of file names. The function `glob' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `glob' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*VECTOR-PTR'. Normally, `glob' sorts the file names alphabetically before returning them. You can turn this off with the flag `GLOB_NOSORT' if you want to get the information as fast as possible. Usually it's a good idea to let `glob' sort them--if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making. If `glob' succeeds, it returns 0. Otherwise, it returns one of these error codes: `GLOB_ABORTED' There was an error opening a directory, and you used the flag `GLOB_ERR' or your specified ERRFUNC returned a nonzero value. *Note Flags for Globbing::, for an explanation of the `GLOB_ERR' flag and ERRFUNC. `GLOB_NOMATCH' The pattern didn't match any existing files. If you use the `GLOB_NOCHECK' flag, then you never get this error code, because that flag tells `glob' to _pretend_ that the pattern matched at least one file. `GLOB_NOSPACE' It was impossible to allocate memory to hold the result. In the event of an error, `glob' stores information in `*VECTOR-PTR' about all the matches it has found so far. It is important to notice that the `glob' function will not fail if it encounters directories or files which cannot be handled without the LFS interfaces. The implementation of `glob' is supposed to use these functions internally. This at least is the assumptions made by the Unix standard. The GNU extension of allowing the user to provide own directory handling and `stat' functions complicates things a bit. If these callback functions are used and a large file or directory is encountered `glob' _can_ fail. - Function: int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR) The `glob64' function was added as part of the Large File Summit extensions but is not part of the original LFS proposal. The reason for this is simple: it is not necessary. The necessity for a `glob64' function is added by the extensions of the GNU `glob' implementation which allows the user to provide own directory handling and `stat' functions. The `readdir' and `stat' functions do depend on the choice of `_FILE_OFFSET_BITS' since the definition of the types `struct dirent' and `struct stat' will change depending on the choice. Beside this difference the `glob64' works just like `glob' in all aspects. This function is a GNU extension. Flags for Globbing ------------------ This section describes the flags that you can specify in the FLAGS argument to `glob'. Choose the flags you want, and combine them with the C bitwise OR operator `|'. `GLOB_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `glob'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `glob'. And, if you set `GLOB_DOOFFS' in the first call to `glob', you must also set it when you append to the results. Note that the pointer stored in `gl_pathv' may no longer be valid after you call `glob' the second time, because `glob' might have relocated the vector. So always fetch `gl_pathv' from the `glob_t' structure after each `glob' call; *never* save the pointer across calls. `GLOB_DOOFFS' Leave blank slots at the beginning of the vector of words. The `gl_offs' field says how many slots to leave. The blank slots contain null pointers. `GLOB_ERR' Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand PATTERN fully. Such difficulties might include a directory in which you don't have the requisite access. Normally, `glob' tries its best to keep on going despite any errors, reading whatever directories it can. You can exercise even more control than this by specifying an error-handler function ERRFUNC when you call `glob'. If ERRFUNC is not a null pointer, then `glob' doesn't give up right away when it can't read a directory; instead, it calls ERRFUNC with two arguments, like this: (*ERRFUNC) (FILENAME, ERROR-CODE) The argument FILENAME is the name of the directory that `glob' couldn't open or couldn't read, and ERROR-CODE is the `errno' value that was reported to `glob'. If the error handler function returns nonzero, then `glob' gives up right away. Otherwise, it continues. `GLOB_MARK' If the pattern matches the name of a directory, append `/' to the directory's name when returning it. `GLOB_NOCHECK' If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched. (Normally, when the pattern doesn't match anything, `glob' returns that there were no matches.) `GLOB_NOSORT' Don't sort the file names; return them in no particular order. (In practice, the order will depend on the order of the entries in the directory.) The only reason _not_ to sort is to save time. `GLOB_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `GLOB_NOESCAPE', then `\' is an ordinary character. `glob' does its work by calling the function `fnmatch' repeatedly. It handles the flag `GLOB_NOESCAPE' by turning on the `FNM_NOESCAPE' flag in calls to `fnmatch'. More Flags for Globbing ----------------------- Beside the flags described in the last section, the GNU implementation of `glob' allows a few more flags which are also defined in the `glob.h' file. Some of the extensions implement functionality which is available in modern shell implementations. `GLOB_PERIOD' The `.' character (period) is treated special. It cannot be matched by wildcards. *Note Wildcard Matching::, `FNM_PERIOD'. `GLOB_MAGCHAR' The `GLOB_MAGCHAR' value is not to be given to `glob' in the FLAGS parameter. Instead, `glob' sets this bit in the GL_FLAGS element of the GLOB_T structure provided as the result if the pattern used for matching contains any wildcard character. `GLOB_ALTDIRFUNC' Instead of the using the using the normal functions for accessing the filesystem the `glob' implementation uses the user-supplied functions specified in the structure pointed to by PGLOB parameter. For more information about the functions refer to the sections about directory handling see *Note Accessing Directories::, and *Note Reading Attributes::. `GLOB_BRACE' If this flag is given the handling of braces in the pattern is changed. It is now required that braces appear correctly grouped. I.e., for each opening brace there must be a closing one. Braces can be used recursively. So it is possible to define one brace expression in another one. It is important to note that the range of each brace expression is completely contained in the outer brace expression (if there is one). The string between the matching braces is separated into single expressions by splitting at `,' (comma) characters. The commas themselves are discarded. Please note what we said above about recursive brace expressions. The commas used to separate the subexpressions must be at the same level. Commas in brace subexpressions are not matched. They are used during expansion of the brace expression of the deeper level. The example below shows this glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result) is equivalent to the sequence glob ("foo/", GLOB_BRACE, NULL, &result) glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result) if we leave aside error handling. `GLOB_NOMAGIC' If the pattern contains no wildcard constructs (it is a literal file name), return it as the sole "matching" word, even if no file exists by that name. `GLOB_TILDE' If this flag is used the character `~' (tilde) is handled special if it appears at the beginning of the pattern. Instead of being taken verbatim it is used to represent the home directory of a known user. If `~' is the only character in pattern or it is followed by a `/' (slash), the home directory of the process owner is substituted. Using `getlogin' and `getpwnam' the information is read from the system databases. As an example take user `bart' with his home directory at `/home/bart'. For him a call like glob ("~/bin/*", GLOB_TILDE, NULL, &result) would return the contents of the directory `/home/bart/bin'. Instead of referring to the own home directory it is also possible to name the home directory of other users. To do so one has to append the user name after the tilde character. So the contents of user `homer''s `bin' directory can be retrieved by glob ("~homer/bin/*", GLOB_TILDE, NULL, &result) If the user name is not valid or the home directory cannot be determined for some reason the pattern is left untouched and itself used as the result. I.e., if in the last example `home' is not available the tilde expansion yields to `"~homer/bin/*"' and `glob' is not looking for a directory named `~homer'. This functionality is equivalent to what is available in C-shells if the `nonomatch' flag is set. `GLOB_TILDE_CHECK' If this flag is used `glob' behaves like as if `GLOB_TILDE' is given. The only difference is that if the user name is not available or the home directory cannot be determined for other reasons this leads to an error. `glob' will return `GLOB_NOMATCH' instead of using the pattern itself as the name. This functionality is equivalent to what is available in C-shells if `nonomatch' flag is not set. `GLOB_ONLYDIR' If this flag is used the globbing function takes this as a *hint* that the caller is only interested in directories matching the pattern. If the information about the type of the file is easily available non-directories will be rejected but no extra work will be done to determine the information for each file. I.e., the caller must still be able to filter directories out. This functionality is only available with the GNU `glob' implementation. It is mainly used internally to increase the performance but might be useful for a user as well and therefore is documented here. Calling `glob' will in most cases allocate resources which are used to represent the result of the function call. If the same object of type `glob_t' is used in multiple call to `glob' the resources are freed or reused so that no leaks appear. But this does not include the time when all `glob' calls are done. - Function: void globfree (glob_t *PGLOB) The `globfree' function frees all resources allocated by previous calls to `glob' associated with the object pointed to by PGLOB. This function should be called whenever the currently used `glob_t' typed object isn't used anymore. - Function: void globfree64 (glob64_t *PGLOB) This function is equivalent to `globfree' but it frees records of type `glob64_t' which were allocated by `glob64'. Regular Expression Matching =========================== The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years. Both interfaces are declared in the header file `regex.h'. If you define `_POSIX_C_SOURCE', then only the POSIX.2 functions, structures, and constants are declared. POSIX Regular Expression Compilation ------------------------------------ Before you can actually match a regular expression, you must "compile" it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (*Note Matching POSIX Regexps::, for how to use the compiled regular expression for matching.) There is a special data type for compiled regular expressions: - Data Type: regex_t This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at: `re_nsub' This field holds the number of parenthetical subexpressions in the regular expression that was compiled. There are several other fields, but we don't describe them here, because only the functions in the library should use them. After you create a `regex_t' object, you can compile a regular expression into it by calling `regcomp'. - Function: int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS) The function `regcomp' "compiles" a regular expression into a data structure that you can use with `regexec' to match against a string. The compiled regular expression format is designed for efficient matching. `regcomp' stores it into `*COMPILED'. It's up to you to allocate an object of type `regex_t' and pass its address to `regcomp'. The argument CFLAGS lets you specify various options that control the syntax and semantics of regular expressions. *Note Flags for POSIX Regexps::. If you use the flag `REG_NOSUB', then `regcomp' omits from the compiled regular expression the information necessary to record how subexpressions actually match. In this case, you might as well pass `0' for the MATCHPTR and NMATCH arguments when you call `regexec'. If you don't use `REG_NOSUB', then the compiled regular expression does have the capacity to record how subexpressions match. Also, `regcomp' tells you how many subexpressions PATTERN has, by storing the number in `COMPILED->re_nsub'. You can use that value to decide how long an array to allocate to hold information about subexpression matches. `regcomp' returns `0' if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below). You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. Here are the possible nonzero values that `regcomp' can return: `REG_BADBR' There was an invalid `\{...\}' construct in the regular expression. A valid `\{...\}' construct must contain either a single number, or two numbers in increasing order separated by a comma. `REG_BADPAT' There was a syntax error in the regular expression. `REG_BADRPT' A repetition operator such as `?' or `*' appeared in a bad position (with no preceding subexpression to act on). `REG_ECOLLATE' The regular expression referred to an invalid collating element (one not defined in the current locale for string collation). *Note Locale Categories::. `REG_ECTYPE' The regular expression referred to an invalid character class name. `REG_EESCAPE' The regular expression ended with `\'. `REG_ESUBREG' There was an invalid number in the `\DIGIT' construct. `REG_EBRACK' There were unbalanced square brackets in the regular expression. `REG_EPAREN' An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced `\(' and `\)'. `REG_EBRACE' The regular expression had unbalanced `\{' and `\}'. `REG_ERANGE' One of the endpoints in a range expression was invalid. `REG_ESPACE' `regcomp' ran out of memory. Flags for POSIX Regular Expressions ----------------------------------- These are the bit flags that you can use in the CFLAGS operand when compiling a regular expression with `regcomp'. `REG_EXTENDED' Treat the pattern as an extended regular expression, rather than as a basic regular expression. `REG_ICASE' Ignore case when matching letters. `REG_NOSUB' Don't bother storing the contents of the MATCHES-PTR array. `REG_NEWLINE' Treat a newline in STRING as dividing STRING into multiple lines, so that `$' can match before the newline and `^' can match after. Also, don't permit `.' to match a newline, and don't permit `[^...]' to match a newline. Otherwise, newline acts like any other ordinary character. Matching a Compiled POSIX Regular Expression -------------------------------------------- Once you have compiled a regular expression, as described in *Note POSIX Regexp Compilation::, you can match it against strings using `regexec'. A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (`^' or `$'). - Function: int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS) This function tries to match the compiled regular expression `*COMPILED' against STRING. `regexec' returns `0' if the regular expression matches; otherwise, it returns a nonzero value. See the table below for what nonzero values mean. You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. The argument EFLAGS is a word of bit flags that enable various options. If you want to get information about what part of STRING actually matched the regular expression or its subexpressions, use the arguments MATCHPTR and NMATCH. Otherwise, pass `0' for NMATCH, and `NULL' for MATCHPTR. *Note Regexp Subexpressions::. You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression. The function `regexec' accepts the following flags in the EFLAGS argument: `REG_NOTBOL' Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it. `REG_NOTEOL' Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it. Here are the possible nonzero values that `regexec' can return: `REG_NOMATCH' The pattern didn't match the string. This isn't really an error. `REG_ESPACE' `regexec' ran out of memory. Match Results with Subexpressions --------------------------------- When `regexec' matches parenthetical subexpressions of PATTERN, it records which parts of STRING they match. It returns that information by storing the offsets into an array whose elements are structures of type `regmatch_t'. The first element of the array (index `0') records the part of the string that matched the entire regular expression. Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression. - Data Type: regmatch_t This is the data type of the MATCHARRAY array that you pass to `regexec'. It contains two structure fields, as follows: `rm_so' The offset in STRING of the beginning of a substring. Add this value to STRING to get the address of that part. `rm_eo' The offset in STRING of the end of the substring. - Data Type: regoff_t `regoff_t' is an alias for another signed integer type. The fields of `regmatch_t' have type `regoff_t'. The `regmatch_t' elements correspond to subexpressions positionally; the first element (index `1') records where the first subexpression matched, the second element records the second subexpression, and so on. The order of the subexpressions is the order in which they begin. When you call `regexec', you specify how long the MATCHPTR array is, with the NMATCH argument. This tells `regexec' how many elements to store. If the actual regular expression has more than NMATCH subexpressions, then you won't get offset information about the rest of them. But this doesn't alter whether the pattern matches a particular string or not. If you don't want `regexec' to return any information about where the subexpressions matched, you can either supply `0' for NMATCH, or use the flag `REG_NOSUB' when you compile the pattern with `regcomp'. Complications in Subexpression Matching --------------------------------------- Sometimes a subexpression matches a substring of no characters. This happens when `f\(o*\)' matches the string `fum'. (It really matches just the `f'.) In this case, both of the offsets identify the point in the string where the null substring was found. In this example, the offsets are both `1'. Sometimes the entire regular expression can match without using some of its subexpressions at all--for example, when `ba\(na\)*' matches the string `ba', the parenthetical subexpression is not used. When this happens, `regexec' stores `-1' in both fields of the element for that subexpression. Sometimes matching the entire regular expression can match a particular subexpression more than once--for example, when `ba\(na\)*' matches the string `bananana', the parenthetical subexpression matches three times. When this happens, `regexec' usually stores the offsets of the last part of the string that matched the subexpression. In the case of `bananana', these offsets are `6' and `8'. But the last match is not always the one that is chosen. It's more accurate to say that the last _opportunity_ to match is the one that takes precedence. What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression. For an example, consider `\(ba\(na\)*s \)*' matching the string `bananas bas '. The last time the inner expression actually matches is near the end of the first word. But it is _considered_ again in the second word, and fails to match there. `regexec' reports nonuse of the "na" subexpression. Another place where this rule applies is when the regular expression \(ba\(na\)*s \|nefer\(ti\)* \)* matches `bananas nefertiti'. The "na" subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there. Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the "na" subexpression is not used. So `regexec' reports nonuse of the "na" subexpression. POSIX Regexp Matching Cleanup ----------------------------- When you are finished using a compiled regular expression, you can free the storage it uses by calling `regfree'. - Function: void regfree (regex_t *COMPILED) Calling `regfree' frees all the storage that `*COMPILED' points to. This includes various internal fields of the `regex_t' structure that aren't documented in this manual. `regfree' does not free the object `*COMPILED' itself. You should always free the space in a `regex_t' structure with `regfree' before using the structure to compile another regular expression. When `regcomp' or `regexec' reports an error, you can use the function `regerror' to turn it into an error message string. - Function: size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH) This function produces an error message string for the error code ERRCODE, and stores the string in LENGTH bytes of memory starting at BUFFER. For the COMPILED argument, supply the same compiled regular expression structure that `regcomp' or `regexec' was working with when it got the error. Alternatively, you can supply `NULL' for COMPILED; you will still get a meaningful error message, but it might not be as detailed. If the error message can't fit in LENGTH bytes (including a terminating null character), then `regerror' truncates it. The string that `regerror' stores is always null-terminated even if it has been truncated. The return value of `regerror' is the minimum length needed to store the entire error message. If this is less than LENGTH, then the error message was not truncated, and you can use it. Otherwise, you should call `regerror' again with a larger buffer. Here is a function which uses `regerror', but always dynamically allocates a buffer for the error message: char *get_regerror (int errcode, regex_t *compiled) { size_t length = regerror (errcode, compiled, NULL, 0); char *buffer = xmalloc (length); (void) regerror (errcode, compiled, buffer, length); return buffer; } Shell-Style Word Expansion ========================== "Word expansion" means the process of splitting a string into "words" and substituting for variables, commands, and wildcards just as the shell does. For example, when you write `ls -l foo.c', this string is split into three separate words--`ls', `-l' and `foo.c'. This is the most basic function of word expansion. When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called "wildcard expansion", and it is also a part of word expansion. When you use `echo $PATH' to print your path, you are taking advantage of "variable substitution", which is also part of word expansion. Ordinary programs can perform word expansion just like the shell by calling the library function `wordexp'. The Stages of Word Expansion ---------------------------- When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here: 1. "Tilde expansion": Replacement of `~foo' with the name of the home directory of `foo'. 2. Next, three different transformations are applied in the same step, from left to right: * "Variable substitution": Environment variables are substituted for references such as `$foo'. * "Command substitution": Constructs such as ``cat foo`' and the equivalent `$(cat foo)' are replaced with the output from the inner command. * "Arithmetic expansion": Constructs such as `$(($x-1))' are replaced with the result of the arithmetic computation. 3. "Field splitting": subdivision of the text into "words". 4. "Wildcard expansion": The replacement of a construct such as `*.c' with a list of `.c' file names. Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words. 5. "Quote removal": The deletion of string-quotes, now that they have done their job by inhibiting the above transformations when appropriate. For the details of these transformations, and how to write the constructs that use them, see `The BASH Manual' (to appear). Calling `wordexp' ----------------- All the functions, constants and data types for word expansion are declared in the header file `wordexp.h'. Word expansion produces a vector of words (strings). To return this vector, `wordexp' uses a special data type, `wordexp_t', which is a structure. You pass `wordexp' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: wordexp_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. `we_wordc' The number of elements in the vector. `we_wordv' The address of the vector. This field has type `char **'. `we_offs' The offset of the first real element of the vector, from its nominal address in the `we_wordv' field. Unlike the other fields, this is always an input to `wordexp', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `wordexp' function fills them with null pointers.) The `we_offs' field is meaningful only if you use the `WRDE_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. - Function: int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS) Perform word expansion on the string WORDS, putting the result in a newly allocated vector, and store the size and address of this vector into `*WORD-VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Wordexp::, for details of the flags. You shouldn't use any of the characters `|&;<>' in the string WORDS unless they are quoted; likewise for newline. If you use these characters unquoted, you will get the `WRDE_BADCHAR' error code. Don't use parentheses or braces unless they are quoted or part of a word expansion construct. If you use quotation characters `'"`', they should come in pairs that balance. The results of word expansion are a sequence of words. The function `wordexp' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `wordexp' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*WORD-VECTOR-PTR'. If `wordexp' succeeds, it returns 0. Otherwise, it returns one of these error codes: `WRDE_BADCHAR' The input string WORDS contains an unquoted invalid character such as `|'. `WRDE_BADVAL' The input string refers to an undefined shell variable, and you used the flag `WRDE_UNDEF' to forbid such references. `WRDE_CMDSUB' The input string uses command substitution, and you used the flag `WRDE_NOCMD' to forbid command substitution. `WRDE_NOSPACE' It was impossible to allocate memory to hold the result. In this case, `wordexp' can store part of the results--as much as it could allocate room for. `WRDE_SYNTAX' There was a syntax error in the input string. For example, an unmatched quoting character is a syntax error. - Function: void wordfree (wordexp_t *WORD-VECTOR-PTR) Free the storage used for the word-strings and vector that `*WORD-VECTOR-PTR' points to. This does not free the structure `*WORD-VECTOR-PTR' itself--only the other data it points to. Flags for Word Expansion ------------------------ This section describes the flags that you can specify in the FLAGS argument to `wordexp'. Choose the flags you want, and combine them with the C operator `|'. `WRDE_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `wordexp'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `wordexp'. And, if you set `WRDE_DOOFFS' in the first call to `wordexp', you must also set it when you append to the results. `WRDE_DOOFFS' Leave blank slots at the beginning of the vector of words. The `we_offs' field says how many slots to leave. The blank slots contain null pointers. `WRDE_NOCMD' Don't do command substitution; if the input requests command substitution, report an error. `WRDE_REUSE' Reuse a word vector made by a previous call to `wordexp'. Instead of allocating a new vector of words, this call to `wordexp' will use the vector that already exists (making it larger if necessary). Note that the vector may move, so it is not safe to save an old pointer and use it again after calling `wordexp'. You must fetch `we_pathv' anew after each call. `WRDE_SHOWERR' Do show any error messages printed by commands run by command substitution. More precisely, allow these commands to inherit the standard error output stream of the current process. By default, `wordexp' gives these commands a standard error stream that discards all output. `WRDE_UNDEF' If the input refers to a shell variable that is not defined, report an error. `wordexp' Example ----------------- Here is an example of using `wordexp' to expand several strings and use the results to run a shell command. It also shows the use of `WRDE_APPEND' to concatenate the expansions and of `wordfree' to free the space allocated by `wordexp'. int expand_and_execute (const char *program, const char *options) { wordexp_t result; pid_t pid int status, i; /* Expand the string for the program to run. */ switch (wordexp (program, &result, 0)) { case 0: /* Successful. */ break; case WRDE_NOSPACE: /* If the error was `WRDE_NOSPACE', then perhaps part of the result was allocated. */ wordfree (&result); default: /* Some other error. */ return -1; } /* Expand the strings specified for the arguments. */ for (i = 0; args[i]; i++) { if (wordexp (options, &result, WRDE_APPEND)) { wordfree (&result); return -1; } } pid = fork (); if (pid == 0) { /* This is the child process. Execute the command. */ execv (result.we_wordv[0], result.we_wordv); exit (EXIT_FAILURE); } else if (pid < 0) /* The fork failed. Report failure. */ status = -1; else /* This is the parent process. Wait for the child to complete. */ if (waitpid (pid, &status, 0) != pid) status = -1; wordfree (&result); return status; } Details of Tilde Expansion -------------------------- It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~USER' to stand for USER's home directory. "Tilde expansion" is the process of converting these abbreviations to the directory names that they stand for. Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way. Plain `~' uses the value of the environment variable `HOME' as the proper home directory name. `~' followed by a user name uses `getpwname' to look up that user in the user database, and uses whatever directory is recorded there. Thus, `~' followed by your own name can give different results from plain `~', if the value of `HOME' is not really your home directory. Details of Variable Substitution -------------------------------- Part of ordinary shell syntax is the use of `$VARIABLE' to substitute the value of a shell variable into a command. This is called "variable substitution", and it is one part of doing word expansion. There are two basic ways you can write a variable reference for substitution: `${VARIABLE}' If you write braces around the variable name, then it is completely unambiguous where the variable name ends. You can concatenate additional letters onto the end of the variable value by writing them immediately after the close brace. For example, `${foo}s' expands into `tractors'. `$VARIABLE' If you do not put braces around the variable name, then the variable name consists of all the alphanumeric characters and underscores that follow the `$'. The next punctuation character ends the variable name. Thus, `$foo-bar' refers to the variable `foo' and expands into `tractor-bar'. When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways. `${VARIABLE:-DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead. `${VARIABLE:=DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead and set the variable to DEFAULT. `${VARIABLE:?MESSAGE}' If VARIABLE is defined and not empty, substitute its value. Otherwise, print MESSAGE as an error message on the standard error stream, and consider word expansion a failure. `${VARIABLE:+REPLACEMENT}' Substitute REPLACEMENT, but only if VARIABLE is defined and nonempty. Otherwise, substitute nothing for this construct. `${#VARIABLE}' Substitute a numeral which expresses in base ten the number of characters in the value of VARIABLE. `${#foo}' stands for `7', because `tractor' is seven characters. These variants of variable substitution let you remove part of the variable's value before substituting it. The PREFIX and SUFFIX are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names. `${VARIABLE%%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the longest possible match. Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'. `${VARIABLE%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the shortest possible alternative. Thus, `${foo%%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'. `${VARIABLE##PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the longest possible match. Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'. `${VARIABLE#PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the shortest possible alternative. Thus, `${foo%%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'. Input/Output Overview ********************* Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate! This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are: * *Note I/O on Streams::, which covers the high-level functions that operate on streams, including formatted input and output. * *Note Low-Level I/O::, which covers the basic I/O and control functions on file descriptors. * *Note File System Interface::, which covers functions for operating on directories and for manipulating file attributes such as access modes and ownership. * *Note Pipes and FIFOs::, which includes information on the basic interprocess communication facilities. * *Note Sockets::, which covers a more complicated interprocess communication facility with support for networking. * *Note Low-Level Terminal Interface::, which covers functions for changing how input and output to terminals or other serial devices are processed. Input/Output Concepts ===================== Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called "opening" the file. You can open a file for reading, writing, or both. The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors. When you have finished reading to or writing from the file, you can terminate the connection by "closing" the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it. Streams and File Descriptors ---------------------------- When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type `int', while streams are represented as `FILE *' objects. File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (*note File Status Flags::). Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (*note Stream Buffering::). The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors. The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (`printf' and `scanf') as well as functions for character- and line-oriented input and output. Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor. In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (*note Formatted Input::) and formatted output functions (*note Formatted Output::). If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however. File Position ------------- One of the attributes of an open file is its "file position" that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file. The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally "sequential". Ordinary files permit read or write operations at any position within the file. Some other kinds of files may also permit this. Files which do permit this are sometimes referred to as "random-access" files. You can change the file position using the `fseek' function on a stream (*note File Positioning::) or the `lseek' function on a file descriptor (*note I/O Primitives::). If you try to change the file position on a file that doesn't support random access, you get the `ESPIPE' error. Streams and descriptors that are opened for "append access" are treated specially for output: output to such files is _always_ appended sequentially to the _end_ of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done. If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do. In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions. By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other. File Names ========== In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called "file names". You specify the file name to say which file you want to open or operate on. This section describes the conventions for file names and how the operating system works with them. Directories ----------- In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories. A "directory" is a file that contains information to associate other files with names; these associations are called "links" or "directory entries". Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves. The name of a file contained in a directory entry is called a "file name component". In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on. Some other documents, such as the POSIX standard, use the term "pathname" for what we call a file name, and either "filename" or "pathname component" for what this manual calls a file name component. We don't use this terminology because a "path" is something completely different (a list of directories to search), and we think that "pathname" used for something else will confuse users. We always use "file name" and "file name component" (or sometimes just "component", where the context is obvious) in GNU documentation. Some macros use the POSIX terminology in their names, such as `PATH_MAX'. These macros are defined by the POSIX standard, so we cannot change their names. You can find more detailed information about operations on directories in *Note File System Interface::. File Name Resolution -------------------- A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character. The process of determining what file a file name refers to is called "file name resolution". This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails. If a file name begins with a `/', the first component in the file name is located in the "root directory" of the process (usually all processes on the system have the same root directory). Such a file name is called an "absolute file name". Otherwise, the first component in the file name is located in the current working directory (*note Working Directory::). This kind of file name is called a "relative file name". The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its "parent directory" (the directory that contains the link for the directory in question). As a special case, `..' in the root directory refers to the root directory itself, since it has no parent; thus `/..' is the same as `/'. Here are some examples of file names: `/a' The file named `a', in the root directory. `/a/b' The file named `b', in the directory named `a' in the root directory. `a' The file named `a', in the current working directory. `/a/./b' This is the same as `/a/b'. `./a' The file named `a', in the current working directory. `../a' The file named `a', in the parent directory of the current working directory. A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'. Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'--but there is nothing in the file system itself that enforces this kind of convention. File Name Errors ---------------- Functions that accept file name arguments usually detect these `errno' error conditions relating to the file name syntax or trouble finding the named file. These errors are referred to throughout this manual as the "usual file name errors". `EACCES' The process does not have search permission for a directory component of the file name. `ENAMETOOLONG' This error is used when either the total length of a file name is greater than `PATH_MAX', or when an individual file name component has a length greater than `NAME_MAX'. *Note Limits for Files::. In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component. `ENOENT' This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist. *Note Symbolic Links::. `ENOTDIR' A file that is referenced as a directory component in the file name exists, but it isn't a directory. `ELOOP' Too many symbolic links were resolved while trying to look up the file name. The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops. *Note Symbolic Links::. Portability of File Names ------------------------- The rules for the syntax of file names discussed in *Note File Names::, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions. There are two reasons why it can be important for you to be aware of file name portability issues: * If your program makes assumptions about file name syntax, or contains embedded literal file name strings, it is more difficult to get it to run under other operating systems that use different syntax conventions. * Even if you are not concerned about running your program on machines that run other operating systems, it may still be possible to access files that use different naming conventions. For example, you may be able to access file systems on another computer running a different operating system over a network, or read and write disks in formats used by other operating systems. The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others. The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings. Input/Output on Streams *********************** This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in *Note I/O Overview::, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process. Streams ======= For historical reasons, the type of the C data structure that represents a stream is called `FILE' rather than "stream". Since most of the library functions deal with objects of type `FILE *', sometimes the term "file pointer" is also used to mean "stream". This leads to unfortunate confusion over terminology in many books on C. This manual, however, is careful to use the terms "file" and "stream" only in the technical sense. The `FILE' type is declared in the header file `stdio.h'. - Data Type: FILE This is the data type used to represent stream objects. A `FILE' object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information. Each stream also has error and end-of-file status indicators that can be tested with the `ferror' and `feof' functions; see *Note EOF and Errors::. `FILE' objects are allocated and managed internally by the input/output library functions. Don't try to create your own objects of type `FILE'; let the library do it. Your programs should deal only with pointers to these objects (that is, `FILE *' values) rather than the objects themselves. Standard Streams ================ When the `main' function of your program is invoked, it already has three predefined streams open and available for use. These represent the "standard" input and output channels that have been established for the process. These streams are declared in the header file `stdio.h'. - Variable: FILE * stdin The "standard input" stream, which is the normal source of input for the program. - Variable: FILE * stdout The "standard output" stream, which is used for normal output from the program. - Variable: FILE * stderr The "standard error" stream, which is used for error messages and diagnostics issued by the program. In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in *Note File System Interface::.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary. In the GNU C library, `stdin', `stdout', and `stderr' are normal variables which you can set just like any others. For example, to redirect the standard output to a file, you could do: fclose (stdout); stdout = fopen ("standard-output-file", "w"); Note however, that in other systems `stdin', `stdout', and `stderr' are macros that you cannot assign to in the normal way. But you can use `freopen' to get the effect of closing one and reopening it. *Note Opening Streams::. The three streams `stdin', `stdout', and `stderr' are not unoriented at program start (*note Streams and I18N::). Opening Streams =============== Opening a file with the `fopen' function creates a new stream and establishes a connection between the stream and a file. This may involve creating a new file. Everything described in this section is declared in the header file `stdio.h'. - Function: FILE * fopen (const char *FILENAME, const char *OPENTYPE) The `fopen' function opens a stream for I/O to the file FILENAME, and returns a pointer to the stream. The OPENTYPE argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters: `r' Open an existing file for reading only. `w' Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created. `a' Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created. `r+' Open an existing file for both reading and writing. The initial contents of the file are unchanged and the initial file position is at the beginning of the file. `w+' Open a file for both reading and writing. If the file already exists, it is truncated to zero length. Otherwise, a new file is created. `a+' Open or create file for both reading and appending. If the file exists, its initial contents are unchanged. Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file. As you can see, `+' requests a stream that can do both input and output. The ISO standard says that when using such a stream, you must call `fflush' (*note Stream Buffering::) or a file positioning function such as `fseek' (*note File Positioning::) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly. The GNU C library does not have this limitation; you can do arbitrary reading and writing operations on a stream in whatever order. Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems. The GNU C library defines one additional character for use in OPENTYPE: the character `x' insists on creating a new file--if a file FILENAME already exists, `fopen' fails rather than opening it. If you use `x' you are guaranteed that you will not clobber an existing file. This is equivalent to the `O_EXCL' option to the `open' function (*note Opening and Closing Files::). The character `b' in OPENTYPE has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. *Note Binary Streams::. If the OPENTYPE string contains the sequence `,ccs=STRING' then STRING is taken as the name of a coded character set and `fopen' will mark the stream as wide-oriented which appropriate conversion functions in place to convert from and to the character set STRING is place. Any other stream is opened initially unoriented and the orientation is decided with the first file operation. If the first operation is a wide character operation, the stream is not only marked as wide-oriented, also the conversion functions to convert to the coded character set used for the current locale are loaded. This will not change anymore from this point on even if the locale selected for the `LC_CTYPE' category is changed. Any other characters in OPENTYPE are simply ignored. They may be meaningful in other systems. If the open fails, `fopen' returns a null pointer. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `fopen64' since the LFS interface replaces transparently the old interface. You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. *Note Stream/Descriptor Precautions::. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. *Note File Locks::. - Function: FILE * fopen64 (const char *FILENAME, const char *OPENTYPE) This function is similar to `fopen' but the stream it returns a pointer for is opened using `open64'. Therefore this stream can be used even on files larger then 2^31 bytes on 32 bit machines. Please note that the return type is still `FILE *'. There is no special `FILE' type for the LFS interface. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fopen' and so transparently replaces the old interface. - Macro: int FOPEN_MAX The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously. You might be able to open more than this many streams, but that is not guaranteed. The value of this constant is at least eight, which includes the three standard streams `stdin', `stdout', and `stderr'. In POSIX.1 systems this value is determined by the `OPEN_MAX' parameter; *note General Limits::. In BSD and GNU, it is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. - Function: FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is like a combination of `fclose' and `fopen'. It first closes the stream referred to by STREAM, ignoring any errors that are detected in the process. (Because errors are ignored, you should not use `freopen' on an output stream if you have actually done any output using the stream.) Then the file named by FILENAME is opened with mode OPENTYPE as for `fopen', and associated with the same stream object STREAM. If the operation fails, a null pointer is returned; otherwise, `freopen' returns STREAM. `freopen' has traditionally been used to connect a standard stream such as `stdin' with a file of your own choice. This is useful in programs in which use of a standard stream for certain purposes is hard-coded. In the GNU C library, you can simply close the standard streams and open new ones with `fopen'. But other systems lack this ability, so using `freopen' is more portable. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `freopen64' since the LFS interface replaces transparently the old interface. - Function: FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is similar to `freopen'. The only difference is that on 32 bit machine the stream returned is able to read beyond the 2^31 bytes limits imposed by the normal interface. It should be noted that the stream pointed to by STREAM need not be opened using `fopen64' or `freopen64' since its mode is not important for this function. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `freopen' and so transparently replaces the old interface. In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library. - Function: int __freadable (FILE *STREAM) The `__freadable' function determines whether the stream STREAM was opened to allow reading. In this case the return value is nonzero. For write-only streams the function returns zero. This function is declared in `stdio_ext.h'. - Function: int __fwritable (FILE *STREAM) The `__fwritable' function determines whether the stream STREAM was opened to allow writing. In this case the return value is nonzero. For read-only streams the function returns zero. This function is declared in `stdio_ext.h'. For slightly different kind of problems there are two more functions. They provide even finer-grained information. - Function: int __freading (FILE *STREAM) The `__freading' function determines whether the stream STREAM was last read from or whether it is opened read-only. In this case the return value is nonzero, otherwise it is zero. Determining whether a stream opened for reading and writing was last used for writing allows to draw conclusions about the content about the buffer, among other things. This function is declared in `stdio_ext.h'. - Function: int __fwriting (FILE *STREAM) The `__fwriting' function determines whether the stream STREAM was last written to or whether it is opened write-only. In this case the return value is nonzero, otherwise it is zero. This function is declared in `stdio_ext.h'. Closing Streams =============== When a stream is closed with `fclose', the connection between the stream and the file is canceled. After you have closed a stream, you cannot perform any additional operations on it. - Function: int fclose (FILE *STREAM) This function causes STREAM to be closed and the connection to the corresponding file to be broken. Any buffered output is written and any buffered input is discarded. The `fclose' function returns a value of `0' if the file was closed successfully, and `EOF' if an error was detected. It is important to check for errors when you call `fclose' to close an output stream, because real, everyday errors can be detected at this time. For example, when `fclose' writes the remaining buffered output, it might get an error because the disk is full. Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS. The function `fclose' is declared in `stdio.h'. To close all streams currently available the GNU C Library provides another function. - Function: int fcloseall (void) This function causes all open streams of the process to be closed and the connection to corresponding files to be broken. All buffered data is written and any buffered input is discarded. The `fcloseall' function returns a value of `0' if all the files were closed successfully, and `EOF' if an error was detected. This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (*note Standard Streams::) will also be closed. The function `fcloseall' is declared in `stdio.h'. If the `main' function to your program returns, or if you call the `exit' function (*note Normal Termination::), all open streams are automatically closed properly. If your program terminates in any other manner, such as by calling the `abort' function (*note Aborting a Program::) or from a fatal signal (*note Signal Handling::), open streams might not be closed properly. Buffered output might not be flushed and files may be incomplete. For more information on buffering of streams, see *Note Stream Buffering::. Streams and Threads =================== Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of a the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming. The POSIX standard requires that by default the stream operations are atomic. I.e., issuing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done. But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code. - Function: void flockfile (FILE *STREAM) The `flockfile' function acquires the internal locking object associated with the stream STREAM. This ensures that no other thread can explicitly through `flockfile'/`ftrylockfile' or implicit through a call of a stream function lock the stream. The thread will block until the lock is acquired. An explicit call to `funlockfile' has to be used to release the lock. - Function: int ftrylockfile (FILE *STREAM) The `ftrylockfile' function tries to acquire the internal locking object associated with the stream STREAM just like `flockfile'. But unlike `flockfile' this function does not block if the lock is not available. `ftrylockfile' returns zero if the lock was successfully acquired. Otherwise the stream is locked by another thread. - Function: void funlockfile (FILE *STREAM) The `funlockfile' function releases the internal locking object of the stream STREAM. The stream must have been locked before by a call to `flockfile' or a successful call of `ftrylockfile'. The implicit locking performed by the stream operations do not count. The `funlockfile' function does not return an error status and the behavior of a call for a stream which is not locked by the current thread is undefined. The following example shows how the functions above can be used to generate an output line atomically even in multi-threaded applications (yes, the same job could be done with one `fprintf' call but it is sometimes not possible): FILE *fp; { ... flockfile (fp); fputs ("This is test number ", fp); fprintf (fp, "%d\n", test); funlockfile (fp) } Without the explicit locking it would be possible for another thread to use the stream FP after the `fputs' call return and before `fprintf' was called with the result that the number does not follow the word `number'. From this description it might already be clear that the locking objects in streams are no simple mutexes. Since locking the same stream twice in the same thread is allowed the locking objects must be equivalent to recursive mutexes. These mutexes keep track of the owner and the number of times the lock is acquired. The same number of `funlockfile' calls by the same threads is necessary to unlock the stream completely. For instance: void foo (FILE *fp) { ftrylockfile (fp); fputs ("in foo\n", fp); /* This is very wrong!!! */ funlockfile (fp); } It is important here that the `funlockfile' function is only called if the `ftrylockfile' function succeeded in locking the stream. It is therefore always wrong to ignore the result of `ftrylockfile'. And it makes no sense since otherwise one would use `flockfile'. The result of code like that above is that either `funlockfile' tries to free a stream that hasn't been locked by the current thread or it frees the stream prematurely. The code should look like this: void foo (FILE *fp) { if (ftrylockfile (fp) == 0) { fputs ("in foo\n", fp); funlockfile (fp); } } Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations that are safe in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is not needed - because the code in question is never used in a context where two or more threads may use a stream at a time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking. There are two basic mechanisms to avoid locking. The first is to use the `_unlocked' variants of the stream operations. The POSIX standard defines quite a few of those and the GNU library adds a few more. These variants of the functions behave just like the functions with the name without the suffix except that they do not lock the stream. Using these functions is very desirable since they are potentially much faster. This is not only because the locking operation itself is avoided. More importantly, functions like `putc' and `getc' are very simple and traditionally (before the introduction of threads) were implemented as macros which are very fast if the buffer is not empty. With the addition of locking requirements these functions are no longer implemented as macros since they would would expand to too much code. But these macros are still available with the same functionality under the new names `putc_unlocked' and `getc_unlocked'. This possibly huge difference of speed also suggests the use of the `_unlocked' functions even if locking is required. The difference is that the locking then has to be performed in the program: void foo (FILE *fp, char *buf) { flockfile (fp); while (*buf != '/') putc_unlocked (*buf++, fp); funlockfile (fp); } If in this example the `putc' function would be used and the explicit locking would be missing the `putc' function would have to acquire the lock in every call, potentially many times depending on when the loop terminates. Writing it the way illustrated above allows the `putc_unlocked' macro to be used which means no locking and direct manipulation of the buffer of the stream. A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well. - Function: int __fsetlocking (FILE *STREAM, int TYPE) The `__fsetlocking' function can be used to select whether the stream operations will implicitly acquire the locking object of the stream STREAM. By default this is done but it can be disabled and reinstated using this function. There are three values defined for the TYPE parameter. `FSETLOCKING_INTERNAL' The stream `stream' will from now on use the default internal locking. Every stream operation with exception of the `_unlocked' variants will implicitly lock the stream. `FSETLOCKING_BYCALLER' After the `__fsetlocking' function returns the user is responsible for locking the stream. None of the stream operations will implicitly do this anymore until the state is set back to `FSETLOCKING_INTERNAL'. `FSETLOCKING_QUERY' `__fsetlocking' only queries the current locking state of the stream. The return value will be `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state. The return value of `__fsetlocking' is either `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state of the stream before the call. This function and the values for the TYPE parameter are declared in `stdio_ext.h'. This function is especially useful when program code has to be used which is written without knowledge about the `_unlocked' functions (or if the programmer was too lazy to use them). Streams in Internationalized Applications ========================================= ISO C90 introduced the new type `wchar_t' to allow handling larger character sets. What was missing was a possibility to output strings of `wchar_t' directly. One had to convert them into multibyte strings using `mbstowcs' (there was no `mbsrtowcs' yet) and then use the normal stream functions. While this is doable it is very cumbersome since performing the conversions is not trivial and greatly increases program complexity and size. The Unix standard early on (I think in XPG4.2) introduced two additional format specifiers for the `printf' and `scanf' families of functions. Printing and reading of single wide characters was made possible using the `%C' specifier and wide character strings can be handled with `%S'. These modifiers behave just like `%c' and `%s' only that they expect the corresponding argument to have the wide character type and that the wide character and string are transformed into/from multibyte strings before being used. This was a beginning but it is still not good enough. Not always is it desirable to use `printf' and `scanf'. The other, smaller and faster functions cannot handle wide characters. Second, it is not possible to have a format string for `printf' and `scanf' consisting of wide characters. The result is that format strings would have to be generated if they have to contain non-basic characters. In the Amendment 1 to ISO C90 a whole new set of functions was added to solve the problem. Most of the stream functions got a counterpart which take a wide character or wide character string instead of a character or string respectively. The new functions operate on the same streams (like `stdout'). This is different from the model of the C++ runtime library where separate streams for wide and normal I/O are used. Being able to use the same stream for wide and normal operations comes with a restriction: a stream can be used either for wide operations or for normal operations. Once it is decided there is no way back. Only a call to `freopen' or `freopen64' can reset the "orientation". The orientation can be decided in three ways: * If any of the normal character functions is used (this includes the `fread' and `fwrite' functions) the stream is marked as not wide oriented. * If any of the wide character functions is used the stream is marked as wide oriented. * The `fwide' function can be used to set the orientation either way. It is important to never mix the use of wide and not wide operations on a stream. There are no diagnostics issued. The application behavior will simply be strange or the application will simply crash. The `fwide' function can help avoiding this. - Function: int fwide (FILE *STREAM, int MODE) The `fwide' function can be used to set and query the state of the orientation of the stream STREAM. If the MODE parameter has a positive value the streams get wide oriented, for negative values narrow oriented. It is not possible to overwrite previous orientations with `fwide'. I.e., if the stream STREAM was already oriented before the call nothing is done. If MODE is zero the current orientation state is queried and nothing is changed. The `fwide' function returns a negative value, zero, or a positive value if the stream is narrow, not at all, or wide oriented respectively. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. It is generally a good idea to orient a stream as early as possible. This can prevent surprise especially for the standard streams `stdin', `stdout', and `stderr'. If some library function in some situations uses one of these streams and this use orients the stream in a different way the rest of the application expects it one might end up with hard to reproduce errors. Remember that no errors are signal if the streams are used incorrectly. Leaving a stream unoriented after creation is normally only necessary for library functions which create streams which can be used in different contexts. When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this. void print_f (FILE *fp) { if (fwide (fp, 0) > 0) /* Positive return value means wide orientation. */ fputwc (L'f', fp); else fputc ('f', fp); } Note that in this case the function `print_f' decides about the orientation of the stream if it was unoriented before (will not happen if the advise above is followed). The encoding used for the `wchar_t' values is unspecified and the user must not make any assumptions about it. For I/O of `wchar_t' values this means that it is impossible to write these values directly to the stream. This is not what follows from the ISO C locale model either. What happens instead is that the bytes read from or written to the underlying media are first converted into the internal encoding chosen by the implementation for `wchar_t'. The external encoding is determined by the `LC_CTYPE' category of the current locale or by the `ccs' part of the mode specification given to `fopen', `fopen64', `freopen', or `freopen64'. How and when the conversion happens is unspecified and it happens invisible to the user. Since a stream is created in the unoriented state it has at that point no conversion associated with it. The conversion which will be used is determined by the `LC_CTYPE' category selected at the time the stream is oriented. If the locales are changed at the runtime this might produce surprising results unless one pays attention. This is just another good reason to orient the stream explicitly as soon as possible, perhaps with a call to `fwide'. Simple Output by Characters or Lines ==================================== This section describes functions for performing character- and line-oriented output. These narrow streams functions are declared in the header file `stdio.h' and the wide stream functions in `wchar.h'. - Function: int fputc (int C, FILE *STREAM) The `fputc' function converts the character C to type `unsigned char', and writes it to the stream STREAM. `EOF' is returned if a write error occurs; otherwise the character C is returned. - Function: wint_t fputwc (wchar_t WC, FILE *STREAM) The `fputwc' function writes the wide character WC to the stream STREAM. `WEOF' is returned if a write error occurs; otherwise the character WC is returned. - Function: int fputc_unlocked (int C, FILE *STREAM) The `fputc_unlocked' function is equivalent to the `fputc' function except that it does not implicitly lock the stream. - Function: wint_t fputwc_unlocked (wint_t WC, FILE *STREAM) The `fputwc_unlocked' function is equivalent to the `fputwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putc (int C, FILE *STREAM) This is just like `fputc', except that most systems implement it as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putc' is usually the best function to use for writing a single character. - Function: wint_t putwc (wchar_t WC, FILE *STREAM) This is just like `fputwc', except that it can be implement as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putwc' is usually the best function to use for writing a single wide character. - Function: int putc_unlocked (int C, FILE *STREAM) The `putc_unlocked' function is equivalent to the `putc' function except that it does not implicitly lock the stream. - Function: wint_t putwc_unlocked (wchar_t WC, FILE *STREAM) The `putwc_unlocked' function is equivalent to the `putwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putchar (int C) The `putchar' function is equivalent to `putc' with `stdout' as the value of the STREAM argument. - Function: wint_t putwchar (wchar_t WC) The `putwchar' function is equivalent to `putwc' with `stdout' as the value of the STREAM argument. - Function: int putchar_unlocked (int C) The `putchar_unlocked' function is equivalent to the `putchar' function except that it does not implicitly lock the stream. - Function: wint_t putwchar_unlocked (wchar_t WC) The `putwchar_unlocked' function is equivalent to the `putwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputs (const char *S, FILE *STREAM) The function `fputs' writes the string S to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `EOF' if a write error occurs, and otherwise a non-negative value. For example: fputs ("Are ", stdout); fputs ("you ", stdout); fputs ("hungry?\n", stdout); outputs the text `Are you hungry?' followed by a newline. - Function: int fputws (const wchar_t *WS, FILE *STREAM) The function `fputws' writes the wide character string WS to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `WEOF' if a write error occurs, and otherwise a non-negative value. - Function: int fputs_unlocked (const char *S, FILE *STREAM) The `fputs_unlocked' function is equivalent to the `fputs' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputws_unlocked (const wchar_t *WS, FILE *STREAM) The `fputws_unlocked' function is equivalent to the `fputws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int puts (const char *S) The `puts' function writes the string S to the stream `stdout' followed by a newline. The terminating null character of the string is not written. (Note that `fputs' does _not_ write a newline as this function does.) `puts' is the most convenient function for printing simple messages. For example: puts ("This is a message."); outputs the text `This is a message.' followed by a newline. - Function: int putw (int W, FILE *STREAM) This function writes the word W (that is, an `int') to STREAM. It is provided for compatibility with SVID, but we recommend you use `fwrite' instead (*note Block Input/Output::). Character Input =============== This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file `stdio.h' and the wide character functions are declared in `wchar.h'. These functions return an `int' or `wint_t' value (for narrow and wide stream functions respectively) that is either a character of input, or the special value `EOF'/`WEOF' (usually -1). For the narrow stream functions it is important to store the result of these functions in a variable of type `int' instead of `char', even when you plan to use it only as a character. Storing `EOF' in a `char' variable truncates its value to the size of a character, so that it is no longer distinguishable from the valid character `(char) -1'. So always use an `int' for the result of `getc' and friends, and check for `EOF' after the call; once you've verified that the result is not `EOF', you can be sure that it will fit in a `char' variable without loss of information. - Function: int fgetc (FILE *STREAM) This function reads the next character as an `unsigned char' from the stream STREAM and returns its value, converted to an `int'. If an end-of-file condition or read error occurs, `EOF' is returned instead. - Function: wint_t fgetwc (FILE *STREAM) This function reads the next wide character from the stream STREAM and returns its value. If an end-of-file condition or read error occurs, `WEOF' is returned instead. - Function: int fgetc_unlocked (FILE *STREAM) The `fgetc_unlocked' function is equivalent to the `fgetc' function except that it does not implicitly lock the stream. - Function: wint_t fgetwc_unlocked (FILE *STREAM) The `fgetwc_unlocked' function is equivalent to the `fgetwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getc (FILE *STREAM) This is just like `fgetc', except that it is permissible (and typical) for it to be implemented as a macro that evaluates the STREAM argument more than once. `getc' is often highly optimized, so it is usually the best function to use to read a single character. - Function: wint_t getwc (FILE *STREAM) This is just like `fgetwc', except that it is permissible for it to be implemented as a macro that evaluates the STREAM argument more than once. `getwc' can be highly optimized, so it is usually the best function to use to read a single wide character. - Function: int getc_unlocked (FILE *STREAM) The `getc_unlocked' function is equivalent to the `getc' function except that it does not implicitly lock the stream. - Function: wint_t getwc_unlocked (FILE *STREAM) The `getwc_unlocked' function is equivalent to the `getwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getchar (void) The `getchar' function is equivalent to `getc' with `stdin' as the value of the STREAM argument. - Function: wint_t getwchar (void) The `getwchar' function is equivalent to `getwc' with `stdin' as the value of the STREAM argument. - Function: int getchar_unlocked (void) The `getchar_unlocked' function is equivalent to the `getchar' function except that it does not implicitly lock the stream. - Function: wint_t getwchar_unlocked (void) The `getwchar_unlocked' function is equivalent to the `getwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. Here is an example of a function that does input using `fgetc'. It would work just as well using `getc' instead, or using `getchar ()' instead of `fgetc (stdin)'. The code would also work the same for the wide character stream functions. int y_or_n_p (const char *question) { fputs (question, stdout); while (1) { int c, answer; /* Write a space to separate answer from question. */ fputc (' ', stdout); /* Read the first character of the line. This should be the answer character, but might not be. */ c = tolower (fgetc (stdin)); answer = c; /* Discard rest of input line. */ while (c != '\n' && c != EOF) c = fgetc (stdin); /* Obey the answer if it was valid. */ if (answer == 'y') return 1; if (answer == 'n') return 0; /* Answer was invalid: ask for valid answer. */ fputs ("Please answer y or n:", stdout); } } - Function: int getw (FILE *STREAM) This function reads a word (that is, an `int') from STREAM. It's provided for compatibility with SVID. We recommend you use `fread' instead (*note Block Input/Output::). Unlike `getc', any `int' value could be a valid result. `getw' returns `EOF' when it encounters end-of-file or an error, but there is no way to distinguish this from an input word with value -1. Line-Oriented Input =================== Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream. Standard C has functions to do this, but they aren't very safe: null characters and even (for `gets') long lines can confuse them. So the GNU library provides the nonstandard `getline' function that makes it easy to read lines reliably. Another GNU extension, `getdelim', generalizes `getline'. It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character. All these functions are declared in `stdio.h'. - Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM) This function reads an entire line from STREAM, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in `*LINEPTR'. Before calling `getline', you should place in `*LINEPTR' the address of a buffer `*N' bytes long, allocated with `malloc'. If this buffer is long enough to hold the line, `getline' stores the line in this buffer. Otherwise, `getline' makes the buffer bigger using `realloc', storing the new buffer address back in `*LINEPTR' and the increased size back in `*N'. *Note Unconstrained Allocation::. If you set `*LINEPTR' to a null pointer, and `*N' to zero, before the call, then `getline' allocates the initial buffer for you by calling `malloc'. In either case, when `getline' returns, `*LINEPTR' is a `char *' which points to the text of the line. When `getline' is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator. This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable. If an error occurs or end of file is reached without any bytes read, `getline' returns `-1'. - Function: ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM) This function is like `getline' except that the character which tells it to stop reading is not necessarily newline. The argument DELIMITER specifies the delimiter character; `getdelim' keeps reading until it sees that character (or end of file). The text is stored in LINEPTR, including the delimiter character and a terminating null. Like `getline', `getdelim' makes LINEPTR bigger if it isn't big enough. `getline' is in fact implemented in terms of `getdelim', just like this: ssize_t getline (char **lineptr, size_t *n, FILE *stream) { return getdelim (lineptr, n, '\n', stream); } - Function: char * fgets (char *S, int COUNT, FILE *STREAM) The `fgets' function reads characters from the stream STREAM up to and including a newline character and stores them in the string S, adding a null character to mark the end of the string. You must supply COUNT characters worth of space in S, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null character at the end of the string. If the system is already at end of file when you call `fgets', then the contents of the array S are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer S. *Warning:* If the input data has a null character, you can't tell. So don't use `fgets' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. We recommend using `getline' instead of `fgets'. - Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws' function reads wide characters from the stream STREAM up to and including a newline character and stores them in the string WS, adding a null wide character to mark the end of the string. You must supply COUNT wide characters worth of space in WS, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null wide character at the end of the string. If the system is already at end of file when you call `fgetws', then the contents of the array WS are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer WS. *Warning:* If the input data has a null wide character (which are null bytes in the input stream), you can't tell. So don't use `fgetws' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. - Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM) The `fgets_unlocked' function is equivalent to the `fgets' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws_unlocked' function is equivalent to the `fgetws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Deprecated function: char * gets (char *S) The function `gets' reads characters from the stream `stdin' up to the next newline character, and stores them in the string S. The newline character is discarded (note that this differs from the behavior of `fgets', which copies the newline character into the string). If `gets' encounters a read error or end-of-file, it returns a null pointer; otherwise it returns S. *Warning:* The `gets' function is *very dangerous* because it provides no protection against overflowing the string S. The GNU library includes it for compatibility only. You should *always* use `fgets' or `getline' instead. To remind you of this, the linker (if using GNU `ld') will issue a warning whenever you use `gets'. Unreading ========= In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next. Using stream I/O, you can peek ahead at input by first reading it and then "unreading" it (also called "pushing it back" on the stream). Unreading a character makes it available to be input again from the stream, by the next call to `fgetc' or other input function on that stream. What Unreading Means -------------------- Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this: f o o b a r ^ so the next input character will be `b'. If instead of reading `b' you unread the letter `o', you get a situation like this: f o o b a r | o-- ^ so that the next input characters will be `o' and `b'. If you unread `9' instead of `o', you get this situation: f o o b a r | 9-- ^ so that the next input characters will be `9' and `b'. Using `ungetc' To Do Unreading ------------------------------ The function to unread a character is called `ungetc', because it reverses the action of `getc'. - Function: int ungetc (int C, FILE *STREAM) The `ungetc' function pushes back the character C onto the input stream STREAM. So the next input from STREAM will read C before anything else. If C is `EOF', `ungetc' does nothing and just returns `EOF'. This lets you call `ungetc' with the return value of `getc' without needing to check for an error from `getc'. The character that you push back doesn't have to be the same as the last character that was actually read from the stream. In fact, it isn't necessary to actually read any characters from the stream before unreading them with `ungetc'! But that is a strange way to write a program; usually `ungetc' is used only to unread a character that was just read from the same stream. The GNU C library only supports one character of pushback--in other words, it does not work to call `ungetc' twice without doing input in between. Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed. Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected. If a file positioning function (such as `fseek', `fseeko' or `rewind'; *note File Positioning::) is called, any pending pushed-back characters are discarded. Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file. - Function: wint_t ungetwc (wint_t WC, FILE *STREAM) The `ungetwc' function behaves just like `ungetc' just that it pushes back a wide character. Here is an example showing the use of `getc' and `ungetc' to skip over whitespace characters. When this function reaches a non-whitespace character, it unreads that character to be seen again on the next read operation on the stream. #include #include void skip_whitespace (FILE *stream) { int c; do /* No need to check for `EOF' because it is not `isspace', and `ungetc' ignores `EOF'. */ c = getc (stream); while (isspace (c)); ungetc (c, stream); } Block Input/Output ================== This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines. Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program. Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers. These functions are declared in `stdio.h'. - Function: size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function reads up to COUNT objects of size SIZE into the array DATA, from the stream STREAM. It returns the number of objects actually read, which might be less than COUNT if a read error occurs or the end of the file is reached. This function returns a value of zero (and doesn't read anything) if either SIZE or COUNT is zero. If `fread' encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object. Therefore, the stream remains at the actual end of the file. - Function: size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fread_unlocked' function is equivalent to the `fread' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function writes up to COUNT objects of size SIZE from the array DATA, to the stream STREAM. The return value is normally COUNT, if the call succeeds. Any other value indicates some sort of error, such as running out of space. - Function: size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fwrite_unlocked' function is equivalent to the `fwrite' function except that it does not implicitly lock the stream. This function is a GNU extension. Formatted Output ================ The functions described in this section (`printf' and related functions) provide a convenient way to perform formatted output. You call `printf' with a "format string" or "template string" that specifies how to format the values of the remaining arguments. Unless your program is a filter that specifically performs line- or character-oriented processing, using `printf' or one of the other related functions described in this section is usually the easiest and most concise way to perform output. These functions are especially useful for printing error messages, tables of data, and the like. Formatted Output Basics ----------------------- The `printf' function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them. Ordinary characters in the template string are simply written to the output stream as-is, while "conversion specifications" introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example, int pct = 37; char filename[] = "foo.txt"; printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", filename, pct); produces output like Processing of `foo.txt' is 37% finished. Please be patient. This example shows the use of the `%d' conversion to specify that an `int' argument should be printed in decimal notation, the `%s' conversion to specify printing of a string argument, and the `%%' conversion to print a literal `%' character. There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c'). Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number. You can control formatting more precisely by writing "modifiers" between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field. The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables. Output Conversion Syntax ------------------------ This section provides details about the precise syntax of conversion specifications that can appear in a `printf' template string. Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (*note Character Set Handling::) are permitted in a template string. The conversion specifications in a `printf' template string have the general form: % [ PARAM-NO $] FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION For example, in the conversion specifier `%-10.8ld', the `-' is a flag, `10' specifies the field width, the precision is `8', the letter `l' is a type modifier, and `d' specifies the conversion style. (This particular type specifier says to print a `long int' argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.) In more detail, output conversion specifications consist of an initial `%' character followed in sequence by: * An optional specification of the parameter used for this format. Normally the parameters to the `printf' function are assigned to the formats in the order of appearance in the format string. But in some situations (such as message translation) this is not desirable and this extension allows an explicit parameter to be specified. The PARAM-NO part of the format must be an integer in the range of 1 to the maximum number of arguments present to the function call. Some implementations limit this number to a certainly upper bound. The exact limit can be retrieved by the following constant. - Macro: NL_ARGMAX The value of `ARGMAX' is the maximum value allowed for the specification of an positional parameter in a `printf' call. The actual value in effect at runtime can be retrieved by using `sysconf' using the `_SC_NL_ARGMAX' parameter *note Sysconf Definition::. Some system have a quite low limit such as 9 for System V systems. The GNU C library has no real limit. If any of the formats has a specification for the parameter position all of them in the format string shall have one. Otherwise the behavior is undefined. * Zero or more "flag characters" that modify the normal behavior of the conversion specification. * An optional decimal integer specifying the "minimum field width". If the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width. This is a _minimum_ value; if the normal conversion produces more characters than this, the field is _not_ truncated. Normally, the output is right-justified within the field. You can also specify a field width of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the field width. The value must be an `int'. If the value is negative, this means to set the `-' flag (see below) and to use the absolute value as the field width. * An optional "precision" to specify the number of digits to be written for the numeric conversions. If the precision is specified, it consists of a period (`.') followed optionally by a decimal integer (which defaults to zero if omitted). You can also specify a precision of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the precision. The value must be an `int', and is ignored if it is negative. If you specify `*' for both the field width and precision, the field width argument precedes the precision argument. Other C library versions may not recognize this syntax. * An optional "type modifier character", which is used to specify the data type of the corresponding argument if it differs from the default type. (For example, the integer conversions assume a type of `int', but you can specify `h', `l', or `L' for other integer types.) * A character that specifies the conversion to be applied. The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use. With the `-Wformat' option, the GNU C compiler checks calls to `printf' and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a `printf'-style format string. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information. Table of Output Conversions --------------------------- Here is a table summarizing what all the different conversions do: `%d', `%i' Print an integer as a signed decimal number. *Note Integer Conversions::, for details. `%d' and `%i' are synonymous for output, but are different when used with `scanf' for input (*note Table of Input Conversions::). `%o' Print an integer as an unsigned octal number. *Note Integer Conversions::, for details. `%u' Print an integer as an unsigned decimal number. *Note Integer Conversions::, for details. `%x', `%X' Print an integer as an unsigned hexadecimal number. `%x' uses lower-case letters and `%X' uses upper-case. *Note Integer Conversions::, for details. `%f' Print a floating-point number in normal (fixed-point) notation. *Note Floating-Point Conversions::, for details. `%e', `%E' Print a floating-point number in exponential notation. `%e' uses lower-case letters and `%E' uses upper-case. *Note Floating-Point Conversions::, for details. `%g', `%G' Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude. `%g' uses lower-case letters and `%G' uses upper-case. *Note Floating-Point Conversions::, for details. `%a', `%A' Print a floating-point number in a hexadecimal fractional notation which the exponent to base 2 represented in decimal digits. `%a' uses lower-case letters and `%A' uses upper-case. *Note Floating-Point Conversions::, for details. `%c' Print a single character. *Note Other Output Conversions::. `%C' This is an alias for `%lc' which is supported for compatibility with the Unix standard. `%s' Print a string. *Note Other Output Conversions::. `%S' This is an alias for `%ls' which is supported for compatibility with the Unix standard. `%p' Print the value of a pointer. *Note Other Output Conversions::. `%n' Get the number of characters printed so far. *Note Other Output Conversions::. Note that this conversion specification never produces any output. `%m' Print the string corresponding to the value of `errno'. (This is a GNU extension.) *Note Other Output Conversions::. `%%' Print a literal `%' character. *Note Other Output Conversions::. If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful. Integer Conversions ------------------- This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats. The `%d' and `%i' conversion specifications both print an `int' argument as a signed decimal number; while `%o', `%u', and `%x' print the argument as an unsigned octal, decimal, or hexadecimal number (respectively). The `%X' conversion specification is just like `%x' except that it uses the characters `ABCDEF' as digits instead of `abcdef'. The following flags are meaningful: `-' Left-justify the result in the field (instead of the normal right-justification). `+' For the signed `%d' and `%i' conversions, print a plus sign if the value is positive. ` ' For the signed `%d' and `%i' conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' For the `%o' conversion, this forces the leading digit to be `0', as if by increasing the precision. For `%x' or `%X', this prefixes a leading `0x' or `0X' (respectively) to the result. This doesn't do anything useful for the `%d', `%i', or `%u' conversions. Using this flag produces output which can be parsed by the `strtoul' function (*note Parsing of Integers::) and `scanf' with the `%i' conversion (*note Numeric Input Conversions::). `'' Separate the digits into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces. The zeros are placed after any indication of sign or base. This flag is ignored if the `-' flag is also specified, or if a precision is specified. If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced. Without a type modifier, the corresponding argument is treated as an `int' (for the signed conversions `%i' and `%d') or `unsigned int' (for the unsigned conversions `%o', `%u', `%x', and `%X'). Recall that since `printf' and friends are variadic, any `char' and `short' arguments are automatically converted to `int' by the default argument promotions. For arguments of other integer types, you can use these modifiers: `hh' Specifies that the argument is a `signed char' or `unsigned char', as appropriate. A `char' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `char' again. This modifier was introduced in ISO C99. `h' Specifies that the argument is a `short int' or `unsigned short int', as appropriate. A `short' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `short' again. `j' Specifies that the argument is a `intmax_t' or `uintmax_t', as appropriate. This modifier was introduced in ISO C99. `l' Specifies that the argument is a `long int' or `unsigned long int', as appropriate. Two `l' characters is like the `L' modifier, below. If used with `%c' or `%s' the corresponding parameter is considered as a wide character or wide character string respectively. This use of `l' was introduced in Amendment 1 to ISO C90. `L' `ll' `q' Specifies that the argument is a `long long int'. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as `long int'.) The `q' modifier is another name for the same thing, which comes from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'. `t' Specifies that the argument is a `ptrdiff_t'. This modifier was introduced in ISO C99. `z' `Z' Specifies that the argument is a `size_t'. `z' was introduced in ISO C99. `Z' is a GNU extension predating this addition and should not be used in new code. Here is an example. Using the template string: "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n" to print numbers using the different options for the `%d' conversion gives results like: | 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000| In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified. Here are some more examples showing how unsigned integers print under various format options, using the template string: "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n" | 0| 0| 0| 0| 0| 0| 0| 00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0| Floating-Point Conversions -------------------------- This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions. The `%f' conversion prints its argument in fixed-point notation, producing output of the form [`-']DDD`.'DDD, where the number of digits following the decimal point is controlled by the precision you specify. The `%e' conversion prints its argument in exponential notation, producing output of the form [`-']D`.'DDD`e'[`+'|`-']DD. Again, the number of digits following the decimal point is controlled by the precision. The exponent always contains at least two digits. The `%E' conversion is similar but the exponent is marked with the letter `E' instead of `e'. The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. A precision of `0', is taken as 1. is Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit. The `%a' and `%A' conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines. The numbers are represented is the form [`-']`0x'H`.'HHH`p'[`+'|`-']DD. At the left of the decimal-point character exactly one digit is print. This character is only `0' if the number is denormalized. Otherwise the value is unspecified; it is implementation dependent how many bits are used. The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision. If the precision is zero it is determined to be large enough to provide an exact representation of the number (or it is large enough to distinguish two adjacent values if the `FLT_RADIX' is not a power of 2, *note Floating Point Parameters::). For the `%a' conversion lower-case characters are used to represent the hexadecimal number and the prefix and exponent sign are printed as `0x' and `p' respectively. Otherwise upper-case characters are used and `0X' and `P' are used for the representation of prefix and exponent string. The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly. If the value to be printed represents infinity or a NaN, the output is [`-']`inf' or `nan' respectively if the conversion specifier is `%a', `%e', `%f', or `%g' and it is [`-']`INF' or `NAN' respectively if the conversion is `%A', `%E', or `%G'. The following flags can be used to modify the behavior: `-' Left-justify the result in the field. Normally the result is right-justified. `+' Always include a plus or minus sign in the result. ` ' If the result doesn't start with a plus or minus sign, prefix it with a space instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' Specifies that the result should always include a decimal point, even if no digits follow it. For the `%g' and `%G' conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed. `'' Separate the digits of the integer part of the result into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the `-' flag is also specified. The precision specifies how many digits follow the decimal-point character for the `%f', `%e', and `%E' conversions. For these conversions, the default precision is `6'. If the precision is explicitly `0', this suppresses the decimal point character entirely. For the `%g' and `%G' conversions, the precision specifies how many significant digits to print. Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is `0' or not specified for `%g' or `%G', it is treated like a value of `1'. If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits. Without a type modifier, the floating-point conversions use an argument of type `double'. (By the default argument promotions, any `float' arguments are automatically converted to `double'.) The following type modifier is supported: `L' An uppercase `L' specifies that the argument is a `long double'. Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string: "|%13.4a|%13.4f|%13.4e|%13.4g|\n" Here is the output: | 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05| Notice how the `%g' conversion drops trailing zeros. Other Output Conversions ------------------------ This section describes miscellaneous conversions for `printf'. The `%c' conversion prints a single character. In case there is no `l' modifier the `int' argument is first converted to an `unsigned char'. Then, if used in a wide stream function, the character is converted into the corresponding wide character. The `-' flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example: printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); prints `hello'. If there is a `l' modifier present the argument is expected to be of type `wint_t'. If used in a multibyte function the wide character is converted into a multibyte character before being added to the output. In this case more than one output byte can be produced. The `%s' conversion prints a string. If no `l' modifier is present the corresponding argument must be of type `char *' (or `const char *'). If used in a wide stream function the string is first converted in a wide character string. A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream. The `-' flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example: printf ("%3s%-6s", "no", "where"); prints ` nowhere '. If there is a `l' modifier present the argument is expected to be of type `wchar_t' (or `const wchar_t *'). If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally. The `%m' conversion prints the string corresponding to the error code in `errno'. *Note Error Messages::. Thus: fprintf (stderr, "can't open `%s': %m\n", filename); is equivalent to: fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); The `%m' conversion is a GNU C library extension. The `%p' conversion prints a pointer value. The corresponding argument must be of type `void *'. In practice, you can use any type of pointer. In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.) For example: printf ("%p", "testing"); prints `0x' followed by a hexadecimal number--the address of the string constant `"testing"'. It does not print the word `testing'. You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined. The `%n' conversion is unlike any of the other output conversions. It uses an argument which must be a pointer to an `int', but instead of printing anything it stores the number of characters printed so far by this call at that location. The `h' and `l' type modifiers are permitted to specify that the argument is of type `short int *' or `long int *' instead of `int *', but no flags, field width, or precision are permitted. For example, int nchar; printf ("%d %s%n\n", 3, "bears", &nchar); prints: 3 bears and sets `nchar' to `7', because `3 bears' is seven characters. The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted. Formatted Output Functions -------------------------- This section describes how to call `printf' and related functions. Prototypes for these functions are in the header file `stdio.h'. Because these functions take a variable number of arguments, you _must_ declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include `stdio.h'. - Function: int printf (const char *TEMPLATE, ...) The `printf' function prints the optional arguments under the control of the template string TEMPLATE to the stream `stdout'. It returns the number of characters printed, or a negative value if there was an output error. - Function: int wprintf (const wchar_t *TEMPLATE, ...) The `wprintf' function prints the optional arguments under the control of the wide template string TEMPLATE to the stream `stdout'. It returns the number of wide characters printed, or a negative value if there was an output error. - Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `printf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wprintf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int sprintf (char *S, const char *TEMPLATE, ...) This is like `printf', except that the output is stored in the character array S instead of written to a stream. A null character is written to mark the end of the string. The `sprintf' function returns the number of characters stored in the array S, not including the terminating null character. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to be printed under control of the `%s' conversion. *Note Copying and Concatenation::. *Warning:* The `sprintf' function can be *dangerous* because it can potentially output more characters than can fit in the allocation size of the string S. Remember that the field width given in a conversion specification is only a _minimum_ value. To avoid this problem, you can use `snprintf' or `asprintf', described below. - Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...) This is like `wprintf', except that the output is stored in the wide character array WS instead of written to a stream. A null wide character is written to mark the end of the string. The SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE wide characters for the string WS. The return value is the number of characters generated for the given input, excluding the trailing null. If not all output fits into the provided buffer a negative value is returned. You should try again with a bigger output string. _Note:_ this is different from how `snprintf' handles this situation. Note that the corresponding narrow stream function takes fewer parameters. `swprintf' in fact corresponds to the `snprintf' function. Since the `sprintf' function can be dangerous and should be avoided the ISO C committee refused to make the same mistake again and decided to not define an function exactly corresponding to `sprintf'. - Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...) The `snprintf' function is similar to `sprintf', except that the SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE characters for the string S. The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to SIZE, not all characters from the result have been stored in S. You should try again with a bigger output string. Here is an example of doing this: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { /* Guess we need no more than 100 chars of space. */ int size = 100; char *buffer = (char *) xmalloc (size); int nchars; if (buffer == NULL) return NULL; /* Try to print in the allocated space. */ nchars = snprintf (buffer, size, "value of %s is %s", name, value); if (nchars >= size) { /* Reallocate buffer now that we know how much space is needed. */ buffer = (char *) xrealloc (buffer, nchars + 1); if (buffer != NULL) /* Try again. */ snprintf (buffer, size, "value of %s is %s", name, value); } /* The last call worked, return the string. */ return buffer; } In practice, it is often easier just to use `asprintf', below. *Attention:* In versions of the GNU C library prior to 2.1 the return value is the number of characters stored, not including the terminating null; unless there was not enough space in S to store the result in which case `-1' is returned. This was changed in order to comply with the ISO C99 standard. Dynamically Allocating Formatted Output --------------------------------------- The functions in this section do formatted output and place the results in dynamically allocated memory. - Function: int asprintf (char **PTR, const char *TEMPLATE, ...) This function is similar to `sprintf', except that it dynamically allocates a string (as with `malloc'; *note Unconstrained Allocation::) to hold the output, instead of putting the output in a buffer you allocate in advance. The PTR argument should be the address of a `char *' object, and `asprintf' stores a pointer to the newly allocated string at that location. The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated. Here is how to use `asprintf' to get the same result as the `snprintf' example, but more easily: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { char *result; if (asprintf (&result, "value of %s is %s", name, value) < 0) return NULL; return result; } - Function: int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...) This function is similar to `asprintf', except that it uses the obstack OBSTACK to allocate the space. *Note Obstacks::. The characters are written onto the end of the current object. To get at them, you must finish the object with `obstack_finish' (*note Growing Objects::). Variable Arguments Output Functions ----------------------------------- The functions `vprintf' and friends are provided so that you can define your own variadic `printf'-like functions that make use of the same internals as the built-in formatted output functions. The most natural way to define such functions would be to use a language construct to say, "Call `printf' and pass this template plus all of my arguments after the first five." But there is no way to do this in C, and it would be hard to provide a way, since at the C language level there is no way to tell how many arguments your function received. Since that method is impossible, we provide alternative functions, the `vprintf' series, which lets you pass a `va_list' to describe "all of my arguments after the first five." When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example: #define myprintf(a, b, c, d, e, rest...) \ printf (mytemplate , ## rest...) *Note Macros with Variable Numbers of Arguments: (gcc.info)Macro Varargs, for details. But this is limited to macros, and does not apply to real functions at all. Before calling `vprintf' or the other functions listed in this section, you _must_ call `va_start' (*note Variadic Functions::) to initialize a pointer to the variable arguments. Then you can call `va_arg' to fetch the arguments that you want to handle yourself. This advances the pointer past those arguments. Once your `va_list' pointer is pointing at the argument of your choice, you are ready to call `vprintf'. That argument and all subsequent arguments that were passed to your function are used by `vprintf' along with the template that you specified separately. In some other systems, the `va_list' pointer may become invalid after the call to `vprintf', so you must not use `va_arg' after you call `vprintf'. Instead, you should call `va_end' to retire the pointer from service. However, you can safely call `va_start' on another pointer variable and begin fetching the arguments again through that pointer. Calling `vprintf' does not destroy the argument list of your function, merely the particular pointer that you passed to it. GNU C does not have such restrictions. You can safely continue to fetch arguments from a `va_list' pointer after passing it to `vprintf', and `va_end' is a no-op. (Note, however, that subsequent `va_arg' calls will fetch the same arguments which `vprintf' previously used.) Prototypes for these functions are declared in `stdio.h'. - Function: int vprintf (const char *TEMPLATE, va_list AP) This function is similar to `printf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vwprintf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wprintf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsprintf (char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP) This is the equivalent of `snprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vasprintf (char **PTR, const char *TEMPLATE, va_list AP) The `vasprintf' function is the equivalent of `asprintf' with the variable argument list specified directly as for `vprintf'. - Function: int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP) The `obstack_vprintf' function is the equivalent of `obstack_printf' with the variable argument list specified directly as for `vprintf'. Here's an example showing how you might use `vfprintf'. This is a function that prints error messages to the stream `stderr', along with a prefix indicating the name of the program (*note Error Messages::, for a description of `program_invocation_short_name'). #include #include void eprintf (const char *template, ...) { va_list ap; extern char *program_invocation_short_name; fprintf (stderr, "%s: ", program_invocation_short_name); va_start (ap, template); vfprintf (stderr, template, ap); va_end (ap); } You could call `eprintf' like this: eprintf ("file `%s' does not exist\n", filename); In GNU C, there is a special construct you can use to let the compiler know that a function uses a `printf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For example, take this declaration of `eprintf': void eprintf (const char *template, ...) __attribute__ ((format (printf, 1, 2))); This tells the compiler that `eprintf' uses a format string like `printf' (as opposed to `scanf'; *note Formatted Input::); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information. Parsing a Template String ------------------------- You can use the function `parse_printf_format' to obtain information about the number and types of arguments that are expected by a given template string. This function permits interpreters that provide interfaces to `printf' to avoid passing along invalid arguments from the user's program, which could cause a crash. All the symbols described in this section are declared in the header file `printf.h'. - Function: size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES) This function returns information about the number and types of arguments expected by the `printf' template string TEMPLATE. The information is stored in the array ARGTYPES; each element of this array describes one argument. This information is encoded using the various `PA_' macros, listed below. The argument N specifies the number of elements in the array ARGTYPES. This is the maximum number of elements that `parse_printf_format' will try to write. `parse_printf_format' returns the total number of arguments required by TEMPLATE. If this number is greater than N, then the information returned describes only the first N arguments. If you want information about additional arguments, allocate a bigger array and call `parse_printf_format' again. The argument types are encoded as a combination of a basic type and modifier flag bits. - Macro: int PA_FLAG_MASK This macro is a bitmask for the type modifier flag bits. You can write the expression `(argtypes[i] & PA_FLAG_MASK)' to extract just the flag bits for an argument, or `(argtypes[i] & ~PA_FLAG_MASK)' to extract just the basic type code. Here are symbolic constants that represent the basic types; they stand for integer values. `PA_INT' This specifies that the base type is `int'. `PA_CHAR' This specifies that the base type is `int', cast to `char'. `PA_STRING' This specifies that the base type is `char *', a null-terminated string. `PA_POINTER' This specifies that the base type is `void *', an arbitrary pointer. `PA_FLOAT' This specifies that the base type is `float'. `PA_DOUBLE' This specifies that the base type is `double'. `PA_LAST' You can define additional base types for your own programs as offsets from `PA_LAST'. For example, if you have data types `foo' and `bar' with their own specialized `printf' conversions, you could define encodings for these types as: #define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1) Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or. `PA_FLAG_PTR' If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value. For example, `PA_INT|PA_FLAG_PTR' represents the type `int *'. `PA_FLAG_SHORT' If this bit is set, it indicates that the base type is modified with `short'. (This corresponds to the `h' type modifier.) `PA_FLAG_LONG' If this bit is set, it indicates that the base type is modified with `long'. (This corresponds to the `l' type modifier.) `PA_FLAG_LONG_LONG' If this bit is set, it indicates that the base type is modified with `long long'. (This corresponds to the `L' type modifier.) `PA_FLAG_LONG_DOUBLE' This is a synonym for `PA_FLAG_LONG_LONG', used by convention with a base type of `PA_DOUBLE' to indicate a type of `long double'. For an example of using these facilities, see *Note Example of Parsing::. Example of Parsing a Template String ------------------------------------ Here is an example of decoding argument types for a format string. We assume this is part of an interpreter which contains arguments of type `NUMBER', `CHAR', `STRING' and `STRUCTURE' (and perhaps others which are not valid here). /* Test whether the NARGS specified objects in the vector ARGS are valid for the format string FORMAT: if so, return 1. If not, return 0 after printing an error message. */ int validate_args (char *format, int nargs, OBJECT *args) { int *argtypes; int nwanted; /* Get the information about the arguments. Each conversion specification must be at least two characters long, so there cannot be more specifications than half the length of the string. */ argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int)); nwanted = parse_printf_format (string, nelts, argtypes); /* Check the number of arguments. */ if (nwanted > nargs) { error ("too few arguments (at least %d required)", nwanted); return 0; } /* Check the C type wanted for each argument and see if the object given is suitable. */ for (i = 0; i < nwanted; i++) { int wanted; if (argtypes[i] & PA_FLAG_PTR) wanted = STRUCTURE; else switch (argtypes[i] & ~PA_FLAG_MASK) { case PA_INT: case PA_FLOAT: case PA_DOUBLE: wanted = NUMBER; break; case PA_CHAR: wanted = CHAR; break; case PA_STRING: wanted = STRING; break; case PA_POINTER: wanted = STRUCTURE; break; } if (TYPE (args[i]) != wanted) { error ("type mismatch for arg number %d", i); return 0; } } return 1; } Customizing `printf' ==================== The GNU C library lets you define your own custom conversion specifiers for `printf' template strings, to teach `printf' clever ways to print the important data structures of your program. The way you do this is by registering the conversion with the function `register_printf_function'; see *Note Registering New Conversions::. One of the arguments you pass to this function is a pointer to a handler function that produces the actual output; see *Note Defining the Output Handler::, for information on how to write this function. You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. *Note Parsing a Template String::, for information about this. The facilities of this section are declared in the header file `printf.h'. *Portability Note:* The ability to extend the syntax of `printf' template strings is a GNU extension. ISO standard C has nothing similar. Registering New Conversions --------------------------- The function to register a new output conversion is `register_printf_function', declared in `printf.h'. - Function: int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION) This function defines the conversion specifier character SPEC. Thus, if SPEC is `'Y'', it defines the conversion `%Y'. You can redefine the built-in conversions like `%s', but flag characters like `#' and type modifiers like `l' can never be used as conversions; calling `register_printf_function' for those characters has no effect. It is advisable not to use lowercase letters, since the ISO C standard warns that additional lowercase letters may be standardized in future editions of the standard. The HANDLER-FUNCTION is the function called by `printf' and friends when this conversion appears in a template string. *Note Defining the Output Handler::, for information about how to define a function to pass as this argument. If you specify a null pointer, any existing handler function for SPEC is removed. The ARGINFO-FUNCTION is the function called by `parse_printf_format' when this conversion appears in a template string. *Note Parsing a Template String::, for information about this. *Attention:* In the GNU C library versions before 2.0 the ARGINFO-FUNCTION function did not need to be installed unless the user used the `parse_printf_format' function. This has changed. Now a call to any of the `printf' functions will call this function when this format specifier appears in the format string. The return value is `0' on success, and `-1' on failure (which occurs if SPEC is out of range). You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this. Conversion Specifier Options ---------------------------- If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template. Both the HANDLER-FUNCTION and ARGINFO-FUNCTION accept an argument that points to a `struct printf_info', which contains information about the options appearing in an instance of the conversion specifier. This data type is declared in the header file `printf.h'. - Type: struct printf_info This structure is used to pass information about the options appearing in an instance of a conversion specifier in a `printf' template string to the handler and arginfo functions for that specifier. It contains the following members: `int prec' This is the precision specified. The value is `-1' if no precision was specified. If the precision was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `int width' This is the minimum field width specified. The value is `0' if no width was specified. If the field width was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `wchar_t spec' This is the conversion specifier character specified. It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called. `unsigned int is_long_double' This is a boolean that is true if the `L', `ll', or `q' type modifier was specified. For integer conversions, this indicates `long long int', as opposed to `long double' for floating point conversions. `unsigned int is_char' This is a boolean that is true if the `hh' type modifier was specified. `unsigned int is_short' This is a boolean that is true if the `h' type modifier was specified. `unsigned int is_long' This is a boolean that is true if the `l' type modifier was specified. `unsigned int alt' This is a boolean that is true if the `#' flag was specified. `unsigned int space' This is a boolean that is true if the ` ' flag was specified. `unsigned int left' This is a boolean that is true if the `-' flag was specified. `unsigned int showsign' This is a boolean that is true if the `+' flag was specified. `unsigned int group' This is a boolean that is true if the `'' flag was specified. `unsigned int extra' This flag has a special meaning depending on the context. It could be used freely by the user-defined handlers but when called from the `printf' function this variable always contains the value `0'. `unsigned int wide' This flag is set if the stream is wide oriented. `wchar_t pad' This is the character to use for padding the output to the minimum field width. The value is `'0'' if the `0' flag was specified, and `' '' otherwise. Defining the Output Handler --------------------------- Now let's look at how to define the handler and arginfo functions which are passed as arguments to `register_printf_function'. *Compatibility Note:* The interface changed in GNU libc version 2.0. Previously the third argument was of type `va_list *'. You should define your handler functions with a prototype like: int FUNCTION (FILE *stream, const struct printf_info *info, const void *const *args) The STREAM argument passed to the handler function is the stream to which it should write output. The INFO argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. *Note Conversion Specifier Options::, for a description of this data structure. The ARGS is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user. Your handler function should return a value just like `printf' does: it should return the number of characters it has written, or a negative value to indicate an error. - Data Type: printf_function This is the data type that a handler function should have. If you are going to use `parse_printf_format' in your application, you must also define a function to pass as the ARGINFO-FUNCTION argument for each new conversion you install with `register_printf_function'. You have to define these functions with a prototype like: int FUNCTION (const struct printf_info *info, size_t n, int *argtypes) The return value from the function should be the number of arguments the conversion expects. The function should also fill in no more than N elements of the ARGTYPES array with information about the types of each of these arguments. This information is encoded using the various `PA_' macros. (You will notice that this is the same calling convention `parse_printf_format' itself uses.) - Data Type: printf_arginfo_function This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier. `printf' Extension Example -------------------------- Here is an example showing how to define a `printf' handler function. This program defines a data structure called a `Widget' and defines the `%W' conversion to print information about `Widget *' arguments, including the pointer value and the name stored in the data structure. The `%W' conversion supports the minimum field width and left-justification options, but ignores everything else. #include #include #include typedef struct { char *name; } Widget; int print_widget (FILE *stream, const struct printf_info *info, const void *const *args) { const Widget *w; char *buffer; int len; /* Format the output into a string. */ w = *((const Widget **) (args[0])); len = asprintf (&buffer, "", w, w->name); if (len == -1) return -1; /* Pad to the minimum field width and print to the stream. */ len = fprintf (stream, "%*s", (info->left ? -info->width : info->width), buffer); /* Clean up and return. */ free (buffer); return len; } int print_widget_arginfo (const struct printf_info *info, size_t n, int *argtypes) { /* We always take exactly one argument and this is a pointer to the structure.. */ if (n > 0) argtypes[0] = PA_POINTER; return 1; } int main (void) { /* Make a widget to print. */ Widget mywidget; mywidget.name = "mywidget"; /* Register the print function for widgets. */ register_printf_function ('W', print_widget, print_widget_arginfo); /* Now print the widget. */ printf ("|%W|\n", &mywidget); printf ("|%35W|\n", &mywidget); printf ("|%-35W|\n", &mywidget); return 0; } The output produced by this program looks like: || | | | | Predefined `printf' Handlers ---------------------------- The GNU libc also contains a concrete and useful application of the `printf' handler extension. There are two functions available which implement a special way to print floating-point numbers. - Function: int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS) Print a given floating point number as for the format `%f' except that there is a postfix character indicating the divisor for the number to make this less than 1000. There are two possible divisors: powers of 1024 or powers of 1000. Which one is used depends on the format character specified while registered this handler. If the character is of lower case, 1024 is used. For upper case characters, 1000 is used. The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is: +------+--------------+--------+--------+---------------+ |low|Multiplier|From|Upper|Multiplier| +------+--------------+--------+--------+---------------+ |' '|1||' '|1| +------+--------------+--------+--------+---------------+ |k|2^10 (1024)|kilo|K|10^3 (1000)| +------+--------------+--------+--------+---------------+ |m|2^20|mega|M|10^6| +------+--------------+--------+--------+---------------+ |g|2^30|giga|G|10^9| +------+--------------+--------+--------+---------------+ |t|2^40|tera|T|10^12| +------+--------------+--------+--------+---------------+ |p|2^50|peta|P|10^15| +------+--------------+--------+--------+---------------+ |e|2^60|exa|E|10^18| +------+--------------+--------+--------+---------------+ |z|2^70|zetta|Z|10^21| +------+--------------+--------+--------+---------------+ |y|2^80|yotta|Y|10^24| +------+--------------+--------+--------+---------------+ The default precision is 3, i.e., 1024 is printed with a lower-case format character as if it were `%.3fk' and will yield `1.000k'. Due to the requirements of `register_printf_function' we must also provide the function which returns information about the arguments. - Function: int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES) This function will return in ARGTYPES the information about the used parameters in the way the `vfprintf' implementation expects it. The format always takes one argument. To use these functions both functions must be registered with a call like register_printf_function ('B', printf_size, printf_size_info); Here we register the functions to print numbers as powers of 1000 since the format character `'B'' is an upper-case character. If we would additionally use `'b'' in a line like register_printf_function ('b', printf_size, printf_size_info); we could also print using a power of 1024. Please note that all that is different in these two lines is the format specifier. The `printf_size' function knows about the difference between lower and upper case format specifiers. The use of `'B'' and `'b'' is no coincidence. Rather it is the preferred way to use this functionality since it is available on some other systems which also use format specifiers. Formatted Input =============== The functions described in this section (`scanf' and related functions) provide facilities for formatted input analogous to the formatted output facilities. These functions provide a mechanism for reading arbitrary values under the control of a "format string" or "template string". Formatted Input Basics ---------------------- Calls to `scanf' are superficially similar to calls to `printf' in that arbitrary arguments are read under the control of a template string. While the syntax of the conversion specifications in the template is very similar to that for `printf', the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting. For example, most `scanf' conversions skip over any amount of "white space" (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions. Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream. Another area of difference between `scanf' and `printf' is that you must remember to supply pointers rather than immediate values as the optional arguments to `scanf'; the values that are read are stored in the objects that the pointers point to. Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to `scanf', you might want to double-check this. When a "matching failure" occurs, `scanf' returns immediately, leaving the first non-matching character as the next character to be read from the stream. The normal return value from `scanf' is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read. The `scanf' function is typically used for things like reading in the contents of tables. For example, here is a function that uses `scanf' to initialize an array of `double': void readarray (double *array, int n) { int i; for (i=0; i scanf ("%a[a-zA-Z0-9] = %a[^\n]\n", &variable, &value)) { invalid_input_error (); return 0; } ... } Other Input Conversions ----------------------- This section describes the miscellaneous input conversions. The `%p' conversion is used to read a pointer value. It recognizes the same syntax used by the `%p' output conversion for `printf' (*note Other Output Conversions::); that is, a hexadecimal number just as the `%x' conversion accepts. The corresponding argument should be of type `void **'; that is, the address of a place to store a pointer. The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in. The `%n' conversion produces the number of characters read so far by this call. The corresponding argument should be of type `int *'. This conversion works in the same way as the `%n' conversion for `printf'; see *Note Other Output Conversions::, for an example. The `%n' conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the `%n' follows the locus of a matching failure, then no value is stored for it since `scanf' returns before processing the `%n'. If you store `-1' in that argument slot before calling `scanf', the presence of `-1' after `scanf' indicates an error occurred before the `%n' was reached. Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified. Formatted Input Functions ------------------------- Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'. - Function: int scanf (const char *TEMPLATE, ...) The `scanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `EOF' is returned. - Function: int wscanf (const wchar_t *TEMPLATE, ...) The `wscanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `WEOF' is returned. - Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `scanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wscanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int sscanf (const char *S, const char *TEMPLATE, ...) This is like `scanf', except that the characters are taken from the null-terminated string S instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion. - Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...) This is like `wscanf', except that the characters are taken from the null-terminated string WS instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if WS is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion. Variable Arguments Input Functions ---------------------------------- The functions `vscanf' and friends are provided so that you can define your own variadic `scanf'-like functions that make use of the same internals as the built-in formatted output functions. These functions are analogous to the `vprintf' series of output functions. *Note Variable Arguments Output::, for important information on how to use them. *Portability Note:* The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions. - Function: int vscanf (const char *TEMPLATE, va_list AP) This function is similar to `scanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wscanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwscanf' with the variable argument list specified directly as for `vwscanf'. - Function: int vsscanf (const char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swscanf' with the variable argument list specified directly as for `vwscanf'. In GNU C, there is a special construct you can use to let the compiler know that a function uses a `scanf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For details, *Note Declaring Attributes of Functions: (gcc.info)Function Attributes. End-Of-File and Errors ====================== Many of the functions described in this chapter return the value of the macro `EOF' to indicate unsuccessful completion of the operation. Since `EOF' is used to report both end of file and random errors, it's often better to use the `feof' function to check explicitly for end of file and `ferror' to check for errors. These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream. - Macro: int EOF This macro is an integer value that is returned by a number of narrow stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `EOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `stdio.h'. - Macro: int WEOF This macro is an integer value that is returned by a number of wide stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `WEOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `wchar.h'. - Function: int feof (FILE *STREAM) The `feof' function returns nonzero if and only if the end-of-file indicator for the stream STREAM is set. This symbol is declared in `stdio.h'. - Function: int feof_unlocked (FILE *STREAM) The `feof_unlocked' function is equivalent to the `feof' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. - Function: int ferror (FILE *STREAM) The `ferror' function returns nonzero if and only if the error indicator for the stream STREAM is set, indicating that an error has occurred on a previous operation on the stream. This symbol is declared in `stdio.h'. - Function: int ferror_unlocked (FILE *STREAM) The `ferror_unlocked' function is equivalent to the `ferror' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. In addition to setting the error indicator associated with the stream, the functions that operate on streams also set `errno' in the same way as the corresponding low-level functions that operate on file descriptors. For example, all of the functions that perform output to a stream--such as `fputc', `printf', and `fflush'--are implemented in terms of `write', and all of the `errno' error conditions defined for `write' are meaningful for these functions. For more information about the descriptor-level I/O functions, see *Note Low-Level I/O::. Recovering from errors ====================== You may explicitly clear the error and EOF flags with the `clearerr' function. - Function: void clearerr (FILE *STREAM) This function clears the end-of-file and error indicators for the stream STREAM. The file positioning functions (*note File Positioning::) also clear the end-of-file indicator for the stream. - Function: void clearerr_unlocked (FILE *STREAM) The `clearerr_unlocked' function is equivalent to the `clearerr' function except that it does not implicitly lock the stream. This function is a GNU extension. Note that it is _not_ correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data. A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying. Most errors that can happen are not recoverable -- a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic. One important exception is `EINTR' (*note Interrupted Primitives::). Many stream I/O implementations will treat it as an ordinary error, which can be quite inconvenient. You can avoid this hassle by installing all signals with the `SA_RESTART' flag. For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable. Text and Binary Streams ======================= The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems. When you open a stream, you can specify either a "text stream" or a "binary stream". You indicate that you want a binary stream by specifying the `b' modifier in the OPENTYPE argument to `fopen'; see *Note Opening Streams::. Without this option, `fopen' opens the file as a text stream. Text and binary streams differ in several ways: * The data read from a text stream is divided into "lines" which are terminated by newline (`'\n'') characters, while a binary stream is simply a long series of characters. A text stream might on some systems fail to handle lines more than 254 characters long (including the terminating newline character). * On some systems, text files can contain only printing characters, horizontal tab characters, and newlines, and so text streams may not support other characters. However, binary streams can handle any character value. * Space characters that are written immediately preceding a newline character in a text stream may disappear when the file is read in again. * More generally, there need not be a one-to-one mapping between characters that are read from or written to a text stream, and the characters in the actual file. Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream. In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have. File Positioning ================ The "file position" of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. *Note File Position::. During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as "random-access" files. You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'. - Function: long int ftell (FILE *STREAM) This function returns the current file position of the stream STREAM. This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a `long int', and possibly for other reasons as well. If a failure occurs, a value of `-1' is returned. - Function: off_t ftello (FILE *STREAM) The `ftello' function is similar to `ftell', except that it returns a value of type `off_t'. Systems which support this type use it to describe all file positions, unlike the POSIX specification which uses a long int. The two are not necessarily the same size. Therefore, using ftell can lead to problems if the implementation is written on top of a POSIX compliant low-level I/O implementation, and using `ftello' is preferable whenever it is available. If this function fails it returns `(off_t) -1'. This can happen due to missing support for file positioning or internal errors. Otherwise the return value is the current file position. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `ftello64'. I.e., the LFS interface transparently replaces the old interface. - Function: off64_t ftello64 (FILE *STREAM) This function is similar to `ftello' with the only difference that the return value is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `ftello' and so transparently replaces the old interface. - Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE) The `fseek' function is used to change the file position of the stream STREAM. The value of WHENCE must be one of the constants `SEEK_SET', `SEEK_CUR', or `SEEK_END', to indicate whether the OFFSET is relative to the beginning of the file, the current file position, or the end of the file, respectively. This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure. A successful call also clears the end-of-file indicator of STREAM and discards any characters that were "pushed back" by the use of `ungetc'. `fseek' either flushes any buffered output before setting the file position or else remembers it so it will be written later in its proper place in the file. - Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE) This function is similar to `fseek' but it corrects a problem with `fseek' in a system with POSIX types. Using a value of type `long int' for the offset is not compatible with POSIX. `fseeko' uses the correct type `off_t' for the OFFSET parameter. For this reason it is a good idea to prefer `ftello' whenever it is available since its functionality is (if different at all) closer the underlying definition. The functionality and return value is the same as for `fseek'. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `fseeko64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE) This function is similar to `fseeko' with the only difference that the OFFSET parameter is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fseeko' and so transparently replaces the old interface. *Portability Note:* In non-POSIX systems, `ftell', `ftello', `fseek' and `fseeko' might work reliably only on binary streams. *Note Binary Streams::. The following symbolic constants are defined for use as the WHENCE argument to `fseek'. They are also used with the `lseek' function (*note I/O Primitives::) and to specify offsets for file locks (*note Control Operations::). - Macro: int SEEK_SET This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the beginning of the file. - Macro: int SEEK_CUR This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the current file position. - Macro: int SEEK_END This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the end of the file. - Function: void rewind (FILE *STREAM) The `rewind' function positions the stream STREAM at the beginning of the file. It is equivalent to calling `fseek' or `fseeko' on the STREAM with an OFFSET argument of `0L' and a WHENCE argument of `SEEK_SET', except that the return value is discarded and the error indicator for the stream is reset. These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'. `L_SET' An alias for `SEEK_SET'. `L_INCR' An alias for `SEEK_CUR'. `L_XTND' An alias for `SEEK_END'. Portable File-Position Functions ================================ On the GNU system, the file position is truly a character count. You can specify any character count value as an argument to `fseek' or `fseeko' and get reliable results for any random access file. However, some ISO C systems do not represent file positions in this way. On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record. As a consequence, if you want your programs to be portable to these systems, you must observe certain rules: * The value returned from `ftell' on a text stream has no predictable relationship to the number of characters you have read so far. The only thing you can rely on is that you can use it subsequently as the OFFSET argument to `fseek' or `fseeko' to move back to the same file position. * In a call to `fseek' or `fseeko' on a text stream, either the OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the OFFSET must be the result of an earlier call to `ftell' on the same stream. * The value of the file position indicator of a text stream is undefined while there are characters that have been pushed back with `ungetc' that haven't been read or discarded. *Note Unreading::. But even if you observe these rules, you may still have trouble for long files, because `ftell' and `fseek' use a `long int' value to represent the file position. This type may not have room to encode all the file positions in a large file. Using the `ftello' and `fseeko' functions might help here since the `off_t' type is expected to be able to hold all file position values but this still does not help to handle additional information which must be associated with a file position. So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions `fgetpos' and `fsetpos' instead. These functions represent the file position using the data type `fpos_t', whose internal representation varies from system to system. These symbols are declared in the header file `stdio.h'. - Data Type: fpos_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos' and `fsetpos'. In the GNU system, `fpos_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this type is in fact equivalent to `fpos64_t' since the LFS interface transparently replaces the old interface. - Data Type: fpos64_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos64' and `fsetpos64'. In the GNU system, `fpos64_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. - Function: int fgetpos (FILE *STREAM, fpos_t *POSITION) This function stores the value of the file position indicator for the stream STREAM in the `fpos_t' object pointed to by POSITION. If successful, `fgetpos' returns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fgetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION) This function is similar to `fgetpos' but the file position is returned in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fgetpos' and so transparently replaces the old interface. - Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION) This function sets the file position indicator for the stream STREAM to the position POSITION, which must have been set by a previous call to `fgetpos' on the same stream. If successful, `fsetpos' clears the end-of-file indicator on the stream, discards any characters that were "pushed back" by the use of `ungetc', and returns a value of zero. Otherwise, `fsetpos' returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fsetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION) This function is similar to `fsetpos' but the file position used for positioning is provided in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fsetpos' and so transparently replaces the old interface. Stream Buffering ================ Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called "buffering". If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior. This section deals only with controlling when characters are transmitted between the stream and the file or device, and _not_ with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see *Note Low-Level Terminal Interface::. You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. *Note Low-Level I/O::. Buffering Concepts ------------------ There are three different kinds of buffering strategies: * Characters written to or read from an "unbuffered" stream are transmitted individually to or from the file as soon as possible. * Characters written to a "line buffered" stream are transmitted to the file in blocks when a newline character is encountered. * Characters written to or read from a "fully buffered" stream are transmitted to or from the file in blocks of arbitrary size. Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. *Note Controlling Buffering::, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open. The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately--which is usually what you want. Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with `fflush', as described in *Note Flushing Buffers::. Flushing Buffers ---------------- "Flushing" output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically: * When you try to do output and the output buffer is full. * When the stream is closed. *Note Closing Streams::. * When the program terminates by calling `exit'. *Note Normal Termination::. * When a newline is written, if the stream is line buffered. * Whenever an input operation on _any_ stream actually reads data from its file. If you want to flush the buffered output at another time, call `fflush', which is declared in the header file `stdio.h'. - Function: int fflush (FILE *STREAM) This function causes any buffered output on STREAM to be delivered to the file. If STREAM is a null pointer, then `fflush' causes buffered output on _all_ open output streams to be flushed. This function returns `EOF' if a write error occurs, or zero otherwise. - Function: int fflush_unlocked (FILE *STREAM) The `fflush_unlocked' function is equivalent to the `fflush' function except that it does not implicitly lock the stream. The `fflush' function can be used to flush all streams currently opened. While this is useful in some situations it does often more than necessary since it might be done in situations when terminal input is required and the program wants to be sure that all output is visible on the terminal. But this means that only line buffered streams have to be flushed. Solaris introduced a function especially for this. It was always available in the GNU C library in some form but never officially exported. - Function: void _flushlbf (void) The `_flushlbf' function flushes all line buffered streams currently opened. This function is declared in the `stdio_ext.h' header. *Compatibility Note:* Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system. In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used. - Function: void __fpurge (FILE *STREAM) The `__fpurge' function causes the buffer of the stream STREAM to be emptied. If the stream is currently in read mode all input in the buffer is lost. If the stream is in output mode the buffered output is not written to the device (or whatever other underlying storage) and the buffer the cleared. This function is declared in `stdio_ext.h'. Controlling Which Kind of Buffering ----------------------------------- After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the `setvbuf' function. The facilities listed in this section are declared in the header file `stdio.h'. - Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE) This function is used to specify that the stream STREAM should have the buffering mode MODE, which can be either `_IOFBF' (for full buffering), `_IOLBF' (for line buffering), or `_IONBF' (for unbuffered input/output). If you specify a null pointer as the BUF argument, then `setvbuf' allocates a buffer itself using `malloc'. This buffer will be freed when you close the stream. Otherwise, BUF should be a character array that can hold at least SIZE characters. You should not free the space for this array as long as the stream remains open and this array remains its buffer. You should usually either allocate it statically, or `malloc' (*note Unconstrained Allocation::) the buffer. Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array. While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering. The `setvbuf' function returns zero on success, or a nonzero value if the value of MODE is not valid or if the request could not be honored. - Macro: int _IOFBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be fully buffered. - Macro: int _IOLBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be line buffered. - Macro: int _IONBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be unbuffered. - Macro: int BUFSIZ The value of this macro is an integer constant expression that is good to use for the SIZE argument to `setvbuf'. This value is guaranteed to be at least `256'. The value of `BUFSIZ' is chosen on each system so as to make stream I/O efficient. So it is a good idea to use `BUFSIZ' as the size for the buffer when you call `setvbuf'. Actually, you can get an even better value to use for the buffer size by means of the `fstat' system call: it is found in the `st_blksize' field of the file attributes. *Note Attribute Meanings::. Sometimes people also use `BUFSIZ' as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with `fgets' (*note Character Input::). There is no particular reason to use `BUFSIZ' for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size. - Function: void setbuf (FILE *STREAM, char *BUF) If BUF is a null pointer, the effect of this function is equivalent to calling `setvbuf' with a MODE argument of `_IONBF'. Otherwise, it is equivalent to calling `setvbuf' with BUF, and a MODE of `_IOFBF' and a SIZE argument of `BUFSIZ'. The `setbuf' function is provided for compatibility with old code; use `setvbuf' in all new programs. - Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE) If BUF is a null pointer, this function makes STREAM unbuffered. Otherwise, it makes STREAM fully buffered using BUF as the buffer. The SIZE argument specifies the length of BUF. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. - Function: void setlinebuf (FILE *STREAM) This function makes STREAM be line buffered, and allocates the buffer for you. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library. - Function: int __flbf (FILE *STREAM) The `__flbf' function will return a nonzero value in case the stream STREAM is line buffered. Otherwise the return value is zero. This function is declared in the `stdio_ext.h' header. Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris. - Function: size_t __fbufsize (FILE *STREAM) The `__fbufsize' function return the size of the buffer in the stream STREAM. This value can be used to optimize the use of the stream. This function is declared in the `stdio_ext.h' header. - Function: size_t __fpending (FILE *STREAM) The `__fpending' function returns the number of bytes currently in the output buffer. For wide-oriented stream the measuring unit is wide characters. This function should not be used on buffers in read mode or opened read-only. This function is declared in the `stdio_ext.h' header. Other Kinds of Streams ====================== The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file. One such type of stream takes input from or writes output to a string. These kinds of streams are used internally to implement the `sprintf' and `sscanf' functions. You can also create such a stream explicitly, using the functions described in *Note String Streams::. More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in *Note Custom Streams::. *Portability Note:* The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality. String Streams -------------- The `fmemopen' and `open_memstream' functions allow you to do I/O to a string or memory buffer. These facilities are declared in `stdio.h'. - Function: FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE) This function opens a stream that allows the access specified by the OPENTYPE argument, that reads from or writes to the buffer specified by the argument BUF. This array must be at least SIZE bytes long. If you specify a null pointer as the BUF argument, `fmemopen' dynamically allocates an array SIZE bytes long (as with `malloc'; *note Unconstrained Allocation::). This is really only useful if you are going to write things to the buffer and then read them back in again, because you have no way of actually getting a pointer to the buffer (for this, try `open_memstream', below). The buffer is freed when the stream is closed. The argument OPENTYPE is the same as in `fopen' (*note Opening Streams::). If the OPENTYPE specifies append mode, then the initial file position is set to the first null character in the buffer. Otherwise the initial file position is at the beginning of the buffer. When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the SIZE argument to account for this. Attempts to write more than SIZE bytes to the buffer result in an error. For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file". Read operations indicate end of file only when the file position advances past SIZE bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the SIZE argument. Here is an example of using `fmemopen' to create a stream for reading from a string: #include static char buffer[] = "foobar"; int main (void) { int ch; FILE *stream; stream = fmemopen (buffer, strlen (buffer), "r"); while ((ch = fgetc (stream)) != EOF) printf ("Got %c\n", ch); fclose (stream); return 0; } This program produces the following output: Got f Got o Got o Got b Got a Got r - Function: FILE * open_memstream (char **PTR, size_t *SIZELOC) This function opens a stream for writing to a buffer. The buffer is allocated dynamically (as with `malloc'; *note Unconstrained Allocation::) and grown as necessary. When the stream is closed with `fclose' or flushed with `fflush', the locations PTR and SIZELOC are updated to contain the pointer to the buffer and its size. The values thus stored remain valid only as long as no further output on the stream takes place. If you do more output, you must flush the stream again to store new values before you use them again. A null character is written at the end of the buffer. This null character is _not_ included in the size value stored at SIZELOC. You can move the stream's file position with `fseek' or `fseeko' (*note File Positioning::). Moving the file position past the end of the data already written fills the intervening space with zeroes. Here is an example of using `open_memstream': #include int main (void) { char *bp; size_t size; FILE *stream; stream = open_memstream (&bp, &size); fprintf (stream, "hello"); fflush (stream); printf ("buf = `%s', size = %d\n", bp, size); fprintf (stream, ", world"); fclose (stream); printf ("buf = `%s', size = %d\n", bp, size); return 0; } This program produces the following output: buf = `hello', size = 5 buf = `hello, world', size = 12 Obstack Streams --------------- You can open an output stream that puts it data in an obstack. *Note Obstacks::. - Function: FILE * open_obstack_stream (struct obstack *OBSTACK) This function opens a stream for writing data into the obstack OBSTACK. This starts an object in the obstack and makes it grow as data is written (*note Growing Objects::). Calling `fflush' on this stream updates the current size of the object to match the amount of data that has been written. After a call to `fflush', you can examine the object temporarily. You can move the file position of an obstack stream with `fseek' or `fseeko' (*note File Positioning::). Moving the file position past the end of the data written fills the intervening space with zeros. To make the object permanent, update the obstack with `fflush', and then use `obstack_finish' to finalize the object and get its address. The following write to the stream starts a new object in the obstack, and later writes add to that object until you do another `fflush' and `obstack_finish'. But how do you find out how long the object is? You can get the length in bytes by calling `obstack_object_size' (*note Status of an Obstack::), or you can null-terminate the object like this: obstack_1grow (OBSTACK, 0); Whichever one you do, you must do it _before_ calling `obstack_finish'. (You can do both if you wish.) Here is a sample function that uses `open_obstack_stream': char * make_message_string (const char *a, int b) { FILE *stream = open_obstack_stream (&message_obstack); output_task (stream); fprintf (stream, ": "); fprintf (stream, a, b); fprintf (stream, "\n"); fclose (stream); obstack_1grow (&message_obstack, 0); return obstack_finish (&message_obstack); } Programming Your Own Custom Streams ----------------------------------- This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these "custom streams". The functions and types described here are all GNU extensions. Custom Streams and Cookies .......................... Inside every custom stream is a special object called the "cookie". This is an object supplied by you which records where to fetch or store the data read or written. It is up to you to define a data type to use for the cookie. The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type `void *'. To implement a custom stream, you must specify _how_ to fetch or store the data in the specified place. You do this by defining "hook functions" to read, write, change "file position", and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know. When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type `cookie_io_functions_t'. These facilities are declared in `stdio.h'. - Data Type: cookie_io_functions_t This is a structure type that holds the functions that define the communications protocol between the stream and its cookie. It has the following members: `cookie_read_function_t *read' This is the function that reads data from the cookie. If the value is a null pointer instead of a function, then read operations on this stream always return `EOF'. `cookie_write_function_t *write' This is the function that writes data to the cookie. If the value is a null pointer instead of a function, then data written to the stream is discarded. `cookie_seek_function_t *seek' This is the function that performs the equivalent of file positioning on the cookie. If the value is a null pointer instead of a function, calls to `fseek' or `fseeko' on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an `ESPIPE' error. `cookie_close_function_t *close' This function performs any appropriate cleanup on the cookie when closing the stream. If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed. - Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS) This function actually creates the stream for communicating with the COOKIE using the functions in the IO-FUNCTIONS argument. The OPENTYPE argument is interpreted as for `fopen'; see *Note Opening Streams::. (But note that the "truncate on open" option is ignored.) The new stream is fully buffered. The `fopencookie' function returns the newly created stream, or a null pointer in case of an error. Custom Stream Hook Functions ............................ Here are more details on how you should define the four hook functions that a custom stream needs. You should define the function to read data from the cookie as: ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE) This is very similar to the `read' function; see *Note I/O Primitives::. Your function should transfer up to SIZE bytes into the BUFFER, and return the number of bytes read, or zero to indicate end-of-file. You can return a value of `-1' to indicate an error. You should define the function to write data to the cookie as: ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE) This is very similar to the `write' function; see *Note I/O Primitives::. Your function should transfer up to SIZE bytes from the buffer, and return the number of bytes written. You can return a value of `-1' to indicate an error. You should define the function to perform seek operations on the cookie as: int SEEKER (void *COOKIE, fpos_t *POSITION, int WHENCE) For this function, the POSITION and WHENCE arguments are interpreted as for `fgetpos'; see *Note Portable Positioning::. In the GNU library, `fpos_t' is equivalent to `off_t' or `long int', and simply represents the number of bytes from the beginning of the file. After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in POSITION. Your function should return a value of `0' on success and `-1' to indicate an error. You should define the function to do cleanup operations on the cookie appropriate for closing the stream as: int CLEANER (void *COOKIE) Your function should return `-1' to indicate an error, and `0' otherwise. - Data Type: cookie_read_function This is the data type that the read function for a custom stream should have. If you declare the function as shown above, this is the type it will have. - Data Type: cookie_write_function The data type of the write function for a custom stream. - Data Type: cookie_seek_function The data type of the seek function for a custom stream. - Data Type: cookie_close_function The data type of the close function for a custom stream. Formatted Messages ================== On systems which are based on System V messages of programs (especially the system tools) are printed in a strict form using the `fmtmsg' function. The uniformity sometimes helps the user to interpret messages and the strictness tests of the `fmtmsg' function ensure that the programmer follows some minimal requirements. Printing Formatted Messages --------------------------- Messages can be printed to standard error and/or to the console. To select the destination the programmer can use the following two values, bitwise OR combined if wanted, for the CLASSIFICATION parameter of `fmtmsg': `MM_PRINT' Display the message in standard error. `MM_CONSOLE' Display the message on the system console. The erroneous piece of the system can be signalled by exactly one of the following values which also is bitwise ORed with the CLASSIFICATION parameter to `fmtmsg': `MM_HARD' The source of the condition is some hardware. `MM_SOFT' The source of the condition is some software. `MM_FIRM' The source of the condition is some firmware. A third component of the CLASSIFICATION parameter to `fmtmsg' can describe the part of the system which detects the problem. This is done by using exactly one of the following values: `MM_APPL' The erroneous condition is detected by the application. `MM_UTIL' The erroneous condition is detected by a utility. `MM_OPSYS' The erroneous condition is detected by the operating system. A last component of CLASSIFICATION can signal the results of this message. Exactly one of the following values can be used: `MM_RECOVER' It is a recoverable error. `MM_NRECOV' It is a non-recoverable error. - Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL, int SEVERITY, const char *TEXT, const char *ACTION, const char *TAG) Display a message described by its parameters on the device(s) specified in the CLASSIFICATION parameter. The LABEL parameter identifies the source of the message. The string should consist of two colon separated parts where the first part has not more than 10 and the second part not more than 14 characters. The TEXT parameter describes the condition of the error, the ACTION parameter possible steps to recover from the error and the TAG parameter is a reference to the online documentation where more information can be found. It should contain the LABEL value and a unique identification number. Each of the parameters can be a special value which means this value is to be omitted. The symbolic names for these values are: `MM_NULLLBL' Ignore LABEL parameter. `MM_NULLSEV' Ignore SEVERITY parameter. `MM_NULLMC' Ignore CLASSIFICATION parameter. This implies that nothing is actually printed. `MM_NULLTXT' Ignore TEXT parameter. `MM_NULLACT' Ignore ACTION parameter. `MM_NULLTAG' Ignore TAG parameter. There is another way certain fields can be omitted from the output to standard error. This is described below in the description of environment variables influencing the behavior. The SEVERITY parameter can have one of the values in the following table: `MM_NOSEV' Nothing is printed, this value is the same as `MM_NULLSEV'. `MM_HALT' This value is printed as `HALT'. `MM_ERROR' This value is printed as `ERROR'. `MM_WARNING' This value is printed as `WARNING'. `MM_INFO' This value is printed as `INFO'. The numeric value of these five macros are between `0' and `4'. Using the environment variable `SEV_LEVEL' or using the `addseverity' function one can add more severity levels with their corresponding string to print. This is described below (*note Adding Severity Classes::). If no parameter is ignored the output looks like this: LABEL: SEVERITY-STRING: TEXT TO FIX: ACTION TAG The colons, new line characters and the `TO FIX' string are inserted if necessary, i.e., if the corresponding parameter is not ignored. This function is specified in the X/Open Portability Guide. It is also available on all systems derived from System V. The function returns the value `MM_OK' if no error occurred. If only the printing to standard error failed, it returns `MM_NOMSG'. If printing to the console fails, it returns `MM_NOCON'. If nothing is printed `MM_NOTOK' is returned. Among situations where all outputs fail this last value is also returned if a parameter value is incorrect. There are two environment variables which influence the behavior of `fmtmsg'. The first is `MSGVERB'. It is used to control the output actually happening on standard error (_not_ the console output). Each of the five fields can explicitly be enabled. To do this the user has to put the `MSGVERB' variable with a format like the following in the environment before calling the `fmtmsg' function the first time: MSGVERB=KEYWORD[:KEYWORD[:...]] Valid KEYWORDs are `label', `severity', `text', `action', and `tag'. If the environment variable is not given or is the empty string, a not supported keyword is given or the value is somehow else invalid, no part of the message is masked out. The second environment variable which influences the behavior of `fmtmsg' is `SEV_LEVEL'. This variable and the change in the behavior of `fmtmsg' is not specified in the X/Open Portability Guide. It is available in System V systems, though. It can be used to introduce new severity levels. By default, only the five severity levels described above are available. Any other numeric value would make `fmtmsg' print nothing. If the user puts `SEV_LEVEL' with a format like SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]] in the environment of the process before the first call to `fmtmsg', where DESCRIPTION has a value of the form SEVERITY-KEYWORD,LEVEL,PRINTSTRING The SEVERITY-KEYWORD part is not used by `fmtmsg' but it has to be present. The LEVEL part is a string representation of a number. The numeric value must be a number greater than 4. This value must be used in the SEVERITY parameter of `fmtmsg' to select this class. It is not possible to overwrite any of the predefined classes. The PRINTSTRING is the string printed when a message of this class is processed by `fmtmsg' (see above, `fmtsmg' does not print the numeric value but instead the string representation). Adding Severity Classes ----------------------- There is another possibility to introduce severity classes besides using the environment variable `SEV_LEVEL'. This simplifies the task of introducing new classes in a running program. One could use the `setenv' or `putenv' function to set the environment variable, but this is toilsome. - Function: int addseverity (int SEVERITY, const char *STRING) This function allows the introduction of new severity classes which can be addressed by the SEVERITY parameter of the `fmtmsg' function. The SEVERITY parameter of `addseverity' must match the value for the parameter with the same name of `fmtmsg', and STRING is the string printed in the actual messages instead of the numeric value. If STRING is `NULL' the severity class with the numeric value according to SEVERITY is removed. It is not possible to overwrite or remove one of the default severity classes. All calls to `addseverity' with SEVERITY set to one of the values for the default classes will fail. The return value is `MM_OK' if the task was successfully performed. If the return value is `MM_NOTOK' something went wrong. This could mean that no more memory is available or a class is not available when it has to be removed. This function is not specified in the X/Open Portability Guide although the `fmtsmg' function is. It is available on System V systems. How to use `fmtmsg' and `addseverity' ------------------------------------- Here is a simple example program to illustrate the use of the both functions described in this section. #include int main (void) { addseverity (5, "NOTE2"); fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2"); fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual", "UX:cat:001"); fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag"); return 0; } The second call to `fmtmsg' illustrates a use of this function as it usually occurs on System V systems, which heavily use this function. It seems worthwhile to give a short explanation here of how this system works on System V. The value of the LABEL field (`UX:cat') says that the error occurred in the Unix program `cat'. The explanation of the error follows and the value for the ACTION parameter is `"refer to manual"'. One could be more specific here, if necessary. The TAG field contains, as proposed above, the value of the string given for the LABEL parameter, and additionally a unique ID (`001' in this case). For a GNU environment this string could contain a reference to the corresponding node in the Info page for the program. Running this program without specifying the `MSGVERB' and `SEV_LEVEL' function produces the following output: UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 We see the different fields of the message and how the extra glue (the colons and the `TO FIX' string) are printed. But only one of the three calls to `fmtmsg' produced output. The first call does not print anything because the LABEL parameter is not in the correct form. The string must contain two fields, separated by a colon (*note Printing Formatted Messages::). The third `fmtmsg' call produced no output since the class with the numeric value `6' is not defined. Although a class with numeric value `5' is also not defined by default, the call to `addseverity' introduces it and the second call to `fmtmsg' produces the above output. When we change the environment of the program to contain `SEV_LEVEL=XXX,6,NOTE' when running it we get a different result: UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 label:foo: NOTE: text TO FIX: action tag Now the third call to `fmtmsg' produced some output and we see how the string `NOTE' from the environment variable appears in the message. Now we can reduce the output by specifying which fields we are interested in. If we additionally set the environment variable `MSGVERB' to the value `severity:label:action' we get the following output: UX:cat: NOTE2 TO FIX: refer to manual label:foo: NOTE TO FIX: action I.e., the output produced by the TEXT and the TAG parameters to `fmtmsg' vanished. Please also note that now there is no colon after the `NOTE' and `NOTE2' strings in the output. This is not necessary since there is no more output on this line because the text is missing. Low-Level Input/Output ********************** This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in *Note I/O on Streams::, as well as functions for performing low-level control operations for which there are no equivalents on streams. Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons: * For reading binary files in large chunks. * For reading an entire file into core before parsing it. * To perform operations other than data transfer, which can only be done with a descriptor. (You can use `fileno' to get the descriptor corresponding to a stream.) * To pass descriptors to a child process. (The child can create its own stream to use a descriptor that it inherits, but cannot inherit a stream directly.) Opening and Closing Files ========================= This section describes the primitives for opening and closing files using file descriptors. The `open' and `creat' functions are declared in the header file `fcntl.h', while `close' is declared in `unistd.h'. - Function: int open (const char *FILENAME, int FLAGS[, mode_t MODE]) The `open' function creates and returns a new file descriptor for the file named by FILENAME. Initially, the file position indicator for the file is at the beginning of the file. The argument MODE is used only when a file is created, but it doesn't hurt to supply the argument in any case. The FLAGS argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). *Note File Status Flags::, for the parameters available. The normal return value from `open' is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' The file exists but is not readable/writable as requested by the FLAGS argument, the file does not exist and the directory is unwritable so it cannot be created. `EEXIST' Both `O_CREAT' and `O_EXCL' are set, and the named file already exists. `EINTR' The `open' operation was interrupted by a signal. *Note Interrupted Primitives::. `EISDIR' The FLAGS argument specified write access, and the file is a directory. `EMFILE' The process has too many files open. The maximum number of file descriptors is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. `ENFILE' The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.) `ENOENT' The named file does not exist, and `O_CREAT' is not specified. `ENOSPC' The directory or file system that would contain the new file cannot be extended, because there is no disk space left. `ENXIO' `O_NONBLOCK' and `O_WRONLY' are both set in the FLAGS argument, the file named by FILENAME is a FIFO (*note Pipes and FIFOs::), and no process has the file open for reading. `EROFS' The file resides on a read-only file system and any of `O_WRONLY', `O_RDWR', and `O_TRUNC' are set in the FLAGS argument, or `O_CREAT' is set and the file does not already exist. If on a 32 bit machine the sources are translated with `_FILE_OFFSET_BITS == 64' the function `open' returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 bytes in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `open' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `open' should be protected using cancellation handlers. The `open' function is the underlying primitive for the `fopen' and `freopen' functions, that create streams. - Function: int open64 (const char *FILENAME, int FLAGS[, mode_t MODE]) This function is similar to `open'. It returns a file descriptor which can be used to access the file named by FILENAME. The only difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `open'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Obsolete function: int creat (const char *FILENAME, mode_t MODE) This function is obsolete. The call: creat (FILENAME, MODE) is equivalent to: open (FILENAME, O_WRONLY | O_CREAT | O_TRUNC, MODE) If on a 32 bit machine the sources are translated with `_FILE_OFFSET_BITS == 64' the function `creat' returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced. - Obsolete function: int creat64 (const char *FILENAME, mode_t MODE) This function is similar to `creat'. It returns a file descriptor which can be used to access the file named by FILENAME. The only the difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits. To use this file descriptor one must not use the normal operations but instead the counterparts named `*64', e.g., `read64'. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `open'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Function: int close (int FILEDES) The function `close' closes the file descriptor FILEDES. Closing a file has the following consequences: * The file descriptor is deallocated. * Any record locks owned by the process on the file are unlocked. * When all file descriptors associated with a pipe or FIFO have been closed, any unread data is discarded. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `close' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `close' should be protected using cancellation handlers. The normal return value from `close' is 0; a value of -1 is returned in case of failure. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINTR' The `close' call was interrupted by a signal. *Note Interrupted Primitives::. Here is an example of how to handle `EINTR' properly: TEMP_FAILURE_RETRY (close (desc)); `ENOSPC' `EIO' `EDQUOT' When the file is accessed by NFS, these errors from `write' can sometimes not be detected until `close'. *Note I/O Primitives::, for details on their meaning. Please note that there is _no_ separate `close64' function. This is not necessary since this function does not determine nor depend on the mode of the file. The kernel which performs the `close' operation knows which mode the descriptor is used for and can handle this situation. To close a stream, call `fclose' (*note Closing Streams::) instead of trying to close its underlying file descriptor with `close'. This flushes any buffered output and updates the stream object to indicate that it is closed. Input and Output Primitives =========================== This section describes the functions for performing primitive input and output operations on file descriptors: `read', `write', and `lseek'. These functions are declared in the header file `unistd.h'. - Data Type: ssize_t This data type is used to represent the sizes of blocks that can be read or written in a single operation. It is similar to `size_t', but must be a signed type. - Function: ssize_t read (int FILEDES, void *BUFFER, size_t SIZE) The `read' function reads up to SIZE bytes from the file with descriptor FILEDES, storing the results in the BUFFER. (This is not necessarily a character string, and no terminating null character is added.) The return value is the number of bytes actually read. This might be less than SIZE; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than SIZE bytes is not an error. A value of zero indicates end-of-file (except if the value of the SIZE argument is also zero). This is not considered an error. If you keep calling `read' while at end-of-file, it will keep returning zero and doing nothing else. If `read' returns at least one character, there is no way you can tell whether end-of-file was reached. But if you did reach the end, the next read will return zero. In case of an error, `read' returns -1. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, when no input is immediately available, `read' waits for some input. But if the `O_NONBLOCK' flag is set for the file (*note File Status Flags::), `read' returns immediately without reading any data, and reports this error. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, reading a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem never happens in the GNU system. Any condition that could result in `EAGAIN' can instead result in a successful `read' which returns fewer bytes than requested. Calling `read' again immediately would result in `EAGAIN'. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for reading. `EINTR' `read' was interrupted by a signal while it was waiting for input. *Note Interrupted Primitives::. A signal will not necessary cause `read' to return `EINTR'; it may instead result in a successful `read' which returns fewer bytes than requested. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `EIO' also occurs when a background process tries to read from the controlling terminal, and the normal action of stopping the process by sending it a `SIGTTIN' signal isn't working. This might happen if the signal is being blocked or ignored, or because the process group is orphaned. *Note Job Control::, for more information about job control, and *Note Signal Handling::, for information about signals. Please note that there is no function named `read64'. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally, the `read' function can be used for all cases. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `read' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `read' should be protected using cancellation handlers. The `read' function is the underlying primitive for all of the functions that read from streams, such as `fgetc'. - Function: ssize_t pread (int FILEDES, void *BUFFER, size_t SIZE, off_t OFFSET) The `pread' function is similar to the `read' function. The first three arguments are identical, and the return values and error codes also correspond. The difference is the fourth argument and its handling. The data block is not read from the current position of the file descriptor `filedes'. Instead the data is read from the file starting at position OFFSET. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `pread' function is in fact `pread64' and the type `off_t' has 64 bits, which makes it possible to handle files up to 2^63 bytes in length. The return value of `pread' describes the number of bytes read. In the error case it returns -1 like `read' does and the error codes are also the same, with these additions: `EINVAL' The value given for OFFSET is negative and therefore illegal. `ESPIPE' The file descriptor FILEDES is associate with a pipe or a FIFO and this device does not allow positioning of the file pointer. The function is an extension defined in the Unix Single Specification version 2. - Function: ssize_t pread64 (int FILEDES, void *BUFFER, size_t SIZE, off64_t OFFSET) This function is similar to the `pread' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is actually available under the name `pread' and so transparently replaces the 32 bit interface. - Function: ssize_t write (int FILEDES, const void *BUFFER, size_t SIZE) The `write' function writes up to SIZE bytes from BUFFER to the file with descriptor FILEDES. The data in BUFFER is not necessarily a character string and a null character is output like any other character. The return value is the number of bytes actually written. This may be SIZE, but can always be smaller. Your program should always call `write' in a loop, iterating until all the data is written. Once `write' returns, the data is enqueued to be written and can be read back right away, but it is not necessarily written out to permanent storage immediately. You can use `fsync' when you need to be sure your data has been permanently stored before continuing. (It is more efficient for the system to batch up consecutive writes and do them all at once when convenient. Normally they will always be written to disk within a minute or less.) Modern systems provide another function `fdatasync' which guarantees integrity only for the file data and is therefore faster. You can use the `O_FSYNC' open mode to make `write' always store the data to disk before returning; *note Operating Modes::. In the case of an error, `write' returns -1. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, `write' blocks until the write operation is complete. But if the `O_NONBLOCK' flag is set for the file (*note Control Operations::), it returns immediately without writing any data and reports this error. An example of a situation that might cause the process to block on output is writing to a terminal device that supports flow control, where output has been suspended by receipt of a STOP character. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, writing a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem does not arise in the GNU system. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for writing. `EFBIG' The size of the file would become larger than the implementation can support. `EINTR' The `write' operation was interrupted by a signal while it was blocked waiting for completion. A signal will not necessarily cause `write' to return `EINTR'; it may instead result in a successful `write' which writes fewer bytes than requested. *Note Interrupted Primitives::. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `ENOSPC' The device containing the file is full. `EPIPE' This error is returned when you try to write to a pipe or FIFO that isn't open for reading by any process. When this happens, a `SIGPIPE' signal is also sent to the process; see *Note Signal Handling::. Unless you have arranged to prevent `EINTR' failures, you should check `errno' after each failing call to `write', and if the error was `EINTR', you should simply repeat the call. *Note Interrupted Primitives::. The easy way to do this is with the macro `TEMP_FAILURE_RETRY', as follows: nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count)); Please note that there is no function named `write64'. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally the `write' function can be used for all cases. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `write' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `write' should be protected using cancellation handlers. The `write' function is the underlying primitive for all of the functions that write to streams, such as `fputc'. - Function: ssize_t pwrite (int FILEDES, const void *BUFFER, size_t SIZE, off_t OFFSET) The `pwrite' function is similar to the `write' function. The first three arguments are identical, and the return values and error codes also correspond. The difference is the fourth argument and its handling. The data block is not written to the current position of the file descriptor `filedes'. Instead the data is written to the file starting at position OFFSET. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `pwrite' function is in fact `pwrite64' and the type `off_t' has 64 bits, which makes it possible to handle files up to 2^63 bytes in length. The return value of `pwrite' describes the number of written bytes. In the error case it returns -1 like `write' does and the error codes are also the same, with these additions: `EINVAL' The value given for OFFSET is negative and therefore illegal. `ESPIPE' The file descriptor FILEDES is associated with a pipe or a FIFO and this device does not allow positioning of the file pointer. The function is an extension defined in the Unix Single Specification version 2. - Function: ssize_t pwrite64 (int FILEDES, const void *BUFFER, size_t SIZE, off64_t OFFSET) This function is similar to the `pwrite' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is actually available under the name `pwrite' and so transparently replaces the 32 bit interface. Setting the File Position of a Descriptor ========================================= Just as you can set the file position of a stream with `fseek', you can set the file position of a descriptor with `lseek'. This specifies the position in the file for the next `read' or `write' operation. *Note File Positioning::, for more information on the file position and what it means. To read the current file position value from a descriptor, use `lseek (DESC, 0, SEEK_CUR)'. - Function: off_t lseek (int FILEDES, off_t OFFSET, int WHENCE) The `lseek' function is used to change the file position of the file with descriptor FILEDES. The WHENCE argument specifies how the OFFSET should be interpreted, in the same way as for the `fseek' function, and it must be one of the symbolic constants `SEEK_SET', `SEEK_CUR', or `SEEK_END'. `SEEK_SET' Specifies that WHENCE is a count of characters from the beginning of the file. `SEEK_CUR' Specifies that WHENCE is a count of characters from the current file position. This count may be positive or negative. `SEEK_END' Specifies that WHENCE is a count of characters from the end of the file. A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end. If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position. The return value from `lseek' is normally the resulting file position, measured in bytes from the beginning of the file. You can use this feature together with `SEEK_CUR' to read the current file position. If you want to append to the file, setting the file position to the current end of file with `SEEK_END' is not sufficient. Another process may write more data after you seek but before you write, extending the file so the position you write onto clobbers their data. Instead, use the `O_APPEND' operating mode; *note Operating Modes::. You can set the file position past the current end of the file. This does not by itself make the file longer; `lseek' never changes the file. But subsequent output at that position will extend the file. Characters between the previous end of file and the new position are filled with zeros. Extending the file in this way can create a "hole": the blocks of zeros are not actually allocated on disk, so the file takes up less space than it appears to; it is then called a "sparse file". If the file position cannot be changed, or the operation is in some way invalid, `lseek' returns a value of -1. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `EINVAL' The WHENCE argument value is not valid, or the resulting file offset is not valid. A file offset is invalid. `ESPIPE' The FILEDES corresponds to an object that cannot be positioned, such as a pipe, FIFO or terminal device. (POSIX.1 specifies this error only for pipes and FIFOs, but in the GNU system, you always get `ESPIPE' if the object is not seekable.) When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `lseek' function is in fact `lseek64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `lseek' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `lseek' should be protected using cancellation handlers. The `lseek' function is the underlying primitive for the `fseek', `fseeko', `ftell', `ftello' and `rewind' functions, which operate on streams instead of file descriptors. - Function: off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE) This function is similar to the `lseek' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `lseek' and so transparently replaces the 32 bit interface. You can have multiple descriptors for the same file if you open the file more than once, or if you duplicate a descriptor with `dup'. Descriptors that come from separate calls to `open' have independent file positions; using `lseek' on one descriptor has no effect on the other. For example, { int d1, d2; char buf[4]; d1 = open ("foo", O_RDONLY); d2 = open ("foo", O_RDONLY); lseek (d1, 1024, SEEK_SET); read (d2, buf, 4); } will read the first four characters of the file `foo'. (The error-checking code necessary for a real program has been omitted here for brevity.) By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example, { int d1, d2, d3; char buf1[4], buf2[4]; d1 = open ("foo", O_RDONLY); d2 = dup (d1); d3 = dup (d2); lseek (d3, 1024, SEEK_SET); read (d1, buf1, 4); read (d2, buf2, 4); } will read four characters starting with the 1024'th character of `foo', and then four more characters starting with the 1028'th character. - Data Type: off_t This is an arithmetic data type used to represent file sizes. In the GNU system, this is equivalent to `fpos_t' or `long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `off64_t'. - Data Type: off64_t This type is used similar to `off_t'. The difference is that even on 32 bit machines, where the `off_t' type would have 32 bits, `off64_t' has 64 bits and so is able to address files up to 2^63 bytes in length. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `off_t'. These aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'. `L_SET' An alias for `SEEK_SET'. `L_INCR' An alias for `SEEK_CUR'. `L_XTND' An alias for `SEEK_END'. Descriptors and Streams ======================= Given an open file descriptor, you can create a stream for it with the `fdopen' function. You can get the underlying file descriptor for an existing stream with the `fileno' function. These functions are declared in the header file `stdio.h'. - Function: FILE * fdopen (int FILEDES, const char *OPENTYPE) The `fdopen' function returns a new stream for the file descriptor FILEDES. The OPENTYPE argument is interpreted in the same way as for the `fopen' function (*note Opening Streams::), except that the `b' option is not permitted; this is because GNU makes no distinction between text and binary files. Also, `"w"' and `"w+"' do not cause truncation of the file; these have an effect only when opening a file, and in this case the file has already been opened. You must make sure that the OPENTYPE argument matches the actual mode of the open file descriptor. The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the OPENTYPE argument), a null pointer is returned instead. In some other systems, `fdopen' may fail to detect that the modes for file descriptor do not permit the access specified by `opentype'. The GNU C library always checks for this. For an example showing the use of the `fdopen' function, see *Note Creating a Pipe::. - Function: int fileno (FILE *STREAM) This function returns the file descriptor associated with the stream STREAM. If an error is detected (for example, if the STREAM is not valid) or if STREAM does not do I/O to a file, `fileno' returns -1. - Function: int fileno_unlocked (FILE *STREAM) The `fileno_unlocked' function is equivalent to the `fileno' function except that it does not implicitly lock the stream if the state is `FSETLOCKING_INTERNAL'. This function is a GNU extension. There are also symbolic constants defined in `unistd.h' for the file descriptors belonging to the standard streams `stdin', `stdout', and `stderr'; see *Note Standard Streams::. `STDIN_FILENO' This macro has value `0', which is the file descriptor for standard input. `STDOUT_FILENO' This macro has value `1', which is the file descriptor for standard output. `STDERR_FILENO' This macro has value `2', which is the file descriptor for standard error output. Dangers of Mixing Streams and Descriptors ========================================= You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: "linked" channels that share a single file position value, and "independent" channels that have their own file positions. It's best to use just one channel in your program for actual data transfer to any given file, except when all the access is for input. For example, if you open a pipe (something you can only do at the file descriptor level), either do all I/O with the descriptor, or construct a stream from the descriptor with `fdopen' and then do all I/O with the stream. Linked Channels --------------- Channels that come from a single opening share the same file position; we call them "linked" channels. Linked channels result when you make a stream from a descriptor using `fdopen', when you get a descriptor from a stream with `fileno', when you copy a descriptor with `dup' or `dup2', and when descriptors are inherited during `fork'. For files that don't support random access, such as terminals and pipes, _all_ channels are effectively linked. On random-access files, all append-type output streams are effectively linked to each other. If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first "clean up" the stream that you have been using. *Note Cleaning Streams::. Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them. Independent Channels -------------------- When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called "independent channels". The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. However, if some of the channels are streams, you must take these precautions: * You should clean an output stream after use, before doing anything else that might read or write from the same part of the file. * You should clean an input stream before reading data that may have been modified using an independent channel. Otherwise, you might read obsolete data that had been in the stream's buffer. If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. You cannot reliably set their file positions to the new end of file before writing, because the file can always be extended by another process between when you set the file position and when you write the data. Instead, use an append-type descriptor or stream; they always output at the current end of the file. In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream. It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see *Note Linked Channels::. Cleaning Streams ---------------- On the GNU system, you can clean up any stream with `fclean': - Function: int fclean (FILE *STREAM) Clean up the stream STREAM so that its buffer is empty. If STREAM is doing output, force it out. If STREAM is doing input, give the data in the buffer back to the system, arranging to reread it. On other systems, you can use `fflush' to clean a stream in most cases. You can skip the `fclean' or `fflush' if you know the stream is already clean. A stream is clean whenever its buffer is empty. For example, an unbuffered stream is always clean. An input stream that is at end-of-file is clean. A line-buffered stream is clean when the last character output was a newline. There is one case in which cleaning a stream is impossible on most systems. This is when the stream is doing input from a file that is not random-access. Such streams typically read ahead, and when the file is not random access, there is no way to give back the excess data already read. When an input stream reads from a random-access file, `fflush' does clean the stream, but leaves the file pointer at an unpredictable place; you must set the file pointer before doing any further I/O. On the GNU system, using `fclean' avoids both of these problems. Closing an output-only stream also does `fflush', so this is a valid way of cleaning an output stream. On the GNU system, closing an input stream does `fclean'. You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. *Note Terminal Modes::. Fast Scatter-Gather I/O ======================= Some applications may need to read or write data to multiple buffers, which are separated in memory. Although this can be done easily enough with multiple calls to `read' and `write', it is inefficient because there is overhead associated with each kernel call. Instead, many platforms provide special high-speed primitives to perform these "scatter-gather" operations in a single kernel call. The GNU C library will provide an emulation on any system that lacks these primitives, so they are not a portability threat. They are defined in `sys/uio.h'. These functions are controlled with arrays of `iovec' structures, which describe the location and size of each buffer. - Data Type: struct iovec The `iovec' structure describes a buffer. It contains two fields: `void *iov_base' Contains the address of a buffer. `size_t iov_len' Contains the length of the buffer. - Function: ssize_t readv (int FILEDES, const struct iovec *VECTOR, int COUNT) The `readv' function reads data from FILEDES and scatters it into the buffers described in VECTOR, which is taken to be COUNT structures long. As each buffer is filled, data is sent to the next. Note that `readv' is not guaranteed to fill all the buffers. It may stop at any point, for the same reasons `read' would. The return value is a count of bytes (_not_ buffers) read, 0 indicating end-of-file, or -1 indicating an error. The possible errors are the same as in `read'. - Function: ssize_t writev (int FILEDES, const struct iovec *VECTOR, int COUNT) The `writev' function gathers data from the buffers described in VECTOR, which is taken to be COUNT structures long, and writes them to `filedes'. As each buffer is written, it moves on to the next. Like `readv', `writev' may stop midstream under the same conditions `write' would. The return value is a count of bytes written, or -1 indicating an error. The possible errors are the same as in `write'. Note that if the buffers are small (under about 1kB), high-level streams may be easier to use than these functions. However, `readv' and `writev' are more efficient when the individual buffers themselves (as opposed to the total output), are large. In that case, a high-level stream would not be able to cache the data effectively. Memory-mapped I/O ================= On modern operating systems, it is possible to "mmap" (pronounced "em-map") a file to a region of memory. When this is done, the file can be accessed just like an array in the program. This is more efficient than `read' or `write', as only the regions of the file that a program actually accesses are loaded. Accesses to not-yet-loaded parts of the mmapped region are handled in the same way as swapped out pages. Since mmapped pages can be stored back to their file when physical memory is low, it is possible to mmap files orders of magnitude larger than both the physical memory _and_ swap space. The only limit is address space. The theoretical limit is 4GB on a 32-bit machine - however, the actual limit will be smaller since some areas will be reserved for other purposes. If the LFS interface is used the file size on 32-bit systems is not limited to 2GB (offsets are signed which reduces the addressable area of 4GB by half); the full 64-bit are available. Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. To determine the size of a page the machine uses one should use size_t page_size = (size_t) sysconf (_SC_PAGESIZE); These functions are declared in `sys/mman.h'. - Function: void * mmap (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off_t OFFSET) The `mmap' function creates a new mapping, connected to bytes (OFFSET) to (OFFSET + LENGTH - 1) in the file open on FILEDES. A new reference for the file specified by FILEDES is created, which is not removed by closing the file. ADDRESS gives a preferred starting address for the mapping. `NULL' expresses no preference. Any previous mapping at that address is automatically removed. The address you give may still be changed, unless you use the `MAP_FIXED' flag. PROTECT contains flags that control what kind of access is permitted. They include `PROT_READ', `PROT_WRITE', and `PROT_EXEC', which permit reading, writing, and execution, respectively. Inappropriate access will cause a segfault (*note Program Error Signals::). Note that most hardware designs cannot support write permission without read permission, and many do not distinguish read and execute permission. Thus, you may receive wider permissions than you ask for, and mappings of write-only files may be denied even if you do not use `PROT_READ'. FLAGS contains flags that control the nature of the map. One of `MAP_SHARED' or `MAP_PRIVATE' must be specified. They include: `MAP_PRIVATE' This specifies that writes to the region should never be written back to the attached file. Instead, a copy is made for the process, and the region will be swapped normally if memory runs low. No other process will see the changes. Since private mappings effectively revert to ordinary memory when written to, you must have enough virtual memory for a copy of the entire mmapped region if you use this mode with `PROT_WRITE'. `MAP_SHARED' This specifies that writes to the region will be written back to the file. Changes made will be shared immediately with other processes mmaping the same file. Note that actual writing may take place at any time. You need to use `msync', described below, if it is important that other processes using conventional I/O get a consistent view of the file. `MAP_FIXED' This forces the system to use the exact mapping address specified in ADDRESS and fail if it can't. `MAP_ANONYMOUS' `MAP_ANON' This flag tells the system to create an anonymous mapping, not connected to a file. FILEDES and OFF are ignored, and the region is initialized with zeros. Anonymous maps are used as the basic primitive to extend the heap on some systems. They are also useful to share data between multiple tasks without creating a file. On some systems using private anonymous mmaps is more efficient than using `malloc' for large blocks. This is not an issue with the GNU C library, as the included `malloc' automatically uses `mmap' where appropriate. `mmap' returns the address of the new mapping, or -1 for an error. Possible errors include: `EINVAL' Either ADDRESS was unusable, or inconsistent FLAGS were given. `EACCES' FILEDES was not open for the type of access specified in PROTECT. `ENOMEM' Either there is not enough memory for the operation, or the process is out of address space. `ENODEV' This file is of a type that doesn't support mapping. `ENOEXEC' The file is on a filesystem that doesn't support mapping. - Function: void * mmap64 (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off64_t OFFSET) The `mmap64' function is equivalent to the `mmap' function but the OFFSET parameter is of type `off64_t'. On 32-bit systems this allows the file associated with the FILEDES descriptor to be larger than 2GB. FILEDES must be a descriptor returned from a call to `open64' or `fopen64' and `freopen64' where the descriptor is retrieved with `fileno'. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `mmap'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Function: int munmap (void *ADDR, size_t LENGTH) `munmap' removes any memory maps from (ADDR) to (ADDR + LENGTH). LENGTH should be the length of the mapping. It is safe to unmap multiple mappings in one command, or include unmapped space in the range. It is also possible to unmap only part of an existing mapping. However, only entire pages can be removed. If LENGTH is not an even number of pages, it will be rounded up. It returns 0 for success and -1 for an error. One error is possible: `EINVAL' The memory range given was outside the user mmap range or wasn't page aligned. - Function: int msync (void *ADDRESS, size_t LENGTH, int FLAGS) When using shared mappings, the kernel can write the file at any time before the mapping is removed. To be certain data has actually been written to the file and will be accessible to non-memory-mapped I/O, it is necessary to use this function. It operates on the region ADDRESS to (ADDRESS + LENGTH). It may be used on part of a mapping or multiple mappings, however the region given should not contain any unmapped space. FLAGS can contain some options: `MS_SYNC' This flag makes sure the data is actually written _to disk_. Normally `msync' only makes sure that accesses to a file with conventional I/O reflect the recent changes. `MS_ASYNC' This tells `msync' to begin the synchronization, but not to wait for it to complete. `msync' returns 0 for success and -1 for error. Errors include: `EINVAL' An invalid region was given, or the FLAGS were invalid. `EFAULT' There is no existing mapping in at least part of the given region. - Function: void * mremap (void *ADDRESS, size_t LENGTH, size_t NEW_LENGTH, int FLAG) This function can be used to change the size of an existing memory area. ADDRESS and LENGTH must cover a region entirely mapped in the same `mmap' statement. A new mapping with the same characteristics will be returned with the length NEW_LENGTH. One option is possible, `MREMAP_MAYMOVE'. If it is given in FLAGS, the system may remove the existing mapping and create a new one of the desired length in another location. The address of the resulting mapping is returned, or -1. Possible error codes include: `EFAULT' There is no existing mapping in at least part of the original region, or the region covers two or more distinct mappings. `EINVAL' The address given is misaligned or inappropriate. `EAGAIN' The region has pages locked, and if extended it would exceed the process's resource limit for locked pages. *Note Limits on Resources::. `ENOMEM' The region is private writable, and insufficient virtual memory is available to extend it. Also, this error will occur if `MREMAP_MAYMOVE' is not given and the extension would collide with another mapped region. This function is only available on a few systems. Except for performing optional optimizations one should not rely on this function. Not all file descriptors may be mapped. Sockets, pipes, and most devices only allow sequential access and do not fit into the mapping abstraction. In addition, some regular files may not be mmapable, and older kernels may not support mapping at all. Thus, programs using `mmap' should have a fallback method to use should it fail. *Note Mmap: (standards)Mmap. - Function: int madvise (void *ADDR, size_t LENGTH, int ADVICE) This function can be used to provide the system with ADVICE about the intended usage patterns of the memory region starting at ADDR and extending LENGTH bytes. The valid BSD values for ADVICE are: `MADV_NORMAL' The region should receive no further special treatment. `MADV_RANDOM' The region will be accessed via random page references. The kernel should page-in the minimal number of pages for each page fault. `MADV_SEQUENTIAL' The region will be accessed via sequential page references. This may cause the kernel to aggressively read-ahead, expecting further sequential references after any page fault within this region. `MADV_WILLNEED' The region will be needed. The pages within this region may be pre-faulted in by the kernel. `MADV_DONTNEED' The region is no longer needed. The kernel may free these pages, causing any changes to the pages to be lost, as well as swapped out pages to be discarded. The POSIX names are slightly different, but with the same meanings: `POSIX_MADV_NORMAL' This corresponds with BSD's `MADV_NORMAL'. `POSIX_MADV_RANDOM' This corresponds with BSD's `MADV_RANDOM'. `POSIX_MADV_SEQUENTIAL' This corresponds with BSD's `MADV_SEQUENTIAL'. `POSIX_MADV_WILLNEED' This corresponds with BSD's `MADV_WILLNEED'. `POSIX_MADV_DONTNEED' This corresponds with BSD's `MADV_DONTNEED'. `msync' returns 0 for success and -1 for error. Errors include: `EINVAL' An invalid region was given, or the ADVICE was invalid. `EFAULT' There is no existing mapping in at least part of the given region. Waiting for Input or Output =========================== Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets. You cannot normally use `read' for this purpose, because this blocks the program until input is available on one particular file descriptor; input on other channels won't wake it up. You could set nonblocking mode and poll each file descriptor in turn, but this is very inefficient. A better solution is to use the `select' function. This blocks the program until input or output is ready on a specified set of file descriptors, or until a timer expires, whichever comes first. This facility is declared in the header file `sys/types.h'. In the case of a server socket (*note Listening::), we say that "input" is available when there are pending connections that could be accepted (*note Accepting Connections::). `accept' for server sockets blocks and interacts with `select' just as `read' does for normal input. The file descriptor sets for the `select' function are specified as `fd_set' objects. Here is the description of the data type and some macros for manipulating these objects. - Data Type: fd_set The `fd_set' data type represents file descriptor sets for the `select' function. It is actually a bit array. - Macro: int FD_SETSIZE The value of this macro is the maximum number of file descriptors that a `fd_set' object can hold information about. On systems with a fixed maximum number, `FD_SETSIZE' is at least that number. On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an `fd_set'; if you get a file descriptor with a value as high as `FD_SETSIZE', you cannot put that descriptor into an `fd_set'. - Macro: void FD_ZERO (fd_set *SET) This macro initializes the file descriptor set SET to be the empty set. - Macro: void FD_SET (int FILEDES, fd_set *SET) This macro adds FILEDES to the file descriptor set SET. - Macro: void FD_CLR (int FILEDES, fd_set *SET) This macro removes FILEDES from the file descriptor set SET. - Macro: int FD_ISSET (int FILEDES, fd_set *SET) This macro returns a nonzero value (true) if FILEDES is a member of the file descriptor set SET, and zero (false) otherwise. Next, here is the description of the `select' function itself. - Function: int select (int NFDS, fd_set *READ-FDS, fd_set *WRITE-FDS, fd_set *EXCEPT-FDS, struct timeval *TIMEOUT) The `select' function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired. The file descriptors specified by the READ-FDS argument are checked to see if they are ready for reading; the WRITE-FDS file descriptors are checked to see if they are ready for writing; and the EXCEPT-FDS file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition. A file descriptor is considered ready for reading if it is not at end of file. A server socket is considered ready for reading if there is a pending connection which can be accepted with `accept'; *note Accepting Connections::. A client socket is ready for writing when its connection is fully established; *note Connecting::. "Exceptional conditions" does not mean errors--errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (*Note Sockets::, for information on urgent messages.) The `select' function checks only the first NFDS file descriptors. The usual thing is to pass `FD_SETSIZE' as the value of this argument. The TIMEOUT specifies the maximum time to wait. If you pass a null pointer for this argument, it means to block indefinitely until one of the file descriptors is ready. Otherwise, you should provide the time in `struct timeval' format; see *Note High-Resolution Calendar::. Specify zero as the time (a `struct timeval' containing all zeros) if you want to find out which descriptors are ready without waiting if none are ready. The normal return value from `select' is the total number of ready file descriptors in all of the sets. Each of the argument sets is overwritten with information about the descriptors that are ready for the corresponding operation. Thus, to see if a particular descriptor DESC has input, use `FD_ISSET (DESC, READ-FDS)' after `select' returns. If `select' returns because the timeout period expires, it returns a value of zero. Any signal will cause `select' to return immediately. So if your program uses signals, you can't rely on `select' to keep waiting for the full time specified. If you want to be sure of waiting for a particular amount of time, you must check for `EINTR' and repeat the `select' with a newly calculated timeout based on the current time. See the example below. See also *Note Interrupted Primitives::. If an error occurs, `select' returns `-1' and does not modify the argument file descriptor sets. The following `errno' error conditions are defined for this function: `EBADF' One of the file descriptor sets specified an invalid file descriptor. `EINTR' The operation was interrupted by a signal. *Note Interrupted Primitives::. `EINVAL' The TIMEOUT argument is invalid; one of the components is negative or too large. *Portability Note:* The `select' function is a BSD Unix feature. Here is an example showing how you can use `select' to establish a timeout period for reading from a file descriptor. The `input_timeout' function blocks the calling process until input is available on the file descriptor, or until the timeout period expires. #include #include #include #include int input_timeout (int filedes, unsigned int seconds) { fd_set set; struct timeval timeout; /* Initialize the file descriptor set. */ FD_ZERO (&set); FD_SET (filedes, &set); /* Initialize the timeout data structure. */ timeout.tv_sec = seconds; timeout.tv_usec = 0; /* `select' returns 0 if timeout, 1 if input available, -1 if error. */ return TEMP_FAILURE_RETRY (select (FD_SETSIZE, &set, NULL, NULL, &timeout)); } int main (void) { fprintf (stderr, "select returned %d.\n", input_timeout (STDIN_FILENO, 5)); return 0; } There is another example showing the use of `select' to multiplex input from multiple sockets in *Note Server Example::. Synchronizing I/O operations ============================ In most modern operating systems, the normal I/O operations are not executed synchronously. I.e., even if a `write' system call returns, this does not mean the data is actually written to the media, e.g., the disk. In situations where synchronization points are necessary, you can use special functions which ensure that all operations finish before they return. - Function: int sync (void) A call to this function will not return as long as there is data which has not been written to the device. All dirty buffers in the kernel will be written and so an overall consistent system can be achieved (if no other process in parallel writes data). A prototype for `sync' can be found in `unistd.h'. The return value is zero to indicate no error. Programs more often want to ensure that data written to a given file is committed, rather than all data in the system. For this, `sync' is overkill. - Function: int fsync (int FILDES) The `fsync' function can be used to make sure all data associated with the open file FILDES is written to the device associated with the descriptor. The function call does not return unless all actions have finished. A prototype for `fsync' can be found in `unistd.h'. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `fsync' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `fsync' should be protected using cancellation handlers. The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable ERRNO is set to the following values: `EBADF' The descriptor FILDES is not valid. `EINVAL' No synchronization is possible since the system does not implement this. Sometimes it is not even necessary to write all data associated with a file descriptor. E.g., in database files which do not change in size it is enough to write all the file content data to the device. Meta-information, like the modification time etc., are not that important and leaving such information uncommitted does not prevent a successful recovering of the file in case of a problem. - Function: int fdatasync (int FILDES) When a call to the `fdatasync' function returns, it is ensured that all of the file data is written to the device. For all pending I/O operations, the parts guaranteeing data integrity finished. Not all systems implement the `fdatasync' operation. On systems missing this functionality `fdatasync' is emulated by a call to `fsync' since the performed actions are a superset of those required by `fdatasync'. The prototype for `fdatasync' is in `unistd.h'. The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable ERRNO is set to the following values: `EBADF' The descriptor FILDES is not valid. `EINVAL' No synchronization is possible since the system does not implement this. Perform I/O Operations in Parallel ================================== The POSIX.1b standard defines a new set of I/O operations which can significantly reduce the time an application spends waiting at I/O. The new functions allow a program to initiate one or more I/O operations and then immediately resume normal work while the I/O operations are executed in parallel. This functionality is available if the `unistd.h' file defines the symbol `_POSIX_ASYNCHRONOUS_IO'. These functions are part of the library with realtime functions named `librt'. They are not actually part of the `libc' binary. The implementation of these functions can be done using support in the kernel (if available) or using an implementation based on threads at userlevel. In the latter case it might be necessary to link applications with the thread library `libpthread' in addition to `librt'. All AIO operations operate on files which were opened previously. There might be arbitrarily many operations running for one file. The asynchronous I/O operations are controlled using a data structure named `struct aiocb' ("AIO control block"). It is defined in `aio.h' as follows. - Data Type: struct aiocb The POSIX.1b standard mandates that the `struct aiocb' structure contains at least the members described in the following table. There might be more elements which are used by the implementation, but depending upon these elements is not portable and is highly deprecated. `int aio_fildes' This element specifies the file descriptor to be used for the operation. It must be a legal descriptor, otherwise the operation will fail. The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an `lseek' call would lead to an error. `off_t aio_offset' This element specifies the offset in the file at which the operation (input or output) is performed. Since the operations are carried out in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor. `volatile void *aio_buf' This is a pointer to the buffer with the data to be written or the place where the read data is stored. `size_t aio_nbytes' This element specifies the length of the buffer pointed to by `aio_buf'. `int aio_reqprio' If the platform has defined `_POSIX_PRIORITIZED_IO' and `_POSIX_PRIORITY_SCHEDULING', the AIO requests are processed based on the current scheduling priority. The `aio_reqprio' element can then be used to lower the priority of the AIO operation. `struct sigevent aio_sigevent' This element specifies how the calling process is notified once the operation terminates. If the `sigev_notify' element is `SIGEV_NONE', no notification is sent. If it is `SIGEV_SIGNAL', the signal determined by `sigev_signo' is sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD'. In this case, a thread is created which starts executing the function pointed to by `sigev_notify_function'. `int aio_lio_opcode' This element is only used by the `lio_listio' and `lio_listio64' functions. Since these functions allow an arbitrary number of operations to start at once, and each operation can be input or output (or nothing), the information must be stored in the control block. The possible values are: `LIO_READ' Start a read operation. Read from the file at position `aio_offset' and store the next `aio_nbytes' bytes in the buffer pointed to by `aio_buf'. `LIO_WRITE' Start a write operation. Write `aio_nbytes' bytes starting at `aio_buf' into the file starting at position `aio_offset'. `LIO_NOP' Do nothing for this control block. This value is useful sometimes when an array of `struct aiocb' values contains holes, i.e., some of the values must not be handled although the whole array is presented to the `lio_listio' function. When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine, this type is in fact `struct aiocb64', since the LFS interface transparently replaces the `struct aiocb' definition. For use with the AIO functions defined in the LFS, there is a similar type defined which replaces the types of the appropriate members with larger types but otherwise is equivalent to `struct aiocb'. Particularly, all member names are the same. - Data Type: struct aiocb64 `int aio_fildes' This element specifies the file descriptor which is used for the operation. It must be a legal descriptor since otherwise the operation fails for obvious reasons. The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an `lseek' call would lead to an error. `off64_t aio_offset' This element specifies at which offset in the file the operation (input or output) is performed. Since the operation are carried in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor. `volatile void *aio_buf' This is a pointer to the buffer with the data to be written or the place where the read data is stored. `size_t aio_nbytes' This element specifies the length of the buffer pointed to by `aio_buf'. `int aio_reqprio' If for the platform `_POSIX_PRIORITIZED_IO' and `_POSIX_PRIORITY_SCHEDULING' are defined the AIO requests are processed based on the current scheduling priority. The `aio_reqprio' element can then be used to lower the priority of the AIO operation. `struct sigevent aio_sigevent' This element specifies how the calling process is notified once the operation terminates. If the `sigev_notify', element is `SIGEV_NONE' no notification is sent. If it is `SIGEV_SIGNAL', the signal determined by `sigev_signo' is sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD' in which case a thread which starts executing the function pointed to by `sigev_notify_function'. `int aio_lio_opcode' This element is only used by the `lio_listio' and `[lio_listio64' functions. Since these functions allow an arbitrary number of operations to start at once, and since each operation can be input or output (or nothing), the information must be stored in the control block. See the description of `struct aiocb' for a description of the possible values. When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine, this type is available under the name `struct aiocb64', since the LFS transparently replaces the old interface. Asynchronous Read and Write Operations -------------------------------------- - Function: int aio_read (struct aiocb *AIOCBP) This function initiates an asynchronous read operation. It immediately returns after the operation was enqueued or when an error was encountered. The first `aiocbp->aio_nbytes' bytes of the file for which `aiocbp->aio_fildes' is a descriptor are written to the buffer starting at `aiocbp->aio_buf'. Reading starts at the absolute position `aiocbp->aio_offset' in the file. If prioritized I/O is supported by the platform the `aiocbp->aio_reqprio' value is used to adjust the priority before the request is actually enqueued. The calling process is notified about the termination of the read request according to the `aiocbp->aio_sigevent' value. When `aio_read' returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found, the function returns -1 and sets `errno' to one of the following values: `EAGAIN' The request was not enqueued due to (temporarily) exceeded resource limitations. `ENOSYS' The `aio_read' function is not implemented. `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously. `EINVAL' The `aiocbp->aio_offset' or `aiocbp->aio_reqpiro' value is invalid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously. If `aio_read' returns zero, the current status of the request can be queried using `aio_error' and `aio_return' functions. As long as the value returned by `aio_error' is `EINPROGRESS' the operation has not yet completed. If `aio_error' returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be obtained using a call to `aio_return'. The returned value is the same as an equivalent call to `read' would have returned. Possible error codes returned by `aio_error' are: `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. `ECANCELED' The operation was canceled before the operation was finished (*note Cancel AIO Operations::) `EINVAL' The `aiocbp->aio_offset' value is invalid. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_read64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_read64 (struct aiocb *AIOCBP) This function is similar to the `aio_read' function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, `aio_read64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the reading, as opposed to `lseek' functionality used in `aio_read'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_read' and so transparently replaces the interface for small files on 32 bit machines. To write data asynchronously to a file, there exists an equivalent pair of functions with a very similar interface. - Function: int aio_write (struct aiocb *AIOCBP) This function initiates an asynchronous write operation. The function call immediately returns after the operation was enqueued or if before this happens an error was encountered. The first `aiocbp->aio_nbytes' bytes from the buffer starting at `aiocbp->aio_buf' are written to the file for which `aiocbp->aio_fildes' is an descriptor, starting at the absolute position `aiocbp->aio_offset' in the file. If prioritized I/O is supported by the platform, the `aiocbp->aio_reqprio' value is used to adjust the priority before the request is actually enqueued. The calling process is notified about the termination of the read request according to the `aiocbp->aio_sigevent' value. When `aio_write' returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found the function returns -1 and sets `errno' to one of the following values. `EAGAIN' The request was not enqueued due to (temporarily) exceeded resource limitations. `ENOSYS' The `aio_write' function is not implemented. `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. This condition may not be recognized before enqueueing the request, and so this error might also be signaled asynchronously. `EINVAL' The `aiocbp->aio_offset' or `aiocbp->aio_reqprio' value is invalid. This condition may not be recognized before enqueueing the request and so this error might also be signaled asynchronously. In the case `aio_write' returns zero, the current status of the request can be queried using `aio_error' and `aio_return' functions. As long as the value returned by `aio_error' is `EINPROGRESS' the operation has not yet completed. If `aio_error' returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be get using a call to `aio_return'. The returned value is the same as an equivalent call to `read' would have returned. Possible error codes returned by `aio_error' are: `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. `ECANCELED' The operation was canceled before the operation was finished. (*note Cancel AIO Operations::) `EINVAL' The `aiocbp->aio_offset' value is invalid. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `aio_write64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_write64 (struct aiocb *AIOCBP) This function is similar to the `aio_write' function. The only difference is that on 32 bit machines the file descriptor should be opened in the large file mode. Internally `aio_write64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the writing, as opposed to `lseek' functionality used in `aio_write'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_write' and so transparently replaces the interface for small files on 32 bit machines. Besides these functions with the more or less traditional interface, POSIX.1b also defines a function which can initiate more than one operation at a time, and which can handle freely mixed read and write operations. It is therefore similar to a combination of `readv' and `writev'. - Function: int lio_listio (int MODE, struct aiocb *const LIST[], int NENT, struct sigevent *SIG) The `lio_listio' function can be used to enqueue an arbitrary number of read and write requests at one time. The requests can all be meant for the same file, all for different files or every solution in between. `lio_listio' gets the NENT requests from the array pointed to by LIST. The operation to be performed is determined by the `aio_lio_opcode' member in each element of LIST. If this field is `LIO_READ' a read operation is enqueued, similar to a call of `aio_read' for this element of the array (except that the way the termination is signalled is different, as we will see below). If the `aio_lio_opcode' member is `LIO_WRITE' a write operation is enqueued. Otherwise the `aio_lio_opcode' must be `LIO_NOP' in which case this element of LIST is simply ignored. This "operation" is useful in situations where one has a fixed array of `struct aiocb' elements from which only a few need to be handled at a time. Another situation is where the `lio_listio' call was canceled before all requests are processed (*note Cancel AIO Operations::) and the remaining requests have to be reissued. The other members of each element of the array pointed to by `list' must have values suitable for the operation as described in the documentation for `aio_read' and `aio_write' above. The MODE argument determines how `lio_listio' behaves after having enqueued all the requests. If MODE is `LIO_WAIT' it waits until all requests terminated. Otherwise MODE must be `LIO_NOWAIT' and in this case the function returns immediately after having enqueued all the requests. In this case the caller gets a notification of the termination of all requests according to the SIG parameter. If SIG is `NULL' no notification is send. Otherwise a signal is sent or a thread is started, just as described in the description for `aio_read' or `aio_write'. If MODE is `LIO_WAIT', the return value of `lio_listio' is 0 when all requests completed successfully. Otherwise the function return -1 and `errno' is set accordingly. To find out which request or requests failed one has to use the `aio_error' function on all the elements of the array LIST. In case MODE is `LIO_NOWAIT', the function returns 0 if all requests were enqueued correctly. The current state of the requests can be found using `aio_error' and `aio_return' as described above. If `lio_listio' returns -1 in this mode, the global variable `errno' is set accordingly. If a request did not yet terminate, a call to `aio_error' returns `EINPROGRESS'. If the value is different, the request is finished and the error value (or 0) is returned and the result of the operation can be retrieved using `aio_return'. Possible values for `errno' are: `EAGAIN' The resources necessary to queue all the requests are not available at the moment. The error status for each element of LIST must be checked to determine which request failed. Another reason could be that the system wide limit of AIO requests is exceeded. This cannot be the case for the implementation on GNU systems since no arbitrary limits exist. `EINVAL' The MODE parameter is invalid or NENT is larger than `AIO_LISTIO_MAX'. `EIO' One or more of the request's I/O operations failed. The error status of each request should be checked to determine which one failed. `ENOSYS' The `lio_listio' function is not supported. If the MODE parameter is `LIO_NOWAIT' and the caller cancels a request, the error status for this request returned by `aio_error' is `ECANCELED'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `lio_listio64' since the LFS interface transparently replaces the normal implementation. - Function: int lio_listio64 (int MODE, struct aiocb *const LIST, int NENT, struct sigevent *SIG) This function is similar to the `lio_listio' function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, `lio_listio64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the reading or writing, as opposed to `lseek' functionality used in `lio_listio'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `lio_listio' and so transparently replaces the interface for small files on 32 bit machines. Getting the Status of AIO Operations ------------------------------------ As already described in the documentation of the functions in the last section, it must be possible to get information about the status of an I/O request. When the operation is performed truly asynchronously (as with `aio_read' and `aio_write' and with `lio_listio' when the mode is `LIO_NOWAIT'), one sometimes needs to know whether a specific request already terminated and if so, what the result was. The following two functions allow you to get this kind of information. - Function: int aio_error (const struct aiocb *AIOCBP) This function determines the error state of the request described by the `struct aiocb' variable pointed to by AIOCBP. If the request has not yet terminated the value returned is always `EINPROGRESS'. Once the request has terminated the value `aio_error' returns is either 0 if the request completed successfully or it returns the value which would be stored in the `errno' variable if the request would have been done using `read', `write', or `fsync'. The function can return `ENOSYS' if it is not implemented. It could also return `EINVAL' if the AIOCBP parameter does not refer to an asynchronous operation whose return status is not yet known. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_error64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_error64 (const struct aiocb64 *AIOCBP) This function is similar to `aio_error' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_error' and so transparently replaces the interface for small files on 32 bit machines. - Function: ssize_t aio_return (const struct aiocb *AIOCBP) This function can be used to retrieve the return status of the operation carried out by the request described in the variable pointed to by AIOCBP. As long as the error status of this request as returned by `aio_error' is `EINPROGRESS' the return of this function is undefined. Once the request is finished this function can be used exactly once to retrieve the return value. Following calls might lead to undefined behavior. The return value itself is the value which would have been returned by the `read', `write', or `fsync' call. The function can return `ENOSYS' if it is not implemented. It could also return `EINVAL' if the AIOCBP parameter does not refer to an asynchronous operation whose return status is not yet known. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_return64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_return64 (const struct aiocb64 *AIOCBP) This function is similar to `aio_return' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_return' and so transparently replaces the interface for small files on 32 bit machines. Getting into a Consistent State ------------------------------- When dealing with asynchronous operations it is sometimes necessary to get into a consistent state. This would mean for AIO that one wants to know whether a certain request or a group of request were processed. This could be done by waiting for the notification sent by the system after the operation terminated, but this sometimes would mean wasting resources (mainly computation time). Instead POSIX.1b defines two functions which will help with most kinds of consistency. The `aio_fsync' and `aio_fsync64' functions are only available if the symbol `_POSIX_SYNCHRONIZED_IO' is defined in `unistd.h'. - Function: int aio_fsync (int OP, struct aiocb *AIOCBP) Calling this function forces all I/O operations operating queued at the time of the function call operating on the file descriptor `aiocbp->aio_fildes' into the synchronized I/O completion state (*note Synchronizing I/O::). The `aio_fsync' function returns immediately but the notification through the method described in `aiocbp->aio_sigevent' will happen only after all requests for this file descriptor have terminated and the file is synchronized. This also means that requests for this very same file descriptor which are queued after the synchronization request are not affected. If OP is `O_DSYNC' the synchronization happens as with a call to `fdatasync'. Otherwise OP should be `O_SYNC' and the synchronization happens as with `fsync'. As long as the synchronization has not happened, a call to `aio_error' with the reference to the object pointed to by AIOCBP returns `EINPROGRESS'. Once the synchronization is done `aio_error' return 0 if the synchronization was not successful. Otherwise the value returned is the value to which the `fsync' or `fdatasync' function would have set the `errno' variable. In this case nothing can be assumed about the consistency for the data written to this file descriptor. The return value of this function is 0 if the request was successfully enqueued. Otherwise the return value is -1 and `errno' is set to one of the following values: `EAGAIN' The request could not be enqueued due to temporary lack of resources. `EBADF' The file descriptor `aiocbp->aio_fildes' is not valid or not open for writing. `EINVAL' The implementation does not support I/O synchronization or the OP parameter is other than `O_DSYNC' and `O_SYNC'. `ENOSYS' This function is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_return64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_fsync64 (int OP, struct aiocb64 *AIOCBP) This function is similar to `aio_fsync' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_fsync' and so transparently replaces the interface for small files on 32 bit machines. Another method of synchronization is to wait until one or more requests of a specific set terminated. This could be achieved by the `aio_*' functions to notify the initiating process about the termination but in some situations this is not the ideal solution. In a program which constantly updates clients somehow connected to the server it is not always the best solution to go round robin since some connections might be slow. On the other hand letting the `aio_*' function notify the caller might also be not the best solution since whenever the process works on preparing data for on client it makes no sense to be interrupted by a notification since the new client will not be handled before the current client is served. For situations like this `aio_suspend' should be used. - Function: int aio_suspend (const struct aiocb *const LIST[], int NENT, const struct timespec *TIMEOUT) When calling this function, the calling thread is suspended until at least one of the requests pointed to by the NENT elements of the array LIST has completed. If any of the requests has already completed at the time `aio_suspend' is called, the function returns immediately. Whether a request has terminated or not is determined by comparing the error status of the request with `EINPROGRESS'. If an element of LIST is `NULL', the entry is simply ignored. If no request has finished, the calling process is suspended. If TIMEOUT is `NULL', the process is not woken until a request has finished. If TIMEOUT is not `NULL', the process remains suspended at least as long as specified in TIMEOUT. In this case, `aio_suspend' returns with an error. The return value of the function is 0 if one or more requests from the LIST have terminated. Otherwise the function returns -1 and `errno' is set to one of the following values: `EAGAIN' None of the requests from the LIST completed in the time specified by TIMEOUT. `EINTR' A signal interrupted the `aio_suspend' function. This signal might also be sent by the AIO implementation while signalling the termination of one of the requests. `ENOSYS' The `aio_suspend' function is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_suspend64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_suspend64 (const struct aiocb64 *const LIST[], int NENT, const struct timespec *TIMEOUT) This function is similar to `aio_suspend' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_suspend' and so transparently replaces the interface for small files on 32 bit machines. Cancellation of AIO Operations ------------------------------ When one or more requests are asynchronously processed, it might be useful in some situations to cancel a selected operation, e.g., if it becomes obvious that the written data is no longer accurate and would have to be overwritten soon. As an example, assume an application, which writes data in files in a situation where new incoming data would have to be written in a file which will be updated by an enqueued request. The POSIX AIO implementation provides such a function, but this function is not capable of forcing the cancellation of the request. It is up to the implementation to decide whether it is possible to cancel the operation or not. Therefore using this function is merely a hint. - Function: int aio_cancel (int FILDES, struct aiocb *AIOCBP) The `aio_cancel' function can be used to cancel one or more outstanding requests. If the AIOCBP parameter is `NULL', the function tries to cancel all of the outstanding requests which would process the file descriptor FILDES (i.e., whose `aio_fildes' member is FILDES). If AIOCBP is not `NULL', `aio_cancel' attempts to cancel the specific request pointed to by AIOCBP. For requests which were successfully canceled, the normal notification about the termination of the request should take place. I.e., depending on the `struct sigevent' object which controls this, nothing happens, a signal is sent or a thread is started. If the request cannot be canceled, it terminates the usual way after performing the operation. After a request is successfully canceled, a call to `aio_error' with a reference to this request as the parameter will return `ECANCELED' and a call to `aio_return' will return -1. If the request wasn't canceled and is still running the error status is still `EINPROGRESS'. The return value of the function is `AIO_CANCELED' if there were requests which haven't terminated and which were successfully canceled. If there is one or more requests left which couldn't be canceled, the return value is `AIO_NOTCANCELED'. In this case `aio_error' must be used to find out which of the, perhaps multiple, requests (in AIOCBP is `NULL') weren't successfully canceled. If all requests already terminated at the time `aio_cancel' is called the return value is `AIO_ALLDONE'. If an error occurred during the execution of `aio_cancel' the function returns -1 and sets `errno' to one of the following values. `EBADF' The file descriptor FILDES is not valid. `ENOSYS' `aio_cancel' is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `aio_cancel64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_cancel64 (int FILDES, struct aiocb64 *AIOCBP) This function is similar to `aio_cancel' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_cancel' and so transparently replaces the interface for small files on 32 bit machines. How to optimize the AIO implementation -------------------------------------- The POSIX standard does not specify how the AIO functions are implemented. They could be system calls, but it is also possible to emulate them at userlevel. At the point of this writing, the available implementation is a userlevel implementation which uses threads for handling the enqueued requests. While this implementation requires making some decisions about limitations, hard limitations are something which is best avoided in the GNU C library. Therefore, the GNU C library provides a means for tuning the AIO implementation according to the individual use. - Data Type: struct aioinit This data type is used to pass the configuration or tunable parameters to the implementation. The program has to initialize the members of this struct and pass it to the implementation using the `aio_init' function. `int aio_threads' This member specifies the maximal number of threads which may be used at any one time. `int aio_num' This number provides an estimate on the maximal number of simultaneously enqueued requests. `int aio_locks' Unused. `int aio_usedba' Unused. `int aio_debug' Unused. `int aio_numusers' Unused. `int aio_reserved[2]' Unused. - Function: void aio_init (const struct aioinit *INIT) This function must be called before any other AIO function. Calling it is completely voluntary, as it is only meant to help the AIO implementation perform better. Before calling the `aio_init', function the members of a variable of type `struct aioinit' must be initialized. Then a reference to this variable is passed as the parameter to `aio_init' which itself may or may not pay attention to the hints. The function has no return value and no error cases are defined. It is a extension which follows a proposal from the SGI implementation in Irix 6. It is not covered by POSIX.1b or Unix98. Control Operations on Files =========================== This section describes how you can perform various other operations on file descriptors, such as inquiring about or setting flags describing the status of the file descriptor, manipulating record locks, and the like. All of these operations are performed by the function `fcntl'. The second argument to the `fcntl' function is a command that specifies which operation to perform. The function and macros that name various flags that are used with it are declared in the header file `fcntl.h'. Many of these flags are also used by the `open' function; see *Note Opening and Closing Files::. - Function: int fcntl (int FILEDES, int COMMAND, ...) The `fcntl' function performs the operation specified by COMMAND on the file descriptor FILEDES. Some commands require additional arguments to be supplied. These additional arguments and the return value and error conditions are given in the detailed descriptions of the individual commands. Briefly, here is a list of what the various commands are. `F_DUPFD' Duplicate the file descriptor (return another file descriptor pointing to the same open file). *Note Duplicating Descriptors::. `F_GETFD' Get flags associated with the file descriptor. *Note Descriptor Flags::. `F_SETFD' Set flags associated with the file descriptor. *Note Descriptor Flags::. `F_GETFL' Get flags associated with the open file. *Note File Status Flags::. `F_SETFL' Set flags associated with the open file. *Note File Status Flags::. `F_GETLK' Get a file lock. *Note File Locks::. `F_SETLK' Set or clear a file lock. *Note File Locks::. `F_SETLKW' Like `F_SETLK', but wait for completion. *Note File Locks::. `F_GETOWN' Get process or process group ID to receive `SIGIO' signals. *Note Interrupt Input::. `F_SETOWN' Set process or process group ID to receive `SIGIO' signals. *Note Interrupt Input::. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `fcntl' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `fcntl' should be protected using cancellation handlers. Duplicating Descriptors ======================= You can "duplicate" a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (*note File Status Flags::), but each has its own set of file descriptor flags (*note Descriptor Flags::). The major use of duplicating a file descriptor is to implement "redirection" of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to. You can perform this operation using the `fcntl' function with the `F_DUPFD' command, but there are also convenient functions `dup' and `dup2' for duplicating descriptors. The `fcntl' function and flags are declared in `fcntl.h', while prototypes for `dup' and `dup2' are in the header file `unistd.h'. - Function: int dup (int OLD) This function copies descriptor OLD to the first available descriptor number (the first number not currently open). It is equivalent to `fcntl (OLD, F_DUPFD, 0)'. - Function: int dup2 (int OLD, int NEW) This function copies the descriptor OLD to descriptor number NEW. If OLD is an invalid descriptor, then `dup2' does nothing; it does not close NEW. Otherwise, the new duplicate of OLD replaces any previous meaning of descriptor NEW, as if NEW were closed first. If OLD and NEW are different numbers, and OLD is a valid descriptor number, then `dup2' is equivalent to: close (NEW); fcntl (OLD, F_DUPFD, NEW) However, `dup2' does this atomically; there is no instant in the middle of calling `dup2' at which NEW is closed and not yet a duplicate of OLD. - Macro: int F_DUPFD This macro is used as the COMMAND argument to `fcntl', to copy the file descriptor given as the first argument. The form of the call in this case is: fcntl (OLD, F_DUPFD, NEXT-FILEDES) The NEXT-FILEDES argument is of type `int' and specifies that the file descriptor returned should be the next available one greater than or equal to this value. The return value from `fcntl' with this command is normally the value of the new file descriptor. A return value of -1 indicates an error. The following `errno' error conditions are defined for this command: `EBADF' The OLD argument is invalid. `EINVAL' The NEXT-FILEDES argument is invalid. `EMFILE' There are no more file descriptors available--your program is already using the maximum. In BSD and GNU, the maximum is controlled by a resource limit that can be changed; *note Limits on Resources::, for more information about the `RLIMIT_NOFILE' limit. `ENFILE' is not a possible error code for `dup2' because `dup2' does not create a new opening of a file; duplicate descriptors do not count toward the limit which `ENFILE' indicates. `EMFILE' is possible because it refers to the limit on distinct descriptor numbers in use in one process. Here is an example showing how to use `dup2' to do redirection. Typically, redirection of the standard streams (like `stdin') is done by a shell or shell-like program before calling one of the `exec' functions (*note Executing a File::) to execute a new program in a child process. When the new program is executed, it creates and initializes the standard streams to point to the corresponding file descriptors, before its `main' function is invoked. So, to redirect standard input to a file, the shell could do something like: pid = fork (); if (pid == 0) { char *filename; char *program; int file; ... file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY)); dup2 (file, STDIN_FILENO); TEMP_FAILURE_RETRY (close (file)); execv (program, NULL); } There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in *Note Launching Jobs::. File Descriptor Flags ===================== "File descriptor flags" are miscellaneous attributes of a file descriptor. These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags. Currently there is just one file descriptor flag: `FD_CLOEXEC', which causes the descriptor to be closed if you use any of the `exec...' functions (*note Executing a File::). The symbols in this section are defined in the header file `fcntl.h'. - Macro: int F_GETFD This macro is used as the COMMAND argument to `fcntl', to specify that it should return the file descriptor flags associated with the FILEDES argument. The normal return value from `fcntl' with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags (except that currently there is only one flag to use). In case of an error, `fcntl' returns -1. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETFD This macro is used as the COMMAND argument to `fcntl', to specify that it should set the file descriptor flags associated with the FILEDES argument. This requires a third `int' argument to specify the new flags, so the form of the call is: fcntl (FILEDES, F_SETFD, NEW-FLAGS) The normal return value from `fcntl' with this command is an unspecified value other than -1, which indicates an error. The flags and error conditions are the same as for the `F_GETFD' command. The following macro is defined for use as a file descriptor flag with the `fcntl' function. The value is an integer constant usable as a bit mask value. - Macro: int FD_CLOEXEC This flag specifies that the file descriptor should be closed when an `exec' function is invoked; see *Note Executing a File::. When a file descriptor is allocated (as with `open' or `dup'), this bit is initially cleared on the new file descriptor, meaning that descriptor will survive into the new program after `exec'. If you want to modify the file descriptor flags, you should get the current flags with `F_GETFD' and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag `FD_CLOEXEC' without altering any other flags: /* Set the `FD_CLOEXEC' flag of DESC if VALUE is nonzero, or clear the flag if VALUE is 0. Return 0 on success, or -1 on error with `errno' set. */ int set_cloexec_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFD, 0); /* If reading the flags failed, return error indication now. if (oldflags < 0) return oldflags; /* Set just the flag we want to set. */ if (value != 0) oldflags |= FD_CLOEXEC; else oldflags &= ~FD_CLOEXEC; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFD, oldflags); } File Status Flags ================= "File status flags" are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in *Note Descriptor Flags::, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file. The file status flags are specified with the FLAGS argument to `open'; *note Opening and Closing Files::. File status flags fall into three categories, which are described in the following sections. * *Note Access Modes::, specify what type of access is allowed to the file: reading, writing, or both. They are set by `open' and are returned by `fcntl', but cannot be changed. * *Note Open-time Flags::, control details of what `open' will do. These flags are not preserved after the `open' call. * *Note Operating Modes::, affect how operations such as `read' and `write' are done. They are set by `open', and can be fetched or changed with `fcntl'. The symbols in this section are defined in the header file `fcntl.h'. File Access Modes ----------------- The file access modes allow a file descriptor to be used for reading, writing, or both. (In the GNU system, they can also allow none of these, and allow execution of the file as a program.) The access modes are chosen when the file is opened, and never change. - Macro: int O_RDONLY Open the file for read access. - Macro: int O_WRONLY Open the file for write access. - Macro: int O_RDWR Open the file for both reading and writing. In the GNU system (and not in other systems), `O_RDONLY' and `O_WRONLY' are independent bits that can be bitwise-ORed together, and it is valid for either bit to be set or clear. This means that `O_RDWR' is the same as `O_RDONLY|O_WRONLY'. A file access mode of zero is permissible; it allows no operations that do input or output to the file, but does allow other operations such as `fchmod'. On the GNU system, since "read-only" or "write-only" is a misnomer, `fcntl.h' defines additional names for the file access modes. These names are preferred when writing GNU-specific code. But most programs will want to be portable to other POSIX.1 systems and should use the POSIX.1 names above instead. - Macro: int O_READ Open the file for reading. Same as `O_RDWR'; only defined on GNU. - Macro: int O_WRITE Open the file for reading. Same as `O_WRONLY'; only defined on GNU. - Macro: int O_EXEC Open the file for executing. Only defined on GNU. To determine the file access mode with `fcntl', you must extract the access mode bits from the retrieved file status flags. In the GNU system, you can just test the `O_READ' and `O_WRITE' bits in the flags word. But in other POSIX.1 systems, reading and writing access modes are not stored as distinct bit flags. The portable way to extract the file access mode bits is with `O_ACCMODE'. - Macro: int O_ACCMODE This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be `O_RDONLY', `O_WRONLY', or `O_RDWR'. (In the GNU system it could also be zero, and it never includes the `O_EXEC' bit.) Open-time Flags --------------- The open-time flags specify options affecting how `open' will behave. These options are not preserved once the file is open. The exception to this is `O_NONBLOCK', which is also an I/O operating mode and so it _is_ saved. *Note Opening and Closing Files::, for how to call `open'. There are two sorts of options specified by open-time flags. * "File name translation flags" affect how `open' looks up the file name to locate the file, and whether the file can be created. * "Open-time action flags" specify extra operations that `open' will perform on the file once it is open. Here are the file name translation flags. - Macro: int O_CREAT If set, the file will be created if it doesn't already exist. - Macro: int O_EXCL If both `O_CREAT' and `O_EXCL' are set, then `open' fails if the specified file already exists. This is guaranteed to never clobber an existing file. - Macro: int O_NONBLOCK This prevents `open' from blocking for a "long time" to open the file. This is only meaningful for some kinds of files, usually devices such as serial ports; when it is not meaningful, it is harmless and ignored. Often opening a port to a modem blocks until the modem reports carrier detection; if `O_NONBLOCK' is specified, `open' will return immediately without a carrier. Note that the `O_NONBLOCK' flag is overloaded as both an I/O operating mode and a file name translation flag. This means that specifying `O_NONBLOCK' in `open' also sets nonblocking I/O mode; *note Operating Modes::. To open the file without blocking but do normal I/O that blocks, you must call `open' with `O_NONBLOCK' set and then call `fcntl' to turn the bit off. - Macro: int O_NOCTTY If the named file is a terminal device, don't make it the controlling terminal for the process. *Note Job Control::, for information about what it means to be the controlling terminal. In the GNU system and 4.4 BSD, opening a file never makes it the controlling terminal and `O_NOCTTY' is zero. However, other systems may use a nonzero value for `O_NOCTTY' and set the controlling terminal when you open a file that is a terminal device; so to be portable, use `O_NOCTTY' when it is important to avoid this. The following three file name translation flags exist only in the GNU system. - Macro: int O_IGNORE_CTTY Do not recognize the named file as the controlling terminal, even if it refers to the process's existing controlling terminal device. Operations on the new file descriptor will never induce job control signals. *Note Job Control::. - Macro: int O_NOLINK If the named file is a symbolic link, open the link itself instead of the file it refers to. (`fstat' on the new file descriptor will return the information returned by `lstat' on the link's name.) - Macro: int O_NOTRANS If the named file is specially translated, do not invoke the translator. Open the bare file the translator itself sees. The open-time action flags tell `open' to do additional operations which are not really related to opening the file. The reason to do them as part of `open' instead of in separate calls is that `open' can do them atomically. - Macro: int O_TRUNC Truncate the file to zero length. This option is only useful for regular files, not special files such as directories or FIFOs. POSIX.1 requires that you open the file for writing to use `O_TRUNC'. In BSD and GNU you must have permission to write the file to truncate it, but you need not open for write access. This is the only open-time action flag specified by POSIX.1. There is no good reason for truncation to be done by `open', instead of by calling `ftruncate' afterwards. The `O_TRUNC' flag existed in Unix before `ftruncate' was invented, and is retained for backward compatibility. The remaining operating modes are BSD extensions. They exist only on some systems. On other systems, these macros are not defined. - Macro: int O_SHLOCK Acquire a shared lock on the file, as with `flock'. *Note File Locks::. If `O_CREAT' is specified, the locking is done atomically when creating the file. You are guaranteed that no other process will get the lock on the new file first. - Macro: int O_EXLOCK Acquire an exclusive lock on the file, as with `flock'. *Note File Locks::. This is atomic like `O_SHLOCK'. I/O Operating Modes ------------------- The operating modes affect how input and output operations using a file descriptor work. These flags are set by `open' and can be fetched and changed with `fcntl'. - Macro: int O_APPEND The bit that enables append mode for the file. If set, then all `write' operations write the data at the end of the file, extending it, regardless of the current file position. This is the only reliable way to append to a file. In append mode, you are guaranteed that the data you write will always go to the current end of the file, regardless of other processes writing to the file. Conversely, if you simply set the file position to the end of file and write, then another process can extend the file after you set the file position but before you write, resulting in your data appearing someplace before the real end of file. - Macro: int O_NONBLOCK The bit that enables nonblocking mode for the file. If this bit is set, `read' requests on the file can return immediately with a failure status if there is no input immediately available, instead of blocking. Likewise, `write' requests can also return immediately with a failure status if the output can't be written immediately. Note that the `O_NONBLOCK' flag is overloaded as both an I/O operating mode and a file name translation flag; *note Open-time Flags::. - Macro: int O_NDELAY This is an obsolete name for `O_NONBLOCK', provided for compatibility with BSD. It is not defined by the POSIX.1 standard. The remaining operating modes are BSD and GNU extensions. They exist only on some systems. On other systems, these macros are not defined. - Macro: int O_ASYNC The bit that enables asynchronous input mode. If set, then `SIGIO' signals will be generated when input is available. *Note Interrupt Input::. Asynchronous input mode is a BSD feature. - Macro: int O_FSYNC The bit that enables synchronous writing for the file. If set, each `write' call will make sure the data is reliably stored on disk before returning. Synchronous writing is a BSD feature. - Macro: int O_SYNC This is another name for `O_FSYNC'. They have the same value. - Macro: int O_NOATIME If this bit is set, `read' will not update the access time of the file. *Note File Times::. This is used by programs that do backups, so that backing a file up does not count as reading it. Only the owner of the file or the superuser may use this bit. This is a GNU extension. Getting and Setting File Status Flags ------------------------------------- The `fcntl' function can fetch or change file status flags. - Macro: int F_GETFL This macro is used as the COMMAND argument to `fcntl', to read the file status flags for the open file with descriptor FILEDES. The normal return value from `fcntl' with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with `O_ACCMODE' to compare them. In case of an error, `fcntl' returns -1. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETFL This macro is used as the COMMAND argument to `fcntl', to set the file status flags for the open file corresponding to the FILEDES argument. This command requires a third `int' argument to specify the new flags, so the call looks like this: fcntl (FILEDES, F_SETFL, NEW-FLAGS) You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing. The normal return value from `fcntl' with this command is an unspecified value other than -1, which indicates an error. The error conditions are the same as for the `F_GETFL' command. If you want to modify the file status flags, you should get the current flags with `F_GETFL' and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag `O_NONBLOCK' without altering any other flags: /* Set the `O_NONBLOCK' flag of DESC if VALUE is nonzero, or clear the flag if VALUE is 0. Return 0 on success, or -1 on error with `errno' set. */ int set_nonblock_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFL, 0); /* If reading the flags failed, return error indication now. */ if (oldflags == -1) return -1; /* Set just the flag we want to set. */ if (value != 0) oldflags |= O_NONBLOCK; else oldflags &= ~O_NONBLOCK; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFL, oldflags); } File Locks ========== The remaining `fcntl' commands are used to support "record locking", which permits multiple cooperating programs to prevent each other from simultaneously accessing parts of a file in error-prone ways. An "exclusive" or "write" lock gives a process exclusive access for writing to the specified part of the file. While a write lock is in place, no other process can lock that part of the file. A "shared" or "read" lock prohibits any other process from requesting a write lock on the specified part of the file. However, other processes can request read locks. The `read' and `write' functions do not actually check to see whether there are any locks in place. If you want to implement a locking protocol for a file shared by multiple processes, your application must do explicit `fcntl' calls to request and clear locks at the appropriate points. Locks are associated with processes. A process can only have one kind of lock set for each byte of a given file. When any file descriptor for that file is closed by the process, all of the locks that process holds on that file are released, even if the locks were made using other descriptors that remain open. Likewise, locks are released when a process exits, and are not inherited by child processes created using `fork' (*note Creating a Process::). When making a lock, use a `struct flock' to specify what kind of lock and where. This data type and the associated macros for the `fcntl' function are declared in the header file `fcntl.h'. - Data Type: struct flock This structure is used with the `fcntl' function to describe a file lock. It has these members: `short int l_type' Specifies the type of the lock; one of `F_RDLCK', `F_WRLCK', or `F_UNLCK'. `short int l_whence' This corresponds to the WHENCE argument to `fseek' or `lseek', and specifies what the offset is relative to. Its value can be one of `SEEK_SET', `SEEK_CUR', or `SEEK_END'. `off_t l_start' This specifies the offset of the start of the region to which the lock applies, and is given in bytes relative to the point specified by `l_whence' member. `off_t l_len' This specifies the length of the region to be locked. A value of `0' is treated specially; it means the region extends to the end of the file. `pid_t l_pid' This field is the process ID (*note Process Creation Concepts::) of the process holding the lock. It is filled in by calling `fcntl' with the `F_GETLK' command, but is ignored when making a lock. - Macro: int F_GETLK This macro is used as the COMMAND argument to `fcntl', to specify that it should get information about a lock. This command requires a third argument of type `struct flock *' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_GETLK, LOCKP) If there is a lock already in place that would block the lock described by the LOCKP argument, information about that lock overwrites `*LOCKP'. Existing locks are not reported if they are compatible with making a new lock as specified. Thus, you should specify a lock type of `F_WRLCK' if you want to find out about both read and write locks, or `F_RDLCK' if you want to find out about write locks only. There might be more than one lock affecting the region specified by the LOCKP argument, but `fcntl' only returns information about one of them. The `l_whence' member of the LOCKP structure is set to `SEEK_SET' and the `l_start' and `l_len' fields set to identify the locked region. If no lock applies, the only change to the LOCKP structure is to update the `l_type' to a value of `F_UNLCK'. The normal return value from `fcntl' with this command is an unspecified value other than -1, which is reserved to indicate an error. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. `EINVAL' Either the LOCKP argument doesn't specify valid lock information, or the file associated with FILEDES doesn't support locks. - Macro: int F_SETLK This macro is used as the COMMAND argument to `fcntl', to specify that it should set or clear a lock. This command requires a third argument of type `struct flock *' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_SETLK, LOCKP) If the process already has a lock on any part of the region, the old lock on that part is replaced with the new lock. You can remove a lock by specifying a lock type of `F_UNLCK'. If the lock cannot be set, `fcntl' returns immediately with a value of -1. This function does not block waiting for other processes to release locks. If `fcntl' succeeds, it return a value other than -1. The following `errno' error conditions are defined for this function: `EAGAIN' `EACCES' The lock cannot be set because it is blocked by an existing lock on the file. Some systems use `EAGAIN' in this case, and other systems use `EACCES'; your program should treat them alike, after `F_SETLK'. (The GNU system always uses `EAGAIN'.) `EBADF' Either: the FILEDES argument is invalid; you requested a read lock but the FILEDES is not open for read access; or, you requested a write lock but the FILEDES is not open for write access. `EINVAL' Either the LOCKP argument doesn't specify valid lock information, or the file associated with FILEDES doesn't support locks. `ENOLCK' The system has run out of file lock resources; there are already too many file locks in place. Well-designed file systems never report this error, because they have no limitation on the number of locks. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine. - Macro: int F_SETLKW This macro is used as the COMMAND argument to `fcntl', to specify that it should set or clear a lock. It is just like the `F_SETLK' command, but causes the process to block (or wait) until the request can be specified. This command requires a third argument of type `struct flock *', as for the `F_SETLK' command. The `fcntl' return values and errors are the same as for the `F_SETLK' command, but these additional `errno' error conditions are defined for this command: `EINTR' The function was interrupted by a signal while it was waiting. *Note Interrupted Primitives::. `EDEADLK' The specified region is being locked by another process. But that process is waiting to lock a region which the current process has locked, so waiting for the lock would result in deadlock. The system does not guarantee that it will detect all such conditions, but it lets you know if it notices one. The following macros are defined for use as values for the `l_type' member of the `flock' structure. The values are integer constants. `F_RDLCK' This macro is used to specify a read (or shared) lock. `F_WRLCK' This macro is used to specify a write (or exclusive) lock. `F_UNLCK' This macro is used to specify that the region is unlocked. As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file. One example of such a program might be a game that uses a file to keep track of high scores. Another example might be a program that records usage or accounting information for billing purposes. Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up. But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file. If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock. While the read lock is set, no other process can lock that part of the file for writing. Remember that file locks are only a _voluntary_ protocol for controlling access to a file. There is still potential for access to the file by programs that don't use the lock protocol. Interrupt-Driven Input ====================== If you set the `O_ASYNC' status flag on a file descriptor (*note File Status Flags::), a `SIGIO' signal is sent whenever input or output becomes possible on that file descriptor. The process or process group to receive the signal can be selected by using the `F_SETOWN' command to the `fcntl' function. If the file descriptor is a socket, this also selects the recipient of `SIGURG' signals that are delivered when out-of-band data arrives on that socket; see *Note Out-of-Band Data::. (`SIGURG' is sent in any situation where `select' would report the socket as having an "exceptional condition". *Note Waiting for I/O::.) If the file descriptor corresponds to a terminal device, then `SIGIO' signals are sent to the foreground process group of the terminal. *Note Job Control::. The symbols in this section are defined in the header file `fcntl.h'. - Macro: int F_GETOWN This macro is used as the COMMAND argument to `fcntl', to specify that it should get information about the process or process group to which `SIGIO' signals are sent. (For a terminal, this is actually the foreground process group ID, which you can get using `tcgetpgrp'; see *Note Terminal Access Functions::.) The return value is interpreted as a process ID; if negative, its absolute value is the process group ID. The following `errno' error condition is defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETOWN This macro is used as the COMMAND argument to `fcntl', to specify that it should set the process or process group to which `SIGIO' signals are sent. This command requires a third argument of type `pid_t' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_SETOWN, PID) The PID argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID. The return value from `fcntl' with this command is -1 in case of error and some other value if successful. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. `ESRCH' There is no process or process group corresponding to PID. Generic I/O Control operations ============================== The GNU system can handle most input/output operations on many different devices and objects in terms of a few file primitives - `read', `write' and `lseek'. However, most devices also have a few peculiar operations which do not fit into this model. Such as: * Changing the character font used on a terminal. * Telling a magnetic tape system to rewind or fast forward. (Since they cannot move in byte increments, `lseek' is inapplicable). * Ejecting a disk from a drive. * Playing an audio track from a CD-ROM drive. * Maintaining routing tables for a network. Although some such objects such as sockets and terminals (1) have special functions of their own, it would not be practical to create functions for all these cases. Instead these minor operations, known as "IOCTL"s, are assigned code numbers and multiplexed through the `ioctl' function, defined in `sys/ioctl.h'. The code numbers themselves are defined in many different headers. - Function: int ioctl (int FILEDES, int COMMAND, ...) The `ioctl' function performs the generic I/O operation COMMAND on FILEDES. A third argument is usually present, either a single number or a pointer to a structure. The meaning of this argument, the returned value, and any error codes depends upon the command used. Often -1 is returned for a failure. On some systems, IOCTLs used by different devices share the same numbers. Thus, although use of an inappropriate IOCTL _usually_ only produces an error, you should not attempt to use device-specific IOCTLs on an unknown device. Most IOCTLs are OS-specific and/or only used in special system utilities, and are thus beyond the scope of this document. For an example of the use of an IOCTL, see *Note Out-of-Band Data::. ---------- Footnotes ---------- (1) Actually, the terminal-specific functions are implemented with IOCTLs on many platforms. File System Interface ********************* This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions (*note I/O on Streams::; *note Low-Level I/O::), these functions are concerned with operating on the files themselves rather than on their contents. Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times. Working Directory ================= Each process has associated with it a directory, called its "current working directory" or simply "working directory", that is used in the resolution of relative file names (*note File Name Resolution::). When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database. You can find any user's home directory using the `getpwuid' or `getpwnam' functions; see *Note User Database::. Users can change the working directory using shell commands like `cd'. The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory. Prototypes for these functions are declared in the header file `unistd.h'. - Function: char * getcwd (char *BUFFER, size_t SIZE) The `getcwd' function returns an absolute file name representing the current working directory, storing it in the character array BUFFER that you provide. The SIZE argument is how you tell the system the allocation size of BUFFER. The GNU library version of this function also permits you to specify a null pointer for the BUFFER argument. Then `getcwd' allocates a buffer automatically, as with `malloc' (*note Unconstrained Allocation::). If the SIZE is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result. The return value is BUFFER on success and a null pointer on failure. The following `errno' error conditions are defined for this function: `EINVAL' The SIZE argument is zero and BUFFER is not a null pointer. `ERANGE' The SIZE argument is less than the length of the working directory name. You need to allocate a bigger array and try again. `EACCES' Permission to read or search a component of the file name was denied. You could implement the behavior of GNU's `getcwd (NULL, 0)' using only the standard behavior of `getcwd': char * gnu_getcwd () { size_t size = 100; while (1) { char *buffer = (char *) xmalloc (size); if (getcwd (buffer, size) == buffer) return buffer; free (buffer); if (errno != ERANGE) return 0; size *= 2; } } *Note Malloc Examples::, for information about `xmalloc', which is not a library function but is a customary name used in most GNU software. - Deprecated Function: char * getwd (char *BUFFER) This is similar to `getcwd', but has no way to specify the size of the buffer. The GNU library provides `getwd' only for backwards compatibility with BSD. The BUFFER argument should be a pointer to an array at least `PATH_MAX' bytes long (*note Limits for Files::). In the GNU system there is no limit to the size of a file name, so this is not necessarily enough space to contain the directory name. That is why this function is deprecated. - Function: char * get_current_dir_name (void) This `get_current_dir_name' function is bascially equivalent to `getcwd (NULL, 0)'. The only difference is that the value of the `PWD' variable is returned if this value is correct. This is a subtle difference which is visible if the path described by the `PWD' value is using one or more symbol links in which case the value returned by `getcwd' can resolve the symbol links and therefore yield a different result. This function is a GNU extension. - Function: int chdir (const char *FILENAME) This function is used to set the process's working directory to FILENAME. The normal, successful return value from `chdir' is `0'. A value of `-1' is returned to indicate an error. The `errno' error conditions defined for this function are the usual file name syntax errors (*note File Name Errors::), plus `ENOTDIR' if the file FILENAME is not a directory. - Function: int fchdir (int FILEDES) This function is used to set the process's working directory to directory associated with the file descriptor FILEDES. The normal, successful return value from `fchdir' is `0'. A value of `-1' is returned to indicate an error. The following `errno' error conditions are defined for this function: `EACCES' Read permission is denied for the directory named by `dirname'. `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTDIR' The file descriptor FILEDES is not associated with a directory. `EINTR' The function call was interrupt by a signal. `EIO' An I/O error occurred. Accessing Directories ===================== The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu. The `opendir' function opens a "directory stream" whose elements are directory entries. You use the `readdir' function on the directory stream to retrieve these entries, represented as `struct dirent' objects. The name of the file for each entry is stored in the `d_name' member of this structure. There are obvious parallels here to the stream facilities for ordinary files, described in *Note I/O on Streams::. Format of a Directory Entry --------------------------- This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file `dirent.h'. - Data Type: struct dirent This is a structure type used to return information about directory entries. It contains the following fields: `char d_name[]' This is the null-terminated file name component. This is the only field you can count on in all POSIX systems. `ino_t d_fileno' This is the file serial number. For BSD compatibility, you can also refer to this member as `d_ino'. In the GNU system and most POSIX systems, for most files this the same as the `st_ino' member that `stat' will return for the file. *Note File Attributes::. `unsigned char d_namlen' This is the length of the file name, not including the terminating null character. Its type is `unsigned char' because that is the integer type of the appropriate size `unsigned char d_type' This is the type of the file, possibly unknown. The following constants are defined for its value: `DT_UNKNOWN' The type is unknown. On some systems this is the only value returned. `DT_REG' A regular file. `DT_DIR' A directory. `DT_FIFO' A named pipe, or FIFO. *Note FIFO Special Files::. `DT_SOCK' A local-domain socket. `DT_CHR' A character device. `DT_BLK' A block device. This member is a BSD extension. The symbol `_DIRENT_HAVE_D_TYPE' is defined if this member is available. On systems where it is used, it corresponds to the file type bits in the `st_mode' member of `struct statbuf'. If the value cannot be determine the member value is DT_UNKNOWN. These two macros convert between `d_type' values and `st_mode' values: - Function: int IFTODT (mode_t MODE) This returns the `d_type' value corresponding to MODE. - Function: mode_t DTTOIF (int DTYPE) This returns the `st_mode' value corresponding to DTYPE. This structure may contain additional members in the future. Their availability is always announced in the compilation environment by a macro names `_DIRENT_HAVE_D_XXX' where XXX is replaced by the name of the new member. For instance, the member `d_reclen' available on some systems is announced through the macro `_DIRENT_HAVE_D_RECLEN'. When a file has multiple names, each name has its own directory entry. The only way you can tell that the directory entries belong to a single file is that they have the same value for the `d_fileno' field. File attributes such as size, modification times etc., are part of the file itself, not of any particular directory entry. *Note File Attributes::. Opening a Directory Stream -------------------------- This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'. - Data Type: DIR The `DIR' data type represents a directory stream. You shouldn't ever allocate objects of the `struct dirent' or `DIR' data types, since the directory access functions do that for you. Instead, you refer to these objects using the pointers returned by the following functions. - Function: DIR * opendir (const char *DIRNAME) The `opendir' function opens and returns a directory stream for reading the directory whose file name is DIRNAME. The stream has type `DIR *'. If unsuccessful, `opendir' returns a null pointer. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Read permission is denied for the directory named by `dirname'. `EMFILE' The process has too many files open. `ENFILE' The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.) The `DIR' type is typically implemented using a file descriptor, and the `opendir' function in terms of the `open' function. *Note Low-Level I/O::. Directory streams and the underlying file descriptors are closed on `exec' (*note Executing a File::). In some situations it can be desirable to get hold of the file descriptor which is created by the `opendir' call. For instance, to switch the current working directory to the directory just read the `fchdir' function could be used. Historically the `DIR' type was exposed and programs could access the fields. This does not happen in the GNU C library. Instead a separate function is provided to allow access. - Function: int dirfd (DIR *DIRSTREAM) The function `dirfd' returns the file descriptor associated with the directory stream DIRSTREAM. This descriptor can be used until the directory is closed with `closedir'. If the directory stream implementation is not using file descriptors the return value is `-1'. Reading and Closing a Directory Stream -------------------------------------- This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file `dirent.h'. - Function: struct dirent * readdir (DIR *DIRSTREAM) This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call. *Portability Note:* On some systems `readdir' may not return entries for `.' and `..', even though these are always valid file names in any directory. *Note File Name Resolution::. If there are no more entries in the directory or an error is detected, `readdir' returns a null pointer. The following `errno' error conditions are defined for this function: `EBADF' The DIRSTREAM argument is not valid. `readdir' is not thread safe. Multiple threads using `readdir' on the same DIRSTREAM may overwrite the return value. Use `readdir_r' when this is critical. - Function: int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY, struct dirent **RESULT) This function is the reentrant version of `readdir'. Like `readdir' it returns the next entry from the directory. But to prevent conflicts between simultaneously running threads the result is not stored in statically allocated memory. Instead the argument ENTRY points to a place to store the result. The return value is `0' in case the next entry was read successfully. In this case a pointer to the result is returned in *RESULT. It is not required that *RESULT is the same as ENTRY. If something goes wrong while executing `readdir_r' the function returns a value indicating the error (as described for `readdir'). If there are no more directory entries, `readdir_r''s return value is `0', and *RESULT is set to `NULL'. *Portability Note:* On some systems `readdir_r' may not return a NUL terminated string for the file name, even when there is no `d_reclen' field in `struct dirent' and the file name is the maximum allowed size. Modern systems all have the `d_reclen' field, and on old systems multi-threading is not critical. In any case there is no such problem with the `readdir' function, so that even on systems without the `d_reclen' member one could use multiple threads by using external locking. It is also important to look at the definition of the `struct dirent' type. Simply passing a pointer to an object of this type for the second parameter of `readdir_r' might not be enough. Some systems don't define the `d_name' element sufficiently long. In this case the user has to provide additional space. There must be room for at least `NAME_MAX + 1' characters in the `d_name' array. Code to call `readdir_r' could look like this: union { struct dirent d; char b[offsetof (struct dirent, d_name) + NAME_MAX + 1]; } u; if (readdir_r (dir, &u.d, &res) == 0) ... To support large filesystems on 32-bit machines there are LFS variants of the last two functions. - Function: struct dirent64 * readdir64 (DIR *DIRSTREAM) The `readdir64' function is just like the `readdir' function except that it returns a pointer to a record of type `struct dirent64'. Some of the members of this data type (notably `d_ino') might have a different size to allow large filesystems. In all other aspects this function is equivalent to `readdir'. - Function: int readdir64_r (DIR *DIRSTREAM, struct dirent64 *ENTRY, struct dirent64 **RESULT) The `readdir64_r' function is equivalent to the `readdir_r' function except that it takes parameters of base type `struct dirent64' instead of `struct dirent' in the second and third position. The same precautions mentioned in the documentation of `readdir_r' also apply here. - Function: int closedir (DIR *DIRSTREAM) This function closes the directory stream DIRSTREAM. It returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' The DIRSTREAM argument is not valid. Simple Program to List a Directory ---------------------------------- Here's a simple program that prints the names of the files in the current working directory: #include #include #include #include int main (void) { DIR *dp; struct dirent *ep; dp = opendir ("./"); if (dp != NULL) { while (ep = readdir (dp)) puts (ep->d_name); (void) closedir (dp); } else perror ("Couldn't open the directory"); return 0; } The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see *Note Scanning Directory Content::, and *Note Array Sort Function::. Random Access in a Directory Stream ----------------------------------- This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file `dirent.h'. - Function: void rewinddir (DIR *DIRSTREAM) The `rewinddir' function is used to reinitialize the directory stream DIRSTREAM, so that if you call `readdir' it returns information about the first entry in the directory again. This function also notices if files have been added or removed to the directory since it was opened with `opendir'. (Entries for these files might or might not be returned by `readdir' if they were added or removed since you last called `opendir' or `rewinddir'.) - Function: off_t telldir (DIR *DIRSTREAM) The `telldir' function returns the file position of the directory stream DIRSTREAM. You can use this value with `seekdir' to restore the directory stream to that position. - Function: void seekdir (DIR *DIRSTREAM, off_t POS) The `seekdir' function sets the file position of the directory stream DIRSTREAM to POS. The value POS must be the result of a previous call to `telldir' on this particular stream; closing and reopening the directory can invalidate values returned by `telldir'. Scanning the Content of a Directory ----------------------------------- A higher-level interface to the directory handling functions is the `scandir' function. With its help one can select a subset of the entries in a directory, possibly sort them and get a list of names as the result. - Function: int scandir (const char *DIR, struct dirent ***NAMELIST, int (*SELECTOR) (const struct dirent *), int (*CMP) (const void *, const void *)) The `scandir' function scans the contents of the directory selected by DIR. The result in *NAMELIST is an array of pointers to structure of type `struct dirent' which describe all selected directory entries and which is allocated using `malloc'. Instead of always getting all directory entries returned, the user supplied function SELECTOR can be used to decide which entries are in the result. Only the entries for which SELECTOR returns a non-zero value are selected. Finally the entries in *NAMELIST are sorted using the user-supplied function CMP. The arguments passed to the CMP function are of type `struct dirent **', therefore one cannot directly use the `strcmp' or `strcoll' functions; instead see the functions `alphasort' and `versionsort' below. The return value of the function is the number of entries placed in *NAMELIST. If it is `-1' an error occurred (either the directory could not be opened for reading or the malloc call failed) and the global variable `errno' contains more information on the error. As described above the fourth argument to the `scandir' function must be a pointer to a sorting function. For the convenience of the programmer the GNU C library contains implementations of functions which are very helpful for this purpose. - Function: int alphasort (const void *A, const void *B) The `alphasort' function behaves like the `strcoll' function (*note String/Array Comparison::). The difference is that the arguments are not string pointers but instead they are of type `struct dirent **'. The return value of `alphasort' is less than, equal to, or greater than zero depending on the order of the two entries A and B. - Function: int versionsort (const void *A, const void *B) The `versionsort' function is like `alphasort' except that it uses the `strverscmp' function internally. If the filesystem supports large files we cannot use the `scandir' anymore since the `dirent' structure might not able to contain all the information. The LFS provides the new type `struct dirent64'. To use this we need a new function. - Function: int scandir64 (const char *DIR, struct dirent64 ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int (*CMP) (const void *, const void *)) The `scandir64' function works like the `scandir' function except that the directory entries it returns are described by elements of type `struct dirent64'. The function pointed to by SELECTOR is again used to select the desired entries, except that SELECTOR now must point to a function which takes a `struct dirent64 *' parameter. Similarly the CMP function should expect its two arguments to be of type `struct dirent64 **'. As CMP is now a function of a different type, the functions `alphasort' and `versionsort' cannot be supplied for that argument. Instead we provide the two replacement functions below. - Function: int alphasort64 (const void *A, const void *B) The `alphasort64' function behaves like the `strcoll' function (*note String/Array Comparison::). The difference is that the arguments are not string pointers but instead they are of type `struct dirent64 **'. Return value of `alphasort64' is less than, equal to, or greater than zero depending on the order of the two entries A and B. - Function: int versionsort64 (const void *A, const void *B) The `versionsort64' function is like `alphasort64', excepted that it uses the `strverscmp' function internally. It is important not to mix the use of `scandir' and the 64-bit comparison functions or vice versa. There are systems on which this works but on others it will fail miserably. Simple Program to List a Directory, Mark II ------------------------------------------- Here is a revised version of the directory lister found above (*note Simple Directory Lister::). Using the `scandir' function we can avoid the functions which work directly with the directory contents. After the call the returned entries are available for direct use. #include #include static int one (struct dirent *unused) { return 1; } int main (void) { struct dirent **eps; int n; n = scandir ("./", &eps, one, alphasort); if (n >= 0) { int cnt; for (cnt = 0; cnt < n; ++cnt) puts (eps[cnt]->d_name); } else perror ("Couldn't open the directory"); return 0; } Note the simple selector function in this example. Since we want to see all directory entries we always return `1'. Working with Directory Trees ============================ The functions described so far for handling the files in a directory have allowed you to either retrieve the information bit by bit, or to process all the files as a group (see `scandir'). Sometimes it is useful to process whole hierarchies of directories and their contained files. The X/Open specification defines two functions to do this. The simpler form is derived from an early definition in System V systems and therefore this function is available on SVID-derived systems. The prototypes and required definitions can be found in the `ftw.h' header. There are four functions in this family: `ftw', `nftw' and their 64-bit counterparts `ftw64' and `nftw64'. These functions take as one of their arguments a pointer to a callback function of the appropriate type. - Data Type: __ftw_func_t int (*) (const char *, const struct stat *, int) The type of callback functions given to the `ftw' function. The first parameter points to the file name, the second parameter to an object of type `struct stat' which is filled in for the file named in the first parameter. The last parameter is a flag giving more information about the current file. It can have the following values: `FTW_F' The item is either a normal file or a file which does not fit into one of the following categories. This could be special files, sockets etc. `FTW_D' The item is a directory. `FTW_NS' The `stat' call failed and so the information pointed to by the second paramater is invalid. `FTW_DNR' The item is a directory which cannot be read. `FTW_SL' The item is a symbolic link. Since symbolic links are normally followed seeing this value in a `ftw' callback function means the referenced file does not exist. The situation for `nftw' is different. This value is only available if the program is compiled with `_BSD_SOURCE' or `_XOPEN_EXTENDED' defined before including the first header. The original SVID systems do not have symbolic links. If the sources are compiled with `_FILE_OFFSET_BITS == 64' this type is in fact `__ftw64_func_t' since this mode changes `struct stat' to be `struct stat64'. For the LFS interface and for use in the function `ftw64', the header `ftw.h' defines another function type. - Data Type: __ftw64_func_t int (*) (const char *, const struct stat64 *, int) This type is used just like `__ftw_func_t' for the callback function, but this time is called from `ftw64'. The second parameter to the function is a pointer to a variable of type `struct stat64' which is able to represent the larger values. - Data Type: __nftw_func_t int (*) (const char *, const struct stat *, int, struct FTW *) The first three arguments are the same as for the `__ftw_func_t' type. However for the third argument some additional values are defined to allow finer differentiation: `FTW_DP' The current item is a directory and all subdirectories have already been visited and reported. This flag is returned instead of `FTW_D' if the `FTW_DEPTH' flag is passed to `nftw' (see below). `FTW_SLN' The current item is a stale symbolic link. The file it points to does not exist. The last parameter of the callback function is a pointer to a structure with some extra information as described below. If the sources are compiled with `_FILE_OFFSET_BITS == 64' this type is in fact `__nftw64_func_t' since this mode changes `struct stat' to be `struct stat64'. For the LFS interface there is also a variant of this data type available which has to be used with the `nftw64' function. - Data Type: __nftw64_func_t int (*) (const char *, const struct stat64 *, int, struct FTW *) This type is used just like `__nftw_func_t' for the callback function, but this time is called from `nftw64'. The second parameter to the function is this time a pointer to a variable of type `struct stat64' which is able to represent the larger values. - Data Type: struct FTW The information contained in this structure helps in interpreting the name parameter and gives some information about the current state of the traversal of the directory hierarchy. `int base' The value is the offset into the string passed in the first parameter to the callback function of the beginning of the file name. The rest of the string is the path of the file. This information is especially important if the `FTW_CHDIR' flag was set in calling `nftw' since then the current directory is the one the current item is found in. `int level' Whilst processing, the code tracks how many directories down it has gone to find the current file. This nesting level starts at 0 for files in the initial directory (or is zero for the initial file if a file was passed). - Function: int ftw (const char *FILENAME, __ftw_func_t FUNC, int DESCRIPTORS) The `ftw' function calls the callback function given in the parameter FUNC for every item which is found in the directory specified by FILENAME and all directories below. The function follows symbolic links if necessary but does not process an item twice. If FILENAME is not a directory then it itself is the only object returned to the callback function. The file name passed to the callback function is constructed by taking the FILENAME parameter and appending the names of all passed directories and then the local file name. So the callback function can use this parameter to access the file. `ftw' also calls `stat' for the file and passes that information on to the callback function. If this `stat' call was not successful the failure is indicated by setting the third argument of the callback function to `FTW_NS'. Otherwise it is set according to the description given in the account of `__ftw_func_t' above. The callback function is expected to return 0 to indicate that no error occurred and that processing should continue. If an error occurred in the callback function or it wants `ftw' to return immediately, the callback function can return a value other than 0. This is the only correct way to stop the function. The program must not use `setjmp' or similar techniques to continue from another place. This would leave resources allocated by the `ftw' function unfreed. The DESCRIPTORS parameter to `ftw' specifies how many file descriptors it is allowed to consume. The function runs faster the more descriptors it can use. For each level in the directory hierarchy at most one descriptor is used, but for very deep ones any limit on open file descriptors for the process or the system may be exceeded. Moreover, file descriptor limits in a multi-threaded program apply to all the threads as a group, and therefore it is a good idea to supply a reasonable limit to the number of open descriptors. The return value of the `ftw' function is 0 if all callback function calls returned 0 and all actions performed by the `ftw' succeeded. If a function call failed (other than calling `stat' on an item) the function returns -1. If a callback function returns a value other than 0 this value is returned as the return value of `ftw'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `ftw64', i.e. the LFS interface transparently replaces the old interface. - Function: int ftw64 (const char *FILENAME, __ftw64_func_t FUNC, int DESCRIPTORS) This function is similar to `ftw' but it can work on filesystems with large files. File information is reported using a variable of type `struct stat64' which is passed by reference to the callback function. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is available under the name `ftw' and transparently replaces the old implementation. - Function: int nftw (const char *FILENAME, __nftw_func_t FUNC, int DESCRIPTORS, int FLAG) The `nftw' function works like the `ftw' functions. They call the callback function FUNC for all items found in the directory FILENAME and below. At most DESCRIPTORS file descriptors are consumed during the `nftw' call. One difference is that the callback function is of a different type. It is of type `struct FTW *' and provides the callback function with the extra information described above. A second difference is that `nftw' takes a fourth argument, which is 0 or a bitwise-OR combination of any of the following values. `FTW_PHYS' While traversing the directory symbolic links are not followed. Instead symbolic links are reported using the `FTW_SL' value for the type parameter to the callback function. If the file referenced by a symbolic link does not exist `FTW_SLN' is returned instead. `FTW_MOUNT' The callback function is only called for items which are on the same mounted filesystem as the directory given by the FILENAME parameter to `nftw'. `FTW_CHDIR' If this flag is given the current working directory is changed to the directory of the reported object before the callback function is called. When `ntfw' finally returns the current directory is restored to its original value. `FTW_DEPTH' If this option is specified then all subdirectories and files within them are processed before processing the top directory itself (depth-first processing). This also means the type flag given to the callback function is `FTW_DP' and not `FTW_D'. The return value is computed in the same way as for `ftw'. `nftw' returns 0 if no failures occurred and all callback functions returned 0. In case of internal errors, such as memory problems, the return value is -1 and ERRNO is set accordingly. If the return value of a callback invocation was non-zero then that value is returned. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `nftw64', i.e. the LFS interface transparently replaces the old interface. - Function: int nftw64 (const char *FILENAME, __nftw64_func_t FUNC, int DESCRIPTORS, int FLAG) This function is similar to `nftw' but it can work on filesystems with large files. File information is reported using a variable of type `struct stat64' which is passed by reference to the callback function. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is available under the name `nftw' and transparently replaces the old implementation. Hard Links ========== In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others. To add a name to a file, use the `link' function. (The new name is also called a "hard link" to the file.) Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file's existing name or names. One file can have names in several directories, so the organization of the file system is not a strict hierarchy or tree. In most implementations, it is not possible to have hard links to the same file in multiple file systems. `link' reports an error if you try to make a hard link to the file from another file system when this cannot be done. The prototype for the `link' function is declared in the header file `unistd.h'. - Function: int link (const char *OLDNAME, const char *NEWNAME) The `link' function makes a new link to the existing file named by OLDNAME, under the new name NEWNAME. This function returns a value of `0' if it is successful and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::) for both OLDNAME and NEWNAME, the following `errno' error conditions are defined for this function: `EACCES' You are not allowed to write to the directory in which the new link is to be written. `EEXIST' There is already a file named NEWNAME. If you want to replace this link with a new link, you must remove the old link explicitly first. `EMLINK' There are already too many links to the file named by OLDNAME. (The maximum number of links to a file is `LINK_MAX'; see *Note Limits for Files::.) `ENOENT' The file named by OLDNAME doesn't exist. You can't make a link to a file that doesn't exist. `ENOSPC' The directory or file system that would contain the new link is full and cannot be extended. `EPERM' In the GNU system and some others, you cannot make links to directories. Many systems allow only privileged users to do so. This error is used to report the problem. `EROFS' The directory containing the new link can't be modified because it's on a read-only file system. `EXDEV' The directory specified in NEWNAME is on a different file system than the existing file. `EIO' A hardware error occurred while trying to read or write the to filesystem. Symbolic Links ============== The GNU system supports "soft links" or "symbolic links". This is a kind of "file" that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file. The reason symbolic links work the way they do is that special things happen when you try to open the link. The `open' function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead. The `stat' function likewise operates on the file that the symbolic link points to, instead of on the link itself. By contrast, other operations such as deleting or renaming the file operate on the link itself. The functions `readlink' and `lstat' also refrain from following symbolic links, because their purpose is to obtain information about the link. `link', the function that makes a hard link, does too. It makes a hard link to the symbolic link, which one rarely wants. Some systems have for some functions operating on files have a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the `sys/param.h' header file. - Macro: int MAXSYMLINKS The macro `MAXSYMLINKS' specifies how many symlinks some function will follow before returning `ELOOP'. Not all functions behave the same and this value is not the same a that returned for `_SC_SYMLOOP' by `sysconf'. In fact, the `sysconf' result can indicate that there is no fixed limit although `MAXSYMLINKS' exists and has a finite value. Prototypes for most of the functions listed in this section are in `unistd.h'. - Function: int symlink (const char *OLDNAME, const char *NEWNAME) The `symlink' function makes a symbolic link to OLDNAME named NEWNAME. The normal return value from `symlink' is `0'. A return value of `-1' indicates an error. In addition to the usual file name syntax errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EEXIST' There is already an existing file named NEWNAME. `EROFS' The file NEWNAME would exist on a read-only file system. `ENOSPC' The directory or file system cannot be extended to make the new link. `EIO' A hardware error occurred while reading or writing data on the disk. - Function: int readlink (const char *FILENAME, char *BUFFER, size_t SIZE) The `readlink' function gets the value of the symbolic link FILENAME. The file name that the link points to is copied into BUFFER. This file name string is _not_ null-terminated; `readlink' normally returns the number of characters copied. The SIZE argument specifies the maximum number of characters to copy, usually the allocation size of BUFFER. If the return value equals SIZE, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call `readlink' again. Here is an example: char * readlink_malloc (const char *filename) { int size = 100; while (1) { char *buffer = (char *) xmalloc (size); int nchars = readlink (filename, buffer, size); if (nchars < 0) return NULL; if (nchars < size) return buffer; free (buffer); size *= 2; } } A value of `-1' is returned in case of error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EINVAL' The named file is not a symbolic link. `EIO' A hardware error occurred while reading or writing data on the disk. In some situations it is desirable to resolve all the to get the real name of a file where no prefix names a symbolic link which is followed and no filename in the path is `.' or `..'. This is for instance desirable if files have to be compare in which case different names can refer to the same inode. - Function: char * canonicalize_file_name (const char *NAME) The `canonicalize_file_name' function returns the absolute name of the file named by NAME which contains no `.', `..' components nor any repeated path separators (`/') or symlinks. The result is passed back as the return value of the function in a block of memory allocated with `malloc'. If the result is not used anymore the memory should be freed with a call to `free'. In any of the path components except the last one is missing the function returns a NULL pointer. This is also what is returned if the length of the path reaches or exceeds `PATH_MAX' characters. In any case `errno' is set accordingly. `ENAMETOOLONG' The resulting path is too long. This error only occurs on systems which have a limit on the file name length. `EACCES' At least one of the path components is not readable. `ENOENT' The input file name is empty. `ENOENT' At least one of the path components does not exist. `ELOOP' More than `MAXSYMLINKS' many symlinks have been followed. This function is a GNU extension and is declared in `stdlib.h'. The Unix standard includes a similar function which differs from `canonicalize_file_name' in that the user has to provide the buffer where the result is placed in. - Function: char * realpath (const char *restrict NAME, char *restrict RESOLVED) The `realpath' function behaves just like `canonicalize_file_name' but instead of allocating a buffer for the result it is placed in the buffer pointed to by RESOLVED. One other difference is that the buffer RESOLVED will contain the part of the path component which does not exist or is not readable if the function returns `NULL' and `errno' is set to `EACCES' or `ENOENT'. This function is declared in `stdlib.h'. The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long path on systems which have no limits on the file name length. Deleting Files ============== You can delete a file with `unlink' or `remove'. Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other remaining names (*note Hard Links::), it remains accessible under those names. - Function: int unlink (const char *FILENAME) The `unlink' function deletes the file name FILENAME. If this is a file's sole name, the file itself is also deleted. (Actually, if any process has the file open when this happens, deletion is postponed until all processes have closed the file.) The function `unlink' is declared in the header file `unistd.h'. This function returns `0' on successful completion, and `-1' on error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Write permission is denied for the directory from which the file is to be removed, or the directory has the sticky bit set and you do not own the file. `EBUSY' This error indicates that the file is being used by the system in such a way that it can't be unlinked. For example, you might see this error if the file name specifies the root directory or a mount point for a file system. `ENOENT' The file name to be deleted doesn't exist. `EPERM' On some systems `unlink' cannot be used to delete the name of a directory, or at least can only be used this way by a privileged user. To avoid such problems, use `rmdir' to delete directories. (In the GNU system `unlink' can never delete the name of a directory.) `EROFS' The directory containing the file name to be deleted is on a read-only file system and can't be modified. - Function: int rmdir (const char *FILENAME) The `rmdir' function deletes a directory. The directory must be empty before it can be removed; in other words, it can only contain entries for `.' and `..'. In most other respects, `rmdir' behaves like `unlink'. There are two additional `errno' error conditions defined for `rmdir': `ENOTEMPTY' `EEXIST' The directory to be deleted is not empty. These two error codes are synonymous; some systems use one, and some use the other. The GNU system always uses `ENOTEMPTY'. The prototype for this function is declared in the header file `unistd.h'. - Function: int remove (const char *FILENAME) This is the ISO C function to remove a file. It works like `unlink' for files and like `rmdir' for directories. `remove' is declared in `stdio.h'. Renaming Files ============== The `rename' function is used to change a file's name. - Function: int rename (const char *OLDNAME, const char *NEWNAME) The `rename' function renames the file OLDNAME to NEWNAME. The file formerly accessible under the name OLDNAME is afterwards accessible as NEWNAME instead. (If the file had any other names aside from OLDNAME, it continues to have those names.) The directory containing the name NEWNAME must be on the same file system as the directory containing the name OLDNAME. One special case for `rename' is when OLDNAME and NEWNAME are two names for the same file. The consistent way to handle this case is to delete OLDNAME. However, in this case POSIX requires that `rename' do nothing and report success--which is inconsistent. We don't know what your operating system will do. If OLDNAME is not a directory, then any existing file named NEWNAME is removed during the renaming operation. However, if NEWNAME is the name of a directory, `rename' fails in this case. If OLDNAME is a directory, then either NEWNAME must not exist or it must name a directory that is empty. In the latter case, the existing directory named NEWNAME is deleted first. The name NEWNAME must not specify a subdirectory of the directory `oldname' which is being renamed. One useful feature of `rename' is that the meaning of NEWNAME changes "atomically" from any previously existing file by that name to its new meaning (i.e. the file that was called OLDNAME). There is no instant at which NEWNAME is non-existent "in between" the old meaning and the new meaning. If there is a system crash during the operation, it is possible for both names to still exist; but NEWNAME will always be intact if it exists at all. If `rename' fails, it returns `-1'. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' One of the directories containing NEWNAME or OLDNAME refuses write permission; or NEWNAME and OLDNAME are directories and write permission is refused for one of them. `EBUSY' A directory named by OLDNAME or NEWNAME is being used by the system in a way that prevents the renaming from working. This includes directories that are mount points for filesystems, and directories that are the current working directories of processes. `ENOTEMPTY' `EEXIST' The directory NEWNAME isn't empty. The GNU system always returns `ENOTEMPTY' for this, but some other systems return `EEXIST'. `EINVAL' OLDNAME is a directory that contains NEWNAME. `EISDIR' NEWNAME is a directory but the OLDNAME isn't. `EMLINK' The parent directory of NEWNAME would have too many links (entries). `ENOENT' The file OLDNAME doesn't exist. `ENOSPC' The directory that would contain NEWNAME has no room for another entry, and there is no space left in the file system to expand it. `EROFS' The operation would involve writing to a directory on a read-only file system. `EXDEV' The two file names NEWNAME and OLDNAME are on different file systems. Creating Directories ==================== Directories are created with the `mkdir' function. (There is also a shell command `mkdir' which does the same thing.) - Function: int mkdir (const char *FILENAME, mode_t MODE) The `mkdir' function creates a new, empty directory with name FILENAME. The argument MODE specifies the file permissions for the new directory file. *Note Permission Bits::, for more information about this. A return value of `0' indicates successful completion, and `-1' indicates failure. In addition to the usual file name syntax errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Write permission is denied for the parent directory in which the new directory is to be added. `EEXIST' A file named FILENAME already exists. `EMLINK' The parent directory has too many links (entries). Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine. `ENOSPC' The file system doesn't have enough room to create the new directory. `EROFS' The parent directory of the directory being created is on a read-only file system and cannot be modified. To use this function, your program should include the header file `sys/stat.h'. File Attributes =============== When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, etc. These are called the "file attributes", and are associated with the file itself and not a particular one of its names. This section contains information about how you can inquire about and modify the attributes of a file. The meaning of the File Attributes ---------------------------------- When you read the attributes of a file, they come back in a structure called `struct stat'. This section describes the names of the attributes, their data types, and what they mean. For the functions to read the attributes of a file, see *Note Reading Attributes::. The header file `sys/stat.h' declares all the symbols defined in this section. - Data Type: struct stat The `stat' structure type is used to return information about the attributes of a file. It contains at least the following members: `mode_t st_mode' Specifies the mode of the file. This includes file type information (*note Testing File Type::) and the file permission bits (*note Permission Bits::). `ino_t st_ino' The file serial number, which distinguishes this file from all other files on the same device. `dev_t st_dev' Identifies the device containing the file. The `st_ino' and `st_dev', taken together, uniquely identify the file. The `st_dev' value is not necessarily consistent across reboots or system crashes, however. `nlink_t st_nlink' The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total. `uid_t st_uid' The user ID of the file's owner. *Note File Owner::. `gid_t st_gid' The group ID of the file. *Note File Owner::. `off_t st_size' This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to. `time_t st_atime' This is the last access time for the file. *Note File Times::. `unsigned long int st_atime_usec' This is the fractional part of the last access time for the file. *Note File Times::. `time_t st_mtime' This is the time of the last modification to the contents of the file. *Note File Times::. `unsigned long int st_mtime_usec' This is the fractional part of the time of the last modification to the contents of the file. *Note File Times::. `time_t st_ctime' This is the time of the last modification to the attributes of the file. *Note File Times::. `unsigned long int st_ctime_usec' This is the fractional part of the time of the last modification to the attributes of the file. *Note File Times::. `blkcnt_t st_blocks' This is the amount of disk space that the file occupies, measured in units of 512-byte blocks. The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk. You can tell (approximately) whether a file is sparse by comparing this value with `st_size', like this: (st.st_blocks * 512 < st.st_size) This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem. `unsigned int st_blksize' The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to `st_blocks'.) The extensions for the Large File Support (LFS) require, even on 32-bit machines, types which can handle file sizes up to 2^63. Therefore a new definition of `struct stat' is necessary. - Data Type: struct stat64 The members of this type are the same and have the same names as those in `struct stat'. The only difference is that the members `st_ino', `st_size', and `st_blocks' have a different type to support larger values. `mode_t st_mode' Specifies the mode of the file. This includes file type information (*note Testing File Type::) and the file permission bits (*note Permission Bits::). `ino64_t st_ino' The file serial number, which distinguishes this file from all other files on the same device. `dev_t st_dev' Identifies the device containing the file. The `st_ino' and `st_dev', taken together, uniquely identify the file. The `st_dev' value is not necessarily consistent across reboots or system crashes, however. `nlink_t st_nlink' The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total. `uid_t st_uid' The user ID of the file's owner. *Note File Owner::. `gid_t st_gid' The group ID of the file. *Note File Owner::. `off64_t st_size' This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to. `time_t st_atime' This is the last access time for the file. *Note File Times::. `unsigned long int st_atime_usec' This is the fractional part of the last access time for the file. *Note File Times::. `time_t st_mtime' This is the time of the last modification to the contents of the file. *Note File Times::. `unsigned long int st_mtime_usec' This is the fractional part of the time of the last modification to the contents of the file. *Note File Times::. `time_t st_ctime' This is the time of the last modification to the attributes of the file. *Note File Times::. `unsigned long int st_ctime_usec' This is the fractional part of the time of the last modification to the attributes of the file. *Note File Times::. `blkcnt64_t st_blocks' This is the amount of disk space that the file occupies, measured in units of 512-byte blocks. `unsigned int st_blksize' The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to `st_blocks'.) Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them. - Data Type: mode_t This is an integer data type used to represent file modes. In the GNU system, this is equivalent to `unsigned int'. - Data Type: ino_t This is an arithmetic data type used to represent file serial numbers. (In Unix jargon, these are sometimes called "inode numbers".) In the GNU system, this type is equivalent to `unsigned long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `ino64_t'. - Data Type: ino64_t This is an arithmetic data type used to represent file serial numbers for the use in LFS. In the GNU system, this type is equivalent to `unsigned long longint'. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `ino_t'. - Data Type: dev_t This is an arithmetic data type used to represent file device numbers. In the GNU system, this is equivalent to `int'. - Data Type: nlink_t This is an arithmetic data type used to represent file link counts. In the GNU system, this is equivalent to `unsigned short int'. - Data Type: blkcnt_t This is an arithmetic data type used to represent block counts. In the GNU system, this is equivalent to `unsigned long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `blkcnt64_t'. - Data Type: blkcnt64_t This is an arithmetic data type used to represent block counts for the use in LFS. In the GNU system, this is equivalent to `unsigned long long int'. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `blkcnt_t'. Reading the Attributes of a File -------------------------------- To examine the attributes of files, use the functions `stat', `fstat' and `lstat'. They return the attribute information in a `struct stat' object. All three functions are declared in the header file `sys/stat.h'. - Function: int stat (const char *FILENAME, struct stat *BUF) The `stat' function returns information about the attributes of the file named by FILENAME in the structure pointed to by BUF. If FILENAME is the name of a symbolic link, the attributes you get describe the file that the link points to. If the link points to a nonexistent file name, then `stat' fails reporting a nonexistent file. The return value is `0' if the operation is successful, or `-1' on failure. In addition to the usual file name errors (*note File Name Errors::, the following `errno' error conditions are defined for this function: `ENOENT' The file named by FILENAME doesn't exist. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `stat64' since the LFS interface transparently replaces the normal implementation. - Function: int stat64 (const char *FILENAME, struct stat64 *BUF) This function is similar to `stat' but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type `struct stat64' to which BUF must point. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `stat' and so transparently replaces the interface for small files on 32-bit machines. - Function: int fstat (int FILEDES, struct stat *BUF) The `fstat' function is like `stat', except that it takes an open file descriptor as an argument instead of a file name. *Note Low-Level I/O::. Like `stat', `fstat' returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for `fstat': `EBADF' The FILEDES argument is not a valid file descriptor. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `fstat64' since the LFS interface transparently replaces the normal implementation. - Function: int fstat64 (int FILEDES, struct stat64 *BUF) This function is similar to `fstat' but is able to work on large files on 32-bit platforms. For large files the file descriptor FILEDES should be obtained by `open64' or `creat64'. The BUF pointer points to a variable of type `struct stat64' which is able to represent the larger values. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `fstat' and so transparently replaces the interface for small files on 32-bit machines. - Function: int lstat (const char *FILENAME, struct stat *BUF) The `lstat' function is like `stat', except that it does not follow symbolic links. If FILENAME is the name of a symbolic link, `lstat' returns information about the link itself; otherwise `lstat' works like `stat'. *Note Symbolic Links::. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `lstat64' since the LFS interface transparently replaces the normal implementation. - Function: int lstat64 (const char *FILENAME, struct stat64 *BUF) This function is similar to `lstat' but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type `struct stat64' to which BUF must point. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `lstat' and so transparently replaces the interface for small files on 32-bit machines. Testing the Type of a File -------------------------- The "file mode", stored in the `st_mode' field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the type code, which you can use to tell whether the file is a directory, socket, symbolic link, and so on. For details about access permissions see *Note Permission Bits::. There are two ways you can access the file type information in a file mode. Firstly, for each file type there is a "predicate macro" which examines a given file mode and returns whether it is of that type or not. Secondly, you can mask out the rest of the file mode to leave just the file type code, and compare this against constants for each of the supported file types. All of the symbols listed in this section are defined in the header file `sys/stat.h'. The following predicate macros test the type of a file, given the value M which is the `st_mode' field returned by `stat' on that file: - Macro: int S_ISDIR (mode_t M) This macro returns non-zero if the file is a directory. - Macro: int S_ISCHR (mode_t M) This macro returns non-zero if the file is a character special file (a device like a terminal). - Macro: int S_ISBLK (mode_t M) This macro returns non-zero if the file is a block special file (a device like a disk). - Macro: int S_ISREG (mode_t M) This macro returns non-zero if the file is a regular file. - Macro: int S_ISFIFO (mode_t M) This macro returns non-zero if the file is a FIFO special file, or a pipe. *Note Pipes and FIFOs::. - Macro: int S_ISLNK (mode_t M) This macro returns non-zero if the file is a symbolic link. *Note Symbolic Links::. - Macro: int S_ISSOCK (mode_t M) This macro returns non-zero if the file is a socket. *Note Sockets::. An alternate non-POSIX method of testing the file type is supported for compatibility with BSD. The mode can be bitwise AND-ed with `S_IFMT' to extract the file type code, and compared to the appropriate constant. For example, S_ISCHR (MODE) is equivalent to: ((MODE & S_IFMT) == S_IFCHR) - Macro: int S_IFMT This is a bit mask used to extract the file type code from a mode value. These are the symbolic names for the different file type codes: `S_IFDIR' This is the file type constant of a directory file. `S_IFCHR' This is the file type constant of a character-oriented device file. `S_IFBLK' This is the file type constant of a block-oriented device file. `S_IFREG' This is the file type constant of a regular file. `S_IFLNK' This is the file type constant of a symbolic link. `S_IFSOCK' This is the file type constant of a socket. `S_IFIFO' This is the file type constant of a FIFO or pipe. The POSIX.1b standard introduced a few more objects which possibly can be implemented as object in the filesystem. These are message queues, semaphores, and shared memory objects. To allow differentiating these objects from other files the POSIX standard introduces three new test macros. But unlike the other macros it does not take the value of the `st_mode' field as the parameter. Instead they expect a pointer to the whole `struct stat' structure. - Macro: int S_TYPEISMQ (struct stat *S) If the system implement POSIX message queues as distinct objects and the file is a message queue object, this macro returns a non-zero value. In all other cases the result is zero. - Macro: int S_TYPEISSEM (struct stat *S) If the system implement POSIX semaphores as distinct objects and the file is a semaphore object, this macro returns a non-zero value. In all other cases the result is zero. - Macro: int S_TYPEISSHM (struct stat *S) If the system implement POSIX shared memory objects as distinct objects and the file is an shared memory object, this macro returns a non-zero value. In all other cases the result is zero. File Owner ---------- Every file has an "owner" which is one of the registered user names defined on the system. Each file also has a "group" which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control. The file owner and group play a role in determining access because the file has one set of access permission bits for the owner, another set that applies to users who belong to the file's group, and a third set of bits that applies to everyone else. *Note Access Permission::, for the details of how access is decided based on this data. When a file is created, its owner is set to the effective user ID of the process that creates it (*note Process Persona::). The file's group ID may be set to either the effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rules, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior no matter what kind of system you run it on. You can change the owner and/or group owner of an existing file using the `chown' function. This is the primitive for the `chown' and `chgrp' shell commands. The prototype for this function is declared in `unistd.h'. - Function: int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP) The `chown' function changes the owner of the file FILENAME to OWNER, and its group owner to GROUP. Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID permission bits. (This is because those bits may not be appropriate for the new owner.) Other file permission bits are not changed. The return value is `0' on success and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EPERM' This process lacks permission to make the requested change. Only privileged users or the file's owner can change the file's group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on. *Note Options for Files::, for information about the `_POSIX_CHOWN_RESTRICTED' macro. `EROFS' The file is on a read-only file system. - Function: int fchown (int FILEDES, int OWNER, int GROUP) This is like `chown', except that it changes the owner of the open file with descriptor FILEDES. The return value from `fchown' is `0' on success and `-1' on failure. The following `errno' error codes are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument corresponds to a pipe or socket, not an ordinary file. `EPERM' This process lacks permission to make the requested change. For details see `chmod' above. `EROFS' The file resides on a read-only file system. The Mode Bits for Access Permission ----------------------------------- The "file mode", stored in the `st_mode' field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the access permission bits, which control who can read or write the file. *Note Testing File Type::, for information about the file type code. All of the symbols listed in this section are defined in the header file `sys/stat.h'. These symbolic constants are defined for the file mode bits that control access permission for the file: `S_IRUSR' `S_IREAD' Read permission bit for the owner of the file. On many systems this bit is 0400. `S_IREAD' is an obsolete synonym provided for BSD compatibility. `S_IWUSR' `S_IWRITE' Write permission bit for the owner of the file. Usually 0200. `S_IWRITE' is an obsolete synonym provided for BSD compatibility. `S_IXUSR' `S_IEXEC' Execute (for ordinary files) or search (for directories) permission bit for the owner of the file. Usually 0100. `S_IEXEC' is an obsolete synonym provided for BSD compatibility. `S_IRWXU' This is equivalent to `(S_IRUSR | S_IWUSR | S_IXUSR)'. `S_IRGRP' Read permission bit for the group owner of the file. Usually 040. `S_IWGRP' Write permission bit for the group owner of the file. Usually 020. `S_IXGRP' Execute or search permission bit for the group owner of the file. Usually 010. `S_IRWXG' This is equivalent to `(S_IRGRP | S_IWGRP | S_IXGRP)'. `S_IROTH' Read permission bit for other users. Usually 04. `S_IWOTH' Write permission bit for other users. Usually 02. `S_IXOTH' Execute or search permission bit for other users. Usually 01. `S_IRWXO' This is equivalent to `(S_IROTH | S_IWOTH | S_IXOTH)'. `S_ISUID' This is the set-user-ID on execute bit, usually 04000. *Note How Change Persona::. `S_ISGID' This is the set-group-ID on execute bit, usually 02000. *Note How Change Persona::. `S_ISVTX' This is the "sticky" bit, usually 01000. For a directory it gives permission to delete a file in that directory only if you own that file. Ordinarily, a user can either delete all the files in a directory or cannot delete any of them (based on whether the user has write permission for the directory). The same restriction applies--you must have both write permission for the directory and own the file you want to delete. The one exception is that the owner of the directory can delete any file in the directory, no matter who owns it (provided the owner has given himself write permission for the directory). This is commonly used for the `/tmp' directory, where anyone may create files but not delete files created by other users. Originally the sticky bit on an executable file modified the swapping policies of the system. Normally, when a program terminated, its pages in core were immediately freed and reused. If the sticky bit was set on the executable file, the system kept the pages in core for a while as if the program were still running. This was advantageous for a program likely to be run many times in succession. This usage is obsolete in modern systems. When a program terminates, its pages always remain in core as long as there is no shortage of memory in the system. When the program is next run, its pages will still be in core if no shortage arose since the last run. On some modern systems where the sticky bit has no useful meaning for an executable file, you cannot set the bit at all for a non-directory. If you try, `chmod' fails with `EFTYPE'; *note Setting Permissions::. Some systems (particularly SunOS) have yet another use for the sticky bit. If the sticky bit is set on a file that is _not_ executable, it means the opposite: never cache the pages of this file at all. The main use of this is for the files on an NFS server machine which are used as the swap area of diskless client machines. The idea is that the pages of the file will be cached in the client's memory, so it is a waste of the server's memory to cache them a second time. With this usage the sticky bit also implies that the filesystem may fail to record the file's modification time onto disk reliably (the idea being that no-one cares for a swap file). This bit is only available on BSD systems (and those derived from them). Therefore one has to use the `_BSD_SOURCE' feature select macro to get the definition (*note Feature Test Macros::). The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed. *Warning:* Writing explicit numbers for file permissions is bad practice. Not only is it not portable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean use the symbolic names. How Your Access to a File is Decided ------------------------------------ Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in *Note Process Persona::. If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits. Otherwise, permissions are controlled by the "other" bits. Privileged users, like `root', can access any file regardless of its permission bits. As a special case, for a file to be executable even by a privileged user, at least one of its execute bits must be set. Assigning File Permissions -------------------------- The primitive functions for creating files (for example, `open' or `mkdir') take a MODE argument, which specifies the file permissions to give the newly created file. This mode is modified by the process's "file creation mask", or "umask", before it is used. The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the MODE argument passed to the create function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant. Programs that create files typically specify a MODE argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask. To change the permission of an existing file given its name, call `chmod'. This function uses the specified permission bits and ignores the file creation mask. In normal use, the file creation mask is initialized by the user's login shell (using the `umask' shell command), and inherited by all subprocesses. Application programs normally don't need to worry about the file creation mask. It will automatically do what it is supposed to do. When your program needs to create a file and bypass the umask for its access permissions, the easiest way to do this is to use `fchmod' after opening the file, rather than changing the umask. In fact, changing the umask is usually done only by shells. They use the `umask' function. The functions in this section are declared in `sys/stat.h'. - Function: mode_t umask (mode_t MASK) The `umask' function sets the file creation mask of the current process to MASK, and returns the previous value of the file creation mask. Here is an example showing how to read the mask with `umask' without changing it permanently: mode_t read_umask (void) { mode_t mask = umask (0); umask (mask); return mask; } However, it is better to use `getumask' if you just want to read the mask value, because it is reentrant (at least if you use the GNU operating system). - Function: mode_t getumask (void) Return the current value of the file creation mask for the current process. This function is a GNU extension. - Function: int chmod (const char *FILENAME, mode_t MODE) The `chmod' function sets the access permission bits for the file named by FILENAME to MODE. If FILENAME is a symbolic link, `chmod' changes the permissions of the file pointed to by the link, not those of the link itself. This function returns `0' if successful and `-1' if not. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `ENOENT' The named file doesn't exist. `EPERM' This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them. `EROFS' The file resides on a read-only file system. `EFTYPE' MODE has the `S_ISVTX' bit (the "sticky bit") set, and the named file is not a directory. Some systems do not allow setting the sticky bit on non-directory files, and some do (and only some of those assign a useful meaning to the bit for non-directory files). You only get `EFTYPE' on systems where the sticky bit has no useful meaning for non-directory files, so it is always safe to just clear the bit in MODE and call `chmod' again. *Note Permission Bits::, for full details on the sticky bit. - Function: int fchmod (int FILEDES, int MODE) This is like `chmod', except that it changes the permissions of the currently open file given by FILEDES. The return value from `fchmod' is `0' on success and `-1' on failure. The following `errno' error codes are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument corresponds to a pipe or socket, or something else that doesn't really have access permissions. `EPERM' This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them. `EROFS' The file resides on a read-only file system. Testing Permission to Access a File ----------------------------------- In some situations it is desirable to allow programs to access files or devices even if this is not possible with the permissions granted to the user. One possible solution is to set the setuid-bit of the program file. If such a program is started the _effective_ user ID of the process is changed to that of the owner of the program file. So to allow write access to files like `/etc/passwd', which normally can be written only by the super-user, the modifying program will have to be owned by `root' and the setuid-bit must be set. But beside the files the program is intended to change the user should not be allowed to access any file to which s/he would not have access anyway. The program therefore must explicitly check whether _the user_ would have the necessary access to a file, before it reads or writes the file. To do this, use the function `access', which checks for access permission based on the process's _real_ user ID rather than the effective user ID. (The setuid feature does not alter the real user ID, so it reflects the user who actually ran the program.) There is another way you could check this access, which is easy to describe, but very hard to use. This is to examine the file mode bits and mimic the system's own access computation. This method is undesirable because many systems have additional access control features; your program cannot portably mimic them, and you would not want to try to keep track of the diverse features that different systems have. Using `access' is simple and automatically does whatever is appropriate for the system you are using. `access' is _only_ only appropriate to use in setuid programs. A non-setuid program will always use the effective ID rather than the real ID. The symbols in this section are declared in `unistd.h'. - Function: int access (const char *FILENAME, int HOW) The `access' function checks to see whether the file named by FILENAME can be accessed in the way specified by the HOW argument. The HOW argument either can be the bitwise OR of the flags `R_OK', `W_OK', `X_OK', or the existence test `F_OK'. This function uses the _real_ user and group IDs of the calling process, rather than the _effective_ IDs, to check for access permission. As a result, if you use the function from a `setuid' or `setgid' program (*note How Change Persona::), it gives information relative to the user who actually ran the program. The return value is `0' if the access is permitted, and `-1' otherwise. (In other words, treated as a predicate function, `access' returns true if the requested access is _denied_.) In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' The access specified by HOW is denied. `ENOENT' The file doesn't exist. `EROFS' Write permission was requested for a file on a read-only file system. These macros are defined in the header file `unistd.h' for use as the HOW argument to the `access' function. The values are integer constants. - Macro: int R_OK Flag meaning test for read permission. - Macro: int W_OK Flag meaning test for write permission. - Macro: int X_OK Flag meaning test for execute/search permission. - Macro: int F_OK Flag meaning test for existence of the file. File Times ---------- Each file has three time stamps associated with it: its access time, its modification time, and its attribute modification time. These correspond to the `st_atime', `st_mtime', and `st_ctime' members of the `stat' structure; see *Note File Attributes::. All of these times are represented in calendar time format, as `time_t' objects. This data type is defined in `time.h'. For more information about representation and manipulation of time values, see *Note Calendar Time::. Reading from a file updates its access time attribute, and writing updates its modification time. When a file is created, all three time stamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated. Adding a new name for a file with the `link' function updates the attribute change time field of the file being linked, and both the attribute change time and modification time fields of the directory containing the new name. These same fields are affected if a file name is deleted with `unlink', `remove' or `rmdir'. Renaming a file with `rename' affects only the attribute change time and modification time fields of the two parent directories involved, and not the times for the file being renamed. Changing the attributes of a file (for example, with `chmod') updates its attribute change time field. You can also change some of the time stamps of a file explicitly using the `utime' function--all except the attribute change time. You need to include the header file `utime.h' to use this facility. - Data Type: struct utimbuf The `utimbuf' structure is used with the `utime' function to specify new access and modification times for a file. It contains the following members: `time_t actime' This is the access time for the file. `time_t modtime' This is the modification time for the file. - Function: int utime (const char *FILENAME, const struct utimbuf *TIMES) This function is used to modify the file times associated with the file named FILENAME. If TIMES is a null pointer, then the access and modification times of the file are set to the current time. Otherwise, they are set to the values from the `actime' and `modtime' members (respectively) of the `utimbuf' structure pointed to by TIMES. The attribute modification time for the file is set to the current time in either case (since changing the time stamps is itself a modification of the file attributes). The `utime' function returns `0' if successful and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' There is a permission problem in the case where a null pointer was passed as the TIMES argument. In order to update the time stamp on the file, you must either be the owner of the file, have write permission for the file, or be a privileged user. `ENOENT' The file doesn't exist. `EPERM' If the TIMES argument is not a null pointer, you must either be the owner of the file or be a privileged user. `EROFS' The file lives on a read-only file system. Each of the three time stamps has a corresponding microsecond part, which extends its resolution. These fields are called `st_atime_usec', `st_mtime_usec', and `st_ctime_usec'; each has a value between 0 and 999,999, which indicates the time in microseconds. They correspond to the `tv_usec' field of a `timeval' structure; see *Note High-Resolution Calendar::. The `utimes' function is like `utime', but also lets you specify the fractional part of the file times. The prototype for this function is in the header file `sys/time.h'. - Function: int utimes (const char *FILENAME, struct timeval TVP[2]) This function sets the file access and modification times of the file FILENAME. The new file access time is specified by `TVP[0]', and the new modification time by `TVP[1]'. This function comes from BSD. The return values and error conditions are the same as for the `utime' function. File Size --------- Normally file sizes are maintained automatically. A file begins with a size of 0 and is automatically extended when data is written past its end. It is also possible to empty a file completely by an `open' or `fopen' call. However, sometimes it is necessary to _reduce_ the size of a file. This can be done with the `truncate' and `ftruncate' functions. They were introduced in BSD Unix. `ftruncate' was later added to POSIX.1. Some systems allow you to extend a file (creating holes) with these functions. This is useful when using memory-mapped I/O (*note Memory-mapped I/O::), where files are not automatically extended. However, it is not portable but must be implemented if `mmap' allows mapping of files (i.e., `_POSIX_MAPPED_FILES' is defined). Using these functions on anything other than a regular file gives _undefined_ results. On many systems, such a call will appear to succeed, without actually accomplishing anything. - Function: int truncate (const char *FILENAME, off_t LENGTH) The `truncate' function changes the size of FILENAME to LENGTH. If LENGTH is shorter than the previous length, data at the end will be lost. The file must be writable by the user to perform this operation. If LENGTH is longer, holes will be added to the end. However, some systems do not support this feature and will leave the file unchanged. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `truncate' function is in fact `truncate64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. The return value is 0 for success, or -1 for an error. In addition to the usual file name errors, the following errors may occur: `EACCES' The file is a directory or not writable. `EINVAL' LENGTH is negative. `EFBIG' The operation would extend the file beyond the limits of the operating system. `EIO' A hardware I/O error occurred. `EPERM' The file is "append-only" or "immutable". `EINTR' The operation was interrupted by a signal. - Function: int truncate64 (const char *NAME, off64_t LENGTH) This function is similar to the `truncate' function. The difference is that the LENGTH argument is 64 bits wide even on 32 bits machines, which allows the handling of files with sizes up to 2^63 bytes. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `truncate' and so transparently replaces the 32 bits interface. - Function: int ftruncate (int FD, off_t LENGTH) This is like `truncate', but it works on a file descriptor FD for an opened file instead of a file name to identify the object. The file must be opened for writing to successfully carry out the operation. The POSIX standard leaves it implementation defined what happens if the specified new LENGTH of the file is bigger than the original size. The `ftruncate' function might simply leave the file alone and do nothing or it can increase the size to the desired size. In this later case the extended area should be zero-filled. So using `ftruncate' is no reliable way to increase the file size but if it is possible it is probably the fastest way. The function also operates on POSIX shared memory segments if these are implemented by the system. `ftruncate' is especially useful in combination with `mmap'. Since the mapped region must have a fixed size one cannot enlarge the file by writing something beyond the last mapped page. Instead one has to enlarge the file itself and then remap the file with the new size. The example below shows how this works. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `ftruncate' function is in fact `ftruncate64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. The return value is 0 for success, or -1 for an error. The following errors may occur: `EBADF' FD does not correspond to an open file. `EACCES' FD is a directory or not open for writing. `EINVAL' LENGTH is negative. `EFBIG' The operation would extend the file beyond the limits of the operating system. `EIO' A hardware I/O error occurred. `EPERM' The file is "append-only" or "immutable". `EINTR' The operation was interrupted by a signal. - Function: int ftruncate64 (int ID, off64_t LENGTH) This function is similar to the `ftruncate' function. The difference is that the LENGTH argument is 64 bits wide even on 32 bits machines which allows the handling of files with sizes up to 2^63 bytes. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `ftruncate' and so transparently replaces the 32 bits interface. As announced here is a little example of how to use `ftruncate' in combination with `mmap': int fd; void *start; size_t len; int add (off_t at, void *block, size_t size) { if (at + size > len) { /* Resize the file and remap. */ size_t ps = sysconf (_SC_PAGESIZE); size_t ns = (at + size + ps - 1) & ~(ps - 1); void *np; if (ftruncate (fd, ns) < 0) return -1; np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (np == MAP_FAILED) return -1; start = np; len = ns; } memcpy ((char *) start + at, block, size); return 0; } The function `add' writes a block of memory at an arbitrary position in the file. If the current size of the file is too small it is extended. Note the it is extended by a round number of pages. This is a requirement of `mmap'. The program has to keep track of the real size, and when it has finished a final `ftruncate' call should set the real size of the file. Making Special Files ==================== The `mknod' function is the primitive for making special files, such as files that correspond to devices. The GNU library includes this function for compatibility with BSD. The prototype for `mknod' is declared in `sys/stat.h'. - Function: int mknod (const char *FILENAME, int MODE, int DEV) The `mknod' function makes a special file with name FILENAME. The MODE specifies the mode of the file, and may include the various special file bits, such as `S_IFCHR' (for a character special file) or `S_IFBLK' (for a block special file). *Note Testing File Type::. The DEV argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created. The return value is `0' on success and `-1' on error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EPERM' The calling process is not privileged. Only the superuser can create special files. `ENOSPC' The directory or file system that would contain the new file is full and cannot be extended. `EROFS' The directory containing the new file can't be modified because it's on a read-only file system. `EEXIST' There is already a file named FILENAME. If you want to replace this file, you must remove the old file explicitly first. Temporary Files =============== If you need to use a temporary file in your program, you can use the `tmpfile' function to open it. Or you can use the `tmpnam' (better: `tmpnam_r') function to provide a name for a temporary file and then you can open it in the usual way with `fopen'. The `tempnam' function is like `tmpnam' but lets you choose what directory temporary files will go in, and something about what their file names will look like. Important for multi-threaded programs is that `tempnam' is reentrant, while `tmpnam' is not since it returns a pointer to a static buffer. These facilities are declared in the header file `stdio.h'. - Function: FILE * tmpfile (void) This function creates a temporary binary file for update mode, as if by calling `fopen' with mode `"wb+"'. The file is deleted automatically when it is closed or when the program terminates. (On some other ISO C systems the file may fail to be deleted if the program terminates abnormally). This function is reentrant. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `tmpfile64', i.e. the LFS interface transparently replaces the old interface. - Function: FILE * tmpfile64 (void) This function is similar to `tmpfile', but the stream it returns a pointer to was opened using `tmpfile64'. Therefore this stream can be used for files larger then 2^31 bytes on 32-bit machines. Please note that the return type is still `FILE *'. There is no special `FILE' type for the LFS interface. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `tmpfile' and so transparently replaces the old interface. - Function: char * tmpnam (char *RESULT) This function constructs and returns a valid file name that does not refer to any existing file. If the RESULT argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls and therefore makes this function non-reentrant. Otherwise, the RESULT argument should be a pointer to an array of at least `L_tmpnam' characters, and the result is written into that array. It is possible for `tmpnam' to fail if you call it too many times without removing previously-created files. This is because the limited length of the temporary file names gives room for only a finite number of different names. If `tmpnam' fails it returns a null pointer. *Warning:* Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `tmpnam', leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL' flag. Using `tmpfile' or `mkstemp' is a safe way to avoid this problem. - Function: char * tmpnam_r (char *RESULT) This function is nearly identical to the `tmpnam' function, except that if RESULT is a null pointer it returns a null pointer. This guarantees reentrancy because the non-reentrant situation of `tmpnam' cannot happen here. *Warning*: This function has the same security problems as `tmpnam'. - Macro: int L_tmpnam The value of this macro is an integer constant expression that represents the minimum size of a string large enough to hold a file name generated by the `tmpnam' function. - Macro: int TMP_MAX The macro `TMP_MAX' is a lower bound for how many temporary names you can create with `tmpnam'. You can rely on being able to call `tmpnam' at least this many times before it might fail saying you have made too many temporary file names. With the GNU library, you can create a very large number of temporary file names. If you actually created the files, you would probably run out of disk space before you ran out of names. Some other systems have a fixed, small limit on the number of temporary files. The limit is never less than `25'. - Function: char * tempnam (const char *DIR, const char *PREFIX) This function generates a unique temporary file name. If PREFIX is not a null pointer, up to five characters of this string are used as a prefix for the file name. The return value is a string newly allocated with `malloc', so you should release its storage with `free' when it is no longer needed. Because the string is dynamically allocated this function is reentrant. The directory prefix for the temporary file name is determined by testing each of the following in sequence. The directory must exist and be writable. * The environment variable `TMPDIR', if it is defined. For security reasons this only happens if the program is not SUID or SGID enabled. * The DIR argument, if it is not a null pointer. * The value of the `P_tmpdir' macro. * The directory `/tmp'. This function is defined for SVID compatibility. *Warning:* Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `tempnam', leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL' flag. Using `tmpfile' or `mkstemp' is a safe way to avoid this problem. - SVID Macro: char * P_tmpdir This macro is the name of the default directory for temporary files. Older Unix systems did not have the functions just described. Instead they used `mktemp' and `mkstemp'. Both of these functions work by modifying a file name template string you pass. The last six characters of this string must be `XXXXXX'. These six `X's are replaced with six characters which make the whole string a unique file name. Usually the template string is something like `/tmp/PREFIXXXXXXX', and each program uses a unique PREFIX. *Note:* Because `mktemp' and `mkstemp' modify the template string, you _must not_ pass string constants to them. String constants are normally in read-only storage, so your program would crash when `mktemp' or `mkstemp' tried to modify the string. - Function: char * mktemp (char *TEMPLATE) The `mktemp' function generates a unique file name by modifying TEMPLATE as described above. If successful, it returns TEMPLATE as modified. If `mktemp' cannot find a unique file name, it makes TEMPLATE an empty string and returns that. If TEMPLATE does not end with `XXXXXX', `mktemp' returns a null pointer. *Warning:* Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `mktemp', leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL' flag. Using `mkstemp' is a safe way to avoid this problem. - Function: int mkstemp (char *TEMPLATE) The `mkstemp' function generates a unique file name just as `mktemp' does, but it also opens the file for you with `open' (*note Opening and Closing Files::). If successful, it modifies TEMPLATE in place and returns a file descriptor for that file open for reading and writing. If `mkstemp' cannot create a uniquely-named file, it returns `-1'. If TEMPLATE does not end with `XXXXXX', `mkstemp' returns `-1' and does not modify TEMPLATE. The file is opened using mode `0600'. If the file is meant to be used by other users this mode must be changed explicitly. Unlike `mktemp', `mkstemp' is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file. This is because it works by calling `open' with the `O_EXCL' flag, which says you want to create a new file and get an error if the file already exists. - Function: char * mkdtemp (char *TEMPLATE) The `mkdtemp' function creates a directory with a unique name. If it succeeds, it overwrites TEMPLATE with the name of the directory, and returns TEMPLATE. As with `mktemp' and `mkstemp', TEMPLATE should be a string ending with `XXXXXX'. If `mkdtemp' cannot create an uniquely named directory, it returns `NULL' and sets ERRNO appropriately. If TEMPLATE does not end with `XXXXXX', `mkdtemp' returns `NULL' and does not modify TEMPLATE. ERRNO will be set to `EINVAL' in this case. The directory is created using mode `0700'. The directory created by `mkdtemp' cannot clash with temporary files or directories created by other users. This is because directory creation always works like `open' with `O_EXCL'. *Note Creating Directories::. The `mkdtemp' function comes from OpenBSD. Pipes and FIFOs *************** A "pipe" is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process. The data is handled in a first-in, first-out (FIFO) order. The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe. A "FIFO special file" is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file. Processes open the FIFO by name in order to communicate through it. A pipe or FIFO has to be open at both ends simultaneously. If you read from a pipe or FIFO file that doesn't have any processes writing to it (perhaps because they have all closed the file, or exited), the read returns end-of-file. Writing to a pipe or FIFO that doesn't have a reading process is treated as an error condition; it generates a `SIGPIPE' signal, and fails with error code `EPIPE' if the signal is handled or blocked. Neither pipes nor FIFO special files allow file positioning. Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end. Creating a Pipe =============== The primitive for creating a pipe is the `pipe' function. This creates both the reading and writing ends of the pipe. It is not very useful for a single process to use a pipe to talk to itself. In typical use, a process creates a pipe just before it forks one or more child processes (*note Creating a Process::). The pipe is then used for communication either between the parent or child processes, or between two sibling processes. The `pipe' function is declared in the header file `unistd.h'. - Function: int pipe (int FILEDES[2]) The `pipe' function creates a pipe and puts the file descriptors for the reading and writing ends of the pipe (respectively) into `FILEDES[0]' and `FILEDES[1]'. An easy way to remember that the input end comes first is that file descriptor `0' is standard input, and file descriptor `1' is standard output. If successful, `pipe' returns a value of `0'. On failure, `-1' is returned. The following `errno' error conditions are defined for this function: `EMFILE' The process has too many files open. `ENFILE' There are too many open files in the entire system. *Note Error Codes::, for more information about `ENFILE'. This error never occurs in the GNU system. Here is an example of a simple program that creates a pipe. This program uses the `fork' function (*note Creating a Process::) to create a child process. The parent process writes data to the pipe, which is read by the child process. #include #include #include #include /* Read characters from the pipe and echo them to `stdout'. */ void read_from_pipe (int file) { FILE *stream; int c; stream = fdopen (file, "r"); while ((c = fgetc (stream)) != EOF) putchar (c); fclose (stream); } /* Write some random text to the pipe. */ void write_to_pipe (int file) { FILE *stream; stream = fdopen (file, "w"); fprintf (stream, "hello, world!\n"); fprintf (stream, "goodbye, world!\n"); fclose (stream); } int main (void) { pid_t pid; int mypipe[2]; /* Create the pipe. */ if (pipe (mypipe)) { fprintf (stderr, "Pipe failed.\n"); return EXIT_FAILURE; } /* Create the child process. */ pid = fork (); if (pid == (pid_t) 0) { /* This is the child process. Close other end first. */ close (mypipe[1]); read_from_pipe (mypipe[0]); return EXIT_SUCCESS; } else if (pid < (pid_t) 0) { /* The fork failed. */ fprintf (stderr, "Fork failed.\n"); return EXIT_FAILURE; } else { /* This is the parent process. Close other end first. */ close (mypipe[0]); write_to_pipe (mypipe[1]); return EXIT_SUCCESS; } } Pipe to a Subprocess ==================== A common use of pipes is to send data to or receive data from a program being run as a subprocess. One way of doing this is by using a combination of `pipe' (to create the pipe), `fork' (to create the subprocess), `dup2' (to force the subprocess to use the pipe as its standard input or output channel), and `exec' (to execute the new program). Or, you can use `popen' and `pclose'. The advantage of using `popen' and `pclose' is that the interface is much simpler and easier to use. But it doesn't offer as much flexibility as using the low-level functions directly. - Function: FILE * popen (const char *COMMAND, const char *MODE) The `popen' function is closely related to the `system' function; see *Note Running a Command::. It executes the shell command COMMAND as a subprocess. However, instead of waiting for the command to complete, it creates a pipe to the subprocess and returns a stream that corresponds to that pipe. If you specify a MODE argument of `"r"', you can read from the stream to retrieve data from the standard output channel of the subprocess. The subprocess inherits its standard input channel from the parent process. Similarly, if you specify a MODE argument of `"w"', you can write to the stream to send data to the standard input channel of the subprocess. The subprocess inherits its standard output channel from the parent process. In the event of an error `popen' returns a null pointer. This might happen if the pipe or stream cannot be created, if the subprocess cannot be forked, or if the program cannot be executed. - Function: int pclose (FILE *STREAM) The `pclose' function is used to close a stream created by `popen'. It waits for the child process to terminate and returns its status value, as for the `system' function. Here is an example showing how to use `popen' and `pclose' to filter output through another program, in this case the paging program `more'. #include #include void write_data (FILE * stream) { int i; for (i = 0; i < 100; i++) fprintf (stream, "%d\n", i); if (ferror (stream)) { fprintf (stderr, "Output to stream failed.\n"); exit (EXIT_FAILURE); } } int main (void) { FILE *output; output = popen ("more", "w"); if (!output) { fprintf (stderr, "incorrect parameters or too many files.\n"); return EXIT_FAILURE; } write_data (output); if (pclose (output) != 0) { fprintf (stderr, "Could not run more or other error.\n"); } return EXIT_SUCCESS; } FIFO Special Files ================== A FIFO special file is similar to a pipe, except that it is created in a different way. Instead of being an anonymous communications channel, a FIFO special file is entered into the file system by calling `mkfifo'. Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa. The `mkfifo' function is declared in the header file `sys/stat.h'. - Function: int mkfifo (const char *FILENAME, mode_t MODE) The `mkfifo' function makes a FIFO special file with name FILENAME. The MODE argument is used to set the file's permissions; see *Note Setting Permissions::. The normal, successful return value from `mkfifo' is `0'. In the case of an error, `-1' is returned. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EEXIST' The named file already exists. `ENOSPC' The directory or file system cannot be extended. `EROFS' The directory that would contain the file resides on a read-only file system. Atomicity of Pipe I/O ===================== Reading or writing pipe data is "atomic" if the size of data written is not greater than `PIPE_BUF'. This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete. Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin it finishes immediately. Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed. Also, once `PIPE_BUF' characters have been written, further writes will block until some characters are read. *Note Limits for Files::, for information about the `PIPE_BUF' parameter. Sockets ******* This chapter describes the GNU facilities for interprocess communication using sockets. A "socket" is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. Unlike pipes sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; `telnet', `rlogin', `ftp', `talk' and the other familiar network programs use sockets. Not all operating systems support sockets. In the GNU library, the header file `sys/socket.h' exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail. *Incomplete:* We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far. Socket Concepts =============== When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The "communication style" of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these: * *What are the units of data transmission?* Some communication styles regard the data as a sequence of bytes with no larger structure; others group the bytes into records (which are known in this context as "packets"). * *Can data be lost during normal operation?* Some communication styles guarantee that all the data sent arrives in the order it was sent (barring system or network crashes); other styles occasionally lose data as a normal part of operation, and may sometimes deliver packets more than once or in the wrong order. Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed. * *Is communication entirely with one partner?* Some communication styles are like a telephone call--you make a "connection" with one remote socket and then exchange data freely. Other styles are like mailing letters--you specify a destination address for each message you send. You must also choose a "namespace" for naming the socket. A socket name ("address") is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called "domains", but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with `PF_'. A corresponding symbolic name starting with `AF_' designates the address format for that namespace. Finally you must choose the "protocol" to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a "protocol family" because of this, which is why the namespace names start with `PF_'. The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system and you need not know about them. What you do need to know about protocols is this: * In order to have communication between two sockets, they must specify the _same_ protocol. * Each protocol is meaningful with particular style/namespace combinations and cannot be used with inappropriate combinations. For example, the TCP protocol fits only the byte stream style of communication and the Internet namespace. * For each combination of style and namespace there is a "default protocol", which you can request by specifying 0 as the protocol number. And that's what you should normally do--use the default. Throughout the following description at various places variables/parameters to denote sizes are required. And here the trouble starts. In the first implementations the type of these variables was simply `int'. On most machines at that time an `int' was 32 bits wide, which created a _de facto_ standard requiring 32-bit variables. This is important since references to variables of this type are passed to the kernel. Then the POSIX people came and unified the interface with the words "all size values are of type `size_t'". On 64-bit machines `size_t' is 64 bits wide, so pointers to variables were no longer possible. The Unix98 specification provides a solution by introducing a type `socklen_t'. This type is used in all of the cases that POSIX changed to use `size_t'. The only requirement of this type is that it be an unsigned type of at least 32 bits. Therefore, implementations which require that references to 32-bit variables be passed can be as happy as implementations which use 64-bit values. Communication Styles ==================== The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in `sys/socket.h'. - Macro: int SOCK_STREAM The `SOCK_STREAM' style is like a pipe (*note Pipes and FIFOs::). It operates over a connection with a particular remote socket and transmits data reliably as a stream of bytes. Use of this style is covered in detail in *Note Connections::. - Macro: int SOCK_DGRAM The `SOCK_DGRAM' style is used for sending individually-addressed packets unreliably. It is the diametrical opposite of `SOCK_STREAM'. Each time you write data to a socket of this kind, that data becomes one packet. Since `SOCK_DGRAM' sockets do not have connections, you must specify the recipient address with each packet. The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth. The typical use for `SOCK_DGRAM' is in situations where it is acceptable to simply re-send a packet if no response is seen in a reasonable amount of time. *Note Datagrams::, for detailed information about how to use datagram sockets. - Macro: int SOCK_RAW This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style. Socket Addresses ================ The name of a socket is normally called an "address". The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term "name" and sometimes using "address". You can regard these terms as synonymous where sockets are concerned. A socket newly created with the `socket' function has no address. Other processes can find it for communication only if you give it an address. We call this "binding" the address to the socket, and the way to do it is with the `bind' function. You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one. Occasionally a client needs to specify an address because the server discriminates based on address; for example, the rsh and rlogin protocols look at the client's socket address and only bypass password checking if it is less than `IPPORT_RESERVED' (*note Ports::). The details of socket addresses vary depending on what namespace you are using. *Note Local Namespace::, or *Note Internet Namespace::, for specific information. Regardless of the namespace, you use the same functions `bind' and `getsockname' to set and examine a socket's address. These functions use a phony data type, `struct sockaddr *', to accept the address. In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to `struct sockaddr *' when you pass it to `bind'. Address Formats --------------- The functions `bind' and `getsockname' use the generic data type `struct sockaddr *' to represent a pointer to a socket address. You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace. Thus, the usual practice is to construct an address of the proper namespace-specific type, then cast a pointer to `struct sockaddr *' when you call `bind' or `getsockname'. The one piece of information that you can get from the `struct sockaddr' data type is the "address format designator". This tells you which data type to use to understand the address fully. The symbols in this section are defined in the header file `sys/socket.h'. - Data Type: struct sockaddr The `struct sockaddr' type itself has the following members: `short int sa_family' This is the code for the address format of this address. It identifies the format of the data which follows. `char sa_data[14]' This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of `sa_data' is essentially arbitrary. Each address format has a symbolic name which starts with `AF_'. Each of them corresponds to a `PF_' symbol which designates the corresponding namespace. Here is a list of address format names: `AF_LOCAL' This designates the address format that goes with the local namespace. (`PF_LOCAL' is the name of that namespace.) *Note Local Namespace Details::, for information about this address format. `AF_UNIX' This is a synonym for `AF_LOCAL'. Although `AF_LOCAL' is mandated by POSIX.1g, `AF_UNIX' is portable to more systems. `AF_UNIX' was the traditional name stemming from BSD, so even most POSIX systems support it. It is also the name of choice in the Unix98 specification. (The same is true for `PF_UNIX' vs. `PF_LOCAL'). `AF_FILE' This is another synonym for `AF_LOCAL', for compatibility. (`PF_FILE' is likewise a synonym for `PF_LOCAL'.) `AF_INET' This designates the address format that goes with the Internet namespace. (`PF_INET' is the name of that namespace.) *Note Internet Address Formats::. `AF_INET6' This is similar to `AF_INET', but refers to the IPv6 protocol. (`PF_INET6' is the name of the corresponding namespace.) `AF_UNSPEC' This designates no particular address format. It is used only in rare cases, such as to clear out the default destination address of a "connected" datagram socket. *Note Sending Datagrams::. The corresponding namespace designator symbol `PF_UNSPEC' exists for completeness, but there is no reason to use it in a program. `sys/socket.h' defines symbols starting with `AF_' for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them. Setting the Address of a Socket ------------------------------- Use the `bind' function to assign an address to a socket. The prototype for `bind' is in the header file `sys/socket.h'. For examples of use, see *Note Local Socket Example::, or see *Note Inet Example::. - Function: int bind (int SOCKET, struct sockaddr *ADDR, socklen_t LENGTH) The `bind' function assigns an address to the socket SOCKET. The ADDR and LENGTH arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format of that namespace. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' The SOCKET argument is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET is not a socket. `EADDRNOTAVAIL' The specified address is not available on this machine. `EADDRINUSE' Some other socket is already using the specified address. `EINVAL' The socket SOCKET already has an address. `EACCES' You do not have permission to access the requested address. (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through `IPPORT_RESERVED' minus one; see *Note Ports::.) Additional conditions may be possible depending on the particular namespace of the socket. Reading the Address of a Socket ------------------------------- Use the function `getsockname' to examine the address of an Internet socket. The prototype for this function is in the header file `sys/socket.h'. - Function: int getsockname (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH-PTR) The `getsockname' function returns information about the address of the socket SOCKET in the locations specified by the ADDR and LENGTH-PTR arguments. Note that the LENGTH-PTR is a pointer; you should initialize it to be the allocation size of ADDR, and on return it contains the actual size of the address data. The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to `struct sockaddr *' to pass it to `getsockname'. The return value is `0' on success and `-1' on error. The following `errno' error conditions are defined for this function: `EBADF' The SOCKET argument is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET is not a socket. `ENOBUFS' There are not enough internal buffers available for the operation. You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file. Interface Naming ================ Each network interface has a name. This usually consists of a few letters that relate to the type of interface, which may be followed by a number if there is more than one interface of that type. Examples might be `lo' (the loopback interface) and `eth0' (the first Ethernet interface). Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its "index", which is an arbitrarily-assigned small positive integer. The following functions, constants and data types are declared in the header file `net/if.h'. - Constant: size_t IFNAMSIZ This constant defines the maximum buffer size needed to hold an interface name, including its terminating zero byte. - Function: unsigned int if_nametoindex (const char *ifname) This function yields the interface index corresponding to a particular name. If no interface exists with the name given, it returns 0. - Function: char * if_indextoname (unsigned int ifindex, char *ifname) This function maps an interface index to its corresponding name. The returned name is placed in the buffer pointed to by `ifname', which must be at least `IFNAMSIZ' bytes in length. If the index was invalid, the function's return value is a null pointer, otherwise it is `ifname'. - Data Type: struct if_nameindex This data type is used to hold the information about a single interface. It has the following members: `unsigned int if_index;' This is the interface index. `char *if_name' This is the null-terminated index name. - Function: struct if_nameindex * if_nameindex (void) This function returns an array of `if_nameindex' structures, one for every interface that is present. The end of the list is indicated by a structure with an interface of 0 and a null name pointer. If an error occurs, this function returns a null pointer. The returned structure must be freed with `if_freenameindex' after use. - Function: void if_freenameindex (struct if_nameindex *ptr) This function frees the structure returned by an earlier call to `if_nameindex'. The Local Namespace =================== This section describes the details of the local namespace, whose symbolic name (required when you create a socket) is `PF_LOCAL'. The local namespace is also known as "Unix domain sockets". Another name is file namespace since socket addresses are normally implemented as file names. Local Namespace Concepts ------------------------ In the local namespace socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. It's common to put these files in the `/tmp' directory. One peculiarity of the local namespace is that the name is only used when opening the connection; once open the address is not meaningful and may not exist. Another peculiarity is that you cannot connect to such a socket from another machine-not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one. After you close a socket in the local namespace, you should delete the file name from the file system. Use `unlink' or `remove' to do this; see *Note Deleting Files::. The local namespace supports just one protocol for any communication style; it is protocol number `0'. Details of Local Namespace -------------------------- To create a socket in the local namespace, use the constant `PF_LOCAL' as the NAMESPACE argument to `socket' or `socketpair'. This constant is defined in `sys/socket.h'. - Macro: int PF_LOCAL This designates the local namespace, in which socket addresses are local names, and its associated family of protocols. `PF_Local' is the macro used by Posix.1g. - Macro: int PF_UNIX This is a synonym for `PF_LOCAL', for compatibility's sake. - Macro: int PF_FILE This is a synonym for `PF_LOCAL', for compatibility's sake. The structure for specifying socket names in the local namespace is defined in the header file `sys/un.h': - Data Type: struct sockaddr_un This structure is used to specify local namespace socket addresses. It has the following members: `short int sun_family' This identifies the address family or format of the socket address. You should store the value `AF_LOCAL' to designate the local namespace. *Note Socket Addresses::. `char sun_path[108]' This is the file name to use. *Incomplete:* Why is 108 a magic number? RMS suggests making this a zero-length array and tweaking the following example to use `alloca' to allocate an appropriate amount of storage based on the length of the filename. You should compute the LENGTH parameter for a socket address in the local namespace as the sum of the size of the `sun_family' component and the string length (_not_ the allocation size!) of the file name string. This can be done using the macro `SUN_LEN': - Macro: int SUN_LEN (_struct sockaddr_un *_ PTR) The macro computes the length of socket address in the local namespace. Example of Local-Namespace Sockets ---------------------------------- Here is an example showing how to create and name a socket in the local namespace. #include #include #include #include #include #include int make_named_socket (const char *filename) { struct sockaddr_un name; int sock; size_t size; /* Create the socket. */ sock = socket (PF_LOCAL, SOCK_DGRAM, 0); if (sock < 0) { perror ("socket"); exit (EXIT_FAILURE); } /* Bind a name to the socket. */ name.sun_family = AF_LOCAL; strncpy (name.sun_path, filename, sizeof (name.sun_path)); /* The size of the address is the offset of the start of the filename, plus its length, plus one for the terminating null byte. Alternatively you can just do: size = SUN_LEN (&name); */ size = (offsetof (struct sockaddr_un, sun_path) + strlen (name.sun_path) + 1); if (bind (sock, (struct sockaddr *) &name, size) < 0) { perror ("bind"); exit (EXIT_FAILURE); } return sock; } The Internet Namespace ====================== This section describes the details of the protocols and socket naming conventions used in the Internet namespace. Originally the Internet namespace used only IP version 4 (IPv4). With the growing number of hosts on the Internet, a new protocol with a larger address space was necessary: IP version 6 (IPv6). IPv6 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other features, and will eventually replace IPv4. To create a socket in the IPv4 Internet namespace, use the symbolic name `PF_INET' of this namespace as the NAMESPACE argument to `socket' or `socketpair'. For IPv6 addresses you need the macro `PF_INET6'. These macros are defined in `sys/socket.h'. - Macro: int PF_INET This designates the IPv4 Internet namespace and associated family of protocols. - Macro: int PF_INET6 This designates the IPv6 Internet namespace and associated family of protocols. A socket address for the Internet namespace includes the following components: * The address of the machine you want to connect to. Internet addresses can be specified in several ways; these are discussed in *Note Internet Address Formats::, *Note Host Addresses:: and *Note Host Names::. * A port number for that machine. *Note Ports::. You must ensure that the address and port number are represented in a canonical format called "network byte order". *Note Byte Order::, for information about this. Internet Socket Address Formats ------------------------------- In the Internet namespace, for both IPv4 (`AF_INET') and IPv6 (`AF_INET6'), a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol. The data types for representing socket addresses in the Internet namespace are defined in the header file `netinet/in.h'. - Data Type: struct sockaddr_in This is the data type used to represent socket addresses in the Internet namespace. It has the following members: `sa_family_t sin_family' This identifies the address family or format of the socket address. You should store the value `AF_INET' in this member. *Note Socket Addresses::. `struct in_addr sin_addr' This is the Internet address of the host machine. *Note Host Addresses::, and *Note Host Names::, for how to get a value to store here. `unsigned short int sin_port' This is the port number. *Note Ports::. When you call `bind' or `getsockname', you should specify `sizeof (struct sockaddr_in)' as the LENGTH parameter if you are using an IPv4 Internet namespace socket address. - Data Type: struct sockaddr_in6 This is the data type used to represent socket addresses in the IPv6 namespace. It has the following members: `sa_family_t sin6_family' This identifies the address family or format of the socket address. You should store the value of `AF_INET6' in this member. *Note Socket Addresses::. `struct in6_addr sin6_addr' This is the IPv6 address of the host machine. *Note Host Addresses::, and *Note Host Names::, for how to get a value to store here. `uint32_t sin6_flowinfo' This is a currently unimplemented field. `uint16_t sin6_port' This is the port number. *Note Ports::. Host Addresses -------------- Each computer on the Internet has one or more "Internet addresses", numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in `128.52.46.32', and IPv6 numeric host addresses as sequences of up to eight numbers separated by colons, as in `5f03:1200:836f:c100::1'. Each computer also has one or more "host names", which are strings of words separated by periods, as in `mescaline.gnu.org'. Programs that let the user specify a host typically accept both numeric addresses and host names. To open a connection a program needs a numeric address, and so must convert a host name to the numeric address it stands for. Internet Host Addresses ....................... Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. An IPv4 Internet host address is a number containing four bytes of data. Historically these are divided into two parts, a "network number" and a "local network address number" within that network. In the mid-1990s classless addresses were introduced which changed this behavior. Since some functions implicitly expect the old definitions, we first describe the class-based network and will then describe classless addresses. IPv6 uses only classless addresses and therefore the following paragraphs don't apply. The class-based IPv4 network number consists of the first one, two or three bytes; the rest of the bytes are the local address. IPv4 network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B and C. The local network address numbers of individual machines are registered with the administrator of the particular network. Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specify a network. The remaining bytes of the Internet address specify the address within that network. The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network. These uses are obsolete now but for compatibility reasons you shouldn't use network 0 and host number 0. The Class A network 127 is reserved for loopback; you can always use the Internet address `127.0.0.1' to refer to the host machine. Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address. There are four forms of the "standard numbers-and-dots notation" for Internet addresses: `A.B.C.D' This specifies all four bytes of the address individually and is the commonly used representation. `A.B.C' The last part of the address, C, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number `A.B'. `A.B' The last part of the address, B, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number A. `A' If only one part is given, this corresponds directly to the host address number. Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading `0x' or `0X' implies hexadecimal radix; a leading `0' implies octal; and otherwise decimal radix is assumed. Classless Addresses ................... IPv4 addresses (and IPv6 addresses also) are now considered classless; the distinction between classes A, B and C can be ignored. Instead an IPv4 host address consists of a 32-bit address and a 32-bit mask. The mask contains set bits for the network part and cleared bits for the host part. The network part is contiguous from the left, with the remaining bits representing the host. As a consequence, the netmask can simply be specified as the number of set bits. Classes A, B and C are just special cases of this general rule. For example, class A addresses have a netmask of `255.0.0.0' or a prefix length of 8. Classless IPv4 network addresses are written in numbers-and-dots notation with the prefix length appended and a slash as separator. For example the class A network 10 is written as `10.0.0.0/8'. IPv6 Addresses .............. IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host address is usually written as eight 16-bit hexadecimal numbers that are separated by colons. Two colons are used to abbreviate strings of consecutive zeros. For example, the IPv6 loopback address `0:0:0:0:0:0:0:1' can just be written as `::1'. Host Address Data Type ...................... IPv4 Internet host addresses are represented in some contexts as integers (type `uint32_t'). In other contexts, the integer is packaged inside a structure of type `struct in_addr'. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure. You will find older code that uses `unsigned long int' for IPv4 Internet host addresses instead of `uint32_t' or `struct in_addr'. Historically `unsigned long int' was a 32-bit number but with 64-bit machines this has changed. Using `unsigned long int' might break the code if it is used on machines where this type doesn't have 32 bits. `uint32_t' is specified by Unix98 and guaranteed to have 32 bits. IPv6 Internet host addresses have 128 bits and are packaged inside a structure of type `struct in6_addr'. The following basic definitions for Internet addresses are declared in the header file `netinet/in.h': - Data Type: struct in_addr This data type is used in certain contexts to contain an IPv4 Internet host address. It has just one field, named `s_addr', which records the host address number as an `uint32_t'. - Macro: uint32_t INADDR_LOOPBACK You can use this constant to stand for "the address of this machine," instead of finding its actual address. It is the IPv4 Internet address `127.0.0.1', which is usually called `localhost'. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements `INADDR_LOOPBACK' specially, avoiding any network traffic for the case of one machine talking to itself. - Macro: uint32_t INADDR_ANY You can use this constant to stand for "any incoming address" when binding to an address. *Note Setting Address::. This is the usual address to give in the `sin_addr' member of `struct sockaddr_in' when you want to accept Internet connections. - Macro: uint32_t INADDR_BROADCAST This constant is the address you use to send a broadcast message. - Macro: uint32_t INADDR_NONE This constant is returned by some functions to indicate an error. - Data Type: struct in6_addr This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways. - Constant: struct in6_addr in6addr_loopback This constant is the IPv6 address `::1', the loopback address. See above for a description of what this means. The macro `IN6ADDR_LOOPBACK_INIT' is provided to allow you to initialize your own variables to this value. - Constant: struct in6_addr in6addr_any This constant is the IPv6 address `::', the unspecified address. See above for a description of what this means. The macro `IN6ADDR_ANY_INIT' is provided to allow you to initialize your own variables to this value. Host Address Functions ...................... These additional functions for manipulating Internet addresses are declared in the header file `arpa/inet.h'. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. *Note Byte Order::, for an explanation of network and host byte order. - Function: int inet_aton (const char *NAME, struct in_addr *ADDR) This function converts the IPv4 Internet host address NAME from the standard numbers-and-dots notation into binary data and stores it in the `struct in_addr' that ADDR points to. `inet_aton' returns nonzero if the address is valid, zero if not. - Function: uint32_t inet_addr (const char *NAME) This function converts the IPv4 Internet host address NAME from the standard numbers-and-dots notation into binary data. If the input is not valid, `inet_addr' returns `INADDR_NONE'. This is an obsolete interface to `inet_aton', described immediately above. It is obsolete because `INADDR_NONE' is a valid address (255.255.255.255), and `inet_aton' provides a cleaner way to indicate error return. - Function: uint32_t inet_network (const char *NAME) This function extracts the network number from the address NAME, given in the standard numbers-and-dots notation. The returned address is in host order. If the input is not valid, `inet_network' returns `-1'. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. - Function: char * inet_ntoa (struct in_addr ADDR) This function converts the IPv4 Internet host address ADDR to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it. In multi-threaded programs each thread has an own statically-allocated buffer. But still subsequent calls of `inet_ntoa' in the same thread will overwrite the result of the last call. Instead of `inet_ntoa' the newer function `inet_ntop' which is described below should be used since it handles both IPv4 and IPv6 addresses. - Function: struct in_addr inet_makeaddr (uint32_t NET, uint32_t LOCAL) This function makes an IPv4 Internet host address by combining the network number NET with the local-address-within-network number LOCAL. - Function: uint32_t inet_lnaof (struct in_addr ADDR) This function returns the local-address-within-network part of the Internet host address ADDR. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. - Function: uint32_t inet_netof (struct in_addr ADDR) This function returns the network number part of the Internet host address ADDR. The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore. - Function: int inet_pton (int AF, const char *CP, void *BUF) This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. AF should be either `AF_INET' or `AF_INET6', as appropriate for the type of address being converted. CP is a pointer to the input string, and BUF is a pointer to a buffer for the result. It is the caller's responsibility to make sure the buffer is large enough. - Function: const char * inet_ntop (int AF, const void *CP, char *BUF, size_t LEN) This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. AF should be either `AF_INET' or `AF_INET6', as appropriate. CP is a pointer to the address to be converted. BUF should be a pointer to a buffer to hold the result, and LEN is the length of this buffer. The return value from the function will be this buffer address. Host Names .......... Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address `158.121.106.19' is also known as `alpha.gnu.org'; and other machines in the `gnu.org' domain can refer to it simply as `alpha'. Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file `/etc/hosts' or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in `netdb.h'. They are BSD features, defined unconditionally if you include `netdb.h'. - Data Type: struct hostent This data type is used to represent an entry in the hosts database. It has the following members: `char *h_name' This is the "official" name of the host. `char **h_aliases' These are alternative names for the host, represented as a null-terminated vector of strings. `int h_addrtype' This is the host address type; in practice, its value is always either `AF_INET' or `AF_INET6', with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than `AF_INET' or `AF_INET6'. *Note Socket Addresses::. `int h_length' This is the length, in bytes, of each address. `char **h_addr_list' This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer. `char *h_addr' This is a synonym for `h_addr_list[0]'; in other words, it is the first host address. As far as the host database is concerned, each address is just a block of memory `h_length' bytes long. But in other contexts there is an implicit assumption that you can convert IPv4 addresses to a `struct in_addr' or an `uint32_t'. Host addresses in a `struct hostent' structure are always given in network byte order; see *Note Byte Order::. You can use `gethostbyname', `gethostbyname2' or `gethostbyaddr' to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use `getaddrinfo' and `getnameinfo' to obtain this information. - Function: struct hostent * gethostbyname (const char *NAME) The `gethostbyname' function returns information about the host named NAME. If the lookup fails, it returns a null pointer. - Function: struct hostent * gethostbyname2 (const char *NAME, int AF) The `gethostbyname2' function is like `gethostbyname', but allows the caller to specify the desired address family (e.g. `AF_INET' or `AF_INET6') of the result. - Function: struct hostent * gethostbyaddr (const char *ADDR, size_t LENGTH, int FORMAT) The `gethostbyaddr' function returns information about the host with Internet address ADDR. The parameter ADDR is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The LENGTH argument is the size (in bytes) of the address at ADDR. FORMAT specifies the address format; for an IPv4 Internet address, specify a value of `AF_INET'; for an IPv6 Internet address, use `AF_INET6'. If the lookup fails, `gethostbyaddr' returns a null pointer. If the name lookup by `gethostbyname' or `gethostbyaddr' fails, you can find out the reason by looking at the value of the variable `h_errno'. (It would be cleaner design for these functions to set `errno', but use of `h_errno' is compatible with other systems.) Here are the error codes that you may find in `h_errno': `HOST_NOT_FOUND' No such host is known in the database. `TRY_AGAIN' This condition happens when the name server could not be contacted. If you try again later, you may succeed then. `NO_RECOVERY' A non-recoverable error occurred. `NO_ADDRESS' The host database contains an entry for the name, but it doesn't have an associated Internet address. The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C library a new set of functions which can be used in this context. - Function: int gethostbyname_r (const char *restrict NAME, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP) The `gethostbyname_r' function returns information about the host named NAME. The caller must pass a pointer to an object of type `struct hostent' in the RESULT_BUF parameter. In addition the function may need extra buffer space and the caller must pass an pointer and the size of the buffer in the BUF and BUFLEN parameters. A pointer to the buffer, in which the result is stored, is available in `*RESULT' after the function call successfully returned. If an error occurs or if no entry is found, the pointer `*RESULT' is a null pointer. Success is signalled by a zero return value. If the function failed the return value is an error number. In addition to the errors defined for `gethostbyname' it can also be `ERANGE'. In this case the call should be repeated with a larger buffer. Additional error information is not stored in the global variable `h_errno' but instead in the object pointed to by H_ERRNOP. Here's a small example: struct hostent * gethostname (char *host) { struct hostent hostbuf, *hp; size_t hstbuflen; char *tmphstbuf; int res; int herr; hstbuflen = 1024; /* Allocate buffer, remember to free it to avoid memory leakage. */ tmphstbuf = malloc (hstbuflen); while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen, &hp, &herr)) == ERANGE) { /* Enlarge the buffer. */ hstbuflen *= 2; tmphstbuf = realloc (tmphstbuf, hstbuflen); } /* Check for errors. */ if (res || hp == NULL) return NULL; return hp; } - Function: int gethostbyname2_r (const char *NAME, int AF, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP) The `gethostbyname2_r' function is like `gethostbyname_r', but allows the caller to specify the desired address family (e.g. `AF_INET' or `AF_INET6') for the result. - Function: int gethostbyaddr_r (const char *ADDR, size_t LENGTH, int FORMAT, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP) The `gethostbyaddr_r' function returns information about the host with Internet address ADDR. The parameter ADDR is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The LENGTH argument is the size (in bytes) of the address at ADDR. FORMAT specifies the address format; for an IPv4 Internet address, specify a value of `AF_INET'; for an IPv6 Internet address, use `AF_INET6'. Similar to the `gethostbyname_r' function, the caller must provide buffers for the result and memory used internally. In case of success the function returns zero. Otherwise the value is an error number where `ERANGE' has the special meaning that the caller-provided buffer is too small. You can also scan the entire hosts database one entry at a time using `sethostent', `gethostent' and `endhostent'. Be careful when using these functions because they are not reentrant. - Function: void sethostent (int STAYOPEN) This function opens the hosts database to begin scanning it. You can then call `gethostent' to read the entries. If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to `gethostbyname' or `gethostbyaddr' will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. - Function: struct hostent * gethostent (void) This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries. - Function: void endhostent (void) This function closes the hosts database. Internet Ports -------------- A socket address in the Internet namespace consists of a machine's Internet address plus a "port number" which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535. Port numbers less than `IPPORT_RESERVED' are reserved for standard servers, such as `finger' and `telnet'. There is a database that keeps track of these, and you can use the `getservbyname' function to map a service name onto a port number; see *Note Services Database::. If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it. Use a number greater than `IPPORT_USERRESERVED'; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you. When you use a socket without specifying its address, the system generates a port number for it. This number is between `IPPORT_RESERVED' and `IPPORT_USERRESERVED'. On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number). You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it. Normally, the system won't let you do it; `bind' normally insists on distinct port numbers. To reuse a port number, you must set the socket option `SO_REUSEADDR'. *Note Socket-Level Options::. These macros are defined in the header file `netinet/in.h'. - Macro: int IPPORT_RESERVED Port numbers less than `IPPORT_RESERVED' are reserved for superuser use. - Macro: int IPPORT_USERRESERVED Port numbers greater than or equal to `IPPORT_USERRESERVED' are reserved for explicit use; they will never be allocated automatically. The Services Database --------------------- The database that keeps track of "well-known" services is usually either the file `/etc/services' or an equivalent from a name server. You can use these utilities, declared in `netdb.h', to access the services database. - Data Type: struct servent This data type holds information about entries from the services database. It has the following members: `char *s_name' This is the "official" name of the service. `char **s_aliases' These are alternate names for the service, represented as an array of strings. A null pointer terminates the array. `int s_port' This is the port number for the service. Port numbers are given in network byte order; see *Note Byte Order::. `char *s_proto' This is the name of the protocol to use with this service. *Note Protocols Database::. To get information about a particular service, use the `getservbyname' or `getservbyport' functions. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. - Function: struct servent * getservbyname (const char *NAME, const char *PROTO) The `getservbyname' function returns information about the service named NAME using protocol PROTO. If it can't find such a service, it returns a null pointer. This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (*note Listening::). - Function: struct servent * getservbyport (int PORT, const char *PROTO) The `getservbyport' function returns information about the service at port PORT using protocol PROTO. If it can't find such a service, it returns a null pointer. You can also scan the services database using `setservent', `getservent' and `endservent'. Be careful when using these functions because they are not reentrant. - Function: void setservent (int STAYOPEN) This function opens the services database to begin scanning it. If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to `getservbyname' or `getservbyport' will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. - Function: struct servent * getservent (void) This function returns the next entry in the services database. If there are no more entries, it returns a null pointer. - Function: void endservent (void) This function closes the services database. Byte Order Conversion --------------------- Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called "big-endian" order), and others put it last ("little-endian" order). So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as "network byte order". When establishing an Internet socket connection, you must make sure that the data in the `sin_port' and `sin_addr' members of the `sockaddr_in' structure are represented in network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines. If you use `getservbyname' and `gethostbyname' or `inet_addr' to get the port number and host address, the values are already in network byte order, and you can copy them directly into the `sockaddr_in' structure. Otherwise, you have to convert the values explicitly. Use `htons' and `ntohs' to convert values for the `sin_port' member. Use `htonl' and `ntohl' to convert IPv4 addresses for the `sin_addr' member. (Remember, `struct in_addr' is equivalent to `uint32_t'.) These functions are declared in `netinet/in.h'. - Function: uint16_t htons (uint16_t HOSTSHORT) This function converts the `uint16_t' integer HOSTSHORT from host byte order to network byte order. - Function: uint16_t ntohs (uint16_t NETSHORT) This function converts the `uint16_t' integer NETSHORT from network byte order to host byte order. - Function: uint32_t htonl (uint32_t HOSTLONG) This function converts the `uint32_t' integer HOSTLONG from host byte order to network byte order. This is used for IPv4 Internet addresses. - Function: uint32_t ntohl (uint32_t NETLONG) This function converts the `uint32_t' integer NETLONG from network byte order to host byte order. This is used for IPv4 Internet addresses. Protocols Database ------------------ The communications protocol used with a socket controls low-level details of how data are exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly. The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP ("transmission control protocol"). For datagram communication, the default is UDP ("user datagram protocol"). For reliable datagram communication, the default is RDP ("reliable datagram protocol"). You should nearly always use the default. Internet protocols are generally specified by a name instead of a number. The network protocols that a host knows about are stored in a database. This is usually either derived from the file `/etc/protocols', or it may be an equivalent provided by a name server. You look up the protocol number associated with a named protocol in the database using the `getprotobyname' function. Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in `netdb.h'. - Data Type: struct protoent This data type is used to represent entries in the network protocols database. It has the following members: `char *p_name' This is the official name of the protocol. `char **p_aliases' These are alternate names for the protocol, specified as an array of strings. The last element of the array is a null pointer. `int p_proto' This is the protocol number (in host byte order); use this member as the PROTOCOL argument to `socket'. You can use `getprotobyname' and `getprotobynumber' to search the protocols database for a specific protocol. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. - Function: struct protoent * getprotobyname (const char *NAME) The `getprotobyname' function returns information about the network protocol named NAME. If there is no such protocol, it returns a null pointer. - Function: struct protoent * getprotobynumber (int PROTOCOL) The `getprotobynumber' function returns information about the network protocol with number PROTOCOL. If there is no such protocol, it returns a null pointer. You can also scan the whole protocols database one protocol at a time by using `setprotoent', `getprotoent' and `endprotoent'. Be careful when using these functions because they are not reentrant. - Function: void setprotoent (int STAYOPEN) This function opens the protocols database to begin scanning it. If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to `getprotobyname' or `getprotobynumber' will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. - Function: struct protoent * getprotoent (void) This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries. - Function: void endprotoent (void) This function closes the protocols database. Internet Socket Example ----------------------- Here is an example showing how to create and name a socket in the Internet namespace. The newly created socket exists on the machine that the program is running on. Rather than finding and using the machine's Internet address, this example specifies `INADDR_ANY' as the host address; the system replaces that with the machine's actual address. #include #include #include #include int make_socket (uint16_t port) { int sock; struct sockaddr_in name; /* Create the socket. */ sock = socket (PF_INET, SOCK_STREAM, 0); if (sock < 0) { perror ("socket"); exit (EXIT_FAILURE); } /* Give the socket a name. */ name.sin_family = AF_INET; name.sin_port = htons (port); name.sin_addr.s_addr = htonl (INADDR_ANY); if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0) { perror ("bind"); exit (EXIT_FAILURE); } return sock; } Here is another example, showing how you can fill in a `sockaddr_in' structure, given a host name string and a port number: #include #include #include #include #include void init_sockaddr (struct sockaddr_in *name, const char *hostname, uint16_t port) { struct hostent *hostinfo; name->sin_family = AF_INET; name->sin_port = htons (port); hostinfo = gethostbyname (hostname); if (hostinfo == NULL) { fprintf (stderr, "Unknown host %s.\n", hostname); exit (EXIT_FAILURE); } name->sin_addr = *(struct in_addr *) hostinfo->h_addr; } Other Namespaces ================ Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. `PF_NS' refers to the Xerox Network Software protocols. `PF_ISO' stands for Open Systems Interconnect. `PF_CCITT' refers to protocols from CCITT. `socket.h' defines these symbols and others naming protocols not actually implemented. `PF_IMPLINK' is used for communicating between hosts and Internet Message Processors. For information on this and `PF_ROUTE', an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future). Opening and Closing Sockets =========================== This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles. Creating a Socket ----------------- The primitive for creating a socket is the `socket' function, declared in `sys/socket.h'. - Function: int socket (int NAMESPACE, int STYLE, int PROTOCOL) This function creates a socket and specifies communication style STYLE, which should be one of the socket styles listed in *Note Communication Styles::. The NAMESPACE argument specifies the namespace; it must be `PF_LOCAL' (*note Local Namespace::) or `PF_INET' (*note Internet Namespace::). PROTOCOL designates the specific protocol (*note Socket Concepts::); zero is usually right for PROTOCOL. The return value from `socket' is the file descriptor for the new socket, or `-1' in case of error. The following `errno' error conditions are defined for this function: `EPROTONOSUPPORT' The PROTOCOL or STYLE is not supported by the NAMESPACE specified. `EMFILE' The process already has too many file descriptors open. `ENFILE' The system already has too many file descriptors open. `EACCES' The process does not have the privilege to create a socket of the specified STYLE or PROTOCOL. `ENOBUFS' The system ran out of internal buffer space. The file descriptor returned by the `socket' function supports both read and write operations. However, like pipes, sockets do not support file positioning operations. For examples of how to call the `socket' function, see *Note Local Socket Example::, or *Note Inet Example::. Closing a Socket ---------------- When you have finished using a socket, you can simply close its file descriptor with `close'; see *Note Opening and Closing Files::. If there is still data waiting to be transmitted over the connection, normally `close' tries to complete this transmission. You can control this behavior using the `SO_LINGER' socket option to specify a timeout period; see *Note Socket Options::. You can also shut down only reception or transmission on a connection by calling `shutdown', which is declared in `sys/socket.h'. - Function: int shutdown (int SOCKET, int HOW) The `shutdown' function shuts down the connection of socket SOCKET. The argument HOW specifies what action to perform: `0' Stop receiving data for this socket. If further data arrives, reject it. `1' Stop trying to transmit data from this socket. Discard any data waiting to be sent. Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost. `2' Stop both reception and transmission. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' SOCKET is not a valid file descriptor. `ENOTSOCK' SOCKET is not a socket. `ENOTCONN' SOCKET is not connected. Socket Pairs ------------ A "socket pair" consists of a pair of connected (but unnamed) sockets. It is very similar to a pipe and is used in much the same way. Socket pairs are created with the `socketpair' function, declared in `sys/socket.h'. A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (*note Pipes and FIFOs::). - Function: int socketpair (int NAMESPACE, int STYLE, int PROTOCOL, int FILEDES[2]) This function creates a socket pair, returning the file descriptors in `FILEDES[0]' and `FILEDES[1]'. The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end. The NAMESPACE, STYLE and PROTOCOL arguments are interpreted as for the `socket' function. STYLE should be one of the communication styles listed in *Note Communication Styles::. The NAMESPACE argument specifies the namespace, which must be `AF_LOCAL' (*note Local Namespace::); PROTOCOL specifies the communications protocol, but zero is the only meaningful value. If STYLE specifies a connectionless communication style, then the two sockets you get are not _connected_, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other. The `socketpair' function returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EMFILE' The process has too many file descriptors open. `EAFNOSUPPORT' The specified namespace is not supported. `EPROTONOSUPPORT' The specified protocol is not supported. `EOPNOTSUPP' The specified protocol does not support the creation of socket pairs. Using Sockets with Connections ============================== The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the "client") acts to request a connection, while the other side (the "server") makes a socket and waits for the connection request. Making a Connection ------------------- In making a connection, the client makes a connection while the server waits for and accepts the connection. Here we discuss what the client program must do with the `connect' function, which is declared in `sys/socket.h'. - Function: int connect (int SOCKET, struct sockaddr *ADDR, socklen_t LENGTH) The `connect' function initiates a connection from the socket with file descriptor SOCKET to the socket whose address is specified by the ADDR and LENGTH arguments. (This socket is typically on another machine, and it must be already set up as a server.) *Note Socket Addresses::, for information about how these arguments are interpreted. Normally, `connect' waits until the server responds to the request before it returns. You can set nonblocking mode on the socket SOCKET to make `connect' return immediately without waiting for the response. *Note File Status Flags::, for information about nonblocking mode. The normal return value from `connect' is `0'. If an error occurs, `connect' returns `-1'. The following `errno' error conditions are defined for this function: `EBADF' The socket SOCKET is not a valid file descriptor. `ENOTSOCK' File descriptor SOCKET is not a socket. `EADDRNOTAVAIL' The specified address is not available on the remote machine. `EAFNOSUPPORT' The namespace of the ADDR is not supported by this socket. `EISCONN' The socket SOCKET is already connected. `ETIMEDOUT' The attempt to establish the connection timed out. `ECONNREFUSED' The server has actively refused to establish the connection. `ENETUNREACH' The network of the given ADDR isn't reachable from this host. `EADDRINUSE' The socket address of the given ADDR is already in use. `EINPROGRESS' The socket SOCKET is non-blocking and the connection could not be established immediately. You can determine when the connection is completely established with `select'; *note Waiting for I/O::. Another `connect' call on the same socket, before the connection is completely established, will fail with `EALREADY'. `EALREADY' The socket SOCKET is non-blocking and already has a pending connection in progress (see `EINPROGRESS' above). This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. Listening for Connections ------------------------- Now let us consider what the server process must do to accept connections on a socket. First it must use the `listen' function to enable connection requests on the socket, and then accept each incoming connection with a call to `accept' (*note Accepting Connections::). Once connection requests are enabled on a server socket, the `select' function reports when the socket has a connection ready to be accepted (*note Waiting for I/O::). The `listen' function is not allowed for sockets using connectionless communication styles. You can write a network server that does not even start running until a connection to it is requested. *Note Inetd Servers::. In the Internet namespace, there are no special protection mechanisms for controlling access to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol. In the local namespace, the ordinary file protection bits control who has access to connect to the socket. - Function: int listen (int SOCKET, unsigned int N) The `listen' function enables the socket SOCKET to accept connections, thus making it a server socket. The argument N specifies the length of the queue for pending connections. When the queue fills, new clients attempting to connect fail with `ECONNREFUSED' until the server calls `accept' to accept a connection from the queue. The `listen' function returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' The argument SOCKET is not a valid file descriptor. `ENOTSOCK' The argument SOCKET is not a socket. `EOPNOTSUPP' The socket SOCKET does not support this operation. Accepting Connections --------------------- When a server receives a connection request, it can complete the connection by accepting the request. Use the function `accept' to do this. A socket that has been established as a server can accept connection requests from multiple clients. The server's original socket _does not become part of the connection_; instead, `accept' makes a new socket which participates in the connection. `accept' returns the descriptor for this socket. The server's original socket remains available for listening for further connection requests. The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with an `ECONNREFUSED' error. You can specify the maximum length of this queue as an argument to the `listen' function, although the system may also impose its own internal limit on the length of this queue. - Function: int accept (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH_PTR) This function is used to accept a connection request on the server socket SOCKET. The `accept' function waits if there are no connections pending, unless the socket SOCKET has nonblocking mode set. (You can use `select' to wait for a pending connection, with a nonblocking socket.) *Note File Status Flags::, for information about nonblocking mode. The ADDR and LENGTH-PTR arguments are used to return information about the name of the client socket that initiated the connection. *Note Socket Addresses::, for information about the format of the information. Accepting a connection does not make SOCKET part of the connection. Instead, it creates a new socket which becomes connected. The normal return value of `accept' is the file descriptor for the new socket. After `accept', the original socket SOCKET remains open and unconnected, and continues listening until you close it. You can accept further connections with SOCKET by calling `accept' again. If an error occurs, `accept' returns `-1'. The following `errno' error conditions are defined for this function: `EBADF' The SOCKET argument is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET argument is not a socket. `EOPNOTSUPP' The descriptor SOCKET does not support this operation. `EWOULDBLOCK' SOCKET has nonblocking mode set, and there are no pending connections immediately available. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. The `accept' function is not allowed for sockets using connectionless communication styles. Who is Connected to Me? ----------------------- - Function: int getpeername (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH-PTR) The `getpeername' function returns the address of the socket that SOCKET is connected to; it stores the address in the memory space specified by ADDR and LENGTH-PTR. It stores the length of the address in `*LENGTH-PTR'. *Note Socket Addresses::, for information about the format of the address. In some operating systems, `getpeername' works only for sockets in the Internet domain. The return value is `0' on success and `-1' on error. The following `errno' error conditions are defined for this function: `EBADF' The argument SOCKET is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET is not a socket. `ENOTCONN' The socket SOCKET is not connected. `ENOBUFS' There are not enough internal buffers available. Transferring Data ----------------- Once a socket has been connected to a peer, you can use the ordinary `read' and `write' operations (*note I/O Primitives::) to transfer data. A socket is a two-way communications channel, so read and write operations can be performed at either end. There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the `recv' and `send' functions instead of the more generic `read' and `write' functions. The `recv' and `send' functions take an additional argument which you can use to specify various flags to control special I/O modes. For example, you can specify the `MSG_OOB' flag to read or write out-of-band data, the `MSG_PEEK' flag to peek at input, or the `MSG_DONTROUTE' flag to control inclusion of routing information on output. Sending Data ............ The `send' function is declared in the header file `sys/socket.h'. If your FLAGS argument is zero, you can just as well use `write' instead of `send'; see *Note I/O Primitives::. If the socket was connected but the connection has broken, you get a `SIGPIPE' signal for any use of `send' or `write' (*note Miscellaneous Signals::). - Function: int send (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS) The `send' function is like `write', but with the additional flags FLAGS. The possible values of FLAGS are described in *Note Socket Data Options::. This function returns the number of bytes transmitted, or `-1' on failure. If the socket is nonblocking, then `send' (like `write') can return after sending just part of the data. *Note File Status Flags::, for information about nonblocking mode. Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error. The following `errno' error conditions are defined for this function: `EBADF' The SOCKET argument is not a valid file descriptor. `EINTR' The operation was interrupted by a signal before any data was sent. *Note Interrupted Primitives::. `ENOTSOCK' The descriptor SOCKET is not a socket. `EMSGSIZE' The socket type requires that the message be sent atomically, but the message is too large for this to be possible. `EWOULDBLOCK' Nonblocking mode has been set on the socket, and the write operation would block. (Normally `send' blocks until the operation can be completed.) `ENOBUFS' There is not enough internal buffer space available. `ENOTCONN' You never connected this socket. `EPIPE' This socket was connected but the connection is now broken. In this case, `send' generates a `SIGPIPE' signal first; if that signal is ignored or blocked, or if its handler returns, then `send' fails with `EPIPE'. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. Receiving Data .............. The `recv' function is declared in the header file `sys/socket.h'. If your FLAGS argument is zero, you can just as well use `read' instead of `recv'; see *Note I/O Primitives::. - Function: int recv (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS) The `recv' function is like `read', but with the additional flags FLAGS. The possible values of FLAGS are described in *Note Socket Data Options::. If nonblocking mode is set for SOCKET, and no data are available to be read, `recv' fails immediately rather than waiting. *Note File Status Flags::, for information about nonblocking mode. This function returns the number of bytes received, or `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' The SOCKET argument is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET is not a socket. `EWOULDBLOCK' Nonblocking mode has been set on the socket, and the read operation would block. (Normally, `recv' blocks until there is input available to be read.) `EINTR' The operation was interrupted by a signal before any data was read. *Note Interrupted Primitives::. `ENOTCONN' You never connected this socket. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. Socket Data Options ................... The FLAGS argument to `send' and `recv' is a bit mask. You can bitwise-OR the values of the following macros together to obtain a value for this argument. All are defined in the header file `sys/socket.h'. - Macro: int MSG_OOB Send or receive out-of-band data. *Note Out-of-Band Data::. - Macro: int MSG_PEEK Look at the data but don't remove it from the input queue. This is only meaningful with input functions such as `recv', not with `send'. - Macro: int MSG_DONTROUTE Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here. Byte Stream Socket Example -------------------------- Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits. This program uses `init_sockaddr' to set up the socket address; see *Note Inet Example::. #include #include #include #include #include #include #include #include #define PORT 5555 #define MESSAGE "Yow!!! Are we having fun yet?!?" #define SERVERHOST "mescaline.gnu.org" void write_to_server (int filedes) { int nbytes; nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1); if (nbytes < 0) { perror ("write"); exit (EXIT_FAILURE); } } int main (void) { extern void init_sockaddr (struct sockaddr_in *name, const char *hostname, uint16_t port); int sock; struct sockaddr_in servername; /* Create the socket. */ sock = socket (PF_INET, SOCK_STREAM, 0); if (sock < 0) { perror ("socket (client)"); exit (EXIT_FAILURE); } /* Connect to the server. */ init_sockaddr (&servername, SERVERHOST, PORT); if (0 > connect (sock, (struct sockaddr *) &servername, sizeof (servername))) { perror ("connect (client)"); exit (EXIT_FAILURE); } /* Send data to the server. */ write_to_server (sock); close (sock); exit (EXIT_SUCCESS); } Byte Stream Connection Server Example ------------------------------------- The server end is much more complicated. Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling `read' or `recv'. Instead, the right thing to do is to use `select' (*note Waiting for I/O::) to wait for input on all of the open sockets. This also allows the server to deal with additional connection requests. This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection). This program uses `make_socket' to set up the socket address; see *Note Inet Example::. #include #include #include #include #include #include #include #include #define PORT 5555 #define MAXMSG 512 int read_from_client (int filedes) { char buffer[MAXMSG]; int nbytes; nbytes = read (filedes, buffer, MAXMSG); if (nbytes < 0) { /* Read error. */ perror ("read"); exit (EXIT_FAILURE); } else if (nbytes == 0) /* End-of-file. */ return -1; else { /* Data read. */ fprintf (stderr, "Server: got message: `%s'\n", buffer); return 0; } } int main (void) { extern int make_socket (uint16_t port); int sock; fd_set active_fd_set, read_fd_set; int i; struct sockaddr_in clientname; size_t size; /* Create the socket and set it up to accept connections. */ sock = make_socket (PORT); if (listen (sock, 1) < 0) { perror ("listen"); exit (EXIT_FAILURE); } /* Initialize the set of active sockets. */ FD_ZERO (&active_fd_set); FD_SET (sock, &active_fd_set); while (1) { /* Block until input arrives on one or more active sockets. */ read_fd_set = active_fd_set; if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0) { perror ("select"); exit (EXIT_FAILURE); } /* Service all the sockets with input pending. */ for (i = 0; i < FD_SETSIZE; ++i) if (FD_ISSET (i, &read_fd_set)) { if (i == sock) { /* Connection request on original socket. */ int new; size = sizeof (clientname); new = accept (sock, (struct sockaddr *) &clientname, &size); if (new < 0) { perror ("accept"); exit (EXIT_FAILURE); } fprintf (stderr, "Server: connect from host %s, port %hd.\n", inet_ntoa (clientname.sin_addr), ntohs (clientname.sin_port)); FD_SET (new, &active_fd_set); } else { /* Data arriving on an already-connected socket. */ if (read_from_client (i) < 0) { close (i); FD_CLR (i, &active_fd_set); } } } } } Out-of-Band Data ---------------- Streams with connections permit "out-of-band" data that is delivered with higher priority than ordinary data. Typically the reason for sending out-of-band data is to send notice of an exceptional condition. To send out-of-band data use `send', specifying the flag `MSG_OOB' (*note Sending Data::). Out-of-band data are received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use `recv' with the `MSG_OOB' flag (*note Receiving Data::). Ordinary read operations do not read out-of-band data; they read only ordinary data. When a socket finds that out-of-band data are on their way, it sends a `SIGURG' signal to the owner process or process group of the socket. You can specify the owner using the `F_SETOWN' command to the `fcntl' function; see *Note Interrupt Input::. You must also establish a handler for this signal, as described in *Note Signal Handling::, in order to take appropriate action such as reading the out-of-band data. Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the `select' function; it can wait for an exceptional condition on the socket. *Note Waiting for I/O::, for more information about `select'. Notification of out-of-band data (whether with `SIGURG' or with `select') indicates that out-of-band data are on the way; the data may not actually arrive until later. If you try to read the out-of-band data before it arrives, `recv' fails with an `EWOULDBLOCK' error. Sending out-of-band data automatically places a "mark" in the stream of ordinary data, showing where in the sequence the out-of-band data "would have been". This is useful when the meaning of out-of-band data is "cancel everything sent so far". Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark: success = ioctl (socket, SIOCATMARK, &atmark); The `integer' variable ATMARK is set to a nonzero value if the socket's read pointer has reached the "mark". Here's a function to discard any ordinary data preceding the out-of-band mark: int discard_until_mark (int socket) { while (1) { /* This is not an arbitrary limit; any size will do. */ char buffer[1024]; int atmark, success; /* If we have reached the mark, return. */ success = ioctl (socket, SIOCATMARK, &atmark); if (success < 0) perror ("ioctl"); if (result) return; /* Otherwise, read a bunch of ordinary data and discard it. This is guaranteed not to read past the mark if it starts before the mark. */ success = read (socket, buffer, sizeof buffer); if (success < 0) perror ("read"); } } If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data. If you try to read out-of-band data and get an `EWOULDBLOCK' error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room. Here is an example: struct buffer { char *buf; int size; struct buffer *next; }; /* Read the out-of-band data from SOCKET and return it as a `struct buffer', which records the address of the data and its size. It may be necessary to read some ordinary data in order to make room for the out-of-band data. If so, the ordinary data are saved as a chain of buffers found in the `next' field of the value. */ struct buffer * read_oob (int socket) { struct buffer *tail = 0; struct buffer *list = 0; while (1) { /* This is an arbitrary limit. Does anyone know how to do this without a limit? */ #define BUF_SZ 1024 char *buf = (char *) xmalloc (BUF_SZ); int success; int atmark; /* Try again to read the out-of-band data. */ success = recv (socket, buf, BUF_SZ, MSG_OOB); if (success >= 0) { /* We got it, so return it. */ struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buf = buf; link->size = success; link->next = list; return link; } /* If we fail, see if we are at the mark. */ success = ioctl (socket, SIOCATMARK, &atmark); if (success < 0) perror ("ioctl"); if (atmark) { /* At the mark; skipping past more ordinary data cannot help. So just wait a while. */ sleep (1); continue; } /* Otherwise, read a bunch of ordinary data and save it. This is guaranteed not to read past the mark if it starts before the mark. */ success = read (socket, buf, BUF_SZ); if (success < 0) perror ("read"); /* Save this data in the buffer list. */ { struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buf = buf; link->size = success; /* Add the new link to the end of the list. */ if (tail) tail->next = link; else list = link; tail = link; } } } Datagram Socket Operations ========================== This section describes how to use communication styles that don't use connections (styles `SOCK_DGRAM' and `SOCK_RDM'). Using these styles, you group data into packets and each packet is an independent communication. You specify the destination for each packet individually. Datagram packets are like letters: you send each one independently with its own destination address, and they may arrive in the wrong order or not at all. The `listen' and `accept' functions are not allowed for sockets using connectionless communication styles. Sending Datagrams ----------------- The normal way of sending data on a datagram socket is by using the `sendto' function, declared in `sys/socket.h'. You can call `connect' on a datagram socket, but this only specifies a default destination for further data transmission on the socket. When a socket has a default destination you can use `send' (*note Sending Data::) or even `write' (*note I/O Primitives::) to send a packet there. You can cancel the default destination by calling `connect' using an address format of `AF_UNSPEC' in the ADDR argument. *Note Connecting::, for more information about the `connect' function. - Function: int sendto (int SOCKET, void *BUFFER. size_t SIZE, int FLAGS, struct sockaddr *ADDR, socklen_t LENGTH) The `sendto' function transmits the data in the BUFFER through the socket SOCKET to the destination address specified by the ADDR and LENGTH arguments. The SIZE argument specifies the number of bytes to be transmitted. The FLAGS are interpreted the same way as for `send'; see *Note Socket Data Options::. The return value and error conditions are also the same as for `send', but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no-one at the specified address to receive it, and the operating system on your machine usually does not know this. It is also possible for one call to `sendto' to report an error owing to a problem related to a previous call. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. Receiving Datagrams ------------------- The `recvfrom' function reads a packet from a datagram socket and also tells you where it was sent from. This function is declared in `sys/socket.h'. - Function: int recvfrom (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS, struct sockaddr *ADDR, socklen_t *LENGTH-PTR) The `recvfrom' function reads one packet from the socket SOCKET into the buffer BUFFER. The SIZE argument specifies the maximum number of bytes to be read. If the packet is longer than SIZE bytes, then you get the first SIZE bytes of the packet and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect. The ADDR and LENGTH-PTR arguments are used to return the address where the packet came from. *Note Socket Addresses::. For a socket in the local domain the address information won't be meaningful, since you can't read the address of such a socket (*note Local Namespace::). You can specify a null pointer as the ADDR argument if you are not interested in this information. The FLAGS are interpreted the same way as for `recv' (*note Socket Data Options::). The return value and error conditions are also the same as for `recv'. This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled. You can use plain `recv' (*note Receiving Data::) instead of `recvfrom' if you don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike). Even `read' can be used if you don't want to specify FLAGS (*note I/O Primitives::). Datagram Socket Example ----------------------- Here is a set of example programs that send messages over a datagram stream in the local namespace. Both the client and server programs use the `make_named_socket' function that was presented in *Note Local Socket Example::, to create and name their sockets. First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously this isn't a particularly useful program, but it does show the general ideas involved. #include #include #include #include #include #define SERVER "/tmp/serversocket" #define MAXMSG 512 int main (void) { int sock; char message[MAXMSG]; struct sockaddr_un name; size_t size; int nbytes; /* Remove the filename first, it's ok if the call fails */ unlink (SERVER); /* Make the socket, then loop endlessly. */ sock = make_named_socket (SERVER); while (1) { /* Wait for a datagram. */ size = sizeof (name); nbytes = recvfrom (sock, message, MAXMSG, 0, (struct sockaddr *) & name, &size); if (nbytes < 0) { perror ("recfrom (server)"); exit (EXIT_FAILURE); } /* Give a diagnostic message. */ fprintf (stderr, "Server: got message: %s\n", message); /* Bounce the message back to the sender. */ nbytes = sendto (sock, message, nbytes, 0, (struct sockaddr *) & name, size); if (nbytes < 0) { perror ("sendto (server)"); exit (EXIT_FAILURE); } } } Example of Reading Datagrams ---------------------------- Here is the client program corresponding to the server above. It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client. #include #include #include #include #include #include #define SERVER "/tmp/serversocket" #define CLIENT "/tmp/mysocket" #define MAXMSG 512 #define MESSAGE "Yow!!! Are we having fun yet?!?" int main (void) { extern int make_named_socket (const char *name); int sock; char message[MAXMSG]; struct sockaddr_un name; size_t size; int nbytes; /* Make the socket. */ sock = make_named_socket (CLIENT); /* Initialize the server socket address. */ name.sun_family = AF_LOCAL; strcpy (name.sun_path, SERVER); size = strlen (name.sun_path) + sizeof (name.sun_family); /* Send the datagram. */ nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0, (struct sockaddr *) & name, size); if (nbytes < 0) { perror ("sendto (client)"); exit (EXIT_FAILURE); } /* Wait for a reply. */ nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0); if (nbytes < 0) { perror ("recfrom (client)"); exit (EXIT_FAILURE); } /* Print a diagnostic message. */ fprintf (stderr, "Client: got message: %s\n", message); /* Clean up. */ remove (CLIENT); close (sock); } Keep in mind that datagram socket communications are unreliable. In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back. It's up to the user running the program to kill and restart it if desired. A more automatic solution could be to use `select' (*note Waiting for I/O::) to establish a timeout period for the reply, and in case of timeout either re-send the message or shut down the socket and exit. The `inetd' Daemon ================== We've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it. Another way to provide a service on an Internet port is to let the daemon program `inetd' do the listening. `inetd' is a program that runs all the time and waits (using `select') for messages on a specified set of ports. When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program. You specify the ports and their programs in the file `/etc/inetd.conf'. `inetd' Servers --------------- Writing a server program to be run by `inetd' is very simple. Each time someone requests a connection to the appropriate port, a new server process starts. The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process. Thus the server program can begin reading and writing data right away. Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by `inetd'. You can also use `inetd' for servers that use connectionless communication styles. For these servers, `inetd' does not try to accept a connection since no connection is possible. It just starts the server program, which can read the incoming datagram packet from descriptor 0. The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit. You must specify which of these two techniques the server uses when you configure `inetd'. Configuring `inetd' ------------------- The file `/etc/inetd.conf' tells `inetd' which ports to listen to and what server programs to run for them. Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace. Lines that start with `#' are comments. Here are two standard entries in `/etc/inetd.conf': ftp stream tcp nowait root /libexec/ftpd ftpd talk dgram udp wait root /libexec/talkd talkd An entry has this format: SERVICE STYLE PROTOCOL WAIT USERNAME PROGRAM ARGUMENTS The SERVICE field says which service this program provides. It should be the name of a service defined in `/etc/services'. `inetd' uses SERVICE to decide which port to listen on for this entry. The fields STYLE and PROTOCOL specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with `SOCK_' deleted--for example, `stream' or `dgram'. PROTOCOL should be one of the protocols listed in `/etc/protocols'. The typical protocol names are `tcp' for byte stream connections and `udp' for unreliable datagrams. The WAIT field should be either `wait' or `nowait'. Use `wait' if STYLE is a connectionless style and the server, once started, handles multiple requests as they come in. Use `nowait' if `inetd' should start a new process for each message or request that comes in. If STYLE uses connections, then WAIT *must* be `nowait'. USER is the user name that the server should run as. `inetd' runs as root, so it can set the user ID of its children arbitrarily. It's best to avoid using `root' for USER if you can; but some servers, such as Telnet and FTP, read a username and password themselves. These servers need to be root initially so they can log in as commanded by the data coming over the network. PROGRAM together with ARGUMENTS specifies the command to run to start the server. PROGRAM should be an absolute file name specifying the executable file to run. ARGUMENTS consists of any number of whitespace-separated words, which become the command-line arguments of PROGRAM. The first word in ARGUMENTS is argument zero, which should by convention be the program name itself (sans directories). If you edit `/etc/inetd.conf', you can tell `inetd' to reread the file and obey its new contents by sending the `inetd' process the `SIGHUP' signal. You'll have to use `ps' to determine the process ID of the `inetd' process as it is not fixed. Socket Options ============== This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols. When you are manipulating a socket option, you must specify which "level" the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface. Socket Option Functions ----------------------- Here are the functions for examining and modifying socket options. They are declared in `sys/socket.h'. - Function: int getsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, socklen_t *OPTLEN-PTR) The `getsockopt' function gets information about the value of option OPTNAME at level LEVEL for socket SOCKET. The option value is stored in a buffer that OPTVAL points to. Before the call, you should supply in `*OPTLEN-PTR' the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer. Most options interpret the OPTVAL buffer as a single `int' value. The actual return value of `getsockopt' is `0' on success and `-1' on failure. The following `errno' error conditions are defined: `EBADF' The SOCKET argument is not a valid file descriptor. `ENOTSOCK' The descriptor SOCKET is not a socket. `ENOPROTOOPT' The OPTNAME doesn't make sense for the given LEVEL. - Function: int setsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, socklen_t OPTLEN) This function is used to set the socket option OPTNAME at level LEVEL for socket SOCKET. The value of the option is passed in the buffer OPTVAL of size OPTLEN. The return value and error codes for `setsockopt' are the same as for `getsockopt'. Socket-Level Options -------------------- - Constant: int SOL_SOCKET Use this constant as the LEVEL argument to `getsockopt' or `setsockopt' to manipulate the socket-level options described in this section. Here is a table of socket-level option names; all are defined in the header file `sys/socket.h'. `SO_DEBUG' This option toggles recording of debugging information in the underlying protocol modules. The value has type `int'; a nonzero value means "yes". `SO_REUSEADDR' This option controls whether `bind' (*note Setting Address::) should permit reuse of local addresses for this socket. If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet. The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same port number. The value has type `int'; a nonzero value means "yes". `SO_KEEPALIVE' This option controls whether the underlying protocol should periodically transmit messages on a connected socket. If the peer fails to respond to these messages, the connection is considered broken. The value has type `int'; a nonzero value means "yes". `SO_DONTROUTE' This option controls whether outgoing messages bypass the normal message routing facilities. If set, messages are sent directly to the network interface instead. The value has type `int'; a nonzero value means "yes". `SO_LINGER' This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see *Note Closing a Socket::. The value has type `struct linger'. - Data Type: struct linger This structure type has the following members: `int l_onoff' This field is interpreted as a boolean. If nonzero, `close' blocks until the data are transmitted or the timeout period has expired. `int l_linger' This specifies the timeout period, in seconds. `SO_BROADCAST' This option controls whether datagrams may be broadcast from the socket. The value has type `int'; a nonzero value means "yes". `SO_OOBINLINE' If this option is set, out-of-band data received on the socket is placed in the normal input queue. This permits it to be read using `read' or `recv' without specifying the `MSG_OOB' flag. *Note Out-of-Band Data::. The value has type `int'; a nonzero value means "yes". `SO_SNDBUF' This option gets or sets the size of the output buffer. The value is a `size_t', which is the size in bytes. `SO_RCVBUF' This option gets or sets the size of the input buffer. The value is a `size_t', which is the size in bytes. `SO_STYLE' `SO_TYPE' This option can be used with `getsockopt' only. It is used to get the socket's communication style. `SO_TYPE' is the historical name, and `SO_STYLE' is the preferred name in GNU. The value has type `int' and its value designates a communication style; see *Note Communication Styles::. `SO_ERROR' This option can be used with `getsockopt' only. It is used to reset the error status of the socket. The value is an `int', which represents the previous error status. Networks Database ================= Many systems come with a database that records a list of networks known to the system developer. This is usually kept either in the file `/etc/networks' or in an equivalent from a name server. This data base is useful for routing programs such as `route', but it is not useful for programs that simply communicate over the network. We provide functions to access this database, which are declared in `netdb.h'. - Data Type: struct netent This data type is used to represent information about entries in the networks database. It has the following members: `char *n_name' This is the "official" name of the network. `char **n_aliases' These are alternative names for the network, represented as a vector of strings. A null pointer terminates the array. `int n_addrtype' This is the type of the network number; this is always equal to `AF_INET' for Internet networks. `unsigned long int n_net' This is the network number. Network numbers are returned in host byte order; see *Note Byte Order::. Use the `getnetbyname' or `getnetbyaddr' functions to search the networks database for information about a specific network. The information is returned in a statically-allocated structure; you must copy the information if you need to save it. - Function: struct netent * getnetbyname (const char *NAME) The `getnetbyname' function returns information about the network named NAME. It returns a null pointer if there is no such network. - Function: struct netent * getnetbyaddr (unsigned long int NET, int TYPE) The `getnetbyaddr' function returns information about the network of type TYPE with number NET. You should specify a value of `AF_INET' for the TYPE argument for Internet networks. `getnetbyaddr' returns a null pointer if there is no such network. You can also scan the networks database using `setnetent', `getnetent' and `endnetent'. Be careful when using these functions because they are not reentrant. - Function: void setnetent (int STAYOPEN) This function opens and rewinds the networks database. If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to `getnetbyname' or `getnetbyaddr' will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call. - Function: struct netent * getnetent (void) This function returns the next entry in the networks database. It returns a null pointer if there are no more entries. - Function: void endnetent (void) This function closes the networks database. Low-Level Terminal Interface **************************** This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions. Most of the functions in this chapter operate on file descriptors. *Note Low-Level I/O::, for more information about what a file descriptor is and how to open a file descriptor for a terminal device. Identifying Terminals ===================== The functions described in this chapter only work on files that correspond to terminal devices. You can find out whether a file descriptor is associated with a terminal by using the `isatty' function. Prototypes for the functions in this section are declared in the header file `unistd.h'. - Function: int isatty (int FILEDES) This function returns `1' if FILEDES is a file descriptor associated with an open terminal device, and 0 otherwise. If a file descriptor is associated with a terminal, you can get its associated file name using the `ttyname' function. See also the `ctermid' function, described in *Note Identifying the Terminal::. - Function: char * ttyname (int FILEDES) If the file descriptor FILEDES is associated with a terminal device, the `ttyname' function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file. The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined. - Function: int ttyname_r (int FILEDES, char *BUF, size_t LEN) The `ttyname_r' function is similar to the `ttyname' function except that it places its result into the user-specified buffer starting at BUF with length LEN. The normal return value from `ttyname_r' is 0. Otherwise an error number is returned to indicate the error. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal. `ERANGE' The buffer length LEN is too small to store the string to be returned. I/O Queues ========== Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering _within the kernel_ independent of the buffering implemented by I/O streams (*note I/O on Streams::). The "terminal input queue" is also sometimes referred to as its "typeahead buffer". It holds the characters that have been received from the terminal but not yet read by any process. The size of the input queue is described by the `MAX_INPUT' and `_POSIX_MAX_INPUT' parameters; see *Note Limits for Files::. You are guaranteed a queue size of at least `MAX_INPUT', but the queue might be larger, and might even dynamically change size. If input flow control is enabled by setting the `IXOFF' input mode bit (*note Input Modes::), the terminal driver transmits STOP and START characters to the terminal when necessary to prevent the queue from overflowing. Otherwise, input may be lost if it comes in too fast from the terminal. In canonical mode, all input stays in the queue until a newline character is received, so the terminal input queue can fill up when you type a very long line. *Note Canonical or Not::. The "terminal output queue" is like the input queue, but for output; it contains characters that have been written by processes, but not yet transmitted to the terminal. If output flow control is enabled by setting the `IXON' input mode bit (*note Input Modes::), the terminal driver obeys START and STOP characters sent by the terminal to stop and restart transmission of output. "Clearing" the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted. Two Styles of Input: Canonical or Not ===================================== POSIX systems support two basic modes of input: canonical and noncanonical. In "canonical input processing" mode, terminal input is processed in lines terminated by newline (`'\n''), EOF, or EOL characters. No input can be read until an entire line has been typed by the user, and the `read' function (*note I/O Primitives::) returns at most a single line of input, no matter how many bytes are requested. In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. *Note Editing Characters::. The constants `_POSIX_MAX_CANON' and `MAX_CANON' parameterize the maximum number of bytes which may appear in a single line of canonical input. *Note Limits for Files::. You are guaranteed a maximum line length of at least `MAX_CANON' bytes, but the maximum might be larger, and might even dynamically change size. In "noncanonical input processing" mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. *Note Noncanonical Input::. Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities. The choice of canonical or noncanonical input is controlled by the `ICANON' flag in the `c_lflag' member of `struct termios'. *Note Local Modes::. Terminal Modes ============== This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file `termios.h'. Don't confuse terminal attributes with file attributes. A device special file which is associated with a terminal has file attributes as described in *Note File Attributes::. These are unrelated to the attributes of the terminal device itself, which are discussed in this section. Terminal Mode Data Types ------------------------ The entire collection of attributes of a terminal is stored in a structure of type `struct termios'. This structure is used with the functions `tcgetattr' and `tcsetattr' to read and set the attributes. - Data Type: struct termios Structure that records all the I/O attributes of a terminal. The structure includes at least the following members: `tcflag_t c_iflag' A bit mask specifying flags for input modes; see *Note Input Modes::. `tcflag_t c_oflag' A bit mask specifying flags for output modes; see *Note Output Modes::. `tcflag_t c_cflag' A bit mask specifying flags for control modes; see *Note Control Modes::. `tcflag_t c_lflag' A bit mask specifying flags for local modes; see *Note Local Modes::. `cc_t c_cc[NCCS]' An array specifying which characters are associated with various control functions; see *Note Special Characters::. The `struct termios' structure also contains members which encode input and output transmission speeds, but the representation is not specified. *Note Line Speed::, for how to examine and store the speed values. The following sections describe the details of the members of the `struct termios' structure. - Data Type: tcflag_t This is an unsigned integer type used to represent the various bit masks for terminal flags. - Data Type: cc_t This is an unsigned integer type used to represent characters associated with various terminal control functions. - Macro: int NCCS The value of this macro is the number of elements in the `c_cc' array. Terminal Mode Functions ----------------------- - Function: int tcgetattr (int FILEDES, struct termios *TERMIOS-P) This function is used to examine the attributes of the terminal device with file descriptor FILEDES. The attributes are returned in the structure that TERMIOS-P points to. If successful, `tcgetattr' returns 0. A return value of -1 indicates an error. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal. - Function: int tcsetattr (int FILEDES, int WHEN, const struct termios *TERMIOS-P) This function sets the attributes of the terminal device with file descriptor FILEDES. The new attributes are taken from the structure that TERMIOS-P points to. The WHEN argument specifies how to deal with input and output already queued. It can be one of the following values: `TCSANOW' Make the change immediately. `TCSADRAIN' Make the change after waiting until all queued output has been written. You should usually use this option when changing parameters that affect output. `TCSAFLUSH' This is like `TCSADRAIN', but also discards any queued input. `TCSASOFT' This is a flag bit that you can add to any of the above alternatives. Its meaning is to inhibit alteration of the state of the terminal hardware. It is a BSD extension; it is only supported on BSD systems and the GNU system. Using `TCSASOFT' is exactly the same as setting the `CIGNORE' bit in the `c_cflag' member of the structure TERMIOS-P points to. *Note Control Modes::, for a description of `CIGNORE'. If this function is called from a background process on its controlling terminal, normally all processes in the process group are sent a `SIGTTOU' signal, in the same way as if the process were trying to write to the terminal. The exception is if the calling process itself is ignoring or blocking `SIGTTOU' signals, in which case the operation is performed and no signal is sent. *Note Job Control::. If successful, `tcsetattr' returns 0. A return value of -1 indicates an error. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal. `EINVAL' Either the value of the `when' argument is not valid, or there is something wrong with the data in the TERMIOS-P argument. Although `tcgetattr' and `tcsetattr' specify the terminal device with a file descriptor, the attributes are those of the terminal device itself and not of the file descriptor. This means that the effects of changing terminal attributes are persistent; if another process opens the terminal file later on, it will see the changed attributes even though it doesn't have anything to do with the open file descriptor you originally specified in changing the attributes. Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes. Setting Terminal Modes Properly ------------------------------- When you set terminal modes, you should call `tcgetattr' first to get the current modes of the particular terminal device, modify only those modes that you are really interested in, and store the result with `tcsetattr'. It's a bad idea to simply initialize a `struct termios' structure to a chosen set of attributes and pass it directly to `tcsetattr'. Your program may be run years from now, on systems that support members not documented in this manual. The way to avoid setting these members to unreasonable values is to avoid changing them. What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another. When a member contains a collection of independent flags, as the `c_iflag', `c_oflag' and `c_cflag' members do, even setting the entire member is a bad idea, because particular operating systems have their own flags. Instead, you should start with the current value of the member and alter only the flags whose values matter in your program, leaving any other flags unchanged. Here is an example of how to set one flag (`ISTRIP') in the `struct termios' structure while properly preserving all the other data in the structure: int set_istrip (int desc, int value) { struct termios settings; int result; result = tcgetattr (desc, &settings); if (result < 0) { perror ("error in tcgetattr"); return 0; } settings.c_iflag &= ~ISTRIP; if (value) settings.c_iflag |= ISTRIP; result = tcsetattr (desc, TCSANOW, &settings); if (result < 0) { perror ("error in tcgetattr"); return; } return 1; } Input Modes ----------- This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and and characters. All of these flags are bits in the `c_iflag' member of the `struct termios' structure. The member is an integer, and you change flags using the operators `&', `|' and `^'. Don't try to specify the entire value for `c_iflag'--instead, change only specific flags and leave the rest untouched (*note Setting Modes::). - Macro: tcflag_t INPCK If this bit is set, input parity checking is enabled. If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application. Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see *Note Control Modes::. For example, you could clear the `INPCK' input mode flag and set the `PARENB' control mode flag to ignore parity errors on input, but still generate parity on output. If this bit is set, what happens when a parity error is detected depends on whether the `IGNPAR' or `PARMRK' bits are set. If neither of these bits are set, a byte with a parity error is passed to the application as a `'\0'' character. - Macro: tcflag_t IGNPAR If this bit is set, any byte with a framing or parity error is ignored. This is only useful if `INPCK' is also set. - Macro: tcflag_t PARMRK If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when `INPCK' is set and `IGNPAR' is not set. The way erroneous bytes are marked is with two preceding bytes, `377' and `0'. Thus, the program actually reads three bytes for one erroneous byte received from the terminal. If a valid byte has the value `0377', and `ISTRIP' (see below) is not set, the program might confuse it with the prefix that marks a parity error. So a valid byte `0377' is passed to the program as two bytes, `0377' `0377', in this case. - Macro: tcflag_t ISTRIP If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read. - Macro: tcflag_t IGNBRK If this bit is set, break conditions are ignored. A "break condition" is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte. - Macro: tcflag_t BRKINT If this bit is set and `IGNBRK' is not set, a break condition clears the terminal input and output queues and raises a `SIGINT' signal for the foreground process group associated with the terminal. If neither `BRKINT' nor `IGNBRK' are set, a break condition is passed to the application as a single `'\0'' character if `PARMRK' is not set, or otherwise as a three-character sequence `'\377'', `'\0'', `'\0''. - Macro: tcflag_t IGNCR If this bit is set, carriage return characters (`'\r'') are discarded on input. Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the key. - Macro: tcflag_t ICRNL If this bit is set and `IGNCR' is not set, carriage return characters (`'\r'') received as input are passed to the application as newline characters (`'\n''). - Macro: tcflag_t INLCR If this bit is set, newline characters (`'\n'') received as input are passed to the application as carriage return characters (`'\r''). - Macro: tcflag_t IXOFF If this bit is set, start/stop control on input is enabled. In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it. The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission. *Note Start/Stop Characters::. - Macro: tcflag_t IXON If this bit is set, start/stop control on output is enabled. In other words, if the computer receives a STOP character, it suspends output until a START character is received. In this case, the STOP and START characters are never passed to the application program. If this bit is not set, then START and STOP can be read as ordinary characters. *Note Start/Stop Characters::. - Macro: tcflag_t IXANY If this bit is set, any input character restarts output when output has been suspended with the STOP character. Otherwise, only the START character restarts output. This is a BSD extension; it exists only on BSD systems and the GNU system. - Macro: tcflag_t IMAXBEL If this bit is set, then filling up the terminal input buffer sends a BEL character (code `007') to the terminal to ring the bell. This is a BSD extension. Output Modes ------------ This section describes the terminal flags and fields that control how output characters are translated and padded for display. All of these are contained in the `c_oflag' member of the `struct termios' structure. The `c_oflag' member itself is an integer, and you change the flags and fields using the operators `&', `|', and `^'. Don't try to specify the entire value for `c_oflag'--instead, change only specific flags and leave the rest untouched (*note Setting Modes::). - Macro: tcflag_t OPOST If this bit is set, output data is processed in some unspecified way so that it is displayed appropriately on the terminal device. This typically includes mapping newline characters (`'\n'') onto carriage return and linefeed pairs. If this bit isn't set, the characters are transmitted as-is. The following three bits are BSD features, and they exist only BSD systems and the GNU system. They are effective only if `OPOST' is set. - Macro: tcflag_t ONLCR If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed. - Macro: tcflag_t OXTABS If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns. - Macro: tcflag_t ONOEOT If this bit is set, discard `C-d' characters (code `004') on output. These characters cause many dial-up terminals to disconnect. Control Modes ------------- This section describes the terminal flags and fields that control parameters usually associated with asynchronous serial data transmission. These flags may not make sense for other kinds of terminal ports (such as a network connection pseudo-terminal). All of these are contained in the `c_cflag' member of the `struct termios' structure. The `c_cflag' member itself is an integer, and you change the flags and fields using the operators `&', `|', and `^'. Don't try to specify the entire value for `c_cflag'--instead, change only specific flags and leave the rest untouched (*note Setting Modes::). - Macro: tcflag_t CLOCAL If this bit is set, it indicates that the terminal is connected "locally" and that the modem status lines (such as carrier detect) should be ignored. On many systems if this bit is not set and you call `open' without the `O_NONBLOCK' flag set, `open' blocks until a modem connection is established. If this bit is not set and a modem disconnect is detected, a `SIGHUP' signal is sent to the controlling process group for the terminal (if it has one). Normally, this causes the process to exit; see *Note Signal Handling::. Reading from the terminal after a disconnect causes an end-of-file condition, and writing causes an `EIO' error to be returned. The terminal device must be closed and reopened to clear the condition. - Macro: tcflag_t HUPCL If this bit is set, a modem disconnect is generated when all processes that have the terminal device open have either closed the file or exited. - Macro: tcflag_t CREAD If this bit is set, input can be read from the terminal. Otherwise, input is discarded when it arrives. - Macro: tcflag_t CSTOPB If this bit is set, two stop bits are used. Otherwise, only one stop bit is used. - Macro: tcflag_t PARENB If this bit is set, generation and detection of a parity bit are enabled. *Note Input Modes::, for information on how input parity errors are handled. If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity. - Macro: tcflag_t PARODD This bit is only useful if `PARENB' is set. If `PARODD' is set, odd parity is used, otherwise even parity is used. The control mode flags also includes a field for the number of bits per character. You can use the `CSIZE' macro as a mask to extract the value, like this: `settings.c_cflag & CSIZE'. - Macro: tcflag_t CSIZE This is a mask for the number of bits per character. - Macro: tcflag_t CS5 This specifies five bits per byte. - Macro: tcflag_t CS6 This specifies six bits per byte. - Macro: tcflag_t CS7 This specifies seven bits per byte. - Macro: tcflag_t CS8 This specifies eight bits per byte. The following four bits are BSD extensions; this exist only on BSD systems and the GNU system. - Macro: tcflag_t CCTS_OFLOW If this bit is set, enable flow control of output based on the CTS wire (RS232 protocol). - Macro: tcflag_t CRTS_IFLOW If this bit is set, enable flow control of input based on the RTS wire (RS232 protocol). - Macro: tcflag_t MDMBUF If this bit is set, enable carrier-based flow control of output. - Macro: tcflag_t CIGNORE If this bit is set, it says to ignore the control modes and line speed values entirely. This is only meaningful in a call to `tcsetattr'. The `c_cflag' member and the line speed values returned by `cfgetispeed' and `cfgetospeed' will be unaffected by the call. `CIGNORE' is useful if you want to set all the software modes in the other members, but leave the hardware details in `c_cflag' unchanged. (This is how the `TCSASOFT' flag to `tcsettattr' works.) This bit is never set in the structure filled in by `tcgetattr'. Local Modes ----------- This section describes the flags for the `c_lflag' member of the `struct termios' structure. These flags generally control higher-level aspects of input processing than the input modes flags described in *Note Input Modes::, such as echoing, signals, and the choice of canonical or noncanonical input. The `c_lflag' member itself is an integer, and you change the flags and fields using the operators `&', `|', and `^'. Don't try to specify the entire value for `c_lflag'--instead, change only specific flags and leave the rest untouched (*note Setting Modes::). - Macro: tcflag_t ICANON This bit, if set, enables canonical input processing mode. Otherwise, input is processed in noncanonical mode. *Note Canonical or Not::. - Macro: tcflag_t ECHO If this bit is set, echoing of input characters back to the terminal is enabled. - Macro: tcflag_t ECHOE If this bit is set, echoing indicates erasure of input with the ERASE character by erasing the last character in the current line from the screen. Otherwise, the character erased is re-echoed to show what has happened (suitable for a printing terminal). This bit only controls the display behavior; the `ICANON' bit by itself controls actual recognition of the ERASE character and erasure of input, without which `ECHOE' is simply irrelevant. - Macro: tcflag_t ECHOPRT This bit is like `ECHOE', enables display of the ERASE character in a way that is geared to a hardcopy terminal. When you type the ERASE character, a `\' character is printed followed by the first character erased. Typing the ERASE character again just prints the next character erased. Then, the next time you type a normal character, a `/' character is printed before the character echoes. This is a BSD extension, and exists only in BSD systems and the GNU system. - Macro: tcflag_t ECHOK This bit enables special display of the KILL character by moving to a new line after echoing the KILL character normally. The behavior of `ECHOKE' (below) is nicer to look at. If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen. This bit only controls the display behavior; the `ICANON' bit by itself controls actual recognition of the KILL character and erasure of input, without which `ECHOK' is simply irrelevant. - Macro: tcflag_t ECHOKE This bit is similar to `ECHOK'. It enables special display of the KILL character by erasing on the screen the entire line that has been killed. This is a BSD extension, and exists only in BSD systems and the GNU system. - Macro: tcflag_t ECHONL If this bit is set and the `ICANON' bit is also set, then the newline (`'\n'') character is echoed even if the `ECHO' bit is not set. - Macro: tcflag_t ECHOCTL If this bit is set and the `ECHO' bit is also set, echo control characters with `^' followed by the corresponding text character. Thus, control-A echoes as `^A'. This is usually the preferred mode for interactive input, because echoing a control character back to the terminal could have some undesired effect on the terminal. This is a BSD extension, and exists only in BSD systems and the GNU system. - Macro: tcflag_t ISIG This bit controls whether the INTR, QUIT, and SUSP characters are recognized. The functions associated with these characters are performed if and only if this bit is set. Being in canonical or noncanonical input mode has no affect on the interpretation of these characters. You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program. *Note Signal Characters::. - Macro: tcflag_t IEXTEN POSIX.1 gives `IEXTEN' implementation-defined meaning, so you cannot rely on this interpretation on all systems. On BSD systems and the GNU system, it enables the LNEXT and DISCARD characters. *Note Other Special::. - Macro: tcflag_t NOFLSH Normally, the INTR, QUIT, and SUSP characters cause input and output queues for the terminal to be cleared. If this bit is set, the queues are not cleared. - Macro: tcflag_t TOSTOP If this bit is set and the system supports job control, then `SIGTTOU' signals are generated by background processes that attempt to write to the terminal. *Note Access to the Terminal::. The following bits are BSD extensions; they exist only in BSD systems and the GNU system. - Macro: tcflag_t ALTWERASE This bit determines how far the WERASE character should erase. The WERASE character erases back to the beginning of a word; the question is, where do words begin? If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those. *Note Editing Characters::, for more information about the WERASE character. - Macro: tcflag_t FLUSHO This is the bit that toggles when the user types the DISCARD character. While this bit is set, all output is discarded. *Note Other Special::. - Macro: tcflag_t NOKERNINFO Setting this bit disables handling of the STATUS character. *Note Other Special::. - Macro: tcflag_t PENDIN If this bit is set, it indicates that there is a line of input that needs to be reprinted. Typing the REPRINT character sets this bit; the bit remains set until reprinting is finished. *Note Editing Characters::. Line Speed ---------- The terminal line speed tells the computer how fast to read and write data on the terminal. If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line--if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals. If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding. There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions. The speed values are stored in the `struct termios' structure, but don't try to access them in the `struct termios' structure directly. Instead, you should use the following functions to read and store them: - Function: speed_t cfgetospeed (const struct termios *TERMIOS-P) This function returns the output line speed stored in the structure `*TERMIOS-P'. - Function: speed_t cfgetispeed (const struct termios *TERMIOS-P) This function returns the input line speed stored in the structure `*TERMIOS-P'. - Function: int cfsetospeed (struct termios *TERMIOS-P, speed_t SPEED) This function stores SPEED in `*TERMIOS-P' as the output speed. The normal return value is 0; a value of -1 indicates an error. If SPEED is not a speed, `cfsetospeed' returns -1. - Function: int cfsetispeed (struct termios *TERMIOS-P, speed_t SPEED) This function stores SPEED in `*TERMIOS-P' as the input speed. The normal return value is 0; a value of -1 indicates an error. If SPEED is not a speed, `cfsetospeed' returns -1. - Function: int cfsetspeed (struct termios *TERMIOS-P, speed_t SPEED) This function stores SPEED in `*TERMIOS-P' as both the input and output speeds. The normal return value is 0; a value of -1 indicates an error. If SPEED is not a speed, `cfsetspeed' returns -1. This function is an extension in 4.4 BSD. - Data Type: speed_t The `speed_t' type is an unsigned integer data type used to represent line speeds. The functions `cfsetospeed' and `cfsetispeed' report errors only for speed values that the system simply cannot handle. If you specify a speed value that is basically acceptable, then those functions will succeed. But they do not check that a particular hardware device can actually support the specified speeds--in fact, they don't know which device you plan to set the speed for. If you use `tcsetattr' to set the speed of a particular device to a value that it cannot handle, `tcsetattr' returns -1. *Portability note:* In the GNU library, the functions above accept speeds measured in bits per second as input, and return speed values measured in bits per second. Other libraries require speeds to be indicated by special codes. For POSIX.1 portability, you must use one of the following symbols to represent the speed; their precise numeric values are system-dependent, but each name has a fixed meaning: `B110' stands for 110 bps, `B300' for 300 bps, and so on. There is no portable way to represent any speed but these, but these are the only speeds that typical serial lines can support. B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400 B57600 B115200 B230400 B460800 BSD defines two additional speed symbols as aliases: `EXTA' is an alias for `B19200' and `EXTB' is an alias for `B38400'. These aliases are obsolete. Special Characters ------------------ In canonical input, the terminal driver recognizes a number of special characters which perform various control functions. These include the ERASE character (usually ) for editing input, and other editing characters. The INTR character (normally `C-c') for sending a `SIGINT' signal, and other signal-raising characters, may be available in either canonical or noncanonical input mode. All these characters are described in this section. The particular characters used are specified in the `c_cc' member of the `struct termios' structure. This member is an array; each element specifies the character for a particular role. Each element has a symbolic constant that stands for the index of that element--for example, `VINTR' is the index of the element that specifies the INTR character, so storing `'='' in `TERMIOS.c_cc[VINTR]' specifies `=' as the INTR character. On some systems, you can disable a particular special character function by specifying the value `_POSIX_VDISABLE' for that role. This value is unequal to any possible character code. *Note Options for Files::, for more information about how to tell whether the operating system you are using supports `_POSIX_VDISABLE'. Characters for Input Editing ............................ These special characters are active only in canonical input mode. *Note Canonical or Not::. - Macro: int VEOF This is the subscript for the EOF character in the special control character array. `TERMIOS.c_cc[VEOF]' holds the character itself. The EOF character is recognized only in canonical input mode. It acts as a line terminator in the same way as a newline character, but if the EOF character is typed at the beginning of a line it causes `read' to return a byte count of zero, indicating end-of-file. The EOF character itself is discarded. Usually, the EOF character is `C-d'. - Macro: int VEOL This is the subscript for the EOL character in the special control character array. `TERMIOS.c_cc[VEOL]' holds the character itself. The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line. You don't need to use the EOL character to make end a line. Just set the ICRNL flag. In fact, this is the default state of affairs. - Macro: int VEOL2 This is the subscript for the EOL2 character in the special control character array. `TERMIOS.c_cc[VEOL2]' holds the character itself. The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, by setting EOL to one of them and EOL2 to the other. The EOL2 character is a BSD extension; it exists only on BSD systems and the GNU system. - Macro: int VERASE This is the subscript for the ERASE character in the special control character array. `TERMIOS.c_cc[VERASE]' holds the character itself. The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded. Usually, the ERASE character is . - Macro: int VWERASE This is the subscript for the WERASE character in the special control character array. `TERMIOS.c_cc[VWERASE]' holds the character itself. The WERASE character is recognized only in canonical mode. It erases an entire word of prior input, and any whitespace after it; whitespace characters before the word are not erased. The definition of a "word" depends on the setting of the `ALTWERASE' mode; *note Local Modes::. If the `ALTWERASE' mode is not set, a word is defined as a sequence of any characters except space or tab. If the `ALTWERASE' mode is set, a word is defined as a sequence of characters containing only letters, numbers, and underscores, optionally followed by one character that is not a letter, number, or underscore. The WERASE character is usually `C-w'. This is a BSD extension. - Macro: int VKILL This is the subscript for the KILL character in the special control character array. `TERMIOS.c_cc[VKILL]' holds the character itself. The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too. The KILL character is usually `C-u'. - Macro: int VREPRINT This is the subscript for the REPRINT character in the special control character array. `TERMIOS.c_cc[VREPRINT]' holds the character itself. The REPRINT character is recognized only in canonical mode. It reprints the current input line. If some asynchronous output has come while you are typing, this lets you see the line you are typing clearly again. The REPRINT character is usually `C-r'. This is a BSD extension. Characters that Cause Signals ............................. These special characters may be active in either canonical or noncanonical input mode, but only when the `ISIG' flag is set (*note Local Modes::). - Macro: int VINTR This is the subscript for the INTR character in the special control character array. `TERMIOS.c_cc[VINTR]' holds the character itself. The INTR (interrupt) character raises a `SIGINT' signal for all processes in the foreground job associated with the terminal. The INTR character itself is then discarded. *Note Signal Handling::, for more information about signals. Typically, the INTR character is `C-c'. - Macro: int VQUIT This is the subscript for the QUIT character in the special control character array. `TERMIOS.c_cc[VQUIT]' holds the character itself. The QUIT character raises a `SIGQUIT' signal for all processes in the foreground job associated with the terminal. The QUIT character itself is then discarded. *Note Signal Handling::, for more information about signals. Typically, the QUIT character is `C-\'. - Macro: int VSUSP This is the subscript for the SUSP character in the special control character array. `TERMIOS.c_cc[VSUSP]' holds the character itself. The SUSP (suspend) character is recognized only if the implementation supports job control (*note Job Control::). It causes a `SIGTSTP' signal to be sent to all processes in the foreground job associated with the terminal. The SUSP character itself is then discarded. *Note Signal Handling::, for more information about signals. Typically, the SUSP character is `C-z'. Few applications disable the normal interpretation of the SUSP character. If your program does this, it should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a `SIGTSTP' signal to the process group of the process, not just to the process itself. *Note Signaling Another Process::. - Macro: int VDSUSP This is the subscript for the DSUSP character in the special control character array. `TERMIOS.c_cc[VDSUSP]' holds the character itself. The DSUSP (suspend) character is recognized only if the implementation supports job control (*note Job Control::). It sends a `SIGTSTP' signal, like the SUSP character, but not right away--only when the program tries to read it as input. Not all systems with job control support DSUSP; only BSD-compatible systems (including the GNU system). *Note Signal Handling::, for more information about signals. Typically, the DSUSP character is `C-y'. Special Characters for Flow Control ................................... These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags `IXON' and `IXOFF' (*note Input Modes::). - Macro: int VSTART This is the subscript for the START character in the special control character array. `TERMIOS.c_cc[VSTART]' holds the character itself. The START character is used to support the `IXON' and `IXOFF' input modes. If `IXON' is set, receiving a START character resumes suspended output; the START character itself is discarded. If `IXANY' is set, receiving any character at all resumes suspended output; the resuming character is not discarded unless it is the START character. `IXOFF' is set, the system may also transmit START characters to the terminal. The usual value for the START character is `C-q'. You may not be able to change this value--the hardware may insist on using `C-q' regardless of what you specify. - Macro: int VSTOP This is the subscript for the STOP character in the special control character array. `TERMIOS.c_cc[VSTOP]' holds the character itself. The STOP character is used to support the `IXON' and `IXOFF' input modes. If `IXON' is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded. If `IXOFF' is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing. The usual value for the STOP character is `C-s'. You may not be able to change this value--the hardware may insist on using `C-s' regardless of what you specify. Other Special Characters ........................ These special characters exist only in BSD systems and the GNU system. - Macro: int VLNEXT This is the subscript for the LNEXT character in the special control character array. `TERMIOS.c_cc[VLNEXT]' holds the character itself. The LNEXT character is recognized only when `IEXTEN' is set, but in both canonical and noncanonical mode. It disables any special significance of the next character the user types. Even if the character would normally perform some editing function or generate a signal, it is read as a plain character. This is the analogue of the `C-q' command in Emacs. "LNEXT" stands for "literal next." The LNEXT character is usually `C-v'. - Macro: int VDISCARD This is the subscript for the DISCARD character in the special control character array. `TERMIOS.c_cc[VDISCARD]' holds the character itself. The DISCARD character is recognized only when `IEXTEN' is set, but in both canonical and noncanonical mode. Its effect is to toggle the discard-output flag. When this flag is set, all program output is discarded. Setting the flag also discards all output currently in the output buffer. Typing any other character resets the flag. - Macro: int VSTATUS This is the subscript for the STATUS character in the special control character array. `TERMIOS.c_cc[VSTATUS]' holds the character itself. The STATUS character's effect is to print out a status message about how the current process is running. The STATUS character is recognized only in canonical mode, and only if `NOKERNINFO' is not set. Noncanonical Input ------------------ In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate. Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting--to return immediately with whatever input is available, or with no input. The MIN and TIME are stored in elements of the `c_cc' array, which is a member of the `struct termios' structure. Each element of this array has a particular role, and each element has a symbolic constant that stands for the index of that element. `VMIN' and `VMAX' are the names for the indices in the array of the MIN and TIME slots. - Macro: int VMIN This is the subscript for the MIN slot in the `c_cc' array. Thus, `TERMIOS.c_cc[VMIN]' is the value itself. The MIN slot is only meaningful in noncanonical input mode; it specifies the minimum number of bytes that must be available in the input queue in order for `read' to return. - Macro: int VTIME This is the subscript for the TIME slot in the `c_cc' array. Thus, `TERMIOS.c_cc[VTIME]' is the value itself. The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds. The MIN and TIME values interact to determine the criterion for when `read' should return; their precise meanings depend on which of them are nonzero. There are four possible cases: * Both TIME and MIN are nonzero. In this case, TIME specifies how long to wait after each input character to see if more input arrives. After the first character received, `read' keeps waiting until either MIN bytes have arrived in all, or TIME elapses with no further input. `read' always blocks until the first character arrives, even if TIME elapses first. `read' can return more than MIN characters if more than MIN happen to be in the queue. * Both MIN and TIME are zero. In this case, `read' always returns immediately with as many characters as are available in the queue, up to the number requested. If no input is immediately available, `read' returns a value of zero. * MIN is zero but TIME has a nonzero value. In this case, `read' waits for time TIME for input to become available; the availability of a single byte is enough to satisfy the read request and cause `read' to return. When it returns, it returns as many characters as are available, up to the number requested. If no input is available before the timer expires, `read' returns a value of zero. * TIME is zero but MIN has a nonzero value. In this case, `read' waits until at least MIN bytes are available in the queue. At that time, `read' returns as many characters as are available, up to the number requested. `read' can return more than MIN characters if more than MIN happen to be in the queue. What happens if MIN is 50 and you ask to read just 10 bytes? Normally, `read' waits until there are 50 bytes in the buffer (or, more generally, the wait condition described above is satisfied), and then reads 10 of them, leaving the other 40 buffered in the operating system for a subsequent call to `read'. *Portability note:* On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. The GNU library allocates separate slots for these uses. - Function: int cfmakeraw (struct termios *TERMIOS-P) This function provides an easy way to set up `*TERMIOS-P' for what has traditionally been called "raw mode" in BSD. This uses noncanonical input, and turns off most processing to give an unmodified channel to the terminal. It does exactly this: TERMIOS-P->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP |INLCR|IGNCR|ICRNL|IXON); TERMIOS-P->c_oflag &= ~OPOST; TERMIOS-P->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN); TERMIOS-P->c_cflag &= ~(CSIZE|PARENB); TERMIOS-P->c_cflag |= CS8; BSD Terminal Modes ================== The usual way to get and set terminal modes is with the functions described in *Note Terminal Modes::. However, on some systems you can use the BSD-derived functions in this section to do some of the same thing. On many systems, these functions do not exist. Even with the GNU C library, the functions simply fail with `errno' = `ENOSYS' with many kernels, including Linux. The symbols used in this section are declared in `sgtty.h'. - Data Type: struct sgttyb This structure is an input or output parameter list for `gtty' and `stty'. `char sg_ispeed' Line speed for input `char sg_ospeed' Line speed for output `char sg_erase' Erase character `char sg_kill' Kill character `int sg_flags' Various flags - Function: int gtty (int FILEDES, struct sgttyb *ATTRIBUTES) This function gets the attributes of a terminal. `gtty' sets *ATTRIBUTES to describe the terminal attributes of the terminal which is open with file descriptor FILEDES. - Function: int stty (int FILEDES, struct sgttyb * attributes) This function sets the attributes of a terminal. `stty' sets the terminal attributes of the terminal which is open with file descriptor FILEDES to those described by *FILEDES. Line Control Functions ====================== These functions perform miscellaneous control actions on terminal devices. As regards terminal access, they are treated like doing output: if any of these functions is used by a background process on its controlling terminal, normally all processes in the process group are sent a `SIGTTOU' signal. The exception is if the calling process itself is ignoring or blocking `SIGTTOU' signals, in which case the operation is performed and no signal is sent. *Note Job Control::. - Function: int tcsendbreak (int FILEDES, int DURATION) This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor FILEDES. The duration of the break is controlled by the DURATION argument. If zero, the duration is between 0.25 and 0.5 seconds. The meaning of a nonzero value depends on the operating system. This function does nothing if the terminal is not an asynchronous serial data port. The return value is normally zero. In the event of an error, a value of -1 is returned. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal device. - Function: int tcdrain (int FILEDES) The `tcdrain' function waits until all queued output to the terminal FILEDES has been transmitted. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `tcdrain' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `tcdrain' should be protected using cancellation handlers. The return value is normally zero. In the event of an error, a value of -1 is returned. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal device. `EINTR' The operation was interrupted by delivery of a signal. *Note Interrupted Primitives::. - Function: int tcflush (int FILEDES, int QUEUE) The `tcflush' function is used to clear the input and/or output queues associated with the terminal file FILEDES. The QUEUE argument specifies which queue(s) to clear, and can be one of the following values: `TCIFLUSH' Clear any input data received, but not yet read. `TCOFLUSH' Clear any output data written, but not yet transmitted. `TCIOFLUSH' Clear both queued input and output. The return value is normally zero. In the event of an error, a value of -1 is returned. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal device. `EINVAL' A bad value was supplied as the QUEUE argument. It is unfortunate that this function is named `tcflush', because the term "flush" is normally used for quite another operation--waiting until all output is transmitted--and using it for discarding input or output would be confusing. Unfortunately, the name `tcflush' comes from POSIX and we cannot change it. - Function: int tcflow (int FILEDES, int ACTION) The `tcflow' function is used to perform operations relating to XON/XOFF flow control on the terminal file specified by FILEDES. The ACTION argument specifies what operation to perform, and can be one of the following values: `TCOOFF' Suspend transmission of output. `TCOON' Restart transmission of output. `TCIOFF' Transmit a STOP character. `TCION' Transmit a START character. For more information about the STOP and START characters, see *Note Special Characters::. The return value is normally zero. In the event of an error, a value of -1 is returned. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `ENOTTY' The FILEDES is not associated with a terminal device. `EINVAL' A bad value was supplied as the ACTION argument. Noncanonical Mode Example ========================= Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo. #include #include #include #include /* Use this variable to remember original terminal attributes. */ struct termios saved_attributes; void reset_input_mode (void) { tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes); } void set_input_mode (void) { struct termios tattr; char *name; /* Make sure stdin is a terminal. */ if (!isatty (STDIN_FILENO)) { fprintf (stderr, "Not a terminal.\n"); exit (EXIT_FAILURE); } /* Save the terminal attributes so we can restore them later. */ tcgetattr (STDIN_FILENO, &saved_attributes); atexit (reset_input_mode); /* Set the funny terminal modes. */ tcgetattr (STDIN_FILENO, &tattr); tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */ tattr.c_cc[VMIN] = 1; tattr.c_cc[VTIME] = 0; tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr); } int main (void) { char c; set_input_mode (); while (1) { read (STDIN_FILENO, &c, 1); if (c == '\004') /* `C-d' */ break; else putchar (c); } return EXIT_SUCCESS; } This program is careful to restore the original terminal modes before exiting or terminating with a signal. It uses the `atexit' function (*note Cleanups on Exit::) to make sure this is done by `exit'. The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see *Note Job Control::. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so. Pseudo-Terminals ================ A "pseudo-terminal" is a special interprocess communication channel that acts like a terminal. One end of the channel is called the "master" side or "master pseudo-terminal device", the other side is called the "slave" side. Data written to the master side is received by the slave side as if it was the result of a user typing at an ordinary terminal, and data written to the slave side is sent to the master side as if it was written on an ordinary terminal. Pseudo terminals are the way programs like `xterm' and `emacs' implement their terminal emulation functionality. Allocating Pseudo-Terminals --------------------------- This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file `stdlib.h'. - Function: int getpt (void) The `getpt' function returns a new file descriptor for the next available master pseudo-terminal. The normal return value from `getpt' is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. The following `errno' conditions are defined for this function: `ENOENT' There are no free master pseudo-terminals available. This function is a GNU extension. - Function: int grantpt (int FILEDES) The `grantpt' function changes the ownership and access permission of the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor FILEDES. The owner is set from the real user ID of the calling process (*note Process Persona::), and the group is set to a special group (typically "tty") or from the real group ID of the calling process. The access permission is set such that the file is both readable and writable by the owner and only writable by the group. On some systems this function is implemented by invoking a special `setuid' root program (*note How Change Persona::). As a consequence, installing a signal handler for the `SIGCHLD' signal (*note Job Control Signals::) may interfere with a call to `grantpt'. The normal return value from `grantpt' is 0; a value of -1 is returned in case of failure. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument is not associated with a master pseudo-terminal device. `EACCES' The slave pseudo-terminal device corresponding to the master associated with FILEDES could not be accessed. - Function: int unlockpt (int FILEDES) The `unlockpt' function unlocks the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor FILEDES. On many systems, the slave can only be opened after unlocking, so portable applications should always call `unlockpt' before trying to open the slave. The normal return value from `unlockpt' is 0; a value of -1 is returned in case of failure. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument is not associated with a master pseudo-terminal device. - Function: char * ptsname (int FILEDES) If the file descriptor FILEDES is associated with a master pseudo-terminal device, the `ptsname' function returns a pointer to a statically-allocated, null-terminated string containing the file name of the associated slave pseudo-terminal file. This string might be overwritten by subsequent calls to `ptsname'. - Function: int ptsname_r (int FILEDES, char *BUF, size_t LEN) The `ptsname_r' function is similar to the `ptsname' function except that it places its result into the user-specified buffer starting at BUF with length LEN. This function is a GNU extension. *Portability Note:* On System V derived systems, the file returned by the `ptsname' and `ptsname_r' functions may be STREAMS-based, and therefore require additional processing after opening before it actually behaves as a pseudo terminal. Typical usage of these functions is illustrated by the following example: int open_pty_pair (int *amaster, int *aslave) { int master, slave; char *name; master = getpt (); if (master < 0) return 0; if (grantpt (master) < 0 || unlockpt (master) < 0) goto close_master; name = ptsname (master); if (name == NULL) goto close_master; slave = open (name, O_RDWR); if (slave == -1) goto close_master; if (isastream (slave)) { if (ioctl (slave, I_PUSH, "ptem") < 0 || ioctl (slave, I_PUSH, "ldterm") < 0) goto close_slave; } *amaster = master; *aslave = slave; return 1; close_slave: close (slave); close_master: close (master); return 0; } Opening a Pseudo-Terminal Pair ------------------------------ These functions, derived from BSD, are available in the separate `libutil' library, and declared in `pty.h'. - Function: int openpty (int *AMASTER, int *ASLAVE, char *NAME, struct termios *TERMP, struct winsize *WINP) This function allocates and opens a pseudo-terminal pair, returning the file descriptor for the master in *AMASTER, and the file descriptor for the slave in *ASLAVE. If the argument NAME is not a null pointer, the file name of the slave pseudo-terminal device is stored in `*name'. If TERMP is not a null pointer, the terminal attributes of the slave are set to the ones specified in the structure that TERMP points to (*note Terminal Modes::). Likewise, if the WINP is not a null pointer, the screen size of the slave is set to the values specified in the structure that WINP points to. The normal return value from `openpty' is 0; a value of -1 is returned in case of failure. The following `errno' conditions are defined for this function: `ENOENT' There are no free pseudo-terminal pairs available. *Warning:* Using the `openpty' function with NAME not set to `NULL' is *very dangerous* because it provides no protection against overflowing the string NAME. You should use the `ttyname' function on the file descriptor returned in *SLAVE to find out the file name of the slave pseudo-terminal device instead. - Function: int forkpty (int *AMASTER, char *NAME, struct termios *TERMP, struct winsize *WINP) This function is similar to the `openpty' function, but in addition, forks a new process (*note Creating a Process::) and makes the newly opened slave pseudo-terminal device the controlling terminal (*note Controlling Terminal::) for the child process. If the operation is successful, there are then both parent and child processes and both see `forkpty' return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process. If the allocation of a pseudo-terminal pair or the process creation failed, `forkpty' returns a value of -1 in the parent process. *Warning:* The `forkpty' function has the same problems with respect to the NAME argument as `openpty'. Syslog ****** This chapter describes facilities for issuing and logging messages of system administration interest. This chapter has nothing to do with programs issuing messages to their own users or keeping private logs (One would typically do that with the facilities described in *Note I/O on Streams::). Most systems have a facility called "Syslog" that allows programs to submit messages of interest to system administrators and can be configured to pass these messages on in various ways, such as printing on the console, mailing to a particular person, or recording in a log file for future reference. A program uses the facilities in this chapter to submit such messages. Overview of Syslog ================== System administrators have to deal with lots of different kinds of messages from a plethora of subsystems within each system, and usually lots of systems as well. For example, an FTP server might report every connection it gets. The kernel might report hardware failures on a disk drive. A DNS server might report usage statistics at regular intervals. Some of these messages need to be brought to a system administrator's attention immediately. And it may not be just any system administrator - there may be a particular system administrator who deals with a particular kind of message. Other messages just need to be recorded for future reference if there is a problem. Still others may need to have information extracted from them by an automated process that generates monthly reports. To deal with these messages, most Unix systems have a facility called "Syslog." It is generally based on a daemon called "Syslogd" Syslogd listens for messages on a Unix domain socket named `/dev/log'. Based on classification information in the messages and its configuration file (usually `/etc/syslog.conf'), Syslogd routes them in various ways. Some of the popular routings are: * Write to the system console * Mail to a specific user * Write to a log file * Pass to another daemon * Discard Syslogd can also handle messages from other systems. It listens on the `syslog' UDP port as well as the local socket for messages. Syslog can handle messages from the kernel itself. But the kernel doesn't write to `/dev/log'; rather, another daemon (sometimes called "Klogd") extracts messages from the kernel and passes them on to Syslog as any other process would (and it properly identifies them as messages from the kernel). Syslog can even handle messages that the kernel issued before Syslogd or Klogd was running. A Linux kernel, for example, stores startup messages in a kernel message ring and they are normally still there when Klogd later starts up. Assuming Syslogd is running by the time Klogd starts, Klogd then passes everything in the message ring to it. In order to classify messages for disposition, Syslog requires any process that submits a message to it to provide two pieces of classification information with it: facility This identifies who submitted the message. There are a small number of facilities defined. The kernel, the mail subsystem, and an FTP server are examples of recognized facilities. For the complete list, *Note syslog; vsyslog::. Keep in mind that these are essentially arbitrary classifications. "Mail subsystem" doesn't have any more meaning than the system administrator gives to it. priority This tells how important the content of the message is. Examples of defined priority values are: debug, informational, warning, critical. For the complete list, *Note syslog; vsyslog::. Except for the fact that the priorities have a defined order, the meaning of each of these priorities is entirely determined by the system administrator. A "facility/priority" is a number that indicates both the facility and the priority. *Warning:* This terminology is not universal. Some people use "level" to refer to the priority and "priority" to refer to the combination of facility and priority. A Linux kernel has a concept of a message "level," which corresponds both to a Syslog priority and to a Syslog facility/priority (It can be both because the facility code for the kernel is zero, and that makes priority and facility/priority the same value). The GNU C library provides functions to submit messages to Syslog. They do it by writing to the `/dev/log' socket. *Note Submitting Syslog Messages::. The GNU C library functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the `syslog' UDP port on that system. *Note Sockets::. Submitting Syslog Messages ========================== The GNU C library provides functions to submit messages to the Syslog facility: These functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the `syslog' UDP port on that system. *Note Sockets::. openlog ------- The symbols referred to in this section are declared in the file `syslog.h'. - Function: void openlog (char *IDENT, int OPTION, int FACILITY) `openlog' opens or reopens a connection to Syslog in preparation for submitting messages. IDENT is an arbitrary identification string which future `syslog' invocations will prefix to each message. This is intended to identify the source of the message, and people conventionally set it to the name of the program that will submit the messages. `openlog' may or may not open the `/dev/log' socket, depending on OPTION. If it does, it tries to open it and connect it as a stream socket. If that doesn't work, it tries to open it and connect it as a datagram socket. The socket has the "Close on Exec" attribute, so the kernel will close it if the process performs an exec. You don't have to use `openlog'. If you call `syslog' without having called `openlog', `syslog' just opens the connection implicitly and uses defaults for the information in IDENT and OPTIONS. OPTIONS is a bit string, with the bits as defined by the following single bit masks: `LOG_PERROR' If on, `openlog' sets up the connection so that any `syslog' on this connection writes its message to the calling process' Standard Error stream in addition to submitting it to Syslog. If off, `syslog' does not write the message to Standard Error. `LOG_CONS' If on, `openlog' sets up the connection so that a `syslog' on this connection that fails to submit a message to Syslog writes the message instead to system console. If off, `syslog' does not write to the system console (but of course Syslog may write messages it receives to the console). `LOG_PID' When on, `openlog' sets up the connection so that a `syslog' on this connection inserts the calling process' Process ID (PID) into the message. When off, `openlog' does not insert the PID. `LOG_NDELAY' When on, `openlog' opens and connects the `/dev/log' socket. When off, a future `syslog' call must open and connect the socket. *Portability note:* In early systems, the sense of this bit was exactly the opposite. `LOG_ODELAY' This bit does nothing. It exists for backward compatibility. If any other bit in OPTIONS is on, the result is undefined. FACILITY is the default facility code for this connection. A `syslog' on this connection that specifies default facility causes this facility to be associated with the message. See `syslog' for possible values. A value of zero means the default default, which is `LOG_USER'. If a Syslog connection is already open when you call `openlog', `openlog' "reopens" the connection. Reopening is like opening except that if you specify zero for the default facility code, the default facility code simply remains unchanged and if you specify LOG_NDELAY and the socket is already open and connected, `openlog' just leaves it that way. syslog, vsyslog --------------- The symbols referred to in this section are declared in the file `syslog.h'. - Function: void syslog (int FACILITY_PRIORITY, char *FORMAT, ...) `syslog' submits a message to the Syslog facility. It does this by writing to the Unix domain socket `/dev/log'. `syslog' submits the message with the facility and priority indicated by FACILITY_PRIORITY. The macro `LOG_MAKEPRI' generates a facility/priority from a facility and a priority, as in the following example: LOG_MAKEPRI(LOG_USER, LOG_WARNING) The possible values for the facility code are (macros): `LOG_USER' A miscellaneous user process `LOG_MAIL' Mail `LOG_DAEMON' A miscellaneous system daemon `LOG_AUTH' Security (authorization) `LOG_SYSLOG' Syslog `LOG_LPR' Central printer `LOG_NEWS' Network news (e.g. Usenet) `LOG_UUCP' UUCP `LOG_CRON' Cron and At `LOG_AUTHPRIV' Private security (authorization) `LOG_FTP' Ftp server `LOG_LOCAL0' Locally defined `LOG_LOCAL1' Locally defined `LOG_LOCAL2' Locally defined `LOG_LOCAL3' Locally defined `LOG_LOCAL4' Locally defined `LOG_LOCAL5' Locally defined `LOG_LOCAL6' Locally defined `LOG_LOCAL7' Locally defined Results are undefined if the facility code is anything else. *note:* `syslog' recognizes one other facility code: that of the kernel. But you can't specify that facility code with these functions. If you try, it looks the same to `syslog' as if you are requesting the default facility. But you wouldn't want to anyway, because any program that uses the GNU C library is not the kernel. You can use just a priority code as FACILITY_PRIORITY. In that case, `syslog' assumes the default facility established when the Syslog connection was opened. *Note Syslog Example::. The possible values for the priority code are (macros): `LOG_EMERG' The message says the system is unusable. `LOG_ALERT' Action on the message must be taken immediately. `LOG_CRIT' The message states a critical condition. `LOG_ERR' The message describes an error. `LOG_WARNING' The message is a warning. `LOG_NOTICE' The message describes a normal but important event. `LOG_INFO' The message is purely informational. `LOG_DEBUG' The message is only for debugging purposes. Results are undefined if the priority code is anything else. If the process does not presently have a Syslog connection open (i.e. it did not call `openlog'), `syslog' implicitly opens the connection the same as `openlog' would, with the following defaults for information that would otherwise be included in an `openlog' call: The default identification string is the program name. The default default facility is `LOG_USER'. The default for all the connection options in OPTIONS is as if those bits were off. `syslog' leaves the Syslog connection open. If the `dev/log' socket is not open and connected, `syslog' opens and connects it, the same as `openlog' with the `LOG_NDELAY' option would. `syslog' leaves `/dev/log' open and connected unless its attempt to send the message failed, in which case `syslog' closes it (with the hope that a future implicit open will restore the Syslog connection to a usable state). Example: #include syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR), "Unable to make network connection to %s. Error=%m", host); - Function: void vsyslog (int FACILITY_PRIORITY, char *FORMAT, va_list arglist) This is functionally identical to `syslog', with the BSD style variable length argument. closelog -------- The symbols referred to in this section are declared in the file `syslog.h'. - Function: void closelog (void) `closelog' closes the current Syslog connection, if there is one. This include closing the `dev/log' socket, if it is open. There is very little reason to use this function. It does not flush any buffers; you can reopen a Syslog connection without closing it first; The connection gets closed automatically on exec or exit. `closelog' has primarily aesthetic value. setlogmask ---------- The symbols referred to in this section are declared in the file `syslog.h'. - Function: int setlogmask (int MASK) `setlogmask' sets a mask (the "logmask") that determines which future `syslog' calls shall be ignored. If a program has not called `setlogmask', `syslog' doesn't ignore any calls. You can use `setlogmask' to specify that messages of particular priorities shall be ignored in the future. A `setlogmask' call overrides any previous `setlogmask' call. Note that the logmask exists entirely independently of opening and closing of Syslog connections. Setting the logmask has a similar effect to, but is not the same as, configuring Syslog. The Syslog configuration may cause Syslog to discard certain messages it receives, but the logmask causes certain messages never to get submitted to Syslog in the first place. MASK is a bit string with one bit corresponding to each of the possible message priorities. If the bit is on, `syslog' handles messages of that priority normally. If it is off, `syslog' discards messages of that priority. Use the message priority macros described in *Note syslog; vsyslog:: and the `LOG_MASK' to construct an appropriate MASK value, as in this example: LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR) or ~(LOG_MASK(LOG_INFO)) There is also a `LOG_UPTO' macro, which generates a mask with the bits on for a certain priority and all priorities above it: LOG_UPTO(LOG_ERROR) The unfortunate naming of the macro is due to the fact that internally, higher numbers are used for lower message priorities. Syslog Example -------------- Here is an example of `openlog', `syslog', and `closelog': This example sets the logmask so that debug and informational messages get discarded without ever reaching Syslog. So the second `syslog' in the example does nothing. #include setlogmask (LOG_UPTO (LOG_NOTICE)); openlog ("exampleprog", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1); syslog (LOG_NOTICE, "Program started by User %d", getuid ()); syslog (LOG_INFO, "A tree falls in a forest"); closelog (); Mathematics *********** This chapter contains information about functions for performing mathematical computations, such as trigonometric functions. Most of these functions have prototypes declared in the header file `math.h'. The complex-valued functions are defined in `complex.h'. All mathematical functions which take a floating-point argument have three variants, one each for `double', `float', and `long double' arguments. The `double' versions are mostly defined in ISO C89. The `float' and `long double' versions are from the numeric extensions to C included in ISO C99. Which of the three versions of a function should be used depends on the situation. For most calculations, the `float' functions are the fastest. On the other hand, the `long double' functions have the highest precision. `double' is somewhere in between. It is usually wise to pick the narrowest type that can accommodate your data. Not all machines have a distinct `long double' type; it may be the same as `double'. Predefined Mathematical Constants ================================= The header `math.h' defines several useful mathematical constants. All values are defined as preprocessor macros starting with `M_'. The values provided are: `M_E' The base of natural logarithms. `M_LOG2E' The logarithm to base `2' of `M_E'. `M_LOG10E' The logarithm to base `10' of `M_E'. `M_LN2' The natural logarithm of `2'. `M_LN10' The natural logarithm of `10'. `M_PI' Pi, the ratio of a circle's circumference to its diameter. `M_PI_2' Pi divided by two. `M_PI_4' Pi divided by four. `M_1_PI' The reciprocal of pi (1/pi) `M_2_PI' Two times the reciprocal of pi. `M_2_SQRTPI' Two times the reciprocal of the square root of pi. `M_SQRT2' The square root of two. `M_SQRT1_2' The reciprocal of the square root of two (also the square root of 1/2). These constants come from the Unix98 standard and were also available in 4.4BSD; therefore they are only defined if `_BSD_SOURCE' or `_XOPEN_SOURCE=500', or a more general feature select macro, is defined. The default set of features includes these constants. *Note Feature Test Macros::. All values are of type `double'. As an extension, the GNU C library also defines these constants with type `long double'. The `long double' macros have a lowercase `l' appended to their names: `M_El', `M_PIl', and so forth. These are only available if `_GNU_SOURCE' is defined. _Note:_ Some programs use a constant named `PI' which has the same value as `M_PI'. This constant is not standard; it may have appeared in some old AT&T headers, and is mentioned in Stroustrup's book on C++. It infringes on the user's name space, so the GNU C library does not define it. Fixing programs written to expect it is simple: replace `PI' with `M_PI' throughout, or put `-DPI=M_PI' on the compiler command line. Trigonometric Functions ======================= These are the familiar `sin', `cos', and `tan' functions. The arguments to all of these functions are in units of radians; recall that pi radians equals 180 degrees. The math library normally defines `M_PI' to a `double' approximation of pi. If strict ISO and/or POSIX compliance are requested this constant is not defined, but you can easily define it yourself: #define M_PI 3.14159265358979323846264338327 You can also compute the value of pi with the expression `acos (-1.0)'. - Function: double sin (double X) - Function: float sinf (float X) - Function: long double sinl (long double X) These functions return the sine of X, where X is given in radians. The return value is in the range `-1' to `1'. - Function: double cos (double X) - Function: float cosf (float X) - Function: long double cosl (long double X) These functions return the cosine of X, where X is given in radians. The return value is in the range `-1' to `1'. - Function: double tan (double X) - Function: float tanf (float X) - Function: long double tanl (long double X) These functions return the tangent of X, where X is given in radians. Mathematically, the tangent function has singularities at odd multiples of pi/2. If the argument X is too close to one of these singularities, `tan' will signal overflow. In many applications where `sin' and `cos' are used, the sine and cosine of the same angle are needed at the same time. It is more efficient to compute them simultaneously, so the library provides a function to do that. - Function: void sincos (double X, double *SINX, double *COSX) - Function: void sincosf (float X, float *SINX, float *COSX) - Function: void sincosl (long double X, long double *SINX, long double *COSX) These functions return the sine of X in `*SINX' and the cosine of X in `*COS', where X is given in radians. Both values, `*SINX' and `*COSX', are in the range of `-1' to `1'. This function is a GNU extension. Portable programs should be prepared to cope with its absence. ISO C99 defines variants of the trig functions which work on complex numbers. The GNU C library provides these functions, but they are only useful if your compiler supports the new complex types defined by the standard. (As of this writing GCC supports complex numbers, but there are bugs in the implementation.) - Function: complex double csin (complex double Z) - Function: complex float csinf (complex float Z) - Function: complex long double csinl (complex long double Z) These functions return the complex sine of Z. The mathematical definition of the complex sine is sin (z) = 1/(2*i) * (exp (z*i) - exp (-z*i)). - Function: complex double ccos (complex double Z) - Function: complex float ccosf (complex float Z) - Function: complex long double ccosl (complex long double Z) These functions return the complex cosine of Z. The mathematical definition of the complex cosine is cos (z) = 1/2 * (exp (z*i) + exp (-z*i)) - Function: complex double ctan (complex double Z) - Function: complex float ctanf (complex float Z) - Function: complex long double ctanl (complex long double Z) These functions return the complex tangent of Z. The mathematical definition of the complex tangent is tan (z) = -i * (exp (z*i) - exp (-z*i)) / (exp (z*i) + exp (-z*i)) The complex tangent has poles at pi/2 + 2n, where n is an integer. `ctan' may signal overflow if Z is too close to a pole. Inverse Trigonometric Functions =============================== These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively. - Function: double asin (double X) - Function: float asinf (float X) - Function: long double asinl (long double X) These functions compute the arc sine of X--that is, the value whose sine is X. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between `-pi/2' and `pi/2' (inclusive). The arc sine function is defined mathematically only over the domain `-1' to `1'. If X is outside the domain, `asin' signals a domain error. - Function: double acos (double X) - Function: float acosf (float X) - Function: long double acosl (long double X) These functions compute the arc cosine of X--that is, the value whose cosine is X. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between `0' and `pi' (inclusive). The arc cosine function is defined mathematically only over the domain `-1' to `1'. If X is outside the domain, `acos' signals a domain error. - Function: double atan (double X) - Function: float atanf (float X) - Function: long double atanl (long double X) These functions compute the arc tangent of X--that is, the value whose tangent is X. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between `-pi/2' and `pi/2' (inclusive). - Function: double atan2 (double Y, double X) - Function: float atan2f (float Y, float X) - Function: long double atan2l (long double Y, long double X) This function computes the arc tangent of Y/X, but the signs of both arguments are used to determine the quadrant of the result, and X is permitted to be zero. The return value is given in radians and is in the range `-pi' to `pi', inclusive. If X and Y are coordinates of a point in the plane, `atan2' returns the signed angle between the line from the origin to that point and the x-axis. Thus, `atan2' is useful for converting Cartesian coordinates to polar coordinates. (To compute the radial coordinate, use `hypot'; see *Note Exponents and Logarithms::.) If both X and Y are zero, `atan2' returns zero. ISO C99 defines complex versions of the inverse trig functions. - Function: complex double casin (complex double Z) - Function: complex float casinf (complex float Z) - Function: complex long double casinl (complex long double Z) These functions compute the complex arc sine of Z--that is, the value whose sine is Z. The value returned is in radians. Unlike the real-valued functions, `casin' is defined for all values of Z. - Function: complex double cacos (complex double Z) - Function: complex float cacosf (complex float Z) - Function: complex long double cacosl (complex long double Z) These functions compute the complex arc cosine of Z--that is, the value whose cosine is Z. The value returned is in radians. Unlike the real-valued functions, `cacos' is defined for all values of Z. - Function: complex double catan (complex double Z) - Function: complex float catanf (complex float Z) - Function: complex long double catanl (complex long double Z) These functions compute the complex arc tangent of Z--that is, the value whose tangent is Z. The value is in units of radians. Exponentiation and Logarithms ============================= - Function: double exp (double X) - Function: float expf (float X) - Function: long double expl (long double X) These functions compute `e' (the base of natural logarithms) raised to the power X. If the magnitude of the result is too large to be representable, `exp' signals overflow. - Function: double exp2 (double X) - Function: float exp2f (float X) - Function: long double exp2l (long double X) These functions compute `2' raised to the power X. Mathematically, `exp2 (x)' is the same as `exp (x * log (2))'. - Function: double exp10 (double X) - Function: float exp10f (float X) - Function: long double exp10l (long double X) - Function: double pow10 (double X) - Function: float pow10f (float X) - Function: long double pow10l (long double X) These functions compute `10' raised to the power X. Mathematically, `exp10 (x)' is the same as `exp (x * log (10))'. These functions are GNU extensions. The name `exp10' is preferred, since it is analogous to `exp' and `exp2'. - Function: double log (double X) - Function: float logf (float X) - Function: long double logl (long double X) These functions compute the natural logarithm of X. `exp (log (X))' equals X, exactly in mathematics and approximately in C. If X is negative, `log' signals a domain error. If X is zero, it returns negative infinity; if X is too close to zero, it may signal overflow. - Function: double log10 (double X) - Function: float log10f (float X) - Function: long double log10l (long double X) These functions return the base-10 logarithm of X. `log10 (X)' equals `log (X) / log (10)'. - Function: double log2 (double X) - Function: float log2f (float X) - Function: long double log2l (long double X) These functions return the base-2 logarithm of X. `log2 (X)' equals `log (X) / log (2)'. - Function: double logb (double X) - Function: float logbf (float X) - Function: long double logbl (long double X) These functions extract the exponent of X and return it as a floating-point value. If `FLT_RADIX' is two, `logb' is equal to `floor (log2 (x))', except it's probably faster. If X is de-normalized, `logb' returns the exponent X would have if it were normalized. If X is infinity (positive or negative), `logb' returns oo. If X is zero, `logb' returns oo. It does not signal. - Function: int ilogb (double X) - Function: int ilogbf (float X) - Function: int ilogbl (long double X) These functions are equivalent to the corresponding `logb' functions except that they return signed integer values. Since integers cannot represent infinity and NaN, `ilogb' instead returns an integer that can't be the exponent of a normal floating-point number. `math.h' defines constants so you can check for this. - Macro: int FP_ILOGB0 `ilogb' returns this value if its argument is `0'. The numeric value is either `INT_MIN' or `-INT_MAX'. This macro is defined in ISO C99. - Macro: int FP_ILOGBNAN `ilogb' returns this value if its argument is `NaN'. The numeric value is either `INT_MIN' or `INT_MAX'. This macro is defined in ISO C99. These values are system specific. They might even be the same. The proper way to test the result of `ilogb' is as follows: i = ilogb (f); if (i == FP_ILOGB0 || i == FP_ILOGBNAN) { if (isnan (f)) { /* Handle NaN. */ } else if (f == 0.0) { /* Handle 0.0. */ } else { /* Some other value with large exponent, perhaps +Inf. */ } } - Function: double pow (double BASE, double POWER) - Function: float powf (float BASE, float POWER) - Function: long double powl (long double BASE, long double POWER) These are general exponentiation functions, returning BASE raised to POWER. Mathematically, `pow' would return a complex number when BASE is negative and POWER is not an integral value. `pow' can't do that, so instead it signals a domain error. `pow' may also underflow or overflow the destination type. - Function: double sqrt (double X) - Function: float sqrtf (float X) - Function: long double sqrtl (long double X) These functions return the nonnegative square root of X. If X is negative, `sqrt' signals a domain error. Mathematically, it should return a complex number. - Function: double cbrt (double X) - Function: float cbrtf (float X) - Function: long double cbrtl (long double X) These functions return the cube root of X. They cannot fail; every representable real value has a representable real cube root. - Function: double hypot (double X, double Y) - Function: float hypotf (float X, float Y) - Function: long double hypotl (long double X, long double Y) These functions return `sqrt (X*X + Y*Y)'. This is the length of the hypotenuse of a right triangle with sides of length X and Y, or the distance of the point (X, Y) from the origin. Using this function instead of the direct formula is wise, since the error is much smaller. See also the function `cabs' in *Note Absolute Value::. - Function: double expm1 (double X) - Function: float expm1f (float X) - Function: long double expm1l (long double X) These functions return a value equivalent to `exp (X) - 1'. They are computed in a way that is accurate even if X is near zero--a case where `exp (X) - 1' would be inaccurate owing to subtraction of two numbers that are nearly equal. - Function: double log1p (double X) - Function: float log1pf (float X) - Function: long double log1pl (long double X) These functions returns a value equivalent to `log (1 + X)'. They are computed in a way that is accurate even if X is near zero. ISO C99 defines complex variants of some of the exponentiation and logarithm functions. - Function: complex double cexp (complex double Z) - Function: complex float cexpf (complex float Z) - Function: complex long double cexpl (complex long double Z) These functions return `e' (the base of natural logarithms) raised to the power of Z. Mathematically, this corresponds to the value exp (z) = exp (creal (z)) * (cos (cimag (z)) + I * sin (cimag (z))) - Function: complex double clog (complex double Z) - Function: complex float clogf (complex float Z) - Function: complex long double clogl (complex long double Z) These functions return the natural logarithm of Z. Mathematically, this corresponds to the value log (z) = log (cabs (z)) + I * carg (z) `clog' has a pole at 0, and will signal overflow if Z equals or is very close to 0. It is well-defined for all other values of Z. - Function: complex double clog10 (complex double Z) - Function: complex float clog10f (complex float Z) - Function: complex long double clog10l (complex long double Z) These functions return the base 10 logarithm of the complex value Z. Mathematically, this corresponds to the value log (z) = log10 (cabs (z)) + I * carg (z) These functions are GNU extensions. - Function: complex double csqrt (complex double Z) - Function: complex float csqrtf (complex float Z) - Function: complex long double csqrtl (complex long double Z) These functions return the complex square root of the argument Z. Unlike the real-valued functions, they are defined for all values of Z. - Function: complex double cpow (complex double BASE, complex double POWER) - Function: complex float cpowf (complex float BASE, complex float POWER) - Function: complex long double cpowl (complex long double BASE, complex long double POWER) These functions return BASE raised to the power of POWER. This is equivalent to `cexp (y * clog (x))' Hyperbolic Functions ==================== The functions in this section are related to the exponential functions; see *Note Exponents and Logarithms::. - Function: double sinh (double X) - Function: float sinhf (float X) - Function: long double sinhl (long double X) These functions return the hyperbolic sine of X, defined mathematically as `(exp (X) - exp (-X)) / 2'. They may signal overflow if X is too large. - Function: double cosh (double X) - Function: float coshf (float X) - Function: long double coshl (long double X) These function return the hyperbolic cosine of X, defined mathematically as `(exp (X) + exp (-X)) / 2'. They may signal overflow if X is too large. - Function: double tanh (double X) - Function: float tanhf (float X) - Function: long double tanhl (long double X) These functions return the hyperbolic tangent of X, defined mathematically as `sinh (X) / cosh (X)'. They may signal overflow if X is too large. There are counterparts for the hyperbolic functions which take complex arguments. - Function: complex double csinh (complex double Z) - Function: complex float csinhf (complex float Z) - Function: complex long double csinhl (complex long double Z) These functions return the complex hyperbolic sine of Z, defined mathematically as `(exp (Z) - exp (-Z)) / 2'. - Function: complex double ccosh (complex double Z) - Function: complex float ccoshf (complex float Z) - Function: complex long double ccoshl (complex long double Z) These functions return the complex hyperbolic cosine of Z, defined mathematically as `(exp (Z) + exp (-Z)) / 2'. - Function: complex double ctanh (complex double Z) - Function: complex float ctanhf (complex float Z) - Function: complex long double ctanhl (complex long double Z) These functions return the complex hyperbolic tangent of Z, defined mathematically as `csinh (Z) / ccosh (Z)'. - Function: double asinh (double X) - Function: float asinhf (float X) - Function: long double asinhl (long double X) These functions return the inverse hyperbolic sine of X--the value whose hyperbolic sine is X. - Function: double acosh (double X) - Function: float acoshf (float X) - Function: long double acoshl (long double X) These functions return the inverse hyperbolic cosine of X--the value whose hyperbolic cosine is X. If X is less than `1', `acosh' signals a domain error. - Function: double atanh (double X) - Function: float atanhf (float X) - Function: long double atanhl (long double X) These functions return the inverse hyperbolic tangent of X--the value whose hyperbolic tangent is X. If the absolute value of X is greater than `1', `atanh' signals a domain error; if it is equal to 1, `atanh' returns infinity. - Function: complex double casinh (complex double Z) - Function: complex float casinhf (complex float Z) - Function: complex long double casinhl (complex long double Z) These functions return the inverse complex hyperbolic sine of Z--the value whose complex hyperbolic sine is Z. - Function: complex double cacosh (complex double Z) - Function: complex float cacoshf (complex float Z) - Function: complex long double cacoshl (complex long double Z) These functions return the inverse complex hyperbolic cosine of Z--the value whose complex hyperbolic cosine is Z. Unlike the real-valued functions, there are no restrictions on the value of Z. - Function: complex double catanh (complex double Z) - Function: complex float catanhf (complex float Z) - Function: complex long double catanhl (complex long double Z) These functions return the inverse complex hyperbolic tangent of Z--the value whose complex hyperbolic tangent is Z. Unlike the real-valued functions, there are no restrictions on the value of Z. Special Functions ================= These are some more exotic mathematical functions which are sometimes useful. Currently they only have real-valued versions. - Function: double erf (double X) - Function: float erff (float X) - Function: long double erfl (long double X) `erf' returns the error function of X. The error function is defined as erf (x) = 2/sqrt(pi) * integral from 0 to x of exp(-t^2) dt - Function: double erfc (double X) - Function: float erfcf (float X) - Function: long double erfcl (long double X) `erfc' returns `1.0 - erf(X)', but computed in a fashion that avoids round-off error when X is large. - Function: double lgamma (double X) - Function: float lgammaf (float X) - Function: long double lgammal (long double X) `lgamma' returns the natural logarithm of the absolute value of the gamma function of X. The gamma function is defined as gamma (x) = integral from 0 to oo of t^(x-1) e^-t dt The sign of the gamma function is stored in the global variable SIGNGAM, which is declared in `math.h'. It is `1' if the intermediate result was positive or zero, or `-1' if it was negative. To compute the real gamma function you can use the `tgamma' function or you can compute the values as follows: lgam = lgamma(x); gam = signgam*exp(lgam); The gamma function has singularities at the non-positive integers. `lgamma' will raise the zero divide exception if evaluated at a singularity. - Function: double lgamma_r (double X, int *SIGNP) - Function: float lgammaf_r (float X, int *SIGNP) - Function: long double lgammal_r (long double X, int *SIGNP) `lgamma_r' is just like `lgamma', but it stores the sign of the intermediate result in the variable pointed to by SIGNP instead of in the SIGNGAM global. This means it is reentrant. - Function: double gamma (double X) - Function: float gammaf (float X) - Function: long double gammal (long double X) These functions exist for compatibility reasons. They are equivalent to `lgamma' etc. It is better to use `lgamma' since for one the name reflects better the actual computation, moreover `lgamma' is standardized in ISO C99 while `gamma' is not. - Function: double tgamma (double X) - Function: float tgammaf (float X) - Function: long double tgammal (long double X) `tgamma' applies the gamma function to X. The gamma function is defined as gamma (x) = integral from 0 to oo of t^(x-1) e^-t dt This function was introduced in ISO C99. - Function: double j0 (double X) - Function: float j0f (float X) - Function: long double j0l (long double X) `j0' returns the Bessel function of the first kind of order 0 of X. It may signal underflow if X is too large. - Function: double j1 (double X) - Function: float j1f (float X) - Function: long double j1l (long double X) `j1' returns the Bessel function of the first kind of order 1 of X. It may signal underflow if X is too large. - Function: double jn (int n, double X) - Function: float jnf (int n, float X) - Function: long double jnl (int n, long double X) `jn' returns the Bessel function of the first kind of order N of X. It may signal underflow if X is too large. - Function: double y0 (double X) - Function: float y0f (float X) - Function: long double y0l (long double X) `y0' returns the Bessel function of the second kind of order 0 of X. It may signal underflow if X is too large. If X is negative, `y0' signals a domain error; if it is zero, `y0' signals overflow and returns -oo. - Function: double y1 (double X) - Function: float y1f (float X) - Function: long double y1l (long double X) `y1' returns the Bessel function of the second kind of order 1 of X. It may signal underflow if X is too large. If X is negative, `y1' signals a domain error; if it is zero, `y1' signals overflow and returns -oo. - Function: double yn (int n, double X) - Function: float ynf (int n, float X) - Function: long double ynl (int n, long double X) `yn' returns the Bessel function of the second kind of order N of X. It may signal underflow if X is too large. If X is negative, `yn' signals a domain error; if it is zero, `yn' signals overflow and returns -oo. Known Maximum Errors in Math Functions ====================================== This section lists the known errors of the functions in the math library. Errors are measured in "units of the last place". This is a measure for the relative error. For a number z with the representation d.d...d*2^e (we assume IEEE floating-point numbers with base 2) the ULP is represented by |d.d...d - (z / 2^e)| / 2^(p - 1) where p is the number of bits in the mantissa of the floating-point number representation. Ideally the error for all functions is always less than 0.5ulps. Using rounding bits this is also possible and normally implemented for the basic operations. To achieve the same for the complex math functions requires a lot more work and this has not yet been done. Therefore many of the functions in the math library have errors. The table lists the maximum error for each function which is exposed by one of the existing tests in the test suite. The table tries to cover as much as possible and list the actual maximum error (or at least a ballpark figure) but this is often not achieved due to the large search space. The table lists the ULP values for different architectures. Different architectures have different results since their hardware support for floating-point operations varies and also the existing hardware support is different. Function Alpha ARM Generic ix86 IA64 acosf - - - - - acos - - - - - acosl - - - 1150 - acoshf - - - - - acosh - - - - - acoshl - - - 1 - asinf 2 2 - - - asin 1 1 - 1 1 asinl - - - 1 - asinhf - - - - - asinh - - - - - asinhl - - - 656 14 atanf - - - - - atan - - - - - atanl - - - 549 - atanhf - - - - - atanh 1 1 - 1 - atanhl - - - 1605 - atan2f 4 - - - - atan2 - - - - - atan2l - - - 549 - cabsf 1 1 - 1 1 cabs 1 1 - 1 1 cabsl - - - 560 1 cacosf 1 + i 1 1 + i 1 - 1 + i 2 1 + i 2 cacos 1 + i 0 1 + i 0 - 1 + i 0 1 + i 0 cacosl - - - 151 + i 329 1 + i 1 cacoshf 7 + i 3 7 + i 3 - 4 + i 4 7 + i 0 cacosh 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 cacoshl - - - 328 + i 151 7 + i 1 cargf - - - - - carg - - - - - cargl - - - - - casinf 2 + i 1 2 + i 1 - 2 + i 2 2 + i 2 casin 3 + i 0 3 + i 0 - 3 + i 0 3 + i 0 casinl - - - 603 + i 329 0 + i 1 casinhf 1 + i 6 1 + i 6 - 1 + i 6 1 + i 6 casinh 5 + i 3 5 + i 3 - 5 + i 3 5 + i 3 casinhl - - - 892 + i 12 5 + i 5 catanf 4 + i 1 4 + i 1 - 0 + i 1 0 + i 1 catan 0 + i 1 0 + i 1 - 0 + i 1 0 + i 1 catanl - - - 251 + i 474 1 + i 0 catanhf 1 + i 6 1 + i 6 - 1 + i 0 - catanh 4 + i 1 4 + i 1 - 2 + i 0 4 + i 0 catanhl - - - 66 + i 447 1 + i 0 cbrtf - - - - - cbrt 1 1 - 1 1 cbrtl - - - 716 - ccosf 0 + i 1 0 + i 1 - 1 + i 1 1 + i 1 ccos 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 ccosl - - - 5 + i 1901 0 + i 1 ccoshf 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 ccosh 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 ccoshl - - - 1467 + i 1183 1 + i 1 ceilf - - - - - ceil - - - - - ceill - - - - - cexpf 1 + i 1 1 + i 1 - 1 + i 0 1 + i 1 cexp 1 + i 0 1 + i 0 - - 1 + i 0 cexpl - - - 940 + i 1067 2 + i 0 cimagf - - - - - cimag - - - - - cimagl - - - - - clogf 0 + i 3 0 + i 3 - - - clog 0 + i 1 0 + i 1 - - - clogl - - - 0 + i 1 - clog10f 1 + i 5 1 + i 5 - 1 + i 1 1 + i 1 clog10 1 + i 1 1 + i 1 - 2 + i 1 2 + i 1 clog10l - - - 1403 + i 186 1 + i 2 conjf - - - - - conj - - - - - conjl - - - - - copysignf - - - - - copysign - - - - - copysignl - - - - - cosf 1 1 - 1 1 cos 2 2 - 2 2 cosl - - - 529 0.5 coshf - - - - - cosh - - - - - coshl - - - 309 2 cpowf 4 + i 2 4 + i 2 - 4 + i 2.5333 5 + i 2.5333 cpow 1 + i 1.1031 1 + i 1.1031 - 1 + i 1.104 1 + i 1.1031 cpowl - - - 2 + i 9 1 + i 4 cprojf - - - - - cproj - - - - - cprojl - - - - - crealf - - - - - creal - - - - - creall - - - - - csinf 0 + i 1 0 + i 1 - - - csin - - - - - csinl - - - 966 + i 168 0 + i 1 csinhf 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 csinh 0 + i 1 0 + i 1 - 1 + i 1 1 + i 1 csinhl - - - 413 + i 477 2 + i 2 csqrtf 1 + i 1 1 + i 1 - - 1 + i 1 csqrt 1 + i 0 1 + i 0 - 1 + i 0 1 + i 0 csqrtl - - - 237 + i 128 - ctanf 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 ctan 1 + i 1 1 + i 1 - 1 + i 1 1 + i 1 ctanl - - - 690 + i 367 436 + i 1 ctanhf 2 + i 1 2 + i 1 - 1 + i 1 1 + i 1 ctanh 2 + i 2 2 + i 2 - 0 + i 1 0 + i 1 ctanhl - - - 286 + i 3074 1 + i 24 erff - - - - - erf - - - - - erfl - - - - - erfcf 12 12 - 12 12 erfc 24 24 - 24 24 erfcl - - - 36 12 expf - - - - - exp - - - - - expl - - - 754 - exp10f 2 2 - - 2 exp10 6 6 - 1 6 exp10l - - - 1182 3 exp2f - - - - - exp2 - - - - - exp2l - - - 462 - expm1f 1 1 - - - expm1 - - - - - expm1l - - - 825 1 fabsf - - - - - fabs - - - - - fabsl - - - - - fdimf - - - - - fdim - - - - - fdiml - - - - - floorf - - - - - floor - - - - - floorl - - - - - fmaf - - - - - fma - - - - - fmal - - - - - fmaxf - - - - - fmax - - - - - fmaxl - - - - - fminf - - - - - fmin - - - - - fminl - - - - - fmodf 1 1 - 1 1 fmod 2 2 - 2 2 fmodl - - - 4096 1 frexpf - - - - - frexp - - - - - frexpl - - - - - gammaf - - - - - gamma - - - 1 - gammal - - - 1 1 hypotf 1 1 - 1 1 hypot 1 1 - 1 1 hypotl - - - 560 1 ilogbf - - - - - ilogb - - - - - ilogbl - - - - - j0f 2 2 - 1 1 j0 2 2 - 2 2 j0l - - - 1 2 j1f 2 2 - 1 2 j1 1 1 - 2 1 j1l - - - 2 - jnf 4 4 - 2 4 jn 6 6 - 5 6 jnl - - - 2 2 lgammaf 2 2 - 2 2 lgamma 1 1 - 1 1 lgammal - - - 1 1 lrintf - - - - - lrint - - - - - lrintl - - - - - llrintf - - - - - llrint - - - - - llrintl - - - - - logf 1 1 - 1 1 log 1 1 - 1 1 logl - - - 2341 1 log10f 1 1 - 1 1 log10 1 1 - 1 1 log10l - - - 2033 1 log1pf 1 1 - 1 1 log1p 1 1 - 1 1 log1pl - - - 585 1 log2f 1 1 - 1 1 log2 1 1 - 1 1 log2l - - - 1688 - logbf - - - - - logb - - - - - logbl - - - - - lroundf - - - - - lround - - - - - lroundl - - - - - llroundf - - - - - llround - - - - - llroundl - - - - - modff - - - - - modf - - - - - modfl - - - - - nearbyintf - - - - - nearbyint - - - - - nearbyintl - - - - - nextafterf - - - - - nextafter - - - - - nextafterl - - - - - nexttowardf - - - - - nexttoward - - - - - nexttowardl - - - - - powf - - - - - pow - - - - - powl - - - 725 1 remainderf - - - - - remainder - - - - - remainderl - - - - - remquof - - - - - remquo - - - - - remquol - - - - - rintf - - - - - rint - - - - - rintl - - - - - roundf - - - - - round - - - - - roundl - - - - - scalbf - - - - - scalb - - - - - scalbl - - - - - scalbnf - - - - - scalbn - - - - - scalbnl - - - - - scalblnf - - - - - scalbln - - - - - scalblnl - - - - - sinf - - - - - sin - - - - - sinl - - - 627 1 sincosf 1 1 - 1 1 sincos 1 1 - 1 1 sincosl - - - 627 1 sinhf 1 1 - 1 1 sinh 1 1 - - - sinhl - - - 1029 1 sqrtf - - - - - sqrt - - - - - sqrtl - - - 489 - tanf - - - - - tan 0.5 0.5 - 0.5 0.5 tanl - - - 1401 1 tanhf 1 1 - - 1 tanh 1 1 - - 1 tanhl - - - 521 1 tgammaf 1 1 - 1 1 tgamma 1 1 - 2 1 tgammal - - - 2 1 truncf - - - - - trunc - - - - - truncl - - - - - y0f 1 1 - 1 1 y0 2 2 - 3 2 y0l - - - 2 2 y1f 2 2 - 2 2 y1 3 3 - 3 3 y1l - - - 2 1 ynf 2 2 - 3 2 yn 3 3 - 6 3 ynl - - - 7 7 Function M68k MIPS PowerPC S/390 SH4 acosf - - - - - acos - - - - - acosl 1 - - - - acoshf - - - - - acosh - - - - - acoshl 1 - - - - asinf - 2 2 2 2 asin 1 1 1 1 1 asinl 1 - - - - asinhf - - - - - asinh - - - - - asinhl 14 - - - - atanf - - - - - atan - - - - - atanl - - - - - atanhf - - - - - atanh 1 1 1 1 1 atanhl - - - - - atan2f - 4 4 4 4 atan2 - - - - - atan2l - - - - - cabsf 1 1 1 1 1 cabs - 1 1 1 1 cabsl 1 - - - - cacosf 1 + i 2 1 + i 1 1 + i 1 1 + i 1 1 + i 1 cacos 1 + i 0 1 + i 0 1 + i 0 1 + i 0 1 + i 0 cacosl 1 + i 1 - - - - cacoshf 7 + i 0 7 + i 3 7 + i 3 7 + i 3 7 + i 3 cacosh 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1 cacoshl 6 + i 2 - - - - cargf - - - - - carg - - - - - cargl - - - - - casinf 2 + i 2 2 + i 1 2 + i 1 2 + i 1 2 + i 1 casin 3 + i 0 3 + i 0 3 + i 0 3 + i 0 3 + i 0 casinl 0 + i 1 - - - - casinhf 19 + i 2 1 + i 6 1 + i 6 1 + i 6 1 + i 6 casinh 6 + i 13 5 + i 3 5 + i 3 5 + i 3 5 + i 3 casinhl 5 + i 6 - - - - catanf 0 + i 1 4 + i 1 4 + i 1 4 + i 1 4 + i 1 catan 0 + i 1 0 + i 1 0 + i 1 0 + i 1 0 + i 1 catanl 1 + i 0 - - - - catanhf - 1 + i 6 0 + i 6 1 + i 6 1 + i 6 catanh - 4 + i 1 4 + i 1 4 + i 1 4 + i 1 catanhl 1 + i 0 - - - - cbrtf - - - - - cbrt 1 1 1 1 1 cbrtl 1 - - - - ccosf 1 + i 1 0 + i 1 0 + i 1 0 + i 1 0 + i 1 ccos 0 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1 ccosl 0 + i 1 - - - - ccoshf 3 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1 ccosh 1 + i 0 1 + i 1 1 + i 1 1 + i 1 1 + i 1 ccoshl 1 + i 2 - - - - ceilf - - - - - ceil - - - - - ceill - - - - - cexpf 3 + i 2 1 + i 1 1 + i 1 1 + i 1 1 + i 1 cexp - 1 + i 0 1 + i 0 1 + i 0 1 + i 0 cexpl 2 + i 0 - - - - cimagf - - - - - cimag - - - - - cimagl - - - - - clogf - 0 + i 3 0 + i 3 0 + i 3 0 + i 3 clog - 0 + i 1 0 + i 1 0 + i 1 0 + i 1 clogl - - - - - clog10f 1 + i 1 1 + i 5 1 + i 5 1 + i 5 1 + i 5 clog10 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1 clog10l 1 + i 3 - - - - conjf - - - - - conj - - - - - conjl - - - - - copysignf - - - - - copysign - - - - - copysignl - - - - - cosf 1 1 1 1 1 cos 2 2 2 2 2 cosl 1 - - - - coshf - - - - - cosh - - - - - coshl 2 - - - - cpowf 1 + i 6 4 + i 2 4 + i 2 4 + i 2 4 + i 2 cpow 1 + i 2 1 + i 1.1031 1 + i 2 1 + i 1.1031 1 + i 1.1031 cpowl 5 + i 2 - - - - cprojf - - - - - cproj - - - - - cprojl - - - - - crealf - - - - - creal - - - - - creall - - - - - csinf 1 + i 1 0 + i 1 0 + i 1 0 + i 1 0 + i 1 csin - - - - - csinl - - - - - csinhf 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1 csinh - 0 + i 1 0 + i 1 0 + i 1 0 + i 1 csinhl 1 + i 2 - - - - csqrtf 1 + i 0 1 + i 1 1 + i 1 1 + i 1 1 + i 1 csqrt - 1 + i 0 1 + i 0 1 + i 0 1 + i 0 csqrtl - - - - - ctanf 1 + i 0 1 + i 1 1 + i 1 1 + i 1 1 + i 1 ctan 1 + i 0 1 + i 1 1 + i 1 1 + i 1 1 + i 1 ctanl 439 + i 2 - - - - ctanhf 1 + i 0 2 + i 1 2 + i 1 2 + i 1 2 + i 1 ctanh 0 + i 1 2 + i 2 2 + i 2 2 + i 2 2 + i 2 ctanhl 2 + i 25 - - - - erff - - - - - erf - - - - - erfl - - - - - erfcf 11 12 12 12 12 erfc 24 24 24 24 24 erfcl 12 - - - - expf - - - - - exp - - - - - expl - - - - - exp10f - 2 2 2 2 exp10 1 6 6 6 6 exp10l 1 - - - - exp2f - - - - - exp2 - - - - - exp2l - - - - - expm1f - 1 1 1 1 expm1 - - - - - expm1l 1 - - - - fabsf - - - - - fabs - - - - - fabsl - - - - - fdimf - - - - - fdim - - - - - fdiml - - - - - floorf - - - - - floor - - - - - floorl - - - - - fmaf - - - - - fma - - - - - fmal - - - - - fmaxf - - - - - fmax - - - - - fmaxl - - - - - fminf - - - - - fmin - - - - - fminl - - - - - fmodf 1 1 1 1 1 fmod 2 2 2 2 2 fmodl 1 - - - - frexpf - - - - - frexp - - - - - frexpl - - - - - gammaf - - - - - gamma - - - - - gammal 1 - - - - hypotf 1 1 1 1 1 hypot - 1 1 1 1 hypotl 1 - - - - ilogbf - - - - - ilogb - - - - - ilogbl - - - - - j0f 1 2 1 2 2 j0 1 2 2 2 2 j0l 1 - - - - j1f 2 2 2 2 2 j1 - 1 1 1 1 j1l 2 - - - - jnf 11 4 4 4 4 jn 4 6 6 6 6 jnl 2 - - - - lgammaf 2 2 2 2 2 lgamma 1 1 1 1 1 lgammal 1 - - - - lrintf - - - - - lrint - - - - - lrintl - - - - - llrintf - - - - - llrint - - - - - llrintl - - - - - logf 1 1 1 1 1 log 1 1 1 1 1 logl 2 - - - - log10f 1 1 1 1 1 log10 1 1 1 1 1 log10l 1 - - - - log1pf 1 1 1 1 1 log1p 1 1 1 1 1 log1pl 2 - - - - log2f 1 1 1 1 1 log2 1 1 1 1 1 log2l 1 - - - - logbf - - - - - logb - - - - - logbl - - - - - lroundf - - - - - lround - - - - - lroundl - - - - - llroundf - - - - - llround - - - - - llroundl - - - - - modff - - - - - modf - - - - - modfl - - - - - nearbyintf - - - - - nearbyint - - - - - nearbyintl - - - - - nextafterf - - - - - nextafter - - - - - nextafterl - - - - - nexttowardf - - - - - nexttoward - - - - - nexttowardl - - - - - powf - - - - - pow - - - - - powl 1 - - - - remainderf - - - - - remainder - - - - - remainderl - - - - - remquof - - - - - remquo - - - - - remquol - - - - - rintf - - - - - rint - - - - - rintl - - - - - roundf - - - - - round - - - - - roundl - - - - - scalbf - - - - - scalb - - - - - scalbl - - - - - scalbnf - - - - - scalbn - - - - - scalbnl - - - - - scalblnf - - - - - scalbln - - - - - scalblnl - - - - - sinf - - - - - sin - - - - - sinl 1 - - - - sincosf 1 1 1 1 1 sincos 1 1 1 1 1 sincosl 1 - - - - sinhf 1 1 1 1 1 sinh - 1 1 1 1 sinhl - - - - - sqrtf - - - - - sqrt - - - - - sqrtl - - - - - tanf - - - - - tan 1 0.5 1 0.5 0.5 tanl 1 - - - - tanhf - 1 1 1 1 tanh - 1 1 1 1 tanhl - - - - - tgammaf 1 1 1 1 1 tgamma 1 1 1 1 1 tgammal 1 - - - - truncf - - - - - trunc - - - - - truncl - - - - - y0f 2 1 1 1 1 y0 2 2 2 2 2 y0l 2 - - - - y1f 2 2 2 2 2 y1 1 3 3 3 3 y1l 2 - - - - ynf 2 2 2 2 2 yn 6 3 3 3 3 ynl 7 - - - - Function Sparc 32-bit Sparc 64-bit x86_64/fpu acosf - - - acos - - - acosl - 1 - acoshf - - - acosh - - - acoshl - - - asinf 2 2 - asin 1 1 1 asinl - - 1 asinhf - - - asinh - - - asinhl - - 15 atanf - - - atan - - - atanl - 1 - atanhf - - - atanh 1 1 1 atanhl - - 1 atan2f 4.0000 4 4 atan2 - - - atan2l - 1 - cabsf 1 1 1 cabs 1 1 1 cabsl - - 1 cacosf 1 + i 1 1 + i 1 1 + i 1 cacos 1 + i 0 1 + i 0 1 + i 0 cacosl - 0 + i 3 1 + i 1 cacoshf 7 + i 3 7 + i 3 7 + i 3 cacosh 1 + i 1 1 + i 1 1 + i 1 cacoshl - 5 + i 1 6 + i 1 cargf - - - carg - - - cargl - - - casinf 2 + i 1 2 + i 1 2 + i 1 casin 3 + i 0 3 + i 0 3 + i 0 casinl - 1 + i 3 0 + i 1 casinhf 1 + i 6 1 + i 6 1 + i 6 casinh 5 + i 3 5 + i 3 5 + i 3 casinhl - 4 + i 2 5 + i 5 catanf 4 + i 1 4 + i 1 4 + i 1 catan 0 + i 1 0 + i 1 0 + i 1 catanl - 0 + i 1 1 + i 0 catanhf 1 + i 6 1 + i 6 1 + i 6 catanh 4 + i 1 4 + i 1 4 + i 0 catanhl - - 1 + i 0 cbrtf - - - cbrt 1 1 1 cbrtl - - 948 ccosf 0 + i 1 0 + i 1 0 + i 1 ccos 1 + i 1 1 + i 1 1 + i 1 ccosl - - 0 + i 1 ccoshf 1 + i 1 1 + i 1 1 + i 1 ccosh 1 + i 1 1 + i 1 1 + i 1 ccoshl - - 1 + i 1 ceilf - - - ceil - - - ceill - - - cexpf 1 + i 1 1 + i 1 1 + i 1 cexp 1 + i 0 1 + i 0 1 + i 0 cexpl - 1 + i 1 2 + i 1 cimagf - - - cimag - - - cimagl - - - clogf 0 + i 3 0 + i 3 0 + i 3 clog 0 + i 1 0 + i 1 - clogl - - - clog10f 1 + i 5 1 + i 5 1 + i 5 clog10 1 + i 1 1 + i 1 1 + i 1 clog10l - - 1 + i 3 conjf - - - conj - - - conjl - - - copysignf - - - copysign - - - copysignl - - - cosf 1 1 1 cos 2 2 2 cosl - 1 0.5 coshf - - - cosh - - - coshl - - 2 cpowf 4 + i 2 4 + i 2 4 + i 2 cpow 1 + i 1.1031 1 + i 1.1031 1 + i 1.1031 cpowl - 3 + i 0.9006 1 + i 2 cprojf - - - cproj - - - cprojl - - - crealf - - - creal - - - creall - - - csinf 0 + i 1 0 + i 1 0 + i 1 csin - - - csinl - - 0 + i 2 csinhf 1 + i 1 1 + i 1 1 + i 1 csinh 0 + i 1 0 + i 1 0 + i 1 csinhl - - 2 + i 2 csqrtf 1 + i 1 1 + i 1 1 + i 1 csqrt 1 + i 0 1 + i 0 1 + i 0 csqrtl - 1 + i 1 - ctanf 1 + i 1 1 + i 1 1 + i 1 ctan 1 + i 1 1 + i 1 1 + i 1 ctanl - - 439 + i 2 ctanhf 2 + i 1 2 + i 1 2 + i 1 ctanh 2 + i 2 2 + i 2 2 + i 2 ctanhl - - 5 + i 25 erff - - - erf - - - erfl - - - erfcf 12 12 12 erfc 24 24 24 erfcl - - 36 expf - - - exp - - - expl - - - exp10f 2 2 2 exp10 6 6 6 exp10l - 1 3 exp2f - - - exp2 - - - exp2l - - - expm1f 1 1 1 expm1 - 1 1 expm1l - - 1 fabsf - - - fabs - - - fabsl - - - fdimf - - - fdim - - - fdiml - - - floorf - - - floor - - - floorl - - - fmaf - - - fma - - - fmal - - - fmaxf - - - fmax - - - fmaxl - - - fminf - - - fmin - - - fminl - - - fmodf 1 1 1 fmod 2 2 2 fmodl - 2 1 frexpf - - - frexp - - - frexpl - - - gammaf - - - gamma - - - gammal - - 1 hypotf 1 1 1 hypot 1 1 1 hypotl - - 1 ilogbf - - - ilogb - - - ilogbl - - - j0f 2 2 2 j0 2 2 2 j0l - - - j1f 2 2 2 j1 1 1 1 j1l - - 2 jnf 4 4 4 jn 6 6 6 jnl - - 2 lgammaf 2 2 2 lgamma 1 1 1 lgammal - - 1 lrintf - - - lrint - - - lrintl - - - llrintf - - - llrint - - - llrintl - - - logf 1 1 1 log 1 1 1 logl - 1 1 log10f 1 1 1 log10 1 1 1 log10l - - 1 log1pf 1 1 1 log1p 1 1 1 log1pl - 1 1 log2f 1 1 1 log2 1 1 1 log2l - - - logbf - - - logb - - - logbl - - - lroundf - - - lround - - - lroundl - - - llroundf - - - llround - - - llroundl - - - modff - - - modf - - - modfl - - - nearbyintf - - - nearbyint - - - nearbyintl - - - nextafterf - - - nextafter - - - nextafterl - - - nexttowardf - - - nexttoward - - - nexttowardl - - - powf - - - pow - - - powl - - - remainderf - - - remainder - - - remainderl - - - remquof - - - remquo - - - remquol - - - rintf - - - rint - - - rintl - - - roundf - - - round - - - roundl - - - scalbf - - - scalb - - - scalbl - - - scalbnf - - - scalbn - - - scalbnl - - - scalblnf - - - scalbln - - - scalblnl - - - sinf - - - sin - - - sinl - - 1 sincosf 1 1 1 sincos 1 1 1 sincosl - 1 1 sinhf 1 1 1 sinh 1 1 1 sinhl - - 1 sqrtf - - - sqrt - - - sqrtl - 1 - tanf - - - tan 0.5 0.5 0.5 tanl - 1 1 tanhf 1 1 1 tanh 1 1 1 tanhl - - 1 tgammaf 1 1 1 tgamma 1 1 1 tgammal - - 2 truncf - - - trunc - - - truncl - - - y0f 1 1 1 y0 2 2 2 y0l - - 2 y1f 2 2 2 y1 3 3 3 y1l - - 2 ynf 2 2 2 yn 3 3 3 ynl - - 7 Pseudo-Random Numbers ===================== This section describes the GNU facilities for generating a series of pseudo-random numbers. The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes. The random number generator works by remembering a "seed" value which it uses to compute the next random number and also to compute a new seed. Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is _exactly the same_ from one run to the next. This is because the initial seed is always the same. This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably. If you want a different pseudo-random series each time your program runs, you must specify a different seed each time. For ordinary purposes, basing the seed on the current time works well. You can obtain repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator. There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers. The GNU library supports the standard ISO C random number functions plus two other sets derived from BSD and SVID. The BSD and ISO C functions provide identical, somewhat limited functionality. If only a small number of random bits are required, we recommend you use the ISO C interface, `rand' and `srand'. The SVID functions provide a more flexible interface, which allows better random number generator algorithms, provides more random bits (up to 48) per call, and can provide random floating-point numbers. These functions are required by the XPG standard and therefore will be present in all modern Unix systems. ISO C Random Number Functions ----------------------------- This section describes the random number functions that are part of the ISO C standard. To use these facilities, you should include the header file `stdlib.h' in your program. - Macro: int RAND_MAX The value of this macro is an integer constant representing the largest value the `rand' function can return. In the GNU library, it is `2147483647', which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as `32767'. - Function: int rand (void) The `rand' function returns the next pseudo-random number in the series. The value ranges from `0' to `RAND_MAX'. - Function: void srand (unsigned int SEED) This function establishes SEED as the seed for a new series of pseudo-random numbers. If you call `rand' before a seed has been established with `srand', it uses the value `1' as a default seed. To produce a different pseudo-random series each time your program is run, do `srand (time (0))'. POSIX.1 extended the C standard functions to support reproducible random numbers in multi-threaded programs. However, the extension is badly designed and unsuitable for serious work. - Function: int rand_r (unsigned int *SEED) This function returns a random number in the range 0 to `RAND_MAX' just as `rand' does. However, all its state is stored in the SEED argument. This means the RNG's state can only have as many bits as the type `unsigned int' has. This is far too few to provide a good RNG. If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random number generator. The POSIX.1 interface should only be used when the GNU extensions are not available. BSD Random Number Functions --------------------------- This section describes a set of random number generation functions that are derived from BSD. There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only. The prototypes for these functions are in `stdlib.h'. - Function: long int random (void) This function returns the next pseudo-random number in the sequence. The value returned ranges from `0' to `RAND_MAX'. *Note:* Temporarily this function was defined to return a `int32_t' value to indicate that the return value always contains 32 bits even if `long int' is wider. The standard demands it differently. Users must always be aware of the 32-bit limitation, though. - Function: void srandom (unsigned int SEED) The `srandom' function sets the state of the random number generator based on the integer SEED. If you supply a SEED value of `1', this will cause `random' to reproduce the default set of random numbers. To produce a different set of pseudo-random numbers each time your program runs, do `srandom (time (0))'. - Function: void * initstate (unsigned int SEED, void *STATE, size_t SIZE) The `initstate' function is used to initialize the random number generator state. The argument STATE is an array of SIZE bytes, used to hold the state information. It is initialized based on SEED. The size must be between 8 and 256 bytes, and should be a power of two. The bigger the STATE array, the better. The return value is the previous value of the state information array. You can use this value later as an argument to `setstate' to restore that state. - Function: void * setstate (void *STATE) The `setstate' function restores the random number state information STATE. The argument must have been the result of a previous call to INITSTATE or SETSTATE. The return value is the previous value of the state information array. You can use this value later as an argument to `setstate' to restore that state. If the function fails the return value is `NULL'. The four functions described so far in this section all work on a state which is shared by all threads. The state is not directly accessible to the user and can only be modified by these functions. This makes it hard to deal with situations where each thread should have its own pseudo-random number generator. The GNU C library contains four additional functions which contain the state as an explicit parameter and therefore make it possible to handle thread-local PRNGs. Beside this there are no difference. In fact, the four functions already discussed are implemented internally using the following interfaces. The `stdlib.h' header contains a definition of the following type: - Data Type: struct random_data Objects of type `struct random_data' contain the information necessary to represent the state of the PRNG. Although a complete definition of the type is present the type should be treated as opaque. The functions modifying the state follow exactly the already described functions. - Function: int random_r (struct random_data *restrict BUF, int32_t *restrict RESULT) The `random_r' function behaves exactly like the `random' function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state. - Function: int srandom_r (unsigned int SEED, struct random_data *BUF) The `srandom_r' function behaves exactly like the `srandom' function except that it uses and modifies the state in the object pointed to by the second parameter instead of the global state. - Function: int initstate_r (unsigned int SEED, char *restrict STATEBUF, size_t STATELEN, struct random_data *restrict BUF) The `initstate_r' function behaves exactly like the `initstate' function except that it uses and modifies the state in the object pointed to by the fourth parameter instead of the global state. - Function: int setstate_r (char *restrict STATEBUF, struct random_data *restrict BUF) The `setstate_r' function behaves exactly like the `setstate' function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state. SVID Random Number Function --------------------------- The C library on SVID systems contains yet another kind of random number generator functions. They use a state of 48 bits of data. The user can choose among a collection of functions which return the random bits in different forms. Generally there are two kinds of function. The first uses a state of the random number generator which is shared among several functions and by all threads of the process. The second requires the user to handle the state. All functions have in common that they use the same congruential formula with the same constants. The formula is Y = (a * X + c) mod m where X is the state of the generator at the beginning and Y the state at the end. `a' and `c' are constants determining the way the generator works. By default they are a = 0x5DEECE66D = 25214903917 c = 0xb = 11 but they can also be changed by the user. `m' is of course 2^48 since the state consists of a 48-bit array. The prototypes for these functions are in `stdlib.h'. - Function: double drand48 (void) This function returns a `double' value in the range of `0.0' to `1.0' (exclusive). The random bits are determined by the global state of the random number generator in the C library. Since the `double' type according to IEEE 754 has a 52-bit mantissa this means 4 bits are not initialized by the random number generator. These are (of course) chosen to be the least significant bits and they are initialized to `0'. - Function: double erand48 (unsigned short int XSUBI[3]) This function returns a `double' value in the range of `0.0' to `1.0' (exclusive), similarly to `drand48'. The argument is an array describing the state of the random number generator. This function can be called subsequently since it updates the array to guarantee random numbers. The array should have been initialized before initial use to obtain reproducible results. - Function: long int lrand48 (void) The `lrand48' function returns an integer value in the range of `0' to `2^31' (exclusive). Even if the size of the `long int' type can take more than 32 bits, no higher numbers are returned. The random bits are determined by the global state of the random number generator in the C library. - Function: long int nrand48 (unsigned short int XSUBI[3]) This function is similar to the `lrand48' function in that it returns a number in the range of `0' to `2^31' (exclusive) but the state of the random number generator used to produce the random bits is determined by the array provided as the parameter to the function. The numbers in the array are updated afterwards so that subsequent calls to this function yield different results (as is expected of a random number generator). The array should have been initialized before the first call to obtain reproducible results. - Function: long int mrand48 (void) The `mrand48' function is similar to `lrand48'. The only difference is that the numbers returned are in the range `-2^31' to `2^31' (exclusive). - Function: long int jrand48 (unsigned short int XSUBI[3]) The `jrand48' function is similar to `nrand48'. The only difference is that the numbers returned are in the range `-2^31' to `2^31' (exclusive). For the `xsubi' parameter the same requirements are necessary. The internal state of the random number generator can be initialized in several ways. The methods differ in the completeness of the information provided. - Function: void srand48 (long int SEEDVAL) The `srand48' function sets the most significant 32 bits of the internal state of the random number generator to the least significant 32 bits of the SEEDVAL parameter. The lower 16 bits are initialized to the value `0x330E'. Even if the `long int' type contains more than 32 bits only the lower 32 bits are used. Owing to this limitation, initialization of the state of this function is not very useful. But it makes it easy to use a construct like `srand48 (time (0))'. A side-effect of this function is that the values `a' and `c' from the internal state, which are used in the congruential formula, are reset to the default values given above. This is of importance once the user has called the `lcong48' function (see below). - Function: unsigned short int * seed48 (unsigned short int SEED16V[3]) The `seed48' function initializes all 48 bits of the state of the internal random number generator from the contents of the parameter SEED16V. Here the lower 16 bits of the first element of SEE16V initialize the least significant 16 bits of the internal state, the lower 16 bits of `SEED16V[1]' initialize the mid-order 16 bits of the state and the 16 lower bits of `SEED16V[2]' initialize the most significant 16 bits of the state. Unlike `srand48' this function lets the user initialize all 48 bits of the state. The value returned by `seed48' is a pointer to an array containing the values of the internal state before the change. This might be useful to restart the random number generator at a certain state. Otherwise the value can simply be ignored. As for `srand48', the values `a' and `c' from the congruential formula are reset to the default values. There is one more function to initialize the random number generator which enables you to specify even more information by allowing you to change the parameters in the congruential formula. - Function: void lcong48 (unsigned short int PARAM[7]) The `lcong48' function allows the user to change the complete state of the random number generator. Unlike `srand48' and `seed48', this function also changes the constants in the congruential formula. From the seven elements in the array PARAM the least significant 16 bits of the entries `PARAM[0]' to `PARAM[2]' determine the initial state, the least significant 16 bits of `PARAM[3]' to `PARAM[5]' determine the 48 bit constant `a' and `PARAM[6]' determines the 16-bit value `c'. All the above functions have in common that they use the global parameters for the congruential formula. In multi-threaded programs it might sometimes be useful to have different parameters in different threads. For this reason all the above functions have a counterpart which works on a description of the random number generator in the user-supplied buffer instead of the global state. Please note that it is no problem if several threads use the global state if all threads use the functions which take a pointer to an array containing the state. The random numbers are computed following the same loop but if the state in the array is different all threads will obtain an individual random number generator. The user-supplied buffer must be of type `struct drand48_data'. This type should be regarded as opaque and not manipulated directly. - Function: int drand48_r (struct drand48_data *BUFFER, double *RESULT) This function is equivalent to the `drand48' function with the difference that it does not modify the global random number generator parameters but instead the parameters in the buffer supplied through the pointer BUFFER. The random number is returned in the variable pointed to by RESULT. The return value of the function indicates whether the call succeeded. If the value is less than `0' an error occurred and ERRNO is set to indicate the problem. This function is a GNU extension and should not be used in portable programs. - Function: int erand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, double *RESULT) The `erand48_r' function works like `erand48', but in addition it takes an argument BUFFER which describes the random number generator. The state of the random number generator is taken from the `xsubi' array, the parameters for the congruential formula from the global random number generator data. The random number is returned in the variable pointed to by RESULT. The return value is non-negative if the call succeeded. This function is a GNU extension and should not be used in portable programs. - Function: int lrand48_r (struct drand48_data *BUFFER, double *RESULT) This function is similar to `lrand48', but in addition it takes a pointer to a buffer describing the state of the random number generator just like `drand48'. If the return value of the function is non-negative the variable pointed to by RESULT contains the result. Otherwise an error occurred. This function is a GNU extension and should not be used in portable programs. - Function: int nrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT) The `nrand48_r' function works like `nrand48' in that it produces a random number in the range `0' to `2^31'. But instead of using the global parameters for the congruential formula it uses the information from the buffer pointed to by BUFFER. The state is described by the values in XSUBI. If the return value is non-negative the variable pointed to by RESULT contains the result. This function is a GNU extension and should not be used in portable programs. - Function: int mrand48_r (struct drand48_data *BUFFER, double *RESULT) This function is similar to `mrand48' but like the other reentrant functions it uses the random number generator described by the value in the buffer pointed to by BUFFER. If the return value is non-negative the variable pointed to by RESULT contains the result. This function is a GNU extension and should not be used in portable programs. - Function: int jrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT) The `jrand48_r' function is similar to `jrand48'. Like the other reentrant functions of this function family it uses the congruential formula parameters from the buffer pointed to by BUFFER. If the return value is non-negative the variable pointed to by RESULT contains the result. This function is a GNU extension and should not be used in portable programs. Before any of the above functions are used the buffer of type `struct drand48_data' should be initialized. The easiest way to do this is to fill the whole buffer with null bytes, e.g. by memset (buffer, '\0', sizeof (struct drand48_data)); Using any of the reentrant functions of this family now will automatically initialize the random number generator to the default values for the state and the parameters of the congruential formula. The other possibility is to use any of the functions which explicitly initialize the buffer. Though it might be obvious how to initialize the buffer from looking at the parameter to the function, it is highly recommended to use these functions since the result might not always be what you expect. - Function: int srand48_r (long int SEEDVAL, struct drand48_data *BUFFER) The description of the random number generator represented by the information in BUFFER is initialized similarly to what the function `srand48' does. The state is initialized from the parameter SEEDVAL and the parameters for the congruential formula are initialized to their default values. If the return value is non-negative the function call succeeded. This function is a GNU extension and should not be used in portable programs. - Function: int seed48_r (unsigned short int SEED16V[3], struct drand48_data *BUFFER) This function is similar to `srand48_r' but like `seed48' it initializes all 48 bits of the state from the parameter SEED16V. If the return value is non-negative the function call succeeded. It does not return a pointer to the previous state of the random number generator like the `seed48' function does. If the user wants to preserve the state for a later re-run s/he can copy the whole buffer pointed to by BUFFER. This function is a GNU extension and should not be used in portable programs. - Function: int lcong48_r (unsigned short int PARAM[7], struct drand48_data *BUFFER) This function initializes all aspects of the random number generator described in BUFFER with the data in PARAM. Here it is especially true that the function does more than just copying the contents of PARAM and BUFFER. More work is required and therefore it is important to use this function rather than initializing the random number generator directly. If the return value is non-negative the function call succeeded. This function is a GNU extension and should not be used in portable programs. Is Fast Code or Small Code preferred? ===================================== If an application uses many floating point functions it is often the case that the cost of the function calls themselves is not negligible. Modern processors can often execute the operations themselves very fast, but the function call disrupts the instruction pipeline. For this reason the GNU C Library provides optimizations for many of the frequently-used math functions. When GNU CC is used and the user activates the optimizer, several new inline functions and macros are defined. These new functions and macros have the same names as the library functions and so are used instead of the latter. In the case of inline functions the compiler will decide whether it is reasonable to use them, and this decision is usually correct. This means that no calls to the library functions may be necessary, and can increase the speed of generated code significantly. The drawback is that code size will increase, and the increase is not always negligible. There are two kind of inline functions: Those that give the same result as the library functions and others that might not set `errno' and might have a reduced precision and/or argument range in comparison with the library functions. The latter inline functions are only available if the flag `-ffast-math' is given to GNU CC. In cases where the inline functions and macros are not wanted the symbol `__NO_MATH_INLINES' should be defined before any system header is included. This will ensure that only library functions are used. Of course, it can be determined for each file in the project whether giving this option is preferable or not. Not all hardware implements the entire IEEE 754 standard, and even if it does there may be a substantial performance penalty for using some of its features. For example, enabling traps on some processors forces the FPU to run un-pipelined, which can more than double calculation time. Arithmetic Functions ******************** This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts or retrieving the imaginary part of a complex value. These functions are declared in the header files `math.h' and `complex.h'. Integers ======== The C language defines several integer data types: integer, short integer, long integer, and character, all in both signed and unsigned varieties. The GNU C compiler extends the language to contain long long integers as well. The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines. The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on. To address this problem, the GNU C library contains C type definitions you can use to declare integers that meet your exact needs. Because the GNU C library header files are customized to a specific machine, your program source code doesn't have to be. These `typedef's are in `stdint.h'. If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness: * int8_t * int16_t * int32_t * int64_t * uint8_t * uint16_t * uint32_t * uint64_t If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist. If you don't need a specific storage size, but want the smallest data structure with _at least_ N bits, use one of these: * int_least8_t * int_least16_t * int_least32_t * int_least64_t * uint_least8_t * uint_least16_t * uint_least32_t * uint_least64_t If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these: * int_fast8_t * int_fast16_t * int_fast32_t * int_fast64_t * uint_fast8_t * uint_fast16_t * uint_fast32_t * uint_fast64_t If you want an integer with the widest range possible on the platform on which it is being used, use one of the following. If you use these, you should write code that takes into account the variable size and range of the integer. * intmax_t * uintmax_t The GNU C library also provides macros that tell you the maximum and minimum possible values for each integer data type. The macro names follow these examples: `INT32_MAX', `UINT8_MAX', `INT_FAST32_MIN', `INT_LEAST64_MIN', `UINTMAX_MAX', `INTMAX_MAX', `INTMAX_MIN'. Note that there are no macros for unsigned integer minima. These are always zero. There are similar macros for use with C's built in integer types which should come with your C compiler. These are described in *Note Data Type Measurements::. Don't forget you can use the C `sizeof' function with any of these data types to get the number of bytes of storage each uses. Integer Division ================ This section describes functions for performing integer division. These functions are redundant when GNU CC is used, because in GNU C the `/' operator always rounds towards zero. But in other C implementations, `/' may round differently with negative arguments. `div' and `ldiv' are useful because they specify how to round the quotient: towards zero. The remainder has the same sign as the numerator. These functions are specified to return a result R such that the value `R.quot*DENOMINATOR + R.rem' equals NUMERATOR. To use these facilities, you should include the header file `stdlib.h' in your program. - Data Type: div_t This is a structure type used to hold the result returned by the `div' function. It has the following members: `int quot' The quotient from the division. `int rem' The remainder from the division. - Function: div_t div (int NUMERATOR, int DENOMINATOR) This function `div' computes the quotient and remainder from the division of NUMERATOR by DENOMINATOR, returning the result in a structure of type `div_t'. If the result cannot be represented (as in a division by zero), the behavior is undefined. Here is an example, albeit not a very useful one. div_t result; result = div (20, -6); Now `result.quot' is `-3' and `result.rem' is `2'. - Data Type: ldiv_t This is a structure type used to hold the result returned by the `ldiv' function. It has the following members: `long int quot' The quotient from the division. `long int rem' The remainder from the division. (This is identical to `div_t' except that the components are of type `long int' rather than `int'.) - Function: ldiv_t ldiv (long int NUMERATOR, long int DENOMINATOR) The `ldiv' function is similar to `div', except that the arguments are of type `long int' and the result is returned as a structure of type `ldiv_t'. - Data Type: lldiv_t This is a structure type used to hold the result returned by the `lldiv' function. It has the following members: `long long int quot' The quotient from the division. `long long int rem' The remainder from the division. (This is identical to `div_t' except that the components are of type `long long int' rather than `int'.) - Function: lldiv_t lldiv (long long int NUMERATOR, long long int DENOMINATOR) The `lldiv' function is like the `div' function, but the arguments are of type `long long int' and the result is returned as a structure of type `lldiv_t'. The `lldiv' function was added in ISO C99. - Data Type: imaxdiv_t This is a structure type used to hold the result returned by the `imaxdiv' function. It has the following members: `intmax_t quot' The quotient from the division. `intmax_t rem' The remainder from the division. (This is identical to `div_t' except that the components are of type `intmax_t' rather than `int'.) See *Note Integers:: for a description of the `intmax_t' type. - Function: imaxdiv_t imaxdiv (intmax_t NUMERATOR, intmax_t DENOMINATOR) The `imaxdiv' function is like the `div' function, but the arguments are of type `intmax_t' and the result is returned as a structure of type `imaxdiv_t'. See *Note Integers:: for a description of the `intmax_t' type. The `imaxdiv' function was added in ISO C99. Floating Point Numbers ====================== Most computer hardware has support for two different kinds of numbers: integers (...-3, -2, -1, 0, 1, 2, 3...) and floating-point numbers. Floating-point numbers have three parts: the "mantissa", the "exponent", and the "sign bit". The real number represented by a floating-point value is given by (s ? -1 : 1) * 2^e * M where s is the sign bit, e the exponent, and M the mantissa. *Note Floating Point Concepts::, for details. (It is possible to have a different "base" for the exponent, but all modern hardware uses 2.) Floating-point numbers can represent a finite subset of the real numbers. While this subset is large enough for most purposes, it is important to remember that the only reals that can be represented exactly are rational numbers that have a terminating binary expansion shorter than the width of the mantissa. Even simple fractions such as 1/5 can only be approximated by floating point. Mathematical operations and functions frequently need to produce values that are not representable. Often these values can be approximated closely enough for practical purposes, but sometimes they can't. Historically there was no way to tell when the results of a calculation were inaccurate. Modern computers implement the IEEE 754 standard for numerical computations, which defines a framework for indicating to the program when the results of calculation are not trustworthy. This framework consists of a set of "exceptions" that indicate why a result could not be represented, and the special values "infinity" and "not a number" (NaN). Floating-Point Number Classification Functions ============================================== ISO C99 defines macros that let you determine what sort of floating-point number a variable holds. - Macro: int fpclassify (_float-type_ X) This is a generic macro which works on all floating-point types and which returns a value of type `int'. The possible values are: `FP_NAN' The floating-point number X is "Not a Number" (*note Infinity and NaN::) `FP_INFINITE' The value of X is either plus or minus infinity (*note Infinity and NaN::) `FP_ZERO' The value of X is zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if X is negative zero. `FP_SUBNORMAL' Numbers whose absolute value is too small to be represented in the normal format are represented in an alternate, "denormalized" format (*note Floating Point Concepts::). This format is less precise but can represent values closer to zero. `fpclassify' returns this value for values of X in this alternate format. `FP_NORMAL' This value is returned for all other values of X. It indicates that there is nothing special about the number. `fpclassify' is most useful if more than one property of a number must be tested. There are more specific macros which only test one property at a time. Generally these macros execute faster than `fpclassify', since there is special hardware support for them. You should therefore use the specific macros whenever possible. - Macro: int isfinite (_float-type_ X) This macro returns a nonzero value if X is finite: not plus or minus infinity, and not NaN. It is equivalent to (fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE) `isfinite' is implemented as a macro which accepts any floating-point type. - Macro: int isnormal (_float-type_ X) This macro returns a nonzero value if X is finite and normalized. It is equivalent to (fpclassify (x) == FP_NORMAL) - Macro: int isnan (_float-type_ X) This macro returns a nonzero value if X is NaN. It is equivalent to (fpclassify (x) == FP_NAN) Another set of floating-point classification functions was provided by BSD. The GNU C library also supports these functions; however, we recommend that you use the ISO C99 macros in new code. Those are standard and will be available more widely. Also, since they are macros, you do not have to worry about the type of their argument. - Function: int isinf (double X) - Function: int isinff (float X) - Function: int isinfl (long double X) This function returns `-1' if X represents negative infinity, `1' if X represents positive infinity, and `0' otherwise. - Function: int isnan (double X) - Function: int isnanf (float X) - Function: int isnanl (long double X) This function returns a nonzero value if X is a "not a number" value, and zero otherwise. *Note:* The `isnan' macro defined by ISO C99 overrides the BSD function. This is normally not a problem, because the two routines behave identically. However, if you really need to get the BSD function for some reason, you can write (isnan) (x) - Function: int finite (double X) - Function: int finitef (float X) - Function: int finitel (long double X) This function returns a nonzero value if X is finite or a "not a number" value, and zero otherwise. *Portability Note:* The functions listed in this section are BSD extensions. Errors in Floating-Point Calculations ===================================== FP Exceptions ------------- The IEEE 754 standard defines five "exceptions" that can occur during a calculation. Each corresponds to a particular sort of error, such as overflow. When exceptions occur (when exceptions are "raised", in the language of the standard), one of two things can happen. By default the exception is simply noted in the floating-point "status word", and the program continues as if nothing had happened. The operation produces a default value, which depends on the exception (see the table below). Your program can check the status word to find out which exceptions happened. Alternatively, you can enable "traps" for exceptions. In that case, when an exception is raised, your program will receive the `SIGFPE' signal. The default action for this signal is to terminate the program. *Note Signal Handling::, for how you can change the effect of the signal. In the System V math library, the user-defined function `matherr' is called when certain exceptions occur inside math library functions. However, the Unix98 standard deprecates this interface. We support it for historical compatibility, but recommend that you do not use it in new programs. The exceptions defined in IEEE 754 are: `Invalid Operation' This exception is raised if the given operands are invalid for the operation to be performed. Examples are (see IEEE 754, section 7): 1. Addition or subtraction: oo - oo. (But oo + oo = oo). 2. Multiplication: 0 * oo. 3. Division: 0/0 or oo/oo. 4. Remainder: x REM y, where y is zero or x is infinite. 5. Square root if the operand is less then zero. More generally, any mathematical function evaluated outside its domain produces this exception. 6. Conversion of a floating-point number to an integer or decimal string, when the number cannot be represented in the target format (due to overflow, infinity, or NaN). 7. Conversion of an unrecognizable input string. 8. Comparison via predicates involving < or >, when one or other of the operands is NaN. You can prevent this exception by using the unordered comparison functions instead; see *Note FP Comparison Functions::. If the exception does not trap, the result of the operation is NaN. `Division by Zero' This exception is raised when a finite nonzero number is divided by zero. If no trap occurs the result is either +oo or -oo, depending on the signs of the operands. `Overflow' This exception is raised whenever the result cannot be represented as a finite value in the precision format of the destination. If no trap occurs the result depends on the sign of the intermediate result and the current rounding mode (IEEE 754, section 7.3): 1. Round to nearest carries all overflows to oo with the sign of the intermediate result. 2. Round toward 0 carries all overflows to the largest representable finite number with the sign of the intermediate result. 3. Round toward -oo carries positive overflows to the largest representable finite number and negative overflows to -oo. 4. Round toward oo carries negative overflows to the most negative representable finite number and positive overflows to oo. Whenever the overflow exception is raised, the inexact exception is also raised. `Underflow' The underflow exception is raised when an intermediate result is too small to be calculated accurately, or if the operation's result rounded to the destination precision is too small to be normalized. When no trap is installed for the underflow exception, underflow is signaled (via the underflow flag) only when both tininess and loss of accuracy have been detected. If no trap handler is installed the operation continues with an imprecise small value, or zero if the destination precision cannot hold the small exact result. `Inexact' This exception is signalled if a rounded result is not exact (such as when calculating the square root of two) or a result overflows without an overflow trap. Infinity and NaN ---------------- IEEE 754 floating point numbers can represent positive or negative infinity, and "NaN" (not a number). These three values arise from calculations whose result is undefined or cannot be represented accurately. You can also deliberately set a floating-point variable to any of them, which is sometimes useful. Some examples of calculations that produce infinity or NaN: 1/0 = oo log (0) = -oo sqrt (-1) = NaN When a calculation produces any of these values, an exception also occurs; see *Note FP Exceptions::. The basic operations and math functions all accept infinity and NaN and produce sensible output. Infinities propagate through calculations as one would expect: for example, 2 + oo = oo, 4/oo = 0, atan (oo) = pi/2. NaN, on the other hand, infects any calculation that involves it. Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN. In comparison operations, positive infinity is larger than all values except itself and NaN, and negative infinity is smaller than all values except itself and NaN. NaN is "unordered": it is not equal to, greater than, or less than anything, _including itself_. `x == x' is false if the value of `x' is NaN. You can use this to test whether a value is NaN or not, but the recommended way to test for NaN is with the `isnan' function (*note Floating Point Classes::). In addition, `<', `>', `<=', and `>=' will raise an exception when applied to NaNs. `math.h' defines macros that allow you to explicitly set a variable to infinity or NaN. - Macro: float INFINITY An expression representing positive infinity. It is equal to the value produced by mathematical operations like `1.0 / 0.0'. `-INFINITY' represents negative infinity. You can test whether a floating-point value is infinite by comparing it to this macro. However, this is not recommended; you should use the `isfinite' macro instead. *Note Floating Point Classes::. This macro was introduced in the ISO C99 standard. - Macro: float NAN An expression representing a value which is "not a number". This macro is a GNU extension, available only on machines that support the "not a number" value--that is to say, on all machines that support IEEE floating point. You can use `#ifdef NAN' to test whether the machine supports NaN. (Of course, you must arrange for GNU extensions to be visible, such as by defining `_GNU_SOURCE', and then you must include `math.h'.) IEEE 754 also allows for another unusual value: negative zero. This value is produced when you divide a positive number by negative infinity, or when a negative result is smaller than the limits of representation. Negative zero behaves identically to zero in all calculations, unless you explicitly test the sign bit with `signbit' or `copysign'. Examining the FPU status word ----------------------------- ISO C99 defines functions to query and manipulate the floating-point status word. You can use these functions to check for untrapped exceptions when it's convenient, rather than worrying about them in the middle of a calculation. These constants represent the various IEEE 754 exceptions. Not all FPUs report all the different exceptions. Each constant is defined if and only if the FPU you are compiling for supports that exception, so you can test for FPU support with `#ifdef'. They are defined in `fenv.h'. `FE_INEXACT' The inexact exception. `FE_DIVBYZERO' The divide by zero exception. `FE_UNDERFLOW' The underflow exception. `FE_OVERFLOW' The overflow exception. `FE_INVALID' The invalid exception. The macro `FE_ALL_EXCEPT' is the bitwise OR of all exception macros which are supported by the FP implementation. These functions allow you to clear exception flags, test for exceptions, and save and restore the set of exceptions flagged. - Function: int feclearexcept (int EXCEPTS) This function clears all of the supported exception flags indicated by EXCEPTS. The function returns zero in case the operation was successful, a non-zero value otherwise. - Function: int feraiseexcept (int EXCEPTS) This function raises the supported exceptions indicated by EXCEPTS. If more than one exception bit in EXCEPTS is set the order in which the exceptions are raised is undefined except that overflow (`FE_OVERFLOW') or underflow (`FE_UNDERFLOW') are raised before inexact (`FE_INEXACT'). Whether for overflow or underflow the inexact exception is also raised is also implementation dependent. The function returns zero in case the operation was successful, a non-zero value otherwise. - Function: int fetestexcept (int EXCEPTS) Test whether the exception flags indicated by the parameter EXCEPT are currently set. If any of them are, a nonzero value is returned which specifies which exceptions are set. Otherwise the result is zero. To understand these functions, imagine that the status word is an integer variable named STATUS. `feclearexcept' is then equivalent to `status &= ~excepts' and `fetestexcept' is equivalent to `(status & excepts)'. The actual implementation may be very different, of course. Exception flags are only cleared when the program explicitly requests it, by calling `feclearexcept'. If you want to check for exceptions from a set of calculations, you should clear all the flags first. Here is a simple example of the way to use `fetestexcept': { double f; int raised; feclearexcept (FE_ALL_EXCEPT); f = compute (); raised = fetestexcept (FE_OVERFLOW | FE_INVALID); if (raised & FE_OVERFLOW) { /* ... */ } if (raised & FE_INVALID) { /* ... */ } /* ... */ } You cannot explicitly set bits in the status word. You can, however, save the entire status word and restore it later. This is done with the following functions: - Function: int fegetexceptflag (fexcept_t *FLAGP, int EXCEPTS) This function stores in the variable pointed to by FLAGP an implementation-defined value representing the current setting of the exception flags indicated by EXCEPTS. The function returns zero in case the operation was successful, a non-zero value otherwise. - Function: int fesetexceptflag (const fexcept_t *FLAGP, int EXCEPTS) This function restores the flags for the exceptions indicated by EXCEPTS to the values stored in the variable pointed to by FLAGP. The function returns zero in case the operation was successful, a non-zero value otherwise. Note that the value stored in `fexcept_t' bears no resemblance to the bit mask returned by `fetestexcept'. The type may not even be an integer. Do not attempt to modify an `fexcept_t' variable. Error Reporting by Mathematical Functions ----------------------------------------- Many of the math functions are defined only over a subset of the real or complex numbers. Even if they are mathematically defined, their result may be larger or smaller than the range representable by their return type. These are known as "domain errors", "overflows", and "underflows", respectively. Math functions do several things when one of these errors occurs. In this manual we will refer to the complete response as "signalling" a domain error, overflow, or underflow. When a math function suffers a domain error, it raises the invalid exception and returns NaN. It also sets ERRNO to `EDOM'; this is for compatibility with old systems that do not support IEEE 754 exception handling. Likewise, when overflow occurs, math functions raise the overflow exception and return oo or -oo as appropriate. They also set ERRNO to `ERANGE'. When underflow occurs, the underflow exception is raised, and zero (appropriately signed) is returned. ERRNO may be set to `ERANGE', but this is not guaranteed. Some of the math functions are defined mathematically to result in a complex value over parts of their domains. The most familiar example of this is taking the square root of a negative number. The complex math functions, such as `csqrt', will return the appropriate complex value in this case. The real-valued functions, such as `sqrt', will signal a domain error. Some older hardware does not support infinities. On that hardware, overflows instead return a particular very large number (usually the largest representable number). `math.h' defines macros you can use to test for overflow on both old and new hardware. - Macro: double HUGE_VAL - Macro: float HUGE_VALF - Macro: long double HUGE_VALL An expression representing a particular very large number. On machines that use IEEE 754 floating point format, `HUGE_VAL' is infinity. On other machines, it's typically the largest positive number that can be represented. Mathematical functions return the appropriately typed version of `HUGE_VAL' or `-HUGE_VAL' when the result is too large to be represented. Rounding Modes ============== Floating-point calculations are carried out internally with extra precision, and then rounded to fit into the destination type. This ensures that results are as precise as the input data. IEEE 754 defines four possible rounding modes: Round to nearest. This is the default mode. It should be used unless there is a specific need for one of the others. In this mode results are rounded to the nearest representable value. If the result is midway between two representable values, the even representable is chosen. "Even" here means the lowest-order bit is zero. This rounding mode prevents statistical bias and guarantees numeric stability: round-off errors in a lengthy calculation will remain smaller than half of `FLT_EPSILON'. Round toward plus Infinity. All results are rounded to the smallest representable value which is greater than the result. Round toward minus Infinity. All results are rounded to the largest representable value which is less than the result. Round toward zero. All results are rounded to the largest representable value whose magnitude is less than that of the result. In other words, if the result is negative it is rounded up; if it is positive, it is rounded down. `fenv.h' defines constants which you can use to refer to the various rounding modes. Each one will be defined if and only if the FPU supports the corresponding rounding mode. `FE_TONEAREST' Round to nearest. `FE_UPWARD' Round toward +oo. `FE_DOWNWARD' Round toward -oo. `FE_TOWARDZERO' Round toward zero. Underflow is an unusual case. Normally, IEEE 754 floating point numbers are always normalized (*note Floating Point Concepts::). Numbers smaller than 2^r (where r is the minimum exponent, `FLT_MIN_RADIX-1' for FLOAT) cannot be represented as normalized numbers. Rounding all such numbers to zero or 2^r would cause some algorithms to fail at 0. Therefore, they are left in denormalized form. That produces loss of precision, since some bits of the mantissa are stolen to indicate the decimal point. If a result is too small to be represented as a denormalized number, it is rounded to zero. However, the sign of the result is preserved; if the calculation was negative, the result is "negative zero". Negative zero can also result from some operations on infinity, such as 4/-oo. Negative zero behaves identically to zero except when the `copysign' or `signbit' functions are used to check the sign bit directly. At any time one of the above four rounding modes is selected. You can find out which one with this function: - Function: int fegetround (void) Returns the currently selected rounding mode, represented by one of the values of the defined rounding mode macros. To change the rounding mode, use this function: - Function: int fesetround (int ROUND) Changes the currently selected rounding mode to ROUND. If ROUND does not correspond to one of the supported rounding modes nothing is changed. `fesetround' returns zero if it changed the rounding mode, a nonzero value if the mode is not supported. You should avoid changing the rounding mode if possible. It can be an expensive operation; also, some hardware requires you to compile your program differently for it to work. The resulting code may run slower. See your compiler documentation for details. Floating-Point Control Functions ================================ IEEE 754 floating-point implementations allow the programmer to decide whether traps will occur for each of the exceptions, by setting bits in the "control word". In C, traps result in the program receiving the `SIGFPE' signal; see *Note Signal Handling::. *Note:* IEEE 754 says that trap handlers are given details of the exceptional situation, and can set the result value. C signals do not provide any mechanism to pass this information back and forth. Trapping exceptions in C is therefore not very useful. It is sometimes necessary to save the state of the floating-point unit while you perform some calculation. The library provides functions which save and restore the exception flags, the set of exceptions that generate traps, and the rounding mode. This information is known as the "floating-point environment". The functions to save and restore the floating-point environment all use a variable of type `fenv_t' to store information. This type is defined in `fenv.h'. Its size and contents are implementation-defined. You should not attempt to manipulate a variable of this type directly. To save the state of the FPU, use one of these functions: - Function: int fegetenv (fenv_t *ENVP) Store the floating-point environment in the variable pointed to by ENVP. The function returns zero in case the operation was successful, a non-zero value otherwise. - Function: int feholdexcept (fenv_t *ENVP) Store the current floating-point environment in the object pointed to by ENVP. Then clear all exception flags, and set the FPU to trap no exceptions. Not all FPUs support trapping no exceptions; if `feholdexcept' cannot set this mode, it returns nonzero value. If it succeeds, it returns zero. The functions which restore the floating-point environment can take these kinds of arguments: * Pointers to `fenv_t' objects, which were initialized previously by a call to `fegetenv' or `feholdexcept'. * The special macro `FE_DFL_ENV' which represents the floating-point environment as it was available at program start. * Implementation defined macros with names starting with `FE_' and having type `fenv_t *'. If possible, the GNU C Library defines a macro `FE_NOMASK_ENV' which represents an environment where every exception raised causes a trap to occur. You can test for this macro using `#ifdef'. It is only defined if `_GNU_SOURCE' is defined. Some platforms might define other predefined environments. To set the floating-point environment, you can use either of these functions: - Function: int fesetenv (const fenv_t *ENVP) Set the floating-point environment to that described by ENVP. The function returns zero in case the operation was successful, a non-zero value otherwise. - Function: int feupdateenv (const fenv_t *ENVP) Like `fesetenv', this function sets the floating-point environment to that described by ENVP. However, if any exceptions were flagged in the status word before `feupdateenv' was called, they remain flagged after the call. In other words, after `feupdateenv' is called, the status word is the bitwise OR of the previous status word and the one saved in ENVP. The function returns zero in case the operation was successful, a non-zero value otherwise. To control for individual exceptions if raising them causes a trap to occur, you can use the following two functions. *Portability Note:* These functions are all GNU extensions. - Function: int feenableexcept (int EXCEPTS) This functions enables traps for each of the exceptions as indicated by the parameter EXCEPT. The individual excepetions are described in *Note Status bit operations::. Only the specified exceptions are enabled, the status of the other exceptions is not changed. The function returns the previous enabled exceptions in case the operation was successful, `-1' otherwise. - Function: int fedisableexcept (int EXCEPTS) This functions disables traps for each of the exceptions as indicated by the parameter EXCEPT. The individual excepetions are described in *Note Status bit operations::. Only the specified exceptions are disabled, the status of the other exceptions is not changed. The function returns the previous enabled exceptions in case the operation was successful, `-1' otherwise. - Function: int fegetexcept (int EXCEPTS) The function returns a bitmask of all currently enabled exceptions. It returns `-1' in case of failure. Arithmetic Functions ==================== The C library provides functions to do basic operations on floating-point numbers. These include absolute value, maximum and minimum, normalization, bit twiddling, rounding, and a few others. Absolute Value -------------- These functions are provided for obtaining the "absolute value" (or "magnitude") of a number. The absolute value of a real number X is X if X is positive, -X if X is negative. For a complex number Z, whose real part is X and whose imaginary part is Y, the absolute value is `sqrt (X*X + Y*Y)'. Prototypes for `abs', `labs' and `llabs' are in `stdlib.h'; `imaxabs' is declared in `inttypes.h'; `fabs', `fabsf' and `fabsl' are declared in `math.h'. `cabs', `cabsf' and `cabsl' are declared in `complex.h'. - Function: int abs (int NUMBER) - Function: long int labs (long int NUMBER) - Function: long long int llabs (long long int NUMBER) - Function: intmax_t imaxabs (intmax_t NUMBER) These functions return the absolute value of NUMBER. Most computers use a two's complement integer representation, in which the absolute value of `INT_MIN' (the smallest possible `int') cannot be represented; thus, `abs (INT_MIN)' is not defined. `llabs' and `imaxdiv' are new to ISO C99. See *Note Integers:: for a description of the `intmax_t' type. - Function: double fabs (double NUMBER) - Function: float fabsf (float NUMBER) - Function: long double fabsl (long double NUMBER) This function returns the absolute value of the floating-point number NUMBER. - Function: double cabs (complex double Z) - Function: float cabsf (complex float Z) - Function: long double cabsl (complex long double Z) These functions return the absolute value of the complex number Z (*note Complex Numbers::). The absolute value of a complex number is: sqrt (creal (Z) * creal (Z) + cimag (Z) * cimag (Z)) This function should always be used instead of the direct formula because it takes special care to avoid losing precision. It may also take advantage of hardware support for this operation. See `hypot' in *Note Exponents and Logarithms::. Normalization Functions ----------------------- The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see *Note Floating Point Concepts::. These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases. All these functions are declared in `math.h'. - Function: double frexp (double VALUE, int *EXPONENT) - Function: float frexpf (float VALUE, int *EXPONENT) - Function: long double frexpl (long double VALUE, int *EXPONENT) These functions are used to split the number VALUE into a normalized fraction and an exponent. If the argument VALUE is not zero, the return value is VALUE times a power of two, and is always in the range 1/2 (inclusive) to 1 (exclusive). The corresponding exponent is stored in `*EXPONENT'; the return value multiplied by 2 raised to this exponent equals the original number VALUE. For example, `frexp (12.8, &exponent)' returns `0.8' and stores `4' in `exponent'. If VALUE is zero, then the return value is zero and zero is stored in `*EXPONENT'. - Function: double ldexp (double VALUE, int EXPONENT) - Function: float ldexpf (float VALUE, int EXPONENT) - Function: long double ldexpl (long double VALUE, int EXPONENT) These functions return the result of multiplying the floating-point number VALUE by 2 raised to the power EXPONENT. (It can be used to reassemble floating-point numbers that were taken apart by `frexp'.) For example, `ldexp (0.8, 4)' returns `12.8'. The following functions, which come from BSD, provide facilities equivalent to those of `ldexp' and `frexp'. See also the ISO C function `logb' which originally also appeared in BSD. - Function: double scalb (double VALUE, int EXPONENT) - Function: float scalbf (float VALUE, int EXPONENT) - Function: long double scalbl (long double VALUE, int EXPONENT) The `scalb' function is the BSD name for `ldexp'. - Function: long long int scalbn (double X, int n) - Function: long long int scalbnf (float X, int n) - Function: long long int scalbnl (long double X, int n) `scalbn' is identical to `scalb', except that the exponent N is an `int' instead of a floating-point number. - Function: long long int scalbln (double X, long int n) - Function: long long int scalblnf (float X, long int n) - Function: long long int scalblnl (long double X, long int n) `scalbln' is identical to `scalb', except that the exponent N is a `long int' instead of a floating-point number. - Function: long long int significand (double X) - Function: long long int significandf (float X) - Function: long long int significandl (long double X) `significand' returns the mantissa of X scaled to the range [1, 2). It is equivalent to `scalb (X, (double) -ilogb (X))'. This function exists mainly for use in certain standardized tests of IEEE 754 conformance. Rounding Functions ------------------ The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating point numbers to integer values. They are all declared in `math.h'. You can also convert floating-point numbers to integers simply by casting them to `int'. This discards the fractional part, effectively rounding towards zero. However, this only works if the result can actually be represented as an `int'--for very large numbers, this is impossible. The functions listed here return the result as a `double' instead to get around this problem. - Function: double ceil (double X) - Function: float ceilf (float X) - Function: long double ceill (long double X) These functions round X upwards to the nearest integer, returning that value as a `double'. Thus, `ceil (1.5)' is `2.0'. - Function: double floor (double X) - Function: float floorf (float X) - Function: long double floorl (long double X) These functions round X downwards to the nearest integer, returning that value as a `double'. Thus, `floor (1.5)' is `1.0' and `floor (-1.5)' is `-2.0'. - Function: double trunc (double X) - Function: float truncf (float X) - Function: long double truncl (long double X) The `trunc' functions round X towards zero to the nearest integer (returned in floating-point format). Thus, `trunc (1.5)' is `1.0' and `trunc (-1.5)' is `-1.0'. - Function: double rint (double X) - Function: float rintf (float X) - Function: long double rintl (long double X) These functions round X to an integer value according to the current rounding mode. *Note Floating Point Parameters::, for information about the various rounding modes. The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicitly select another. If X was not initially an integer, these functions raise the inexact exception. - Function: double nearbyint (double X) - Function: float nearbyintf (float X) - Function: long double nearbyintl (long double X) These functions return the same value as the `rint' functions, but do not raise the inexact exception if X is not an integer. - Function: double round (double X) - Function: float roundf (float X) - Function: long double roundl (long double X) These functions are similar to `rint', but they round halfway cases away from zero instead of to the nearest even integer. - Function: long int lrint (double X) - Function: long int lrintf (float X) - Function: long int lrintl (long double X) These functions are just like `rint', but they return a `long int' instead of a floating-point number. - Function: long long int llrint (double X) - Function: long long int llrintf (float X) - Function: long long int llrintl (long double X) These functions are just like `rint', but they return a `long long int' instead of a floating-point number. - Function: long int lround (double X) - Function: long int lroundf (float X) - Function: long int lroundl (long double X) These functions are just like `round', but they return a `long int' instead of a floating-point number. - Function: long long int llround (double X) - Function: long long int llroundf (float X) - Function: long long int llroundl (long double X) These functions are just like `round', but they return a `long long int' instead of a floating-point number. - Function: double modf (double VALUE, double *INTEGER-PART) - Function: float modff (float VALUE, float *INTEGER-PART) - Function: long double modfl (long double VALUE, long double *INTEGER-PART) These functions break the argument VALUE into an integer part and a fractional part (between `-1' and `1', exclusive). Their sum equals VALUE. Each of the parts has the same sign as VALUE, and the integer part is always rounded toward zero. `modf' stores the integer part in `*INTEGER-PART', and returns the fractional part. For example, `modf (2.5, &intpart)' returns `0.5' and stores `2.0' into `intpart'. Remainder Functions ------------------- The functions in this section compute the remainder on division of two floating-point numbers. Each is a little different; pick the one that suits your problem. - Function: double fmod (double NUMERATOR, double DENOMINATOR) - Function: float fmodf (float NUMERATOR, float DENOMINATOR) - Function: long double fmodl (long double NUMERATOR, long double DENOMINATOR) These functions compute the remainder from the division of NUMERATOR by DENOMINATOR. Specifically, the return value is `NUMERATOR - N * DENOMINATOR', where N is the quotient of NUMERATOR divided by DENOMINATOR, rounded towards zero to an integer. Thus, `fmod (6.5, 2.3)' returns `1.9', which is `6.5' minus `4.6'. The result has the same sign as the NUMERATOR and has magnitude less than the magnitude of the DENOMINATOR. If DENOMINATOR is zero, `fmod' signals a domain error. - Function: double drem (double NUMERATOR, double DENOMINATOR) - Function: float dremf (float NUMERATOR, float DENOMINATOR) - Function: long double dreml (long double NUMERATOR, long double DENOMINATOR) These functions are like `fmod' except that they rounds the internal quotient N to the nearest integer instead of towards zero to an integer. For example, `drem (6.5, 2.3)' returns `-0.4', which is `6.5' minus `6.9'. The absolute value of the result is less than or equal to half the absolute value of the DENOMINATOR. The difference between `fmod (NUMERATOR, DENOMINATOR)' and `drem (NUMERATOR, DENOMINATOR)' is always either DENOMINATOR, minus DENOMINATOR, or zero. If DENOMINATOR is zero, `drem' signals a domain error. - Function: double remainder (double NUMERATOR, double DENOMINATOR) - Function: float remainderf (float NUMERATOR, float DENOMINATOR) - Function: long double remainderl (long double NUMERATOR, long double DENOMINATOR) This function is another name for `drem'. Setting and modifying single bits of FP values ---------------------------------------------- There are some operations that are too complicated or expensive to perform by hand on floating-point numbers. ISO C99 defines functions to do these operations, which mostly involve changing single bits. - Function: double copysign (double X, double Y) - Function: float copysignf (float X, float Y) - Function: long double copysignl (long double X, long double Y) These functions return X but with the sign of Y. They work even if X or Y are NaN or zero. Both of these can carry a sign (although not all implementations support it) and this is one of the few operations that can tell the difference. `copysign' never raises an exception. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854). - Function: int signbit (_float-type_ X) `signbit' is a generic macro which can work on all floating-point types. It returns a nonzero value if the value of X has its sign bit set. This is not the same as `x < 0.0', because IEEE 754 floating point allows zero to be signed. The comparison `-0.0 < 0.0' is false, but `signbit (-0.0)' will return a nonzero value. - Function: double nextafter (double X, double Y) - Function: float nextafterf (float X, float Y) - Function: long double nextafterl (long double X, long double Y) The `nextafter' function returns the next representable neighbor of X in the direction towards Y. The size of the step between X and the result depends on the type of the result. If X = Y the function simply returns Y. If either value is `NaN', `NaN' is returned. Otherwise a value corresponding to the value of the least significant bit in the mantissa is added or subtracted, depending on the direction. `nextafter' will signal overflow or underflow if the result goes outside of the range of normalized numbers. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854). - Function: double nexttoward (double X, long double Y) - Function: float nexttowardf (float X, long double Y) - Function: long double nexttowardl (long double X, long double Y) These functions are identical to the corresponding versions of `nextafter' except that their second argument is a `long double'. - Function: double nan (const char *TAGP) - Function: float nanf (const char *TAGP) - Function: long double nanl (const char *TAGP) The `nan' function returns a representation of NaN, provided that NaN is supported by the target platform. `nan ("N-CHAR-SEQUENCE")' is equivalent to `strtod ("NAN(N-CHAR-SEQUENCE)")'. The argument TAGP is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and TAGP selects one. On other systems it may do nothing. Floating-Point Comparison Functions ----------------------------------- The standard C comparison operators provoke exceptions when one or other of the operands is NaN. For example, int v = a < 1.0; will raise an exception if A is NaN. (This does _not_ happen with `==' and `!='; those merely return false and true, respectively, when NaN is examined.) Frequently this exception is undesirable. ISO C99 therefore defines comparison functions that do not raise exceptions when NaN is examined. All of the functions are implemented as macros which allow their arguments to be of any floating-point type. The macros are guaranteed to evaluate their arguments only once. - Macro: int isgreater (_real-floating_ X, _real-floating_ Y) This macro determines whether the argument X is greater than Y. It is equivalent to `(X) > (Y)', but no exception is raised if X or Y are NaN. - Macro: int isgreaterequal (_real-floating_ X, _real-floating_ Y) This macro determines whether the argument X is greater than or equal to Y. It is equivalent to `(X) >= (Y)', but no exception is raised if X or Y are NaN. - Macro: int isless (_real-floating_ X, _real-floating_ Y) This macro determines whether the argument X is less than Y. It is equivalent to `(X) < (Y)', but no exception is raised if X or Y are NaN. - Macro: int islessequal (_real-floating_ X, _real-floating_ Y) This macro determines whether the argument X is less than or equal to Y. It is equivalent to `(X) <= (Y)', but no exception is raised if X or Y are NaN. - Macro: int islessgreater (_real-floating_ X, _real-floating_ Y) This macro determines whether the argument X is less or greater than Y. It is equivalent to `(X) < (Y) || (X) > (Y)' (although it only evaluates X and Y once), but no exception is raised if X or Y are NaN. This macro is not equivalent to `X != Y', because that expression is true if X or Y are NaN. - Macro: int isunordered (_real-floating_ X, _real-floating_ Y) This macro determines whether its arguments are unordered. In other words, it is true if X or Y are NaN, and false otherwise. Not all machines provide hardware support for these operations. On machines that don't, the macros can be very slow. Therefore, you should not use these functions when NaN is not a concern. *Note:* There are no macros `isequal' or `isunequal'. They are unnecessary, because the `==' and `!=' operators do _not_ throw an exception if one or both of the operands are NaN. Miscellaneous FP arithmetic functions ------------------------------------- The functions in this section perform miscellaneous but common operations that are awkward to express with C operators. On some processors these functions can use special machine instructions to perform these operations faster than the equivalent C code. - Function: double fmin (double X, double Y) - Function: float fminf (float X, float Y) - Function: long double fminl (long double X, long double Y) The `fmin' function returns the lesser of the two values X and Y. It is similar to the expression ((x) < (y) ? (x) : (y)) except that X and Y are only evaluated once. If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned. - Function: double fmax (double X, double Y) - Function: float fmaxf (float X, float Y) - Function: long double fmaxl (long double X, long double Y) The `fmax' function returns the greater of the two values X and Y. If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned. - Function: double fdim (double X, double Y) - Function: float fdimf (float X, float Y) - Function: long double fdiml (long double X, long double Y) The `fdim' function returns the positive difference between X and Y. The positive difference is X - Y if X is greater than Y, and 0 otherwise. If X, Y, or both are NaN, NaN is returned. - Function: double fma (double X, double Y, double Z) - Function: float fmaf (float X, float Y, float Z) - Function: long double fmal (long double X, long double Y, long double Z) The `fma' function performs floating-point multiply-add. This is the operation (X * Y) + Z, but the intermediate result is not rounded to the destination type. This can sometimes improve the precision of a calculation. This function was introduced because some processors have a special instruction to perform multiply-add. The C compiler cannot use it directly, because the expression `x*y + z' is defined to round the intermediate result. `fma' lets you choose when you want to round only once. On processors which do not implement multiply-add in hardware, `fma' can be very slow since it must avoid intermediate rounding. `math.h' defines the symbols `FP_FAST_FMA', `FP_FAST_FMAF', and `FP_FAST_FMAL' when the corresponding version of `fma' is no slower than the expression `x*y + z'. In the GNU C library, this always means the operation is implemented in hardware. Complex Numbers =============== ISO C99 introduces support for complex numbers in C. This is done with a new type qualifier, `complex'. It is a keyword if and only if `complex.h' has been included. There are three complex types, corresponding to the three real types: `float complex', `double complex', and `long double complex'. To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, `complex.h' defines two macros that can be used to create complex numbers. - Macro: const float complex _Complex_I This macro is a representation of the complex number "0+1i". Multiplying a real floating-point value by `_Complex_I' gives a complex number whose value is purely imaginary. You can use this to construct complex constants: 3.0 + 4.0i = `3.0 + 4.0 * _Complex_I' Note that `_Complex_I * _Complex_I' has the value `-1', but the type of that value is `complex'. `_Complex_I' is a bit of a mouthful. `complex.h' also defines a shorter name for the same constant. - Macro: const float complex I This macro has exactly the same value as `_Complex_I'. Most of the time it is preferable. However, it causes problems if you want to use the identifier `I' for something else. You can safely write #include #undef I if you need `I' for your own purposes. (In that case we recommend you also define some other short name for `_Complex_I', such as `J'.) Projections, Conjugates, and Decomposing of Complex Numbers =========================================================== ISO C99 also defines functions that perform basic operations on complex numbers, such as decomposition and conjugation. The prototypes for all these functions are in `complex.h'. All functions are available in three variants, one for each of the three complex types. - Function: double creal (complex double Z) - Function: float crealf (complex float Z) - Function: long double creall (complex long double Z) These functions return the real part of the complex number Z. - Function: double cimag (complex double Z) - Function: float cimagf (complex float Z) - Function: long double cimagl (complex long double Z) These functions return the imaginary part of the complex number Z. - Function: complex double conj (complex double Z) - Function: complex float conjf (complex float Z) - Function: complex long double conjl (complex long double Z) These functions return the conjugate value of the complex number Z. The conjugate of a complex number has the same real part and a negated imaginary part. In other words, `conj(a + bi) = a + -bi'. - Function: double carg (complex double Z) - Function: float cargf (complex float Z) - Function: long double cargl (complex long double Z) These functions return the argument of the complex number Z. The argument of a complex number is the angle in the complex plane between the positive real axis and a line passing through zero and the number. This angle is measured in the usual fashion and ranges from 0 to 2pi. `carg' has a branch cut along the positive real axis. - Function: complex double cproj (complex double Z) - Function: complex float cprojf (complex float Z) - Function: complex long double cprojl (complex long double Z) These functions return the projection of the complex value Z onto the Riemann sphere. Values with a infinite imaginary part are projected to positive infinity on the real axis, even if the real part is NaN. If the real part is infinite, the result is equivalent to INFINITY + I * copysign (0.0, cimag (z)) Parsing of Numbers ================== This section describes functions for "reading" integer and floating-point numbers from a string. It may be more convenient in some cases to use `sscanf' or one of the related functions; see *Note Formatted Input::. But often you can make a program more robust by finding the tokens in the string by hand, then converting the numbers one by one. Parsing of Integers ------------------- The `str' functions are declared in `stdlib.h' and those beginning with `wcs' are declared in `wchar.h'. One might wonder about the use of `restrict' in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well. - Function: long int strtol (const char *restrict STRING, char **restrict TAILPTR, int BASE) The `strtol' ("string-to-long") function converts the initial part of STRING to a signed integer, which is returned as a value of type `long int'. This function attempts to decompose STRING as follows: * A (possibly empty) sequence of whitespace characters. Which characters are whitespace is determined by the `isspace' function (*note Classification of Characters::). These are discarded. * An optional plus or minus sign (`+' or `-'). * A nonempty sequence of digits in the radix specified by BASE. If BASE is zero, decimal radix is assumed unless the series of digits begins with `0' (specifying octal radix), or `0x' or `0X' (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C. Otherwise BASE must have a value between `2' and `36'. If BASE is `16', the digits may optionally be preceded by `0x' or `0X'. If base has no legal value the value returned is `0l' and the global variable `errno' is set to `EINVAL'. * Any remaining characters in the string. If TAILPTR is not a null pointer, `strtol' stores a pointer to this tail in `*TAILPTR'. If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for an integer in the specified BASE, no conversion is performed. In this case, `strtol' returns a value of zero and the value stored in `*TAILPTR' is the value of STRING. In a locale other than the standard `"C"' locale, this function may recognize additional implementation-dependent syntax. If the string has valid syntax for an integer but the value is not representable because of overflow, `strtol' returns either `LONG_MAX' or `LONG_MIN' (*note Range of Type::), as appropriate for the sign of the value. It also sets `errno' to `ERANGE' to indicate there was overflow. You should not check for errors by examining the return value of `strtol', because the string might be a valid representation of `0l', `LONG_MAX', or `LONG_MIN'. Instead, check whether TAILPTR points to what you expect after the number (e.g. `'\0'' if the string should end after the number). You also need to clear ERRNO before the call and check it afterward, in case there was overflow. There is an example at the end of this section. - Function: long int wcstol (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstol' function is equivalent to the `strtol' function in nearly all aspects but handles wide character strings. The `wcstol' function was introduced in Amendment 1 of ISO C90. - Function: unsigned long int strtoul (const char *retrict STRING, char **restrict TAILPTR, int BASE) The `strtoul' ("string-to-unsigned-long") function is like `strtol' except it converts to an `unsigned long int' value. The syntax is the same as described above for `strtol'. The value returned on overflow is `ULONG_MAX' (*note Range of Type::). If STRING depicts a negative number, `strtoul' acts the same as STRTOL but casts the result to an unsigned integer. That means for example that `strtoul' on `"-1"' returns `ULONG_MAX' and an input more negative than `LONG_MIN' returns (`ULONG_MAX' + 1) / 2. `strtoul' sets ERRNO to `EINVAL' if BASE is out of range, or `ERANGE' on overflow. - Function: unsigned long int wcstoul (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoul' function is equivalent to the `strtoul' function in nearly all aspects but handles wide character strings. The `wcstoul' function was introduced in Amendment 1 of ISO C90. - Function: long long int strtoll (const char *restrict STRING, char **restrict TAILPTR, int BASE) The `strtoll' function is like `strtol' except that it returns a `long long int' value, and accepts numbers with a correspondingly larger range. If the string has valid syntax for an integer but the value is not representable because of overflow, `strtoll' returns either `LONG_LONG_MAX' or `LONG_LONG_MIN' (*note Range of Type::), as appropriate for the sign of the value. It also sets `errno' to `ERANGE' to indicate there was overflow. The `strtoll' function was introduced in ISO C99. - Function: long long int wcstoll (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoll' function is equivalent to the `strtoll' function in nearly all aspects but handles wide character strings. The `wcstoll' function was introduced in Amendment 1 of ISO C90. - Function: long long int strtoq (const char *restrict STRING, char **restrict TAILPTR, int BASE) `strtoq' ("string-to-quad-word") is the BSD name for `strtoll'. - Function: long long int wcstoq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoq' function is equivalent to the `strtoq' function in nearly all aspects but handles wide character strings. The `wcstoq' function is a GNU extension. - Function: unsigned long long int strtoull (const char *restrict STRING, char **restrict TAILPTR, int BASE) The `strtoull' function is related to `strtoll' the same way `strtoul' is related to `strtol'. The `strtoull' function was introduced in ISO C99. - Function: unsigned long long int wcstoull (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoull' function is equivalent to the `strtoull' function in nearly all aspects but handles wide character strings. The `wcstoull' function was introduced in Amendment 1 of ISO C90. - Function: unsigned long long int strtouq (const char *restrict STRING, char **restrict TAILPTR, int BASE) `strtouq' is the BSD name for `strtoull'. - Function: unsigned long long int wcstouq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstouq' function is equivalent to the `strtouq' function in nearly all aspects but handles wide character strings. The `wcstoq' function is a GNU extension. - Function: intmax_t strtoimax (const char *restrict STRING, char **restrict TAILPTR, int BASE) The `strtoimax' function is like `strtol' except that it returns a `intmax_t' value, and accepts numbers of a corresponding range. If the string has valid syntax for an integer but the value is not representable because of overflow, `strtoimax' returns either `INTMAX_MAX' or `INTMAX_MIN' (*note Integers::), as appropriate for the sign of the value. It also sets `errno' to `ERANGE' to indicate there was overflow. See *Note Integers:: for a description of the `intmax_t' type. The `strtoimax' function was introduced in ISO C99. - Function: intmax_t wcstoimax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoimax' function is equivalent to the `strtoimax' function in nearly all aspects but handles wide character strings. The `wcstoimax' function was introduced in ISO C99. - Function: uintmax_t strtoumax (const char *restrict STRING, char **restrict TAILPTR, int BASE) The `strtoumax' function is related to `strtoimax' the same way that `strtoul' is related to `strtol'. See *Note Integers:: for a description of the `intmax_t' type. The `strtoumax' function was introduced in ISO C99. - Function: uintmax_t wcstoumax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE) The `wcstoumax' function is equivalent to the `strtoumax' function in nearly all aspects but handles wide character strings. The `wcstoumax' function was introduced in ISO C99. - Function: long int atol (const char *STRING) This function is similar to the `strtol' function with a BASE argument of `10', except that it need not detect overflow errors. The `atol' function is provided mostly for compatibility with existing code; using `strtol' is more robust. - Function: int atoi (const char *STRING) This function is like `atol', except that it returns an `int'. The `atoi' function is also considered obsolete; use `strtol' instead. - Function: long long int atoll (const char *STRING) This function is similar to `atol', except it returns a `long long int'. The `atoll' function was introduced in ISO C99. It too is obsolete (despite having just been added); use `strtoll' instead. All the functions mentioned in this section so far do not handle alternative representations of characters as described in the locale data. Some locales specify thousands separator and the way they have to be used which can help to make large numbers more readable. To read such numbers one has to use the `scanf' functions with the `'' flag. Here is a function which parses a string as a sequence of integers and returns the sum of them: int sum_ints_from_string (char *string) { int sum = 0; while (1) { char *tail; int next; /* Skip whitespace by hand, to detect the end. */ while (isspace (*string)) string++; if (*string == 0) break; /* There is more nonwhitespace, */ /* so it ought to be another number. */ errno = 0; /* Parse it. */ next = strtol (string, &tail, 0); /* Add it in, if not overflow. */ if (errno) printf ("Overflow\n"); else sum += next; /* Advance past it. */ string = tail; } return sum; } Parsing of Floats ----------------- The `str' functions are declared in `stdlib.h' and those beginning with `wcs' are declared in `wchar.h'. One might wonder about the use of `restrict' in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well. - Function: double strtod (const char *restrict STRING, char **restrict TAILPTR) The `strtod' ("string-to-double") function converts the initial part of STRING to a floating-point number, which is returned as a value of type `double'. This function attempts to decompose STRING as follows: * A (possibly empty) sequence of whitespace characters. Which characters are whitespace is determined by the `isspace' function (*note Classification of Characters::). These are discarded. * An optional plus or minus sign (`+' or `-'). * A floating point number in decimal or hexadecimal format. The decimal format is: - A nonempty sequence of digits optionally containing a decimal-point character--normally `.', but it depends on the locale (*note General Numeric::). - An optional exponent part, consisting of a character `e' or `E', an optional sign, and a sequence of digits. The hexadecimal format is as follows: - A 0x or 0X followed by a nonempty sequence of hexadecimal digits optionally containing a decimal-point character--normally `.', but it depends on the locale (*note General Numeric::). - An optional binary-exponent part, consisting of a character `p' or `P', an optional sign, and a sequence of digits. * Any remaining characters in the string. If TAILPTR is not a null pointer, a pointer to this tail of the string is stored in `*TAILPTR'. If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for a floating-point number, no conversion is performed. In this case, `strtod' returns a value of zero and the value returned in `*TAILPTR' is the value of STRING. In a locale other than the standard `"C"' or `"POSIX"' locales, this function may recognize additional locale-dependent syntax. If the string has valid syntax for a floating-point number but the value is outside the range of a `double', `strtod' will signal overflow or underflow as described in *Note Math Error Reporting::. `strtod' recognizes four special input strings. The strings `"inf"' and `"infinity"' are converted to oo, or to the largest representable value if the floating-point format doesn't support infinities. You can prepend a `"+"' or `"-"' to specify the sign. Case is ignored when scanning these strings. The strings `"nan"' and `"nan(CHARS...)"' are converted to NaN. Again, case is ignored. If CHARS... are provided, they are used in some unspecified fashion to select a particular representation of NaN (there can be several). Since zero is a valid result as well as the value returned on error, you should check for errors in the same way as for `strtol', by examining ERRNO and TAILPTR. - Function: float strtof (const char *STRING, char **TAILPTR) - Function: long double strtold (const char *STRING, char **TAILPTR) These functions are analogous to `strtod', but return `float' and `long double' values respectively. They report errors in the same way as `strtod'. `strtof' can be substantially faster than `strtod', but has less precision; conversely, `strtold' can be much slower but has more precision (on systems where `long double' is a separate type). These functions have been GNU extensions and are new to ISO C99. - Function: double wcstod (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR) - Function: float wcstof (const wchar_t *STRING, wchar_t **TAILPTR) - Function: long double wcstold (const wchar_t *STRING, wchar_t **TAILPTR) The `wcstod', `wcstof', and `wcstol' functions are equivalent in nearly all aspect to the `strtod', `strtof', and `strtold' functions but it handles wide character string. The `wcstod' function was introduced in Amendment 1 of ISO C90. The `wcstof' and `wcstold' functions were introduced in ISO C99. - Function: double atof (const char *STRING) This function is similar to the `strtod' function, except that it need not detect overflow and underflow errors. The `atof' function is provided mostly for compatibility with existing code; using `strtod' is more robust. The GNU C library also provides `_l' versions of these functions, which take an additional argument, the locale to use in conversion. *Note Parsing of Integers::. Old-fashioned System V number-to-string functions ================================================= The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics. The GNU C library also provides these functions and some natural extensions. These functions are only available in glibc and on systems descended from AT&T Unix. Therefore, unless these functions do precisely what you need, it is better to use `sprintf', which is standard. All these functions are defined in `stdlib.h'. - Function: char * ecvt (double VALUE, int NDIGIT, int *DECPT, int *NEG) The function `ecvt' converts the floating-point number VALUE to a string with at most NDIGIT decimal digits. The returned string contains no decimal point or sign. The first digit of the string is non-zero (unless VALUE is actually zero) and the last digit is rounded to nearest. `*DECPT' is set to the index in the string of the first digit after the decimal point. `*NEG' is set to a nonzero value if VALUE is negative, zero otherwise. If NDIGIT decimal digits would exceed the precision of a `double' it is reduced to a system-specific value. The returned string is statically allocated and overwritten by each call to `ecvt'. If VALUE is zero, it is implementation defined whether `*DECPT' is `0' or `1'. For example: `ecvt (12.3, 5, &d, &n)' returns `"12300"' and sets D to `2' and N to `0'. - Function: char * fcvt (double VALUE, int NDIGIT, int *DECPT, int *NEG) The function `fcvt' is like `ecvt', but NDIGIT specifies the number of digits after the decimal point. If NDIGIT is less than zero, VALUE is rounded to the NDIGIT+1'th place to the left of the decimal point. For example, if NDIGIT is `-1', VALUE will be rounded to the nearest 10. If NDIGIT is negative and larger than the number of digits to the left of the decimal point in VALUE, VALUE will be rounded to one significant digit. If NDIGIT decimal digits would exceed the precision of a `double' it is reduced to a system-specific value. The returned string is statically allocated and overwritten by each call to `fcvt'. - Function: char * gcvt (double VALUE, int NDIGIT, char *BUF) `gcvt' is functionally equivalent to `sprintf(buf, "%*g", ndigit, value'. It is provided only for compatibility's sake. It returns BUF. If NDIGIT decimal digits would exceed the precision of a `double' it is reduced to a system-specific value. As extensions, the GNU C library provides versions of these three functions that take `long double' arguments. - Function: char * qecvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG) This function is equivalent to `ecvt' except that it takes a `long double' for the first parameter and that NDIGIT is restricted by the precision of a `long double'. - Function: char * qfcvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG) This function is equivalent to `fcvt' except that it takes a `long double' for the first parameter and that NDIGIT is restricted by the precision of a `long double'. - Function: char * qgcvt (long double VALUE, int NDIGIT, char *BUF) This function is equivalent to `gcvt' except that it takes a `long double' for the first parameter and that NDIGIT is restricted by the precision of a `long double'. The `ecvt' and `fcvt' functions, and their `long double' equivalents, all return a string located in a static buffer which is overwritten by the next call to the function. The GNU C library provides another set of extended functions which write the converted string into a user-supplied buffer. These have the conventional `_r' suffix. `gcvt_r' is not necessary, because `gcvt' already uses a user-supplied buffer. - Function: char * ecvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN) The `ecvt_r' function is the same as `ecvt', except that it places its result into the user-specified buffer pointed to by BUF, with length LEN. This function is a GNU extension. - Function: char * fcvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN) The `fcvt_r' function is the same as `fcvt', except that it places its result into the user-specified buffer pointed to by BUF, with length LEN. This function is a GNU extension. - Function: char * qecvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN) The `qecvt_r' function is the same as `qecvt', except that it places its result into the user-specified buffer pointed to by BUF, with length LEN. This function is a GNU extension. - Function: char * qfcvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN) The `qfcvt_r' function is the same as `qfcvt', except that it places its result into the user-specified buffer pointed to by BUF, with length LEN. This function is a GNU extension. Date and Time ************* This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations. Time Basics =========== Discussing time in a technical manual can be difficult because the word "time" in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word "time" for is to talk about the abstract concept. A "calendar time" is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called "absolute time". We don't speak of a "date", because that is inherent in a calendar time. An "interval" is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980. An "elapsed time" is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word "interval" to refer to the elapsed time of some interval. An "amount of time" is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read. A "period" is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events. "CPU time" is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process. "Processor time" is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above. Elapsed Time ============ One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times. This function is declared in `time.h'. - Function: double difftime (time_t TIME1, time_t TIME0) The `difftime' function returns the number of seconds of elapsed time between calendar time TIME1 and calendar time TIME0, as a value of type `double'. The difference ignores leap seconds unless leap second support is enabled. In the GNU system, you can simply subtract `time_t' values. But on other systems, the `time_t' data type might use some other encoding where subtraction doesn't work directly. The GNU C library provides two data types specifically for representing an elapsed time. They are used by various GNU C library functions, and you can use them for your own purposes too. They're exactly the same except that one has a resolution in microseconds, and the other, newer one, is in nanoseconds. - Data Type: struct timeval The `struct timeval' structure represents an elapsed time. It is declared in `sys/time.h' and has the following members: `long int tv_sec' This represents the number of whole seconds of elapsed time. `long int tv_usec' This is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds. It is always less than one million. - Data Type: struct timespec The `struct timespec' structure represents an elapsed time. It is declared in `time.h' and has the following members: `long int tv_sec' This represents the number of whole seconds of elapsed time. `long int tv_nsec' This is the rest of the elapsed time (a fraction of a second), represented as the number of nanoseconds. It is always less than one billion. It is often necessary to subtract two values of type `struct timeval' or `struct timespec'. Here is the best way to do this. It works even on some peculiar operating systems where the `tv_sec' member has an unsigned type. /* Subtract the `struct timeval' values X and Y, storing the result in RESULT. Return 1 if the difference is negative, otherwise 0. */ int timeval_subtract (result, x, y) struct timeval *result, *x, *y; { /* Perform the carry for the later subtraction by updating Y. */ if (x->tv_usec < y->tv_usec) { int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1; y->tv_usec -= 1000000 * nsec; y->tv_sec += nsec; } if (x->tv_usec - y->tv_usec > 1000000) { int nsec = (x->tv_usec - y->tv_usec) / 1000000; y->tv_usec += 1000000 * nsec; y->tv_sec -= nsec; } /* Compute the time remaining to wait. `tv_usec' is certainly positive. */ result->tv_sec = x->tv_sec - y->tv_sec; result->tv_usec = x->tv_usec - y->tv_usec; /* Return 1 if result is negative. */ return x->tv_sec < y->tv_sec; } Common functions that use `struct timeval' are `gettimeofday' and `settimeofday'. There are no GNU C library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time, and alarm and sleeping functions have a lot to do with them. Processor And CPU Time ====================== If you're trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses. For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section. CPU time (*note Time Basics::) is represented by the data type `clock_t', which is a number of "clock ticks". It gives the total amount of time a process has actively used a CPU since some arbitrary event. On the GNU system, that event is the creation of the process. While arbitrary in general, the event is always the same event for any particular process, so you can always measure how much time on the CPU a particular computation takes by examinining the process' CPU time before and after the computation. In the GNU system, `clock_t' is equivalent to `long int' and `CLOCKS_PER_SEC' is an integer value. But in other systems, both `clock_t' and the macro `CLOCKS_PER_SEC' can be either integer or floating-point types. Casting CPU time values to `double', as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is. Note that the clock can wrap around. On a 32bit system with `CLOCKS_PER_SEC' set to one million this function will return the same value approximately every 72 minutes. For additional functions to examine a process' use of processor time, and to control it, *Note Resource Usage And Limitation::. CPU Time Inquiry ---------------- To get a process' CPU time, you can use the `clock' function. This facility is declared in the header file `time.h'. In typical usage, you call the `clock' function at the beginning and end of the interval you want to time, subtract the values, and then divide by `CLOCKS_PER_SEC' (the number of clock ticks per second) to get processor time, like this: #include clock_t start, end; double cpu_time_used; start = clock(); ... /* Do the work. */ end = clock(); cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC; Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. *Note Processor Time::. Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second. - Macro: int CLOCKS_PER_SEC The value of this macro is the number of clock ticks per second measured by the `clock' function. POSIX requires that this value be one million independent of the actual resolution. - Macro: int CLK_TCK This is an obsolete name for `CLOCKS_PER_SEC'. - Data Type: clock_t This is the type of the value returned by the `clock' function. Values of type `clock_t' are numbers of clock ticks. - Function: clock_t clock (void) This function returns the calling process' current CPU time. If the CPU time is not available or cannot be represented, `clock' returns the value `(clock_t)(-1)'. Processor Time Inquiry ---------------------- The `times' function returns information about a process' consumption of processor time in a `struct tms' object, in addition to the process' CPU time. *Note Time Basics::. You should include the header file `sys/times.h' to use this facility. - Data Type: struct tms The `tms' structure is used to return information about process times. It contains at least the following members: `clock_t tms_utime' This is the total processor time the calling process has used in executing the instructions of its program. `clock_t tms_stime' This is the processor time the system has used on behalf of the calling process. `clock_t tms_cutime' This is the sum of the `tms_utime' values and the `tms_cutime' values of all terminated child processes of the calling process, whose status has been reported to the parent process by `wait' or `waitpid'; see *Note Process Completion::. In other words, it represents the total processor time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by `wait' or `waitpid'. `clock_t tms_cstime' This is similar to `tms_cutime', but represents the total processor time system has used on behalf of all the terminated child processes of the calling process. All of the times are given in numbers of clock ticks. Unlike CPU time, these are the actual amounts of time; not relative to any event. *Note Creating a Process::. - Function: clock_t times (struct tms *BUFFER) The `times' function stores the processor time information for the calling process in BUFFER. The return value is the calling process' CPU time (the same value you get from `clock()'. `times' returns `(clock_t)(-1)' to indicate failure. *Portability Note:* The `clock' function described in *Note CPU Time:: is specified by the ISO C standard. The `times' function is a feature of POSIX.1. In the GNU system, the CPU time is defined to be equivalent to the sum of the `tms_utime' and `tms_stime' fields returned by `times'. Calendar Time ============= This section describes facilities for keeping track of calendar time. *Note Time Basics::. The GNU C library represents calendar time three ways: * "Simple time" (the `time_t' data type) is a compact representation, typically giving the number of seconds of elapsed time since some implementation-specific base time. * There is also a "high-resolution time" representation. Like simple time, this represents a calendar time as an elapsed time since a base time, but instead of measuring in whole seconds, it uses a `struct timeval' data type, which includes fractions of a second. Use this time representation instead of simple time when you need greater precision. * "Local time" or "broken-down time" (the `struct tm' data type) represents a calendar time as a set of components specifying the year, month, and so on in the Gregorian calendar, for a specific time zone. This calendar time representation is usually used only to communicate with people. Simple Calendar Time -------------------- This section describes the `time_t' data type for representing calendar time as simple time, and the functions which operate on simple time objects. These facilities are declared in the header file `time.h'. - Data Type: time_t This is the data type used to represent simple time. Sometimes, it also represents an elapsed time. When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This calendar time is sometimes referred to as the "epoch".) POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set `TZ' to certain values (*note TZ Variable::). Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is. In the GNU C library, `time_t' is equivalent to `long int'. In other systems, `time_t' might be either an integer or floating-point type. The function `difftime' tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting. *Note Elapsed Time::. - Function: time_t time (time_t *RESULT) The `time' function returns the current calendar time as a value of type `time_t'. If the argument RESULT is not a null pointer, the calendar time value is also stored in `*RESULT'. If the current calendar time is not available, the value `(time_t)(-1)' is returned. - Function: int stime (time_t *NEWTIME) `stime' sets the system clock, i.e. it tells the system that the current calendar time is NEWTIME, where `newtime' is interpreted as described in the above definition of `time_t'. `settimeofday' is a newer function which sets the system clock to better than one second precision. `settimeofday' is generally a better choice than `stime'. *Note High-Resolution Calendar::. Only the superuser can set the system clock. If the function succeeds, the return value is zero. Otherwise, it is `-1' and `errno' is set accordingly: `EPERM' The process is not superuser. High-Resolution Calendar ------------------------ The `time_t' data type used to represent simple times has a resolution of only one second. Some applications need more precision. So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'. - Data Type: struct timezone The `struct timezone' structure is used to hold minimal information about the local time zone. It has the following members: `int tz_minuteswest' This is the number of minutes west of UTC. `int tz_dsttime' If nonzero, Daylight Saving Time applies during some part of the year. The `struct timezone' type is obsolete and should never be used. Instead, use the facilities described in *Note Time Zone Functions::. - Function: int gettimeofday (struct timeval *TP, struct timezone *TZP) The `gettimeofday' function returns the current calendar time as the elapsed time since the epoch in the `struct timeval' structure indicated by TP. (*note Elapsed Time:: for a description of `struct timespec'). Information about the time zone is returned in the structure pointed at TZP. If the TZP argument is a null pointer, time zone information is ignored. The return value is `0' on success and `-1' on failure. The following `errno' error condition is defined for this function: `ENOSYS' The operating system does not support getting time zone information, and TZP is not a null pointer. The GNU operating system does not support using `struct timezone' to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in *Note Time Zone Functions::. - Function: int settimeofday (const struct timeval *TP, const struct timezone *TZP) The `settimeofday' function sets the current calendar time in the system clock according to the arguments. As for `gettimeofday', the calendar time is represented as the elapsed time since the epoch. As for `gettimeofday', time zone information is ignored if TZP is a null pointer. You must be a privileged user in order to use `settimeofday'. Some kernels automatically set the system clock from some source such as a hardware clock when they start up. Others, including Linux, place the system clock in an "invalid" state (in which attempts to read the clock fail). A call of `stime' removes the system clock from an invalid state, and system startup scripts typically run a program that calls `stime'. `settimeofday' causes a sudden jump forwards or backwards, which can cause a variety of problems in a system. Use `adjtime' (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock. With a Linux kernel, `adjtimex' does the same thing and can also make permanent changes to the speed of the system clock so it doesn't need to be corrected as often. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EPERM' This process cannot set the clock because it is not privileged. `ENOSYS' The operating system does not support setting time zone information, and TZP is not a null pointer. - Function: int adjtime (const struct timeval *DELTA, struct timeval *OLDDELTA) This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock. The DELTA argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while. If the OLDDELTA argument is not a null pointer, the `adjtime' function returns information about any previous time adjustment that has not yet completed. This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it. With a Linux kernel, you can use the `adjtimex' function to permanently change the clock speed. The return value is `0' on success and `-1' on failure. The following `errno' error condition is defined for this function: `EPERM' You do not have privilege to set the time. *Portability Note:* The `gettimeofday', `settimeofday', and `adjtime' functions are derived from BSD. Symbols for the following function are declared in `sys/timex.h'. - Function: int adjtimex (struct timex *TIMEX) `adjtimex' is functionally identical to `ntp_adjtime'. *Note High Accuracy Clock::. This function is present only with a Linux kernel. Broken-down Time ---------------- Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, "broken-down time" is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information. A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is. The symbols in this section are declared in the header file `time.h'. - Data Type: struct tm This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order. `int tm_sec' This is the number of full seconds since the top of the minute (normally in the range `0' through `59', but the actual upper limit is `60', to allow for leap seconds if leap second support is available). `int tm_min' This is the number of full minutes since the top of the hour (in the range `0' through `59'). `int tm_hour' This is the number of full hours past midnight (in the range `0' through `23'). `int tm_mday' This is the ordinal day of the month (in the range `1' through `31'). Watch out for this one! As the only ordinal number in the structure, it is inconsistent with the rest of the structure. `int tm_mon' This is the number of full calendar months since the beginning of the year (in the range `0' through `11'). Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1). `int tm_year' This is the number of full calendar years since 1900. `int tm_wday' This is the number of full days since Sunday (in the range `0' through `6'). `int tm_yday' This is the number of full days since the beginning of the year (in the range `0' through `365'). `int tm_isdst' This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available. `long int tm_gmtoff' This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is `-5*60*60'. The `tm_gmtoff' field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment. `const char *tm_zone' This field is the name for the time zone that was used to compute this broken-down time value. Like `tm_gmtoff', this field is a BSD and GNU extension, and is not visible in a strict ISO C environment. - Function: struct tm * localtime (const time_t *TIME) The `localtime' function converts the simple time pointed to by TIME to broken-down time representation, expressed relative to the user's specified time zone. The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to `ctime', `gmtime', or `localtime'. (But no other library function overwrites the contents of this object.) The return value is the null pointer if TIME cannot be represented as a broken-down time; typically this is because the year cannot fit into an `int'. Calling `localtime' has one other effect: it sets the variable `tzname' with information about the current time zone. *Note Time Zone Functions::. Using the `localtime' function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a variant of this function. - Function: struct tm * localtime_r (const time_t *TIME, struct tm *RESULTP) The `localtime_r' function works just like the `localtime' function. It takes a pointer to a variable containing a simple time and converts it to the broken-down time format. But the result is not placed in a static buffer. Instead it is placed in the object of type `struct tm' to which the parameter RESULTP points. If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns RESULTP. - Function: struct tm * gmtime (const time_t *TIME) This function is similar to `localtime', except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)) rather than relative to a local time zone. As for the `localtime' function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for `gmtime'. - Function: struct tm * gmtime_r (const time_t *TIME, struct tm *RESULTP) This function is similar to `localtime_r', except that it converts just like `gmtime' the given time as Coordinated Universal Time. If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns RESULTP. - Function: time_t mktime (struct tm *BROKENTIME) The `mktime' function is used to convert a broken-down time structure to a simple time representation. It also "normalizes" the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components. The `mktime' function ignores the specified contents of the `tm_wday' and `tm_yday' members of the broken-down time structure. It uses the values of the other components to determine the calendar time; it's permissible for these components to have unnormalized values outside their normal ranges. The last thing that `mktime' does is adjust the components of the BROKENTIME structure (including the `tm_wday' and `tm_yday'). If the specified broken-down time cannot be represented as a simple time, `mktime' returns a value of `(time_t)(-1)' and does not modify the contents of BROKENTIME. Calling `mktime' also sets the variable `tzname' with information about the current time zone. *Note Time Zone Functions::. - Function: time_t timelocal (struct tm *BROKENTIME) `timelocal' is functionally identical to `mktime', but more mnemonically named. Note that it is the inverse of the `localtime' function. *Portability note:* `mktime' is essentially universally available. `timelocal' is rather rare. - Function: time_t timegm (struct tm *BROKENTIME) `timegm' is functionally identical to `mktime' except it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting. Note that `timegm' is the inverse of `gmtime'. *Portability note:* `mktime' is essentially universally available. `timegm' is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the `TZ' environment variable to UTC, call `mktime', then set `TZ' back. High Accuracy Clock ------------------- The `ntp_gettime' and `ntp_adjtime' functions provide an interface to monitor and manipulate the system clock to maintain high accuracy time. For example, you can fine tune the speed of the clock or synchronize it with another time source. A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks. These functions are declared in `sys/timex.h'. - Data Type: struct ntptimeval This structure is used for information about the system clock. It contains the following members: `struct timeval time' This is the current calendar time, expressed as the elapsed time since the epoch. The `struct timeval' data type is described in *Note Elapsed Time::. `long int maxerror' This is the maximum error, measured in microseconds. Unless updated via `ntp_adjtime' periodically, this value will reach some platform-specific maximum value. `long int esterror' This is the estimated error, measured in microseconds. This value can be set by `ntp_adjtime' to indicate the estimated offset of the system clock from the true calendar time. - Function: int ntp_gettime (struct ntptimeval *TPTR) The `ntp_gettime' function sets the structure pointed to by TPTR to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a `ntp_adjtime' call is necessary. The return value is `0' on success and other values on failure. The following `errno' error conditions are defined for this function: `TIME_ERROR' The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. - Data Type: struct timex This structure is used to control and monitor the system clock. It contains the following members: `unsigned int modes' This variable controls whether and which values are set. Several symbolic constants have to be combined with _binary or_ to specify the effective mode. These constants start with `MOD_'. `long int offset' This value indicates the current offset of the system clock from the true calendar time. The value is given in microseconds. If bit `MOD_OFFSET' is set in `modes', the offset (and possibly other dependent values) can be set. The offset's absolute value must not exceed `MAXPHASE'. `long int frequency' This value indicates the difference in frequency between the true calendar time and the system clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is `1 << SHIFT_USEC'. The value can be set with bit `MOD_FREQUENCY', but the absolute value must not exceed `MAXFREQ'. `long int maxerror' This is the maximum error, measured in microseconds. A new value can be set using bit `MOD_MAXERROR'. Unless updated via `ntp_adjtime' periodically, this value will increase steadily and reach some platform-specific maximum value. `long int esterror' This is the estimated error, measured in microseconds. This value can be set using bit `MOD_ESTERROR'. `int status' This variable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with `STA_'. Some of these flags can be updated using the `MOD_STATUS' bit. `long int constant' This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit `MOD_TIMECONST'. `long int precision' This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds. `long int tolerance' This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the `maxerror' every second. `struct timeval time' The current calendar time. `long int tick' The elapsed time between clock ticks in microseconds. A clock tick is a periodic timer interrupt on which the system clock is based. `long int ppsfreq' This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal. `long int jitter' This value expresses a median filtered average of the PPS signal's dispersion in microseconds. `int shift' This value is a binary exponent for the duration of the PPS calibration interval, ranging from `PPS_SHIFT' to `PPS_SHIFTMAX'. `long int stabil' This value represents the median filtered dispersion of the PPS frequency in scaled PPM. `long int jitcnt' This counter represents the number of pulses where the jitter exceeded the allowed maximum `MAXTIME'. `long int calcnt' This counter reflects the number of successful calibration intervals. `long int errcnt' This counter represents the number of calibration errors (caused by large offsets or jitter). `long int stbcnt' This counter denotes the number of of calibrations where the stability exceeded the threshold. - Function: int ntp_adjtime (struct timex *TPTR) The `ntp_adjtime' function sets the structure specified by TPTR to current values. In addition, `ntp_adjtime' updates some settings to match what you pass to it in *TPTR. Use the `modes' element of *TPTR to select what settings to update. You can set `offset', `freq', `maxerror', `esterror', `status', `constant', and `tick'. `modes' = zero means set nothing. Only the superuser can update settings. The return value is `0' on success and other values on failure. The following `errno' error conditions are defined for this function: `TIME_ERROR' The high accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed. `EPERM' The process specified a settings update, but is not superuser. For more details see RFC1305 (Network Time Protocol, Version 3) and related documents. *Portability note:* Early versions of the GNU C library did not have this function but did have the synonymous `adjtimex'. Formatting Calendar Time ------------------------ The functions described in this section format calendar time values as strings. These functions are declared in the header file `time.h'. - Function: char * asctime (const struct tm *BROKENTIME) The `asctime' function converts the broken-down time value that BROKENTIME points to into a string in a standard format: "Tue May 21 13:46:22 1991\n" The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'. The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'. The return value points to a statically allocated string, which might be overwritten by subsequent calls to `asctime' or `ctime'. (But no other library function overwrites the contents of this string.) - Function: char * asctime_r (const struct tm *BROKENTIME, char *BUFFER) This function is similar to `asctime' but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter BUFFER. This buffer should have room for at least 26 bytes, including the terminating null. If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns BUFFER. Otherwise return `NULL'. - Function: char * ctime (const time_t *TIME) The `ctime' function is similar to `asctime', except that you specify the calendar time argument as a `time_t' simple time value rather than in broken-down local time format. It is equivalent to asctime (localtime (TIME)) `ctime' sets the variable `tzname', because `localtime' does so. *Note Time Zone Functions::. - Function: char * ctime_r (const time_t *TIME, char *BUFFER) This function is similar to `ctime', but places the result in the string pointed to by BUFFER. It is equivalent to (written using gcc extensions, *note Statement Exprs: (gcc)Statement Exprs.): ({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); }) If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns BUFFER. Otherwise return `NULL'. - Function: size_t strftime (char *S, size_t SIZE, const char *TEMPLATE, const struct tm *BROKENTIME) This function is similar to the `sprintf' function (*note Formatted Input::), but the conversion specifications that can appear in the format template TEMPLATE are specialized for printing components of the date and time BROKENTIME according to the locale currently specified for time conversion (*note Locales::). Ordinary characters appearing in the TEMPLATE are copied to the output string S; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers: `_' The number is padded with spaces. `-' The number is not padded at all. `0' The number is padded with zeros even if the format specifies padding with spaces. `^' The output uses uppercase characters, but only if this is possible (*note Case Conversion::). The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them. Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size. An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are: `E' Use the locale's alternate representation for date and time. This modifier applies to the `%c', `%C', `%x', `%X', `%y' and `%Y' format specifiers. In a Japanese locale, for example, `%Ex' might yield a date format based on the Japanese Emperors' reigns. `O' Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers. If the format supports the modifier but no alternate representation is available, it is ignored. The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows: `%a' The abbreviated weekday name according to the current locale. `%A' The full weekday name according to the current locale. `%b' The abbreviated month name according to the current locale. `%B' The full month name according to the current locale. `%c' The preferred calendar time representation for the current locale. `%C' The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100. This format is a POSIX.2 extension and also appears in ISO C99. `%d' The day of the month as a decimal number (range `01' through `31'). `%D' The date using the format `%m/%d/%y'. This format is a POSIX.2 extension and also appears in ISO C99. `%e' The day of the month like with `%d', but padded with blank (range ` 1' through `31'). This format is a POSIX.2 extension and also appears in ISO C99. `%F' The date using the format `%Y-%m-%d'. This is the form specified in the ISO 8601 standard and is the preferred form for all uses. This format is a ISO C99 extension. `%g' The year corresponding to the ISO week number, but without the century (range `00' through `99'). This has the same format and value as `%y', except that if the ISO week number (see `%V') belongs to the previous or next year, that year is used instead. This format was introduced in ISO C99. `%G' The year corresponding to the ISO week number. This has the same format and value as `%Y', except that if the ISO week number (see `%V') belongs to the previous or next year, that year is used instead. This format was introduced in ISO C99 but was previously available as a GNU extension. `%h' The abbreviated month name according to the current locale. The action is the same as for `%b'. This format is a POSIX.2 extension and also appears in ISO C99. `%H' The hour as a decimal number, using a 24-hour clock (range `00' through `23'). `%I' The hour as a decimal number, using a 12-hour clock (range `01' through `12'). `%j' The day of the year as a decimal number (range `001' through `366'). `%k' The hour as a decimal number, using a 24-hour clock like `%H', but padded with blank (range ` 0' through `23'). This format is a GNU extension. `%l' The hour as a decimal number, using a 12-hour clock like `%I', but padded with blank (range ` 1' through `12'). This format is a GNU extension. `%m' The month as a decimal number (range `01' through `12'). `%M' The minute as a decimal number (range `00' through `59'). `%n' A single `\n' (newline) character. This format is a POSIX.2 extension and also appears in ISO C99. `%p' Either `AM' or `PM', according to the given time value; or the corresponding strings for the current locale. Noon is treated as `PM' and midnight as `AM'. `%P' Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as `pm' and midnight as `am'. This format was introduced in ISO C99 but was previously available as a GNU extension. `%r' The complete calendar time using the AM/PM format of the current locale. This format is a POSIX.2 extension and also appears in ISO C99. `%R' The hour and minute in decimal numbers using the format `%H:%M'. This format was introduced in ISO C99 but was previously available as a GNU extension. `%s' The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. This format is a GNU extension. `%S' The seconds as a decimal number (range `00' through `60'). `%t' A single `\t' (tabulator) character. This format is a POSIX.2 extension and also appears in ISO C99. `%T' The time of day using decimal numbers using the format `%H:%M:%S'. This format is a POSIX.2 extension. `%u' The day of the week as a decimal number (range `1' through `7'), Monday being `1'. This format is a POSIX.2 extension and also appears in ISO C99. `%U' The week number of the current year as a decimal number (range `00' through `53'), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week `00'. `%V' The ISO 8601:1988 week number as a decimal number (range `01' through `53'). ISO weeks start with Monday and end with Sunday. Week `01' of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week `01' of a year can contain days from the previous year. The week before week `01' of a year is the last week (`52' or `53') of the previous year even if it contains days from the new year. This format is a POSIX.2 extension and also appears in ISO C99. `%w' The day of the week as a decimal number (range `0' through `6'), Sunday being `0'. `%W' The week number of the current year as a decimal number (range `00' through `53'), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week `00'. `%x' The preferred date representation for the current locale. `%X' The preferred time of day representation for the current locale. `%y' The year without a century as a decimal number (range `00' through `99'). This is equivalent to the year modulo 100. `%Y' The year as a decimal number, using the Gregorian calendar. Years before the year `1' are numbered `0', `-1', and so on. `%z' RFC 822/ISO 8601:1988 style numeric time zone (e.g., `-0600' or `+0100'), or nothing if no time zone is determinable. This format was introduced in ISO C99 but was previously available as a GNU extension. A full RFC 822 timestamp is generated by the format `"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent `"%a, %d %b %Y %T %z"'). `%Z' The time zone abbreviation (empty if the time zone can't be determined). `%%' A literal `%' character. The SIZE parameter can be used to specify the maximum number of characters to be stored in the array S, including the terminating null character. If the formatted time requires more than SIZE characters, `strftime' returns zero and the contents of the array S are undefined. Otherwise the return value indicates the number of characters placed in the array S, not including the terminating null character. _Warning:_ This convention for the return value which is prescribed in ISO C can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales `"%p"' will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used: buf[0] = '\1'; len = strftime (buf, bufsize, format, tp); if (len == 0 && buf[0] != '\0') { /* Something went wrong in the strftime call. */ ... } If S is a null pointer, `strftime' does not actually write anything, but instead returns the number of characters it would have written. According to POSIX.1 every call to `strftime' implies a call to `tzset'. So the contents of the environment variable `TZ' is examined before any output is produced. For an example of `strftime', see *Note Time Functions Example::. - Function: size_t wcsftime (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, const struct tm *BROKENTIME) The `wcsftime' function is equivalent to the `strftime' function with the difference that it operates on wide character strings. The buffer where the result is stored, pointed to by S, must be an array of wide characters. The parameter SIZE which specifies the size of the output buffer gives the number of wide character, not the number of bytes. Also the format string TEMPLATE is a wide character string. Since all characters needed to specify the format string are in the basic character set it is portably possible to write format strings in the C source code using the `L"..."' notation. The parameter BROKENTIME has the same meaning as in the `strftime' call. The `wcsftime' function supports the same flags, modifiers, and format specifiers as the `strftime' function. The return value of `wcsftime' is the number of wide characters stored in `s'. When more characters would have to be written than can be placed in the buffer S the return value is zero, with the same problems indicated in the `strftime' documentation. Convert textual time and date information back ---------------------------------------------- The ISO C standard does not specify any functions which can convert the output of the `strftime' function back into a binary format. This led to a variety of more-or-less successful implementations with different interfaces over the years. Then the Unix standard was extended by the addition of two functions: `strptime' and `getdate'. Both have strange interfaces but at least they are widely available. Interpret string according to given format .......................................... he first function is rather low-level. It is nevertheless frequently used in software since it is better known. Its interface and implementation are heavily influenced by the `getdate' function, which is defined and implemented in terms of calls to `strptime'. - Function: char * strptime (const char *S, const char *FMT, struct tm *TP) The `strptime' function parses the input string S according to the format string FMT and stores its results in the structure TP. The input string could be generated by a `strftime' call or obtained any other way. It does not need to be in a human-recognizable format; e.g. a date passed as `"02:1999:9"' is acceptable, even though it is ambiguous without context. As long as the format string FMT matches the input string the function will succeed. The user has to make sure, though, that the input can be parsed in a unambiguous way. The string `"1999112"' can be parsed using the format `"%Y%m%d"' as 1999-1-12, 1999-11-2, or even 19991-1-2. It is necessary to add appropriate separators to reliably get results. The format string consists of the same components as the format string of the `strftime' function. The only difference is that the flags `_', `-', `0', and `^' are not allowed. Several of the distinct formats of `strftime' do the same work in `strptime' since differences like case of the input do not matter. For reasons of symmetry all formats are supported, though. The modifiers `E' and `O' are also allowed everywhere the `strftime' function allows them. The formats are: `%a' `%A' The weekday name according to the current locale, in abbreviated form or the full name. `%b' `%B' `%h' The month name according to the current locale, in abbreviated form or the full name. `%c' The date and time representation for the current locale. `%Ec' Like `%c' but the locale's alternative date and time format is used. `%C' The century of the year. It makes sense to use this format only if the format string also contains the `%y' format. `%EC' The locale's representation of the period. Unlike `%C' it sometimes makes sense to use this format since some cultures represent years relative to the beginning of eras instead of using the Gregorian years. `%d' `%e' The day of the month as a decimal number (range `1' through `31'). Leading zeroes are permitted but not required. `%Od' `%Oe' Same as `%d' but using the locale's alternative numeric symbols. Leading zeroes are permitted but not required. `%D' Equivalent to `%m/%d/%y'. `%F' Equivalent to `%Y-%m-%d', which is the ISO 8601 date format. This is a GNU extension following an ISO C99 extension to `strftime'. `%g' The year corresponding to the ISO week number, but without the century (range `00' through `99'). _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. This format is a GNU extension following a GNU extension of `strftime'. `%G' The year corresponding to the ISO week number. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. This format is a GNU extension following a GNU extension of `strftime'. `%H' `%k' The hour as a decimal number, using a 24-hour clock (range `00' through `23'). `%k' is a GNU extension following a GNU extension of `strftime'. `%OH' Same as `%H' but using the locale's alternative numeric symbols. `%I' `%l' The hour as a decimal number, using a 12-hour clock (range `01' through `12'). `%l' is a GNU extension following a GNU extension of `strftime'. `%OI' Same as `%I' but using the locale's alternative numeric symbols. `%j' The day of the year as a decimal number (range `1' through `366'). Leading zeroes are permitted but not required. `%m' The month as a decimal number (range `1' through `12'). Leading zeroes are permitted but not required. `%Om' Same as `%m' but using the locale's alternative numeric symbols. `%M' The minute as a decimal number (range `0' through `59'). Leading zeroes are permitted but not required. `%OM' Same as `%M' but using the locale's alternative numeric symbols. `%n' `%t' Matches any white space. `%p' `%P' The locale-dependent equivalent to `AM' or `PM'. This format is not useful unless `%I' or `%l' is also used. Another complication is that the locale might not define these values at all and therefore the conversion fails. `%P' is a GNU extension following a GNU extension to `strftime'. `%r' The complete time using the AM/PM format of the current locale. A complication is that the locale might not define this format at all and therefore the conversion fails. `%R' The hour and minute in decimal numbers using the format `%H:%M'. `%R' is a GNU extension following a GNU extension to `strftime'. `%s' The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. `%s' is a GNU extension following a GNU extension to `strftime'. `%S' The seconds as a decimal number (range `0' through `60'). Leading zeroes are permitted but not required. *Note:* The Unix specification says the upper bound on this value is `61', a result of a decision to allow double leap seconds. You will not see the value `61' because no minute has more than one leap second, but the myth persists. `%OS' Same as `%S' but using the locale's alternative numeric symbols. `%T' Equivalent to the use of `%H:%M:%S' in this place. `%u' The day of the week as a decimal number (range `1' through `7'), Monday being `1'. Leading zeroes are permitted but not required. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. `%U' The week number of the current year as a decimal number (range `0' through `53'). Leading zeroes are permitted but not required. `%OU' Same as `%U' but using the locale's alternative numeric symbols. `%V' The ISO 8601:1988 week number as a decimal number (range `1' through `53'). Leading zeroes are permitted but not required. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. `%w' The day of the week as a decimal number (range `0' through `6'), Sunday being `0'. Leading zeroes are permitted but not required. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. `%Ow' Same as `%w' but using the locale's alternative numeric symbols. `%W' The week number of the current year as a decimal number (range `0' through `53'). Leading zeroes are permitted but not required. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. `%OW' Same as `%W' but using the locale's alternative numeric symbols. `%x' The date using the locale's date format. `%Ex' Like `%x' but the locale's alternative data representation is used. `%X' The time using the locale's time format. `%EX' Like `%X' but the locale's alternative time representation is used. `%y' The year without a century as a decimal number (range `0' through `99'). Leading zeroes are permitted but not required. Note that it is questionable to use this format without the `%C' format. The `strptime' function does regard input values in the range 68 to 99 as the years 1969 to 1999 and the values 0 to 68 as the years 2000 to 2068. But maybe this heuristic fails for some input data. Therefore it is best to avoid `%y' completely and use `%Y' instead. `%Ey' The offset from `%EC' in the locale's alternative representation. `%Oy' The offset of the year (from `%C') using the locale's alternative numeric symbols. `%Y' The year as a decimal number, using the Gregorian calendar. `%EY' The full alternative year representation. `%z' The offset from GMT in ISO 8601/RFC822 format. `%Z' The timezone name. _Note:_ Currently, this is not fully implemented. The format is recognized, input is consumed but no field in TM is set. `%%' A literal `%' character. All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more white space characters in the format string. The `strptime' function processes the input string from right to left. Each of the three possible input elements (white space, literal, or format) are handled one after the other. If the input cannot be matched to the format string the function stops. The remainder of the format and input strings are not processed. The function returns a pointer to the first character it was unable to process. If the input string contains more characters than required by the format string the return value points right after the last consumed input character. If the whole input string is consumed the return value points to the `NULL' byte at the end of the string. If an error occurs, i.e. `strptime' fails to match all of the format string, the function returns `NULL'. The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information. Most importantly, it does not specify what happens to those elements of TM which are not directly initialized by the different formats. The implementations on different Unix systems vary here. The GNU libc implementation does not touch those fields which are not directly initialized. Exceptions are the `tm_wday' and `tm_yday' elements, which are recomputed if any of the year, month, or date elements changed. This has two implications: * Before calling the `strptime' function for a new input string, you should prepare the TM structure you pass. Normally this will mean initializing all values are to zero. Alternatively, you can set all fields to values like `INT_MAX', allowing you to determine which elements were set by the function call. Zero does not work here since it is a valid value for many of the fields. Careful initialization is necessary if you want to find out whether a certain field in TM was initialized by the function call. * You can construct a `struct tm' value with several consecutive `strptime' calls. A useful application of this is e.g. the parsing of two separate strings, one containing date information and the other time information. By parsing one after the other without clearing the structure in-between, you can construct a complete broken-down time. The following example shows a function which parses a string which is contains the date information in either US style or ISO 8601 form: const char * parse_date (const char *input, struct tm *tm) { const char *cp; /* First clear the result structure. */ memset (tm, '\0', sizeof (*tm)); /* Try the ISO format first. */ cp = strptime (input, "%F", tm); if (cp == NULL) { /* Does not match. Try the US form. */ cp = strptime (input, "%D", tm); } return cp; } A More User-friendly Way to Parse Times and Dates ................................................. The Unix standard defines another function for parsing date strings. The interface is weird, but if the function happens to suit your application it is just fine. It is problematic to use this function in multi-threaded programs or libraries, since it returns a pointer to a static variable, and uses a global variable and global state (an environment variable). - Variable: getdate_err This variable of type `int' contains the error code of the last unsuccessful call to `getdate'. Defined values are: 1 The environment variable `DATEMSK' is not defined or null. 2 The template file denoted by the `DATEMSK' environment variable cannot be opened. 3 Information about the template file cannot retrieved. 4 The template file is not a regular file. 5 An I/O error occurred while reading the template file. 6 Not enough memory available to execute the function. 7 The template file contains no matching template. 8 The input date is invalid, but would match a template otherwise. This includes dates like February 31st, and dates which cannot be represented in a `time_t' variable. - Function: struct tm * getdate (const char *STRING) The interface to `getdate' is the simplest possible for a function to parse a string and return the value. STRING is the input string and the result is returned in a statically-allocated variable. The details about how the string is processed are hidden from the user. In fact, they can be outside the control of the program. Which formats are recognized is controlled by the file named by the environment variable `DATEMSK'. This file should contain lines of valid format strings which could be passed to `strptime'. The `getdate' function reads these format strings one after the other and tries to match the input string. The first line which completely matches the input string is used. Elements not initialized through the format string retain the values present at the time of the `getdate' function call. The formats recognized by `getdate' are the same as for `strptime'. See above for an explanation. There are only a few extensions to the `strptime' behavior: * If the `%Z' format is given the broken-down time is based on the current time of the timezone matched, not of the current timezone of the runtime environment. _Note_: This is not implemented (currently). The problem is that timezone names are not unique. If a fixed timezone is assumed for a given string (say `EST' meaning US East Coast time), then uses for countries other than the USA will fail. So far we have found no good solution to this. * If only the weekday is specified the selected day depends on the current date. If the current weekday is greater or equal to the `tm_wday' value the current week's day is chosen, otherwise the day next week is chosen. * A similar heuristic is used when only the month is given and not the year. If the month is greater than or equal to the current month, then the current year is used. Otherwise it wraps to next year. The first day of the month is assumed if one is not explicitly specified. * The current hour, minute, and second are used if the appropriate value is not set through the format. * If no date is given tomorrow's date is used if the time is smaller than the current time. Otherwise today's date is taken. It should be noted that the format in the template file need not only contain format elements. The following is a list of possible format strings (taken from the Unix standard): %m %A %B %d, %Y %H:%M:%S %A %B %m/%d/%y %I %p %d,%m,%Y %H:%M at %A the %dst of %B in %Y run job at %I %p,%B %dnd %A den %d. %B %Y %H.%M Uhr As you can see, the template list can contain very specific strings like `run job at %I %p,%B %dnd'. Using the above list of templates and assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the following results for the given input. Input Match Result Mon %a Mon Sep 22 12:19:47 EDT 1986 Sun %a Sun Sep 28 12:19:47 EDT 1986 Fri %a Fri Sep 26 12:19:47 EDT 1986 September %B Mon Sep 1 12:19:47 EDT 1986 January %B Thu Jan 1 12:19:47 EST 1987 December %B Mon Dec 1 12:19:47 EST 1986 Sep Mon %b %a Mon Sep 1 12:19:47 EDT 1986 Jan Fri %b %a Fri Jan 2 12:19:47 EST 1987 Dec Mon %b %a Mon Dec 1 12:19:47 EST 1986 Jan Wed 1989 %b %a %Y Wed Jan 4 12:19:47 EST 1989 Fri 9 %a %H Fri Sep 26 09:00:00 EDT 1986 Feb 10:30 %b %H:%S Sun Feb 1 10:00:30 EST 1987 10:30 %H:%M Tue Sep 23 10:30:00 EDT 1986 13:30 %H:%M Mon Sep 22 13:30:00 EDT 1986 The return value of the function is a pointer to a static variable of type `struct tm', or a null pointer if an error occurred. The result is only valid until the next `getdate' call, making this function unusable in multi-threaded applications. The `errno' variable is _not_ changed. Error conditions are stored in the global variable `getdate_err'. See the description above for a list of the possible error values. _Warning:_ The `getdate' function should _never_ be used in SUID-programs. The reason is obvious: using the `DATEMSK' environment variable you can get the function to open any arbitrary file and chances are high that with some bogus input (such as a binary file) the program will crash. - Function: int getdate_r (const char *STRING, struct tm *TP) The `getdate_r' function is the reentrant counterpart of `getdate'. It does not use the global variable `getdate_err' to signal an error, but instead returns an error code. The same error codes as described in the `getdate_err' documentation above are used, with 0 meaning success. Moreover, `getdate_r' stores the broken-down time in the variable of type `struct tm' pointed to by the second argument, rather than in a static variable. This function is not defined in the Unix standard. Nevertheless it is available on some other Unix systems as well. The warning against using `getdate' in SUID-programs applies to `getdate_r' as well. Specifying the Time Zone with `TZ' ---------------------------------- In POSIX systems, a user can specify the time zone by means of the `TZ' environment variable. For information about how to set environment variables, see *Note Environment Variables::. The functions for accessing the time zone are declared in `time.h'. You should not normally need to set `TZ'. If the system is configured properly, the default time zone will be correct. You might set `TZ' if you are using a computer over a network from a different time zone, and would like times reported to you in the time zone local to you, rather than what is local to the computer. In POSIX.1 systems the value of the `TZ' variable can be in one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database. The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone: STD OFFSET The STD string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon, embedded digits, commas, nor plus and minus signs. There is no space character separating the time zone name from the OFFSET, so these restrictions are necessary to parse the specification correctly. The OFFSET specifies the time value you must add to the local time to get a Coordinated Universal Time value. It has syntax like [`+'|`-']HH[`:'MM[`:'SS]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between `0' and `23', and the minute and seconds between `0' and `59'. For example, here is how we would specify Eastern Standard Time, but without any Daylight Saving Time alternative: EST+5 The second format is used when there is Daylight Saving Time: STD OFFSET DST [OFFSET]`,'START[`/'TIME]`,'END[`/'TIME] The initial STD and OFFSET specify the standard time zone, as described above. The DST string and OFFSET specify the name and offset for the corresponding Daylight Saving Time zone; if the OFFSET is omitted, it defaults to one hour ahead of standard time. The remainder of the specification describes when Daylight Saving Time is in effect. The START field is when Daylight Saving Time goes into effect and the END field is when the change is made back to standard time. The following formats are recognized for these fields: `JN' This specifies the Julian day, with N between `1' and `365'. February 29 is never counted, even in leap years. `N' This specifies the Julian day, with N between `0' and `365'. February 29 is counted in leap years. `MM.W.D' This specifies day D of week W of month M. The day D must be between `0' (Sunday) and `6'. The week W must be between `1' and `5'; week `1' is the first week in which day D occurs, and week `5' specifies the _last_ D day in the month. The month M should be between `1' and `12'. The TIME fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is `02:00:00'. For example, here is how you would specify the Eastern time zone in the United States, including the appropriate Daylight Saving Time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am. EST+5EDT,M4.1.0/2,M10.5.0/2 The schedule of Daylight Saving Time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below). The third format looks like this: :CHARACTERS Each operating system interprets this format differently; in the GNU C library, CHARACTERS is the name of a file which describes the time zone. If the `TZ' environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification `TZ=:/etc/localtime' (or `TZ=:/usr/local/etc/localtime', depending on how GNU C library was configured; *note Installation::). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them. If CHARACTERS begins with a slash, it is an absolute file name; otherwise the library looks for the file `/share/lib/zoneinfo/CHARACTERS'. The `zoneinfo' directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, `America/New_York', `Europe/London', `Asia/Hong_Kong'. These data files are installed by the system administrator, who also sets `/etc/localtime' to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain. Functions and Variables for Time Zones -------------------------------------- - Variable: char * tzname [2] The array `tzname' contains two strings, which are the standard names of the pair of time zones (standard and Daylight Saving) that the user has selected. `tzname[0]' is the name of the standard time zone (for example, `"EST"'), and `tzname[1]' is the name for the time zone when Daylight Saving Time is in use (for example, `"EDT"'). These correspond to the STD and DST strings (respectively) from the `TZ' environment variable. If Daylight Saving Time is never used, `tzname[1]' is the empty string. The `tzname' array is initialized from the `TZ' environment variable whenever `tzset', `ctime', `strftime', `mktime', or `localtime' is called. If multiple abbreviations have been used (e.g. `"EWT"' and `"EDT"' for U.S. Eastern War Time and Eastern Daylight Time), the array contains the most recent abbreviation. The `tzname' array is required for POSIX.1 compatibility, but in GNU programs it is better to use the `tm_zone' member of the broken-down time structure, since `tm_zone' reports the correct abbreviation even when it is not the latest one. Though the strings are declared as `char *' the user must refrain from modifying these strings. Modifying the strings will almost certainly lead to trouble. - Function: void tzset (void) The `tzset' function initializes the `tzname' variable from the value of the `TZ' environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone. The following variables are defined for compatibility with System V Unix. Like `tzname', these variables are set by calling `tzset' or the other time conversion functions. - Variable: long int timezone This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is `5*60*60'. Unlike the `tm_gmtoff' member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use `tm_gmtoff', since it contains the correct offset even when it is not the latest one. - Variable: int daylight This variable has a nonzero value if Daylight Saving Time rules apply. A nonzero value does not necessarily mean that Daylight Saving Time is now in effect; it means only that Daylight Saving Time is sometimes in effect. Time Functions Example ---------------------- Here is an example program showing the use of some of the calendar time functions. #include #include #define SIZE 256 int main (void) { char buffer[SIZE]; time_t curtime; struct tm *loctime; /* Get the current time. */ curtime = time (NULL); /* Convert it to local time representation. */ loctime = localtime (&curtime); /* Print out the date and time in the standard format. */ fputs (asctime (loctime), stdout); /* Print it out in a nice format. */ strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime); fputs (buffer, stdout); strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime); fputs (buffer, stdout); return 0; } It produces output like this: Wed Jul 31 13:02:36 1991 Today is Wednesday, July 31. The time is 01:02 PM. Setting an Alarm ================ The `alarm' and `setitimer' functions provide a mechanism for a process to interrupt itself in the future. They do this by setting a timer; when the timer expires, the process receives a signal. Each process has three independent interval timers available: * A real-time timer that counts elapsed time. This timer sends a `SIGALRM' signal to the process when it expires. * A virtual timer that counts processor time used by the process. This timer sends a `SIGVTALRM' signal to the process when it expires. * A profiling timer that counts both processor time used by the process, and processor time spent in system calls on behalf of the process. This timer sends a `SIGPROF' signal to the process when it expires. This timer is useful for profiling in interpreters. The interval timer mechanism does not have the fine granularity necessary for profiling native code. You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value. You should establish a handler for the appropriate alarm signal using `signal' or `sigaction' before issuing a call to `setitimer' or `alarm'. Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler. In this case it would be terminated, since termination is the default action for the alarm signals. *Note Signal Handling::. The `setitimer' function is the primary means for setting an alarm. This facility is declared in the header file `sys/time.h'. The `alarm' function, declared in `unistd.h', provides a somewhat simpler interface for setting the real-time timer. - Data Type: struct itimerval This structure is used to specify when a timer should expire. It contains the following members: `struct timeval it_interval' This is the period between successive timer interrupts. If zero, the alarm will only be sent once. `struct timeval it_value' This is the period between now and the first timer interrupt. If zero, the alarm is disabled. The `struct timeval' data type is described in *Note Elapsed Time::. - Function: int setitimer (int WHICH, struct itimerval *NEW, struct itimerval *OLD) The `setitimer' function sets the timer specified by WHICH according to NEW. The WHICH argument can have a value of `ITIMER_REAL', `ITIMER_VIRTUAL', or `ITIMER_PROF'. If OLD is not a null pointer, `setitimer' returns information about any previous unexpired timer of the same kind in the structure it points to. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EINVAL' The timer period is too large. - Function: int getitimer (int WHICH, struct itimerval *OLD) The `getitimer' function stores information about the timer specified by WHICH in the structure pointed at by OLD. The return value and error conditions are the same as for `setitimer'. `ITIMER_REAL' This constant can be used as the WHICH argument to the `setitimer' and `getitimer' functions to specify the real-time timer. `ITIMER_VIRTUAL' This constant can be used as the WHICH argument to the `setitimer' and `getitimer' functions to specify the virtual timer. `ITIMER_PROF' This constant can be used as the WHICH argument to the `setitimer' and `getitimer' functions to specify the profiling timer. - Function: unsigned int alarm (unsigned int SECONDS) The `alarm' function sets the real-time timer to expire in SECONDS seconds. If you want to cancel any existing alarm, you can do this by calling `alarm' with a SECONDS argument of zero. The return value indicates how many seconds remain before the previous alarm would have been sent. If there is no previous alarm, `alarm' returns zero. The `alarm' function could be defined in terms of `setitimer' like this: unsigned int alarm (unsigned int seconds) { struct itimerval old, new; new.it_interval.tv_usec = 0; new.it_interval.tv_sec = 0; new.it_value.tv_usec = 0; new.it_value.tv_sec = (long int) seconds; if (setitimer (ITIMER_REAL, &new, &old) < 0) return 0; else return old.it_value.tv_sec; } There is an example showing the use of the `alarm' function in *Note Handler Returns::. If you simply want your process to wait for a given number of seconds, you should use the `sleep' function. *Note Sleeping::. You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved. *Portability Note:* The `setitimer' and `getitimer' functions are derived from BSD Unix, while the `alarm' function is specified by the POSIX.1 standard. `setitimer' is more powerful than `alarm', but `alarm' is more widely used. Sleeping ======== The function `sleep' gives a simple way to make the program wait for a short interval. If your program doesn't use signals (except to terminate), then you can expect `sleep' to wait reliably throughout the specified interval. Otherwise, `sleep' can return sooner if a signal arrives; if you want to wait for a given interval regardless of signals, use `select' (*note Waiting for I/O::) and don't specify any descriptors to wait for. - Function: unsigned int sleep (unsigned int SECONDS) The `sleep' function waits for SECONDS or until a signal is delivered, whichever happens first. If `sleep' function returns because the requested interval is over, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep interval. The `sleep' function is declared in `unistd.h'. Resist the temptation to implement a sleep for a fixed amount of time by using the return value of `sleep', when nonzero, to call `sleep' again. This will work with a certain amount of accuracy as long as signals arrive infrequently. But each signal can cause the eventual wakeup time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck--there is no limit on how much this could shorten or lengthen the wait. Instead, compute the calendar time at which the program should stop waiting, and keep trying to wait until that calendar time. This won't be off by more than a second. With just a little more work, you can use `select' and make the waiting period quite accurate. (Of course, heavy system load can cause additional unavoidable delays--unless the machine is dedicated to one application, there is no way you can avoid this.) On some systems, `sleep' can do strange things if your program uses `SIGALRM' explicitly. Even if `SIGALRM' signals are being ignored or blocked when `sleep' is called, `sleep' might return prematurely on delivery of a `SIGALRM' signal. If you have established a handler for `SIGALRM' signals and a `SIGALRM' signal is delivered while the process is sleeping, the action taken might be just to cause `sleep' to return instead of invoking your handler. And, if `sleep' is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of `SIGALRM', this handler and `sleep' will interfere. On the GNU system, it is safe to use `sleep' and `SIGALRM' in the same program, because `sleep' does not work by means of `SIGALRM'. - Function: int nanosleep (const struct timespec *REQUESTED_TIME, struct timespec *REMAINING) If resolution to seconds is not enough the `nanosleep' function can be used. As the name suggests the sleep interval can be specified in nanoseconds. The actual elapsed time of the sleep interval might be longer since the system rounds the elapsed time you request up to the next integer multiple of the actual resolution the system can deliver. *`requested_time' is the elapsed time of the interval you want to sleep. The function returns as *`remaining' the elapsed time left in the interval for which you requested to sleep. If the interval completed without getting interrupted by a signal, this is zero. `struct timespec' is described in *Note Elapsed Time::. If the function returns because the interval is over the return value is zero. If the function returns -1 the global variable ERRNO is set to the following values: `EINTR' The call was interrupted because a signal was delivered to the thread. If the REMAINING parameter is not the null pointer the structure pointed to by REMAINING is updated to contain the remaining elapsed time. `EINVAL' The nanosecond value in the REQUESTED_TIME parameter contains an illegal value. Either the value is negative or greater than or equal to 1000 million. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `nanosleep' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `nanosleep' should be protected using cancellation handlers. The `nanosleep' function is declared in `time.h'. Resource Usage And Limitation ***************************** This chapter describes functions for examining how much of various kinds of resources (CPU time, memory, etc.) a process has used and getting and setting limits on future usage. Resource Usage ============== The function `getrusage' and the data type `struct rusage' are used to examine the resource usage of a process. They are declared in `sys/resource.h'. - Function: int getrusage (int PROCESSES, struct rusage *RUSAGE) This function reports resource usage totals for processes specified by PROCESSES, storing the information in `*RUSAGE'. In most systems, PROCESSES has only two valid values: `RUSAGE_SELF' Just the current process. `RUSAGE_CHILDREN' All child processes (direct and indirect) that have already terminated. In the GNU system, you can also inquire about a particular child process by specifying its process ID. The return value of `getrusage' is zero for success, and `-1' for failure. `EINVAL' The argument PROCESSES is not valid. One way of getting resource usage for a particular child process is with the function `wait4', which returns totals for a child when it terminates. *Note BSD Wait Functions::. - Data Type: struct rusage This data type stores various resource usage statistics. It has the following members, and possibly others: `struct timeval ru_utime' Time spent executing user instructions. `struct timeval ru_stime' Time spent in operating system code on behalf of PROCESSES. `long int ru_maxrss' The maximum resident set size used, in kilobytes. That is, the maximum number of kilobytes of physical memory that PROCESSES used simultaneously. `long int ru_ixrss' An integral value expressed in kilobytes times ticks of execution, which indicates the amount of memory used by text that was shared with other processes. `long int ru_idrss' An integral value expressed the same way, which is the amount of unshared memory used for data. `long int ru_isrss' An integral value expressed the same way, which is the amount of unshared memory used for stack space. `long int ru_minflt' The number of page faults which were serviced without requiring any I/O. `long int ru_majflt' The number of page faults which were serviced by doing I/O. `long int ru_nswap' The number of times PROCESSES was swapped entirely out of main memory. `long int ru_inblock' The number of times the file system had to read from the disk on behalf of PROCESSES. `long int ru_oublock' The number of times the file system had to write to the disk on behalf of PROCESSES. `long int ru_msgsnd' Number of IPC messages sent. `long int ru_msgrcv' Number of IPC messages received. `long int ru_nsignals' Number of signals received. `long int ru_nvcsw' The number of times PROCESSES voluntarily invoked a context switch (usually to wait for some service). `long int ru_nivcsw' The number of times an involuntary context switch took place (because a time slice expired, or another process of higher priority was scheduled). `vtimes' is a historical function that does some of what `getrusage' does. `getrusage' is a better choice. `vtimes' and its `vtimes' data structure are declared in `sys/vtimes.h'. - Function: int vtimes (struct vtimes CURRENT, struct vtimes CHILD) `vtimes' reports resource usage totals for a process. If CURRENT is non-null, `vtimes' stores resource usage totals for the invoking process alone in the structure to which it points. If CHILD is non-null, `vtimes' stores resource usage totals for all past children (which have terminated) of the invoking process in the structure to which it points. - Data Type: struct vtimes This data type contains information about the resource usage of a process. Each member corresponds to a member of the `struct rusage' data type described above. `vm_utime' User CPU time. Analogous to `ru_utime' in `struct rusage' `vm_stime' System CPU time. Analogous to `ru_stime' in `struct rusage' `vm_idsrss' Data and stack memory. The sum of the values that would be reported as `ru_idrss' and `ru_isrss' in `struct rusage' `vm_ixrss' Shared memory. Analogous to `ru_ixrss' in `struct rusage' `vm_maxrss' Maximent resident set size. Analogous to `ru_maxrss' in `struct rusage' `vm_majflt' Major page faults. Analogous to `ru_majflt' in `struct rusage' `vm_minflt' Minor page faults. Analogous to `ru_minflt' in `struct rusage' `vm_nswap' Swap count. Analogous to `ru_nswap' in `struct rusage' `vm_inblk' Disk reads. Analogous to `ru_inblk' in `struct rusage' `vm_oublk' Disk writes. Analogous to `ru_oublk' in `struct rusage' The return value is zero if the function succeeds; `-1' otherwise. An additional historical function for examining resource usage, `vtimes', is supported but not documented here. It is declared in `sys/vtimes.h'. Limiting Resource Usage ======================= You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the resource. Each process initially inherits its limit values from its parent, but it can subsequently change them. There are two per-process limits associated with a resource: "current limit" The current limit is the value the system will not allow usage to exceed. It is also called the "soft limit" because the process being limited can generally raise the current limit at will. "maximum limit" The maximum limit is the maximum value to which a process is allowed to set its current limit. It is also called the "hard limit" because there is no way for a process to get around it. A process may lower its own maximum limit, but only the superuser may increase a maximum limit. The symbols for use with `getrlimit', `setrlimit', `getrlimit64', and `setrlimit64' are defined in `sys/resource.h'. - Function: int getrlimit (int RESOURCE, struct rlimit *RLP) Read the current and maximum limits for the resource RESOURCE and store them in `*RLP'. The return value is `0' on success and `-1' on failure. The only possible `errno' error condition is `EFAULT'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `getrlimit64'. Thus, the LFS interface transparently replaces the old interface. - Function: int getrlimit64 (int RESOURCE, struct rlimit64 *RLP) This function is similar to `getrlimit' but its second parameter is a pointer to a variable of type `struct rlimit64', which allows it to read values which wouldn't fit in the member of a `struct rlimit'. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit machine, this function is available under the name `getrlimit' and so transparently replaces the old interface. - Function: int setrlimit (int RESOURCE, const struct rlimit *RLP) Store the current and maximum limits for the resource RESOURCE in `*RLP'. The return value is `0' on success and `-1' on failure. The following `errno' error condition is possible: `EPERM' * The process tried to raise a current limit beyond the maximum limit. * The process tried to raise a maximum limit, but is not superuser. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `setrlimit64'. Thus, the LFS interface transparently replaces the old interface. - Function: int setrlimit64 (int RESOURCE, const struct rlimit64 *RLP) This function is similar to `setrlimit' but its second parameter is a pointer to a variable of type `struct rlimit64' which allows it to set values which wouldn't fit in the member of a `struct rlimit'. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit machine this function is available under the name `setrlimit' and so transparently replaces the old interface. - Data Type: struct rlimit This structure is used with `getrlimit' to receive limit values, and with `setrlimit' to specify limit values for a particular process and resource. It has two fields: `rlim_t rlim_cur' The current limit `rlim_t rlim_max' The maximum limit. For `getrlimit', the structure is an output; it receives the current values. For `setrlimit', it specifies the new values. For the LFS functions a similar type is defined in `sys/resource.h'. - Data Type: struct rlimit64 This structure is analogous to the `rlimit' structure above, but its components have wider ranges. It has two fields: `rlim64_t rlim_cur' This is analogous to `rlimit.rlim_cur', but with a different type. `rlim64_t rlim_max' This is analogous to `rlimit.rlim_max', but with a different type. Here is a list of resources for which you can specify a limit. Memory and file sizes are measured in bytes. `RLIMIT_CPU' The maximum amount of CPU time the process can use. If it runs for longer than this, it gets a signal: `SIGXCPU'. The value is measured in seconds. *Note Operation Error Signals::. `RLIMIT_FSIZE' The maximum size of file the process can create. Trying to write a larger file causes a signal: `SIGXFSZ'. *Note Operation Error Signals::. `RLIMIT_DATA' The maximum size of data memory for the process. If the process tries to allocate data memory beyond this amount, the allocation function fails. `RLIMIT_STACK' The maximum stack size for the process. If the process tries to extend its stack past this size, it gets a `SIGSEGV' signal. *Note Program Error Signals::. `RLIMIT_CORE' The maximum size core file that this process can create. If the process terminates and would dump a core file larger than this, then no core file is created. So setting this limit to zero prevents core files from ever being created. `RLIMIT_RSS' The maximum amount of physical memory that this process should get. This parameter is a guide for the system's scheduler and memory allocator; the system may give the process more memory when there is a surplus. `RLIMIT_MEMLOCK' The maximum amount of memory that can be locked into physical memory (so it will never be paged out). `RLIMIT_NPROC' The maximum number of processes that can be created with the same user ID. If you have reached the limit for your user ID, `fork' will fail with `EAGAIN'. *Note Creating a Process::. `RLIMIT_NOFILE' `RLIMIT_OFILE' The maximum number of files that the process can open. If it tries to open more files than this, its open attempt fails with `errno' `EMFILE'. *Note Error Codes::. Not all systems support this limit; GNU does, and 4.4 BSD does. `RLIMIT_AS' The maximum size of total memory that this process should get. If the process tries to allocate more memory beyond this amount with, for example, `brk', `malloc', `mmap' or `sbrk', the allocation function fails. `RLIM_NLIMITS' The number of different resource limits. Any valid RESOURCE operand must be less than `RLIM_NLIMITS'. - Constant: int RLIM_INFINITY This constant stands for a value of "infinity" when supplied as the limit value in `setrlimit'. The following are historical functions to do some of what the functions above do. The functions above are better choices. `ulimit' and the command symbols are declared in `ulimit.h'. - Function: int ulimit (int CMD, ...) `ulimit' gets the current limit or sets the current and maximum limit for a particular resource for the calling process according to the command CMD.a If you are getting a limit, the command argument is the only argument. If you are setting a limit, there is a second argument: `long int' LIMIT which is the value to which you are setting the limit. The CMD values and the operations they specify are: `GETFSIZE' Get the current limit on the size of a file, in units of 512 bytes. `SETFSIZE' Set the current and maximum limit on the size of a file to LIMIT * 512 bytes. There are also some other CMD values that may do things on some systems, but they are not supported. Only the superuser may increase a maximum limit. When you successfully get a limit, the return value of `ulimit' is that limit, which is never negative. When you successfully set a limit, the return value is zero. When the function fails, the return value is `-1' and `errno' is set according to the reason: `EPERM' A process tried to increase a maximum limit, but is not superuser. `vlimit' and its resource symbols are declared in `sys/vlimit.h'. - Function: int vlimit (int RESOURCE, int LIMIT) `vlimit' sets the current limit for a resource for a process. RESOURCE identifies the resource: `LIM_CPU' Maximum CPU time. Same as `RLIMIT_CPU' for `setrlimit'. `LIM_FSIZE' Maximum file size. Same as `RLIMIT_FSIZE' for `setrlimit'. `LIM_DATA' Maximum data memory. Same as `RLIMIT_DATA' for `setrlimit'. `LIM_STACK' Maximum stack size. Same as `RLIMIT_STACK' for `setrlimit'. `LIM_CORE' Maximum core file size. Same as `RLIMIT_COR' for `setrlimit'. `LIM_MAXRSS' Maximum physical memory. Same as `RLIMIT_RSS' for `setrlimit'. The return value is zero for success, and `-1' with `errno' set accordingly for failure: `EPERM' The process tried to set its current limit beyond its maximum limit. Process CPU Priority And Scheduling =================================== When multiple processes simultaneously require CPU time, the system's scheduling policy and process CPU priorities determine which processes get it. This section describes how that determination is made and GNU C library functions to control it. It is common to refer to CPU scheduling simply as scheduling and a process' CPU priority simply as the process' priority, with the CPU resource being implied. Bear in mind, though, that CPU time is not the only resource a process uses or that processes contend for. In some cases, it is not even particularly important. Giving a process a high "priority" may have very little effect on how fast a process runs with respect to other processes. The priorities discussed in this section apply only to CPU time. CPU scheduling is a complex issue and different systems do it in wildly different ways. New ideas continually develop and find their way into the intricacies of the various systems' scheduling algorithms. This section discusses the general concepts, some specifics of systems that commonly use the GNU C library, and some standards. For simplicity, we talk about CPU contention as if there is only one CPU in the system. But all the same principles apply when a processor has multiple CPUs, and knowing that the number of processes that can run at any one time is equal to the number of CPUs, you can easily extrapolate the information. The functions described in this section are all defined by the POSIX.1 and POSIX.1b standards (the `sched...' functions are POSIX.1b). However, POSIX does not define any semantics for the values that these functions get and set. In this chapter, the semantics are based on the Linux kernel's implementation of the POSIX standard. As you will see, the Linux implementation is quite the inverse of what the authors of the POSIX syntax had in mind. Absolute Priority ----------------- Every process has an absolute priority, and it is represented by a number. The higher the number, the higher the absolute priority. On systems of the past, and most systems today, all processes have absolute priority 0 and this section is irrelevant. In that case, *Note Traditional Scheduling::. Absolute priorities were invented to accommodate realtime systems, in which it is vital that certain processes be able to respond to external events happening in real time, which means they cannot wait around while some other process that _wants to_, but doesn't _need to_ run occupies the CPU. When two processes are in contention to use the CPU at any instant, the one with the higher absolute priority always gets it. This is true even if the process with the lower priority is already using the CPU (i.e. the scheduling is preemptive). Of course, we're only talking about processes that are running or "ready to run," which means they are ready to execute instructions right now. When a process blocks to wait for something like I/O, its absolute priority is irrelevant. *Note:* The term "runnable" is a synonym for "ready to run." When two processes are running or ready to run and both have the same absolute priority, it's more interesting. In that case, who gets the CPU is determined by the scheduling policy. If the processes have absolute priority 0, the traditional scheduling policy described in *Note Traditional Scheduling:: applies. Otherwise, the policies described in *Note Realtime Scheduling:: apply. You normally give an absolute priority above 0 only to a process that can be trusted not to hog the CPU. Such processes are designed to block (or terminate) after relatively short CPU runs. A process begins life with the same absolute priority as its parent process. Functions described in *Note Basic Scheduling Functions:: can change it. Only a privileged process can change a process' absolute priority to something other than `0'. Only a privileged process or the target process' owner can change its absolute priority at all. POSIX requires absolute priority values used with the realtime scheduling policies to be consecutive with a range of at least 32. On Linux, they are 1 through 99. The functions `sched_get_priority_max' and `sched_set_priority_min' portably tell you what the range is on a particular system. Using Absolute Priority ....................... One thing you must keep in mind when designing real time applications is that having higher absolute priority than any other process doesn't guarantee the process can run continuously. Two things that can wreck a good CPU run are interrupts and page faults. Interrupt handlers live in that limbo between processes. The CPU is executing instructions, but they aren't part of any process. An interrupt will stop even the highest priority process. So you must allow for slight delays and make sure that no device in the system has an interrupt handler that could cause too long a delay between instructions for your process. Similarly, a page fault causes what looks like a straightforward sequence of instructions to take a long time. The fact that other processes get to run while the page faults in is of no consequence, because as soon as the I/O is complete, the high priority process will kick them out and run again, but the wait for the I/O itself could be a problem. To neutralize this threat, use `mlock' or `mlockall'. There are a few ramifications of the absoluteness of this priority on a single-CPU system that you need to keep in mind when you choose to set a priority and also when you're working on a program that runs with high absolute priority. Consider a process that has higher absolute priority than any other process in the system and due to a bug in its program, it gets into an infinite loop. It will never cede the CPU. You can't run a command to kill it because your command would need to get the CPU in order to run. The errant program is in complete control. It controls the vertical, it controls the horizontal. There are two ways to avoid this: 1) keep a shell running somewhere with a higher absolute priority. 2) keep a controlling terminal attached to the high priority process group. All the priority in the world won't stop an interrupt handler from running and delivering a signal to the process if you hit Control-C. Some systems use absolute priority as a means of allocating a fixed percentage of CPU time to a process. To do this, a super high priority privileged process constantly monitors the process' CPU usage and raises its absolute priority when the process isn't getting its entitled share and lowers it when the process is exceeding it. *Note:* The absolute priority is sometimes called the "static priority." We don't use that term in this manual because it misses the most important feature of the absolute priority: its absoluteness. Realtime Scheduling ------------------- Whenever two processes with the same absolute priority are ready to run, the kernel has a decision to make, because only one can run at a time. If the processes have absolute priority 0, the kernel makes this decision as described in *Note Traditional Scheduling::. Otherwise, the decision is as described in this section. If two processes are ready to run but have different absolute priorities, the decision is much simpler, and is described in *Note Absolute Priority::. Each process has a scheduling policy. For processes with absolute priority other than zero, there are two available: 1. First Come First Served 2. Round Robin The most sensible case is where all the processes with a certain absolute priority have the same scheduling policy. We'll discuss that first. In Round Robin, processes share the CPU, each one running for a small quantum of time ("time slice") and then yielding to another in a circular fashion. Of course, only processes that are ready to run and have the same absolute priority are in this circle. In First Come First Served, the process that has been waiting the longest to run gets the CPU, and it keeps it until it voluntarily relinquishes the CPU, runs out of things to do (blocks), or gets preempted by a higher priority process. First Come First Served, along with maximal absolute priority and careful control of interrupts and page faults, is the one to use when a process absolutely, positively has to run at full CPU speed or not at all. Judicious use of `sched_yield' function invocations by processes with First Come First Served scheduling policy forms a good compromise between Round Robin and First Come First Served. To understand how scheduling works when processes of different scheduling policies occupy the same absolute priority, you have to know the nitty gritty details of how processes enter and exit the ready to run list: In both cases, the ready to run list is organized as a true queue, where a process gets pushed onto the tail when it becomes ready to run and is popped off the head when the scheduler decides to run it. Note that ready to run and running are two mutually exclusive states. When the scheduler runs a process, that process is no longer ready to run and no longer in the ready to run list. When the process stops running, it may go back to being ready to run again. The only difference between a process that is assigned the Round Robin scheduling policy and a process that is assigned First Come First Serve is that in the former case, the process is automatically booted off the CPU after a certain amount of time. When that happens, the process goes back to being ready to run, which means it enters the queue at the tail. The time quantum we're talking about is small. Really small. This is not your father's timesharing. For example, with the Linux kernel, the round robin time slice is a thousand times shorter than its typical time slice for traditional scheduling. A process begins life with the same scheduling policy as its parent process. Functions described in *Note Basic Scheduling Functions:: can change it. Only a privileged process can set the scheduling policy of a process that has absolute priority higher than 0. Basic Scheduling Functions -------------------------- This section describes functions in the GNU C library for setting the absolute priority and scheduling policy of a process. *Portability Note:* On systems that have the functions in this section, the macro _POSIX_PRIORITY_SCHEDULING is defined in `'. For the case that the scheduling policy is traditional scheduling, more functions to fine tune the scheduling are in *Note Traditional Scheduling::. Don't try to make too much out of the naming and structure of these functions. They don't match the concepts described in this manual because the functions are as defined by POSIX.1b, but the implementation on systems that use the GNU C library is the inverse of what the POSIX structure contemplates. The POSIX scheme assumes that the primary scheduling parameter is the scheduling policy and that the priority value, if any, is a parameter of the scheduling policy. In the implementation, though, the priority value is king and the scheduling policy, if anything, only fine tunes the effect of that priority. The symbols in this section are declared by including file `sched.h'. - Data Type: struct sched_param This structure describes an absolute priority. `int sched_priority' absolute priority value - Function: int sched_setscheduler (pid_t PID, int POLICY, const struct sched_param *PARAM) This function sets both the absolute priority and the scheduling policy for a process. It assigns the absolute priority value given by PARAM and the scheduling policy POLICY to the process with Process ID PID, or the calling process if PID is zero. If POLICY is negative, `sched_setscheduler' keeps the existing scheduling policy. The following macros represent the valid values for POLICY: `SCHED_OTHER' Traditional Scheduling `SCHED_FIFO' First In First Out `SCHED_RR' Round Robin On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `EPERM' * The calling process does not have `CAP_SYS_NICE' permission and POLICY is not `SCHED_OTHER' (or it's negative and the existing policy is not `SCHED_OTHER'. * The calling process does not have `CAP_SYS_NICE' permission and its owner is not the target process' owner. I.e. the effective uid of the calling process is neither the effective nor the real uid of process PID. `ESRCH' There is no process with pid PID and PID is not zero. `EINVAL' * POLICY does not identify an existing scheduling policy. * The absolute priority value identified by *PARAM is outside the valid range for the scheduling policy POLICY (or the existing scheduling policy if POLICY is negative) or PARAM is null. `sched_get_priority_max' and `sched_get_priority_min' tell you what the valid range is. * PID is negative. - Function: int sched_getscheduler (pid_t PID) This function returns the scheduling policy assigned to the process with Process ID (pid) PID, or the calling process if PID is zero. The return value is the scheduling policy. See `sched_setscheduler' for the possible values. If the function fails, the return value is instead `-1' and `errno' is set accordingly. The `errno' values specific to this function are: `ESRCH' There is no process with pid PID and it is not zero. `EINVAL' PID is negative. Note that this function is not an exact mate to `sched_setscheduler' because while that function sets the scheduling policy and the absolute priority, this function gets only the scheduling policy. To get the absolute priority, use `sched_getparam'. - Function: int sched_setparam (pid_t PID, const struct sched_param *PARAM) This function sets a process' absolute priority. It is functionally identical to `sched_setscheduler' with POLICY = `-1'. - Function: int sched_getparam (pid_t PID, const struct sched_param *PARAM) This function returns a process' absolute priority. PID is the Process ID (pid) of the process whose absolute priority you want to know. PARAM is a pointer to a structure in which the function stores the absolute priority of the process. On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `ESRCH' There is no process with pid PID and it is not zero. `EINVAL' PID is negative. - Function: int sched_get_priority_min (int *POLICY); This function returns the lowest absolute priority value that is allowable for a process with scheduling policy POLICY. On Linux, it is 0 for SCHED_OTHER and 1 for everything else. On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `EINVAL' POLICY does not identify an existing scheduling policy. - Function: int sched_get_priority_max (int *POLICY); This function returns the highest absolute priority value that is allowable for a process that with scheduling policy POLICY. On Linux, it is 0 for SCHED_OTHER and 99 for everything else. On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `EINVAL' POLICY does not identify an existing scheduling policy. - Function: int sched_rr_get_interval (pid_t PID, struct timespec *INTERVAL) This function returns the length of the quantum (time slice) used with the Round Robin scheduling policy, if it is used, for the process with Process ID PID. It returns the length of time as INTERVAL. With a Linux kernel, the round robin time slice is always 150 microseconds, and PID need not even be a real pid. The return value is `0' on success and in the pathological case that it fails, the return value is `-1' and `errno' is set accordingly. There is nothing specific that can go wrong with this function, so there are no specific `errno' values. - Function: int sched_yield (void) This function voluntarily gives up the process' claim on the CPU. Technically, `sched_yield' causes the calling process to be made immediately ready to run (as opposed to running, which is what it was before). This means that if it has absolute priority higher than 0, it gets pushed onto the tail of the queue of processes that share its absolute priority and are ready to run, and it will run again when its turn next arrives. If its absolute priority is 0, it is more complicated, but still has the effect of yielding the CPU to other processes. If there are no other processes that share the calling process' absolute priority, this function doesn't have any effect. To the extent that the containing program is oblivious to what other processes in the system are doing and how fast it executes, this function appears as a no-op. The return value is `0' on success and in the pathological case that it fails, the return value is `-1' and `errno' is set accordingly. There is nothing specific that can go wrong with this function, so there are no specific `errno' values. Traditional Scheduling ---------------------- This section is about the scheduling among processes whose absolute priority is 0. When the system hands out the scraps of CPU time that are left over after the processes with higher absolute priority have taken all they want, the scheduling described herein determines who among the great unwashed processes gets them. Introduction To Traditional Scheduling ...................................... Long before there was absolute priority (See *Note Absolute Priority::), Unix systems were scheduling the CPU using this system. When Posix came in like the Romans and imposed absolute priorities to accommodate the needs of realtime processing, it left the indigenous Absolute Priority Zero processes to govern themselves by their own familiar scheduling policy. Indeed, absolute priorities higher than zero are not available on many systems today and are not typically used when they are, being intended mainly for computers that do realtime processing. So this section describes the only scheduling many programmers need to be concerned about. But just to be clear about the scope of this scheduling: Any time a process with a absolute priority of 0 and a process with an absolute priority higher than 0 are ready to run at the same time, the one with absolute priority 0 does not run. If it's already running when the higher priority ready-to-run process comes into existence, it stops immediately. In addition to its absolute priority of zero, every process has another priority, which we will refer to as "dynamic priority" because it changes over time. The dynamic priority is meaningless for processes with an absolute priority higher than zero. The dynamic priority sometimes determines who gets the next turn on the CPU. Sometimes it determines how long turns last. Sometimes it determines whether a process can kick another off the CPU. In Linux, the value is a combination of these things, but mostly it is just determines the length of the time slice. The higher a process' dynamic priority, the longer a shot it gets on the CPU when it gets one. If it doesn't use up its time slice before giving up the CPU to do something like wait for I/O, it is favored for getting the CPU back when it's ready for it, to finish out its time slice. Other than that, selection of processes for new time slices is basically round robin. But the scheduler does throw a bone to the low priority processes: A process' dynamic priority rises every time it is snubbed in the scheduling process. In Linux, even the fat kid gets to play. The fluctuation of a process' dynamic priority is regulated by another value: The "nice" value. The nice value is an integer, usually in the range -20 to 20, and represents an upper limit on a process' dynamic priority. The higher the nice number, the lower that limit. On a typical Linux system, for example, a process with a nice value of 20 can get only 10 milliseconds on the CPU at a time, whereas a process with a nice value of -20 can achieve a high enough priority to get 400 milliseconds. The idea of the nice value is deferential courtesy. In the beginning, in the Unix garden of Eden, all processes shared equally in the bounty of the computer system. But not all processes really need the same share of CPU time, so the nice value gave a courteous process the ability to refuse its equal share of CPU time that others might prosper. Hence, the higher a process' nice value, the nicer the process is. (Then a snake came along and offered some process a negative nice value and the system became the crass resource allocation system we know today). Dynamic priorities tend upward and downward with an objective of smoothing out allocation of CPU time and giving quick response time to infrequent requests. But they never exceed their nice limits, so on a heavily loaded CPU, the nice value effectively determines how fast a process runs. In keeping with the socialistic heritage of Unix process priority, a process begins life with the same nice value as its parent process and can raise it at will. A process can also raise the nice value of any other process owned by the same user (or effective user). But only a privileged process can lower its nice value. A privileged process can also raise or lower another process' nice value. GNU C Library functions for getting and setting nice values are described in *Note Traditional Scheduling Functions::. Functions For Traditional Scheduling .................................... This section describes how you can read and set the nice value of a process. All these symbols are declared in `sys/resource.h'. The function and macro names are defined by POSIX, and refer to "priority," but the functions actually have to do with nice values, as the terms are used both in the manual and POSIX. The range of valid nice values depends on the kernel, but typically it runs from `-20' to `20'. A lower nice value corresponds to higher priority for the process. These constants describe the range of priority values: `PRIO_MIN' The lowest valid nice value. `PRIO_MAX' The highest valid nice value. - Function: int getpriority (int CLASS, int ID) Return the nice value of a set of processes; CLASS and ID specify which ones (see below). If the processes specified do not all have the same nice value, this returns the lowest value that any of them has. On success, the return value is `0'. Otherwise, it is `-1' and `ERRNO' is set accordingly. The `errno' values specific to this function are: `ESRCH' The combination of CLASS and ID does not match any existing process. `EINVAL' The value of CLASS is not valid. If the return value is `-1', it could indicate failure, or it could be the nice value. The only way to make certain is to set `errno = 0' before calling `getpriority', then use `errno != 0' afterward as the criterion for failure. - Function: int setpriority (int CLASS, int ID, int NICEVAL) Set the nice value of a set of processes to NICEVAL; CLASS and ID specify which ones (see below). The return value is `0' on success, and `-1' on failure. The following `errno' error condition are possible for this function: `ESRCH' The combination of CLASS and ID does not match any existing process. `EINVAL' The value of CLASS is not valid. `EPERM' The call would set the nice value of a process which is owned by a different user than the calling process (i.e. the target process' real or effective uid does not match the calling process' effective uid) and the calling process does not have `CAP_SYS_NICE' permission. `EACCES' The call would lower the process' nice value and the process does not have `CAP_SYS_NICE' permission. The arguments CLASS and ID together specify a set of processes in which you are interested. These are the possible values of CLASS: `PRIO_PROCESS' One particular process. The argument ID is a process ID (pid). `PRIO_PGRP' All the processes in a particular process group. The argument ID is a process group ID (pgid). `PRIO_USER' All the processes owned by a particular user (i.e. whose real uid indicates the user). The argument ID is a user ID (uid). If the argument ID is 0, it stands for the calling process, its process group, or its owner (real uid), according to CLASS. - Function: int nice (int INCREMENT) Increment the nice value of the calling process by INCREMENT. The return value is the new nice value on success, and `-1' on failure. In the case of failure, `errno' will be set to the same values as for `setpriority'. Here is an equivalent definition of `nice': int nice (int increment) { int result, old = getpriority (PRIO_PROCESS, 0); result = setpriority (PRIO_PROCESS, 0, old + increment); if (result != -1) return old + increment; else return -1; } Querying memory available resources =================================== The amount of memory available in the system and the way it is organized determines oftentimes the way programs can and have to work. For functions like `mmap' it is necessary to know about the size of individual memory pages and knowing how much memory is available enables a program to select appropriate sizes for, say, caches. Before we get into these details a few words about memory subsystems in traditional Unix systems will be given. Overview about traditional Unix memory handling ----------------------------------------------- Unix systems normally provide processes virtual address spaces. This means that the addresses of the memory regions do not have to correspond directly to the addresses of the actual physical memory which stores the data. An extra level of indirection is introduced which translates virtual addresses into physical addresses. This is normally done by the hardware of the processor. Using a virtual address space has several advantage. The most important is process isolation. The different processes running on the system cannot interfere directly with each other. No process can write into the address space of another process (except when shared memory is used but then it is wanted and controlled). Another advantage of virtual memory is that the address space the processes see can actually be larger than the physical memory available. The physical memory can be extended by storage on an external media where the content of currently unused memory regions is stored. The address translation can then intercept accesses to these memory regions and make memory content available again by loading the data back into memory. This concept makes it necessary that programs which have to use lots of memory know the difference between available virtual address space and available physical memory. If the working set of virtual memory of all the processes is larger than the available physical memory the system will slow down dramatically due to constant swapping of memory content from the memory to the storage media and back. This is called "thrashing". A final aspect of virtual memory which is important and follows from what is said in the last paragraph is the granularity of the virtual address space handling. When we said that the virtual address handling stores memory content externally it cannot do this on a byte-by-byte basis. The administrative overhead does not allow this (leaving alone the processor hardware). Instead several thousand bytes are handled together and form a "page". The size of each page is always a power of two byte. The smallest page size in use today is 4096, with 8192, 16384, and 65536 being other popular sizes. How to get information about the memory subsystem? -------------------------------------------------- The page size of the virtual memory the process sees is essential to know in several situations. Some programming interface (e.g., `mmap', *note Memory-mapped I/O::) require the user to provide information adjusted to the page size. In the case of `mmap' is it necessary to provide a length argument which is a multiple of the page size. Another place where the knowledge about the page size is useful is in memory allocation. If one allocates pieces of memory in larger chunks which are then subdivided by the application code it is useful to adjust the size of the larger blocks to the page size. If the total memory requirement for the block is close (but not larger) to a multiple of the page size the kernel's memory handling can work more effectively since it only has to allocate memory pages which are fully used. (To do this optimization it is necessary to know a bit about the memory allocator which will require a bit of memory itself for each block and this overhead must not push the total size over the page size multiple. The page size traditionally was a compile time constant. But recent development of processors changed this. Processors now support different page sizes and they can possibly even vary among different processes on the same system. Therefore the system should be queried at runtime about the current page size and no assumptions (except about it being a power of two) should be made. The correct interface to query about the page size is `sysconf' (*note Sysconf Definition::) with the parameter `_SC_PAGESIZE'. There is a much older interface available, too. - Function: int getpagesize (void) The `getpagesize' function returns the page size of the process. This value is fixed for the runtime of the process but can vary in different runs of the application. The function is declared in `unistd.h'. Widely available on System V derived systems is a method to get information about the physical memory the system has. The call sysconf (_SC_PHYS_PAGES) returns the total number of pages of physical the system has. This does not mean all this memory is available. This information can be found using sysconf (_SC_AVPHYS_PAGES) These two values help to optimize applications. The value returned for `_SC_AVPHYS_PAGES' is the amount of memory the application can use without hindering any other process (given that no other process increases its memory usage). The value returned for `_SC_PHYS_PAGES' is more or less a hard limit for the working set. If all applications together constantly use more than that amount of memory the system is in trouble. The GNU C library provides in addition to these already described way to get this information two functions. They are declared in the file `sys/sysinfo.h'. Programmers should prefer to use the `sysconf' method described above. - Function: long int get_phys_pages (void) The `get_phys_pages' function returns the total number of pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size. This function is a GNU extension. - Function: long int get_avphys_pages (void) The `get_phys_pages' function returns the number of available pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size. This function is a GNU extension. Learn about the processors available ==================================== The use of threads or processes with shared memory allows an application to take advantage of all the processing power a system can provide. If the task can be parallelized the optimal way to write an application is to have at any time as many processes running as there are processors. To determine the number of processors available to the system one can run sysconf (_SC_NPROCESSORS_CONF) which returns the number of processors the operating system configured. But it might be possible for the operating system to disable individual processors and so the call sysconf (_SC_NPROCESSORS_ONLN) returns the number of processors which are currently inline (i.e., available). For these two pieces of information the GNU C library also provides functions to get the information directly. The functions are declared in `sys/sysinfo.h'. - Function: int get_nprocs_conf (void) The `get_nprocs_conf' function returns the number of processors the operating system configured. This function is a GNU extension. - Function: int get_nprocs (void) The `get_nprocs' function returns the number of available processors. This function is a GNU extension. Before starting more threads it should be checked whether the processors are not already overused. Unix systems calculate something called the "load average". This is a number indicating how many processes were running. This number is average over different periods of times (normally 1, 5, and 15 minutes). - Function: int getloadavg (double LOADAVG[], int NELEM) This function gets the 1, 5 and 15 minute load averages of the system. The values are placed in LOADAVG. `getloadavg' will place at most NELEM elements into the array but never more than three elements. The return value is the number of elements written to LOADAVG, or -1 on error. This function is declared in `stdlib.h'. Non-Local Exits *************** Sometimes when your program detects an unusual situation inside a deeply nested set of function calls, you would like to be able to immediately return to an outer level of control. This section describes how you can do such "non-local exits" using the `setjmp' and `longjmp' functions. Introduction to Non-Local Exits =============================== As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a "main loop" that prompts for and executes commands. Suppose the "read" command reads input from a file, doing some lexical analysis and parsing of the input while processing it. If a low-level input error is detected, it would be useful to be able to return immediately to the "main loop" instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls. (On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits--such as closing files, deallocating buffers or other data structures, and the like--then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely. Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the "main loop".) In some ways, a non-local exit is similar to using the `return' statement to return from a function. But while `return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls. You identify return points for non-local exits by calling the function `setjmp'. This function saves information about the execution environment in which the call to `setjmp' appears in an object of type `jmp_buf'. Execution of the program continues normally after the call to `setjmp', but if an exit is later made to this return point by calling `longjmp' with the corresponding `jmp_buf' object, control is transferred back to the point where `setjmp' was called. The return value from `setjmp' is used to distinguish between an ordinary return and a return made by a call to `longjmp', so calls to `setjmp' usually appear in an `if' statement. Here is how the example program described above might be set up: #include #include #include jmp_buf main_loop; void abort_to_main_loop (int status) { longjmp (main_loop, status); } int main (void) { while (1) if (setjmp (main_loop)) puts ("Back at main loop...."); else do_command (); } void do_command (void) { char buffer[128]; if (fgets (buffer, 128, stdin) == NULL) abort_to_main_loop (-1); else exit (EXIT_SUCCESS); } The function `abort_to_main_loop' causes an immediate transfer of control back to the main loop of the program, no matter where it is called from. The flow of control inside the `main' function may appear a little mysterious at first, but it is actually a common idiom with `setjmp'. A normal call to `setjmp' returns zero, so the "else" clause of the conditional is executed. If `abort_to_main_loop' is called somewhere within the execution of `do_command', then it actually appears as if the _same_ call to `setjmp' in `main' were returning a second time with a value of `-1'. So, the general pattern for using `setjmp' looks something like: if (setjmp (BUFFER)) /* Code to clean up after premature return. */ ... else /* Code to be executed normally after setting up the return point. */ ... Details of Non-Local Exits ========================== Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in `setjmp.h'. - Data Type: jmp_buf Objects of type `jmp_buf' hold the state information to be restored by a non-local exit. The contents of a `jmp_buf' identify a specific place to return to. - Macro: int setjmp (jmp_buf STATE) When called normally, `setjmp' stores information about the execution state of the program in STATE and returns zero. If `longjmp' is later used to perform a non-local exit to this STATE, `setjmp' returns a nonzero value. - Function: void longjmp (jmp_buf STATE, int VALUE) This function restores current execution to the state saved in STATE, and continues execution from the call to `setjmp' that established that return point. Returning from `setjmp' by means of `longjmp' returns the VALUE argument that was passed to `longjmp', rather than `0'. (But if VALUE is given as `0', `setjmp' returns `1'). There are a lot of obscure but important restrictions on the use of `setjmp' and `longjmp'. Most of these restrictions are present because non-local exits require a fair amount of magic on the part of the C compiler and can interact with other parts of the language in strange ways. The `setjmp' function is actually a macro without an actual function definition, so you shouldn't try to `#undef' it or take its address. In addition, calls to `setjmp' are safe in only the following contexts: * As the test expression of a selection or iteration statement (such as `if', `switch', or `while'). * As one operand of a equality or comparison operator that appears as the test expression of a selection or iteration statement. The other operand must be an integer constant expression. * As the operand of a unary `!' operator, that appears as the test expression of a selection or iteration statement. * By itself as an expression statement. Return points are valid only during the dynamic extent of the function that called `setjmp' to establish them. If you `longjmp' to a return point that was established in a function that has already returned, unpredictable and disastrous things are likely to happen. You should use a nonzero VALUE argument to `longjmp'. While `longjmp' refuses to pass back a zero argument as the return value from `setjmp', this is intended as a safety net against accidental misuse and is not really good programming style. When you perform a non-local exit, accessible objects generally retain whatever values they had at the time `longjmp' was called. The exception is that the values of automatic variables local to the function containing the `setjmp' call that have been changed since the call to `setjmp' are indeterminate, unless you have declared them `volatile'. Non-Local Exits and Signals =========================== In BSD Unix systems, `setjmp' and `longjmp' also save and restore the set of blocked signals; see *Note Blocking Signals::. However, the POSIX.1 standard requires `setjmp' and `longjmp' not to change the set of blocked signals, and provides an additional pair of functions (`sigsetjmp' and `siglongjmp') to get the BSD behavior. The behavior of `setjmp' and `longjmp' in the GNU library is controlled by feature test macros; see *Note Feature Test Macros::. The default in the GNU system is the POSIX.1 behavior rather than the BSD behavior. The facilities in this section are declared in the header file `setjmp.h'. - Data Type: sigjmp_buf This is similar to `jmp_buf', except that it can also store state information about the set of blocked signals. - Function: int sigsetjmp (sigjmp_buf STATE, int SAVESIGS) This is similar to `setjmp'. If SAVESIGS is nonzero, the set of blocked signals is saved in STATE and will be restored if a `siglongjmp' is later performed with this STATE. - Function: void siglongjmp (sigjmp_buf STATE, int VALUE) This is similar to `longjmp' except for the type of its STATE argument. If the `sigsetjmp' call that set this STATE used a nonzero SAVESIGS flag, `siglongjmp' also restores the set of blocked signals. Complete Context Control ======================== The Unix standard one more set of function to control the execution path and these functions are more powerful than those discussed in this chapter so far. These function were part of the original System V API and by this route were added to the Unix API. Beside on branded Unix implementations these interfaces are not widely available. Not all platforms and/or architectures the GNU C Library is available on provide this interface. Use `configure' to detect the availability. Similar to the `jmp_buf' and `sigjmp_buf' types used for the variables to contain the state of the `longjmp' functions the interfaces of interest here have an appropriate type as well. Objects of this type are normally much larger since more information is contained. The type is also used in a few more places as we will see. The types and functions described in this section are all defined and declared respectively in the `ucontext.h' header file. - Data Type: ucontext_t The `ucontext_t' type is defined as a structure with as least the following elements: `ucontext_t *uc_link' This is a pointer to the next context structure which is used if the context described in the current structure returns. `sigset_t uc_sigmask' Set of signals which are blocked when this context is used. `stack_t uc_stack' Stack used for this context. The value need not be (and normally is not) the stack pointer. *Note Signal Stack::. `mcontext_t uc_mcontext' This element contains the actual state of the process. The `mcontext_t' type is also defined in this header but the definition should be treated as opaque. Any use of knowledge of the type makes applications less portable. Objects of this type have to be created by the user. The initialization and modification happens through one of the following functions: - Function: int getcontext (ucontext_t *UCP) The `getcontext' function initializes the variable pointed to by UCP with the context of the calling thread. The context contains the content of the registers, the signal mask, and the current stack. Executing the contents would start at the point where the `getcontext' call just returned. The function returns `0' if successful. Otherwise it returns `-1' and sets ERRNO accordingly. The `getcontext' function is similar to `setjmp' but it does not provide an indication of whether the function returns for the first time or whether the initialized context was used and the execution is resumed at just that point. If this is necessary the user has to take determine this herself. This must be done carefully since the context contains registers which might contain register variables. This is a good situation to define variables with `volatile'. Once the context variable is initialized it can be used as is or it can be modified. The latter is normally done to implement co-routines or similar constructs. The `makecontext' function is what has to be used to do that. - Function: void makecontext (ucontext_t *UCP, void (*FUNC) (void), int ARGC, ...) The UCP parameter passed to the `makecontext' shall be initialized by a call to `getcontext'. The context will be modified to in a way so that if the context is resumed it will start by calling the function `func' which gets ARGC integer arguments passed. The integer arguments which are to be passed should follow the ARGC parameter in the call to `makecontext'. Before the call to this function the `uc_stack' and `uc_link' element of the UCP structure should be initialized. The `uc_stack' element describes the stack which is used for this context. No two contexts which are used at the same time should use the same memory region for a stack. The `uc_link' element of the object pointed to by UCP should be a pointer to the context to be executed when the function FUNC returns or it should be a null pointer. See `setcontext' for more information about the exact use. While allocating the memory for the stack one has to be careful. Most modern processors keep track of whether a certain memory region is allowed to contain code which is executed or not. Data segments and heap memory is normally not tagged to allow this. The result is that programs would fail. Examples for such code include the calling sequences the GNU C compiler generates for calls to nested functions. Safe ways to allocate stacks correctly include using memory on the original threads stack or explicitly allocate memory tagged for execution using (*note Memory-mapped I/O::). *Compatibility note*: The current Unix standard is very imprecise about the way the stack is allocated. All implementations seem to agree that the `uc_stack' element must be used but the values stored in the elements of the `stack_t' value are unclear. The GNU C library and most other Unix implementations require the `ss_sp' value of the `uc_stack' element to point to the base of the memory region allocated for the stack and the size of the memory region is stored in `ss_size'. There are implements out there which require `ss_sp' to be set to the value the stack pointer will have (which can depending on the direction the stack grows be different). This difference makes the `makecontext' function hard to use and it requires detection of the platform at compile time. - Function: int setcontext (const ucontext_t *UCP) The `setcontext' function restores the context described by UCP. The context is not modified and can be reused as often as wanted. If the context was created by `getcontext' execution resumes with the registers filled with the same values and the same stack as if the `getcontext' call just returned. If the context was modified with a call to `makecontext' execution continues with the function passed to `makecontext' which gets the specified parameters passed. If this function returns execution is resumed in the context which was referenced by the `uc_link' element of the context structure passed to `makecontext' at the time of the call. If `uc_link' was a null pointer the application terminates in this case. Since the context contains information about the stack no two threads should use the same context at the same time. The result in most cases would be disastrous. The `setcontext' function does not return unless an error occurred in which case it returns `-1'. The `setcontext' function simply replaces the current context with the one described by the UCP parameter. This is often useful but there are situations where the current context has to be preserved. - Function: int swapcontext (ucontext_t *restrict OUCP, const ucontext_t *restrict UCP) The `swapcontext' function is similar to `setcontext' but instead of just replacing the current context the latter is first saved in the object pointed to by OUCP as if this was a call to `getcontext'. The saved context would resume after the call to `swapcontext'. Once the current context is saved the context described in UCP is installed and execution continues as described in this context. If `swapcontext' succeeds the function does not return unless the context OUCP is used without prior modification by `makecontext'. The return value in this case is `0'. If the function fails it returns `-1' and set ERRNO accordingly. Example for SVID Context Handling ================================= The easiest way to use the context handling functions is as a replacement for `setjmp' and `longjmp'. The context contains on most platforms more information which might lead to less surprises but this also means using these functions is more expensive (beside being less portable). int random_search (int n, int (*fp) (int, ucontext_t *)) { volatile int cnt = 0; ucontext_t uc; /* Safe current context. */ if (getcontext (&uc) < 0) return -1; /* If we have not tried N times try again. */ if (cnt++ < n) /* Call the function with a new random number and the context. */ if (fp (rand (), &uc) != 0) /* We found what we were looking for. */ return 1; /* Not found. */ return 0; } Using contexts in such a way enables emulating exception handling. The search functions passed in the FP parameter could be very large, nested, and complex which would make it complicated (or at least would require a lot of code) to leave the function with an error value which has to be passed down to the caller. By using the context it is possible to leave the search function in one step and allow restarting the search which also has the nice side effect that it can be significantly faster. Something which is harder to implement with `setjmp' and `longjmp' is to switch temporarily to a different execution path and then resume where execution was stopped. #include #include #include #include #include /* Set by the signal handler. */ static volatile int expired; /* The contexts. */ static ucontext_t uc[3]; /* We do only a certain number of switches. */ static int switches; /* This is the function doing the work. It is just a skeleton, real code has to be filled in. */ static void f (int n) { int m = 0; while (1) { /* This is where the work would be done. */ if (++m % 100 == 0) { putchar ('.'); fflush (stdout); } /* Regularly the EXPIRE variable must be checked. */ if (expired) { /* We do not want the program to run forever. */ if (++switches == 20) return; printf ("\nswitching from %d to %d\n", n, 3 - n); expired = 0; /* Switch to the other context, saving the current one. */ swapcontext (&uc[n], &uc[3 - n]); } } } /* This is the signal handler which simply set the variable. */ void handler (int signal) { expired = 1; } int main (void) { struct sigaction sa; struct itimerval it; char st1[8192]; char st2[8192]; /* Initialize the data structures for the interval timer. */ sa.sa_flags = SA_RESTART; sigfillset (&sa.sa_mask); sa.sa_handler = handler; it.it_interval.tv_sec = 0; it.it_interval.tv_usec = 1; it.it_value = it.it_interval; /* Install the timer and get the context we can manipulate. */ if (sigaction (SIGPROF, &sa, NULL) < 0 || setitimer (ITIMER_PROF, &it, NULL) < 0 || getcontext (&uc[1]) == -1 || getcontext (&uc[2]) == -1) abort (); /* Create a context with a separate stack which causes the function `f' to be call with the parameter `1'. Note that the `uc_link' points to the main context which will cause the program to terminate once the function return. */ uc[1].uc_link = &uc[0]; uc[1].uc_stack.ss_sp = st1; uc[1].uc_stack.ss_size = sizeof st1; makecontext (&uc[1], (void (*) (void)) f, 1, 1); /* Similarly, but `2' is passed as the parameter to `f'. */ uc[2].uc_link = &uc[0]; uc[2].uc_stack.ss_sp = st2; uc[2].uc_stack.ss_size = sizeof st2; makecontext (&uc[2], (void (*) (void)) f, 1, 2); /* Start running. */ swapcontext (&uc[0], &uc[1]); putchar ('\n'); return 0; } This an example how the context functions can be used to implement co-routines or cooperative multi-threading. All that has to be done is to call every once in a while `swapcontext' to continue running a different context. It is not allowed to do the context switching from the signal handler directly since neither `setcontext' nor `swapcontext' are functions which can be called from a signal handler. But setting a variable in the signal handler and checking it in the body of the functions which are executed. Since `swapcontext' is saving the current context it is possible to have multiple different scheduling points in the code. Execution will always resume where it was left. Signal Handling *************** A "signal" is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line. The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default. If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives. Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize. Basic Concepts of Signals ========================= This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals. Some Kinds of Signals --------------------- A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or "generate", or "raise") a signal: * A program error such as dividing by zero or issuing an address outside the valid range. * A user request to interrupt or terminate the program. Most environments are set up to let a user suspend the program by typing `C-z', or terminate it with `C-c'. Whatever key sequence is used, the operating system sends the proper signal to interrupt the process. * The termination of a child process. * Expiration of a timer or alarm. * A call to `kill' or `raise' by the same process. * A call to `kill' from another process. Signals are a limited but useful form of interprocess communication. * An attempt to perform an I/O operation that cannot be done. Examples are reading from a pipe that has no writer (*note Pipes and FIFOs::), and reading or writing to a terminal in certain situations (*note Job Control::). Each of these kinds of events (excepting explicit calls to `kill' and `raise') generates its own particular kind of signal. The various kinds of signals are listed and described in detail in *Note Standard Signals::. Concepts of Signal Generation ----------------------------- In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests. An error means that a program has done something invalid and cannot continue execution. But not all kinds of errors generate signals--in fact, most do not. For example, opening a nonexistent file is an error, but it does not raise a signal; instead, `open' returns `-1'. In general, errors that are necessarily associated with certain library functions are reported by returning a value that indicates an error. The errors which raise signals are those which can happen anywhere in the program, not just in library calls. These include division by zero and invalid memory addresses. An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process. An explicit request means the use of a library function such as `kill' whose purpose is specifically to generate a signal. Signals may be generated "synchronously" or "asynchronously". A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later. Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process. A given type of signal is either typically synchronous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request. How Signals Are Delivered ------------------------- When a signal is generated, it becomes "pending". Normally it remains pending for just a short period of time and then is "delivered" to the process that was signaled. However, if that kind of signal is currently "blocked", it may remain pending indefinitely--until signals of that kind are "unblocked". Once unblocked, it will be delivered immediately. *Note Blocking Signals::. When the signal is delivered, whether right away or after a long delay, the "specified action" for that signal is taken. For certain signals, such as `SIGKILL' and `SIGSTOP', the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a "handler function", or accept the "default action" for that kind of signal. The program specifies its choice using functions such as `signal' or `sigaction' (*note Signal Actions::). We sometimes say that a handler "catches" the signal. While the handler is running, that particular signal is normally blocked. If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it. If a signal arrives which the program has neither handled nor ignored, its "default action" takes place. Each kind of signal has its own default action, documented below (*note Standard Signals::). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing. When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the `wait' or `waitpid' functions. (This is discussed in more detail in *Note Process Completion::.) The information it can get includes the fact that termination was due to a signal and the kind of signal involved. If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message. The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a "core dump file" which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error. If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error. Standard Signals ================ This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the "signal number" for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform. The signal names are defined in the header file `signal.h'. - Macro: int NSIG The value of this symbolic constant is the total number of signals defined. Since the signal numbers are allocated consecutively, `NSIG' is also one greater than the largest defined signal number. Program Error Signals --------------------- The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error. Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (*Note Termination in Handler::.) Termination is the sensible ultimate outcome from a program error in most programs. However, programming systems such as Lisp that can load compiled user programs might need to keep executing even if a user program incurs an error. These programs have handlers which use `longjmp' to return control to the command level. The default action for all of these signals is to cause the process to terminate. If you block or ignore these signals or establish handlers for them that return normally, your program will probably break horribly when such signals happen, unless they are generated by `raise' or `kill' instead of a real error. When one of these program error signals terminates a process, it also writes a "core dump file" which records the state of the process at the time of termination. The core dump file is named `core' and is written in whichever directory is current in the process at the time. (On the GNU system, you can specify the file name for core dumps with the environment variable `COREFILE'.) The purpose of core dump files is so that you can examine them with a debugger to investigate what caused the error. - Macro: int SIGFPE The `SIGFPE' signal reports a fatal arithmetic error. Although the name is derived from "floating-point exception", this signal actually covers all arithmetic errors, including division by zero and overflow. If a program stores integer data in a location which is then used in a floating-point operation, this often causes an "invalid operation" exception, because the processor cannot recognize the data as a floating-point number. Actual floating-point exceptions are a complicated subject because there are many types of exceptions with subtly different meanings, and the `SIGFPE' signal doesn't distinguish between them. The `IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985 and ANSI/IEEE Std 854-1987)' defines various floating-point exceptions and requires conforming computer systems to report their occurrences. However, this standard does not specify how the exceptions are reported, or what kinds of handling and control the operating system can offer to the programmer. BSD systems provide the `SIGFPE' handler with an extra argument that distinguishes various causes of the exception. In order to access this argument, you must define the handler to accept two arguments, which means you must cast it to a one-argument function type in order to establish the handler. The GNU library does provide this extra argument, but the value is meaningful only on operating systems that provide the information (BSD systems and GNU systems). `FPE_INTOVF_TRAP' Integer overflow (impossible in a C program unless you enable overflow trapping in a hardware-specific fashion). `FPE_INTDIV_TRAP' Integer division by zero. `FPE_SUBRNG_TRAP' Subscript-range (something that C programs never check for). `FPE_FLTOVF_TRAP' Floating overflow trap. `FPE_FLTDIV_TRAP' Floating/decimal division by zero. `FPE_FLTUND_TRAP' Floating underflow trap. (Trapping on floating underflow is not normally enabled.) `FPE_DECOVF_TRAP' Decimal overflow trap. (Only a few machines have decimal arithmetic and C never uses it.) - Macro: int SIGILL The name of this signal is derived from "illegal instruction"; it usually means your program is trying to execute garbage or a privileged instruction. Since the C compiler generates only valid instructions, `SIGILL' typically indicates that the executable file is corrupted, or that you are trying to execute data. Some common ways of getting into the latter situation are by passing an invalid object where a pointer to a function was expected, or by writing past the end of an automatic array (or similar problems with pointers to automatic variables) and corrupting other data on the stack such as the return address of a stack frame. `SIGILL' can also be generated when the stack overflows, or when the system has trouble running the handler for a signal. - Macro: int SIGSEGV This signal is generated when a program tries to read or write outside the memory that is allocated for it, or to write memory that can only be read. (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.) The name is an abbreviation for "segmentation violation". Common ways of getting a `SIGSEGV' condition include dereferencing a null or uninitialized pointer, or when you use a pointer to step through an array, but fail to check for the end of the array. It varies among systems whether dereferencing a null pointer generates `SIGSEGV' or `SIGBUS'. - Macro: int SIGBUS This signal is generated when an invalid pointer is dereferenced. Like `SIGSEGV', this signal is typically the result of dereferencing an uninitialized pointer. The difference between the two is that `SIGSEGV' indicates an invalid access to valid memory, while `SIGBUS' indicates an access to an invalid address. In particular, `SIGBUS' signals often result from dereferencing a misaligned pointer, such as referring to a four-word integer at an address not divisible by four. (Each kind of computer has its own requirements for address alignment.) The name of this signal is an abbreviation for "bus error". - Macro: int SIGABRT This signal indicates an error detected by the program itself and reported by calling `abort'. *Note Aborting a Program::. - Macro: int SIGIOT Generated by the PDP-11 "iot" instruction. On most machines, this is just another name for `SIGABRT'. - Macro: int SIGTRAP Generated by the machine's breakpoint instruction, and possibly other trap instructions. This signal is used by debuggers. Your program will probably only see `SIGTRAP' if it is somehow executing bad instructions. - Macro: int SIGEMT Emulator trap; this results from certain unimplemented instructions which might be emulated in software, or the operating system's failure to properly emulate them. - Macro: int SIGSYS Bad system call; that is to say, the instruction to trap to the operating system was executed, but the code number for the system call to perform was invalid. Termination Signals ------------------- These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently. The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (*Note Termination in Handler::.) The (obvious) default action for all of these signals is to cause the process to terminate. - Macro: int SIGTERM The `SIGTERM' signal is a generic signal used to cause program termination. Unlike `SIGKILL', this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate. The shell command `kill' generates `SIGTERM' by default. - Macro: int SIGINT The `SIGINT' ("program interrupt") signal is sent when the user types the INTR character (normally `C-c'). *Note Special Characters::, for information about terminal driver support for `C-c'. - Macro: int SIGQUIT The `SIGQUIT' signal is similar to `SIGINT', except that it's controlled by a different key--the QUIT character, usually `C-\'--and produces a core dump when it terminates the process, just like a program error signal. You can think of this as a program error condition "detected" by the user. *Note Program Error Signals::, for information about core dumps. *Note Special Characters::, for information about terminal driver support. Certain kinds of cleanups are best omitted in handling `SIGQUIT'. For example, if the program creates temporary files, it should handle the other termination requests by deleting the temporary files. But it is better for `SIGQUIT' not to delete them, so that the user can examine them in conjunction with the core dump. - Macro: int SIGKILL The `SIGKILL' signal is used to cause immediate program termination. It cannot be handled or ignored, and is therefore always fatal. It is also not possible to block this signal. This signal is usually generated only by explicit request. Since it cannot be handled, you should generate it only as a last resort, after first trying a less drastic method such as `C-c' or `SIGTERM'. If a process does not respond to any other termination signals, sending it a `SIGKILL' signal will almost always cause it to go away. In fact, if `SIGKILL' fails to terminate a process, that by itself constitutes an operating system bug which you should report. The system will generate `SIGKILL' for a process itself under some unusual conditions where the program cannot possibly continue to run (even to run a signal handler). - Macro: int SIGHUP The `SIGHUP' ("hang-up") signal is used to report that the user's terminal is disconnected, perhaps because a network or telephone connection was broken. For more information about this, see *Note Control Modes::. This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see *Note Termination Internals::. Alarm Signals ------------- These signals are used to indicate the expiration of timers. *Note Setting an Alarm::, for information about functions that cause these signals to be sent. The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case. - Macro: int SIGALRM This signal typically indicates expiration of a timer that measures real or clock time. It is used by the `alarm' function, for example. - Macro: int SIGVTALRM This signal typically indicates expiration of a timer that measures CPU time used by the current process. The name is an abbreviation for "virtual time alarm". - Macro: int SIGPROF This signal typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system. Such a timer is used to implement code profiling facilities, hence the name of this signal. Asynchronous I/O Signals ------------------------ The signals listed in this section are used in conjunction with asynchronous I/O facilities. You have to take explicit action by calling `fcntl' to enable a particular file descriptor to generate these signals (*note Interrupt Input::). The default action for these signals is to ignore them. - Macro: int SIGIO This signal is sent when a file descriptor is ready to perform input or output. On most operating systems, terminals and sockets are the only kinds of files that can generate `SIGIO'; other kinds, including ordinary files, never generate `SIGIO' even if you ask them to. In the GNU system `SIGIO' will always be generated properly if you successfully set asynchronous mode with `fcntl'. - Macro: int SIGURG This signal is sent when "urgent" or out-of-band data arrives on a socket. *Note Out-of-Band Data::. - Macro: int SIGPOLL This is a System V signal name, more or less similar to `SIGIO'. It is defined only for compatibility. Job Control Signals ------------------- These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled. You should generally leave these signals alone unless you really understand how job control works. *Note Job Control::. - Macro: int SIGCHLD This signal is sent to a parent process whenever one of its child processes terminates or stops. The default action for this signal is to ignore it. If you establish a handler for this signal while there are child processes that have terminated but not reported their status via `wait' or `waitpid' (*note Process Completion::), whether your new handler applies to those processes or not depends on the particular operating system. - Macro: int SIGCLD This is an obsolete name for `SIGCHLD'. - Macro: int SIGCONT You can send a `SIGCONT' signal to a process to make it continue. This signal is special--it always makes the process continue if it is stopped, before the signal is delivered. The default behavior is to do nothing else. You cannot block this signal. You can set a handler, but `SIGCONT' always makes the process continue regardless. Most programs have no reason to handle `SIGCONT'; they simply resume execution without realizing they were ever stopped. You can use a handler for `SIGCONT' to make a program do something special when it is stopped and continued--for example, to reprint a prompt when it is suspended while waiting for input. - Macro: int SIGSTOP The `SIGSTOP' signal stops the process. It cannot be handled, ignored, or blocked. - Macro: int SIGTSTP The `SIGTSTP' signal is an interactive stop signal. Unlike `SIGSTOP', this signal can be handled and ignored. Your program should handle this signal if you have a special need to leave files or system tables in a secure state when a process is stopped. For example, programs that turn off echoing should handle `SIGTSTP' so they can turn echoing back on before stopping. This signal is generated when the user types the SUSP character (normally `C-z'). For more information about terminal driver support, see *Note Special Characters::. - Macro: int SIGTTIN A process cannot read from the user's terminal while it is running as a background job. When any process in a background job tries to read from the terminal, all of the processes in the job are sent a `SIGTTIN' signal. The default action for this signal is to stop the process. For more information about how this interacts with the terminal driver, see *Note Access to the Terminal::. - Macro: int SIGTTOU This is similar to `SIGTTIN', but is generated when a process in a background job attempts to write to the terminal or set its modes. Again, the default action is to stop the process. `SIGTTOU' is only generated for an attempt to write to the terminal if the `TOSTOP' output mode is set; *note Output Modes::. While a process is stopped, no more signals can be delivered to it until it is continued, except `SIGKILL' signals and (obviously) `SIGCONT' signals. The signals are marked as pending, but not delivered until the process is continued. The `SIGKILL' signal always causes termination of the process and can't be blocked, handled or ignored. You can ignore `SIGCONT', but it always causes the process to be continued anyway if it is stopped. Sending a `SIGCONT' signal to a process causes any pending stop signals for that process to be discarded. Likewise, any pending `SIGCONT' signals for a process are discarded when it receives a stop signal. When a process in an orphaned process group (*note Orphaned Process Groups::) receives a `SIGTSTP', `SIGTTIN', or `SIGTTOU' signal and does not handle it, the process does not stop. Stopping the process would probably not be very useful, since there is no shell program that will notice it stop and allow the user to continue it. What happens instead depends on the operating system you are using. Some systems may do nothing; others may deliver another signal instead, such as `SIGKILL' or `SIGHUP'. In the GNU system, the process dies with `SIGKILL'; this avoids the problem of many stopped, orphaned processes lying around the system. Operation Error Signals ----------------------- These signals are used to report various errors generated by an operation done by the program. They do not necessarily indicate a programming error in the program, but an error that prevents an operating system call from completing. The default action for all of them is to cause the process to terminate. - Macro: int SIGPIPE Broken pipe. If you use pipes or FIFOs, you have to design your application so that one process opens the pipe for reading before another starts writing. If the reading process never starts, or terminates unexpectedly, writing to the pipe or FIFO raises a `SIGPIPE' signal. If `SIGPIPE' is blocked, handled or ignored, the offending call fails with `EPIPE' instead. Pipes and FIFO special files are discussed in more detail in *Note Pipes and FIFOs::. Another cause of `SIGPIPE' is when you try to output to a socket that isn't connected. *Note Sending Data::. - Macro: int SIGLOST Resource lost. This signal is generated when you have an advisory lock on an NFS file, and the NFS server reboots and forgets about your lock. In the GNU system, `SIGLOST' is generated when any server program dies unexpectedly. It is usually fine to ignore the signal; whatever call was made to the server that died just returns an error. - Macro: int SIGXCPU CPU time limit exceeded. This signal is generated when the process exceeds its soft resource limit on CPU time. *Note Limits on Resources::. - Macro: int SIGXFSZ File size limit exceeded. This signal is generated when the process attempts to extend a file so it exceeds the process's soft resource limit on file size. *Note Limits on Resources::. Miscellaneous Signals --------------------- These signals are used for various other purposes. In general, they will not affect your program unless it explicitly uses them for something. - Macro: int SIGUSR1 - Macro: int SIGUSR2 The `SIGUSR1' and `SIGUSR2' signals are set aside for you to use any way you want. They're useful for simple interprocess communication, if you write a signal handler for them in the program that receives the signal. There is an example showing the use of `SIGUSR1' and `SIGUSR2' in *Note Signaling Another Process::. The default action is to terminate the process. - Macro: int SIGWINCH Window size change. This is generated on some systems (including GNU) when the terminal driver's record of the number of rows and columns on the screen is changed. The default action is to ignore it. If a program does full-screen display, it should handle `SIGWINCH'. When the signal arrives, it should fetch the new screen size and reformat its display accordingly. - Macro: int SIGINFO Information request. In 4.4 BSD and the GNU system, this signal is sent to all the processes in the foreground process group of the controlling terminal when the user types the STATUS character in canonical mode; *note Signal Characters::. If the process is the leader of the process group, the default action is to print some status information about the system and what the process is doing. Otherwise the default is to do nothing. Signal Messages --------------- We mentioned above that the shell prints a message describing the signal that terminated a child process. The clean way to print a message describing a signal is to use the functions `strsignal' and `psignal'. These functions use a signal number to specify which kind of signal to describe. The signal number may come from the termination status of a child process (*note Process Completion::) or it may come from a signal handler in the same process. - Function: char * strsignal (int SIGNUM) This function returns a pointer to a statically-allocated string containing a message describing the signal SIGNUM. You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later. This function is a GNU extension, declared in the header file `string.h'. - Function: void psignal (int SIGNUM, const char *MESSAGE) This function prints a message describing the signal SIGNUM to the standard error output stream `stderr'; see *Note Standard Streams::. If you call `psignal' with a MESSAGE that is either a null pointer or an empty string, `psignal' just prints the message corresponding to SIGNUM, adding a trailing newline. If you supply a non-null MESSAGE argument, then `psignal' prefixes its output with this string. It adds a colon and a space character to separate the MESSAGE from the string corresponding to SIGNUM. This function is a BSD feature, declared in the header file `signal.h'. There is also an array `sys_siglist' which contains the messages for the various signal codes. This array exists on BSD systems, unlike `strsignal'. Specifying Signal Actions ========================= The simplest way to change the action for a signal is to use the `signal' function. You can specify a built-in action (such as to ignore the signal), or you can "establish a handler". The GNU library also implements the more versatile `sigaction' facility. This section describes both facilities and gives suggestions on which to use when. Basic Signal Handling --------------------- The `signal' function provides a simple interface for establishing an action for a particular signal. The function and associated macros are declared in the header file `signal.h'. - Data Type: sighandler_t This is the type of signal handler functions. Signal handlers take one integer argument specifying the signal number, and have return type `void'. So, you should define handler functions like this: void HANDLER (int `signum') { ... } The name `sighandler_t' for this data type is a GNU extension. - Function: sighandler_t signal (int SIGNUM, sighandler_t ACTION) The `signal' function establishes ACTION as the action for the signal SIGNUM. The first argument, SIGNUM, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names (*note Standard Signals::)--don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system. The second argument, ACTION, specifies the action to use for the signal SIGNUM. This can be one of the following: `SIG_DFL' `SIG_DFL' specifies the default action for the particular signal. The default actions for various kinds of signals are stated in *Note Standard Signals::. `SIG_IGN' `SIG_IGN' specifies that the signal should be ignored. Your program generally should not ignore signals that represent serious events or that are normally used to request termination. You cannot ignore the `SIGKILL' or `SIGSTOP' signals at all. You can ignore program error signals like `SIGSEGV', but ignoring the error won't enable the program to continue executing meaningfully. Ignoring user requests such as `SIGINT', `SIGQUIT', and `SIGTSTP' is unfriendly. When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them. *Note Blocking Signals::. `HANDLER' Supply the address of a handler function in your program, to specify running this handler as the way to deliver the signal. For more information about defining signal handler functions, see *Note Defining Handlers::. If you set the action for a signal to `SIG_IGN', or if you set it to `SIG_DFL' and the default action is to ignore that signal, then any pending signals of that type are discarded (even if they are blocked). Discarding the pending signals means that they will never be delivered, not even if you subsequently specify another action and unblock this kind of signal. The `signal' function returns the action that was previously in effect for the specified SIGNUM. You can save this value and restore it later by calling `signal' again. If `signal' can't honor the request, it returns `SIG_ERR' instead. The following `errno' error conditions are defined for this function: `EINVAL' You specified an invalid SIGNUM; or you tried to ignore or provide a handler for `SIGKILL' or `SIGSTOP'. *Compatibility Note:* A problem encountered when working with the `signal' function is that it has different semantics on BSD and SVID systems. The difference is that on SVID systems the signal handler is deinstalled after signal delivery. On BSD systems the handler must be explicitly deinstalled. In the GNU C Library we use the BSD version by default. To use the SVID version you can either use the function `sysv_signal' (see below) or use the `_XOPEN_SOURCE' feature select macro (*note Feature Test Macros::). In general, use of these functions should be avoided because of compatibility problems. It is better to use `sigaction' if it is available since the results are much more reliable. Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen: #include void termination_handler (int signum) { struct temp_file *p; for (p = temp_file_list; p; p = p->next) unlink (p->name); } int main (void) { ... if (signal (SIGINT, termination_handler) == SIG_IGN) signal (SIGINT, SIG_IGN); if (signal (SIGHUP, termination_handler) == SIG_IGN) signal (SIGHUP, SIG_IGN); if (signal (SIGTERM, termination_handler) == SIG_IGN) signal (SIGTERM, SIG_IGN); ... } Note that if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this. We do not handle `SIGQUIT' or the program error signals in this example because these are designed to provide information for debugging (a core dump), and the temporary files may give useful information. - Function: sighandler_t sysv_signal (int SIGNUM, sighandler_t ACTION) The `sysv_signal' implements the behavior of the standard `signal' function as found on SVID systems. The difference to BSD systems is that the handler is deinstalled after a delivery of a signal. *Compatibility Note:* As said above for `signal', this function should be avoided when possible. `sigaction' is the preferred method. - Function: sighandler_t ssignal (int SIGNUM, sighandler_t ACTION) The `ssignal' function does the same thing as `signal'; it is provided only for compatibility with SVID. - Macro: sighandler_t SIG_ERR The value of this macro is used as the return value from `signal' to indicate an error. Advanced Signal Handling ------------------------ The `sigaction' function has the same basic effect as `signal': to specify how a signal should be handled by the process. However, `sigaction' offers more control, at the expense of more complexity. In particular, `sigaction' allows you to specify additional flags to control when the signal is generated and how the handler is invoked. The `sigaction' function is declared in `signal.h'. - Data Type: struct sigaction Structures of type `struct sigaction' are used in the `sigaction' function to specify all the information about how to handle a particular signal. This structure contains at least the following members: `sighandler_t sa_handler' This is used in the same way as the ACTION argument to the `signal' function. The value can be `SIG_DFL', `SIG_IGN', or a function pointer. *Note Basic Signal Handling::. `sigset_t sa_mask' This specifies a set of signals to be blocked while the handler runs. Blocking is explained in *Note Blocking for Handler::. Note that the signal that was delivered is automatically blocked by default before its handler is started; this is true regardless of the value in `sa_mask'. If you want that signal not to be blocked within its handler, you must write code in the handler to unblock it. `int sa_flags' This specifies various flags which can affect the behavior of the signal. These are described in more detail in *Note Flags for Sigaction::. - Function: int sigaction (int SIGNUM, const struct sigaction *restrict ACTION, struct sigaction *restrict OLD-ACTION) The ACTION argument is used to set up a new action for the signal SIGNUM, while the OLD-ACTION argument is used to return information about the action previously associated with this symbol. (In other words, OLD-ACTION has the same purpose as the `signal' function's return value--you can check to see what the old action in effect for the signal was, and restore it later if you want.) Either ACTION or OLD-ACTION can be a null pointer. If OLD-ACTION is a null pointer, this simply suppresses the return of information about the old action. If ACTION is a null pointer, the action associated with the signal SIGNUM is unchanged; this allows you to inquire about how a signal is being handled without changing that handling. The return value from `sigaction' is zero if it succeeds, and `-1' on failure. The following `errno' error conditions are defined for this function: `EINVAL' The SIGNUM argument is not valid, or you are trying to trap or ignore `SIGKILL' or `SIGSTOP'. Interaction of `signal' and `sigaction' --------------------------------------- It's possible to use both the `signal' and `sigaction' functions within a single program, but you have to be careful because they can interact in slightly strange ways. The `sigaction' function specifies more information than the `signal' function, so the return value from `signal' cannot express the full range of `sigaction' possibilities. Therefore, if you use `signal' to save and later reestablish an action, it may not be able to reestablish properly a handler that was established with `sigaction'. To avoid having problems as a result, always use `sigaction' to save and restore a handler if your program uses `sigaction' at all. Since `sigaction' is more general, it can properly save and reestablish any action, regardless of whether it was established originally with `signal' or `sigaction'. On some systems if you establish an action with `signal' and then examine it with `sigaction', the handler address that you get may not be the same as what you specified with `signal'. It may not even be suitable for use as an action argument with `signal'. But you can rely on using it as an argument to `sigaction'. This problem never happens on the GNU system. So, you're better off using one or the other of the mechanisms consistently within a single program. *Portability Note:* The basic `signal' function is a feature of ISO C, while `sigaction' is part of the POSIX.1 standard. If you are concerned about portability to non-POSIX systems, then you should use the `signal' function instead. `sigaction' Function Example ---------------------------- In *Note Basic Signal Handling::, we gave an example of establishing a simple handler for termination signals using `signal'. Here is an equivalent example using `sigaction': #include void termination_handler (int signum) { struct temp_file *p; for (p = temp_file_list; p; p = p->next) unlink (p->name); } int main (void) { ... struct sigaction new_action, old_action; /* Set up the structure to specify the new action. */ new_action.sa_handler = termination_handler; sigemptyset (&new_action.sa_mask); new_action.sa_flags = 0; sigaction (SIGINT, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGINT, &new_action, NULL); sigaction (SIGHUP, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGHUP, &new_action, NULL); sigaction (SIGTERM, NULL, &old_action); if (old_action.sa_handler != SIG_IGN) sigaction (SIGTERM, &new_action, NULL); ... } The program just loads the `new_action' structure with the desired parameters and passes it in the `sigaction' call. The usage of `sigemptyset' is described later; see *Note Blocking Signals::. As in the example using `signal', we avoid handling signals previously set to be ignored. Here we can avoid altering the signal handler even momentarily, by using the feature of `sigaction' that lets us examine the current action without specifying a new one. Here is another example. It retrieves information about the current action for `SIGINT' without changing that action. struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /* `sigaction' returns -1 in case of error. */ else if (query_action.sa_handler == SIG_DFL) /* `SIGINT' is handled in the default, fatal manner. */ else if (query_action.sa_handler == SIG_IGN) /* `SIGINT' is ignored. */ else /* A programmer-defined signal handler is in effect. */ Flags for `sigaction' --------------------- The `sa_flags' member of the `sigaction' structure is a catch-all for special features. Most of the time, `SA_RESTART' is a good value to use for this field. The value of `sa_flags' is interpreted as a bit mask. Thus, you should choose the flags you want to set, OR those flags together, and store the result in the `sa_flags' member of your `sigaction' structure. Each signal number has its own set of flags. Each call to `sigaction' affects one particular signal number, and the flags that you specify apply only to that particular signal. In the GNU C library, establishing a handler with `signal' sets all the flags to zero except for `SA_RESTART', whose value depends on the settings you have made with `siginterrupt'. *Note Interrupted Primitives::, to see what this is about. These macros are defined in the header file `signal.h'. - Macro: int SA_NOCLDSTOP This flag is meaningful only for the `SIGCHLD' signal. When the flag is set, the system delivers the signal for a terminated child process but not for one that is stopped. By default, `SIGCHLD' is delivered for both terminated children and stopped children. Setting this flag for a signal other than `SIGCHLD' has no effect. - Macro: int SA_ONSTACK If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. *Note Signal Stack::. If a signal with this flag arrives and you have not set a signal stack, the system terminates the program with `SIGILL'. - Macro: int SA_RESTART This flag controls what happens when a signal is delivered during certain primitives (such as `open', `read' or `write'), and the signal handler returns normally. There are two alternatives: the library function can resume, or it can return failure with error code `EINTR'. The choice is controlled by the `SA_RESTART' flag for the particular kind of signal that was delivered. If the flag is set, returning from a handler resumes the library function. If the flag is clear, returning from a handler makes the function fail. *Note Interrupted Primitives::. Initial Signal Actions ---------------------- When a new process is created (*note Creating a Process::), it inherits handling of signals from its parent process. However, when you load a new process image using the `exec' function (*note Executing a File::), any signals that you've defined your own handlers for revert to their `SIG_DFL' handling. (If you think about it a little, this makes sense; the handler functions from the old program are specific to that program, and aren't even present in the address space of the new program image.) Of course, the new program can establish its own handlers. When a program is run by a shell, the shell normally sets the initial actions for the child process to `SIG_DFL' or `SIG_IGN', as appropriate. It's a good idea to check to make sure that the shell has not set up an initial action of `SIG_IGN' before you establish your own signal handlers. Here is an example of how to establish a handler for `SIGHUP', but not if `SIGHUP' is currently ignored: ... struct sigaction temp; sigaction (SIGHUP, NULL, &temp); if (temp.sa_handler != SIG_IGN) { temp.sa_handler = handle_sighup; sigemptyset (&temp.sa_mask); sigaction (SIGHUP, &temp, NULL); } Defining Signal Handlers ======================== This section describes how to write a signal handler function that can be established with the `signal' or `sigaction' functions. A signal handler is just a function that you compile together with the rest of the program. Instead of directly invoking the function, you use `signal' or `sigaction' to tell the operating system to call it when a signal arrives. This is known as "establishing" the handler. *Note Signal Actions::. There are two basic strategies you can use in signal handler functions: * You can have the handler function note that the signal arrived by tweaking some global data structures, and then return normally. * You can have the handler function terminate the program or transfer control to a point where it can recover from the situation that caused the signal. You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid. Signal Handlers that Return --------------------------- Handlers which return normally are usually used for signals such as `SIGALRM' and the I/O and interprocess communication signals. But a handler for `SIGINT' might also return normally after setting a flag that tells the program to exit at a convenient time. It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. *Note Program Error Signals::. Handlers that return normally must modify some global variable in order to have any effect. Typically, the variable is one that is examined periodically by the program during normal operation. Its data type should be `sig_atomic_t' for reasons described in *Note Atomic Data Access::. Here is a simple example of such a program. It executes the body of the loop until it has noticed that a `SIGALRM' signal has arrived. This technique is useful because it allows the iteration in progress when the signal arrives to complete before the loop exits. #include #include #include /* This flag controls termination of the main loop. */ volatile sig_atomic_t keep_going = 1; /* The signal handler just clears the flag and re-enables itself. */ void catch_alarm (int sig) { keep_going = 0; signal (sig, catch_alarm); } void do_stuff (void) { puts ("Doing stuff while waiting for alarm...."); } int main (void) { /* Establish a handler for SIGALRM signals. */ signal (SIGALRM, catch_alarm); /* Set an alarm to go off in a little while. */ alarm (2); /* Check the flag once in a while to see when to quit. */ while (keep_going) do_stuff (); return EXIT_SUCCESS; } Handlers That Terminate the Process ----------------------------------- Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts. The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this: volatile sig_atomic_t fatal_error_in_progress = 0; void fatal_error_signal (int sig) { /* Since this handler is established for more than one kind of signal, it might still get invoked recursively by delivery of some other kind of signal. Use a static variable to keep track of that. */ if (fatal_error_in_progress) raise (sig); fatal_error_in_progress = 1; /* Now do the clean up actions: - reset terminal modes - kill child processes - remove lock files */ ... /* Now reraise the signal. We reactivate the signal's default handling, which is to terminate the process. We could just call `exit' or `abort', but reraising the signal sets the return status from the process correctly. */ signal (sig, SIG_DFL); raise (sig); } Nonlocal Control Transfer in Handlers ------------------------------------- You can do a nonlocal transfer of control out of a signal handler using the `setjmp' and `longjmp' facilities (*note Non-Local Exits::). When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on. There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. *Note Blocking Signals::. The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent. Here is a rather schematic example showing the reinitialization of one global variable. #include #include jmp_buf return_to_top_level; volatile sig_atomic_t waiting_for_input; void handle_sigint (int signum) { /* We may have been waiting for input when the signal arrived, but we are no longer waiting once we transfer control. */ waiting_for_input = 0; longjmp (return_to_top_level, 1); } int main (void) { ... signal (SIGINT, sigint_handler); ... while (1) { prepare_for_command (); if (setjmp (return_to_top_level) == 0) read_and_execute_command (); } } /* Imagine this is a subroutine used by various commands. */ char * read_data () { if (input_from_terminal) { waiting_for_input = 1; ... waiting_for_input = 0; } else { ... } } Signals Arriving While a Handler Runs ------------------------------------- What happens if another signal arrives while your signal handler function is running? When the handler for a particular signal is invoked, that signal is automatically blocked until the handler returns. That means that if two signals of the same kind arrive close together, the second one will be held until the first has been handled. (The handler can explicitly unblock the signal using `sigprocmask', if you want to allow more signals of this type to arrive; see *Note Process Signal Mask::.) However, your handler can still be interrupted by delivery of another kind of signal. To avoid this, you can use the `sa_mask' member of the action structure passed to `sigaction' to explicitly specify which signals should be blocked while the signal handler runs. These signals are in addition to the signal for which the handler was invoked, and any other signals that are normally blocked by the process. *Note Blocking for Handler::. When the handler returns, the set of blocked signals is restored to the value it had before the handler ran. So using `sigprocmask' inside the handler only affects what signals can arrive during the execution of the handler itself, not what signals can arrive once the handler returns. *Portability Note:* Always use `sigaction' to establish a handler for a signal that you expect to receive asynchronously, if you want your program to work properly on System V Unix. On this system, the handling of a signal whose handler was established with `signal' automatically sets the signal's action back to `SIG_DFL', and the handler must re-establish itself each time it runs. This practice, while inconvenient, does work when signals cannot arrive in succession. However, if another signal can arrive right away, it may arrive before the handler can re-establish itself. Then the second signal would receive the default handling, which could terminate the process. Signals Close Together Merge into One ------------------------------------- If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past. Here is an example of a handler for `SIGCHLD' that compensates for the fact that the number of signals received may not equal the number of child processes that generate them. It assumes that the program keeps track of all the child processes with a chain of structures as follows: struct process { struct process *next; /* The process ID of this child. */ int pid; /* The descriptor of the pipe or pseudo terminal on which output comes from this child. */ int input_descriptor; /* Nonzero if this process has stopped or terminated. */ sig_atomic_t have_status; /* The status of this child; 0 if running, otherwise a status value from `waitpid'. */ int status; }; struct process *process_list; This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero. /* Nonzero means some child's status has changed so look at `process_list' for the details. */ int process_status_change; Here is the handler itself: void sigchld_handler (int signo) { int old_errno = errno; while (1) { register int pid; int w; struct process *p; /* Keep asking for a status until we get a definitive result. */ do { errno = 0; pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED); } while (pid <= 0 && errno == EINTR); if (pid <= 0) { /* A real failure means there are no more stopped or terminated child processes, so return. */ errno = old_errno; return; } /* Find the process that signaled us, and record its status. */ for (p = process_list; p; p = p->next) if (p->pid == pid) { p->status = w; /* Indicate that the `status' field has data to look at. We do this only after storing it. */ p->have_status = 1; /* If process has terminated, stop waiting for its output. */ if (WIFSIGNALED (w) || WIFEXITED (w)) if (p->input_descriptor) FD_CLR (p->input_descriptor, &input_wait_mask); /* The program should check this flag from time to time to see if there is any news in `process_list'. */ ++process_status_change; } /* Loop around to handle all the processes that have something to tell us. */ } } Here is the proper way to check the flag `process_status_change': if (process_status_change) { struct process *p; process_status_change = 0; for (p = process_list; p; p = p->next) if (p->have_status) { ... Examine `p->status' ... } } It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order. The loop which checks process status avoids examining `p->status' until it sees that status has been validly stored. This is to make sure that the status cannot change in the middle of accessing it. Once `p->have_status' is set, it means that the child process is stopped or terminated, and in either case, it cannot stop or terminate again until the program has taken notice. *Note Atomic Usage::, for more information about coping with interruptions during accesses of a variable. Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked. sig_atomic_t process_status_change; sig_atomic_t last_process_status_change; ... { sig_atomic_t prev = last_process_status_change; last_process_status_change = process_status_change; if (last_process_status_change != prev) { struct process *p; for (p = process_list; p; p = p->next) if (p->have_status) { ... Examine `p->status' ... } } } Signal Handling and Nonreentrant Functions ------------------------------------------ Handler functions usually don't do very much. The best practice is to write a handler that does nothing but set an external variable that the program checks regularly, and leave all serious work to the program. This is best because the handler can be called asynchronously, at unpredictable times--perhaps in the middle of a primitive function, or even between the beginning and the end of a C operator that requires multiple instructions. The data structures being manipulated might therefore be in an inconsistent state when the handler function is invoked. Even copying one `int' variable into another can take two instructions on most machines. This means you have to be very careful about what you do in a signal handler. * If your handler needs to access any global variables from your program, declare those variables `volatile'. This tells the compiler that the value of the variable might change asynchronously, and inhibits certain optimizations that would be invalidated by such modifications. * If you call a function in the handler, make sure it is "reentrant" with respect to signals, or else make sure that the signal cannot interrupt a call to a related function. A function can be non-reentrant if it uses memory that is not on the stack. * If a function uses a static variable or a global variable, or a dynamically-allocated object that it finds for itself, then it is non-reentrant and any two calls to the function can interfere. For example, suppose that the signal handler uses `gethostbyname'. This function returns its value in a static object, reusing the same object each time. If the signal happens to arrive during a call to `gethostbyname', or even after one (while the program is still using the value), it will clobber the value that the program asked for. However, if the program does not use `gethostbyname' or any other function that returns information in the same object, or if it always blocks signals around each use, then you are safe. There are a large number of library functions that return values in a fixed object, always reusing the same object in this fashion, and all of them cause the same problem. Function descriptions in this manual always mention this behavior. * If a function uses and modifies an object that you supply, then it is potentially non-reentrant; two calls can interfere if they use the same object. This case arises when you do I/O using streams. Suppose that the signal handler prints a message with `fprintf'. Suppose that the program was in the middle of an `fprintf' call using the same stream when the signal was delivered. Both the signal handler's message and the program's data could be corrupted, because both calls operate on the same data structure--the stream itself. However, if you know that the stream that the handler uses cannot possibly be used by the program at a time when signals can arrive, then you are safe. It is no problem if the program uses some other stream. * On most systems, `malloc' and `free' are not reentrant, because they use a static data structure which records what memory blocks are free. As a result, no library functions that allocate or free memory are reentrant. This includes functions that allocate space to store a result. The best way to avoid the need to allocate memory in a handler is to allocate in advance space for signal handlers to use. The best way to avoid freeing memory in a handler is to flag or record the objects to be freed, and have the program check from time to time whether anything is waiting to be freed. But this must be done with care, because placing an object on a chain is not atomic, and if it is interrupted by another signal handler that does the same thing, you could "lose" one of the objects. * Any function that modifies `errno' is non-reentrant, but you can correct for this: in the handler, save the original value of `errno' and restore it before returning normally. This prevents errors that occur within the signal handler from being confused with errors from system calls at the point the program is interrupted to run the handler. This technique is generally applicable; if you want to call in a handler a function that modifies a particular object in memory, you can make this safe by saving and restoring that object. * Merely reading from a memory object is safe provided that you can deal with any of the values that might appear in the object at a time when the signal can be delivered. Keep in mind that assignment to some data types requires more than one instruction, which means that the handler could run "in the middle of" an assignment to the variable if its type is not atomic. *Note Atomic Data Access::. * Merely writing into a memory object is safe as long as a sudden change in the value, at any time when the handler might run, will not disturb anything. Atomic Data Access and Signal Handling -------------------------------------- Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily "atomic". This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler might be invoked in the middle of reading or writing the object. There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (*note Blocking Signals::). Problems with Non-Atomic Access ............................... Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.) #include #include struct two_words { int a, b; } memory; void handler(int signum) { printf ("%d,%d\n", memory.a, memory.b); alarm (1); } int main (void) { static struct two_words zeros = { 0, 0 }, ones = { 1, 1 }; signal (SIGALRM, handler); memory = zeros; alarm (1); while (1) { memory = zeros; memory = ones; } } This program fills `memory' with zeros, ones, zeros, ones, alternating forever; meanwhile, once per second, the alarm signal handler prints the current contents. (Calling `printf' in the handler is safe in this program because it is certainly not being called outside the handler when the signal happens.) Clearly, this program can print a pair of zeros or a pair of ones. But that's not all it can do! On most machines, it takes several instructions to store a new value in `memory', and the value is stored one word at a time. If the signal is delivered in between these instructions, the handler might find that `memory.a' is zero and `memory.b' is one (or vice versa). On some machines it may be possible to store a new value in `memory' with just one instruction that cannot be interrupted. On these machines, the handler will always print two zeros or two ones. Atomic Types ............ To avoid uncertainty about interrupting access to a variable, you can use a particular data type for which access is always atomic: `sig_atomic_t'. Reading and writing this data type is guaranteed to happen in a single instruction, so there's no way for a handler to run "in the middle" of an access. The type `sig_atomic_t' is always an integer data type, but which one it is, and how many bits it contains, may vary from machine to machine. - Data Type: sig_atomic_t This is an integer data type. Objects of this type are always accessed atomically. In practice, you can assume that `int' and other integer types no longer than `int' are atomic. You can also assume that pointer types are atomic; that is very convenient. Both of these assumptions are true on all of the machines that the GNU C library supports and on all POSIX systems we know of. Atomic Usage Patterns ..................... Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted. An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested. An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.) Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. *Note Merged Signals::, for an example. Primitives Interrupted by Signals ================================= A signal can arrive and be handled while an I/O primitive such as `open' or `read' is waiting for an I/O device. If the signal handler returns, the system faces the question: what should happen next? POSIX specifies one approach: make the primitive fail right away. The error code for this kind of failure is `EINTR'. This is flexible, but usually inconvenient. Typically, POSIX applications that use signal handlers must check for `EINTR' after each library function that can return it, in order to try the call again. Often programmers forget to check, which is a common source of error. The GNU library provides a convenient way to retry a call after a temporary failure, with the macro `TEMP_FAILURE_RETRY': - Macro: TEMP_FAILURE_RETRY (EXPRESSION) This macro evaluates EXPRESSION once. If it fails and reports error code `EINTR', `TEMP_FAILURE_RETRY' evaluates it again, and over and over until the result is not a temporary failure. The value returned by `TEMP_FAILURE_RETRY' is whatever value EXPRESSION produced. BSD avoids `EINTR' entirely and provides a more convenient approach: to restart the interrupted primitive, instead of making it fail. If you choose this approach, you need not be concerned with `EINTR'. You can choose either approach with the GNU library. If you use `sigaction' to establish a signal handler, you can specify how that handler should behave. If you specify the `SA_RESTART' flag, return from that handler will resume a primitive; otherwise, return from that handler will cause `EINTR'. *Note Flags for Sigaction::. Another way to specify the choice is with the `siginterrupt' function. *Note BSD Handler::. When you don't specify with `sigaction' or `siginterrupt' what a particular handler should do, it uses a default choice. The default choice in the GNU library depends on the feature test macros you have defined. If you define `_BSD_SOURCE' or `_GNU_SOURCE' before calling `signal', the default is to resume primitives; otherwise, the default is to make them fail with `EINTR'. (The library contains alternate versions of the `signal' function, and the feature test macros determine which one you really call.) *Note Feature Test Macros::. The description of each primitive affected by this issue lists `EINTR' among the error codes it can return. There is one situation where resumption never happens no matter which choice you make: when a data-transfer function such as `read' or `write' is interrupted by a signal after transferring part of the data. In this case, the function returns the number of bytes already transferred, indicating partial success. This might at first appear to cause unreliable behavior on record-oriented devices (including datagram sockets; *note Datagrams::), where splitting one `read' or `write' into two would read or write two records. Actually, there is no problem, because interruption after a partial transfer cannot happen on such devices; they always transfer an entire record in one burst, with no waiting once data transfer has started. Generating Signals ================== Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process. Signaling Yourself ------------------ A process can send itself a signal with the `raise' function. This function is declared in `signal.h'. - Function: int raise (int SIGNUM) The `raise' function sends the signal SIGNUM to the calling process. It returns zero if successful and a nonzero value if it fails. About the only reason for failure would be if the value of SIGNUM is invalid. - Function: int gsignal (int SIGNUM) The `gsignal' function does the same thing as `raise'; it is provided only for compatibility with SVID. One convenient use for `raise' is to reproduce the default behavior of a signal that you have trapped. For instance, suppose a user of your program types the SUSP character (usually `C-z'; *note Special Characters::) to send it an interactive stop signal (`SIGTSTP'), and you want to clean up some internal data buffers before stopping. You might set this up like this: #include /* When a stop signal arrives, set the action back to the default and then resend the signal after doing cleanup actions. */ void tstp_handler (int sig) { signal (SIGTSTP, SIG_DFL); /* Do cleanup actions here. */ ... raise (SIGTSTP); } /* When the process is continued again, restore the signal handler. */ void cont_handler (int sig) { signal (SIGCONT, cont_handler); signal (SIGTSTP, tstp_handler); } /* Enable both handlers during program initialization. */ int main (void) { signal (SIGCONT, cont_handler); signal (SIGTSTP, tstp_handler); ... } *Portability note:* `raise' was invented by the ISO C committee. Older systems may not support it, so using `kill' may be more portable. *Note Signaling Another Process::. Signaling Another Process ------------------------- The `kill' function can be used to send a signal to another process. In spite of its name, it can be used for a lot of things other than causing a process to terminate. Some examples of situations where you might want to send signals between processes are: * A parent process starts a child to perform a task--perhaps having the child running an infinite loop--and then terminates the child when the task is no longer needed. * A process executes as part of a group, and needs to terminate or notify the other processes in the group when an error or other event occurs. * Two processes need to synchronize while working together. This section assumes that you know a little bit about how processes work. For more information on this subject, see *Note Processes::. The `kill' function is declared in `signal.h'. - Function: int kill (pid_t PID, int SIGNUM) The `kill' function sends the signal SIGNUM to the process or process group specified by PID. Besides the signals listed in *Note Standard Signals::, SIGNUM can also have a value of zero to check the validity of the PID. The PID specifies the process or process group to receive the signal: `PID > 0' The process whose identifier is PID. `PID == 0' All processes in the same process group as the sender. `PID < -1' The process group whose identifier is -PID. `PID == -1' If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID. A process can send a signal to itself with a call like `kill (getpid(), SIGNUM)'. If `kill' is used by a process to send a signal to itself, and the signal is not blocked, then `kill' delivers at least one signal (which might be some other pending unblocked signal instead of the signal SIGNUM) to that process before it returns. The return value from `kill' is zero if the signal can be sent successfully. Otherwise, no signal is sent, and a value of `-1' is returned. If PID specifies sending a signal to several processes, `kill' succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did. The following `errno' error conditions are defined for this function: `EINVAL' The SIGNUM argument is an invalid or unsupported number. `EPERM' You do not have the privilege to send a signal to the process or any of the processes in the process group named by PID. `ESCRH' The PID argument does not refer to an existing process or group. - Function: int killpg (int PGID, int SIGNUM) This is similar to `kill', but sends signal SIGNUM to the process group PGID. This function is provided for compatibility with BSD; using `kill' to do this is more portable. As a simple example of `kill', the call `kill (getpid (), SIG)' has the same effect as `raise (SIG)'. Permission for using `kill' --------------------------- There are restrictions that prevent you from using `kill' to send signals to any random process. These are intended to prevent antisocial behavior such as arbitrarily killing off processes belonging to another user. In typical use, `kill' is used to pass signals between parent, child, and sibling processes, and in these situations you normally do have permission to send signals. The only common exception is when you run a setuid program in a child process; if the program changes its real UID as well as its effective UID, you may not have permission to send a signal. The `su' program does this. Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in *Note Process Persona::. Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions. The `SIGCONT' signal is a special case. It can be sent if the sender is part of the same session as the receiver, regardless of user IDs. Using `kill' for Communication ------------------------------ Here is a longer example showing how signals can be used for interprocess communication. This is what the `SIGUSR1' and `SIGUSR2' signals are provided for. Since these signals are fatal by default, the process that is supposed to receive them must trap them through `signal' or `sigaction'. In this example, a parent process forks a child process and then waits for the child to complete its initialization. The child process tells the parent when it is ready by sending it a `SIGUSR1' signal, using the `kill' function. #include #include #include #include /* When a `SIGUSR1' signal arrives, set this variable. */ volatile sig_atomic_t usr_interrupt = 0; void synch_signal (int sig) { usr_interrupt = 1; } /* The child process executes this function. */ void child_function (void) { /* Perform initialization. */ printf ("I'm here!!! My pid is %d.\n", (int) getpid ()); /* Let parent know you're done. */ kill (getppid (), SIGUSR1); /* Continue with execution. */ puts ("Bye, now...."); exit (0); } int main (void) { struct sigaction usr_action; sigset_t block_mask; pid_t child_id; /* Establish the signal handler. */ sigfillset (&block_mask); usr_action.sa_handler = synch_signal; usr_action.sa_mask = block_mask; usr_action.sa_flags = 0; sigaction (SIGUSR1, &usr_action, NULL); /* Create the child process. */ child_id = fork (); if (child_id == 0) child_function (); /* Does not return. */ /* Busy wait for the child to send a signal. */ while (!usr_interrupt) ; /* Now continue execution. */ puts ("That's all, folks!"); return 0; } This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in *Note Waiting for a Signal::. Blocking Signals ================ Blocking a signal means telling the operating system to hold it and deliver it later. Generally, a program does not block signals indefinitely--it might as well ignore them by setting their actions to `SIG_IGN'. But it is useful to block signals briefly, to prevent them from interrupting sensitive operations. For instance: * You can use the `sigprocmask' function to block signals while you modify global variables that are also modified by the handlers for these signals. * You can set `sa_mask' in your `sigaction' call to block certain signals while a particular signal handler runs. This way, the signal handler can run without being interrupted itself by signals. Why Blocking Signals is Useful ------------------------------ Temporary blocking of signals with `sigprocmask' gives you a way to prevent interrupts during critical parts of your code. If signals arrive in that part of the program, they are delivered later, after you unblock them. One example where this is useful is for sharing data between a signal handler and the rest of the program. If the type of the data is not `sig_atomic_t' (*note Atomic Data Access::), then the signal handler could run when the rest of the program has only half finished reading or writing the data. This would lead to confusing consequences. To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data. Blocking signals is also necessary when you want to perform a certain action only if a signal has not arrived. Suppose that the handler for the signal sets a flag of type `sig_atomic_t'; you would like to test the flag and perform the action if the flag is not set. This is unreliable. Suppose the signal is delivered immediately after you test the flag, but before the consequent action: then the program will perform the action even though the signal has arrived. The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked. Signal Sets ----------- All of the signal blocking functions use a data structure called a "signal set" to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function. These facilities are declared in the header file `signal.h'. - Data Type: sigset_t The `sigset_t' data type is used to represent a signal set. Internally, it may be implemented as either an integer or structure type. For portability, use only the functions described in this section to initialize, change, and retrieve information from `sigset_t' objects--don't try to manipulate them directly. There are two ways to initialize a signal set. You can initially specify it to be empty with `sigemptyset' and then add specified signals individually. Or you can specify it to be full with `sigfillset' and then delete specified signals individually. You must always initialize the signal set with one of these two functions before using it in any other way. Don't try to set all the signals explicitly because the `sigset_t' object might include some other information (like a version field) that needs to be initialized as well. (In addition, it's not wise to put into your program an assumption that the system has no signals aside from the ones you know about.) - Function: int sigemptyset (sigset_t *SET) This function initializes the signal set SET to exclude all of the defined signals. It always returns `0'. - Function: int sigfillset (sigset_t *SET) This function initializes the signal set SET to include all of the defined signals. Again, the return value is `0'. - Function: int sigaddset (sigset_t *SET, int SIGNUM) This function adds the signal SIGNUM to the signal set SET. All `sigaddset' does is modify SET; it does not block or unblock any signals. The return value is `0' on success and `-1' on failure. The following `errno' error condition is defined for this function: `EINVAL' The SIGNUM argument doesn't specify a valid signal. - Function: int sigdelset (sigset_t *SET, int SIGNUM) This function removes the signal SIGNUM from the signal set SET. All `sigdelset' does is modify SET; it does not block or unblock any signals. The return value and error conditions are the same as for `sigaddset'. Finally, there is a function to test what signals are in a signal set: - Function: int sigismember (const sigset_t *SET, int SIGNUM) The `sigismember' function tests whether the signal SIGNUM is a member of the signal set SET. It returns `1' if the signal is in the set, `0' if not, and `-1' if there is an error. The following `errno' error condition is defined for this function: `EINVAL' The SIGNUM argument doesn't specify a valid signal. Process Signal Mask ------------------- The collection of signals that are currently blocked is called the "signal mask". Each process has its own signal mask. When you create a new process (*note Creating a Process::), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask. The prototype for the `sigprocmask' function is in `signal.h'. - Function: int sigprocmask (int HOW, const sigset_t *restrict SET, sigset_t *restrict OLDSET) The `sigprocmask' function is used to examine or change the calling process's signal mask. The HOW argument determines how the signal mask is changed, and must be one of the following values: `SIG_BLOCK' Block the signals in `set'--add them to the existing mask. In other words, the new mask is the union of the existing mask and SET. `SIG_UNBLOCK' Unblock the signals in SET--remove them from the existing mask. `SIG_SETMASK' Use SET for the mask; ignore the previous value of the mask. The last argument, OLDSET, is used to return information about the old process signal mask. If you just want to change the mask without looking at it, pass a null pointer as the OLDSET argument. Similarly, if you want to know what's in the mask without changing it, pass a null pointer for SET (in this case the HOW argument is not significant). The OLDSET argument is often used to remember the previous signal mask in order to restore it later. (Since the signal mask is inherited over `fork' and `exec' calls, you can't predict what its contents are when your program starts running.) If invoking `sigprocmask' causes any pending signals to be unblocked, at least one of those signals is delivered to the process before `sigprocmask' returns. The order in which pending signals are delivered is not specified, but you can control the order explicitly by making multiple `sigprocmask' calls to unblock various signals one at a time. The `sigprocmask' function returns `0' if successful, and `-1' to indicate an error. The following `errno' error conditions are defined for this function: `EINVAL' The HOW argument is invalid. You can't block the `SIGKILL' and `SIGSTOP' signals, but if the signal set includes these, `sigprocmask' just ignores them instead of returning an error status. Remember, too, that blocking program error signals such as `SIGFPE' leads to undesirable results for signals generated by an actual program error (as opposed to signals sent with `raise' or `kill'). This is because your program may be too broken to be able to continue executing to a point where the signal is unblocked again. *Note Program Error Signals::. Blocking to Test for Delivery of a Signal ----------------------------------------- Now for a simple example. Suppose you establish a handler for `SIGALRM' signals that sets a flag whenever a signal arrives, and your main program checks this flag from time to time and then resets it. You can prevent additional `SIGALRM' signals from arriving in the meantime by wrapping the critical part of the code with calls to `sigprocmask', like this: /* This variable is set by the SIGALRM signal handler. */ volatile sig_atomic_t flag = 0; int main (void) { sigset_t block_alarm; ... /* Initialize the signal mask. */ sigemptyset (&block_alarm); sigaddset (&block_alarm, SIGALRM); while (1) { /* Check if a signal has arrived; if so, reset the flag. */ sigprocmask (SIG_BLOCK, &block_alarm, NULL); if (flag) { ACTIONS-IF-NOT-ARRIVED flag = 0; } sigprocmask (SIG_UNBLOCK, &block_alarm, NULL); ... } } Blocking Signals for a Handler ------------------------------ When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data. When a handler function is invoked on a signal, that signal is automatically blocked (in addition to any other signals that are already in the process's signal mask) during the time the handler is running. If you set up a handler for `SIGTSTP', for instance, then the arrival of that signal forces further `SIGTSTP' signals to wait during the execution of the handler. However, by default, other kinds of signals are not blocked; they can arrive during handler execution. The reliable way to block other kinds of signals during the execution of the handler is to use the `sa_mask' member of the `sigaction' structure. Here is an example: #include #include void catch_stop (); void install_handler (void) { struct sigaction setup_action; sigset_t block_mask; sigemptyset (&block_mask); /* Block other terminal-generated signals while handler runs. */ sigaddset (&block_mask, SIGINT); sigaddset (&block_mask, SIGQUIT); setup_action.sa_handler = catch_stop; setup_action.sa_mask = block_mask; setup_action.sa_flags = 0; sigaction (SIGTSTP, &setup_action, NULL); } This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicitly in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked. You cannot remove signals from the process's current mask using this mechanism. However, you can make calls to `sigprocmask' within your handler to block or unblock signals as you wish. In any case, when the handler returns, the system restores the mask that was in place before the handler was entered. If any signals that become unblocked by this restoration are pending, the process will receive those signals immediately, before returning to the code that was interrupted. Checking for Pending Signals ---------------------------- You can find out which signals are pending at any time by calling `sigpending'. This function is declared in `signal.h'. - Function: int sigpending (sigset_t *SET) The `sigpending' function stores information about pending signals in SET. If there is a pending signal that is blocked from delivery, then that signal is a member of the returned set. (You can test whether a particular signal is a member of this set using `sigismember'; see *Note Signal Sets::.) The return value is `0' if successful, and `-1' on failure. Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design. Here is an example. #include #include sigset_t base_mask, waiting_mask; sigemptyset (&base_mask); sigaddset (&base_mask, SIGINT); sigaddset (&base_mask, SIGTSTP); /* Block user interrupts while doing other processing. */ sigprocmask (SIG_SETMASK, &base_mask, NULL); ... /* After a while, check to see whether any signals are pending. */ sigpending (&waiting_mask); if (sigismember (&waiting_mask, SIGINT)) { /* User has tried to kill the process. */ } else if (sigismember (&waiting_mask, SIGTSTP)) { /* User has tried to stop the process. */ } Remember that if there is a particular signal pending for your process, additional signals of that same type that arrive in the meantime might be discarded. For example, if a `SIGINT' signal is pending when another `SIGINT' signal arrives, your program will probably only see one of them when you unblock this signal. *Portability Note:* The `sigpending' function is new in POSIX.1. Older systems have no equivalent facility. Remembering a Signal to Act On Later ------------------------------------ Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock". Here is an example: /* If this flag is nonzero, don't handle the signal right away. */ volatile sig_atomic_t signal_pending; /* This is nonzero if a signal arrived and was not handled. */ volatile sig_atomic_t defer_signal; void handler (int signum) { if (defer_signal) signal_pending = signum; else ... /* "Really" handle the signal. */ } ... void update_mumble (int frob) { /* Prevent signals from having immediate effect. */ defer_signal++; /* Now update `mumble', without worrying about interruption. */ mumble.a = 1; mumble.b = hack (); mumble.c = frob; /* We have updated `mumble'. Handle any signal that came in. */ defer_signal--; if (defer_signal == 0 && signal_pending != 0) raise (signal_pending); } Note how the particular signal that arrives is stored in `signal_pending'. That way, we can handle several types of inconvenient signals with the same mechanism. We increment and decrement `defer_signal' so that nested critical sections will work properly; thus, if `update_mumble' were called with `signal_pending' already nonzero, signals would be deferred not only within `update_mumble', but also within the caller. This is also why we do not check `signal_pending' if `defer_signal' is still nonzero. The incrementing and decrementing of `defer_signal' each require more than one instruction; it is possible for a signal to happen in the middle. But that does not cause any problem. If the signal happens early enough to see the value from before the increment or decrement, that is equivalent to a signal which came before the beginning of the increment or decrement, which is a case that works properly. It is absolutely vital to decrement `defer_signal' before testing `signal_pending', because this avoids a subtle bug. If we did these things in the other order, like this, if (defer_signal == 1 && signal_pending != 0) raise (signal_pending); defer_signal--; then a signal arriving in between the `if' statement and the decrement would be effectively "lost" for an indefinite amount of time. The handler would merely set `defer_signal', but the program having already tested this variable, it would not test the variable again. Bugs like these are called "timing errors". They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them. (You would not be tempted to write the code in this order, given the use of `defer_signal' as a counter which must be tested along with `signal_pending'. After all, testing for zero is cleaner than testing for one. But if you did not use `defer_signal' as a counter, and gave it values of zero and one only, then either order might seem equally simple. This is a further advantage of using a counter for `defer_signal': it will reduce the chance you will write the code in the wrong order and create a subtle bug.) Waiting for a Signal ==================== If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives. Using `pause' ------------- The simple way to wait until a signal arrives is to call `pause'. Please read about its disadvantages, in the following section, before you use it. - Function: int pause () The `pause' function suspends program execution until a signal arrives whose action is either to execute a handler function, or to terminate the process. If the signal causes a handler function to be executed, then `pause' returns. This is considered an unsuccessful return (since "successful" behavior would be to suspend the program forever), so the return value is `-1'. Even if you specify that other primitives should resume when a system handler returns (*note Interrupted Primitives::), this has no effect on `pause'; it always fails when a signal is handled. The following `errno' error conditions are defined for this function: `EINTR' The function was interrupted by delivery of a signal. If the signal causes program termination, `pause' doesn't return (obviously). This function is a cancellation point in multithreaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `pause' is called. If the thread gets cancelled these resources stay allocated until the program ends. To avoid this calls to `pause' should be protected using cancellation handlers. The `pause' function is declared in `unistd.h'. Problems with `pause' --------------------- The simplicity of `pause' can conceal serious timing errors that can make a program hang mysteriously. It is safe to use `pause' if the real work of your program is done by the signal handlers themselves, and the "main program" does nothing but call `pause'. Each time a signal is delivered, the handler will do the next batch of work that is to be done, and then return, so that the main loop of the program can call `pause' again. You can't safely use `pause' to wait until one more signal arrives, and then resume real work. Even if you arrange for the signal handler to cooperate by setting a flag, you still can't use `pause' reliably. Here is an example of this problem: /* `usr_interrupt' is set by the signal handler. */ if (!usr_interrupt) pause (); /* Do work once the signal arrives. */ ... This has a bug: the signal could arrive after the variable `usr_interrupt' is checked, but before the call to `pause'. If no further signals arrive, the process would never wake up again. You can put an upper limit on the excess waiting by using `sleep' in a loop, instead of using `pause'. (*Note Sleeping::, for more about `sleep'.) Here is what this looks like: /* `usr_interrupt' is set by the signal handler. while (!usr_interrupt) sleep (1); /* Do work once the signal arrives. */ ... For some purposes, that is good enough. But with a little more complexity, you can wait reliably until a particular signal handler is run, using `sigsuspend'. *Note Sigsuspend::. Using `sigsuspend' ------------------ The clean and reliable way to wait for a signal to arrive is to block it and then use `sigsuspend'. By using `sigsuspend' in a loop, you can wait for certain kinds of signals, while letting other kinds of signals be handled by their handlers. - Function: int sigsuspend (const sigset_t *SET) This function replaces the process's signal mask with SET and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function. In other words, the program is effectively suspended until one of the signals that is not a member of SET arrives. If the process is woken up by delivery of a signal that invokes a handler function, and the handler function returns, then `sigsuspend' also returns. The mask remains SET only as long as `sigsuspend' is waiting. The function `sigsuspend' always restores the previous signal mask when it returns. The return value and error conditions are the same as for `pause'. With `sigsuspend', you can replace the `pause' or `sleep' loop in the previous section with something completely reliable: sigset_t mask, oldmask; ... /* Set up the mask of signals to temporarily block. */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); ... /* Wait for a signal to arrive. */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL); This last piece of code is a little tricky. The key point to remember here is that when `sigsuspend' returns, it resets the process's signal mask to the original value, the value from before the call to `sigsuspend'--in this case, the `SIGUSR1' signal is once again blocked. The second call to `sigprocmask' is necessary to explicitly unblock this signal. One other point: you may be wondering why the `while' loop is necessary at all, since the program is apparently only waiting for one `SIGUSR1' signal. The answer is that the mask passed to `sigsuspend' permits the process to be woken up by the delivery of other kinds of signals, as well--for example, job control signals. If the process is woken up by a signal that doesn't set `usr_interrupt', it just suspends itself again until the "right" kind of signal eventually arrives. This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines. Using a Separate Signal Stack ============================= A signal stack is a special area of memory to be used as the execution stack during signal handlers. It should be fairly large, to avoid any danger that it will overflow in turn; the macro `SIGSTKSZ' is defined to a canonical size for signal stacks. You can use `malloc' to allocate the space for the stack. Then call `sigaltstack' or `sigstack' to tell the system to use that space for the signal stack. You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. (Some non-GNU debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.) There are two interfaces for telling the system to use a separate signal stack. `sigstack' is the older interface, which comes from 4.2 BSD. `sigaltstack' is the newer interface, and comes from 4.4 BSD. The `sigaltstack' interface has the advantage that it does not require your program to know which direction the stack grows, which depends on the specific machine and operating system. - Data Type: stack_t This structure describes a signal stack. It contains the following members: `void *ss_sp' This points to the base of the signal stack. `size_t ss_size' This is the size (in bytes) of the signal stack which `ss_sp' points to. You should set this to however much space you allocated for the stack. There are two macros defined in `signal.h' that you should use in calculating this size: `SIGSTKSZ' This is the canonical size for a signal stack. It is judged to be sufficient for normal uses. `MINSIGSTKSZ' This is the amount of signal stack space the operating system needs just to implement signal delivery. The size of a signal stack *must* be greater than this. For most cases, just using `SIGSTKSZ' for `ss_size' is sufficient. But if you know how much stack space your program's signal handlers will need, you may want to use a different size. In this case, you should allocate `MINSIGSTKSZ' additional bytes for the signal stack and increase `ss_size' accordingly. `int ss_flags' This field contains the bitwise OR of these flags: `SS_DISABLE' This tells the system that it should not use the signal stack. `SS_ONSTACK' This is set by the system, and indicates that the signal stack is currently in use. If this bit is not set, then signals will be delivered on the normal user stack. - Function: int sigaltstack (const stack_t *restrict STACK, stack_t *restrict OLDSTACK) The `sigaltstack' function specifies an alternate stack for use during signal handling. When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed. If OLDSTACK is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If STACK is not a null pointer, then this is installed as the new stack for use by signal handlers. The return value is `0' on success and `-1' on failure. If `sigaltstack' fails, it sets `errno' to one of these values: `EINVAL' You tried to disable a stack that was in fact currently in use. `ENOMEM' The size of the alternate stack was too small. It must be greater than `MINSIGSTKSZ'. Here is the older `sigstack' interface. You should use `sigaltstack' instead on systems that have it. - Data Type: struct sigstack This structure describes a signal stack. It contains the following members: `void *ss_sp' This is the stack pointer. If the stack grows downwards on your machine, this should point to the top of the area you allocated. If the stack grows upwards, it should point to the bottom. `int ss_onstack' This field is true if the process is currently using this stack. - Function: int sigstack (const struct sigstack *STACK, struct sigstack *OLDSTACK) The `sigstack' function specifies an alternate stack for use during signal handling. When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed. If OLDSTACK is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If STACK is not a null pointer, then this is installed as the new stack for use by signal handlers. The return value is `0' on success and `-1' on failure. BSD Signal Handling =================== This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix. There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are: * BSD Unix represents signal masks as an `int' bit mask, rather than as a `sigset_t' object. * The BSD facilities use a different default for whether an interrupted primitive should fail or resume. The POSIX facilities make system calls fail unless you specify that they should resume. With the BSD facility, the default is to make system calls resume unless you say they should fail. *Note Interrupted Primitives::. The BSD facilities are declared in `signal.h'. BSD Function to Establish a Handler ----------------------------------- - Data Type: struct sigvec This data type is the BSD equivalent of `struct sigaction' (*note Advanced Signal Handling::); it is used to specify signal actions to the `sigvec' function. It contains the following members: `sighandler_t sv_handler' This is the handler function. `int sv_mask' This is the mask of additional signals to be blocked while the handler function is being called. `int sv_flags' This is a bit mask used to specify various flags which affect the behavior of the signal. You can also refer to this field as `sv_onstack'. These symbolic constants can be used to provide values for the `sv_flags' field of a `sigvec' structure. This field is a bit mask value, so you bitwise-OR the flags of interest to you together. - Macro: int SV_ONSTACK If this bit is set in the `sv_flags' field of a `sigvec' structure, it means to use the signal stack when delivering the signal. - Macro: int SV_INTERRUPT If this bit is set in the `sv_flags' field of a `sigvec' structure, it means that system calls interrupted by this kind of signal should not be restarted if the handler returns; instead, the system calls should return with a `EINTR' error status. *Note Interrupted Primitives::. - Macro: int SV_RESETHAND If this bit is set in the `sv_flags' field of a `sigvec' structure, it means to reset the action for the signal back to `SIG_DFL' when the signal is received. - Function: int sigvec (int SIGNUM, const struct sigvec *ACTION,struct sigvec *OLD-ACTION) This function is the equivalent of `sigaction' (*note Advanced Signal Handling::); it installs the action ACTION for the signal SIGNUM, returning information about the previous action in effect for that signal in OLD-ACTION. - Function: int siginterrupt (int SIGNUM, int FAILFLAG) This function specifies which approach to use when certain primitives are interrupted by handling signal SIGNUM. If FAILFLAG is false, signal SIGNUM restarts primitives. If FAILFLAG is true, handling SIGNUM causes these primitives to fail with error code `EINTR'. *Note Interrupted Primitives::. BSD Functions for Blocking Signals ---------------------------------- - Macro: int sigmask (int SIGNUM) This macro returns a signal mask that has the bit for signal SIGNUM set. You can bitwise-OR the results of several calls to `sigmask' together to specify more than one signal. For example, (sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU)) specifies a mask that includes all the job-control stop signals. - Function: int sigblock (int MASK) This function is equivalent to `sigprocmask' (*note Process Signal Mask::) with a HOW argument of `SIG_BLOCK': it adds the signals specified by MASK to the calling process's set of blocked signals. The return value is the previous set of blocked signals. - Function: int sigsetmask (int MASK) This function equivalent to `sigprocmask' (*note Process Signal Mask::) with a HOW argument of `SIG_SETMASK': it sets the calling process's signal mask to MASK. The return value is the previous set of blocked signals. - Function: int sigpause (int MASK) This function is the equivalent of `sigsuspend' (*note Waiting for a Signal::): it sets the calling process's signal mask to MASK, and waits for a signal to arrive. On return the previous set of blocked signals is restored. The Basic Program/System Interface ********************************** "Processes" are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies. Though it may have multiple threads of control within the same program and a program may be composed of multiple logically separate modules, a process always executes exactly one program. Note that we are using a specific definition of "program" for the purposes of this manual, which corresponds to a common definition in the context of Unix system. In popular usage, "program" enjoys a much broader definition; it can refer for example to a system's kernel, an editor macro, a complex package of software, or a discrete section of code executing within a process. Writing the program is what this manual is all about. This chapter explains the most basic interface between your program and the system that runs, or calls, it. This includes passing of parameters (arguments and environment) from the system, requesting basic services from the system, and telling the system the program is done. A program starts another program with the `exec' family of system calls. This chapter looks at program startup from the execee's point of view. To see the event from the execor's point of view, *Note Executing a File::. Program Arguments ================= The system starts a C program by calling the function `main'. It is up to you to write a function named `main'--otherwise, you won't even be able to link your program without errors. In ISO C you can define `main' either to take no arguments, or to take two arguments that represent the command line arguments to the program, like this: int main (int ARGC, char *ARGV[]) The command line arguments are the whitespace-separated tokens given in the shell command used to invoke the program; thus, in `cat foo bar', the arguments are `foo' and `bar'. The only way a program can look at its command line arguments is via the arguments of `main'. If `main' doesn't take arguments, then you cannot get at the command line. The value of the ARGC argument is the number of command line arguments. The ARGV argument is a vector of C strings; its elements are the individual command line argument strings. The file name of the program being run is also included in the vector as the first element; the value of ARGC counts this element. A null pointer always follows the last element: `ARGV[ARGC]' is this null pointer. For the command `cat foo bar', ARGC is 3 and ARGV has three elements, `"cat"', `"foo"' and `"bar"'. In Unix systems you can define `main' a third way, using three arguments: int main (int ARGC, char *ARGV[], char *ENVP[]) The first two arguments are just the same. The third argument ENVP gives the program's environment; it is the same as the value of `environ'. *Note Environment Variables::. POSIX.1 does not allow this three-argument form, so to be portable it is best to write `main' to take two arguments, and use the value of `environ'. Program Argument Syntax Conventions ----------------------------------- POSIX recommends these conventions for command line arguments. `getopt' (*note Getopt::) and `argp_parse' (*note Argp::) make it easy to implement them. * Arguments are options if they begin with a hyphen delimiter (`-'). * Multiple options may follow a hyphen delimiter in a single token if the options do not take arguments. Thus, `-abc' is equivalent to `-a -b -c'. * Option names are single alphanumeric characters (as for `isalnum'; *note Classification of Characters::). * Certain options require an argument. For example, the `-o' command of the `ld' command requires an argument--an output file name. * An option and its argument may or may not appear as separate tokens. (In other words, the whitespace separating them is optional.) Thus, `-o foo' and `-ofoo' are equivalent. * Options typically precede other non-option arguments. The implementations of `getopt' and `argp_parse' in the GNU C library normally make it appear as if all the option arguments were specified before all the non-option arguments for the purposes of parsing, even if the user of your program intermixed option and non-option arguments. They do this by reordering the elements of the ARGV array. This behavior is nonstandard; if you want to suppress it, define the `_POSIX_OPTION_ORDER' environment variable. *Note Standard Environment::. * The argument `--' terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen. * A token consisting of a single hyphen character is interpreted as an ordinary non-option argument. By convention, it is used to specify input from or output to the standard input and output streams. * Options may be supplied in any order, or appear multiple times. The interpretation is left up to the particular application program. GNU adds "long options" to these conventions. Long options consist of `--' followed by a name made of alphanumeric characters and dashes. Option names are typically one to three words long, with hyphens to separate words. Users can abbreviate the option names as long as the abbreviations are unique. To specify an argument for a long option, write `--NAME=VALUE'. This syntax enables a long option to accept an argument that is itself optional. Eventually, the GNU system will provide completion for long option names in the shell. Parsing Program Arguments ------------------------- If the syntax for the command line arguments to your program is simple enough, you can simply pick the arguments off from ARGV by hand. But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using `getopt' (*note Getopt::) or `argp_parse' (*note Argp::) to do the parsing. `getopt' is more standard (the short-option only version of it is a part of the POSIX standard), but using `argp_parse' is often easier, both for very simple and very complex option structures, because it does more of the dirty work for you. Parsing program options using `getopt' ====================================== The `getopt' and `getopt_long' functions automate some of the chore involved in parsing typical unix command line options. Using the `getopt' function --------------------------- Here are the details about how to call the `getopt' function. To use this facility, your program must include the header file `unistd.h'. - Variable: int opterr If the value of this variable is nonzero, then `getopt' prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument. This is the default behavior. If you set this variable to zero, `getopt' does not print any messages, but it still returns the character `?' to indicate an error. - Variable: int optopt When `getopt' encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable. You can use this for providing your own diagnostic messages. - Variable: int optind This variable is set by `getopt' to the index of the next element of the ARGV array to be processed. Once `getopt' has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin. The initial value of this variable is `1'. - Variable: char * optarg This variable is set by `getopt' to point at the value of the option argument, for those options that accept arguments. - Function: int getopt (int ARGC, char **ARGV, const char *OPTIONS) The `getopt' function gets the next option argument from the argument list specified by the ARGV and ARGC arguments. Normally these values come directly from the arguments received by `main'. The OPTIONS argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (`:') to indicate that it takes a required argument. If an option character is followed by two colons (`::'), its argument is optional; this is a GNU extension. `getopt' has three ways to deal with options that follow non-options ARGV elements. The special argument `--' forces in all cases the end of option scanning. * The default is to permute the contents of ARGV while scanning it so that eventually all the non-options are at the end. This allows options to be given in any order, even with programs that were not written to expect this. * If the OPTIONS argument string begins with a hyphen (`-'), this is treated specially. It permits arguments that are not options to be returned as if they were associated with option character `\1'. * POSIX demands the following behavior: The first non-option stops option processing. This mode is selected by either setting the environment variable `POSIXLY_CORRECT' or beginning the OPTIONS argument string with a plus sign (`+'). The `getopt' function returns the option character for the next command line option. When no more option arguments are available, it returns `-1'. There may still be more non-option arguments; you must compare the external variable `optind' against the ARGC parameter to check this. If the option has an argument, `getopt' returns the argument by storing it in the variable OPTARG. You don't ordinarily need to copy the `optarg' string, since it is a pointer into the original ARGV array, not into a static area that might be overwritten. If `getopt' finds an option character in ARGV that was not included in OPTIONS, or a missing option argument, it returns `?' and sets the external variable `optopt' to the actual option character. If the first character of OPTIONS is a colon (`:'), then `getopt' returns `:' instead of `?' to indicate a missing option argument. In addition, if the external variable `opterr' is nonzero (which is the default), `getopt' prints an error message. Example of Parsing Arguments with `getopt' ------------------------------------------ Here is an example showing how `getopt' is typically used. The key points to notice are: * Normally, `getopt' is called in a loop. When `getopt' returns `-1', indicating no more options are present, the loop terminates. * A `switch' statement is used to dispatch on the return value from `getopt'. In typical use, each case just sets a variable that is used later in the program. * A second loop is used to process the remaining non-option arguments. #include #include int main (int argc, char **argv) { int aflag = 0; int bflag = 0; char *cvalue = NULL; int index; int c; opterr = 0; while ((c = getopt (argc, argv, "abc:")) != -1) switch (c) { case 'a': aflag = 1; break; case 'b': bflag = 1; break; case 'c': cvalue = optarg; break; case '?': if (isprint (optopt)) fprintf (stderr, "Unknown option `-%c'.\n", optopt); else fprintf (stderr, "Unknown option character `\\x%x'.\n", optopt); return 1; default: abort (); } printf ("aflag = %d, bflag = %d, cvalue = %s\n", aflag, bflag, cvalue); for (index = optind; index < argc; index++) printf ("Non-option argument %s\n", argv[index]); return 0; } Here are some examples showing what this program prints with different combinations of arguments: % testopt aflag = 0, bflag = 0, cvalue = (null) % testopt -a -b aflag = 1, bflag = 1, cvalue = (null) % testopt -ab aflag = 1, bflag = 1, cvalue = (null) % testopt -c foo aflag = 0, bflag = 0, cvalue = foo % testopt -cfoo aflag = 0, bflag = 0, cvalue = foo % testopt arg1 aflag = 0, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -a arg1 aflag = 1, bflag = 0, cvalue = (null) Non-option argument arg1 % testopt -c foo arg1 aflag = 0, bflag = 0, cvalue = foo Non-option argument arg1 % testopt -a -- -b aflag = 1, bflag = 0, cvalue = (null) Non-option argument -b % testopt -a - aflag = 1, bflag = 0, cvalue = (null) Non-option argument - Parsing Long Options with `getopt_long' --------------------------------------- To accept GNU-style long options as well as single-character options, use `getopt_long' instead of `getopt'. This function is declared in `getopt.h', not `unistd.h'. You should make every program accept long options if it uses any options, for this takes little extra work and helps beginners remember how to use the program. - Data Type: struct option This structure describes a single long option name for the sake of `getopt_long'. The argument LONGOPTS must be an array of these structures, one for each long option. Terminate the array with an element containing all zeros. The `struct option' structure has these fields: `const char *name' This field is the name of the option. It is a string. `int has_arg' This field says whether the option takes an argument. It is an integer, and there are three legitimate values: `no_argument', `required_argument' and `optional_argument'. `int *flag' `int val' These fields control how to report or act on the option when it occurs. If `flag' is a null pointer, then the `val' is a value which identifies this option. Often these values are chosen to uniquely identify particular long options. If `flag' is not a null pointer, it should be the address of an `int' variable which is the flag for this option. The value in `val' is the value to store in the flag to indicate that the option was seen. - Function: int getopt_long (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR) Decode options from the vector ARGV (whose length is ARGC). The argument SHORTOPTS describes the short options to accept, just as it does in `getopt'. The argument LONGOPTS describes the long options to accept (see above). When `getopt_long' encounters a short option, it does the same thing that `getopt' would do: it returns the character code for the option, and stores the options argument (if it has one) in `optarg'. When `getopt_long' encounters a long option, it takes actions based on the `flag' and `val' fields of the definition of that option. If `flag' is a null pointer, then `getopt_long' returns the contents of `val' to indicate which option it found. You should arrange distinct values in the `val' field for options with different meanings, so you can decode these values after `getopt_long' returns. If the long option is equivalent to a short option, you can use the short option's character code in `val'. If `flag' is not a null pointer, that means this option should just set a flag in the program. The flag is a variable of type `int' that you define. Put the address of the flag in the `flag' field. Put in the `val' field the value you would like this option to store in the flag. In this case, `getopt_long' returns `0'. For any long option, `getopt_long' tells you the index in the array LONGOPTS of the options definition, by storing it into `*INDEXPTR'. You can get the name of the option with `LONGOPTS[*INDEXPTR].name'. So you can distinguish among long options either by the values in their `val' fields or by their indices. You can also distinguish in this way among long options that set flags. When a long option has an argument, `getopt_long' puts the argument value in the variable `optarg' before returning. When the option has no argument, the value in `optarg' is a null pointer. This is how you can tell whether an optional argument was supplied. When `getopt_long' has no more options to handle, it returns `-1', and leaves in the variable `optind' the index in ARGV of the next remaining argument. Since long option names were used before before the `getopt_long' options was invented there are program interfaces which require programs to recognize options like `-option value' instead of `--option value'. To enable these programs to use the GNU getopt functionality there is one more function available. - Function: int getopt_long_only (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR) The `getopt_long_only' function is equivalent to the `getopt_long' function but it allows to specify the user of the application to pass long options with only `-' instead of `--'. The `--' prefix is still recognized but instead of looking through the short options if a `-' is seen it is first tried whether this parameter names a long option. If not, it is parsed as a short option. Assuming `getopt_long_only' is used starting an application with app -foo the `getopt_long_only' will first look for a long option named `foo'. If this is not found, the short options `f', `o', and again `o' are recognized. Example of Parsing Long Options with `getopt_long' -------------------------------------------------- #include #include #include /* Flag set by `--verbose'. */ static int verbose_flag; int main (argc, argv) int argc; char **argv; { int c; while (1) { static struct option long_options[] = { /* These options set a flag. */ {"verbose", no_argument, &verbose_flag, 1}, {"brief", no_argument, &verbose_flag, 0}, /* These options don't set a flag. We distinguish them by their indices. */ {"add", no_argument, 0, 'a'}, {"append", no_argument, 0, 'b'}, {"delete", required_argument, 0, 'd'}, {"create", required_argument, 0, 'c'}, {"file", required_argument, 0, 'f'}, {0, 0, 0, 0} }; /* `getopt_long' stores the option index here. */ int option_index = 0; c = getopt_long (argc, argv, "abc:d:f:", long_options, &option_index); /* Detect the end of the options. */ if (c == -1) break; switch (c) { case 0: /* If this option set a flag, do nothing else now. */ if (long_options[option_index].flag != 0) break; printf ("option %s", long_options[option_index].name); if (optarg) printf (" with arg %s", optarg); printf ("\n"); break; case 'a': puts ("option -a\n"); break; case 'b': puts ("option -b\n"); break; case 'c': printf ("option -c with value `%s'\n", optarg); break; case 'd': printf ("option -d with value `%s'\n", optarg); break; case 'f': printf ("option -f with value `%s'\n", optarg); break; case '?': /* `getopt_long' already printed an error message. */ break; default: abort (); } } /* Instead of reporting `--verbose' and `--brief' as they are encountered, we report the final status resulting from them. */ if (verbose_flag) puts ("verbose flag is set"); /* Print any remaining command line arguments (not options). */ if (optind < argc) { printf ("non-option ARGV-elements: "); while (optind < argc) printf ("%s ", argv[optind++]); putchar ('\n'); } exit (0); } Parsing Program Options with Argp ================================= "Argp" is an interface for parsing unix-style argument vectors. *Note Program Arguments::. Argp provides features unavailable in the more commonly used `getopt' interface. These features include automatically producing output in response to the `--help' and `--version' options, as described in the GNU coding standards. Using argp makes it less likely that programmers will neglect to implement these additional options or keep them up to date. Argp also provides the ability to merge several independently defined option parsers into one, mediating conflicts between them and making the result appear seamless. A library can export an argp option parser that user programs might employ in conjunction with their own option parsers, resulting in less work for the user programs. Some programs may use only argument parsers exported by libraries, thereby achieving consistent and efficient option-parsing for abstractions implemented by the libraries. The header file `' should be included to use argp. The `argp_parse' Function ------------------------- The main interface to argp is the `argp_parse' function. In many cases, calling `argp_parse' is the only argument-parsing code needed in `main'. *Note Program Arguments::. - Function: error_t argp_parse (const struct argp *ARGP, int ARGC, char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT) The `argp_parse' function parses the arguments in ARGV, of length ARGC, using the argp parser ARGP. *Note Argp Parsers::. A value of zero is the same as a `struct argp'containing all zeros. FLAGS is a set of flag bits that modify the parsing behavior. *Note Argp Flags::. INPUT is passed through to the argp parser ARGP, and has meaning defined by ARGP. A typical usage is to pass a pointer to a structure which is used for specifying parameters to the parser and passing back the results. Unless the `ARGP_NO_EXIT' or `ARGP_NO_HELP' flags are included in FLAGS, calling `argp_parse' may result in the program exiting. This behavior is true if an error is detected, or when an unknown option is encountered. *Note Program Termination::. If ARG_INDEX is non-null, the index of the first unparsed option in ARGV is returned as a value. The return value is zero for successful parsing, or an error code (*note Error Codes::) if an error is detected. Different argp parsers may return arbitrary error codes, but the standard error codes are: `ENOMEM' if a memory allocation error occurred, or `EINVAL' if an unknown option or option argument is encountered. Argp Global Variables --------------------- These variables make it easy for user programs to implement the `--version' option and provide a bug-reporting address in the `--help' output. These are implemented in argp by default. - Variable: const char * argp_program_version If defined or set by the user program to a non-zero value, then a `--version' option is added when parsing with `argp_parse', which will print the `--version' string followed by a newline and exit. The exception to this is if the `ARGP_NO_EXIT' flag is used. - Variable: const char * argp_program_bug_address If defined or set by the user program to a non-zero value, `argp_program_bug_address' should point to a string that will be printed at the end of the standard output for the `--help' option, embedded in a sentence that says `Report bugs to ADDRESS.'. - Variable: argp_program_version_hook If defined or set by the user program to a non-zero value, a `--version' option is added when parsing with `arg_parse', which prints the program version and exits with a status of zero. This is not the case if the `ARGP_NO_HELP' flag is used. If the `ARGP_NO_EXIT' flag is set, the exit behavior of the program is suppressed or modified, as when the argp parser is going to be used by other programs. It should point to a function with this type of signature: void PRINT-VERSION (FILE *STREAM, struct argp_state *STATE) *Note Argp Parsing State::, for an explanation of STATE. This variable takes precedence over `argp_program_version', and is useful if a program has version information not easily expressed in a simple string. - Variable: error_t argp_err_exit_status This is the exit status used when argp exits due to a parsing error. If not defined or set by the user program, this defaults to: `EX_USAGE' from `'. Specifying Argp Parsers ----------------------- The first argument to the `argp_parse' function is a pointer to a `struct argp', which is known as an "argp parser": - Data Type: struct argp This structure specifies how to parse a given set of options and arguments, perhaps in conjunction with other argp parsers. It has the following fields: `const struct argp_option *options' A pointer to a vector of `argp_option' structures specifying which options this argp parser understands; it may be zero if there are no options at all. *Note Argp Option Vectors::. `argp_parser_t parser' A pointer to a function that defines actions for this parser; it is called for each option parsed, and at other well-defined points in the parsing process. A value of zero is the same as a pointer to a function that always returns `ARGP_ERR_UNKNOWN'. *Note Argp Parser Functions::. `const char *args_doc' If non-zero, a string describing what non-option arguments are called by this parser. This is only used to print the `Usage:' message. If it contains newlines, the strings separated by them are considered alternative usage patterns and printed on separate lines. Lines after the first are prefixed by ` or: ' instead of `Usage:'. `const char *doc' If non-zero, a string containing extra text to be printed before and after the options in a long help message, with the two sections separated by a vertical tab (`'\v'', `'\013'') character. By convention, the documentation before the options is just a short string explaining what the program does. Documentation printed after the options describe behavior in more detail. `const struct argp_child *children' A pointer to a vector of `argp_children' structures. This pointer specifies which additional argp parsers should be combined with this one. *Note Argp Children::. `char *(*help_filter)(int KEY, const char *TEXT, void *INPUT)' If non-zero, a pointer to a function that filters the output of help messages. *Note Argp Help Filtering::. `const char *argp_domain' If non-zero, the strings used in the argp library are translated using the domain described by this string. If zero, the current default domain is used. Of the above group, `options', `parser', `args_doc', and the `doc' fields are usually all that are needed. If an argp parser is defined as an initialized C variable, only the fields used need be specified in the initializer. The rest will default to zero due to the way C structure initialization works. This design is exploited in most argp structures; the most-used fields are grouped near the beginning, the unused fields left unspecified. Specifying Options in an Argp Parser ------------------------------------ The `options' field in a `struct argp' points to a vector of `struct argp_option' structures, each of which specifies an option that the argp parser supports. Multiple entries may be used for a single option provided it has multiple names. This should be terminated by an entry with zero in all fields. Note that when using an initialized C array for options, writing `{ 0 }' is enough to achieve this. - Data Type: struct argp_option This structure specifies a single option that an argp parser understands, as well as how to parse and document that option. It has the following fields: `const char *name' The long name for this option, corresponding to the long option `--NAME'; this field may be zero if this option _only_ has a short name. To specify multiple names for an option, additional entries may follow this one, with the `OPTION_ALIAS' flag set. *Note Argp Option Flags::. `int key' The integer key provided by the current option to the option parser. If KEY has a value that is a printable ASCII character (i.e., `isascii (KEY)' is true), it _also_ specifies a short option `-CHAR', where CHAR is the ASCII character with the code KEY. `const char *arg' If non-zero, this is the name of an argument associated with this option, which must be provided (e.g., with the `--NAME=VALUE' or `-CHAR VALUE' syntaxes), unless the `OPTION_ARG_OPTIONAL' flag (*note Argp Option Flags::) is set, in which case it _may_ be provided. `int flags' Flags associated with this option, some of which are referred to above. *Note Argp Option Flags::. `const char *doc' A documentation string for this option, for printing in help messages. If both the `name' and `key' fields are zero, this string will be printed tabbed left from the normal option column, making it useful as a group header. This will be the first thing printed in its group. In this usage, it's conventional to end the string with a `:' character. `int group' Group identity for this option. In a long help message, options are sorted alphabetically within each group, and the groups presented in the order 0, 1, 2, ..., N, -M, ..., -2, -1. Every entry in an options array with this field 0 will inherit the group number of the previous entry, or zero if it's the first one. If it's a group header with `name' and `key' fields both zero, the previous entry + 1 is the default. Automagic options such as `--help' are put into group -1. Note that because of C structure initialization rules, this field often need not be specified, because 0 is the correct value. Flags for Argp Options ...................... The following flags may be or'd together in the `flags' field of a `struct argp_option'. These flags control various aspects of how that option is parsed or displayed in help messages: `OPTION_ARG_OPTIONAL' The argument associated with this option is optional. `OPTION_HIDDEN' This option isn't displayed in any help messages. `OPTION_ALIAS' This option is an alias for the closest previous non-alias option. This means that it will be displayed in the same help entry, and will inherit fields other than `name' and `key' from the option being aliased. `OPTION_DOC' This option isn't actually an option and should be ignored by the actual option parser. It is an arbitrary section of documentation that should be displayed in much the same manner as the options. This is known as a "documentation option". If this flag is set, then the option `name' field is displayed unmodified (e.g., no `--' prefix is added) at the left-margin where a _short_ option would normally be displayed, and this documentation string is left in it's usual place. For purposes of sorting, any leading whitespace and punctuation is ignored, unless the first non-whitespace character is `-'. This entry is displayed after all options, after `OPTION_DOC' entries with a leading `-', in the same group. `OPTION_NO_USAGE' This option shouldn't be included in `long' usage messages, but should still be included in other help messages. This is intended for options that are completely documented in an argp's `args_doc' field. *Note Argp Parsers::. Including this option in the generic usage list would be redundant, and should be avoided. For instance, if `args_doc' is `"FOO BAR\n-x BLAH"', and the `-x' option's purpose is to distinguish these two cases, `-x' should probably be marked `OPTION_NO_USAGE'. Argp Parser Functions --------------------- The function pointed to by the `parser' field in a `struct argp' (*note Argp Parsers::) defines what actions take place in response to each option or argument parsed. It is also used as a hook, allowing a parser to perform tasks at certain other points during parsing. Argp parser functions have the following type signature: error_t PARSER (int KEY, char *ARG, struct argp_state *STATE) where the arguments are as follows: KEY For each option that is parsed, PARSER is called with a value of KEY from that option's `key' field in the option vector. *Note Argp Option Vectors::. PARSER is also called at other times with special reserved keys, such as `ARGP_KEY_ARG' for non-option arguments. *Note Argp Special Keys::. ARG If KEY is an option, ARG is its given value. This defaults to zero if no value is specified. Only options that have a non-zero `arg' field can ever have a value. These must _always_ have a value unless the `OPTION_ARG_OPTIONAL' flag is specified. If the input being parsed specifies a value for an option that doesn't allow one, an error results before PARSER ever gets called. If KEY is `ARGP_KEY_ARG', ARG is a non-option argument. Other special keys always have a zero ARG. STATE STATE points to a `struct argp_state', containing useful information about the current parsing state for use by PARSER. *Note Argp Parsing State::. When PARSER is called, it should perform whatever action is appropriate for KEY, and return `0' for success, `ARGP_ERR_UNKNOWN' if the value of KEY is not handled by this parser function, or a unix error code if a real error occurred. *Note Error Codes::. - Macro: int ARGP_ERR_UNKNOWN Argp parser functions should return `ARGP_ERR_UNKNOWN' for any KEY value they do not recognize, or for non-option arguments (`KEY == ARGP_KEY_ARG') that they are not equipped to handle. A typical parser function uses a switch statement on KEY: error_t parse_opt (int key, char *arg, struct argp_state *state) { switch (key) { case OPTION_KEY: ACTION break; ... default: return ARGP_ERR_UNKNOWN; } return 0; } Special Keys for Argp Parser Functions ...................................... In addition to key values corresponding to user options, the KEY argument to argp parser functions may have a number of other special values. In the following example ARG and STATE refer to parser function arguments. *Note Argp Parser Functions::. `ARGP_KEY_ARG' This is not an option at all, but rather a command line argument, whose value is pointed to by ARG. When there are multiple parser functions in play due to argp parsers being combined, it's impossible to know which one will handle a specific argument. Each is called until one returns 0 or an error other than `ARGP_ERR_UNKNOWN'; if an argument is not handled, `argp_parse' immediately returns success, without parsing any more arguments. Once a parser function returns success for this key, that fact is recorded, and the `ARGP_KEY_NO_ARGS' case won't be used. _However_, if while processing the argument a parser function decrements the `next' field of its STATE argument, the option won't be considered processed; this is to allow you to actually modify the argument, perhaps into an option, and have it processed again. `ARGP_KEY_ARGS' If a parser function returns `ARGP_ERR_UNKNOWN' for `ARGP_KEY_ARG', it is immediately called again with the key `ARGP_KEY_ARGS', which has a similar meaning, but is slightly more convenient for consuming all remaining arguments. ARG is 0, and the tail of the argument vector may be found at `STATE->argv + STATE->next'. If success is returned for this key, and `STATE->next' is unchanged, all remaining arguments are considered to have been consumed. Otherwise, the amount by which `STATE->next' has been adjusted indicates how many were used. Here's an example that uses both, for different args: ... case ARGP_KEY_ARG: if (STATE->arg_num == 0) /* First argument */ first_arg = ARG; else /* Let the next case parse it. */ return ARGP_KEY_UNKNOWN; break; case ARGP_KEY_ARGS: remaining_args = STATE->argv + STATE->next; num_remaining_args = STATE->argc - STATE->next; break; `ARGP_KEY_END' This indicates that there are no more command line arguments. Parser functions are called in a different order, children first. This allows each parser to clean up its state for the parent. `ARGP_KEY_NO_ARGS' Because it's common to do some special processing if there aren't any non-option args, parser functions are called with this key if they didn't successfully process any non-option arguments. This is called just before `ARGP_KEY_END', where more general validity checks on previously parsed arguments take place. `ARGP_KEY_INIT' This is passed in before any parsing is done. Afterwards, the values of each element of the `child_input' field of STATE, if any, are copied to each child's state to be the initial value of the `input' when _their_ parsers are called. `ARGP_KEY_SUCCESS' Passed in when parsing has successfully been completed, even if arguments remain. `ARGP_KEY_ERROR' Passed in if an error has occurred and parsing is terminated. In this case a call with a key of `ARGP_KEY_SUCCESS' is never made. `ARGP_KEY_FINI' The final key ever seen by any parser, even after `ARGP_KEY_SUCCESS' and `ARGP_KEY_ERROR'. Any resources allocated by `ARGP_KEY_INIT' may be freed here. At times, certain resources allocated are to be returned to the caller after a successful parse. In that case, those particular resources can be freed in the `ARGP_KEY_ERROR' case. In all cases, `ARGP_KEY_INIT' is the first key seen by parser functions, and `ARGP_KEY_FINI' the last, unless an error was returned by the parser for `ARGP_KEY_INIT'. Other keys can occur in one the following orders. OPT refers to an arbitrary option key: OPT... `ARGP_KEY_NO_ARGS' `ARGP_KEY_END' `ARGP_KEY_SUCCESS' The arguments being parsed did not contain any non-option arguments. ( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_END' `ARGP_KEY_SUCCESS' All non-option arguments were successfully handled by a parser function. There may be multiple parser functions if multiple argp parsers were combined. ( OPT | `ARGP_KEY_ARG' )... `ARGP_KEY_SUCCESS' Some non-option argument went unrecognized. This occurs when every parser function returns `ARGP_KEY_UNKNOWN' for an argument, in which case parsing stops at that argument if ARG_INDEX is a null pointer. Otherwise an error occurs. In all cases, if a non-null value for ARG_INDEX gets passed to `argp_parse', the index of the first unparsed command-line argument is passed back in that value. If an error occurs and is either detected by argp or because a parser function returned an error value, each parser is called with `ARGP_KEY_ERROR'. No further calls are made, except the final call with `ARGP_KEY_FINI'. Functions For Use in Argp Parsers ................................. Argp provides a number of functions available to the user of argp (*note Argp Parser Functions::), mostly for producing error messages. These take as their first argument the STATE argument to the parser function. *Note Argp Parsing State::. - Function: void argp_usage (const struct argp_state *STATE) Outputs the standard usage message for the argp parser referred to by STATE to `STATE->err_stream' and terminate the program with `exit (argp_err_exit_status)'. *Note Argp Global Variables::. - Function: void argp_error (const struct argp_state *STATE, const char *FMT, ...) Prints the printf format string FMT and following args, preceded by the program name and `:', and followed by a `Try ... --help' message, and terminates the program with an exit status of `argp_err_exit_status'. *Note Argp Global Variables::. - Function: void argp_failure (const struct argp_state *STATE, int STATUS, int ERRNUM, const char *FMT, ...) Similar to the standard gnu error-reporting function `error', this prints the program name and `:', the printf format string FMT, and the appropriate following args. If it is non-zero, the standard unix error text for ERRNUM is printed. If STATUS is non-zero, it terminates the program with that value as its exit status. The difference between `argp_failure' and `argp_error' is that `argp_error' is for _parsing errors_, whereas `argp_failure' is for other problems that occur during parsing but don't reflect a syntactic problem with the input, such as illegal values for options, bad phase of the moon, etc. - Function: void argp_state_help (const struct argp_state *STATE, FILE *STREAM, unsigned FLAGS) Outputs a help message for the argp parser referred to by STATE, to STREAM. The FLAGS argument determines what sort of help message is produced. *Note Argp Help Flags::. Error output is sent to `STATE->err_stream', and the program name printed is `STATE->name'. The output or program termination behavior of these functions may be suppressed if the `ARGP_NO_EXIT' or `ARGP_NO_ERRS' flags are passed to `argp_parse'. *Note Argp Flags::. This behavior is useful if an argp parser is exported for use by other programs (e.g., by a library), and may be used in a context where it is not desirable to terminate the program in response to parsing errors. In argp parsers intended for such general use, and for the case where the program _doesn't_ terminate, calls to any of these functions should be followed by code that returns the appropriate error code: if (BAD ARGUMENT SYNTAX) { argp_usage (STATE); return EINVAL; } If a parser function will _only_ be used when `ARGP_NO_EXIT' is not set, the return may be omitted. Argp Parsing State .................. The third argument to argp parser functions (*note Argp Parser Functions::) is a pointer to a `struct argp_state', which contains information about the state of the option parsing. - Data Type: struct argp_state This structure has the following fields, which may be modified as noted: `const struct argp *const root_argp' The top level argp parser being parsed. Note that this is often _not_ the same `struct argp' passed into `argp_parse' by the invoking program. *Note Argp::. It is an internal argp parser that contains options implemented by `argp_parse' itself, such as `--help'. `int argc' `char **argv' The argument vector being parsed. This may be modified. `int next' The index in `argv' of the next argument to be parsed. This may be modified. One way to consume all remaining arguments in the input is to set `STATE->next = STATE->argc', perhaps after recording the value of the `next' field to find the consumed arguments. The current option can be re-parsed immediately by decrementing this field, then modifying `STATE->argv[STATE->next]' to reflect the option that should be reexamined. `unsigned flags' The flags supplied to `argp_parse'. These may be modified, although some flags may only take effect when `argp_parse' is first invoked. *Note Argp Flags::. `unsigned arg_num' While calling a parsing function with the KEY argument `ARGP_KEY_ARG', this represents the number of the current arg, starting at 0. It is incremented after each `ARGP_KEY_ARG' call returns. At all other times, this is the number of `ARGP_KEY_ARG' arguments that have been processed. `int quoted' If non-zero, the index in `argv' of the first argument following a special `--' argument. This prevents anything that follows from being interpreted as an option. It is only set after argument parsing has proceeded past this point. `void *input' An arbitrary pointer passed in from the caller of `argp_parse', in the INPUT argument. `void **child_inputs' These are values that will be passed to child parsers. This vector will be the same length as the number of children in the current parser. Each child parser will be given the value of `STATE->child_inputs[I]' as _its_ `STATE->input' field, where I is the index of the child in the this parser's `children' field. *Note Argp Children::. `void *hook' For the parser function's use. Initialized to 0, but otherwise ignored by argp. `char *name' The name used when printing messages. This is initialized to `argv[0]', or `program_invocation_name' if `argv[0]' is unavailable. `FILE *err_stream' `FILE *out_stream' The stdio streams used when argp prints. Error messages are printed to `err_stream', all other output, such as `--help' output) to `out_stream'. These are initialized to `stderr' and `stdout' respectively. *Note Standard Streams::. `void *pstate' Private, for use by the argp implementation. Combining Multiple Argp Parsers ------------------------------- The `children' field in a `struct argp' enables other argp parsers to be combined with the referencing one for the parsing of a single set of arguments. This field should point to a vector of `struct argp_child', which is terminated by an entry having a value of zero in the `argp' field. Where conflicts between combined parsers arise, as when two specify an option with the same name, the parser conflicts are resolved in favor of the parent argp parser(s), or the earlier of the argp parsers in the list of children. - Data Type: struct argp_child An entry in the list of subsidiary argp parsers pointed to by the `children' field in a `struct argp'. The fields are as follows: `const struct argp *argp' The child argp parser, or zero to end of the list. `int flags' Flags for this child. `const char *header' If non-zero, this is an optional header to be printed within help output before the child options. As a side-effect, a non-zero value forces the child options to be grouped together. To achieve this effect without actually printing a header string, use a value of `""'. As with header strings specified in an option entry, the conventional value of the last character is `:'. *Note Argp Option Vectors::. `int group' This is where the child options are grouped relative to the other `consolidated' options in the parent argp parser. The values are the same as the `group' field in `struct argp_option'. *Note Argp Option Vectors::. All child-groupings follow parent options at a particular group level. If both this field and `header' are zero, then the child's options aren't grouped together, they are merged with parent options at the parent option group level. Flags for `argp_parse' ---------------------- The default behavior of `argp_parse' is designed to be convenient for the most common case of parsing program command line argument. To modify these defaults, the following flags may be or'd together in the FLAGS argument to `argp_parse': `ARGP_PARSE_ARGV0' Don't ignore the first element of the ARGV argument to `argp_parse'. Unless `ARGP_NO_ERRS' is set, the first element of the argument vector is skipped for option parsing purposes, as it corresponds to the program name in a command line. `ARGP_NO_ERRS' Don't print error messages for unknown options to `stderr'; unless this flag is set, `ARGP_PARSE_ARGV0' is ignored, as `argv[0]' is used as the program name in the error messages. This flag implies `ARGP_NO_EXIT'. This is based on the assumption that silent exiting upon errors is bad behavior. `ARGP_NO_ARGS' Don't parse any non-option args. Normally these are parsed by calling the parse functions with a key of `ARGP_KEY_ARG', the actual argument being the value. This flag needn't normally be set, as the default behavior is to stop parsing as soon as an argument fails to be parsed. *Note Argp Parser Functions::. `ARGP_IN_ORDER' Parse options and arguments in the same order they occur on the command line. Normally they're rearranged so that all options come first. `ARGP_NO_HELP' Don't provide the standard long option `--help', which ordinarily causes usage and option help information to be output to `stdout' and `exit (0)'. `ARGP_NO_EXIT' Don't exit on errors, although they may still result in error messages. `ARGP_LONG_ONLY' Use the gnu getopt `long-only' rules for parsing arguments. This allows long-options to be recognized with only a single `-' (i.e. `-help'). This results in a less useful interface, and its use is discouraged as it conflicts with the way most GNU programs work as well as the GNU coding standards. `ARGP_SILENT' Turns off any message-printing/exiting options, specifically `ARGP_NO_EXIT', `ARGP_NO_ERRS', and `ARGP_NO_HELP'. Customizing Argp Help Output ---------------------------- The `help_filter' field in a `struct argp' is a pointer to a function that filters the text of help messages before displaying them. They have a function signature like: char *HELP-FILTER (int KEY, const char *TEXT, void *INPUT) Where KEY is either a key from an option, in which case TEXT is that option's help text. *Note Argp Option Vectors::. Alternately, one of the special keys with names beginning with `ARGP_KEY_HELP_' might be used, describing which other help text TEXT will contain. *Note Argp Help Filter Keys::. The function should return either TEXT if it remains as-is, or a replacement string allocated using `malloc'. This will be either be freed by argp or zero, which prints nothing. The value of TEXT is supplied _after_ any translation has been done, so if any of the replacement text needs translation, it will be done by the filter function. INPUT is either the input supplied to `argp_parse' or it is zero, if `argp_help' was called directly by the user. Special Keys for Argp Help Filter Functions ........................................... The following special values may be passed to an argp help filter function as the first argument in addition to key values for user options. They specify which help text the TEXT argument contains: `ARGP_KEY_HELP_PRE_DOC' The help text preceding options. `ARGP_KEY_HELP_POST_DOC' The help text following options. `ARGP_KEY_HELP_HEADER' The option header string. `ARGP_KEY_HELP_EXTRA' This is used after all other documentation; TEXT is zero for this key. `ARGP_KEY_HELP_DUP_ARGS_NOTE' The explanatory note printed when duplicate option arguments have been suppressed. `ARGP_KEY_HELP_ARGS_DOC' The argument doc string; formally the `args_doc' field from the argp parser. *Note Argp Parsers::. The `argp_help' Function ------------------------ Normally programs using argp need not be written with particular printing argument-usage-type help messages in mind as the standard `--help' option is handled automatically by argp. Typical error cases can be handled using `argp_usage' and `argp_error'. *Note Argp Helper Functions::. However, if it's desirable to print a help message in some context other than parsing the program options, argp offers the `argp_help' interface. - Function: void argp_help (const struct argp *ARGP, FILE *STREAM, unsigned FLAGS, char *NAME) This outputs a help message for the argp parser ARGP to STREAM. The type of messages printed will be determined by FLAGS. Any options such as `--help' that are implemented automatically by argp itself will _not_ be present in the help output; for this reason it is best to use `argp_state_help' if calling from within an argp parser function. *Note Argp Helper Functions::. Flags for the `argp_help' Function ---------------------------------- When calling `argp_help' (*note Argp Help::) or `argp_state_help' (*note Argp Helper Functions::) the exact output is determined by the FLAGS argument. This should consist of any of the following flags, or'd together: `ARGP_HELP_USAGE' A unix `Usage:' message that explicitly lists all options. `ARGP_HELP_SHORT_USAGE' A unix `Usage:' message that displays an appropriate placeholder to indicate where the options go; useful for showing the non-option argument syntax. `ARGP_HELP_SEE' A `Try ... for more help' message; `...' contains the program name and `--help'. `ARGP_HELP_LONG' A verbose option help message that gives each option available along with its documentation string. `ARGP_HELP_PRE_DOC' The part of the argp parser doc string preceding the verbose option help. `ARGP_HELP_POST_DOC' The part of the argp parser doc string that following the verbose option help. `ARGP_HELP_DOC' `(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)' `ARGP_HELP_BUG_ADDR' A message that prints where to report bugs for this program, if the `argp_program_bug_address' variable contains this information. `ARGP_HELP_LONG_ONLY' This will modify any output to reflect the `ARGP_LONG_ONLY' mode. The following flags are only understood when used with `argp_state_help'. They control whether the function returns after printing its output, or terminates the program: `ARGP_HELP_EXIT_ERR' This will terminate the program with `exit (argp_err_exit_status)'. `ARGP_HELP_EXIT_OK' This will terminate the program with `exit (0)'. The following flags are combinations of the basic flags for printing standard messages: `ARGP_HELP_STD_ERR' Assuming that an error message for a parsing error has printed, this prints a message on how to get help, and terminates the program with an error. `ARGP_HELP_STD_USAGE' This prints a standard usage message and terminates the program with an error. This is used when no other specific error messages are appropriate or available. `ARGP_HELP_STD_HELP' This prints the standard response for a `--help' option, and terminates the program successfully. Argp Examples ------------- These example programs demonstrate the basic usage of argp. A Minimal Program Using Argp ............................ This is perhaps the smallest program possible that uses argp. It won't do much except give an error messages and exit when there are any arguments, and prints a rather pointless message for `--help'. /* Argp example #1 - a minimal program using argp */ /* This is (probably) the smallest possible program that uses argp. It won't do much except give an error messages and exit when there are any arguments, and print a (rather pointless) messages for -help. */ #include int main (int argc, char **argv) { argp_parse (0, argc, argv, 0, 0, 0); exit (0); } A Program Using Argp with Only Default Options .............................................. This program doesn't use any options or arguments, it uses argp to be compliant with the GNU standard command line format. In addition to giving no arguments and implementing a `--help' option, this example has a `--version' option, which will put the given documentation string and bug address in the `--help' output, as per GNU standards. The variable `argp' contains the argument parser specification. Adding fields to this structure is the way most parameters are passed to `argp_parse'. The first three fields are normally used, but they are not in this small program. There are also two global variables that argp can use defined here, `argp_program_version' and `argp_program_bug_address'. They are considered global variables because they will almost always be constant for a given program, even if they use different argument parsers for various tasks. /* Argp example #2 - a pretty minimal program using argp */ /* This program doesn't use any options or arguments, but uses argp to be compliant with the GNU standard command line format. In addition to making sure no arguments are given, and implementing a -help option, this example will have a -version option, and will put the given documentation string and bug address in the -help output, as per GNU standards. The variable ARGP contains the argument parser specification; adding fields to this structure is the way most parameters are passed to argp_parse (the first three fields are usually used, but not in this small program). There are also two global variables that argp knows about defined here, ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are global variables because they will almost always be constant for a given program, even if it uses different argument parsers for various tasks). */ #include const char *argp_program_version = "argp-ex2 1.0"; const char *argp_program_bug_address = ""; /* Program documentation. */ static char doc[] = "Argp example #2 -- a pretty minimal program using argp"; /* Our argument parser. The `options', `parser', and `args_doc' fields are zero because we have neither options or arguments; `doc' and `argp_program_bug_address' will be used in the output for `--help', and the `--version' option will print out `argp_program_version'. */ static struct argp argp = { 0, 0, 0, doc }; int main (int argc, char **argv) { argp_parse (&argp, argc, argv, 0, 0, 0); exit (0); } A Program Using Argp with User Options ...................................... This program uses the same features as example 2, adding user options and arguments. We now use the first four fields in `argp' (*note Argp Parsers::) and specify `parse_opt' as the parser function. *Note Argp Parser Functions::. Note that in this example, `main' uses a structure to communicate with the `parse_opt' function, a pointer to which it passes in the `input' argument to `argp_parse'. *Note Argp::. It is retrieved by `parse_opt' through the `input' field in its `state' argument. *Note Argp Parsing State::. Of course, it's also possible to use global variables instead, but using a structure like this is somewhat more flexible and clean. /* Argp example #3 - a program with options and arguments using argp */ /* This program uses the same features as example 2, and uses options and arguments. We now use the first four fields in ARGP, so here's a description of them: OPTIONS - A pointer to a vector of struct argp_option (see below) PARSER - A function to parse a single option, called by argp ARGS_DOC - A string describing how the non-option arguments should look DOC - A descriptive string about this program; if it contains a vertical tab character (\v), the part after it will be printed *following* the options The function PARSER takes the following arguments: KEY - An integer specifying which option this is (taken from the KEY field in each struct argp_option), or a special key specifying something else; the only special keys we use here are ARGP_KEY_ARG, meaning a non-option argument, and ARGP_KEY_END, meaning that all arguments have been parsed ARG - For an option KEY, the string value of its argument, or NULL if it has none STATE- A pointer to a struct argp_state, containing various useful information about the parsing state; used here are the INPUT field, which reflects the INPUT argument to argp_parse, and the ARG_NUM field, which is the number of the current non-option argument being parsed It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the given KEY wasn't recognized, or an errno value indicating some other error. Note that in this example, main uses a structure to communicate with the parse_opt function, a pointer to which it passes in the INPUT argument to argp_parse. Of course, it's also possible to use global variables instead, but this is somewhat more flexible. The OPTIONS field contains a pointer to a vector of struct argp_option's; that structure has the following fields (if you assign your option structures using array initialization like this example, unspecified fields will be defaulted to 0, and need not be specified): NAME - The name of this option's long option (may be zero) KEY - The KEY to pass to the PARSER function when parsing this option, *and* the name of this option's short option, if it is a printable ascii character ARG - The name of this option's argument, if any FLAGS - Flags describing this option; some of them are: OPTION_ARG_OPTIONAL - The argument to this option is optional OPTION_ALIAS - This option is an alias for the previous option OPTION_HIDDEN - Don't show this option in -help output DOC - A documentation string for this option, shown in -help output An options vector should be terminated by an option with all fields zero. */ #include const char *argp_program_version = "argp-ex3 1.0"; const char *argp_program_bug_address = ""; /* Program documentation. */ static char doc[] = "Argp example #3 -- a program with options and arguments using argp"; /* A description of the arguments we accept. */ static char args_doc[] = "ARG1 ARG2"; /* The options we understand. */ static struct argp_option options[] = { {"verbose", 'v', 0, 0, "Produce verbose output" }, {"quiet", 'q', 0, 0, "Don't produce any output" }, {"silent", 's', 0, OPTION_ALIAS }, {"output", 'o', "FILE", 0, "Output to FILE instead of standard output" }, { 0 } }; /* Used by `main' to communicate with `parse_opt'. */ struct arguments { char *args[2]; /* ARG1 & ARG2 */ int silent, verbose; char *output_file; }; /* Parse a single option. */ static error_t parse_opt (int key, char *arg, struct argp_state *state) { /* Get the INPUT argument from `argp_parse', which we know is a pointer to our arguments structure. */ struct arguments *arguments = state->input; switch (key) { case 'q': case 's': arguments->silent = 1; break; case 'v': arguments->verbose = 1; break; case 'o': arguments->output_file = arg; break; case ARGP_KEY_ARG: if (state->arg_num >= 2) /* Too many arguments. */ argp_usage (state); arguments->args[state->arg_num] = arg; break; case ARGP_KEY_END: if (state->arg_num < 2) /* Not enough arguments. */ argp_usage (state); break; default: return ARGP_ERR_UNKNOWN; } return 0; } /* Our argp parser. */ static struct argp argp = { options, parse_opt, args_doc, doc }; int main (int argc, char **argv) { struct arguments arguments; /* Default values. */ arguments.silent = 0; arguments.verbose = 0; arguments.output_file = "-"; /* Parse our arguments; every option seen by `parse_opt' will be reflected in `arguments'. */ argp_parse (&argp, argc, argv, 0, 0, &arguments); printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n" "VERBOSE = %s\nSILENT = %s\n", arguments.args[0], arguments.args[1], arguments.output_file, arguments.verbose ? "yes" : "no", arguments.silent ? "yes" : "no"); exit (0); } A Program Using Multiple Combined Argp Parsers .............................................. This program uses the same features as example 3, but has more options, and presents more structure in the `--help' output. It also illustrates how you can `steal' the remainder of the input arguments past a certain point for programs that accept a list of items. It also illustrates the KEY value `ARGP_KEY_NO_ARGS', which is only given if no non-option arguments were supplied to the program. *Note Argp Special Keys::. For structuring help output, two features are used: _headers_ and a two part option string. The _headers_ are entries in the options vector. *Note Argp Option Vectors::. The first four fields are zero. The two part documentation string are in the variable `doc', which allows documentation both before and after the options. *Note Argp Parsers::, the two parts of `doc' are separated by a vertical-tab character (`'\v'', or `'\013''). By convention, the documentation before the options is a short string stating what the program does, and after any options it is longer, describing the behavior in more detail. All documentation strings are automatically filled for output, although newlines may be included to force a line break at a particular point. In addition, documentation strings are passed to the `gettext' function, for possible translation into the current locale. /* Argp example #4 - a program with somewhat more complicated options */ /* This program uses the same features as example 3, but has more options, and somewhat more structure in the -help output. It also shows how you can `steal' the remainder of the input arguments past a certain point, for programs that accept a list of items. It also shows the special argp KEY value ARGP_KEY_NO_ARGS, which is only given if no non-option arguments were supplied to the program. For structuring the help output, two features are used, *headers* which are entries in the options vector with the first four fields being zero, and a two part documentation string (in the variable DOC), which allows documentation both before and after the options; the two parts of DOC are separated by a vertical-tab character ('\v', or '\013'). By convention, the documentation before the options is just a short string saying what the program does, and that afterwards is longer, describing the behavior in more detail. All documentation strings are automatically filled for output, although newlines may be included to force a line break at a particular point. All documentation strings are also passed to the `gettext' function, for possible translation into the current locale. */ #include #include #include const char *argp_program_version = "argp-ex4 1.0"; const char *argp_program_bug_address = ""; /* Program documentation. */ static char doc[] = "Argp example #4 -- a program with somewhat more complicated\ options\ \vThis part of the documentation comes *after* the options;\ note that the text is automatically filled, but it's possible\ to force a line-break, e.g.\n<-- here."; /* A description of the arguments we accept. */ static char args_doc[] = "ARG1 [STRING...]"; /* Keys for options without short-options. */ #define OPT_ABORT 1 /* -abort */ /* The options we understand. */ static struct argp_option options[] = { {"verbose", 'v', 0, 0, "Produce verbose output" }, {"quiet", 'q', 0, 0, "Don't produce any output" }, {"silent", 's', 0, OPTION_ALIAS }, {"output", 'o', "FILE", 0, "Output to FILE instead of standard output" }, {0,0,0,0, "The following options should be grouped together:" }, {"repeat", 'r', "COUNT", OPTION_ARG_OPTIONAL, "Repeat the output COUNT (default 10) times"}, {"abort", OPT_ABORT, 0, 0, "Abort before showing any output"}, { 0 } }; /* Used by `main' to communicate with `parse_opt'. */ struct arguments { char *arg1; /* ARG1 */ char **strings; /* [STRING...] */ int silent, verbose, abort; /* `-s', `-v', `--abort' */ char *output_file; /* FILE arg to `--output' */ int repeat_count; /* COUNT arg to `--repeat' */ }; /* Parse a single option. */ static error_t parse_opt (int key, char *arg, struct argp_state *state) { /* Get the `input' argument from `argp_parse', which we know is a pointer to our arguments structure. */ struct arguments *arguments = state->input; switch (key) { case 'q': case 's': arguments->silent = 1; break; case 'v': arguments->verbose = 1; break; case 'o': arguments->output_file = arg; break; case 'r': arguments->repeat_count = arg ? atoi (arg) : 10; break; case OPT_ABORT: arguments->abort = 1; break; case ARGP_KEY_NO_ARGS: argp_usage (state); case ARGP_KEY_ARG: /* Here we know that `state->arg_num == 0', since we force argument parsing to end before any more arguments can get here. */ arguments->arg1 = arg; /* Now we consume all the rest of the arguments. `state->next' is the index in `state->argv' of the next argument to be parsed, which is the first STRING we're interested in, so we can just use `&state->argv[state->next]' as the value for arguments->strings. _In addition_, by setting `state->next' to the end of the arguments, we can force argp to stop parsing here and return. */ arguments->strings = &state->argv[state->next]; state->next = state->argc; break; default: return ARGP_ERR_UNKNOWN; } return 0; } /* Our argp parser. */ static struct argp argp = { options, parse_opt, args_doc, doc }; int main (int argc, char **argv) { int i, j; struct arguments arguments; /* Default values. */ arguments.silent = 0; arguments.verbose = 0; arguments.output_file = "-"; arguments.repeat_count = 1; arguments.abort = 0; /* Parse our arguments; every option seen by `parse_opt' will be reflected in `arguments'. */ argp_parse (&argp, argc, argv, 0, 0, &arguments); if (arguments.abort) error (10, 0, "ABORTED"); for (i = 0; i < arguments.repeat_count; i++) { printf ("ARG1 = %s\n", arguments.arg1); printf ("STRINGS = "); for (j = 0; arguments.strings[j]; j++) printf (j == 0 ? "%s" : ", %s", arguments.strings[j]); printf ("\n"); printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n", arguments.output_file, arguments.verbose ? "yes" : "no", arguments.silent ? "yes" : "no"); } exit (0); } Argp User Customization ----------------------- The formatting of argp `--help' output may be controlled to some extent by a program's users, by setting the `ARGP_HELP_FMT' environment variable to a comma-separated list of tokens. Whitespace is ignored: `dup-args' `no-dup-args' These turn "duplicate-argument-mode" on or off. In duplicate argument mode, if an option that accepts an argument has multiple names, the argument is shown for each name. Otherwise, it is only shown for the first long option. A note is subsequently printed so the user knows that it applies to other names as well. The default is `no-dup-args', which is less consistent, but prettier. `dup-args-note' `no-dup-args-note' These will enable or disable the note informing the user of suppressed option argument duplication. The default is `dup-args-note'. `short-opt-col=N' This prints the first short option in column N. The default is 2. `long-opt-col=N' This prints the first long option in column N. The default is 6. `doc-opt-col=N' This prints `documentation options' (*note Argp Option Flags::) in column N. The default is 2. `opt-doc-col=N' This prints the documentation for options starting in column N. The default is 29. `header-col=N' This will indent the group headers that document groups of options to column N. The default is 1. `usage-indent=N' This will indent continuation lines in `Usage:' messages to column N. The default is 12. `rmargin=N' This will word wrap help output at or before column N. The default is 79. Parsing of Suboptions ..................... Having a single level of options is sometimes not enough. There might be too many options which have to be available or a set of options is closely related. For this case some programs use suboptions. One of the most prominent programs is certainly `mount'(8). The `-o' option take one argument which itself is a comma separated list of options. To ease the programming of code like this the function `getsubopt' is available. - Function: int getsubopt (char **OPTIONP, const char* const *TOKENS, char **VALUEP) The OPTIONP parameter must be a pointer to a variable containing the address of the string to process. When the function returns the reference is updated to point to the next suboption or to the terminating `\0' character if there is no more suboption available. The TOKENS parameter references an array of strings containing the known suboptions. All strings must be `\0' terminated and to mark the end a null pointer must be stored. When `getsubopt' finds a possible legal suboption it compares it with all strings available in the TOKENS array and returns the index in the string as the indicator. In case the suboption has an associated value introduced by a `=' character, a pointer to the value is returned in VALUEP. The string is `\0' terminated. If no argument is available VALUEP is set to the null pointer. By doing this the caller can check whether a necessary value is given or whether no unexpected value is present. In case the next suboption in the string is not mentioned in the TOKENS array the starting address of the suboption including a possible value is returned in VALUEP and the return value of the function is `-1'. Parsing of Suboptions Example ----------------------------- The code which might appear in the `mount'(8) program is a perfect example of the use of `getsubopt': #include #include #include int do_all; const char *type; int read_size; int write_size; int read_only; enum { RO_OPTION = 0, RW_OPTION, READ_SIZE_OPTION, WRITE_SIZE_OPTION, THE_END }; const char *mount_opts[] = { [RO_OPTION] = "ro", [RW_OPTION] = "rw", [READ_SIZE_OPTION] = "rsize", [WRITE_SIZE_OPTION] = "wsize", [THE_END] = NULL }; int main (int argc, char *argv[]) { char *subopts, *value; int opt; while ((opt = getopt (argc, argv, "at:o:")) != -1) switch (opt) { case 'a': do_all = 1; break; case 't': type = optarg; break; case 'o': subopts = optarg; while (*subopts != '\0') switch (getsubopt (&subopts, mount_opts, &value)) { case RO_OPTION: read_only = 1; break; case RW_OPTION: read_only = 0; break; case READ_SIZE_OPTION: if (value == NULL) abort (); read_size = atoi (value); break; case WRITE_SIZE_OPTION: if (value == NULL) abort (); write_size = atoi (value); break; default: /* Unknown suboption. */ printf ("Unknown suboption `%s'\n", value); break; } break; default: abort (); } /* Do the real work. */ return 0; } Environment Variables ===================== When a program is executed, it receives information about the context in which it was invoked in two ways. The first mechanism uses the ARGV and ARGC arguments to its `main' function, and is discussed in *Note Program Arguments::. The second mechanism uses "environment variables" and is discussed in this section. The ARGV mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently used. The environment variables discussed in this section are the same environment variables that you set using assignments and the `export' command in the shell. Programs executed from the shell inherit all of the environment variables from the shell. Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes. The set of all environment variables that have values is collectively known as the "environment". Names of environment variables are case-sensitive and must not contain the character `='. System-defined environment variables are invariably uppercase. The values of environment variables can be anything that can be represented as a string. A value must not contain an embedded null character, since this is assumed to terminate the string. Environment Access ------------------ The value of an environment variable can be accessed with the `getenv' function. This is declared in the header file `stdlib.h'. All of the following functions can be safely used in multi-threaded programs. It is made sure that concurrent modifications to the environment do not lead to errors. - Function: char * getenv (const char *NAME) This function returns a string that is the value of the environment variable NAME. You must not modify this string. In some non-Unix systems not using the GNU library, it might be overwritten by subsequent calls to `getenv' (but not by any other library function). If the environment variable NAME is not defined, the value is a null pointer. - Function: int putenv (char *STRING) The `putenv' function adds or removes definitions from the environment. If the STRING is of the form `NAME=VALUE', the definition is added to the environment. Otherwise, the STRING is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed. The difference to the `setenv' function is that the exact string given as the parameter STRING is put into the environment. If the user should change the string after the `putenv' call this will reflect in automatically in the environment. This also requires that STRING is no automatic variable which scope is left before the variable is removed from the environment. The same applies of course to dynamically allocated variables which are freed later. This function is part of the extended Unix interface. Since it was also available in old SVID libraries you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header. - Function: int setenv (const char *NAME, const char *VALUE, int REPLACE) The `setenv' function can be used to add a new definition to the environment. The entry with the name NAME is replaced by the value `NAME=VALUE'. Please note that this is also true if VALUE is the empty string. To do this a new string is created and the strings NAME and VALUE are copied. A null pointer for the VALUE parameter is illegal. If the environment already contains an entry with key NAME the REPLACE parameter controls the action. If replace is zero, nothing happens. Otherwise the old entry is replaced by the new one. Please note that you cannot remove an entry completely using this function. This function was originally part of the BSD library but is now part of the Unix standard. - Function: int unsetenv (const char *NAME) Using this function one can remove an entry completely from the environment. If the environment contains an entry with the key NAME this whole entry is removed. A call to this function is equivalent to a call to `putenv' when the VALUE part of the string is empty. The function return `-1' if NAME is a null pointer, points to an empty string, or points to a string containing a `=' character. It returns `0' if the call succeeded. This function was originally part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though. There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments. - Function: int clearenv (void) The `clearenv' function removes all entries from the environment. Using `putenv' and `setenv' new entries can be added again later. If the function is successful it returns `0'. Otherwise the return value is nonzero. You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; *note Executing a File::). - Variable: char ** environ The environment is represented as an array of strings. Each string is of the format `NAME=VALUE'. The order in which strings appear in the environment is not significant, but the same NAME must not appear more than once. The last element of the array is a null pointer. This variable is declared in the header file `unistd.h'. If you just want to get the value of an environment variable, use `getenv'. Unix systems, and the GNU system, pass the initial value of `environ' as the third argument to `main'. *Note Program Arguments::. Standard Environment Variables ------------------------------ These environment variables have standard meanings. This doesn't mean that they are always present in the environment; but if these variables _are_ present, they have these meanings. You shouldn't try to use these environment variable names for some other purpose. `HOME' This is a string representing the user's "home directory", or initial default working directory. The user can set `HOME' to any value. If you need to make sure to obtain the proper home directory for a particular user, you should not use `HOME'; instead, look up the user's name in the user database (*note User Database::). For most purposes, it is better to use `HOME', precisely because this lets the user specify the value. `LOGNAME' This is the name that the user used to log in. Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a program; a function like `getlogin' (*note Who Logged In::) is better for that purpose. For most purposes, it is better to use `LOGNAME', precisely because this lets the user specify the value. `PATH' A "path" is a sequence of directory names which is used for searching for a file. The variable `PATH' holds a path used for searching for programs to be run. The `execlp' and `execvp' functions (*note Executing a File::) use this environment variable, as do many shells and other utilities which are implemented in terms of those functions. The syntax of a path is a sequence of directory names separated by colons. An empty string instead of a directory name stands for the current directory (*note Working Directory::). A typical value for this environment variable might be a string like: :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin This means that if the user tries to execute a program named `foo', the system will look for files named `foo', `/bin/foo', `/etc/foo', and so on. The first of these files that exists is the one that is executed. `TERM' This specifies the kind of terminal that is receiving program output. Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals. Many programs which use the termcap library (*note Find: (termcap)Finding a Terminal Description.) use the `TERM' environment variable, for example. `TZ' This specifies the time zone. *Note TZ Variable::, for information about the format of this string and how it is used. `LANG' This specifies the default locale to use for attribute categories where neither `LC_ALL' nor the specific environment variable for that category is set. *Note Locales::, for more information about locales. `LC_ALL' If this environment variable is set it overrides the selection for all the locales done using the other `LC_*' environment variables. The value of the other `LC_*' environment variables is simply ignored in this case. `LC_COLLATE' This specifies what locale to use for string sorting. `LC_CTYPE' This specifies what locale to use for character sets and character classification. `LC_MESSAGES' This specifies what locale to use for printing messages and to parse responses. `LC_MONETARY' This specifies what locale to use for formatting monetary values. `LC_NUMERIC' This specifies what locale to use for formatting numbers. `LC_TIME' This specifies what locale to use for formatting date/time values. `NLSPATH' This specifies the directories in which the `catopen' function looks for message translation catalogs. `_POSIX_OPTION_ORDER' If this environment variable is defined, it suppresses the usual reordering of command line arguments by `getopt' and `argp_parse'. *Note Argument Syntax::. System Calls ============ A system call is a request for service that a program makes of the kernel. The service is generally something that only the kernel has the privilege to do, such as doing I/O. Programmers don't normally need to be concerned with system calls because there are functions in the GNU C library to do virtually everything that system calls do. These functions work by making system calls themselves. For example, there is a system call that changes the permissions of a file, but you don't need to know about it because you can just use the GNU C library's `chmod' function. System calls are sometimes called kernel calls. However, there are times when you want to make a system call explicitly, and for that, the GNU C library provides the `syscall' function. `syscall' is harder to use and less portable than functions like `chmod', but easier and more portable than coding the system call in assembler instructions. `syscall' is most useful when you are working with a system call which is special to your system or is newer than the GNU C library you are using. `syscall' is implemented in an entirely generic way; the function does not know anything about what a particular system call does or even if it is valid. The description of `syscall' in this section assumes a certain protocol for system calls on the various platforms on which the GNU C library runs. That protocol is not defined by any strong authority, but we won't describe it here either because anyone who is coding `syscall' probably won't accept anything less than kernel and C library source code as a specification of the interface between them anyway. `syscall' is declared in `unistd.h'. - Function: long int syscall (long int SYSNO, ...) `syscall' performs a generic system call. SYSNO is the system call number. Each kind of system call is identified by a number. Macros for all the possible system call numbers are defined in `sys/syscall.h' The remaining arguments are the arguments for the system call, in order, and their meanings depend on the kind of system call. Each kind of system call has a definite number of arguments, from zero to five. If you code more arguments than the system call takes, the extra ones to the right are ignored. The return value is the return value from the system call, unless the system call failed. In that case, `syscall' returns `-1' and sets `errno' to an error code that the system call returned. Note that system calls do not return `-1' when they succeed. If you specify an invalid SYSNO, `syscall' returns `-1' with `errno' = `ENOSYS'. Example: #include #include #include ... int rc; rc = syscall(SYS_chmod, "/etc/passwd", 0444); if (rc == -1) fprintf(stderr, "chmod failed, errno = %d\n", errno); This, if all the compatibility stars are aligned, is equivalent to the following preferable code: #include #include #include ... int rc; rc = chmod("/etc/passwd", 0444); if (rc == -1) fprintf(stderr, "chmod failed, errno = %d\n", errno); Program Termination =================== The usual way for a program to terminate is simply for its `main' function to return. The "exit status value" returned from the `main' function is used to report information back to the process's parent process or shell. A program can also terminate normally by calling the `exit' function. In addition, programs can be terminated by signals; this is discussed in more detail in *Note Signal Handling::. The `abort' function causes a signal that kills the program. Normal Termination ------------------ A process terminates normally when its program signals it is done by calling `exit'. Returning from `main' is equivalent to calling `exit', and the value that `main' returns is used as the argument to `exit'. - Function: void exit (int STATUS) The `exit' function tells the system that the program is done, which causes it to terminate the process. STATUS is the program's exit status, which becomes part of the process' termination status. This function does not return. Normal termination causes the following actions: 1. Functions that were registered with the `atexit' or `on_exit' functions are called in the reverse order of their registration. This mechanism allows your application to specify its own "cleanup" actions to be performed at program termination. Typically, this is used to do things like saving program state information in a file, or unlocking locks in shared data bases. 2. All open streams are closed, writing out any buffered output data. See *Note Closing Streams::. In addition, temporary files opened with the `tmpfile' function are removed; see *Note Temporary Files::. 3. `_exit' is called, terminating the program. *Note Termination Internals::. Exit Status ----------- When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the "exit status". This is a value between 0 and 255 that the exiting process passes as an argument to `exit'. Normally you should use the exit status to report very broad information about success or failure. You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway. There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it. A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs. *Warning:* Don't try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors--that is, success. For the same reason, it does not work to use the value of `errno' as the exit status--these can exceed 255. *Portability note:* Some non-POSIX systems use different conventions for exit status values. For greater portability, you can use the macros `EXIT_SUCCESS' and `EXIT_FAILURE' for the conventional status value for success and failure, respectively. They are declared in the file `stdlib.h'. - Macro: int EXIT_SUCCESS This macro can be used with the `exit' function to indicate successful program completion. On POSIX systems, the value of this macro is `0'. On other systems, the value might be some other (possibly non-constant) integer expression. - Macro: int EXIT_FAILURE This macro can be used with the `exit' function to indicate unsuccessful program completion in a general sense. On POSIX systems, the value of this macro is `1'. On other systems, the value might be some other (possibly non-constant) integer expression. Other nonzero status values also indicate failures. Certain programs use different nonzero status values to indicate particular kinds of "non-success". For example, `diff' uses status value `1' to mean that the files are different, and `2' or more to mean that there was difficulty in opening the files. Don't confuse a program's exit status with a process' termination status. There are lots of ways a process can terminate besides having it's program finish. In the event that the process termination _is_ caused by program termination (i.e. `exit'), though, the program's exit status becomes part of the process' termination status. Cleanups on Exit ---------------- Your program can arrange to run its own cleanup functions if normal termination happens. If you are writing a library for use in various application programs, then it is unreliable to insist that all applications call the library's cleanup functions explicitly before exiting. It is much more robust to make the cleanup invisible to the application, by setting up a cleanup function in the library itself using `atexit' or `on_exit'. - Function: int atexit (void (*FUNCTION) (void)) The `atexit' function registers the function FUNCTION to be called at normal program termination. The FUNCTION is called with no arguments. The return value from `atexit' is zero on success and nonzero if the function cannot be registered. - Function: int on_exit (void (*FUNCTION)(int STATUS, void *ARG), void *ARG) This function is a somewhat more powerful variant of `atexit'. It accepts two arguments, a function FUNCTION and an arbitrary pointer ARG. At normal program termination, the FUNCTION is called with two arguments: the STATUS value passed to `exit', and the ARG. This function is included in the GNU C library only for compatibility for SunOS, and may not be supported by other implementations. Here's a trivial program that illustrates the use of `exit' and `atexit': #include #include void bye (void) { puts ("Goodbye, cruel world...."); } int main (void) { atexit (bye); exit (EXIT_SUCCESS); } When this program is executed, it just prints the message and exits. Aborting a Program ------------------ You can abort your program using the `abort' function. The prototype for this function is in `stdlib.h'. - Function: void abort (void) The `abort' function causes abnormal program termination. This does not execute cleanup functions registered with `atexit' or `on_exit'. This function actually terminates the process by raising a `SIGABRT' signal, and your program can include a handler to intercept this signal; see *Note Signal Handling::. *Future Change Warning:* Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function. We would be required to say that this is not an acceptable way of terminating a program. Termination Internals --------------------- The `_exit' function is the primitive used for process termination by `exit'. It is declared in the header file `unistd.h'. - Function: void _exit (int STATUS) The `_exit' function is the primitive for causing a process to terminate with status STATUS. Calling this function does not execute cleanup functions registered with `atexit' or `on_exit'. - Function: void _Exit (int STATUS) The `_Exit' function is the ISO C equivalent to `_exit'. The ISO C committee members were not sure whether the definitions of `_exit' and `_Exit' were compatible so they have not used the POSIX name. This function was introduced in ISO C99 and is declared in `stdlib.h'. When a process terminates for any reason--either because the program terminates, or as a result of a signal--the following things happen: * All open file descriptors in the process are closed. *Note Low-Level I/O::. Note that streams are not flushed automatically when the process terminates; see *Note I/O on Streams::. * A process exit status is saved to be reported back to the parent process via `wait' or `waitpid'; see *Note Process Completion::. If the program exited, this status includes as its low-order 8 bits the program exit status. * Any child processes of the process being terminated are assigned a new parent process. (On most systems, including GNU, this is the `init' process, with process ID 1.) * A `SIGCHLD' signal is sent to the parent process. * If the process is a session leader that has a controlling terminal, then a `SIGHUP' signal is sent to each process in the foreground job, and the controlling terminal is disassociated from that session. *Note Job Control::. * If termination of a process causes a process group to become orphaned, and any member of that process group is stopped, then a `SIGHUP' signal and a `SIGCONT' signal are sent to each process in the group. *Note Job Control::. Processes ********* "Processes" are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies. Processes are organized hierarchically. Each process has a "parent process" which explicitly arranged to create it. The processes created by a given parent are called its "child processes". A child inherits many of its attributes from the parent process. This chapter describes how a program can create, terminate, and control child processes. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program. The `system' function provides a simple, portable mechanism for running another program; it does all three steps automatically. If you need more control over the details of how this is done, you can use the primitive functions to do each step individually instead. Running a Command ================= The easy way to run another program is to use the `system' function. This function does all the work of running a subprogram, but it doesn't give you much control over the details: you have to wait until the subprogram terminates before you can do anything else. - Function: int system (const char *COMMAND) This function executes COMMAND as a shell command. In the GNU C library, it always uses the default shell `sh' to run the command. In particular, it searches the directories in `PATH' to find programs to execute. The return value is `-1' if it wasn't possible to create the shell process, and otherwise is the status of the shell process. *Note Process Completion::, for details on how this status code can be interpreted. If the COMMAND argument is a null pointer, a return value of zero indicates that no command processor is available. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `system' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `system' should be protected using cancellation handlers. The `system' function is declared in the header file `stdlib.h'. *Portability Note:* Some C implementations may not have any notion of a command processor that can execute other programs. You can determine whether a command processor exists by executing `system (NULL)'; if the return value is nonzero, a command processor is available. The `popen' and `pclose' functions (*note Pipe to a Subprocess::) are closely related to the `system' function. They allow the parent process to communicate with the standard input and output channels of the command being executed. Process Creation Concepts ========================= This section gives an overview of processes and of the steps involved in creating a process and making it run another program. Each process is named by a "process ID" number. A unique process ID is allocated to each process when it is created. The "lifetime" of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed. Processes are created with the `fork' system call (so the operation of creating a new process is sometimes called "forking" a process). The "child process" created by `fork' is a copy of the original "parent process", except that it has its own process ID. After forking a child process, both the parent and child processes continue to execute normally. If you want your program to wait for a child process to finish executing before continuing, you must do this explicitly after the fork operation, by calling `wait' or `waitpid' (*note Process Completion::). These functions give you limited information about why the child terminated--for example, its exit status code. A newly forked child process continues to execute the same program as its parent process, at the point where the `fork' call returns. You can use the return value from `fork' to tell whether the program is running in the parent process or the child. Having several processes run the same program is only occasionally useful. But the child can execute another program using one of the `exec' functions; see *Note Executing a File::. The program that the process is executing is called its "process image". Starting execution of a new program causes the process to forget all about its previous process image; when the new program exits, the process exits too, instead of returning to the previous process image. Process Identification ====================== The `pid_t' data type represents process IDs. You can get the process ID of a process by calling `getpid'. The function `getppid' returns the process ID of the parent of the current process (this is also known as the "parent process ID"). Your program should include the header files `unistd.h' and `sys/types.h' to use these functions. - Data Type: pid_t The `pid_t' data type is a signed integer type which is capable of representing a process ID. In the GNU library, this is an `int'. - Function: pid_t getpid (void) The `getpid' function returns the process ID of the current process. - Function: pid_t getppid (void) The `getppid' function returns the process ID of the parent of the current process. Creating a Process ================== The `fork' function is the primitive for creating a process. It is declared in the header file `unistd.h'. - Function: pid_t fork (void) The `fork' function creates a new process. If the operation is successful, there are then both parent and child processes and both see `fork' return, but with different values: it returns a value of `0' in the child process and returns the child's process ID in the parent process. If process creation failed, `fork' returns a value of `-1' in the parent process. The following `errno' error conditions are defined for `fork': `EAGAIN' There aren't enough system resources to create another process, or the user already has too many processes running. This means exceeding the `RLIMIT_NPROC' resource limit, which can usually be increased; *note Limits on Resources::. `ENOMEM' The process requires more space than the system can supply. The specific attributes of the child process that differ from the parent process are: * The child process has its own unique process ID. * The parent process ID of the child process is the process ID of its parent process. * The child process gets its own copies of the parent process's open file descriptors. Subsequently changing attributes of the file descriptors in the parent process won't affect the file descriptors in the child, and vice versa. *Note Control Operations::. However, the file position associated with each descriptor is shared by both processes; *note File Position::. * The elapsed processor times for the child process are set to zero; see *Note Processor Time::. * The child doesn't inherit file locks set by the parent process. *Note Control Operations::. * The child doesn't inherit alarms set by the parent process. *Note Setting an Alarm::. * The set of pending signals (*note Delivery of Signal::) for the child process is cleared. (The child process inherits its mask of blocked signals and signal actions from the parent process.) - Function: pid_t vfork (void) The `vfork' function is similar to `fork' but on some systems it is more efficient; however, there are restrictions you must follow to use it safely. While `fork' makes a complete copy of the calling process's address space and allows both the parent and child to execute independently, `vfork' does not make this copy. Instead, the child process created with `vfork' shares its parent's address space until it calls `_exit' or one of the `exec' functions. In the meantime, the parent process suspends execution. You must be very careful not to allow the child process created with `vfork' to modify any global data or even local variables shared with the parent. Furthermore, the child process cannot return from (or do a long jump out of) the function that called `vfork'! This would leave the parent process's control information very confused. If in doubt, use `fork' instead. Some operating systems don't really implement `vfork'. The GNU C library permits you to use `vfork' on all systems, but actually executes `fork' if `vfork' isn't available. If you follow the proper precautions for using `vfork', your program will still work even if the system uses `fork' instead. Executing a File ================ This section describes the `exec' family of functions, for executing a file as a process image. You can use these functions to make a child process execute a new program after it has been forked. To see the effects of `exec' from the point of view of the called program, *Note Program Basics::. The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file `unistd.h'. - Function: int execv (const char *FILENAME, char *const ARGV[]) The `execv' function executes the file named by FILENAME as a new process image. The ARGV argument is an array of null-terminated strings that is used to provide a value for the `argv' argument to the `main' function of the program to be executed. The last element of this array must be a null pointer. By convention, the first element of this array is the file name of the program sans directory names. *Note Program Arguments::, for full details on how programs can access these arguments. The environment for the new process image is taken from the `environ' variable of the current process image; see *Note Environment Variables::, for information about environments. - Function: int execl (const char *FILENAME, const char *ARG0, ...) This is similar to `execv', but the ARGV strings are specified individually instead of as an array. A null pointer must be passed as the last such argument. - Function: int execve (const char *FILENAME, char *const ARGV[], char *const ENV[]) This is similar to `execv', but permits you to specify the environment for the new program explicitly as the ENV argument. This should be an array of strings in the same format as for the `environ' variable; see *Note Environment Access::. - Function: int execle (const char *FILENAME, const char *ARG0, char *const ENV[], ...) This is similar to `execl', but permits you to specify the environment for the new program explicitly. The environment argument is passed following the null pointer that marks the last ARGV argument, and should be an array of strings in the same format as for the `environ' variable. - Function: int execvp (const char *FILENAME, char *const ARGV[]) The `execvp' function is similar to `execv', except that it searches the directories listed in the `PATH' environment variable (*note Standard Environment::) to find the full file name of a file from FILENAME if FILENAME does not contain a slash. This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type. - Function: int execlp (const char *FILENAME, const char *ARG0, ...) This function is like `execl', except that it performs the same file name searching as the `execvp' function. The size of the argument list and environment list taken together must not be greater than `ARG_MAX' bytes. *Note General Limits::. In the GNU system, the size (which compares against `ARG_MAX') includes, for each string, the number of characters in the string, plus the size of a `char *', plus one, rounded up to a multiple of the size of a `char *'. Other systems may have somewhat different rules for counting. These functions normally don't return, since execution of a new program causes the currently executing program to go away completely. A value of `-1' is returned in the event of a failure. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for these functions: `E2BIG' The combined size of the new program's argument list and environment list is larger than `ARG_MAX' bytes. The GNU system has no specific limit on the argument list size, so this error code cannot result, but you may get `ENOMEM' instead if the arguments are too big for available memory. `ENOEXEC' The specified file can't be executed because it isn't in the right format. `ENOMEM' Executing the specified file requires more storage than is available. If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. *Note File Times::, for more details about access times of files. The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed. Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged: * The process ID and the parent process ID. *Note Process Creation Concepts::. * Session and process group membership. *Note Concepts of Job Control::. * Real user ID and group ID, and supplementary group IDs. *Note Process Persona::. * Pending alarms. *Note Setting an Alarm::. * Current working directory and root directory. *Note Working Directory::. In the GNU system, the root directory is not copied when executing a setuid program; instead the system default root directory is used for the new program. * File mode creation mask. *Note Setting Permissions::. * Process signal mask; see *Note Process Signal Mask::. * Pending signals; see *Note Blocking Signals::. * Elapsed processor time associated with the process; see *Note Processor Time::. If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in *Note Process Persona::. Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see *Note Signal Handling::. File descriptors open in the existing process image remain open in the new process image, unless they have the `FD_CLOEXEC' (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in *Note Low-Level I/O::. Streams, by contrast, cannot survive through `exec' functions, because they are located in the memory of the process itself. The new process image has no streams except those it creates afresh. Each of the streams in the pre-`exec' process image has a descriptor inside it, and these descriptors do survive through `exec' (provided that they do not have `FD_CLOEXEC' set). The new process image can reconnect these to new streams using `fdopen' (*note Descriptors and Streams::). Process Completion ================== The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file `sys/wait.h'. - Function: pid_t waitpid (pid_t PID, int *STATUS-PTR, int OPTIONS) The `waitpid' function is used to request status information from a child process whose process ID is PID. Normally, the calling process is suspended until the child process makes status information available by terminating. Other values for the PID argument have special interpretations. A value of `-1' or `WAIT_ANY' requests status information for any child process; a value of `0' or `WAIT_MYPGRP' requests information for any child process in the same process group as the calling process; and any other negative value - PGID requests information for any child process whose process group ID is PGID. If status information for a child process is available immediately, this function returns immediately without waiting. If more than one eligible child process has status information available, one of them is chosen randomly, and its status is returned immediately. To get the status from the other eligible child processes, you need to call `waitpid' again. The OPTIONS argument is a bit mask. Its value should be the bitwise OR (that is, the `|' operator) of zero or more of the `WNOHANG' and `WUNTRACED' flags. You can use the `WNOHANG' flag to indicate that the parent process shouldn't wait; and the `WUNTRACED' flag to request status information from stopped processes as well as processes that have terminated. The status information from the child process is stored in the object that STATUS-PTR points to, unless STATUS-PTR is a null pointer. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `waitpid' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `waitpid' should be protected using cancellation handlers. The return value is normally the process ID of the child process whose status is reported. If there are child processes but none of them is waiting to be noticed, `waitpid' will block until one is. However, if the `WNOHANG' option was specified, `waitpid' will return zero instead of blocking. If a specific PID to wait for was given to `waitpid', it will ignore all other children (if any). Therefore if there are children waiting to be noticed but the child whose PID was specified is not one of them, `waitpid' will block or return zero as described above. A value of `-1' is returned in case of error. The following `errno' error conditions are defined for this function: `EINTR' The function was interrupted by delivery of a signal to the calling process. *Note Interrupted Primitives::. `ECHILD' There are no child processes to wait for, or the specified PID is not a child of the calling process. `EINVAL' An invalid value was provided for the OPTIONS argument. These symbolic constants are defined as values for the PID argument to the `waitpid' function. `WAIT_ANY' This constant macro (whose value is `-1') specifies that `waitpid' should return status information about any child process. `WAIT_MYPGRP' This constant (with value `0') specifies that `waitpid' should return status information about any child process in the same process group as the calling process. These symbolic constants are defined as flags for the OPTIONS argument to the `waitpid' function. You can bitwise-OR the flags together to obtain a value to use as the argument. `WNOHANG' This flag specifies that `waitpid' should return immediately instead of waiting, if there is no child process ready to be noticed. `WUNTRACED' This flag specifies that `waitpid' should report the status of any child processes that have been stopped as well as those that have terminated. - Function: pid_t wait (int *STATUS-PTR) This is a simplified version of `waitpid', and is used to wait until any one child process terminates. The call: wait (&status) is exactly equivalent to: waitpid (-1, &status, 0) This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `wait' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `wait' should be protected using cancellation handlers. - Function: pid_t wait4 (pid_t PID, int *STATUS-PTR, int OPTIONS, struct rusage *USAGE) If USAGE is a null pointer, `wait4' is equivalent to `waitpid (PID, STATUS-PTR, OPTIONS)'. If USAGE is not null, `wait4' stores usage figures for the child process in `*RUSAGE' (but only if the child has terminated, not if it has stopped). *Note Resource Usage::. This function is a BSD extension. Here's an example of how to use `waitpid' to get the status from all child processes that have terminated, without ever waiting. This function is designed to be a handler for `SIGCHLD', the signal that indicates that at least one child process has terminated. void sigchld_handler (int signum) { int pid, status, serrno; serrno = errno; while (1) { pid = waitpid (WAIT_ANY, &status, WNOHANG); if (pid < 0) { perror ("waitpid"); break; } if (pid == 0) break; notice_termination (pid, status); } errno = serrno; } Process Completion Status ========================= If the exit status value (*note Program Termination::) of the child process is zero, then the status value reported by `waitpid' or `wait' is also zero. You can test for other kinds of information encoded in the returned status value using the following macros. These macros are defined in the header file `sys/wait.h'. - Macro: int WIFEXITED (int STATUS) This macro returns a nonzero value if the child process terminated normally with `exit' or `_exit'. - Macro: int WEXITSTATUS (int STATUS) If `WIFEXITED' is true of STATUS, this macro returns the low-order 8 bits of the exit status value from the child process. *Note Exit Status::. - Macro: int WIFSIGNALED (int STATUS) This macro returns a nonzero value if the child process terminated because it received a signal that was not handled. *Note Signal Handling::. - Macro: int WTERMSIG (int STATUS) If `WIFSIGNALED' is true of STATUS, this macro returns the signal number of the signal that terminated the child process. - Macro: int WCOREDUMP (int STATUS) This macro returns a nonzero value if the child process terminated and produced a core dump. - Macro: int WIFSTOPPED (int STATUS) This macro returns a nonzero value if the child process is stopped. - Macro: int WSTOPSIG (int STATUS) If `WIFSTOPPED' is true of STATUS, this macro returns the signal number of the signal that caused the child process to stop. BSD Process Wait Functions ========================== The GNU library also provides these related facilities for compatibility with BSD Unix. BSD uses the `union wait' data type to represent status values rather than an `int'. The two representations are actually interchangeable; they describe the same bit patterns. The GNU C Library defines macros such as `WEXITSTATUS' so that they will work on either kind of object, and the `wait' function is defined to accept either type of pointer as its STATUS-PTR argument. These functions are declared in `sys/wait.h'. - Data Type: union wait This data type represents program termination status values. It has the following members: `int w_termsig' The value of this member is the same as that of the `WTERMSIG' macro. `int w_coredump' The value of this member is the same as that of the `WCOREDUMP' macro. `int w_retcode' The value of this member is the same as that of the `WEXITSTATUS' macro. `int w_stopsig' The value of this member is the same as that of the `WSTOPSIG' macro. Instead of accessing these members directly, you should use the equivalent macros. The `wait3' function is the predecessor to `wait4', which is more flexible. `wait3' is now obsolete. - Function: pid_t wait3 (union wait *STATUS-PTR, int OPTIONS, struct rusage *USAGE) If USAGE is a null pointer, `wait3' is equivalent to `waitpid (-1, STATUS-PTR, OPTIONS)'. If USAGE is not null, `wait3' stores usage figures for the child process in `*RUSAGE' (but only if the child has terminated, not if it has stopped). *Note Resource Usage::. Process Creation Example ======================== Here is an example program showing how you might write a function similar to the built-in `system'. It executes its COMMAND argument using the equivalent of `sh -c COMMAND'. #include #include #include #include #include /* Execute the command using this shell program. */ #define SHELL "/bin/sh" int my_system (const char *command) { int status; pid_t pid; pid = fork (); if (pid == 0) { /* This is the child process. Execute the shell command. */ execl (SHELL, SHELL, "-c", command, NULL); _exit (EXIT_FAILURE); } else if (pid < 0) /* The fork failed. Report failure. */ status = -1; else /* This is the parent process. Wait for the child to complete. */ if (waitpid (pid, &status, 0) != pid) status = -1; return status; } There are a couple of things you should pay attention to in this example. Remember that the first `argv' argument supplied to the program represents the name of the program being executed. That is why, in the call to `execl', `SHELL' is supplied once to name the program to execute and a second time to supply a value for `argv[0]'. The `execl' call in the child process doesn't return if it is successful. If it fails, you must do something to make the child process terminate. Just returning a bad status code with `return' would leave two processes running the original program. Instead, the right behavior is for the child process to report failure to its parent process. Call `_exit' to accomplish this. The reason for using `_exit' instead of `exit' is to avoid flushing fully buffered streams such as `stdout'. The buffers of these streams probably contain data that was copied from the parent process by the `fork', data that will be output eventually by the parent process. Calling `exit' in the child would output the data twice. *Note Termination Internals::. Job Control *********** "Job control" refers to the protocol for allowing a user to move between multiple "process groups" (or "jobs") within a single "login session". The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control. So you can probably ignore the material in this chapter unless you are writing a shell or login program. You need to be familiar with concepts relating to process creation (*note Process Creation Concepts::) and signal handling (*note Signal Handling::) in order to understand this material presented in this chapter. Concepts of Job Control ======================= The fundamental purpose of an interactive shell is to read commands from the user's terminal and create processes to execute the programs specified by those commands. It can do this using the `fork' (*note Creating a Process::) and `exec' (*note Executing a File::) functions. A single command may run just one process--but often one command uses several processes. If you use the `|' operator in a shell command, you explicitly request several programs in their own processes. But even if you run just one program, it can use multiple processes internally. For example, a single compilation command such as `cc -c foo.c' typically uses four processes (though normally only two at any given time). If you run `make', its job is to run other programs in separate processes. The processes belonging to a single command are called a "process group" or "job". This is so that you can operate on all of them at once. For example, typing `C-c' sends the signal `SIGINT' to terminate all the processes in the foreground process group. A "session" is a larger group of processes. Normally all the processes that stem from a single login belong to the same session. Every process belongs to a process group. When a process is created, it becomes a member of the same process group and session as its parent process. You can put it in another process group using the `setpgid' function, provided the process group belongs to the same session. The only way to put a process in a different session is to make it the initial process of a new session, or a "session leader", using the `setsid' function. This also puts the session leader into a new process group, and you can't move it out of that process group again. Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell. A shell that supports job control must arrange to control which job can use the terminal at any time. Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user. To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter. The shell can give unlimited access to the controlling terminal to only one process group at a time. This is called the "foreground job" on that controlling terminal. Other process groups managed by the shell that are executing without such access to the terminal are called "background jobs". If a background job needs to read from its controlling terminal, it is "stopped" by the terminal driver; if the `TOSTOP' mode is set, likewise for writing. The user can stop a foreground job by typing the SUSP character (*note Special Characters::) and a program can stop any job by sending it a `SIGSTOP' signal. It's the responsibility of the shell to notice when jobs stop, to notify the user about them, and to provide mechanisms for allowing the user to interactively continue stopped jobs and switch jobs between foreground and background. *Note Access to the Terminal::, for more information about I/O to the controlling terminal, Job Control is Optional ======================= Not all operating systems support job control. The GNU system does support job control, but if you are using the GNU library on some other system, that system may not support job control itself. You can use the `_POSIX_JOB_CONTROL' macro to test at compile-time whether the system supports job control. *Note System Options::. If job control is not supported, then there can be only one process group per session, which behaves as if it were always in the foreground. The functions for creating additional process groups simply fail with the error code `ENOSYS'. The macros naming the various job control signals (*note Job Control Signals::) are defined even if job control is not supported. However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing. Controlling Terminal of a Process ================================= One of the attributes of a process is its controlling terminal. Child processes created with `fork' inherit the controlling terminal from their parent process. In this way, all the processes in a session inherit the controlling terminal from the session leader. A session leader that has control of a terminal is called the "controlling process" of that terminal. You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in. An individual process disconnects from its controlling terminal when it calls `setsid' to become the leader of a new session. *Note Process Group Functions::. Access to the Controlling Terminal ================================== Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background processes do not. This section describes in more detail what happens when a process in a background job tries to access its controlling terminal. When a process in a background job tries to read from its controlling terminal, the process group is usually sent a `SIGTTIN' signal. This normally causes all of the processes in that group to stop (unless they handle the signal and don't stop themselves). However, if the reading process is ignoring or blocking this signal, then `read' fails with an `EIO' error instead. Similarly, when a process in a background job tries to write to its controlling terminal, the default behavior is to send a `SIGTTOU' signal to the process group. However, the behavior is modified by the `TOSTOP' bit of the local modes flags (*note Local Modes::). If this bit is not set (which is the default), then writing to the controlling terminal is always permitted without sending a signal. Writing is also permitted if the `SIGTTOU' signal is being ignored or blocked by the writing process. Most other terminal operations that a program can do are treated as reading or as writing. (The description of each operation should say which.) For more information about the primitive `read' and `write' functions, see *Note I/O Primitives::. Orphaned Process Groups ======================= When a controlling process terminates, its terminal becomes free and a new session can be established on it. (In fact, another user could log in on the terminal.) This could cause a problem if any processes from the old session are still trying to use that terminal. To prevent problems, process groups that continue running even after the session leader has terminated are marked as "orphaned process groups". When a process group becomes an orphan, its processes are sent a `SIGHUP' signal. Ordinarily, this causes the processes to terminate. However, if a program ignores this signal or establishes a handler for it (*note Signal Handling::), it can continue running as in the orphan process group even after its controlling process terminates; but it still cannot access the terminal any more. Implementing a Job Control Shell ================================ This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved. Data Structures for the Shell ----------------------------- All of the program examples included in this chapter are part of a simple shell program. This section presents data structures and utility functions which are used throughout the example. The sample shell deals mainly with two data structures. The `job' type contains information about a job, which is a set of subprocesses linked together with pipes. The `process' type holds information about a single subprocess. Here are the relevant data structure declarations: /* A process is a single process. */ typedef struct process { struct process *next; /* next process in pipeline */ char **argv; /* for exec */ pid_t pid; /* process ID */ char completed; /* true if process has completed */ char stopped; /* true if process has stopped */ int status; /* reported status value */ } process; /* A job is a pipeline of processes. */ typedef struct job { struct job *next; /* next active job */ char *command; /* command line, used for messages */ process *first_process; /* list of processes in this job */ pid_t pgid; /* process group ID */ char notified; /* true if user told about stopped job */ struct termios tmodes; /* saved terminal modes */ int stdin, stdout, stderr; /* standard i/o channels */ } job; /* The active jobs are linked into a list. This is its head. */ job *first_job = NULL; Here are some utility functions that are used for operating on `job' objects. /* Find the active job with the indicated PGID. */ job * find_job (pid_t pgid) { job *j; for (j = first_job; j; j = j->next) if (j->pgid == pgid) return j; return NULL; } /* Return true if all processes in the job have stopped or completed. */ int job_is_stopped (job *j) { process *p; for (p = j->first_process; p; p = p->next) if (!p->completed && !p->stopped) return 0; return 1; } /* Return true if all processes in the job have completed. */ int job_is_completed (job *j) { process *p; for (p = j->first_process; p; p = p->next) if (!p->completed) return 0; return 1; } Initializing the Shell ---------------------- When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control. A subshell that runs interactively has to ensure that it has been placed in the foreground by its parent shell before it can enable job control itself. It does this by getting its initial process group ID with the `getpgrp' function, and comparing it to the process group ID of the current foreground job associated with its controlling terminal (which can be retrieved using the `tcgetpgrp' function). If the subshell is not running as a foreground job, it must stop itself by sending a `SIGTTIN' signal to its own process group. It may not arbitrarily put itself into the foreground; it must wait for the user to tell the parent shell to do this. If the subshell is continued again, it should repeat the check and stop itself again if it is still not in the foreground. Once the subshell has been placed into the foreground by its parent shell, it can enable its own job control. It does this by calling `setpgid' to put itself into its own process group, and then calling `tcsetpgrp' to place this process group into the foreground. When a shell enables job control, it should set itself to ignore all the job control stop signals so that it doesn't accidentally stop itself. You can do this by setting the action for all the stop signals to `SIG_IGN'. A subshell that runs non-interactively cannot and should not support job control. It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell. This is easy to do--just don't use any of the job control primitives--but you must remember to make the shell do it. Here is the initialization code for the sample shell that shows how to do all of this. /* Keep track of attributes of the shell. */ #include #include #include pid_t shell_pgid; struct termios shell_tmodes; int shell_terminal; int shell_is_interactive; /* Make sure the shell is running interactively as the foreground job before proceeding. */ void init_shell () { /* See if we are running interactively. */ shell_terminal = STDIN_FILENO; shell_is_interactive = isatty (shell_terminal); if (shell_is_interactive) { /* Loop until we are in the foreground. */ while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ())) kill (- shell_pgid, SIGTTIN); /* Ignore interactive and job-control signals. */ signal (SIGINT, SIG_IGN); signal (SIGQUIT, SIG_IGN); signal (SIGTSTP, SIG_IGN); signal (SIGTTIN, SIG_IGN); signal (SIGTTOU, SIG_IGN); signal (SIGCHLD, SIG_IGN); /* Put ourselves in our own process group. */ shell_pgid = getpid (); if (setpgid (shell_pgid, shell_pgid) < 0) { perror ("Couldn't put the shell in its own process group"); exit (1); } /* Grab control of the terminal. */ tcsetpgrp (shell_terminal, shell_pgid); /* Save default terminal attributes for shell. */ tcgetattr (shell_terminal, &shell_tmodes); } } Launching Jobs -------------- Once the shell has taken responsibility for performing job control on its controlling terminal, it can launch jobs in response to commands typed by the user. To create the processes in a process group, you use the same `fork' and `exec' functions described in *Note Process Creation Concepts::. Since there are multiple child processes involved, though, things are a little more complicated and you must be careful to do things in the right order. Otherwise, nasty race conditions can result. You have two choices for how to structure the tree of parent-child relationships among the processes. You can either make all the processes in the process group be children of the shell process, or you can make one process in group be the ancestor of all the other processes in that group. The sample shell program presented in this chapter uses the first approach because it makes bookkeeping somewhat simpler. As each process is forked, it should put itself in the new process group by calling `setpgid'; see *Note Process Group Functions::. The first process in the new group becomes its "process group leader", and its process ID becomes the "process group ID" for the group. The shell should also call `setpgid' to put each of its child processes into the new process group. This is because there is a potential timing problem: each child process must be put in the process group before it begins executing a new program, and the shell depends on having all the child processes in the group before it continues executing. If both the child processes and the shell call `setpgid', this ensures that the right things happen no matter which process gets to it first. If the job is being launched as a foreground job, the new process group also needs to be put into the foreground on the controlling terminal using `tcsetpgrp'. Again, this should be done by the shell as well as by each of its child processes, to avoid race conditions. The next thing each child process should do is to reset its signal actions. During initialization, the shell process set itself to ignore job control signals; see *Note Initializing the Shell::. As a result, any child processes it creates also ignore these signals by inheritance. This is definitely undesirable, so each child process should explicitly set the actions for these signals back to `SIG_DFL' just after it is forked. Since shells follow this convention, applications can assume that they inherit the correct handling of these signals from the parent process. But every application has a responsibility not to mess up the handling of stop signals. Applications that disable the normal interpretation of the SUSP character should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a `SIGTSTP' signal to the process group of the process, not just to the process itself. *Note Signaling Another Process::. Finally, each child process should call `exec' in the normal way. This is also the point at which redirection of the standard input and output channels should be handled. *Note Duplicating Descriptors::, for an explanation of how to do this. Here is the function from the sample shell program that is responsible for launching a program. The function is executed by each child process immediately after it has been forked by the shell, and never returns. void launch_process (process *p, pid_t pgid, int infile, int outfile, int errfile, int foreground) { pid_t pid; if (shell_is_interactive) { /* Put the process into the process group and give the process group the terminal, if appropriate. This has to be done both by the shell and in the individual child processes because of potential race conditions. */ pid = getpid (); if (pgid == 0) pgid = pid; setpgid (pid, pgid); if (foreground) tcsetpgrp (shell_terminal, pgid); /* Set the handling for job control signals back to the default. */ signal (SIGINT, SIG_DFL); signal (SIGQUIT, SIG_DFL); signal (SIGTSTP, SIG_DFL); signal (SIGTTIN, SIG_DFL); signal (SIGTTOU, SIG_DFL); signal (SIGCHLD, SIG_DFL); } /* Set the standard input/output channels of the new process. */ if (infile != STDIN_FILENO) { dup2 (infile, STDIN_FILENO); close (infile); } if (outfile != STDOUT_FILENO) { dup2 (outfile, STDOUT_FILENO); close (outfile); } if (errfile != STDERR_FILENO) { dup2 (errfile, STDERR_FILENO); close (errfile); } /* Exec the new process. Make sure we exit. */ execvp (p->argv[0], p->argv); perror ("execvp"); exit (1); } If the shell is not running interactively, this function does not do anything with process groups or signals. Remember that a shell not performing job control must keep all of its subprocesses in the same process group as the shell itself. Next, here is the function that actually launches a complete job. After creating the child processes, this function calls some other functions to put the newly created job into the foreground or background; these are discussed in *Note Foreground and Background::. void launch_job (job *j, int foreground) { process *p; pid_t pid; int mypipe[2], infile, outfile; infile = j->stdin; for (p = j->first_process; p; p = p->next) { /* Set up pipes, if necessary. */ if (p->next) { if (pipe (mypipe) < 0) { perror ("pipe"); exit (1); } outfile = mypipe[1]; } else outfile = j->stdout; /* Fork the child processes. */ pid = fork (); if (pid == 0) /* This is the child process. */ launch_process (p, j->pgid, infile, outfile, j->stderr, foreground); else if (pid < 0) { /* The fork failed. */ perror ("fork"); exit (1); } else { /* This is the parent process. */ p->pid = pid; if (shell_is_interactive) { if (!j->pgid) j->pgid = pid; setpgid (pid, j->pgid); } } /* Clean up after pipes. */ if (infile != j->stdin) close (infile); if (outfile != j->stdout) close (outfile); infile = mypipe[0]; } format_job_info (j, "launched"); if (!shell_is_interactive) wait_for_job (j); else if (foreground) put_job_in_foreground (j, 0); else put_job_in_background (j, 0); } Foreground and Background ------------------------- Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched. When a foreground job is launched, the shell must first give it access to the controlling terminal by calling `tcsetpgrp'. Then, the shell should wait for processes in that process group to terminate or stop. This is discussed in more detail in *Note Stopped and Terminated Jobs::. When all of the processes in the group have either completed or stopped, the shell should regain control of the terminal for its own process group by calling `tcsetpgrp' again. Since stop signals caused by I/O from a background process or a SUSP character typed by the user are sent to the process group, normally all the processes in the job stop together. The foreground job may have left the terminal in a strange state, so the shell should restore its own saved terminal modes before continuing. In case the job is merely stopped, the shell should first save the current terminal modes so that it can restore them later if the job is continued. The functions for dealing with terminal modes are `tcgetattr' and `tcsetattr'; these are described in *Note Terminal Modes::. Here is the sample shell's function for doing all of this. /* Put job J in the foreground. If CONT is nonzero, restore the saved terminal modes and send the process group a `SIGCONT' signal to wake it up before we block. */ void put_job_in_foreground (job *j, int cont) { /* Put the job into the foreground. */ tcsetpgrp (shell_terminal, j->pgid); /* Send the job a continue signal, if necessary. */ if (cont) { tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes); if (kill (- j->pgid, SIGCONT) < 0) perror ("kill (SIGCONT)"); } /* Wait for it to report. */ wait_for_job (j); /* Put the shell back in the foreground. */ tcsetpgrp (shell_terminal, shell_pgid); /* Restore the shell's terminal modes. */ tcgetattr (shell_terminal, &j->tmodes); tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes); } If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal. In the sample shell, there is not much that needs to be done to put a job into the background. Here is the function it uses: /* Put a job in the background. If the cont argument is true, send the process group a `SIGCONT' signal to wake it up. */ void put_job_in_background (job *j, int cont) { /* Send the job a continue signal, if necessary. */ if (cont) if (kill (-j->pgid, SIGCONT) < 0) perror ("kill (SIGCONT)"); } Stopped and Terminated Jobs --------------------------- When a foreground process is launched, the shell must block until all of the processes in that job have either terminated or stopped. It can do this by calling the `waitpid' function; see *Note Process Completion::. Use the `WUNTRACED' option so that status is reported for processes that stop as well as processes that terminate. The shell must also check on the status of background jobs so that it can report terminated and stopped jobs to the user; this can be done by calling `waitpid' with the `WNOHANG' option. A good place to put a such a check for terminated and stopped jobs is just before prompting for a new command. The shell can also receive asynchronous notification that there is status information available for a child process by establishing a handler for `SIGCHLD' signals. *Note Signal Handling::. In the sample shell program, the `SIGCHLD' signal is normally ignored. This is to avoid reentrancy problems involving the global data structures the shell manipulates. But at specific times when the shell is not using these data structures--such as when it is waiting for input on the terminal--it makes sense to enable a handler for `SIGCHLD'. The same function that is used to do the synchronous status checks (`do_job_notification', in this case) can also be called from within this handler. Here are the parts of the sample shell program that deal with checking the status of jobs and reporting the information to the user. /* Store the status of the process PID that was returned by waitpid. Return 0 if all went well, nonzero otherwise. */ int mark_process_status (pid_t pid, int status) { job *j; process *p; if (pid > 0) { /* Update the record for the process. */ for (j = first_job; j; j = j->next) for (p = j->first_process; p; p = p->next) if (p->pid == pid) { p->status = status; if (WIFSTOPPED (status)) p->stopped = 1; else { p->completed = 1; if (WIFSIGNALED (status)) fprintf (stderr, "%d: Terminated by signal %d.\n", (int) pid, WTERMSIG (p->status)); } return 0; } fprintf (stderr, "No child process %d.\n", pid); return -1; } else if (pid == 0 || errno == ECHILD) /* No processes ready to report. */ return -1; else { /* Other weird errors. */ perror ("waitpid"); return -1; } } /* Check for processes that have status information available, without blocking. */ void update_status (void) { int status; pid_t pid; do pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG); while (!mark_process_status (pid, status)); } /* Check for processes that have status information available, blocking until all processes in the given job have reported. */ void wait_for_job (job *j) { int status; pid_t pid; do pid = waitpid (WAIT_ANY, &status, WUNTRACED); while (!mark_process_status (pid, status) && !job_is_stopped (j) && !job_is_completed (j)); } /* Format information about job status for the user to look at. */ void format_job_info (job *j, const char *status) { fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command); } /* Notify the user about stopped or terminated jobs. Delete terminated jobs from the active job list. */ void do_job_notification (void) { job *j, *jlast, *jnext; process *p; /* Update status information for child processes. */ update_status (); jlast = NULL; for (j = first_job; j; j = jnext) { jnext = j->next; /* If all processes have completed, tell the user the job has completed and delete it from the list of active jobs. */ if (job_is_completed (j)) { format_job_info (j, "completed"); if (jlast) jlast->next = jnext; else first_job = jnext; free_job (j); } /* Notify the user about stopped jobs, marking them so that we won't do this more than once. */ else if (job_is_stopped (j) && !j->notified) { format_job_info (j, "stopped"); j->notified = 1; jlast = j; } /* Don't say anything about jobs that are still running. */ else jlast = j; } } Continuing Stopped Jobs ----------------------- The shell can continue a stopped job by sending a `SIGCONT' signal to its process group. If the job is being continued in the foreground, the shell should first invoke `tcsetpgrp' to give the job access to the terminal, and restore the saved terminal settings. After continuing a job in the foreground, the shell should wait for the job to stop or complete, as if the job had just been launched in the foreground. The sample shell program handles both newly created and continued jobs with the same pair of functions, `put_job_in_foreground' and `put_job_in_background'. The definitions of these functions were given in *Note Foreground and Background::. When continuing a stopped job, a nonzero value is passed as the CONT argument to ensure that the `SIGCONT' signal is sent and the terminal modes reset, as appropriate. This leaves only a function for updating the shell's internal bookkeeping about the job being continued: /* Mark a stopped job J as being running again. */ void mark_job_as_running (job *j) { Process *p; for (p = j->first_process; p; p = p->next) p->stopped = 0; j->notified = 0; } /* Continue the job J. */ void continue_job (job *j, int foreground) { mark_job_as_running (j); if (foreground) put_job_in_foreground (j, 1); else put_job_in_background (j, 1); } The Missing Pieces ------------------ The code extracts for the sample shell included in this chapter are only a part of the entire shell program. In particular, nothing at all has been said about how `job' and `program' data structures are allocated and initialized. Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like. All of this is far too complicated to explain here! Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell. Here is a table summarizing the major entry points we have presented: `void init_shell (void)' Initialize the shell's internal state. *Note Initializing the Shell::. `void launch_job (job *J, int FOREGROUND)' Launch the job J as either a foreground or background job. *Note Launching Jobs::. `void do_job_notification (void)' Check for and report any jobs that have terminated or stopped. Can be called synchronously or within a handler for `SIGCHLD' signals. *Note Stopped and Terminated Jobs::. `void continue_job (job *J, int FOREGROUND)' Continue the job J. *Note Continuing Stopped Jobs::. Of course, a real shell would also want to provide other functions for managing jobs. For example, it would be useful to have commands to list all active jobs or to send a signal (such as `SIGKILL') to a job. Functions for Job Control ========================= This section contains detailed descriptions of the functions relating to job control. Identifying the Controlling Terminal ------------------------------------ You can use the `ctermid' function to get a file name that you can use to open the controlling terminal. In the GNU library, it returns the same string all the time: `"/dev/tty"'. That is a special "magic" file name that refers to the controlling terminal of the current process (if it has one). To find the name of the specific terminal device, use `ttyname'; *note Is It a Terminal::. The function `ctermid' is declared in the header file `stdio.h'. - Function: char * ctermid (char *STRING) The `ctermid' function returns a string containing the file name of the controlling terminal for the current process. If STRING is not a null pointer, it should be an array that can hold at least `L_ctermid' characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned, which might get overwritten on subsequent calls to this function. An empty string is returned if the file name cannot be determined for any reason. Even if a file name is returned, access to the file it represents is not guaranteed. - Macro: int L_ctermid The value of this macro is an integer constant expression that represents the size of a string large enough to hold the file name returned by `ctermid'. See also the `isatty' and `ttyname' functions, in *Note Is It a Terminal::. Process Group Functions ----------------------- Here are descriptions of the functions for manipulating process groups. Your program should include the header files `sys/types.h' and `unistd.h' to use these functions. - Function: pid_t setsid (void) The `setsid' function creates a new session. The calling process becomes the session leader, and is put in a new process group whose process group ID is the same as the process ID of that process. There are initially no other processes in the new process group, and no other process groups in the new session. This function also makes the calling process have no controlling terminal. The `setsid' function returns the new process group ID of the calling process if successful. A return value of `-1' indicates an error. The following `errno' error conditions are defined for this function: `EPERM' The calling process is already a process group leader, or there is already another process group around that has the same process group ID. - Function: pid_t getsid (pid_t PID) The `getsid' function returns the process group ID of the session leader of the specified process. If a PID is `0', the process group ID of the session leader of the current process is returned. In case of error `-1' is returned and `errno' is set. The following `errno' error conditions are defined for this function: `ESRCH' There is no process with the given process ID PID. `EPERM' The calling process and the process specified by PID are in different sessions, and the implementation doesn't allow to access the process group ID of the session leader of the process with ID PID from the calling process. The `getpgrp' function has two definitions: one derived from BSD Unix, and one from the POSIX.1 standard. The feature test macros you have selected (*note Feature Test Macros::) determine which definition you get. Specifically, you get the BSD version if you define `_BSD_SOURCE'; otherwise, you get the POSIX version if you define `_POSIX_SOURCE' or `_GNU_SOURCE'. Programs written for old BSD systems will not include `unistd.h', which defines `getpgrp' specially under `_BSD_SOURCE'. You must link such programs with the `-lbsd-compat' option to get the BSD definition. - POSIX.1 Function: pid_t getpgrp (void) The POSIX.1 definition of `getpgrp' returns the process group ID of the calling process. - BSD Function: pid_t getpgrp (pid_t PID) The BSD definition of `getpgrp' returns the process group ID of the process PID. You can supply a value of `0' for the PID argument to get information about the calling process. - System V Function: int getpgid (pid_t PID) `getpgid' is the same as the BSD function `getpgrp'. It returns the process group ID of the process PID. You can supply a value of `0' for the PID argument to get information about the calling process. In case of error `-1' is returned and `errno' is set. The following `errno' error conditions are defined for this function: `ESRCH' There is no process with the given process ID PID. The calling process and the process specified by PID are in different sessions, and the implementation doesn't allow to access the process group ID of the process with ID PID from the calling process. - Function: int setpgid (pid_t PID, pid_t PGID) The `setpgid' function puts the process PID into the process group PGID. As a special case, either PID or PGID can be zero to indicate the process ID of the calling process. This function fails on a system that does not support job control. *Note Job Control is Optional::, for more information. If the operation is successful, `setpgid' returns zero. Otherwise it returns `-1'. The following `errno' error conditions are defined for this function: `EACCES' The child process named by PID has executed an `exec' function since it was forked. `EINVAL' The value of the PGID is not valid. `ENOSYS' The system doesn't support job control. `EPERM' The process indicated by the PID argument is a session leader, or is not in the same session as the calling process, or the value of the PGID argument doesn't match a process group ID in the same session as the calling process. `ESRCH' The process indicated by the PID argument is not the calling process or a child of the calling process. - Function: int setpgrp (pid_t PID, pid_t PGID) This is the BSD Unix name for `setpgid'. Both functions do exactly the same thing. Functions for Controlling Terminal Access ----------------------------------------- These are the functions for reading or setting the foreground process group of a terminal. You should include the header files `sys/types.h' and `unistd.h' in your application to use these functions. Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor. - Function: pid_t tcgetpgrp (int FILEDES) This function returns the process group ID of the foreground process group associated with the terminal open on descriptor FILEDES. If there is no foreground process group, the return value is a number greater than `1' that does not match the process group ID of any existing process group. This can happen if all of the processes in the job that was formerly the foreground job have terminated, and no other job has yet been moved into the foreground. In case of an error, a value of `-1' is returned. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `ENOSYS' The system doesn't support job control. `ENOTTY' The terminal file associated with the FILEDES argument isn't the controlling terminal of the calling process. - Function: int tcsetpgrp (int FILEDES, pid_t PGID) This function is used to set a terminal's foreground process group ID. The argument FILEDES is a descriptor which specifies the terminal; PGID specifies the process group. The calling process must be a member of the same session as PGID and must have the same controlling terminal. For terminal access purposes, this function is treated as output. If it is called from a background process on its controlling terminal, normally all processes in the process group are sent a `SIGTTOU' signal. The exception is if the calling process itself is ignoring or blocking `SIGTTOU' signals, in which case the operation is performed and no signal is sent. If successful, `tcsetpgrp' returns `0'. A return value of `-1' indicates an error. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The PGID argument is not valid. `ENOSYS' The system doesn't support job control. `ENOTTY' The FILEDES isn't the controlling terminal of the calling process. `EPERM' The PGID isn't a process group in the same session as the calling process. - Function: pid_t tcgetsid (int FILDES) This function is used to obtain the process group ID of the session for which the terminal specified by FILDES is the controlling terminal. If the call is successful the group ID is returned. Otherwise the return value is `(pid_t) -1' and the global variable ERRNO is set to the following value: `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTTY' The calling process does not have a controlling terminal, or the file is not the controlling terminal. System Databases and Name Service Switch **************************************** Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done by using files (e.g., `/etc/passwd'), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service (DNS)) became popular, and were hacked into the C library, usually with a fixed search order (*note frobnicate: (jargon)frobnicate.). The GNU C Library contains a cleaner solution of this problem. It is designed after a method used by Sun Microsystems in the C library of Solaris 2. GNU C Library follows their name and calls this scheme "Name Service Switch" (NSS). Though the interface might be similar to Sun's version there is no common code. We never saw any source code of Sun's implementation and so the internal interface is incompatible. This also manifests in the file names we use as we will see later. NSS Basics ========== The basic idea is to put the implementation of the different services offered to access the databases in separate modules. This has some advantages: 1. Contributors can add new services without adding them to GNU C Library. 2. The modules can be updated separately. 3. The C library image is smaller. To fulfill the first goal above the ABI of the modules will be described below. For getting the implementation of a new service right it is important to understand how the functions in the modules get called. They are in no way designed to be used by the programmer directly. Instead the programmer should only use the documented and standardized functions to access the databases. The databases available in the NSS are `aliases' Mail aliases `ethers' Ethernet numbers, `group' Groups of users, *note Group Database::. `hosts' Host names and numbers, *note Host Names::. `netgroup' Network wide list of host and users, *note Netgroup Database::. `networks' Network names and numbers, *note Networks Database::. `protocols' Network protocols, *note Protocols Database::. `passwd' User passwords, *note User Database::. `rpc' Remote procedure call names and numbers, `services' Network services, *note Services Database::. `shadow' Shadow user passwords, There will be some more added later (`automount', `bootparams', `netmasks', and `publickey'). The NSS Configuration File ========================== Somehow the NSS code must be told about the wishes of the user. For this reason there is the file `/etc/nsswitch.conf'. For each database this file contain a specification how the lookup process should work. The file could look like this: # /etc/nsswitch.conf # # Name Service Switch configuration file. # passwd: db files nis shadow: files group: db files nis hosts: files nisplus nis dns networks: nisplus [NOTFOUND=return] files ethers: nisplus [NOTFOUND=return] db files protocols: nisplus [NOTFOUND=return] db files rpc: nisplus [NOTFOUND=return] db files services: nisplus [NOTFOUND=return] db files The first column is the database as you can guess from the table above. The rest of the line specifies how the lookup process works. Please note that you specify the way it works for each database individually. This cannot be done with the old way of a monolithic implementation. The configuration specification for each database can contain two different items: * the service specification like `files', `db', or `nis'. * the reaction on lookup result like `[NOTFOUND=return]'. Services in the NSS configuration File -------------------------------------- The above example file mentions four different services: `files', `db', `nis', and `nisplus'. This does not mean these services are available on all sites and it does also not mean these are all the services which will ever be available. In fact, these names are simply strings which the NSS code uses to find the implicitly addressed functions. The internal interface will be described later. Visible to the user are the modules which implement an individual service. Assume the service NAME shall be used for a lookup. The code for this service is implemented in a module called `libnss_NAME'. On a system supporting shared libraries this is in fact a shared library with the name (for example) `libnss_NAME.so.2'. The number at the end is the currently used version of the interface which will not change frequently. Normally the user should not have to be cognizant of these files since they should be placed in a directory where they are found automatically. Only the names of all available services are important. Actions in the NSS configuration -------------------------------- The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two service names and are written within brackets. The general form is `[' ( `!'? STATUS `=' ACTION )+ `]' where STATUS => success | notfound | unavail | tryagain ACTION => return | continue The case of the keywords is insignificant. The STATUS values are the results of a call to a lookup function of a specific service. They mean `success' No error occurred and the wanted entry is returned. The default action for this is `return'. `notfound' The lookup process works ok but the needed value was not found. The default action is `continue'. `unavail' The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is not available or does not allow queries. The default action is `continue'. `tryagain' The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connections. The default action is `continue'. If we have a line like ethers: nisplus [NOTFOUND=return] db files this is equivalent to ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue TRYAGAIN=continue] db [SUCCESS=return NOTFOUND=continue UNAVAIL=continue TRYAGAIN=continue] files (except that it would have to be written on one line). The default value for the actions are normally what you want, and only need to be changed in exceptional cases. If the optional `!' is placed before the STATUS this means the following action is used for all statuses but STATUS itself. I.e., `!' is negation as in the C language (and others). Before we explain the exception which makes this action item necessary one more remark: obviously it makes no sense to add another action item after the `files' service. Since there is no other service following the action _always_ is `return'. Now, why is this `[NOTFOUND=return]' action useful? To understand this we should know that the `nisplus' service is often complete; i.e., if an entry is not available in the NIS+ tables it is not available anywhere else. This is what is expressed by this action item: it is useless to examine further services since they will not give us a result. The situation would be different if the NIS+ service is not available because the machine is booting. In this case the return value of the lookup function is not `notfound' but instead `unavail'. And as you can see in the complete form above: in this situation the `db' and `files' services are used. Neat, isn't it? The system administrator need not pay special care for the time the system is not completely ready to work (while booting or shutdown or network problems). Notes on the NSS Configuration File ----------------------------------- Finally a few more hints. The NSS implementation is not completely helpless if `/etc/nsswitch.conf' does not exist. For all supported databases there is a default value so it should normally be possible to get the system running even if the file is corrupted or missing. For the `hosts' and `networks' databases the default value is `dns [!UNAVAIL=return] files'. I.e., the system is prepared for the DNS service not to be available but if it is available the answer it returns is definitive. The `passwd', `group', and `shadow' databases are traditionally handled in a special way. The appropriate files in the `/etc' directory are read but if an entry with a name starting with a `+' character is found NIS is used. This kind of lookup remains possible by using the special lookup service `compat' and the default value for the three databases above is `compat [NOTFOUND=return] files'. For all other databases the default value is `nis [NOTFOUND=return] files'. This solution give the best chance to be correct since NIS and file based lookup is used. A second point is that the user should try to optimize the lookup process. The different service have different response times. A simple file look up on a local file could be fast, but if the file is long and the needed entry is near the end of the file this may take quite some time. In this case it might be better to use the `db' service which allows fast local access to large data sets. Often the situation is that some global information like NIS must be used. So it is unavoidable to use service entries like `nis' etc. But one should avoid slow services like this if possible. NSS Module Internals ==================== Now it is time to describe what the modules look like. The functions contained in a module are identified by their names. I.e., there is no jump table or the like. How this is done is of no interest here; those interested in this topic should read about Dynamic Linking. The Naming Scheme of the NSS Modules ------------------------------------ The name of each function consist of various parts: _nss_SERVICE_FUNCTION SERVICE of course corresponds to the name of the module this function is found in.(1) The FUNCTION part is derived from the interface function in the C library itself. If the user calls the function `gethostbyname' and the service used is `files' the function _nss_files_gethostbyname_r in the module libnss_files.so.2 is used. You see, what is explained above in not the whole truth. In fact the NSS modules only contain reentrant versions of the lookup functions. I.e., if the user would call the `gethostbyname_r' function this also would end in the above function. For all user interface functions the C library maps this call to a call to the reentrant function. For reentrant functions this is trivial since the interface is (nearly) the same. For the non-reentrant version The library keeps internal buffers which are used to replace the user supplied buffer. I.e., the reentrant functions _can_ have counterparts. No service module is forced to have functions for all databases and all kinds to access them. If a function is not available it is simply treated as if the function would return `unavail' (*note Actions in the NSS configuration::). The file name `libnss_files.so.2' would be on a Solaris 2 system `nss_files.so.2'. This is the difference mentioned above. Sun's NSS modules are usable as modules which get indirectly loaded only. The NSS modules in the GNU C Library are prepared to be used as normal libraries themselves. This is _not_ true at the moment, though. However, the organization of the name space in the modules does not make it impossible like it is for Solaris. Now you can see why the modules are still libraries.(2) ---------- Footnotes ---------- (1) Now you might ask why this information is duplicated. The answer is that we want to make it possible to link directly with these shared objects. (2) There is a second explanation: we were too lazy to change the Makefiles to allow the generation of shared objects not starting with `lib' but don't tell this to anybody. The Interface of the Function in NSS Modules -------------------------------------------- Now we know about the functions contained in the modules. It is now time to describe the types. When we mentioned the reentrant versions of the functions above, this means there are some additional arguments (compared with the standard, non-reentrant version). The prototypes for the non-reentrant and reentrant versions of our function above are: struct hostent *gethostbyname (const char *name) int gethostbyname_r (const char *name, struct hostent *result_buf, char *buf, size_t buflen, struct hostent **result, int *h_errnop) The actual prototype of the function in the NSS modules in this case is enum nss_status _nss_files_gethostbyname_r (const char *name, struct hostent *result_buf, char *buf, size_t buflen, int *errnop, int *h_errnop) I.e., the interface function is in fact the reentrant function with the change of the return value and the omission of the RESULT parameter. While the user-level function returns a pointer to the result the reentrant function return an `enum nss_status' value: `NSS_STATUS_TRYAGAIN' numeric value `-2' `NSS_STATUS_UNAVAIL' numeric value `-1' `NSS_STATUS_NOTFOUND' numeric value `0' `NSS_STATUS_SUCCESS' numeric value `1' Now you see where the action items of the `/etc/nsswitch.conf' file are used. If you study the source code you will find there is a fifth value: `NSS_STATUS_RETURN'. This is an internal use only value, used by a few functions in places where none of the above value can be used. If necessary the source code should be examined to learn about the details. In case the interface function has to return an error it is important that the correct error code is stored in `*ERRNOP'. Some return status value have only one associated error code, others have more. `NSS_STATUS_TRYAGAIN' `EAGAIN' One of the functions used ran temporarily out of resources or a service is currently not available. `ERANGE' The provided buffer is not large enough. The function should be called again with a larger buffer. `NSS_STATUS_UNAVAIL' `ENOENT' A necessary input file cannot be found. `NSS_STATUS_NOTFOUND' `ENOENT' The requested entry is not available. These are proposed values. There can be other error codes and the described error codes can have different meaning. *With one exception:* when returning `NSS_STATUS_TRYAGAIN' the error code `ERANGE' _must_ mean that the user provided buffer is too small. Everything is non-critical. The above function has something special which is missing for almost all the other module functions. There is an argument H_ERRNOP. This points to a variable which will be filled with the error code in case the execution of the function fails for some reason. The reentrant function cannot use the global variable H_ERRNO; `gethostbyname' calls `gethostbyname_r' with the last argument set to `&h_errno'. The `getXXXbyYYY' functions are the most important functions in the NSS modules. But there are others which implement the other ways to access system databases (say for the password database, there are `setpwent', `getpwent', and `endpwent'). These will be described in more detail later. Here we give a general way to determine the signature of the module function: * the return value is `int'; * the name is as explained in *note NSS Module Names::; * the first arguments are identical to the arguments of the non-reentrant function; * the next three arguments are: `STRUCT_TYPE *result_buf' pointer to buffer where the result is stored. `STRUCT_TYPE' is normally a struct which corresponds to the database. `char *buffer' pointer to a buffer where the function can store additional data for the result etc. `size_t buflen' length of the buffer pointed to by BUFFER. * possibly a last argument H_ERRNOP, for the host name and network name lookup functions. This table is correct for all functions but the `set...ent' and `end...ent' functions. Extending NSS ============= One of the advantages of NSS mentioned above is that it can be extended quite easily. There are two ways in which the extension can happen: adding another database or adding another service. The former is normally done only by the C library developers. It is here only important to remember that adding another database is independent from adding another service because a service need not support all databases or lookup functions. A designer/implementor of a new service is therefore free to choose the databases s/he is interested in and leave the rest for later (or completely aside). Adding another Service to NSS ----------------------------- The sources for a new service need not (and should not) be part of the GNU C Library itself. The developer retains complete control over the sources and its development. The links between the C library and the new service module consists solely of the interface functions. Each module is designed following a specific interface specification. For now the version is 2 (the interface in version 1 was not adequate) and this manifests in the version number of the shared library object of the NSS modules: they have the extension `.2'. If the interface changes again in an incompatible way, this number will be increased. Modules using the old interface will still be usable. Developers of a new service will have to make sure that their module is created using the correct interface number. This means the file itself must have the correct name and on ELF systems the "soname" (Shared Object Name) must also have this number. Building a module from a bunch of object files on an ELF system using GNU CC could be done like this: gcc -shared -o libnss_NAME.so.2 -Wl,-soname,libnss_NAME.so.2 OBJECTS *Note Options for Linking: (gcc)Link Options, to learn more about this command line. To use the new module the library must be able to find it. This can be achieved by using options for the dynamic linker so that it will search the directory where the binary is placed. For an ELF system this could be done by adding the wanted directory to the value of `LD_LIBRARY_PATH'. But this is not always possible since some programs (those which run under IDs which do not belong to the user) ignore this variable. Therefore the stable version of the module should be placed into a directory which is searched by the dynamic linker. Normally this should be the directory `$prefix/lib', where `$prefix' corresponds to the value given to configure using the `--prefix' option. But be careful: this should only be done if it is clear the module does not cause any harm. System administrators should be careful. Internals of the NSS Module Functions ------------------------------------- Until now we only provided the syntactic interface for the functions in the NSS module. In fact there is not much more we can say since the implementation obviously is different for each function. But a few general rules must be followed by all functions. In fact there are four kinds of different functions which may appear in the interface. All derive from the traditional ones for system databases. DB in the following table is normally an abbreviation for the database (e.g., it is `pw' for the password database). `enum nss_status _nss_DATABASE_setDBent (void)' This function prepares the service for following operations. For a simple file based lookup this means files could be opened, for other services this function simply is a noop. One special case for this function is that it takes an additional argument for some DATABASEs (i.e., the interface is `int setDBent (int)'). *Note Host Names::, which describes the `sethostent' function. The return value should be NSS_STATUS_SUCCESS or according to the table above in case of an error (*note NSS Modules Interface::). `enum nss_status _nss_DATABASE_endDBent (void)' This function simply closes all files which are still open or removes buffer caches. If there are no files or buffers to remove this is again a simple noop. There normally is no return value different to NSS_STATUS_SUCCESS. `enum nss_status _nss_DATABASE_getDBent_r (STRUCTURE *result, char *buffer, size_t buflen, int *errnop)' Since this function will be called several times in a row to retrieve one entry after the other it must keep some kind of state. But this also means the functions are not really reentrant. They are reentrant only in that simultaneous calls to this function will not try to write the retrieved data in the same place (as it would be the case for the non-reentrant functions); instead, it writes to the structure pointed to by the RESULT parameter. But the calls share a common state and in the case of a file access this means they return neighboring entries in the file. The buffer of length BUFLEN pointed to by BUFFER can be used for storing some additional data for the result. It is _not_ guaranteed that the same buffer will be passed for the next call of this function. Therefore one must not misuse this buffer to save some state information from one call to another. Before the function returns the implementation should store the value of the local ERRNO variable in the variable pointed to be ERRNOP. This is important to guarantee the module working in statically linked programs. As explained above this function could also have an additional last argument. This depends on the database used; it happens only for `host' and `networks'. The function shall return `NSS_STATUS_SUCCESS' as long as there are more entries. When the last entry was read it should return `NSS_STATUS_NOTFOUND'. When the buffer given as an argument is too small for the data to be returned `NSS_STATUS_TRYAGAIN' should be returned. When the service was not formerly initialized by a call to `_nss_DATABASE_setDBent' all return value allowed for this function can also be returned here. `enum nss_status _nss_DATABASE_getDBbyXX_r (PARAMS, STRUCTURE *result, char *buffer, size_t buflen, int *errnop)' This function shall return the entry from the database which is addressed by the PARAMS. The type and number of these arguments vary. It must be individually determined by looking to the user-level interface functions. All arguments given to the non-reentrant version are here described by PARAMS. The result must be stored in the structure pointed to by RESULT. If there is additional data to return (say strings, where the RESULT structure only contains pointers) the function must use the BUFFER or length BUFLEN. There must not be any references to non-constant global data. The implementation of this function should honor the STAYOPEN flag set by the `setDBent' function whenever this makes sense. Before the function returns the implementation should store the value of the local ERRNO variable in the variable pointed to be ERRNOP. This is important to guarantee the module working in statically linked programs. Again, this function takes an additional last argument for the `host' and `networks' database. The return value should as always follow the rules given above (*note NSS Modules Interface::). Users and Groups **************** Every user who can log in on the system is identified by a unique number called the "user ID". Each process has an effective user ID which says which user's access permissions it has. Users are classified into "groups" for access control purposes. Each process has one or more "group ID values" which say which groups the process can use for access to files. The effective user and group IDs of a process collectively form its "persona". This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions. Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process. The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases. User and Group IDs ================== Each user account on a computer system is identified by a "user name" (or "login name") and "user ID". Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in *Note User Database::. Users are classified in "groups". Each user name belongs to one "default group" and may also belong to any number of "supplementary groups". Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group. Each group has a "group name" and "group ID". *Note Group Database::, for how to find information about a group ID or group name. The Persona of a Process ======================== At any time, each process has an "effective user ID", a "effective group ID", and a set of "supplementary group IDs". These IDs determine the privileges of the process. They are collectively called the "persona" of the process, because they determine "who it is" for purposes of access control. Your login shell starts out with a persona which consists of your user ID, your default group ID, and your supplementary group IDs (if you are in more than one group). In normal circumstances, all your other processes inherit these values. A process also has a "real user ID" which identifies the user who created the process, and a "real group ID" which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important. Both the real and effective user ID can be changed during the lifetime of a process. *Note Why Change Persona::. For details on how a process's effective user ID and group IDs affect its permission to access files, see *Note Access Permission::. The effective user ID of a process also controls permissions for sending signals using the `kill' function. *Note Signaling Another Process::. Finally, there are many operations which can only be performed by a process whose effective user ID is zero. A process with this user ID is a "privileged process". Commonly the user name `root' is associated with user ID 0, but there may be other user names with this ID. Why Change the Persona of a Process? ==================================== The most obvious situation where it is necessary for a process to change its user and/or group IDs is the `login' program. When `login' starts running, its user ID is `root'. Its job is to start a shell whose user and group IDs are those of the user who is logging in. (To accomplish this fully, `login' must set the real user and group IDs as well as its persona. But this is a special case.) The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it. For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource. Thus, imagine a game program that saves scores in a file. The game program itself needs to be able to update this file no matter who is running it, but if users can write the file without going through the game, they can give themselves any scores they like. Some people consider this undesirable, or even reprehensible. It can be prevented by creating a new user ID and login name (say, `games') to own the scores file, and make the file writable only by this user. Then, when the game program wants to update this file, it can change its effective user ID to be that for `games'. In effect, the program must adopt the persona of `games' so it can write the scores file. How an Application Can Change Persona ===================================== The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances. You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. *Note Permission Bits::. When the setuid bit of an executable file is on, executing that file gives the process a third user ID: the "file user ID". This ID is set to the owner ID of the file. The system then changes the effective user ID to the file user ID. The real user ID remains as it was. Likewise, if the setgid bit is on, the process is given a "file group ID" equal to the group ID of the file, and its effective group ID is changed to the file group ID. If a process has a file ID (user or group), then it can at any time change its effective ID to its real ID and back to its file ID. Programs use this feature to relinquish their special privileges except when they actually need them. This makes it less likely that they can be tricked into doing something inappropriate with their privileges. *Portability Note:* Older systems do not have file IDs. To determine if a system has this feature, you can test the compiler define `_POSIX_SAVED_IDS'. (In the POSIX standard, file IDs are known as saved IDs.) *Note File Attributes::, for a more general discussion of file modes and accessibility. Reading the Persona of a Process ================================ Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'. - Data Type: uid_t This is an integer data type used to represent user IDs. In the GNU library, this is an alias for `unsigned int'. - Data Type: gid_t This is an integer data type used to represent group IDs. In the GNU library, this is an alias for `unsigned int'. - Function: uid_t getuid (void) The `getuid' function returns the real user ID of the process. - Function: gid_t getgid (void) The `getgid' function returns the real group ID of the process. - Function: uid_t geteuid (void) The `geteuid' function returns the effective user ID of the process. - Function: gid_t getegid (void) The `getegid' function returns the effective group ID of the process. - Function: int getgroups (int COUNT, gid_t *GROUPS) The `getgroups' function is used to inquire about the supplementary group IDs of the process. Up to COUNT of these group IDs are stored in the array GROUPS; the return value from the function is the number of group IDs actually stored. If COUNT is smaller than the total number of supplementary group IDs, then `getgroups' returns a value of `-1' and `errno' is set to `EINVAL'. If COUNT is zero, then `getgroups' just returns the total number of supplementary group IDs. On systems that do not support supplementary groups, this will always be zero. Here's how to use `getgroups' to read all the supplementary group IDs: gid_t * read_all_groups (void) { int ngroups = getgroups (0, NULL); gid_t *groups = (gid_t *) xmalloc (ngroups * sizeof (gid_t)); int val = getgroups (ngroups, groups); if (val < 0) { free (groups); return NULL; } return groups; } Setting the User ID =================== This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'. - Function: int seteuid (uid_t NEWEUID) This function sets the effective user ID of a process to NEWUID, provided that the process is allowed to change its effective user ID. A privileged process (effective user ID zero) can change its effective user ID to any legal value. An unprivileged process with a file user ID can change its effective user ID to its real user ID or to its file user ID. Otherwise, a process may not change its effective user ID at all. The `seteuid' function returns a value of `0' to indicate successful completion, and a value of `-1' to indicate an error. The following `errno' error conditions are defined for this function: `EINVAL' The value of the NEWUID argument is invalid. `EPERM' The process may not change to the specified ID. Older systems (those without the `_POSIX_SAVED_IDS' feature) do not have this function. - Function: int setuid (uid_t NEWUID) If the calling process is privileged, this function sets both the real and effective user ID of the process to NEWUID. It also deletes the file user ID of the process, if any. NEWUID may be any legal value. (Once this has been done, there is no way to recover the old effective user ID.) If the process is not privileged, and the system supports the `_POSIX_SAVED_IDS' feature, then this function behaves like `seteuid'. The return values and error conditions are the same as for `seteuid'. - Function: int setreuid (uid_t RUID, uid_t EUID) This function sets the real user ID of the process to RUID and the effective user ID to EUID. If RUID is `-1', it means not to change the real user ID; likewise if EUID is `-1', it means not to change the effective user ID. The `setreuid' function exists for compatibility with 4.3 BSD Unix, which does not support file IDs. You can use this function to swap the effective and real user IDs of the process. (Privileged processes are not limited to this particular usage.) If file IDs are supported, you should use that feature instead of this function. *Note Enable/Disable Setuid::. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EPERM' The process does not have the appropriate privileges; you do not have permission to change to the specified ID. Setting the Group IDs ===================== This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files `sys/types.h' and `unistd.h'. - Function: int setegid (gid_t NEWGID) This function sets the effective group ID of the process to NEWGID, provided that the process is allowed to change its group ID. Just as with `seteuid', if the process is privileged it may change its effective group ID to any value; if it isn't, but it has a file group ID, then it may change to its real group ID or file group ID; otherwise it may not change its effective group ID. Note that a process is only privileged if its effective _user_ ID is zero. The effective group ID only affects access permissions. The return values and error conditions for `setegid' are the same as those for `seteuid'. This function is only present if `_POSIX_SAVED_IDS' is defined. - Function: int setgid (gid_t NEWGID) This function sets both the real and effective group ID of the process to NEWGID, provided that the process is privileged. It also deletes the file group ID, if any. If the process is not privileged, then `setgid' behaves like `setegid'. The return values and error conditions for `setgid' are the same as those for `seteuid'. - Function: int setregid (gid_t RGID, gid_t EGID) This function sets the real group ID of the process to RGID and the effective group ID to EGID. If RGID is `-1', it means not to change the real group ID; likewise if EGID is `-1', it means not to change the effective group ID. The `setregid' function is provided for compatibility with 4.3 BSD Unix, which does not support file IDs. You can use this function to swap the effective and real group IDs of the process. (Privileged processes are not limited to this usage.) If file IDs are supported, you should use that feature instead of using this function. *Note Enable/Disable Setuid::. The return values and error conditions for `setregid' are the same as those for `setreuid'. `setuid' and `setgid' behave differently depending on whether the effective user ID at the time is zero. If it is not zero, they behave like `seteuid' and `setegid'. If it is, they change both effective and real IDs and delete the file ID. To avoid confusion, we recommend you always use `seteuid' and `setegid' except when you know the effective user ID is zero and your intent is to change the persona permanently. This case is rare--most of the programs that need it, such as `login' and `su', have already been written. Note that if your program is setuid to some user other than `root', there is no way to drop privileges permanently. The system also lets privileged processes change their supplementary group IDs. To use `setgroups' or `initgroups', your programs should include the header file `grp.h'. - Function: int setgroups (size_t COUNT, gid_t *GROUPS) This function sets the process's supplementary group IDs. It can only be called from privileged processes. The COUNT argument specifies the number of group IDs in the array GROUPS. This function returns `0' if successful and `-1' on error. The following `errno' error conditions are defined for this function: `EPERM' The calling process is not privileged. - Function: int initgroups (const char *USER, gid_t GROUP) The `initgroups' function sets the process's supplementary group IDs to be the normal default for the user name USER. The group GROUP is automatically included. This function works by scanning the group database for all the groups USER belongs to. It then calls `setgroups' with the list it has constructed. The return values and error conditions are the same as for `setgroups'. If you are interested in the groups a particular user belongs to, but do not want to change the process's supplementary group IDs, you can use `getgrouplist'. To use `getgrouplist', your programs should include the header file `grp.h'. - Function: int getgrouplist (const char *USER, gid_t GROUP, gid_t *GROUPS, int *NGROUPS) The `getgrouplist' function scans the group database for all the groups USER belongs to. Up to *NGROUPS group IDs corresponding to these groups are stored in the array GROUPS; the return value from the function is the number of group IDs actually stored. If *NGROUPS is smaller than the total number of groups found, then `getgrouplist' returns a value of `-1' and stores the actual number of groups in *NGROUPS. The group GROUP is automatically included in the list of groups returned by `getgrouplist'. Here's how to use `getgrouplist' to read all supplementary groups for USER: gid_t * supplementary_groups (char *user) { int ngroups = 16; gid_t *groups = (gid_t *) xmalloc (ngroups * sizeof (gid_t)); struct passwd *pw = getpwnam (user); if (pw == NULL) return NULL; if (getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups) < 0) { groups = xrealloc (ngroups * sizeof (gid_t)); getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups); } return groups; } Enabling and Disabling Setuid Access ==================================== A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access. If the system supports the `_POSIX_SAVED_IDS' feature, you can accomplish this with `seteuid'. When the game program starts, its real user ID is `jdoe', its effective user ID is `games', and its saved user ID is also `games'. The program should record both user ID values once at the beginning, like this: user_user_id = getuid (); game_user_id = geteuid (); Then it can turn off game file access with seteuid (user_user_id); and turn it on with seteuid (game_user_id); Throughout this process, the real user ID remains `jdoe' and the file user ID remains `games', so the program can always set its effective user ID to either one. On other systems that don't support file user IDs, you can turn setuid access on and off by using `setreuid' to swap the real and effective user IDs of the process, as follows: setreuid (geteuid (), getuid ()); This special case is always allowed--it cannot fail. Why does this have the effect of toggling the setuid access? Suppose a game program has just started, and its real user ID is `jdoe' while its effective user ID is `games'. In this state, the game can write the scores file. If it swaps the two uids, the real becomes `games' and the effective becomes `jdoe'; now the program has only `jdoe' access. Another swap brings `games' back to the effective user ID and restores access to the scores file. In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this: #ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif Setuid Program Example ====================== Here's an example showing how to set up a program that changes its effective user ID. This is part of a game program called `caber-toss' that manipulates a file `scores' that should be writable only by the game program itself. The program assumes that its executable file will be installed with the setuid bit set and owned by the same user as the `scores' file. Typically, a system administrator will set up an account like `games' for this purpose. The executable file is given mode `4755', so that doing an `ls -l' on it produces output like: -rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss The setuid bit shows up in the file modes as the `s'. The scores file is given mode `644', and doing an `ls -l' on it shows: -rw-r--r-- 1 games 0 Jul 31 15:33 scores Here are the parts of the program that show how to set up the changed user ID. This program is conditionalized so that it makes use of the file IDs feature if it is supported, and otherwise uses `setreuid' to swap the effective and real user IDs. #include #include #include #include /* Remember the effective and real UIDs. */ static uid_t euid, ruid; /* Restore the effective UID to its original value. */ void do_setuid (void) { int status; #ifdef _POSIX_SAVED_IDS status = seteuid (euid); #else status = setreuid (ruid, euid); #endif if (status < 0) { fprintf (stderr, "Couldn't set uid.\n"); exit (status); } } /* Set the effective UID to the real UID. */ void undo_setuid (void) { int status; #ifdef _POSIX_SAVED_IDS status = seteuid (ruid); #else status = setreuid (euid, ruid); #endif if (status < 0) { fprintf (stderr, "Couldn't set uid.\n"); exit (status); } } /* Main program. */ int main (void) { /* Remember the real and effective user IDs. */ ruid = getuid (); euid = geteuid (); undo_setuid (); /* Do the game and record the score. */ ... } Notice how the first thing the `main' function does is to set the effective user ID back to the real user ID. This is so that any other file accesses that are performed while the user is playing the game use the real user ID for determining permissions. Only when the program needs to open the scores file does it switch back to the file user ID, like this: /* Record the score. */ int record_score (int score) { FILE *stream; char *myname; /* Open the scores file. */ do_setuid (); stream = fopen (SCORES_FILE, "a"); undo_setuid (); /* Write the score to the file. */ if (stream) { myname = cuserid (NULL); if (score < 0) fprintf (stream, "%10s: Couldn't lift the caber.\n", myname); else fprintf (stream, "%10s: %d feet.\n", myname, score); fclose (stream); return 0; } else return -1; } Tips for Writing Setuid Programs ================================ It is easy for setuid programs to give the user access that isn't intended--in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur: * Don't have `setuid' programs with privileged user IDs such as `root' unless it is absolutely necessary. If the resource is specific to your particular program, it's better to define a new, nonprivileged user ID or group ID just to manage that resource. It's better if you can write your program to use a special group than a special user. * Be cautious about using the `exec' functions in combination with changing the effective user ID. Don't let users of your program execute arbitrary programs under a changed user ID. Executing a shell is especially bad news. Less obviously, the `execlp' and `execvp' functions are a potential risk (since the program they execute depends on the user's `PATH' environment variable). If you must `exec' another program under a changed ID, specify an absolute file name (*note File Name Resolution::) for the executable, and make sure that the protections on that executable and _all_ containing directories are such that ordinary users cannot replace it with some other program. You should also check the arguments passed to the program to make sure they do not have unexpected effects. Likewise, you should examine the environment variables. Decide which arguments and variables are safe, and reject all others. You should never use `system' in a privileged program, because it invokes a shell. * Only use the user ID controlling the resource in the part of the program that actually uses that resource. When you're finished with it, restore the effective user ID back to the actual user's user ID. *Note Enable/Disable Setuid::. * If the `setuid' part of your program needs to access other files besides the controlled resource, it should verify that the real user would ordinarily have permission to access those files. You can use the `access' function (*note Access Permission::) to check this; it uses the real user and group IDs, rather than the effective IDs. Identifying Who Logged In ========================= You can use the functions listed in this section to determine the login name of the user who is running a process, and the name of the user who logged in the current session. See also the function `getuid' and friends (*note Reading Persona::). How this information is collected by the system and how to control/add/remove information from the background storage is described in *Note User Accounting Database::. The `getlogin' function is declared in `unistd.h', while `cuserid' and `L_cuserid' are declared in `stdio.h'. - Function: char * getlogin (void) The `getlogin' function returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to `cuserid'. - Function: char * cuserid (char *STRING) The `cuserid' function returns a pointer to a string containing a user name associated with the effective ID of the process. If STRING is not a null pointer, it should be an array that can hold at least `L_cuserid' characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned. This string is statically allocated and might be overwritten on subsequent calls to this function or to `getlogin'. The use of this function is deprecated since it is marked to be withdrawn in XPG4.2 and has already been removed from newer revisions of POSIX.1. - Macro: int L_cuserid An integer constant that indicates how long an array you might need to store a user name. These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; see *Note Process Persona::.) The user cannot do anything to fool these functions. For most purposes, it is more useful to use the environment variable `LOGNAME' to find out who the user is. This is more flexible precisely because the user can set `LOGNAME' arbitrarily. *Note Standard Environment::. The User Accounting Database ============================ Most Unix-like operating systems keep track of logged in users by maintaining a user accounting database. This user accounting database stores for each terminal, who has logged on, at what time, the process ID of the user's login shell, etc., etc., but also stores information about the run level of the system, the time of the last system reboot, and possibly more. The user accounting database typically lives in `/etc/utmp', `/var/adm/utmp' or `/var/run/utmp'. However, these files should *never* be accessed directly. For reading information from and writing information to the user accounting database, the functions described in this section should be used. Manipulating the User Accounting Database ----------------------------------------- These functions and the corresponding data structures are declared in the header file `utmp.h'. - Data Type: struct exit_status The `exit_status' data structure is used to hold information about the exit status of processes marked as `DEAD_PROCESS' in the user accounting database. `short int e_termination' The exit status of the process. `short int e_exit' The exit status of the process. - Data Type: struct utmp The `utmp' data structure is used to hold information about entries in the user accounting database. On the GNU system it has the following members: `short int ut_type' Specifies the type of login; one of `EMPTY', `RUN_LVL', `BOOT_TIME', `OLD_TIME', `NEW_TIME', `INIT_PROCESS', `LOGIN_PROCESS', `USER_PROCESS', `DEAD_PROCESS' or `ACCOUNTING'. `pid_t ut_pid' The process ID number of the login process. `char ut_line[]' The device name of the tty (without `/dev/'). `char ut_id[]' The inittab ID of the process. `char ut_user[]' The user's login name. `char ut_host[]' The name of the host from which the user logged in. `struct exit_status ut_exit' The exit status of a process marked as `DEAD_PROCESS'. `long ut_session' The Session ID, used for windowing. `struct timeval ut_tv' Time the entry was made. For entries of type `OLD_TIME' this is the time when the system clock changed, and for entries of type `NEW_TIME' this is the time the system clock was set to. `int32_t ut_addr_v6[4]' The Internet address of a remote host. The `ut_type', `ut_pid', `ut_id', `ut_tv', and `ut_host' fields are not available on all systems. Portable applications therefore should be prepared for these situations. To help doing this the `utmp.h' header provides macros `_HAVE_UT_TYPE', `_HAVE_UT_PID', `_HAVE_UT_ID', `_HAVE_UT_TV', and `_HAVE_UT_HOST' if the respective field is available. The programmer can handle the situations by using `#ifdef' in the program code. The following macros are defined for use as values for the `ut_type' member of the `utmp' structure. The values are integer constants. `EMPTY' This macro is used to indicate that the entry contains no valid user accounting information. `RUN_LVL' This macro is used to identify the systems runlevel. `BOOT_TIME' This macro is used to identify the time of system boot. `OLD_TIME' This macro is used to identify the time when the system clock changed. `NEW_TIME' This macro is used to identify the time after the system changed. `INIT_PROCESS' This macro is used to identify a process spawned by the init process. `LOGIN_PROCESS' This macro is used to identify the session leader of a logged in user. `USER_PROCESS' This macro is used to identify a user process. `DEAD_PROCESS' This macro is used to identify a terminated process. `ACCOUNTING' ??? The size of the `ut_line', `ut_id', `ut_user' and `ut_host' arrays can be found using the `sizeof' operator. Many older systems have, instead of an `ut_tv' member, an `ut_time' member, usually of type `time_t', for representing the time associated with the entry. Therefore, for backwards compatibility only, `utmp.h' defines `ut_time' as an alias for `ut_tv.tv_sec'. - Function: void setutent (void) This function opens the user accounting database to begin scanning it. You can then call `getutent', `getutid' or `getutline' to read entries and `pututline' to write entries. If the database is already open, it resets the input to the beginning of the database. - Function: struct utmp * getutent (void) The `getutent' function reads the next entry from the user accounting database. It returns a pointer to the entry, which is statically allocated and may be overwritten by subsequent calls to `getutent'. You must copy the contents of the structure if you wish to save the information or you can use the `getutent_r' function which stores the data in a user-provided buffer. A null pointer is returned in case no further entry is available. - Function: void endutent (void) This function closes the user accounting database. - Function: struct utmp * getutid (const struct utmp *ID) This function searches forward from the current point in the database for an entry that matches ID. If the `ut_type' member of the ID structure is one of `RUN_LVL', `BOOT_TIME', `OLD_TIME' or `NEW_TIME' the entries match if the `ut_type' members are identical. If the `ut_type' member of the ID structure is `INIT_PROCESS', `LOGIN_PROCESS', `USER_PROCESS' or `DEAD_PROCESS', the entries match if the `ut_type' member of the entry read from the database is one of these four, and the `ut_id' members match. However if the `ut_id' member of either the ID structure or the entry read from the database is empty it checks if the `ut_line' members match instead. If a matching entry is found, `getutid' returns a pointer to the entry, which is statically allocated, and may be overwritten by a subsequent call to `getutent', `getutid' or `getutline'. You must copy the contents of the structure if you wish to save the information. A null pointer is returned in case the end of the database is reached without a match. The `getutid' function may cache the last read entry. Therefore, if you are using `getutid' to search for multiple occurrences, it is necessary to zero out the static data after each call. Otherwise `getutid' could just return a pointer to the same entry over and over again. - Function: struct utmp * getutline (const struct utmp *LINE) This function searches forward from the current point in the database until it finds an entry whose `ut_type' value is `LOGIN_PROCESS' or `USER_PROCESS', and whose `ut_line' member matches the `ut_line' member of the LINE structure. If it finds such an entry, it returns a pointer to the entry which is statically allocated, and may be overwritten by a subsequent call to `getutent', `getutid' or `getutline'. You must copy the contents of the structure if you wish to save the information. A null pointer is returned in case the end of the database is reached without a match. The `getutline' function may cache the last read entry. Therefore if you are using `getutline' to search for multiple occurrences, it is necessary to zero out the static data after each call. Otherwise `getutline' could just return a pointer to the same entry over and over again. - Function: struct utmp * pututline (const struct utmp *UTMP) The `pututline' function inserts the entry `*UTMP' at the appropriate place in the user accounting database. If it finds that it is not already at the correct place in the database, it uses `getutid' to search for the position to insert the entry, however this will not modify the static structure returned by `getutent', `getutid' and `getutline'. If this search fails, the entry is appended to the database. The `pututline' function returns a pointer to a copy of the entry inserted in the user accounting database, or a null pointer if the entry could not be added. The following `errno' error conditions are defined for this function: `EPERM' The process does not have the appropriate privileges; you cannot modify the user accounting database. All the `get*' functions mentioned before store the information they return in a static buffer. This can be a problem in multi-threaded programs since the data returned for the request is overwritten by the return value data in another thread. Therefore the GNU C Library provides as extensions three more functions which return the data in a user-provided buffer. - Function: int getutent_r (struct utmp *BUFFER, struct utmp **RESULT) The `getutent_r' is equivalent to the `getutent' function. It returns the next entry from the database. But instead of storing the information in a static buffer it stores it in the buffer pointed to by the parameter BUFFER. If the call was successful, the function returns `0' and the pointer variable pointed to by the parameter RESULT contains a pointer to the buffer which contains the result (this is most probably the same value as BUFFER). If something went wrong during the execution of `getutent_r' the function returns `-1'. This function is a GNU extension. - Function: int getutid_r (const struct utmp *ID, struct utmp *BUFFER, struct utmp **RESULT) This function retrieves just like `getutid' the next entry matching the information stored in ID. But the result is stored in the buffer pointed to by the parameter BUFFER. If successful the function returns `0' and the pointer variable pointed to by the parameter RESULT contains a pointer to the buffer with the result (probably the same as RESULT. If not successful the function return `-1'. This function is a GNU extension. - Function: int getutline_r (const struct utmp *LINE, struct utmp *BUFFER, struct utmp **RESULT) This function retrieves just like `getutline' the next entry matching the information stored in LINE. But the result is stored in the buffer pointed to by the parameter BUFFER. If successful the function returns `0' and the pointer variable pointed to by the parameter RESULT contains a pointer to the buffer with the result (probably the same as RESULT. If not successful the function return `-1'. This function is a GNU extension. In addition to the user accounting database, most systems keep a number of similar databases. For example most systems keep a log file with all previous logins (usually in `/etc/wtmp' or `/var/log/wtmp'). For specifying which database to examine, the following function should be used. - Function: int utmpname (const char *FILE) The `utmpname' function changes the name of the database to be examined to FILE, and closes any previously opened database. By default `getutent', `getutid', `getutline' and `pututline' read from and write to the user accounting database. The following macros are defined for use as the FILE argument: - Macro: char * _PATH_UTMP This macro is used to specify the user accounting database. - Macro: char * _PATH_WTMP This macro is used to specify the user accounting log file. The `utmpname' function returns a value of `0' if the new name was successfully stored, and a value of `-1' to indicate an error. Note that `utmpname' does not try to open the database, and that therefore the return value does not say anything about whether the database can be successfully opened. Specially for maintaining log-like databases the GNU C Library provides the following function: - Function: void updwtmp (const char *WTMP_FILE, const struct utmp *UTMP) The `updwtmp' function appends the entry *UTMP to the database specified by WTMP_FILE. For possible values for the WTMP_FILE argument see the `utmpname' function. *Portability Note:* Although many operating systems provide a subset of these functions, they are not standardized. There are often subtle differences in the return types, and there are considerable differences between the various definitions of `struct utmp'. When programming for the GNU system, it is probably best to stick with the functions described in this section. If however, you want your program to be portable, consider using the XPG functions described in *Note XPG Functions::, or take a look at the BSD compatible functions in *Note Logging In and Out::. XPG User Accounting Database Functions -------------------------------------- These functions, described in the X/Open Portability Guide, are declared in the header file `utmpx.h'. - Data Type: struct utmpx The `utmpx' data structure contains at least the following members: `short int ut_type' Specifies the type of login; one of `EMPTY', `RUN_LVL', `BOOT_TIME', `OLD_TIME', `NEW_TIME', `INIT_PROCESS', `LOGIN_PROCESS', `USER_PROCESS' or `DEAD_PROCESS'. `pid_t ut_pid' The process ID number of the login process. `char ut_line[]' The device name of the tty (without `/dev/'). `char ut_id[]' The inittab ID of the process. `char ut_user[]' The user's login name. `struct timeval ut_tv' Time the entry was made. For entries of type `OLD_TIME' this is the time when the system clock changed, and for entries of type `NEW_TIME' this is the time the system clock was set to. On the GNU system, `struct utmpx' is identical to `struct utmp' except for the fact that including `utmpx.h' does not make visible the declaration of `struct exit_status'. The following macros are defined for use as values for the `ut_type' member of the `utmpx' structure. The values are integer constants and are, on the GNU system, identical to the definitions in `utmp.h'. `EMPTY' This macro is used to indicate that the entry contains no valid user accounting information. `RUN_LVL' This macro is used to identify the systems runlevel. `BOOT_TIME' This macro is used to identify the time of system boot. `OLD_TIME' This macro is used to identify the time when the system clock changed. `NEW_TIME' This macro is used to identify the time after the system changed. `INIT_PROCESS' This macro is used to identify a process spawned by the init process. `LOGIN_PROCESS' This macro is used to identify the session leader of a logged in user. `USER_PROCESS' This macro is used to identify a user process. `DEAD_PROCESS' This macro is used to identify a terminated process. The size of the `ut_line', `ut_id' and `ut_user' arrays can be found using the `sizeof' operator. - Function: void setutxent (void) This function is similar to `setutent'. On the GNU system it is simply an alias for `setutent'. - Function: struct utmpx * getutxent (void) The `getutxent' function is similar to `getutent', but returns a pointer to a `struct utmpx' instead of `struct utmp'. On the GNU system it simply is an alias for `getutent'. - Function: void endutxent (void) This function is similar to `endutent'. On the GNU system it is simply an alias for `endutent'. - Function: struct utmpx * getutxid (const struct utmpx *ID) This function is similar to `getutid', but uses `struct utmpx' instead of `struct utmp'. On the GNU system it is simply an alias for `getutid'. - Function: struct utmpx * getutxline (const struct utmpx *LINE) This function is similar to `getutid', but uses `struct utmpx' instead of `struct utmp'. On the GNU system it is simply an alias for `getutline'. - Function: struct utmpx * pututxline (const struct utmpx *UTMP) The `pututxline' function is functionally identical to `pututline', but uses `struct utmpx' instead of `struct utmp'. On the GNU system, `pututxline' is simply an alias for `pututline'. - Function: int utmpxname (const char *FILE) The `utmpxname' function is functionally identical to `utmpname'. On the GNU system, `utmpxname' is simply an alias for `utmpname'. You can translate between a traditional `struct utmp' and an XPG `struct utmpx' with the following functions. On the GNU system, these functions are merely copies, since the two structures are identical. - Function: int getutmp (const struct utmpx *utmpx, struct utmp *utmp) `getutmp' copies the information, insofar as the structures are compatible, from UTMPX to UTMP. - Function: int getutmpx (const struct utmp *utmp, struct utmpx *utmpx) `getutmpx' copies the information, insofar as the structures are compatible, from UTMP to UTMPX. Logging In and Out ------------------ These functions, derived from BSD, are available in the separate `libutil' library, and declared in `utmp.h'. Note that the `ut_user' member of `struct utmp' is called `ut_name' in BSD. Therefore, `ut_name' is defined as an alias for `ut_user' in `utmp.h'. - Function: int login_tty (int FILEDES) This function makes FILEDES the controlling terminal of the current process, redirects standard input, standard output and standard error output to this terminal, and closes FILEDES. This function returns `0' on successful completion, and `-1' on error. - Function: void login (const struct utmp *ENTRY) The `login' functions inserts an entry into the user accounting database. The `ut_line' member is set to the name of the terminal on standard input. If standard input is not a terminal `login' uses standard output or standard error output to determine the name of the terminal. If `struct utmp' has a `ut_type' member, `login' sets it to `USER_PROCESS', and if there is an `ut_pid' member, it will be set to the process ID of the current process. The remaining entries are copied from ENTRY. A copy of the entry is written to the user accounting log file. - Function: int logout (const char *UT_LINE) This function modifies the user accounting database to indicate that the user on UT_LINE has logged out. The `logout' function returns `1' if the entry was successfully written to the database, or `0' on error. - Function: void logwtmp (const char *UT_LINE, const char *UT_NAME, const char *UT_HOST) The `logwtmp' function appends an entry to the user accounting log file, for the current time and the information provided in the UT_LINE, UT_NAME and UT_HOST arguments. *Portability Note:* The BSD `struct utmp' only has the `ut_line', `ut_name', `ut_host' and `ut_time' members. Older systems do not even have the `ut_host' member. User Database ============= This section describes how to search and scan the database of registered users. The database itself is kept in the file `/etc/passwd' on most systems, but on some systems a special network server gives access to it. The Data Structure that Describes a User ---------------------------------------- The functions and data structures for accessing the system user database are declared in the header file `pwd.h'. - Data Type: struct passwd The `passwd' data structure is used to hold information about entries in the system user data base. It has at least the following members: `char *pw_name' The user's login name. `char *pw_passwd.' The encrypted password string. `uid_t pw_uid' The user ID number. `gid_t pw_gid' The user's default group ID number. `char *pw_gecos' A string typically containing the user's real name, and possibly other information such as a phone number. `char *pw_dir' The user's home directory, or initial working directory. This might be a null pointer, in which case the interpretation is system-dependent. `char *pw_shell' The user's default shell, or the initial program run when the user logs in. This might be a null pointer, indicating that the system default should be used. Looking Up One User ------------------- You can search the system user database for information about a specific user using `getpwuid' or `getpwnam'. These functions are declared in `pwd.h'. - Function: struct passwd * getpwuid (uid_t UID) This function returns a pointer to a statically-allocated structure containing information about the user whose user ID is UID. This structure may be overwritten on subsequent calls to `getpwuid'. A null pointer value indicates there is no user in the data base with user ID UID. - Function: int getpwuid_r (uid_t UID, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT) This function is similar to `getpwuid' in that it returns information about the user whose user ID is UID. However, it fills the user supplied structure pointed to by RESULT_BUF with the information instead of using a static buffer. The first BUFLEN bytes of the additional buffer pointed to by BUFFER are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If a user with ID UID is found, the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). If no user is found or if an error occurred, the pointer returned in RESULT is a null pointer. The function returns zero or an error code. If the buffer BUFFER is too small to contain all the needed information, the error code `ERANGE' is returned and ERRNO is set to `ERANGE'. - Function: struct passwd * getpwnam (const char *NAME) This function returns a pointer to a statically-allocated structure containing information about the user whose user name is NAME. This structure may be overwritten on subsequent calls to `getpwnam'. A null pointer return indicates there is no user named NAME. - Function: int getpwnam_r (const char *NAME, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT) This function is similar to `getpwnam' in that is returns information about the user whose user name is NAME. However, like `getpwuid_r', it fills the user supplied buffers in RESULT_BUF and BUFFER with the information instead of using a static buffer. The return values are the same as for `getpwuid_r'. Scanning the List of All Users ------------------------------ This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in `pwd.h'. You can use the `fgetpwent' function to read user entries from a particular file. - Function: struct passwd * fgetpwent (FILE *STREAM) This function reads the next user entry from STREAM and returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to `fgetpwent'. You must copy the contents of the structure if you wish to save the information. The stream must correspond to a file in the same format as the standard password database file. - Function: int fgetpwent_r (FILE *STREAM, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT) This function is similar to `fgetpwent' in that it reads the next user entry from STREAM. But the result is returned in the structure pointed to by RESULT_BUF. The first BUFLEN bytes of the additional buffer pointed to by BUFFER are used to contain additional information, normally strings which are pointed to by the elements of the result structure. The stream must correspond to a file in the same format as the standard password database file. If the function returns zero RESULT points to the structure with the wanted data (normally this is in RESULT_BUF). If errors occurred the return value is nonzero and RESULT contains a null pointer. The way to scan all the entries in the user database is with `setpwent', `getpwent', and `endpwent'. - Function: void setpwent (void) This function initializes a stream which `getpwent' and `getpwent_r' use to read the user database. - Function: struct passwd * getpwent (void) The `getpwent' function reads the next entry from the stream initialized by `setpwent'. It returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to `getpwent'. You must copy the contents of the structure if you wish to save the information. A null pointer is returned when no more entries are available. - Function: int getpwent_r (struct passwd *RESULT_BUF, char *BUFFER, int BUFLEN, struct passwd **RESULT) This function is similar to `getpwent' in that it returns the next entry from the stream initialized by `setpwent'. Like `fgetpwent_r', it uses the user-supplied buffers in RESULT_BUF and BUFFER to return the information requested. The return values are the same as for `fgetpwent_r'. - Function: void endpwent (void) This function closes the internal stream used by `getpwent' or `getpwent_r'. Writing a User Entry -------------------- - Function: int putpwent (const struct passwd *P, FILE *STREAM) This function writes the user entry `*P' to the stream STREAM, in the format used for the standard user database file. The return value is zero on success and nonzero on failure. This function exists for compatibility with SVID. We recommend that you avoid using it, because it makes sense only on the assumption that the `struct passwd' structure has no members except the standard ones; on a system which merges the traditional Unix data base with other extended information about users, adding an entry using this function would inevitably leave out much of the important information. The function `putpwent' is declared in `pwd.h'. Group Database ============== This section describes how to search and scan the database of registered groups. The database itself is kept in the file `/etc/group' on most systems, but on some systems a special network service provides access to it. The Data Structure for a Group ------------------------------ The functions and data structures for accessing the system group database are declared in the header file `grp.h'. - Data Type: struct group The `group' structure is used to hold information about an entry in the system group database. It has at least the following members: `char *gr_name' The name of the group. `gid_t gr_gid' The group ID of the group. `char **gr_mem' A vector of pointers to the names of users in the group. Each user name is a null-terminated string, and the vector itself is terminated by a null pointer. Looking Up One Group -------------------- You can search the group database for information about a specific group using `getgrgid' or `getgrnam'. These functions are declared in `grp.h'. - Function: struct group * getgrgid (gid_t GID) This function returns a pointer to a statically-allocated structure containing information about the group whose group ID is GID. This structure may be overwritten by subsequent calls to `getgrgid'. A null pointer indicates there is no group with ID GID. - Function: int getgrgid_r (gid_t GID, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT) This function is similar to `getgrgid' in that it returns information about the group whose group ID is GID. However, it fills the user supplied structure pointed to by RESULT_BUF with the information instead of using a static buffer. The first BUFLEN bytes of the additional buffer pointed to by BUFFER are used to contain additional information, normally strings which are pointed to by the elements of the result structure. If a group with ID GID is found, the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). If no group is found or if an error occurred, the pointer returned in RESULT is a null pointer. The function returns zero or an error code. If the buffer BUFFER is too small to contain all the needed information, the error code `ERANGE' is returned and ERRNO is set to `ERANGE'. - Function: struct group * getgrnam (const char *NAME) This function returns a pointer to a statically-allocated structure containing information about the group whose group name is NAME. This structure may be overwritten by subsequent calls to `getgrnam'. A null pointer indicates there is no group named NAME. - Function: int getgrnam_r (const char *NAME, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT) This function is similar to `getgrnam' in that is returns information about the group whose group name is NAME. Like `getgrgid_r', it uses the user supplied buffers in RESULT_BUF and BUFFER, not a static buffer. The return values are the same as for `getgrgid_r' `ERANGE'. Scanning the List of All Groups ------------------------------- This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in `grp.h'. You can use the `fgetgrent' function to read group entries from a particular file. - Function: struct group * fgetgrent (FILE *STREAM) The `fgetgrent' function reads the next entry from STREAM. It returns a pointer to the entry. The structure is statically allocated and is overwritten on subsequent calls to `fgetgrent'. You must copy the contents of the structure if you wish to save the information. The stream must correspond to a file in the same format as the standard group database file. - Function: int fgetgrent_r (FILE *STREAM, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT) This function is similar to `fgetgrent' in that it reads the next user entry from STREAM. But the result is returned in the structure pointed to by RESULT_BUF. The first BUFLEN bytes of the additional buffer pointed to by BUFFER are used to contain additional information, normally strings which are pointed to by the elements of the result structure. This stream must correspond to a file in the same format as the standard group database file. If the function returns zero RESULT points to the structure with the wanted data (normally this is in RESULT_BUF). If errors occurred the return value is non-zero and RESULT contains a null pointer. The way to scan all the entries in the group database is with `setgrent', `getgrent', and `endgrent'. - Function: void setgrent (void) This function initializes a stream for reading from the group data base. You use this stream by calling `getgrent' or `getgrent_r'. - Function: struct group * getgrent (void) The `getgrent' function reads the next entry from the stream initialized by `setgrent'. It returns a pointer to the entry. The structure is statically allocated and is overwritten on subsequent calls to `getgrent'. You must copy the contents of the structure if you wish to save the information. - Function: int getgrent_r (struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT) This function is similar to `getgrent' in that it returns the next entry from the stream initialized by `setgrent'. Like `fgetgrent_r', it places the result in user-supplied buffers pointed to RESULT_BUF and BUFFER. If the function returns zero RESULT contains a pointer to the data (normally equal to RESULT_BUF). If errors occurred the return value is non-zero and RESULT contains a null pointer. - Function: void endgrent (void) This function closes the internal stream used by `getgrent' or `getgrent_r'. User and Group Database Example =============================== Here is an example program showing the use of the system database inquiry functions. The program prints some information about the user running the program. #include #include #include #include #include int main (void) { uid_t me; struct passwd *my_passwd; struct group *my_group; char **members; /* Get information about the user ID. */ me = getuid (); my_passwd = getpwuid (me); if (!my_passwd) { printf ("Couldn't find out about user %d.\n", (int) me); exit (EXIT_FAILURE); } /* Print the information. */ printf ("I am %s.\n", my_passwd->pw_gecos); printf ("My login name is %s.\n", my_passwd->pw_name); printf ("My uid is %d.\n", (int) (my_passwd->pw_uid)); printf ("My home directory is %s.\n", my_passwd->pw_dir); printf ("My default shell is %s.\n", my_passwd->pw_shell); /* Get information about the default group ID. */ my_group = getgrgid (my_passwd->pw_gid); if (!my_group) { printf ("Couldn't find out about group %d.\n", (int) my_passwd->pw_gid); exit (EXIT_FAILURE); } /* Print the information. */ printf ("My default group is %s (%d).\n", my_group->gr_name, (int) (my_passwd->pw_gid)); printf ("The members of this group are:\n"); members = my_group->gr_mem; while (*members) { printf (" %s\n", *(members)); members++; } return EXIT_SUCCESS; } Here is some output from this program: I am Throckmorton Snurd. My login name is snurd. My uid is 31093. My home directory is /home/fsg/snurd. My default shell is /bin/sh. My default group is guest (12). The members of this group are: friedman tami Netgroup Database ================= Netgroup Data ------------- Sometimes it is useful to group users according to other criteria (*note Group Database::). E.g., it is useful to associate a certain group of users with a certain machine. On the other hand grouping of host names is not supported so far. In Sun Microsystems SunOS appeared a new kind of database, the netgroup database. It allows grouping hosts, users, and domains freely, giving them individual names. To be more concrete, a netgroup is a list of triples consisting of a host name, a user name, and a domain name where any of the entries can be a wildcard entry matching all inputs. A last possibility is that names of other netgroups can also be given in the list specifying a netgroup. So one can construct arbitrary hierarchies without loops. Sun's implementation allows netgroups only for the `nis' or `nisplus' service, *note Services in the NSS configuration::. The implementation in the GNU C library has no such restriction. An entry in either of the input services must have the following form: GROUPNAME ( GROUPNAME | `('HOSTNAME`,'USERNAME`,'`domainname'`)' )+ Any of the fields in the triple can be empty which means anything matches. While describing the functions we will see that the opposite case is useful as well. I.e., there may be entries which will not match any input. For entries like this, a name consisting of the single character `-' shall be used. Looking up one Netgroup ----------------------- The lookup functions for netgroups are a bit different to all other system database handling functions. Since a single netgroup can contain many entries a two-step process is needed. First a single netgroup is selected and then one can iterate over all entries in this netgroup. These functions are declared in `netdb.h'. - Function: int setnetgrent (const char *NETGROUP) A call to this function initializes the internal state of the library to allow following calls of the `getnetgrent' to iterate over all entries in the netgroup with name NETGROUP. When the call is successful (i.e., when a netgroup with this name exists) the return value is `1'. When the return value is `0' no netgroup of this name is known or some other error occurred. It is important to remember that there is only one single state for iterating the netgroups. Even if the programmer uses the `getnetgrent_r' function the result is not really reentrant since always only one single netgroup at a time can be processed. If the program needs to process more than one netgroup simultaneously she must protect this by using external locking. This problem was introduced in the original netgroups implementation in SunOS and since we must stay compatible it is not possible to change this. Some other functions also use the netgroups state. Currently these are the `innetgr' function and parts of the implementation of the `compat' service part of the NSS implementation. - Function: int getnetgrent (char **HOSTP, char **USERP, char **DOMAINP) This function returns the next unprocessed entry of the currently selected netgroup. The string pointers, in which addresses are passed in the arguments HOSTP, USERP, and DOMAINP, will contain after a successful call pointers to appropriate strings. If the string in the next entry is empty the pointer has the value `NULL'. The returned string pointers are only valid if none of the netgroup related functions are called. The return value is `1' if the next entry was successfully read. A value of `0' means no further entries exist or internal errors occurred. - Function: int getnetgrent_r (char **HOSTP, char **USERP, char **DOMAINP, char *BUFFER, int BUFLEN) This function is similar to `getnetgrent' with only one exception: the strings the three string pointers HOSTP, USERP, and DOMAINP point to, are placed in the buffer of BUFLEN bytes starting at BUFFER. This means the returned values are valid even after other netgroup related functions are called. The return value is `1' if the next entry was successfully read and the buffer contains enough room to place the strings in it. `0' is returned in case no more entries are found, the buffer is too small, or internal errors occurred. This function is a GNU extension. The original implementation in the SunOS libc does not provide this function. - Function: void endnetgrent (void) This function frees all buffers which were allocated to process the last selected netgroup. As a result all string pointers returned by calls to `getnetgrent' are invalid afterwards. Testing for Netgroup Membership ------------------------------- It is often not necessary to scan the whole netgroup since often the only interesting question is whether a given entry is part of the selected netgroup. - Function: int innetgr (const char *NETGROUP, const char *HOST, const char *USER, const char *DOMAIN) This function tests whether the triple specified by the parameters HOSTP, USERP, and DOMAINP is part of the netgroup NETGROUP. Using this function has the advantage that 1. no other netgroup function can use the global netgroup state since internal locking is used and 2. the function is implemented more efficiently than successive calls to the other `set'/`get'/`endnetgrent' functions. Any of the pointers HOSTP, USERP, and DOMAINP can be `NULL' which means any value is accepted in this position. This is also true for the name `-' which should not match any other string otherwise. The return value is `1' if an entry matching the given triple is found in the netgroup. The return value is `0' if the netgroup itself is not found, the netgroup does not contain the triple or internal errors occurred. System Management ***************** This chapter describes facilities for controlling the system that underlies a process (including the operating system and hardware) and for getting information about it. Anyone can generally use the informational facilities, but usually only a properly privileged process can make changes. To get information on parameters of the system that are built into the system, such as the maximum length of a filename, *Note System Configuration::. Host Identification =================== This section explains how to identify the particular system on which your program is running. First, let's review the various ways computer systems are named, which is a little complicated because of the history of the development of the Internet. Every Unix system (also known as a host) has a host name, whether it's connected to a network or not. In its simplest form, as used before computer networks were an issue, it's just a word like `chicken'. But any system attached to the Internet or any network like it conforms to a more rigorous naming convention as part of the Domain Name System (DNS). In DNS, every host name is composed of two parts: 1. hostname 2. domain name You will note that "hostname" looks a lot like "host name", but is not the same thing, and that people often incorrectly refer to entire host names as "domain names." In DNS, the full host name is properly called the FQDN (Fully Qualified Domain Name) and consists of the hostname, then a period, then the domain name. The domain name itself usually has multiple components separated by periods. So for example, a system's hostname may be `chicken' and its domain name might be `ai.mit.edu', so its FQDN (which is its host name) is `chicken.ai.mit.edu'. Adding to the confusion, though, is that DNS is not the only name space in which a computer needs to be known. Another name space is the NIS (aka YP) name space. For NIS purposes, there is another domain name, which is called the NIS domain name or the YP domain name. It need not have anything to do with the DNS domain name. Confusing things even more is the fact that in DNS, it is possible for multiple FQDNs to refer to the same system. However, there is always exactly one of them that is the true host name, and it is called the canonical FQDN. In some contexts, the host name is called a "node name." For more information on DNS host naming, *Note Host Names::. Prototypes for these functions appear in `unistd.h'. The programs `hostname', `hostid', and `domainname' work by calling these functions. - Function: int gethostname (char *NAME, size_t SIZE) This function returns the host name of the system on which it is called, in the array NAME. The SIZE argument specifies the size of this array, in bytes. Note that this is _not_ the DNS hostname. If the system participates in DNS, this is the FQDN (see above). The return value is `0' on success and `-1' on failure. In the GNU C library, `gethostname' fails if SIZE is not large enough; then you can try again with a larger array. The following `errno' error condition is defined for this function: `ENAMETOOLONG' The SIZE argument is less than the size of the host name plus one. On some systems, there is a symbol for the maximum possible host name length: `MAXHOSTNAMELEN'. It is defined in `sys/param.h'. But you can't count on this to exist, so it is cleaner to handle failure and try again. `gethostname' stores the beginning of the host name in NAME even if the host name won't entirely fit. For some purposes, a truncated host name is good enough. If it is, you can ignore the error code. - Function: int sethostname (const char *NAME, size_t LENGTH) The `sethostname' function sets the host name of the system that calls it to NAME, a string with length LENGTH. Only privileged processes are permitted to do this. Usually `sethostname' gets called just once, at system boot time. Often, the program that calls it sets it to the value it finds in the file `/etc/hostname'. Be sure to set the host name to the full host name, not just the DNS hostname (see above). The return value is `0' on success and `-1' on failure. The following `errno' error condition is defined for this function: `EPERM' This process cannot set the host name because it is not privileged. - Function: int getdomainnname (char *NAME, size_t LENGTH) `getdomainname' returns the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Get that with `gethostname'. The specifics of this function are analogous to `gethostname', above. - Function: int setdomainname (const char *NAME, size_t LENGTH) `getdomainname' sets the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Set that with `sethostname'. The specifics of this function are analogous to `sethostname', above. - Function: long int gethostid (void) This function returns the "host ID" of the machine the program is running on. By convention, this is usually the primary Internet IP address of that machine, converted to a `long int'. However, on some systems it is a meaningless but unique number which is hard-coded for each machine. This is not widely used. It arose in BSD 4.2, but was dropped in BSD 4.4. It is not required by POSIX. The proper way to query the IP address is to use `gethostbyname' on the results of `gethostname'. For more information on IP addresses, *Note Host Addresses::. - Function: int sethostid (long int ID) The `sethostid' function sets the "host ID" of the host machine to ID. Only privileged processes are permitted to do this. Usually it happens just once, at system boot time. The proper way to establish the primary IP address of a system is to configure the IP address resolver to associate that IP address with the system's host name as returned by `gethostname'. For example, put a record for the system in `/etc/hosts'. See `gethostid' above for more information on host ids. The return value is `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EPERM' This process cannot set the host name because it is not privileged. `ENOSYS' The operating system does not support setting the host ID. On some systems, the host ID is a meaningless but unique number hard-coded for each machine. Platform Type Identification ============================ You can use the `uname' function to find out some information about the type of computer your program is running on. This function and the associated data type are declared in the header file `sys/utsname.h'. As a bonus, `uname' also gives some information identifying the particular system your program is running on. This is the same information which you can get with functions targetted to this purpose described in *Note Host Identification::. - Data Type: struct utsname The `utsname' structure is used to hold information returned by the `uname' function. It has the following members: `char sysname[]' This is the name of the operating system in use. `char release[]' This is the current release level of the operating system implementation. `char version[]' This is the current version level within the release of the operating system. `char machine[]' This is a description of the type of hardware that is in use. Some systems provide a mechanism to interrogate the kernel directly for this information. On systems without such a mechanism, the GNU C library fills in this field based on the configuration name that was specified when building and installing the library. GNU uses a three-part name to describe a system configuration; the three parts are CPU, MANUFACTURER and SYSTEM-TYPE, and they are separated with dashes. Any possible combination of three names is potentially meaningful, but most such combinations are meaningless in practice and even the meaningful ones are not necessarily supported by any particular GNU program. Since the value in `machine' is supposed to describe just the hardware, it consists of the first two parts of the configuration name: `CPU-MANUFACTURER'. For example, it might be one of these: `"sparc-sun"', `"i386-ANYTHING"', `"m68k-hp"', `"m68k-sony"', `"m68k-sun"', `"mips-dec"' `char nodename[]' This is the host name of this particular computer. In the GNU C library, the value is the same as that returned by `gethostname'; see *Note Host Identification::. gethostname() is implemented with a call to uname(). `char domainname[]' This is the NIS or YP domain name. It is the same value returned by `getdomainname'; see *Note Host Identification::. This element is a relatively recent invention and use of it is not as portable as use of the rest of the structure. - Function: int uname (struct utsname *INFO) The `uname' function fills in the structure pointed to by INFO with information about the operating system and host machine. A non-negative value indicates that the data was successfully stored. `-1' as the value indicates an error. The only error possible is `EFAULT', which we normally don't mention as it is always a possibility. Controlling and Querying Mounts =============================== All files are in filesystems, and before you can access any file, its filesystem must be mounted. Because of Unix's concept of _Everything is a file_, mounting of filesystems is central to doing almost anything. This section explains how to find out what filesystems are currently mounted and what filesystems are available for mounting, and how to change what is mounted. The classic filesystem is the contents of a disk drive. The concept is considerably more abstract, though, and lots of things other than disk drives can be mounted. Some block devices don't correspond to traditional devices like disk drives. For example, a loop device is a block device whose driver uses a regular file in another filesystem as its medium. So if that regular file contains appropriate data for a filesystem, you can by mounting the loop device essentially mount a regular file. Some filesystems aren't based on a device of any kind. The "proc" filesystem, for example, contains files whose data is made up by the filesystem driver on the fly whenever you ask for it. And when you write to it, the data you write causes changes in the system. No data gets stored. Mount Information ----------------- For some programs it is desirable and necessary to access information about whether a certain filesystem is mounted and, if it is, where, or simply to get lists of all the available filesystems. The GNU libc provides some functions to retrieve this information portably. Traditionally Unix systems have a file named `/etc/fstab' which describes all possibly mounted filesystems. The `mount' program uses this file to mount at startup time of the system all the necessary filesystems. The information about all the filesystems actually mounted is normally kept in a file named `/etc/mtab'. Both files share the same syntax and it is crucial that this syntax is followed all the time. Therefore it is best to never directly write the files. The functions described in this section can do this and they also provide the functionality to convert the external textual representation to the internal representation. Note that the `fstab' and `mtab' files are maintained on a system by _convention_. It is possible for the files not to exist or not to be consistent with what is really mounted or available to mount, if the system's administration policy allows it. But programs that mount and unmount filesystems typically maintain and use these files as described herein. The filenames given above should never be used directly. The portable way to handle these file is to use the macros `_PATH_FSTAB', defined in `fstab.h' and `_PATH_MNTTAB', defined in `mntent.h', respectively. There are also two alternate macro names `FSTAB' and `_PATH_MOUNTED' defined but both names are deprecated and kept only for backward compatibility. The two former names should always be used. The `fstab' file ................ The internal representation for entries of the file is `struct fstab', defined in `fstab.h'. - Data Type: struct fstab This structure is used with the `getfsent', `getfsspec', and `getfsfile' functions. `char *fs_spec' This element describes the device from which the filesystem is mounted. Normally this is the name of a special device, such as a hard disk partition, but it could also be a more or less generic string. For "NFS" it would be a hostname and directory name combination. Even though the element is not declared `const' it shouldn't be modified. The missing `const' has historic reasons, since this function predates ISO C. The same is true for the other string elements of this structure. `char *fs_file' This describes the mount point on the local system. I.e., accessing any file in this filesystem has implicitly or explicitly this string as a prefix. `char *fs_vfstype' This is the type of the filesystem. Depending on what the underlying kernel understands it can be any string. `char *fs_mntops' This is a string containing options passed to the kernel with the `mount' call. Again, this can be almost anything. There can be more than one option, separated from the others by a comma. Each option consists of a name and an optional value part, introduced by an `=' character. If the value of this element must be processed it should ideally be done using the `getsubopt' function; see *Note Suboptions::. `const char *fs_type' This name is poorly chosen. This element points to a string (possibly in the `fs_mntops' string) which describes the modes with which the filesystem is mounted. `fstab' defines five macros to describe the possible values: `FSTAB_RW' The filesystems gets mounted with read and write enabled. `FSTAB_RQ' The filesystems gets mounted with read and write enabled. Write access is restricted by quotas. `FSTAB_RO' The filesystem gets mounted read-only. `FSTAB_SW' This is not a real filesystem, it is a swap device. `FSTAB_XX' This entry from the `fstab' file is totally ignored. Testing for equality with these value must happen using `strcmp' since these are all strings. Comparing the pointer will probably always fail. `int fs_freq' This element describes the dump frequency in days. `int fs_passno' This element describes the pass number on parallel dumps. It is closely related to the `dump' utility used on Unix systems. To read the entire content of the of the `fstab' file the GNU libc contains a set of three functions which are designed in the usual way. - Function: int setfsent (void) This function makes sure that the internal read pointer for the `fstab' file is at the beginning of the file. This is done by either opening the file or resetting the read pointer. Since the file handle is internal to the libc this function is not thread-safe. This function returns a non-zero value if the operation was successful and the `getfs*' functions can be used to read the entries of the file. - Function: void endfsent (void) This function makes sure that all resources acquired by a prior call to `setfsent' (explicitly or implicitly by calling `getfsent') are freed. - Function: struct fstab * getfsent (void) This function returns the next entry of the `fstab' file. If this is the first call to any of the functions handling `fstab' since program start or the last call of `endfsent', the file will be opened. The function returns a pointer to a variable of type `struct fstab'. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred `getfsent' returns a `NULL' pointer. - Function: struct fstab * getfsspec (const char *NAME) This function returns the next entry of the `fstab' file which has a string equal to NAME pointed to by the `fs_spec' element. Since there is normally exactly one entry for each special device it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling `fstab' since program start or the last call of `endfsent', the file will be opened. The function returns a pointer to a variable of type `struct fstab'. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred `getfsent' returns a `NULL' pointer. - Function: struct fstab * getfsfile (const char *NAME) This function returns the next entry of the `fstab' file which has a string equal to NAME pointed to by the `fs_file' element. Since there is normally exactly one entry for each mount point it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling `fstab' since program start or the last call of `endfsent', the file will be opened. The function returns a pointer to a variable of type `struct fstab'. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred `getfsent' returns a `NULL' pointer. The `mtab' file ............... The following functions and data structure access the `mtab' file. - Data Type: struct mntent This structure is used with the `getmntent', `getmntent_t', `addmntent', and `hasmntopt' functions. `char *mnt_fsname' This element contains a pointer to a string describing the name of the special device from which the filesystem is mounted. It corresponds to the `fs_spec' element in `struct fstab'. `char *mnt_dir' This element points to a string describing the mount point of the filesystem. It corresponds to the `fs_file' element in `struct fstab'. `char *mnt_type' `mnt_type' describes the filesystem type and is therefore equivalent to `fs_vfstype' in `struct fstab'. `mntent.h' defines a few symbolic names for some of the values this string can have. But since the kernel can support arbitrary filesystems it does not make much sense to give them symbolic names. If one knows the symbol name one also knows the filesystem name. Nevertheless here follows the list of the symbols provided in `mntent.h'. `MNTTYPE_IGNORE' This symbol expands to `"ignore"'. The value is sometime used in `fstab' files to make sure entries are not used without removing them. `MNTTYPE_NFS' Expands to `"nfs"'. Using this macro sometimes could make sense since it names the default NFS implementation, in case both version 2 and 3 are supported. `MNTTYPE_SWAP' This symbol expands to `"swap"'. It names the special `fstab' entry which names one of the possibly multiple swap partitions. `char *mnt_opts' The element contains a string describing the options used while mounting the filesystem. As for the equivalent element `fs_mntops' of `struct fstab' it is best to use the function `getsubopt' (*note Suboptions::) to access the parts of this string. The `mntent.h' file defines a number of macros with string values which correspond to some of the options understood by the kernel. There might be many more options which are possible so it doesn't make much sense to rely on these macros but to be consistent here is the list: `MNTOPT_DEFAULTS' Expands to `"defaults"'. This option should be used alone since it indicates all values for the customizable values are chosen to be the default. `MNTOPT_RO' Expands to `"ro"'. See the `FSTAB_RO' value, it means the filesystem is mounted read-only. `MNTOPT_RW' Expand to `"rw"'. See the `FSTAB_RW' value, it means the filesystem is mounted with read and write permissions. `MNTOPT_SUID' Expands to `"suid"'. This means that the SUID bit (*note How Change Persona::) is respected when a program from the filesystem is started. `MNTOPT_NOSUID' Expands to `"nosuid"'. This is the opposite of `MNTOPT_SUID', the SUID bit for all files from the filesystem is ignored. `MNTOPT_NOAUTO' Expands to `"noauto"'. At startup time the `mount' program will ignore this entry if it is started with the `-a' option to mount all filesystems mentioned in the `fstab' file. As for the `FSTAB_*' entries introduced above it is important to use `strcmp' to check for equality. `mnt_freq' This elements corresponds to `fs_freq' and also specifies the frequency in days in which dumps are made. `mnt_passno' This element is equivalent to `fs_passno' with the same meaning which is uninteresting for all programs beside `dump'. For accessing the `mtab' file there is again a set of three functions to access all entries in a row. Unlike the functions to handle `fstab' these functions do not access a fixed file and there is even a thread safe variant of the get function. Beside this the GNU libc contains functions to alter the file and test for specific options. - Function: FILE * setmntent (const char *FILE, const char *MODE) The `setmntent' function prepares the file named FILE which must be in the format of a `fstab' and `mtab' file for the upcoming processing through the other functions of the family. The MODE parameter can be chosen in the way the OPENTYPE parameter for `fopen' (*note Opening Streams::) can be chosen. If the file is opened for writing the file is also allowed to be empty. If the file was successfully opened `setmntent' returns a file descriptor for future use. Otherwise the return value is `NULL' and `errno' is set accordingly. - Function: int endmntent (FILE *STREAM) This function takes for the STREAM parameter a file handle which previously was returned from the `setmntent' call. `endmntent' closes the stream and frees all resources. The return value is 1 unless an error occurred in which case it is 0. - Function: struct mntent * getmntent (FILE *STREAM) The `getmntent' function takes as the parameter a file handle previously returned by successful call to `setmntent'. It returns a pointer to a static variable of type `struct mntent' which is filled with the information from the next entry from the file currently read. The file format used prescribes the use of spaces or tab characters to separate the fields. This makes it harder to use name containing one of these characters (e.g., mount points using spaces). Therefore these characters are encoded in the files and the `getmntent' function takes care of the decoding while reading the entries back in. `'\040'' is used to encode a space character, `'\012'' to encode a tab character and `'\\'' to encode a backslash. If there was an error or the end of the file is reached the return value is `NULL'. This function is not thread-safe since all calls to this function return a pointer to the same static variable. `getmntent_r' should be used in situations where multiple threads access the file. - Function: struct mntent * getmntent_r (FILE *STREAM, struct mentent *RESULT, char *BUFFER, int BUFSIZE) The `getmntent_r' function is the reentrant variant of `getmntent'. It also returns the next entry from the file and returns a pointer. The actual variable the values are stored in is not static, though. Instead the function stores the values in the variable pointed to by the RESULT parameter. Additional information (e.g., the strings pointed to by the elements of the result) are kept in the buffer of size BUFSIZE pointed to by BUFFER. Escaped characters (space, tab, backslash) are converted back in the same way as it happens for `getmentent'. The function returns a `NULL' pointer in error cases. Errors could be: * error while reading the file, * end of file reached, * BUFSIZE is too small for reading a complete new entry. - Function: int addmntent (FILE *STREAM, const struct mntent *MNT) The `addmntent' function allows adding a new entry to the file previously opened with `setmntent'. The new entries are always appended. I.e., even if the position of the file descriptor is not at the end of the file this function does not overwrite an existing entry following the current position. The implication of this is that to remove an entry from a file one has to create a new file while leaving out the entry to be removed and after closing the file remove the old one and rename the new file to the chosen name. This function takes care of spaces and tab characters in the names to be written to the file. It converts them and the backslash character into the format describe in the `getmntent' description above. This function returns 0 in case the operation was successful. Otherwise the return value is 1 and `errno' is set appropriately. - Function: char * hasmntopt (const struct mntent *MNT, const char *OPT) This function can be used to check whether the string pointed to by the `mnt_opts' element of the variable pointed to by MNT contains the option OPT. If this is true a pointer to the beginning of the option in the `mnt_opts' element is returned. If no such option exists the function returns `NULL'. This function is useful to test whether a specific option is present but when all options have to be processed one is better off with using the `getsubopt' function to iterate over all options in the string. Other (Non-libc) Sources of Mount Information ............................................. On a system with a Linux kernel and the `proc' filesystem, you can get information on currently mounted filesystems from the file `mounts' in the `proc' filesystem. Its format is similar to that of the `mtab' file, but represents what is truly mounted without relying on facilities outside the kernel to keep `mtab' up to date. Mount, Unmount, Remount ----------------------- This section describes the functions for mounting, unmounting, and remounting filesystems. Only the superuser can mount, unmount, or remount a filesystem. These functions do not access the `fstab' and `mtab' files. You should maintain and use these separately. *Note Mount Information::. The symbols in this section are declared in `sys/mount.h'. - Function: int mount (const char *SPECIAL_FILE, const char *DIR, const char *FSTYPE, unsigned long int OPTIONS, const void *DATA) `mount' mounts or remounts a filesystem. The two operations are quite different and are merged rather unnaturally into this one function. The `MS_REMOUNT' option, explained below, determines whether `mount' mounts or remounts. For a mount, the filesystem on the block device represented by the device special file named SPECIAL_FILE gets mounted over the mount point DIR. This means that the directory DIR (along with any files in it) is no longer visible; in its place (and still with the name DIR) is the root directory of the filesystem on the device. As an exception, if the filesystem type (see below) is one which is not based on a device (e.g. "proc"), `mount' instantiates a filesystem and mounts it over DIR and ignores SPECIAL_FILE. For a remount, DIR specifies the mount point where the filesystem to be remounted is (and remains) mounted and SPECIAL_FILE is ignored. Remounting a filesystem means changing the options that control operations on the filesystem while it is mounted. It does not mean unmounting and mounting again. For a mount, you must identify the type of the filesystem as FSTYPE. This type tells the kernel how to access the filesystem and can be thought of as the name of a filesystem driver. The acceptable values are system dependent. On a system with a Linux kernel and the `proc' filesystem, the list of possible values is in the file `filesystems' in the `proc' filesystem (e.g. type `cat /proc/filesystems' to see the list). With a Linux kernel, the types of filesystems that `mount' can mount, and their type names, depends on what filesystem drivers are configured into the kernel or loaded as loadable kernel modules. An example of a common value for FSTYPE is `ext2'. For a remount, `mount' ignores FSTYPE. OPTIONS specifies a variety of options that apply until the filesystem is unmounted or remounted. The precise meaning of an option depends on the filesystem and with some filesystems, an option may have no effect at all. Furthermore, for some filesystems, some of these options (but never `MS_RDONLY') can be overridden for individual file accesses via `ioctl'. OPTIONS is a bit string with bit fields defined using the following mask and masked value macros: `MS_MGC_MASK' This multibit field contains a magic number. If it does not have the value `MS_MGC_VAL', `mount' assumes all the following bits are zero and the DATA argument is a null string, regardless of their actual values. `MS_REMOUNT' This bit on means to remount the filesystem. Off means to mount it. `MS_RDONLY' This bit on specifies that no writing to the filesystem shall be allowed while it is mounted. This cannot be overridden by `ioctl'. This option is available on nearly all filesystems. `S_IMMUTABLE' This bit on specifies that no writing to the files in the filesystem shall be allowed while it is mounted. This can be overridden for a particular file access by a properly privileged call to `ioctl'. This option is a relatively new invention and is not available on many filesystems. `S_APPEND' This bit on specifies that the only file writing that shall be allowed while the filesystem is mounted is appending. Some filesystems allow this to be overridden for a particular process by a properly privileged call to `ioctl'. This is a relatively new invention and is not available on many filesystems. `MS_NOSUID' This bit on specifies that Setuid and Setgid permissions on files in the filesystem shall be ignored while it is mounted. `MS_NOEXEC' This bit on specifies that no files in the filesystem shall be executed while the filesystem is mounted. `MS_NODEV' This bit on specifies that no device special files in the filesystem shall be accessible while the filesystem is mounted. `MS_SYNCHRONOUS' This bit on specifies that all writes to the filesystem while it is mounted shall be synchronous; i.e. data shall be synced before each write completes rather than held in the buffer cache. `MS_MANDLOCK' This bit on specifies that mandatory locks on files shall be permitted while the filesystem is mounted. `MS_NOATIME' This bit on specifies that access times of files shall not be updated when the files are accessed while the filesystem is mounted. `MS_NODIRATIME' This bit on specifies that access times of directories shall not be updated when the directories are accessed while the filesystem in mounted. Any bits not covered by the above masks should be set off; otherwise, results are undefined. The meaning of DATA depends on the filesystem type and is controlled entirely by the filesystem driver in the kernel. Example: #include mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, ""); mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, ""); Appropriate arguments for `mount' are conventionally recorded in the `fstab' table. *Note Mount Information::. The return value is zero if the mount or remount is successful. Otherwise, it is `-1' and `errno' is set appropriately. The values of `errno' are filesystem dependent, but here is a general list: `EPERM' The process is not superuser. `ENODEV' The file system type FSTYPE is not known to the kernel. `ENOTBLK' The file DEV is not a block device special file. `EBUSY' * The device is already mounted. * The mount point is busy. (E.g. it is some process' working directory or has a filesystem mounted on it already). * The request is to remount read-only, but there are files open for write. `EINVAL' * A remount was attempted, but there is no filesystem mounted over the specified mount point. * The supposed filesystem has an invalid superblock. `EACCES' * The filesystem is inherently read-only (possibly due to a switch on the device) and the process attempted to mount it read/write (by setting the `MS_RDONLY' bit off). * SPECIAL_FILE or DIR is not accessible due to file permissions. * SPECIAL_FILE is not accessible because it is in a filesystem that is mounted with the `MS_NODEV' option. `EM_FILE' The table of dummy devices is full. `mount' needs to create a dummy device (aka "unnamed" device) if the filesystem being mounted is not one that uses a device. - Function: int umount2 (const char *FILE, int FLAGS) `umount2' unmounts a filesystem. You can identify the filesystem to unmount either by the device special file that contains the filesystem or by the mount point. The effect is the same. Specify either as the string FILE. FLAGS contains the one-bit field identified by the following mask macro: `MNT_FORCE' This bit on means to force the unmounting even if the filesystem is busy, by making it unbusy first. If the bit is off and the filesystem is busy, `umount2' fails with `errno' = `EBUSY'. Depending on the filesystem, this may override all, some, or no busy conditions. All other bits in FLAGS should be set to zero; otherwise, the result is undefined. Example: #include umount2("/mnt", MNT_FORCE); umount2("/dev/hdd1", 0); After the filesystem is unmounted, the directory that was the mount point is visible, as are any files in it. As part of unmounting, `umount2' syncs the filesystem. If the unmounting is successful, the return value is zero. Otherwise, it is `-1' and `errno' is set accordingly: `EPERM' The process is not superuser. `EBUSY' The filesystem cannot be unmounted because it is busy. E.g. it contains a directory that is some process's working directory or a file that some process has open. With some filesystems in some cases, you can avoid this failure with the `MNT_FORCE' option. `EINVAL' FILE validly refers to a file, but that file is neither a mount point nor a device special file of a currently mounted filesystem. This function is not available on all systems. - Function: int umount (const char *FILE) `umount' does the same thing as `umount2' with FLAGS set to zeroes. It is more widely available than `umount2' but since it lacks the possibility to forcefully unmount a filesystem is deprecated when `umount2' is also available. System Parameters ================= This section describes the `sysctl' function, which gets and sets a variety of system parameters. The symbols used in this section are declared in the file `sysctl.h'. - Function: int sysctl (int *NAMES, int NLEN, void *OLDVAL, size_t *OLDLENP, void *NEWVAL, size_t NEWLEN) `sysctl' gets or sets a specified system parameter. There are so many of these parameters that it is not practical to list them all here, but here are some examples: * network domain name * paging parameters * network Address Resolution Protocol timeout time * maximum number of files that may be open * root filesystem device * when kernel was built The set of available parameters depends on the kernel configuration and can change while the system is running, particularly when you load and unload loadable kernel modules. The system parameters with which `syslog' is concerned are arranged in a hierarchical structure like a hierarchical filesystem. To identify a particular parameter, you specify a path through the structure in a way analogous to specifying the pathname of a file. Each component of the path is specified by an integer and each of these integers has a macro defined for it by `sysctl.h'. NAMES is the path, in the form of an array of integers. Each component of the path is one element of the array, in order. NLEN is the number of components in the path. For example, the first component of the path for all the paging parameters is the value `CTL_VM'. For the free page thresholds, the second component of the path is `VM_FREEPG'. So to get the free page threshold values, make NAMES an array containing the two elements `CTL_VM' and `VM_FREEPG' and make NLEN = 2. The format of the value of a parameter depends on the parameter. Sometimes it is an integer; sometimes it is an ASCII string; sometimes it is an elaborate structure. In the case of the free page thresholds used in the example above, the parameter value is a structure containing several integers. In any case, you identify a place to return the parameter's value with OLDVAL and specify the amount of storage available at that location as *OLDLENP. *OLDLENP does double duty because it is also the output location that contains the actual length of the returned value. If you don't want the parameter value returned, specify a null pointer for OLDVAL. To set the parameter, specify the address and length of the new value as NEWVAL and NEWLEN. If you don't want to set the parameter, specify a null pointer as NEWVAL. If you get and set a parameter in the same `sysctl' call, the value returned is the value of the parameter before it was set. Each system parameter has a set of permissions similar to the permissions for a file (including the permissions on directories in its path) that determine whether you may get or set it. For the purposes of these permissions, every parameter is considered to be owned by the superuser and Group 0 so processes with that effective uid or gid may have more access to system parameters. Unlike with files, the superuser does not invariably have full permission to all system parameters, because some of them are designed not to be changed ever. `sysctl' returns a zero return value if it succeeds. Otherwise, it returns `-1' and sets `errno' appropriately. Besides the failures that apply to all system calls, the following are the `errno' codes for all possible failures: `EPERM' The process is not permitted to access one of the components of the path of the system parameter or is not permitted to access the system parameter itself in the way (read or write) that it requested. `ENOTDIR' There is no system parameter corresponding to NAME. `EFAULT' OLDVAL is not null, which means the process wanted to read the parameter, but *OLDLENP is zero, so there is no place to return it. `EINVAL' * The process attempted to set a system parameter to a value that is not valid for that parameter. * The space provided for the return of the system parameter is not the right size for that parameter. `ENOMEM' This value may be returned instead of the more correct `EINVAL' in some cases where the space provided for the return of the system parameter is too small. If you have a Linux kernel with the `proc' filesystem, you can get and set most of the same parameters by reading and writing to files in the `sys' directory of the `proc' filesystem. In the `sys' directory, the directory structure represents the hierarchical structure of the parameters. E.g. you can display the free page thresholds with cat /proc/sys/vm/freepages Some more traditional and more widely available, though less general, GNU C library functions for getting and setting some of the same system parameters are: * `getdomainname', `setdomainname' * `gethostname', `sethostname' (*Note Host Identification::.) * `uname' (*Note Platform Type::.) * `bdflush' System Configuration Parameters ******************************* The functions and macros listed in this chapter give information about configuration parameters of the operating system--for example, capacity limits, presence of optional POSIX features, and the default path for executable files (*note String Parameters::). General Capacity Limits ======================= The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system. These limits can be fixed constants for a given operating system, or they can vary from machine to machine. For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs. Each of the following limit parameters has a macro that is defined in `limits.h' only if the system has a fixed, uniform limit for the parameter in question. If the system allows different file systems or files to have different limits, then the macro is undefined; use `sysconf' to find out the limit that applies at a particular time on a particular machine. *Note Sysconf::. Each of these parameters also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on _any_ POSIX system. *Note Minimums::. - Macro: int ARG_MAX If defined, the unvarying maximum combined length of the ARGV and ENVIRON arguments that can be passed to the `exec' functions. - Macro: int CHILD_MAX If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time. In BSD and GNU, this is controlled by the `RLIMIT_NPROC' resource limit; *note Limits on Resources::. - Macro: int OPEN_MAX If defined, the unvarying maximum number of files that a single process can have open simultaneously. In BSD and GNU, this is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. - Macro: int STREAM_MAX If defined, the unvarying maximum number of streams that a single process can have open simultaneously. *Note Opening Streams::. - Macro: int TZNAME_MAX If defined, the unvarying maximum length of a time zone name. *Note Time Zone Functions::. These limit macros are always defined in `limits.h'. - Macro: int NGROUPS_MAX The maximum number of supplementary group IDs that one process can have. The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many supplementary group IDs, but a particular machine might let you have even more. You can use `sysconf' to see whether a particular machine will let you have more (*note Sysconf::). - Macro: int SSIZE_MAX The largest value that can fit in an object of type `ssize_t'. Effectively, this is the limit on the number of bytes that can be read or written in a single operation. This macro is defined in all POSIX systems because this limit is never configurable. - Macro: int RE_DUP_MAX The largest number of repetitions you are guaranteed is allowed in the construct `\{MIN,MAX\}' in a regular expression. The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many repetitions, but a particular machine might let you have even more. You can use `sysconf' to see whether a particular machine will let you have more (*note Sysconf::). And even the value that `sysconf' tells you is just a lower bound--larger values might work. This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit. Overall System Options ====================== POSIX defines certain system-specific options that not all POSIX systems support. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using. You can test for the availability of a given option using the macros in this section, together with the function `sysconf'. The macros are defined only if you include `unistd.h'. For the following macros, if the macro is defined in `unistd.h', then the option is supported. Otherwise, the option may or may not be supported; use `sysconf' to find out. *Note Sysconf::. - Macro: int _POSIX_JOB_CONTROL If this symbol is defined, it indicates that the system supports job control. Otherwise, the implementation behaves as if all processes within a session belong to a single process group. *Note Job Control::. - Macro: int _POSIX_SAVED_IDS If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted. If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again. *Note Enable/Disable Setuid::. For the following macros, if the macro is defined in `unistd.h', then its value indicates whether the option is supported. A value of `-1' means no, and any other value means yes. If the macro is not defined, then the option may or may not be supported; use `sysconf' to find out. *Note Sysconf::. - Macro: int _POSIX2_C_DEV If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, `c89'. The GNU C library always defines this as `1', on the assumption that you would not have installed it if you didn't have a C compiler. - Macro: int _POSIX2_FORT_DEV If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, `fort77'. The GNU C library never defines this, because we don't know what the system has. - Macro: int _POSIX2_FORT_RUN If this symbol is defined, it indicates that the system has the POSIX.2 `asa' command to interpret Fortran carriage control. The GNU C library never defines this, because we don't know what the system has. - Macro: int _POSIX2_LOCALEDEF If this symbol is defined, it indicates that the system has the POSIX.2 `localedef' command. The GNU C library never defines this, because we don't know what the system has. - Macro: int _POSIX2_SW_DEV If this symbol is defined, it indicates that the system has the POSIX.2 commands `ar', `make', and `strip'. The GNU C library always defines this as `1', on the assumption that you had to have `ar' and `make' to install the library, and it's unlikely that `strip' would be absent when those are present. Which Version of POSIX is Supported =================================== - Macro: long int _POSIX_VERSION This constant represents the version of the POSIX.1 standard to which the implementation conforms. For an implementation conforming to the 1995 POSIX.1 standard, the value is the integer `199506L'. `_POSIX_VERSION' is always defined (in `unistd.h') in any POSIX system. *Usage Note:* Don't try to test whether the system supports POSIX by including `unistd.h' and then checking whether `_POSIX_VERSION' is defined. On a non-POSIX system, this will probably fail because there is no `unistd.h'. We do not know of _any_ way you can reliably test at compilation time whether your target system supports POSIX or whether `unistd.h' exists. The GNU C compiler predefines the symbol `__POSIX__' if the target system is a POSIX system. Provided you do not use any other compilers on POSIX systems, testing `defined (__POSIX__)' will reliably detect such systems. - Macro: long int _POSIX2_C_VERSION This constant represents the version of the POSIX.2 standard which the library and system kernel support. We don't know what value this will be for the first version of the POSIX.2 standard, because the value is based on the year and month in which the standard is officially adopted. The value of this symbol says nothing about the utilities installed on the system. *Usage Note:* You can use this macro to tell whether a POSIX.1 system library supports POSIX.2 as well. Any POSIX.1 system contains `unistd.h', so include that file and then test `defined (_POSIX2_C_VERSION)'. Using `sysconf' =============== When your system has configurable system limits, you can use the `sysconf' function to find out the value that applies to any particular machine. The function and the associated PARAMETER constants are declared in the header file `unistd.h'. Definition of `sysconf' ----------------------- - Function: long int sysconf (int PARAMETER) This function is used to inquire about runtime system parameters. The PARAMETER argument should be one of the `_SC_' symbols listed below. The normal return value from `sysconf' is the value you requested. A value of `-1' is returned both if the implementation does not impose a limit, and in case of an error. The following `errno' error conditions are defined for this function: `EINVAL' The value of the PARAMETER is invalid. Constants for `sysconf' Parameters ---------------------------------- Here are the symbolic constants for use as the PARAMETER argument to `sysconf'. The values are all integer constants (more specifically, enumeration type values). `_SC_ARG_MAX' Inquire about the parameter corresponding to `ARG_MAX'. `_SC_CHILD_MAX' Inquire about the parameter corresponding to `CHILD_MAX'. `_SC_OPEN_MAX' Inquire about the parameter corresponding to `OPEN_MAX'. `_SC_STREAM_MAX' Inquire about the parameter corresponding to `STREAM_MAX'. `_SC_TZNAME_MAX' Inquire about the parameter corresponding to `TZNAME_MAX'. `_SC_NGROUPS_MAX' Inquire about the parameter corresponding to `NGROUPS_MAX'. `_SC_JOB_CONTROL' Inquire about the parameter corresponding to `_POSIX_JOB_CONTROL'. `_SC_SAVED_IDS' Inquire about the parameter corresponding to `_POSIX_SAVED_IDS'. `_SC_VERSION' Inquire about the parameter corresponding to `_POSIX_VERSION'. `_SC_CLK_TCK' Inquire about the parameter corresponding to `CLOCKS_PER_SEC'; *note CPU Time::. `_SC_CHARCLASS_NAME_MAX' Inquire about the parameter corresponding to maximal length allowed for a character class name in an extended locale specification. These extensions are not yet standardized and so this option is not standardized as well. `_SC_REALTIME_SIGNALS' Inquire about the parameter corresponding to `_POSIX_REALTIME_SIGNALS'. `_SC_PRIORITY_SCHEDULING' Inquire about the parameter corresponding to `_POSIX_PRIORITY_SCHEDULING'. `_SC_TIMERS' Inquire about the parameter corresponding to `_POSIX_TIMERS'. `_SC_ASYNCHRONOUS_IO' Inquire about the parameter corresponding to `_POSIX_ASYNCHRONOUS_IO'. `_SC_PRIORITIZED_IO' Inquire about the parameter corresponding to `_POSIX_PRIORITIZED_IO'. `_SC_SYNCHRONIZED_IO' Inquire about the parameter corresponding to `_POSIX_SYNCHRONIZED_IO'. `_SC_FSYNC' Inquire about the parameter corresponding to `_POSIX_FSYNC'. `_SC_MAPPED_FILES' Inquire about the parameter corresponding to `_POSIX_MAPPED_FILES'. `_SC_MEMLOCK' Inquire about the parameter corresponding to `_POSIX_MEMLOCK'. `_SC_MEMLOCK_RANGE' Inquire about the parameter corresponding to `_POSIX_MEMLOCK_RANGE'. `_SC_MEMORY_PROTECTION' Inquire about the parameter corresponding to `_POSIX_MEMORY_PROTECTION'. `_SC_MESSAGE_PASSING' Inquire about the parameter corresponding to `_POSIX_MESSAGE_PASSING'. `_SC_SEMAPHORES' Inquire about the parameter corresponding to `_POSIX_SEMAPHORES'. `_SC_SHARED_MEMORY_OBJECTS' Inquire about the parameter corresponding to `_POSIX_SHARED_MEMORY_OBJECTS'. `_SC_AIO_LISTIO_MAX' Inquire about the parameter corresponding to `_POSIX_AIO_LISTIO_MAX'. `_SC_AIO_MAX' Inquire about the parameter corresponding to `_POSIX_AIO_MAX'. `_SC_AIO_PRIO_DELTA_MAX' Inquire the value by which a process can decrease its asynchronous I/O priority level from its own scheduling priority. This corresponds to the run-time invariant value `AIO_PRIO_DELTA_MAX'. `_SC_DELAYTIMER_MAX' Inquire about the parameter corresponding to `_POSIX_DELAYTIMER_MAX'. `_SC_MQ_OPEN_MAX' Inquire about the parameter corresponding to `_POSIX_MQ_OPEN_MAX'. `_SC_MQ_PRIO_MAX' Inquire about the parameter corresponding to `_POSIX_MQ_PRIO_MAX'. `_SC_RTSIG_MAX' Inquire about the parameter corresponding to `_POSIX_RTSIG_MAX'. `_SC_SEM_NSEMS_MAX' Inquire about the parameter corresponding to `_POSIX_SEM_NSEMS_MAX'. `_SC_SEM_VALUE_MAX' Inquire about the parameter corresponding to `_POSIX_SEM_VALUE_MAX'. `_SC_SIGQUEUE_MAX' Inquire about the parameter corresponding to `_POSIX_SIGQUEUE_MAX'. `_SC_TIMER_MAX' Inquire about the parameter corresponding to `_POSIX_TIMER_MAX'. `_SC_PII' Inquire about the parameter corresponding to `_POSIX_PII'. `_SC_PII_XTI' Inquire about the parameter corresponding to `_POSIX_PII_XTI'. `_SC_PII_SOCKET' Inquire about the parameter corresponding to `_POSIX_PII_SOCKET'. `_SC_PII_INTERNET' Inquire about the parameter corresponding to `_POSIX_PII_INTERNET'. `_SC_PII_OSI' Inquire about the parameter corresponding to `_POSIX_PII_OSI'. `_SC_SELECT' Inquire about the parameter corresponding to `_POSIX_SELECT'. `_SC_UIO_MAXIOV' Inquire about the parameter corresponding to `_POSIX_UIO_MAXIOV'. `_SC_PII_INTERNET_STREAM' Inquire about the parameter corresponding to `_POSIX_PII_INTERNET_STREAM'. `_SC_PII_INTERNET_DGRAM' Inquire about the parameter corresponding to `_POSIX_PII_INTERNET_DGRAM'. `_SC_PII_OSI_COTS' Inquire about the parameter corresponding to `_POSIX_PII_OSI_COTS'. `_SC_PII_OSI_CLTS' Inquire about the parameter corresponding to `_POSIX_PII_OSI_CLTS'. `_SC_PII_OSI_M' Inquire about the parameter corresponding to `_POSIX_PII_OSI_M'. `_SC_T_IOV_MAX' Inquire the value of the value associated with the `T_IOV_MAX' variable. `_SC_THREADS' Inquire about the parameter corresponding to `_POSIX_THREADS'. `_SC_THREAD_SAFE_FUNCTIONS' Inquire about the parameter corresponding to `_POSIX_THREAD_SAFE_FUNCTIONS'. `_SC_GETGR_R_SIZE_MAX' Inquire about the parameter corresponding to `_POSIX_GETGR_R_SIZE_MAX'. `_SC_GETPW_R_SIZE_MAX' Inquire about the parameter corresponding to `_POSIX_GETPW_R_SIZE_MAX'. `_SC_LOGIN_NAME_MAX' Inquire about the parameter corresponding to `_POSIX_LOGIN_NAME_MAX'. `_SC_TTY_NAME_MAX' Inquire about the parameter corresponding to `_POSIX_TTY_NAME_MAX'. `_SC_THREAD_DESTRUCTOR_ITERATIONS' Inquire about the parameter corresponding to `_POSIX_THREAD_DESTRUCTOR_ITERATIONS'. `_SC_THREAD_KEYS_MAX' Inquire about the parameter corresponding to `_POSIX_THREAD_KEYS_MAX'. `_SC_THREAD_STACK_MIN' Inquire about the parameter corresponding to `_POSIX_THREAD_STACK_MIN'. `_SC_THREAD_THREADS_MAX' Inquire about the parameter corresponding to `_POSIX_THREAD_THREADS_MAX'. `_SC_THREAD_ATTR_STACKADDR' Inquire about the parameter corresponding to a `_POSIX_THREAD_ATTR_STACKADDR'. `_SC_THREAD_ATTR_STACKSIZE' Inquire about the parameter corresponding to `_POSIX_THREAD_ATTR_STACKSIZE'. `_SC_THREAD_PRIORITY_SCHEDULING' Inquire about the parameter corresponding to `_POSIX_THREAD_PRIORITY_SCHEDULING'. `_SC_THREAD_PRIO_INHERIT' Inquire about the parameter corresponding to `_POSIX_THREAD_PRIO_INHERIT'. `_SC_THREAD_PRIO_PROTECT' Inquire about the parameter corresponding to `_POSIX_THREAD_PRIO_PROTECT'. `_SC_THREAD_PROCESS_SHARED' Inquire about the parameter corresponding to `_POSIX_THREAD_PROCESS_SHARED'. `_SC_2_C_DEV' Inquire about whether the system has the POSIX.2 C compiler command, `c89'. `_SC_2_FORT_DEV' Inquire about whether the system has the POSIX.2 Fortran compiler command, `fort77'. `_SC_2_FORT_RUN' Inquire about whether the system has the POSIX.2 `asa' command to interpret Fortran carriage control. `_SC_2_LOCALEDEF' Inquire about whether the system has the POSIX.2 `localedef' command. `_SC_2_SW_DEV' Inquire about whether the system has the POSIX.2 commands `ar', `make', and `strip'. `_SC_BC_BASE_MAX' Inquire about the maximum value of `obase' in the `bc' utility. `_SC_BC_DIM_MAX' Inquire about the maximum size of an array in the `bc' utility. `_SC_BC_SCALE_MAX' Inquire about the maximum value of `scale' in the `bc' utility. `_SC_BC_STRING_MAX' Inquire about the maximum size of a string constant in the `bc' utility. `_SC_COLL_WEIGHTS_MAX' Inquire about the maximum number of weights that can necessarily be used in defining the collating sequence for a locale. `_SC_EXPR_NEST_MAX' Inquire about the maximum number of expressions nested within parentheses when using the `expr' utility. `_SC_LINE_MAX' Inquire about the maximum size of a text line that the POSIX.2 text utilities can handle. `_SC_EQUIV_CLASS_MAX' Inquire about the maximum number of weights that can be assigned to an entry of the `LC_COLLATE' category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions. `_SC_VERSION' Inquire about the version number of POSIX.1 that the library and kernel support. `_SC_2_VERSION' Inquire about the version number of POSIX.2 that the system utilities support. `_SC_PAGESIZE' Inquire about the virtual memory page size of the machine. `getpagesize' returns the same value (*note Query Memory Parameters::). `_SC_NPROCESSORS_CONF' Inquire about the number of configured processors. `_SC_NPROCESSORS_ONLN' Inquire about the number of processors online. `_SC_PHYS_PAGES' Inquire about the number of physical pages in the system. `_SC_AVPHYS_PAGES' Inquire about the number of available physical pages in the system. `_SC_ATEXIT_MAX' Inquire about the number of functions which can be registered as termination functions for `atexit'; *note Cleanups on Exit::. `_SC_XOPEN_VERSION' Inquire about the parameter corresponding to `_XOPEN_VERSION'. `_SC_XOPEN_XCU_VERSION' Inquire about the parameter corresponding to `_XOPEN_XCU_VERSION'. `_SC_XOPEN_UNIX' Inquire about the parameter corresponding to `_XOPEN_UNIX'. `_SC_XOPEN_REALTIME' Inquire about the parameter corresponding to `_XOPEN_REALTIME'. `_SC_XOPEN_REALTIME_THREADS' Inquire about the parameter corresponding to `_XOPEN_REALTIME_THREADS'. `_SC_XOPEN_LEGACY' Inquire about the parameter corresponding to `_XOPEN_LEGACY'. `_SC_XOPEN_CRYPT' Inquire about the parameter corresponding to `_XOPEN_CRYPT'. `_SC_XOPEN_ENH_I18N' Inquire about the parameter corresponding to `_XOPEN_ENH_I18N'. `_SC_XOPEN_SHM' Inquire about the parameter corresponding to `_XOPEN_SHM'. `_SC_XOPEN_XPG2' Inquire about the parameter corresponding to `_XOPEN_XPG2'. `_SC_XOPEN_XPG3' Inquire about the parameter corresponding to `_XOPEN_XPG3'. `_SC_XOPEN_XPG4' Inquire about the parameter corresponding to `_XOPEN_XPG4'. `_SC_CHAR_BIT' Inquire about the number of bits in a variable of type `char'. `_SC_CHAR_MAX' Inquire about the maximum value which can be stored in a variable of type `char'. `_SC_CHAR_MIN' Inquire about the minimum value which can be stored in a variable of type `char'. `_SC_INT_MAX' Inquire about the maximum value which can be stored in a variable of type `int'. `_SC_INT_MIN' Inquire about the minimum value which can be stored in a variable of type `int'. `_SC_LONG_BIT' Inquire about the number of bits in a variable of type `long int'. `_SC_WORD_BIT' Inquire about the number of bits in a variable of a register word. `_SC_MB_LEN_MAX' Inquire the maximum length of a multi-byte representation of a wide character value. `_SC_NZERO' Inquire about the value used to internally represent the zero priority level for the process execution. `SC_SSIZE_MAX' Inquire about the maximum value which can be stored in a variable of type `ssize_t'. `_SC_SCHAR_MAX' Inquire about the maximum value which can be stored in a variable of type `signed char'. `_SC_SCHAR_MIN' Inquire about the minimum value which can be stored in a variable of type `signed char'. `_SC_SHRT_MAX' Inquire about the maximum value which can be stored in a variable of type `short int'. `_SC_SHRT_MIN' Inquire about the minimum value which can be stored in a variable of type `short int'. `_SC_UCHAR_MAX' Inquire about the maximum value which can be stored in a variable of type `unsigned char'. `_SC_UINT_MAX' Inquire about the maximum value which can be stored in a variable of type `unsigned int'. `_SC_ULONG_MAX' Inquire about the maximum value which can be stored in a variable of type `unsigned long int'. `_SC_USHRT_MAX' Inquire about the maximum value which can be stored in a variable of type `unsigned short int'. `_SC_NL_ARGMAX' Inquire about the parameter corresponding to `NL_ARGMAX'. `_SC_NL_LANGMAX' Inquire about the parameter corresponding to `NL_LANGMAX'. `_SC_NL_MSGMAX' Inquire about the parameter corresponding to `NL_MSGMAX'. `_SC_NL_NMAX' Inquire about the parameter corresponding to `NL_NMAX'. `_SC_NL_SETMAX' Inquire about the parameter corresponding to `NL_SETMAX'. `_SC_NL_TEXTMAX' Inquire about the parameter corresponding to `NL_TEXTMAX'. Examples of `sysconf' --------------------- We recommend that you first test for a macro definition for the parameter you are interested in, and call `sysconf' only if the macro is not defined. For example, here is how to test whether job control is supported: int have_job_control (void) { #ifdef _POSIX_JOB_CONTROL return 1; #else int value = sysconf (_SC_JOB_CONTROL); if (value < 0) /* If the system is that badly wedged, there's no use trying to go on. */ fatal (strerror (errno)); return value; #endif } Here is how to get the value of a numeric limit: int get_child_max () { #ifdef CHILD_MAX return CHILD_MAX; #else int value = sysconf (_SC_CHILD_MAX); if (value < 0) fatal (strerror (errno)); return value; #endif } Minimum Values for General Capacity Limits ========================================== Here are the names for the POSIX minimum upper bounds for the system limit parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far. `_POSIX_AIO_LISTIO_MAX' The most restrictive limit permitted by POSIX for the maximum number of I/O operations that can be specified in a list I/O call. The value of this constant is `2'; thus you can add up to two new entries of the list of outstanding operations. `_POSIX_AIO_MAX' The most restrictive limit permitted by POSIX for the maximum number of outstanding asynchronous I/O operations. The value of this constant is `1'. So you cannot expect that you can issue more than one operation and immediately continue with the normal work, receiving the notifications asynchronously. `_POSIX_ARG_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum combined length of the ARGV and ENVIRON arguments that can be passed to the `exec' functions. Its value is `4096'. `_POSIX_CHILD_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of simultaneous processes per real user ID. Its value is `6'. `_POSIX_NGROUPS_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of supplementary group IDs per process. Its value is `0'. `_POSIX_OPEN_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of files that a single process can have open simultaneously. Its value is `16'. `_POSIX_SSIZE_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum value that can be stored in an object of type `ssize_t'. Its value is `32767'. `_POSIX_STREAM_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of streams that a single process can have open simultaneously. Its value is `8'. `_POSIX_TZNAME_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the maximum length of a time zone name. Its value is `3'. `_POSIX2_RE_DUP_MAX' The value of this macro is the most restrictive limit permitted by POSIX for the numbers used in the `\{MIN,MAX\}' construct in a regular expression. Its value is `255'. Limits on File System Capacity ============================== The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system. It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case. On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits. For example, this is very likely if you use NFS to mount some of the file systems from other machines. Each of the following macros is defined in `limits.h' only if the system has a fixed, uniform limit for the parameter in question. If the system allows different file systems or files to have different limits, then the macro is undefined; use `pathconf' or `fpathconf' to find out the limit that applies to a particular file. *Note Pathconf::. Each parameter also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on _any_ POSIX system. *Note File Minimums::. - Macro: int LINK_MAX The uniform system limit (if any) for the number of names for a given file. *Note Hard Links::. - Macro: int MAX_CANON The uniform system limit (if any) for the amount of text in a line of input when input editing is enabled. *Note Canonical or Not::. - Macro: int MAX_INPUT The uniform system limit (if any) for the total number of characters typed ahead as input. *Note I/O Queues::. - Macro: int NAME_MAX The uniform system limit (if any) for the length of a file name component. - Macro: int PATH_MAX The uniform system limit (if any) for the length of an entire file name (that is, the argument given to system calls such as `open'). - Macro: int PIPE_BUF The uniform system limit (if any) for the number of bytes that can be written atomically to a pipe. If multiple processes are writing to the same pipe simultaneously, output from different processes might be interleaved in chunks of this size. *Note Pipes and FIFOs::. These are alternative macro names for some of the same information. - Macro: int MAXNAMLEN This is the BSD name for `NAME_MAX'. It is defined in `dirent.h'. - Macro: int FILENAME_MAX The value of this macro is an integer constant expression that represents the maximum length of a file name string. It is defined in `stdio.h'. Unlike `PATH_MAX', this macro is defined even if there is no actual limit imposed. In such a case, its value is typically a very large number. *This is always the case on the GNU system.* *Usage Note:* Don't use `FILENAME_MAX' as the size of an array in which to store a file name! You can't possibly make an array that big! Use dynamic allocation (*note Memory Allocation::) instead. Optional Features in File Support ================================= POSIX defines certain system-specific options in the system calls for operating on files. Some systems support these options and others do not. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee that any of these features is supported; it depends on the system you are using. They can also vary between file systems on a single machine. This section describes the macros you can test to determine whether a particular option is supported on your machine. If a given macro is defined in `unistd.h', then its value says whether the corresponding feature is supported. (A value of `-1' indicates no; any other value indicates yes.) If the macro is undefined, it means particular files may or may not support the feature. Since all the machines that support the GNU C library also support NFS, one can never make a general statement about whether all file systems support the `_POSIX_CHOWN_RESTRICTED' and `_POSIX_NO_TRUNC' features. So these names are never defined as macros in the GNU C library. - Macro: int _POSIX_CHOWN_RESTRICTED If this option is in effect, the `chown' function is restricted so that the only changes permitted to nonprivileged processes is to change the group owner of a file to either be the effective group ID of the process, or one of its supplementary group IDs. *Note File Owner::. - Macro: int _POSIX_NO_TRUNC If this option is in effect, file name components longer than `NAME_MAX' generate an `ENAMETOOLONG' error. Otherwise, file name components that are too long are silently truncated. - Macro: unsigned char _POSIX_VDISABLE This option is only meaningful for files that are terminal devices. If it is enabled, then handling for special control characters can be disabled individually. *Note Special Characters::. If one of these macros is undefined, that means that the option might be in effect for some files and not for others. To inquire about a particular file, call `pathconf' or `fpathconf'. *Note Pathconf::. Minimum Values for File System Limits ===================================== Here are the names for the POSIX minimum upper bounds for some of the above parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far. In most cases GNU systems do not have these strict limitations. The actual limit should be requested if necessary. `_POSIX_LINK_MAX' The most restrictive limit permitted by POSIX for the maximum value of a file's link count. The value of this constant is `8'; thus, you can always make up to eight names for a file without running into a system limit. `_POSIX_MAX_CANON' The most restrictive limit permitted by POSIX for the maximum number of bytes in a canonical input line from a terminal device. The value of this constant is `255'. `_POSIX_MAX_INPUT' The most restrictive limit permitted by POSIX for the maximum number of bytes in a terminal device input queue (or typeahead buffer). *Note Input Modes::. The value of this constant is `255'. `_POSIX_NAME_MAX' The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name component. The value of this constant is `14'. `_POSIX_PATH_MAX' The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name. The value of this constant is `256'. `_POSIX_PIPE_BUF' The most restrictive limit permitted by POSIX for the maximum number of bytes that can be written atomically to a pipe. The value of this constant is `512'. `SYMLINK_MAX' Maximum number of bytes in a symbolic link. `POSIX_REC_INCR_XFER_SIZE' Recommended increment for file transfer sizes between the `POSIX_REC_MIN_XFER_SIZE' and `POSIX_REC_MAX_XFER_SIZE' values. `POSIX_REC_MAX_XFER_SIZE' Maximum recommended file transfer size. `POSIX_REC_MIN_XFER_SIZE' Minimum recommended file transfer size. `POSIX_REC_XFER_ALIGN' Recommended file transfer buffer alignment. Using `pathconf' ================ When your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file. These functions and the associated constants for the PARAMETER argument are declared in the header file `unistd.h'. - Function: long int pathconf (const char *FILENAME, int PARAMETER) This function is used to inquire about the limits that apply to the file named FILENAME. The PARAMETER argument should be one of the `_PC_' constants listed below. The normal return value from `pathconf' is the value you requested. A value of `-1' is returned both if the implementation does not impose a limit, and in case of an error. In the former case, `errno' is not set, while in the latter case, `errno' is set to indicate the cause of the problem. So the only way to use this function robustly is to store `0' into `errno' just before calling it. Besides the usual file name errors (*note File Name Errors::), the following error condition is defined for this function: `EINVAL' The value of PARAMETER is invalid, or the implementation doesn't support the PARAMETER for the specific file. - Function: long int fpathconf (int FILEDES, int PARAMETER) This is just like `pathconf' except that an open file descriptor is used to specify the file for which information is requested, instead of a file name. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The value of PARAMETER is invalid, or the implementation doesn't support the PARAMETER for the specific file. Here are the symbolic constants that you can use as the PARAMETER argument to `pathconf' and `fpathconf'. The values are all integer constants. `_PC_LINK_MAX' Inquire about the value of `LINK_MAX'. `_PC_MAX_CANON' Inquire about the value of `MAX_CANON'. `_PC_MAX_INPUT' Inquire about the value of `MAX_INPUT'. `_PC_NAME_MAX' Inquire about the value of `NAME_MAX'. `_PC_PATH_MAX' Inquire about the value of `PATH_MAX'. `_PC_PIPE_BUF' Inquire about the value of `PIPE_BUF'. `_PC_CHOWN_RESTRICTED' Inquire about the value of `_POSIX_CHOWN_RESTRICTED'. `_PC_NO_TRUNC' Inquire about the value of `_POSIX_NO_TRUNC'. `_PC_VDISABLE' Inquire about the value of `_POSIX_VDISABLE'. `_PC_SYNC_IO' Inquire about the value of `_POSIX_SYNC_IO'. `_PC_ASYNC_IO' Inquire about the value of `_POSIX_ASYNC_IO'. `_PC_PRIO_IO' Inquire about the value of `_POSIX_PRIO_IO'. `_PC_SOCK_MAXBUF' Inquire about the value of `_POSIX_PIPE_BUF'. `_PC_FILESIZEBITS' Inquire about the availability of large files on the filesystem. `_PC_REC_INCR_XFER_SIZE' Inquire about the value of `POSIX_REC_INCR_XFER_SIZE'. `_PC_REC_MAX_XFER_SIZE' Inquire about the value of `POSIX_REC_MAX_XFER_SIZE'. `_PC_REC_MIN_XFER_SIZE' Inquire about the value of `POSIX_REC_MIN_XFER_SIZE'. `_PC_REC_XFER_ALIGN' Inquire about the value of `POSIX_REC_XFER_ALIGN'. Utility Program Capacity Limits =============================== The POSIX.2 standard specifies certain system limits that you can access through `sysconf' that apply to utility behavior rather than the behavior of the library or the operating system. The GNU C library defines macros for these limits, and `sysconf' returns values for them if you ask; but these values convey no meaningful information. They are simply the smallest values that POSIX.2 permits. - Macro: int BC_BASE_MAX The largest value of `obase' that the `bc' utility is guaranteed to support. - Macro: int BC_DIM_MAX The largest number of elements in one array that the `bc' utility is guaranteed to support. - Macro: int BC_SCALE_MAX The largest value of `scale' that the `bc' utility is guaranteed to support. - Macro: int BC_STRING_MAX The largest number of characters in one string constant that the `bc' utility is guaranteed to support. - Macro: int COLL_WEIGHTS_MAX The largest number of weights that can necessarily be used in defining the collating sequence for a locale. - Macro: int EXPR_NEST_MAX The maximum number of expressions that can be nested within parenthesis by the `expr' utility. - Macro: int LINE_MAX The largest text line that the text-oriented POSIX.2 utilities can support. (If you are using the GNU versions of these utilities, then there is no actual limit except that imposed by the available virtual memory, but there is no way that the library can tell you this.) - Macro: int EQUIV_CLASS_MAX The maximum number of weights that can be assigned to an entry of the `LC_COLLATE' category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions. Minimum Values for Utility Limits ================================= `_POSIX2_BC_BASE_MAX' The most restrictive limit permitted by POSIX.2 for the maximum value of `obase' in the `bc' utility. Its value is `99'. `_POSIX2_BC_DIM_MAX' The most restrictive limit permitted by POSIX.2 for the maximum size of an array in the `bc' utility. Its value is `2048'. `_POSIX2_BC_SCALE_MAX' The most restrictive limit permitted by POSIX.2 for the maximum value of `scale' in the `bc' utility. Its value is `99'. `_POSIX2_BC_STRING_MAX' The most restrictive limit permitted by POSIX.2 for the maximum size of a string constant in the `bc' utility. Its value is `1000'. `_POSIX2_COLL_WEIGHTS_MAX' The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can necessarily be used in defining the collating sequence for a locale. Its value is `2'. `_POSIX2_EXPR_NEST_MAX' The most restrictive limit permitted by POSIX.2 for the maximum number of expressions nested within parenthesis when using the `expr' utility. Its value is `32'. `_POSIX2_LINE_MAX' The most restrictive limit permitted by POSIX.2 for the maximum size of a text line that the text utilities can handle. Its value is `2048'. `_POSIX2_EQUIV_CLASS_MAX' The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can be assigned to an entry of the `LC_COLLATE' category `order' keyword in a locale definition. Its value is `2'. The GNU C library does not presently support locale definitions. String-Valued Parameters ======================== POSIX.2 defines a way to get string-valued parameters from the operating system with the function `confstr': - Function: size_t confstr (int PARAMETER, char *BUF, size_t LEN) This function reads the value of a string-valued system parameter, storing the string into LEN bytes of memory space starting at BUF. The PARAMETER argument should be one of the `_CS_' symbols listed below. The normal return value from `confstr' is the length of the string value that you asked for. If you supply a null pointer for BUF, then `confstr' does not try to store the string; it just returns its length. A value of `0' indicates an error. If the string you asked for is too long for the buffer (that is, longer than `LEN - 1'), then `confstr' stores just that much (leaving room for the terminating null character). You can tell that this has happened because `confstr' returns a value greater than or equal to LEN. The following `errno' error conditions are defined for this function: `EINVAL' The value of the PARAMETER is invalid. Currently there is just one parameter you can read with `confstr': `_CS_PATH' This parameter's value is the recommended default path for searching for executable files. This is the path that a user has by default just after logging in. `_CS_LFS_CFLAGS' The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the `_LARGEFILE_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS_LDFLAGS' The returned string specifies which additional flags must be given to the linker if a source is compiled using the `_LARGEFILE_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS_LIBS' The returned string specifies which additional libraries must be linked to the application if a source is compiled using the `_LARGEFILE_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS_LINTFLAGS' The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the `_LARGEFILE_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS64_CFLAGS' The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the `_LARGEFILE64_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS64_LDFLAGS' The returned string specifies which additional flags must be given to the linker if a source is compiled using the `_LARGEFILE64_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS64_LIBS' The returned string specifies which additional libraries must be linked to the application if a source is compiled using the `_LARGEFILE64_SOURCE' feature select macro; *note Feature Test Macros::. `_CS_LFS64_LINTFLAGS' The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the `_LARGEFILE64_SOURCE' feature select macro; *note Feature Test Macros::. The way to use `confstr' without any arbitrary limit on string size is to call it twice: first call it to get the length, allocate the buffer accordingly, and then call `confstr' again to fill the buffer, like this: char * get_default_path (void) { size_t len = confstr (_CS_PATH, NULL, 0); char *buffer = (char *) xmalloc (len); if (confstr (_CS_PATH, buf, len + 1) == 0) { free (buffer); return NULL; } return buffer; } DES Encryption and Password Handling ************************************ On many systems, it is unnecessary to have any kind of user authentication; for instance, a workstation which is not connected to a network probably does not need any user authentication, because to use the machine an intruder must have physical access. Sometimes, however, it is necessary to be sure that a user is authorized to use some service a machine provides--for instance, to log in as a particular user id (*note Users and Groups::). One traditional way of doing this is for each user to choose a secret "password"; then, the system can ask someone claiming to be a user what the user's password is, and if the person gives the correct password then the system can grant the appropriate privileges. If all the passwords are just stored in a file somewhere, then this file has to be very carefully protected. To avoid this, passwords are run through a "one-way function", a function which makes it difficult to work out what its input was by looking at its output, before storing in the file. The GNU C library already provides a one-way function based on MD5 and for compatibility with Unix systems the standard one-way function based on the Data Encryption Standard. It also provides support for Secure RPC, and some library functions that can be used to perform normal DES encryption. Legal Problems ============== Because of the continuously changing state of the law, it's not possible to provide a definitive survey of the laws affecting cryptography. Instead, this section warns you of some of the known trouble spots; this may help you when you try to find out what the laws of your country are. Some countries require that you have a licence to use, possess, or import cryptography. These countries are believed to include Byelorussia, Burma, India, Indonesia, Israel, Kazakhstan, Pakistan, Russia, and Saudi Arabia. Some countries restrict the transmission of encrypted messages by radio; some telecommunications carriers restrict the transmission of encrypted messages over their network. Many countries have some form of export control for encryption software. The Wassenaar Arrangement is a multilateral agreement between 33 countries (Argentina, Australia, Austria, Belgium, Bulgaria, Canada, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway, Poland, Portugal, the Republic of Korea, Romania, the Russian Federation, the Slovak Republic, Spain, Sweden, Switzerland, Turkey, Ukraine, the United Kingdom and the United States) which restricts some kinds of encryption exports. Different countries apply the arrangement in different ways; some do not allow the exception for certain kinds of "public domain" software (which would include this library), some only restrict the export of software in tangible form, and others impose significant additional restrictions. The United States has additional rules. This software would generally be exportable under 15 CFR 740.13(e), which permits exports of "encryption source code" which is "publicly available" and which is "not subject to an express agreement for the payment of a licensing fee or royalty for commercial production or sale of any product developed with the source code" to most countries. The rules in this area are continuously changing. If you know of any information in this manual that is out-of-date, please report it using the `glibcbug' script. *Note Reporting Bugs::. Reading Passwords ================= When reading in a password, it is desirable to avoid displaying it on the screen, to help keep it secret. The following function handles this in a convenient way. - Function: char * getpass (const char *PROMPT) `getpass' outputs PROMPT, then reads a string in from the terminal without echoing it. It tries to connect to the real terminal, `/dev/tty', if possible, to encourage users not to put plaintext passwords in files; otherwise, it uses `stdin' and `stderr'. `getpass' also disables the INTR, QUIT, and SUSP characters on the terminal using the `ISIG' terminal attribute (*note Local Modes::). The terminal is flushed before and after `getpass', so that characters of a mistyped password are not accidentally visible. In other C libraries, `getpass' may only return the first `PASS_MAX' bytes of a password. The GNU C library has no limit, so `PASS_MAX' is undefined. The prototype for this function is in `unistd.h'. `PASS_MAX' would be defined in `limits.h'. This precise set of operations may not suit all possible situations. In this case, it is recommended that users write their own `getpass' substitute. For instance, a very simple substitute is as follows: #include #include ssize_t my_getpass (char **lineptr, size_t *n, FILE *stream) { struct termios old, new; int nread; /* Turn echoing off and fail if we can't. */ if (tcgetattr (fileno (stream), &old) != 0) return -1; new = old; new.c_lflag &= ~ECHO; if (tcsetattr (fileno (stream), TCSAFLUSH, &new) != 0) return -1; /* Read the password. */ nread = getline (lineptr, n, stream); /* Restore terminal. */ (void) tcsetattr (fileno (stream), TCSAFLUSH, &old); return nread; } The substitute takes the same parameters as `getline' (*note Line Input::); the user must print any prompt desired. Encrypting Passwords ==================== - Function: char * crypt (const char *KEY, const char *SALT) The `crypt' function takes a password, KEY, as a string, and a SALT character array which is described below, and returns a printable ASCII string which starts with another salt. It is believed that, given the output of the function, the best way to find a KEY that will produce that output is to guess values of KEY until the original value of KEY is found. The SALT parameter does two things. Firstly, it selects which algorithm is used, the MD5-based one or the DES-based one. Secondly, it makes life harder for someone trying to guess passwords against a file containing many passwords; without a SALT, an intruder can make a guess, run `crypt' on it once, and compare the result with all the passwords. With a SALT, the intruder must run `crypt' once for each different salt. For the MD5-based algorithm, the SALT should consist of the string `$1$', followed by up to 8 characters, terminated by either another `$' or the end of the string. The result of `crypt' will be the SALT, followed by a `$' if the salt didn't end with one, followed by 22 characters from the alphabet `./0-9A-Za-z', up to 34 characters total. Every character in the KEY is significant. For the DES-based algorithm, the SALT should consist of two characters from the alphabet `./0-9A-Za-z', and the result of `crypt' will be those two characters followed by 11 more from the same alphabet, 13 in total. Only the first 8 characters in the KEY are significant. The MD5-based algorithm has no limit on the useful length of the password used, and is slightly more secure. It is therefore preferred over the DES-based algorithm. When the user enters their password for the first time, the SALT should be set to a new string which is reasonably random. To verify a password against the result of a previous call to `crypt', pass the result of the previous call as the SALT. The following short program is an example of how to use `crypt' the first time a password is entered. Note that the SALT generation is just barely acceptable; in particular, it is not unique between machines, and in many applications it would not be acceptable to let an attacker know what time the user's password was last set. #include #include #include #include int main(void) { unsigned long seed[2]; char salt[] = "$1$........"; const char *const seedchars = "./0123456789ABCDEFGHIJKLMNOPQRST" "UVWXYZabcdefghijklmnopqrstuvwxyz"; char *password; int i; /* Generate a (not very) random seed. You should do it better than this... */ seed[0] = time(NULL); seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000); /* Turn it into printable characters from `seedchars'. */ for (i = 0; i < 8; i++) salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f]; /* Read in the user's password and encrypt it. */ password = crypt(getpass("Password:"), salt); /* Print the results. */ puts(password); return 0; } The next program shows how to verify a password. It prompts the user for a password and prints "Access granted." if the user types `GNU libc manual'. #include #include #include #include int main(void) { /* Hashed form of "GNU libc manual". */ const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/"; char *result; int ok; /* Read in the user's password and encrypt it, passing the expected password in as the salt. */ result = crypt(getpass("Password:"), pass); /* Test the result. */ ok = strcmp (result, pass) == 0; puts(ok ? "Access granted." : "Access denied."); return ok ? 0 : 1; } - Function: char * crypt_r (const char *KEY, const char *SALT, struct crypt_data * DATA) The `crypt_r' function does the same thing as `crypt', but takes an extra parameter which includes space for its result (among other things), so it can be reentrant. `data->initialized' must be cleared to zero before the first time `crypt_r' is called. The `crypt_r' function is a GNU extension. The `crypt' and `crypt_r' functions are prototyped in the header `crypt.h'. DES Encryption ============== The Data Encryption Standard is described in the US Government Federal Information Processing Standards (FIPS) 46-3 published by the National Institute of Standards and Technology. The DES has been very thoroughly analyzed since it was developed in the late 1970s, and no new significant flaws have been found. However, the DES uses only a 56-bit key (plus 8 parity bits), and a machine has been built in 1998 which can search through all possible keys in about 6 days, which cost about US$200000; faster searches would be possible with more money. This makes simple DES insecure for most purposes, and NIST no longer permits new US government systems to use simple DES. For serious encryption functionality, it is recommended that one of the many free encryption libraries be used instead of these routines. The DES is a reversible operation which takes a 64-bit block and a 64-bit key, and produces another 64-bit block. Usually the bits are numbered so that the most-significant bit, the first bit, of each block is numbered 1. Under that numbering, every 8th bit of the key (the 8th, 16th, and so on) is not used by the encryption algorithm itself. But the key must have odd parity; that is, out of bits 1 through 8, and 9 through 16, and so on, there must be an odd number of `1' bits, and this completely specifies the unused bits. - Function: void setkey (const char *KEY) The `setkey' function sets an internal data structure to be an expanded form of KEY. KEY is specified as an array of 64 bits each stored in a `char', the first bit is `key[0]' and the 64th bit is `key[63]'. The KEY should have the correct parity. - Function: void encrypt (char *BLOCK, int EDFLAG) The `encrypt' function encrypts BLOCK if EDFLAG is 0, otherwise it decrypts BLOCK, using a key previously set by `setkey'. The result is placed in BLOCK. Like `setkey', BLOCK is specified as an array of 64 bits each stored in a `char', but there are no parity bits in BLOCK. - Function: void setkey_r (const char *KEY, struct crypt_data * DATA) - Function: void encrypt_r (char *BLOCK, int EDFLAG, struct crypt_data * DATA) These are reentrant versions of `setkey' and `encrypt'. The only difference is the extra parameter, which stores the expanded version of KEY. Before calling `setkey_r' the first time, `data->initialized' must be cleared to zero. The `setkey_r' and `encrypt_r' functions are GNU extensions. `setkey', `encrypt', `setkey_r', and `encrypt_r' are defined in `crypt.h'. - Function: int ecb_crypt (char *KEY, char *BLOCKS, unsigned LEN, unsigned MODE) The function `ecb_crypt' encrypts or decrypts one or more blocks using DES. Each block is encrypted independently. The BLOCKS and the KEY are stored packed in 8-bit bytes, so that the first bit of the key is the most-significant bit of `key[0]' and the 63rd bit of the key is stored as the least-significant bit of `key[7]'. The KEY should have the correct parity. LEN is the number of bytes in BLOCKS. It should be a multiple of 8 (so that there is a whole number of blocks to encrypt). LEN is limited to a maximum of `DES_MAXDATA' bytes. The result of the encryption replaces the input in BLOCKS. The MODE parameter is the bitwise OR of two of the following: `DES_ENCRYPT' This constant, used in the MODE parameter, specifies that BLOCKS is to be encrypted. `DES_DECRYPT' This constant, used in the MODE parameter, specifies that BLOCKS is to be decrypted. `DES_HW' This constant, used in the MODE parameter, asks to use a hardware device. If no hardware device is available, encryption happens anyway, but in software. `DES_SW' This constant, used in the MODE parameter, specifies that no hardware device is to be used. The result of the function will be one of these values: `DESERR_NONE' The encryption succeeded. `DESERR_NOHWDEVICE' The encryption succeeded, but there was no hardware device available. `DESERR_HWERROR' The encryption failed because of a hardware problem. `DESERR_BADPARAM' The encryption failed because of a bad parameter, for instance LEN is not a multiple of 8 or LEN is larger than `DES_MAXDATA'. - Function: int DES_FAILED (int ERR) This macro returns 1 if ERR is a `success' result code from `ecb_crypt' or `cbc_crypt', and 0 otherwise. - Function: int cbc_crypt (char *KEY, char *BLOCKS, unsigned LEN, unsigned MODE, char *IVEC) The function `cbc_crypt' encrypts or decrypts one or more blocks using DES in Cipher Block Chaining mode. For encryption in CBC mode, each block is exclusive-ored with IVEC before being encrypted, then IVEC is replaced with the result of the encryption, then the next block is processed. Decryption is the reverse of this process. This has the advantage that blocks which are the same before being encrypted are very unlikely to be the same after being encrypted, making it much harder to detect patterns in the data. Usually, IVEC is set to 8 random bytes before encryption starts. Then the 8 random bytes are transmitted along with the encrypted data (without themselves being encrypted), and passed back in as IVEC for decryption. Another possibility is to set IVEC to 8 zeroes initially, and have the first the block encrypted consist of 8 random bytes. Otherwise, all the parameters are similar to those for `ecb_crypt'. - Function: void des_setparity (char *KEY) The function `des_setparity' changes the 64-bit KEY, stored packed in 8-bit bytes, to have odd parity by altering the low bits of each byte. The `ecb_crypt', `cbc_crypt', and `des_setparity' functions and their accompanying macros are all defined in the header `rpc/des_crypt.h'. Debugging support ***************** Applications are usually debugged using dedicated debugger programs. But sometimes this is not possible and, in any case, it is useful to provide the developer with as much information as possible at the time the problems are experienced. For this reason a few functions are provided which a program can use to help the developer more easily locate the problem. Backtraces ========== A "backtrace" is a list of the function calls that are currently active in a thread. The usual way to inspect a backtrace of a program is to use an external debugger such as gdb. However, sometimes it is useful to obtain a backtrace programatically from within a program, e.g., for the purposes of logging or diagnostics. The header file `execinfo.h' declares three functions that obtain and manipulate backtraces of the current thread. - Function: int backtrace (void **BUFFER, int SIZE) The `backtrace' function obtains a backtrace for the current thread, as a list of pointers, and places the information into BUFFER. The argument SIZE should be the number of `void *' elements that will fit into BUFFER. The return value is the actual number of entries of BUFFER that are obtained, and is at most SIZE. The pointers placed in BUFFER are actually return addresses obtained by inspecting the stack, one return address per stack frame. Note that certain compiler optimizations may interfere with obtaining a valid backtrace. Function inlining causes the inlined function to not have a stack frame; tail call optimization replaces one stack frame with another; frame pointer elimination will stop `backtrace' from interpreting the stack contents correctly. - Function: char ** backtrace_symbols (void *const *BUFFER, int SIZE) The `backtrace_symbols' function translates the information obtained from the `backtrace' function into an array of strings. The argument BUFFER should be a pointer to an array of addresses obtained via the `backtrace' function, and SIZE is the number of entries in that array (the return value of `backtrace'). The return value is a pointer to an array of strings, which has SIZE entries just like the array BUFFER. Each string contains a printable representation of the corresponding element of BUFFER. It includes the function name (if this can be determined), an offset into the function, and the actual return address (in hexadecimal). Currently, the function name and offset only be obtained on systems that use the ELF binary format for programs and libraries. On other systems, only the hexadecimal return address will be present. Also, you may need to pass additional flags to the linker to make the function names available to the program. (For example, on systems using GNU ld, you must pass (`-rdynamic'.) The return value of `backtrace_symbols' is a pointer obtained via the `malloc' function, and it is the responsibility of the caller to `free' that pointer. Note that only the return value need be freed, not the individual strings. The return value is `NULL' if sufficient memory for the strings cannot be obtained. - Function: void backtrace_symbols_fd (void *const *BUFFER, int SIZE, int FD) The `backtrace_symbols_fd' function performs the same translation as the function `backtrace_symbols' function. Instead of returning the strings to the caller, it writes the strings to the file descriptor FD, one per line. It does not use the `malloc' function, and can therefore be used in situations where that function might fail. The following program illustrates the use of these functions. Note that the array to contain the return addresses returned by `backtrace' is allocated on the stack. Therefore code like this can be used in situations where the memory handling via `malloc' does not work anymore (in which case the `backtrace_symbols' has to be replaced by a `backtrace_symbols_fd' call as well). The number of return addresses is normally not very large. Even complicated programs rather seldom have a nesting level of more than, say, 50 and with 200 possible entries probably all programs should be covered. #include #include #include /* Obtain a backtrace and print it to `stdout'. */ void print_trace (void) { void *array[10]; size_t size; char **strings; size_t i; size = backtrace (array, 10); strings = backtrace_symbols (array, size); printf ("Obtained %zd stack frames.\n", size); for (i = 0; i < size; i++) printf ("%s\n", strings[i]); free (strings); } /* A dummy function to make the backtrace more interesting. */ void dummy_function (void) { print_trace (); } int main (void) { dummy_function (); return 0; } C Language Facilities in the Library ************************************ Some of the facilities implemented by the C library really should be thought of as parts of the C language itself. These facilities ought to be documented in the C Language Manual, not in the library manual; but since we don't have the language manual yet, and documentation for these features has been written, we are publishing it here. Explicitly Checking Internal Consistency ======================================== When you're writing a program, it's often a good idea to put in checks at strategic places for "impossible" errors or violations of basic assumptions. These kinds of checks are helpful in debugging problems with the interfaces between different parts of the program, for example. The `assert' macro, defined in the header file `assert.h', provides a convenient way to abort the program while printing a message about where in the program the error was detected. Once you think your program is debugged, you can disable the error checks performed by the `assert' macro by recompiling with the macro `NDEBUG' defined. This means you don't actually have to change the program source code to disable these checks. But disabling these consistency checks is undesirable unless they make the program significantly slower. All else being equal, more error checking is good no matter who is running the program. A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong. - Macro: void assert (int EXPRESSION) Verify the programmer's belief that EXPRESSION is nonzero at this point in the program. If `NDEBUG' is not defined, `assert' tests the value of EXPRESSION. If it is false (zero), `assert' aborts the program (*note Aborting a Program::) after printing a message of the form: `FILE':LINENUM: FUNCTION: Assertion `EXPRESSION' failed. on the standard error stream `stderr' (*note Standard Streams::). The filename and line number are taken from the C preprocessor macros `__FILE__' and `__LINE__' and specify where the call to `assert' was made. When using the GNU C compiler, the name of the function which calls `assert' is taken from the built-in variable `__PRETTY_FUNCTION__'; with older compilers, the function name and following colon are omitted. If the preprocessor macro `NDEBUG' is defined before `assert.h' is included, the `assert' macro is defined to do absolutely nothing. *Warning:* Even the argument expression EXPRESSION is not evaluated if `NDEBUG' is in effect. So never use `assert' with arguments that involve side effects. For example, `assert (++i > 0);' is a bad idea, because `i' will not be incremented if `NDEBUG' is defined. Sometimes the "impossible" condition you want to check for is an error return from an operating system function. Then it is useful to display not only where the program crashes, but also what error was returned. The `assert_perror' macro makes this easy. - Macro: void assert_perror (int ERRNUM) Similar to `assert', but verifies that ERRNUM is zero. If `NDEBUG' is defined, `assert_perror' tests the value of ERRNUM. If it is nonzero, `assert_perror' aborts the program after printing a message of the form: `FILE':LINENUM: FUNCTION: ERROR TEXT on the standard error stream. The file name, line number, and function name are as for `assert'. The error text is the result of `strerror (ERRNUM)'. *Note Error Messages::. Like `assert', if `NDEBUG' is defined before `assert.h' is included, the `assert_perror' macro does absolutely nothing. It does not evaluate the argument, so ERRNUM should not have any side effects. It is best for ERRNUM to be just a simple variable reference; often it will be `errno'. This macro is a GNU extension. *Usage note:* The `assert' facility is designed for detecting _internal inconsistency_; it is not suitable for reporting invalid input or improper usage by the _user_ of the program. The information in the diagnostic messages printed by the `assert' and `assert_perror' macro is intended to help you, the programmer, track down the cause of a bug, but is not really useful for telling a user of your program why his or her input was invalid or why a command could not be carried out. What's more, your program should not abort when given invalid input, as `assert' would do--it should exit with nonzero status (*note Exit Status::) after printing its error messages, or perhaps read another command or move on to the next input file. *Note Error Messages::, for information on printing error messages for problems that _do not_ represent bugs in the program. Variadic Functions ================== ISO C defines a syntax for declaring a function to take a variable number or type of arguments. (Such functions are referred to as "varargs functions" or "variadic functions".) However, the language itself provides no mechanism for such functions to access their non-required arguments; instead, you use the variable arguments macros defined in `stdarg.h'. This section describes how to declare variadic functions, how to write them, and how to call them properly. *Compatibility Note:* Many older C dialects provide a similar, but incompatible, mechanism for defining functions with variable numbers of arguments, using `varargs.h'. Why Variadic Functions are Used ------------------------------- Ordinary C functions take a fixed number of arguments. When you define a function, you specify the data type for each argument. Every call to the function should supply the expected number of arguments, with types that can be converted to the specified ones. Thus, if the function `foo' is declared with `int foo (int, char *);' then you must call it with two arguments, a number (any kind will do) and a string pointer. But some functions perform operations that can meaningfully accept an unlimited number of arguments. In some cases a function can handle any number of values by operating on all of them as a block. For example, consider a function that allocates a one-dimensional array with `malloc' to hold a specified set of values. This operation makes sense for any number of values, as long as the length of the array corresponds to that number. Without facilities for variable arguments, you would have to define a separate function for each possible array size. The library function `printf' (*note Formatted Output::) is an example of another class of function where variable arguments are useful. This function prints its arguments (which can vary in type as well as number) under the control of a format template string. These are good reasons to define a "variadic" function which can handle as many arguments as the caller chooses to pass. Some functions such as `open' take a fixed set of arguments, but occasionally ignore the last few. Strict adherence to ISO C requires these functions to be defined as variadic; in practice, however, the GNU C compiler and most other C compilers let you define such a function to take a fixed set of arguments--the most it can ever use--and then only _declare_ the function as variadic (or not declare its arguments at all!). How Variadic Functions are Defined and Used ------------------------------------------- Defining and using a variadic function involves three steps: * _Define_ the function as variadic, using an ellipsis (`...') in the argument list, and using special macros to access the variable arguments. *Note Receiving Arguments::. * _Declare_ the function as variadic, using a prototype with an ellipsis (`...'), in all the files which call it. *Note Variadic Prototypes::. * _Call_ the function by writing the fixed arguments followed by the additional variable arguments. *Note Calling Variadics::. Syntax for Variable Arguments ............................. A function that accepts a variable number of arguments must be declared with a prototype that says so. You write the fixed arguments as usual, and then tack on `...' to indicate the possibility of additional arguments. The syntax of ISO C requires at least one fixed argument before the `...'. For example, int func (const char *a, int b, ...) { ... } defines a function `func' which returns an `int' and takes two required arguments, a `const char *' and an `int'. These are followed by any number of anonymous arguments. *Portability note:* For some C compilers, the last required argument must not be declared `register' in the function definition. Furthermore, this argument's type must be "self-promoting": that is, the default promotions must not change its type. This rules out array and function types, as well as `float', `char' (whether signed or not) and `short int' (whether signed or not). This is actually an ISO C requirement. Receiving the Argument Values ............................. Ordinary fixed arguments have individual names, and you can use these names to access their values. But optional arguments have no names--nothing but `...'. How can you access them? The only way to access them is sequentially, in the order they were written, and you must use special macros from `stdarg.h' in the following three step process: 1. You initialize an argument pointer variable of type `va_list' using `va_start'. The argument pointer when initialized points to the first optional argument. 2. You access the optional arguments by successive calls to `va_arg'. The first call to `va_arg' gives you the first optional argument, the next call gives you the second, and so on. You can stop at any time if you wish to ignore any remaining optional arguments. It is perfectly all right for a function to access fewer arguments than were supplied in the call, but you will get garbage values if you try to access too many arguments. 3. You indicate that you are finished with the argument pointer variable by calling `va_end'. (In practice, with most C compilers, calling `va_end' does nothing. This is always true in the GNU C compiler. But you might as well call `va_end' just in case your program is someday compiled with a peculiar compiler.) *Note Argument Macros::, for the full definitions of `va_start', `va_arg' and `va_end'. Steps 1 and 3 must be performed in the function that accepts the optional arguments. However, you can pass the `va_list' variable as an argument to another function and perform all or part of step 2 there. You can perform the entire sequence of three steps multiple times within a single function invocation. If you want to ignore the optional arguments, you can do these steps zero times. You can have more than one argument pointer variable if you like. You can initialize each variable with `va_start' when you wish, and then you can fetch arguments with each argument pointer as you wish. Each argument pointer variable will sequence through the same set of argument values, but at its own pace. *Portability note:* With some compilers, once you pass an argument pointer value to a subroutine, you must not keep using the same argument pointer value after that subroutine returns. For full portability, you should just pass it to `va_end'. This is actually an ISO C requirement, but most ANSI C compilers work happily regardless. How Many Arguments Were Supplied ................................ There is no general way for a function to determine the number and type of the optional arguments it was called with. So whoever designs the function typically designs a convention for the caller to specify the number and type of arguments. It is up to you to define an appropriate calling convention for each variadic function, and write all calls accordingly. One kind of calling convention is to pass the number of optional arguments as one of the fixed arguments. This convention works provided all of the optional arguments are of the same type. A similar alternative is to have one of the required arguments be a bit mask, with a bit for each possible purpose for which an optional argument might be supplied. You would test the bits in a predefined sequence; if the bit is set, fetch the value of the next argument, otherwise use a default value. A required argument can be used as a pattern to specify both the number and types of the optional arguments. The format string argument to `printf' is one example of this (*note Formatted Output Functions::). Another possibility is to pass an "end marker" value as the last optional argument. For example, for a function that manipulates an arbitrary number of pointer arguments, a null pointer might indicate the end of the argument list. (This assumes that a null pointer isn't otherwise meaningful to the function.) The `execl' function works in just this way; see *Note Executing a File::. Calling Variadic Functions .......................... You don't have to do anything special to call a variadic function. Just put the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual. But you must declare the function with a prototype and know how the argument values are converted. In principle, functions that are _defined_ to be variadic must also be _declared_ to be variadic using a function prototype whenever you call them. (*Note Variadic Prototypes::, for how.) This is because some C compilers use a different calling convention to pass the same set of argument values to a function depending on whether that function takes variable arguments or fixed arguments. In practice, the GNU C compiler always passes a given set of argument types in the same way regardless of whether they are optional or required. So, as long as the argument types are self-promoting, you can safely omit declaring them. Usually it is a good idea to declare the argument types for variadic functions, and indeed for all functions. But there are a few functions which it is extremely convenient not to have to declare as variadic--for example, `open' and `printf'. Since the prototype doesn't specify types for optional arguments, in a call to a variadic function the "default argument promotions" are performed on the optional argument values. This means the objects of type `char' or `short int' (whether signed or not) are promoted to either `int' or `unsigned int', as appropriate; and that objects of type `float' are promoted to type `double'. So, if the caller passes a `char' as an optional argument, it is promoted to an `int', and the function can access it with `va_arg (AP, int)'. Conversion of the required arguments is controlled by the function prototype in the usual way: the argument expression is converted to the declared argument type as if it were being assigned to a variable of that type. Argument Access Macros ...................... Here are descriptions of the macros used to retrieve variable arguments. These macros are defined in the header file `stdarg.h'. - Data Type: va_list The type `va_list' is used for argument pointer variables. - Macro: void va_start (va_list AP, LAST-REQUIRED) This macro initializes the argument pointer variable AP to point to the first of the optional arguments of the current function; LAST-REQUIRED must be the last required argument to the function. *Note Old Varargs::, for an alternate definition of `va_start' found in the header file `varargs.h'. - Macro: TYPE va_arg (va_list AP, TYPE) The `va_arg' macro returns the value of the next optional argument, and modifies the value of AP to point to the subsequent argument. Thus, successive uses of `va_arg' return successive optional arguments. The type of the value returned by `va_arg' is TYPE as specified in the call. TYPE must be a self-promoting type (not `char' or `short int' or `float') that matches the type of the actual argument. - Macro: void va_end (va_list AP) This ends the use of AP. After a `va_end' call, further `va_arg' calls with the same AP may not work. You should invoke `va_end' before returning from the function in which `va_start' was invoked with the same AP argument. In the GNU C library, `va_end' does nothing, and you need not ever use it except for reasons of portability. Sometimes it is necessary to parse the list of parameters more than once or one wants to remember a certain position in the parameter list. To do this, one will have to make a copy of the current value of the argument. But `va_list' is an opaque type and one cannot necessarily assign the value of one variable of type `va_list' to another variable of the same type. - Macro: void __va_copy (va_list DEST, va_list SRC) The `__va_copy' macro allows copying of objects of type `va_list' even if this is not an integral type. The argument pointer in DEST is initialized to point to the same argument as the pointer in SRC. This macro is a GNU extension but it will hopefully also be available in the next update of the ISO C standard. If you want to use `__va_copy' you should always be prepared for the possibility that this macro will not be available. On architectures where a simple assignment is invalid, hopefully `__va_copy' _will_ be available, so one should always write something like this: { va_list ap, save; ... #ifdef __va_copy __va_copy (save, ap); #else save = ap; #endif ... } Example of a Variadic Function ------------------------------ Here is a complete sample function that accepts a variable number of arguments. The first argument to the function is the count of remaining arguments, which are added up and the result returned. While trivial, this function is sufficient to illustrate how to use the variable arguments facility. #include #include int add_em_up (int count,...) { va_list ap; int i, sum; va_start (ap, count); /* Initialize the argument list. */ sum = 0; for (i = 0; i < count; i++) sum += va_arg (ap, int); /* Get the next argument value. */ va_end (ap); /* Clean up. */ return sum; } int main (void) { /* This call prints 16. */ printf ("%d\n", add_em_up (3, 5, 5, 6)); /* This call prints 55. */ printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)); return 0; } Old-Style Variadic Functions ............................ Before ISO C, programmers used a slightly different facility for writing variadic functions. The GNU C compiler still supports it; currently, it is more portable than the ISO C facility, since support for ISO C is still not universal. The header file which defines the old-fashioned variadic facility is called `varargs.h'. Using `varargs.h' is almost the same as using `stdarg.h'. There is no difference in how you call a variadic function; see *Note Calling Variadics::. The only difference is in how you define them. First of all, you must use old-style non-prototype syntax, like this: tree build (va_alist) va_dcl { Secondly, you must give `va_start' only one argument, like this: va_list p; va_start (p); These are the special macros used for defining old-style variadic functions: - Macro: va_alist This macro stands for the argument name list required in a variadic function. - Macro: va_dcl This macro declares the implicit argument or arguments for a variadic function. - Macro: void va_start (va_list AP) This macro, as defined in `varargs.h', initializes the argument pointer variable AP to point to the first argument of the current function. The other argument macros, `va_arg' and `va_end', are the same in `varargs.h' as in `stdarg.h'; see *Note Argument Macros::, for details. It does not work to include both `varargs.h' and `stdarg.h' in the same compilation; they define `va_start' in conflicting ways. Null Pointer Constant ===================== The null pointer constant is guaranteed not to point to any real object. You can assign it to any pointer variable since it has type `void *'. The preferred way to write a null pointer constant is with `NULL'. - Macro: void * NULL This is a null pointer constant. You can also use `0' or `(void *)0' as a null pointer constant, but using `NULL' is cleaner because it makes the purpose of the constant more evident. If you use the null pointer constant as a function argument, then for complete portability you should make sure that the function has a prototype declaration. Otherwise, if the target machine has two different pointer representations, the compiler won't know which representation to use for that argument. You can avoid the problem by explicitly casting the constant to the proper pointer type, but we recommend instead adding a prototype for the function you are calling. Important Data Types ==================== The result of subtracting two pointers in C is always an integer, but the precise data type varies from C compiler to C compiler. Likewise, the data type of the result of `sizeof' also varies between compilers. ISO defines standard aliases for these two types, so you can refer to them in a portable fashion. They are defined in the header file `stddef.h'. - Data Type: ptrdiff_t This is the signed integer type of the result of subtracting two pointers. For example, with the declaration `char *p1, *p2;', the expression `p2 - p1' is of type `ptrdiff_t'. This will probably be one of the standard signed integer types (`short int', `int' or `long int'), but might be a nonstandard type that exists only for this purpose. - Data Type: size_t This is an unsigned integer type used to represent the sizes of objects. The result of the `sizeof' operator is of this type, and functions such as `malloc' (*note Unconstrained Allocation::) and `memcpy' (*note Copying and Concatenation::) accept arguments of this type to specify object sizes. *Usage Note:* `size_t' is the preferred way to declare any arguments or variables that hold the size of an object. In the GNU system `size_t' is equivalent to either `unsigned int' or `unsigned long int'. These types have identical properties on the GNU system and, for most purposes, you can use them interchangeably. However, they are distinct as data types, which makes a difference in certain contexts. For example, when you specify the type of a function argument in a function prototype, it makes a difference which one you use. If the system header files declare `malloc' with an argument of type `size_t' and you declare `malloc' with an argument of type `unsigned int', you will get a compilation error if `size_t' happens to be `unsigned long int' on your system. To avoid any possibility of error, when a function argument or value is supposed to have type `size_t', never declare its type in any other way. *Compatibility Note:* Implementations of C before the advent of ISO C generally used `unsigned int' for representing object sizes and `int' for pointer subtraction results. They did not necessarily define either `size_t' or `ptrdiff_t'. Unix systems did define `size_t', in `sys/types.h', but the definition was usually a signed type. Data Type Measurements ====================== Most of the time, if you choose the proper C data type for each object in your program, you need not be concerned with just how it is represented or how many bits it uses. When you do need such information, the C language itself does not provide a way to get it. The header files `limits.h' and `float.h' contain macros which give you this information in full detail. Computing the Width of an Integer Data Type ------------------------------------------- The most common reason that a program needs to know how many bits are in an integer type is for using an array of `long int' as a bit vector. You can access the bit at index N with vector[N / LONGBITS] & (1 << (N % LONGBITS)) provided you define `LONGBITS' as the number of bits in a `long int'. There is no operator in the C language that can give you the number of bits in an integer data type. But you can compute it from the macro `CHAR_BIT', defined in the header file `limits.h'. `CHAR_BIT' This is the number of bits in a `char'--eight, on most systems. The value has type `int'. You can compute the number of bits in any data type TYPE like this: sizeof (TYPE) * CHAR_BIT Range of an Integer Type ------------------------ Suppose you need to store an integer value which can range from zero to one million. Which is the smallest type you can use? There is no general rule; it depends on the C compiler and target machine. You can use the `MIN' and `MAX' macros in `limits.h' to determine which type will work. Each signed integer type has a pair of macros which give the smallest and largest values that it can hold. Each unsigned integer type has one such macro, for the maximum value; the minimum value is, of course, zero. The values of these macros are all integer constant expressions. The `MAX' and `MIN' macros for `char' and `short int' types have values of type `int'. The `MAX' and `MIN' macros for the other types have values of the same type described by the macro--thus, `ULONG_MAX' has type `unsigned long int'. `SCHAR_MIN' This is the minimum value that can be represented by a `signed char'. `SCHAR_MAX' `UCHAR_MAX' These are the maximum values that can be represented by a `signed char' and `unsigned char', respectively. `CHAR_MIN' This is the minimum value that can be represented by a `char'. It's equal to `SCHAR_MIN' if `char' is signed, or zero otherwise. `CHAR_MAX' This is the maximum value that can be represented by a `char'. It's equal to `SCHAR_MAX' if `char' is signed, or `UCHAR_MAX' otherwise. `SHRT_MIN' This is the minimum value that can be represented by a `signed short int'. On most machines that the GNU C library runs on, `short' integers are 16-bit quantities. `SHRT_MAX' `USHRT_MAX' These are the maximum values that can be represented by a `signed short int' and `unsigned short int', respectively. `INT_MIN' This is the minimum value that can be represented by a `signed int'. On most machines that the GNU C system runs on, an `int' is a 32-bit quantity. `INT_MAX' `UINT_MAX' These are the maximum values that can be represented by, respectively, the type `signed int' and the type `unsigned int'. `LONG_MIN' This is the minimum value that can be represented by a `signed long int'. On most machines that the GNU C system runs on, `long' integers are 32-bit quantities, the same size as `int'. `LONG_MAX' `ULONG_MAX' These are the maximum values that can be represented by a `signed long int' and `unsigned long int', respectively. `LONG_LONG_MIN' This is the minimum value that can be represented by a `signed long long int'. On most machines that the GNU C system runs on, `long long' integers are 64-bit quantities. `LONG_LONG_MAX' `ULONG_LONG_MAX' These are the maximum values that can be represented by a `signed long long int' and `unsigned long long int', respectively. `WCHAR_MAX' This is the maximum value that can be represented by a `wchar_t'. *Note Extended Char Intro::. The header file `limits.h' also defines some additional constants that parameterize various operating system and file system limits. These constants are described in *Note System Configuration::. Floating Type Macros -------------------- The specific representation of floating point numbers varies from machine to machine. Because floating point numbers are represented internally as approximate quantities, algorithms for manipulating floating point data often need to take account of the precise details of the machine's floating point representation. Some of the functions in the C library itself need this information; for example, the algorithms for printing and reading floating point numbers (*note I/O on Streams::) and for calculating trigonometric and irrational functions (*note Mathematics::) use it to avoid round-off error and loss of accuracy. User programs that implement numerical analysis techniques also often need this information in order to minimize or compute error bounds. The header file `float.h' describes the format used by your machine. Floating Point Representation Concepts ...................................... This section introduces the terminology for describing floating point representations. You are probably already familiar with most of these concepts in terms of scientific or exponential notation for floating point numbers. For example, the number `123456.0' could be expressed in exponential notation as `1.23456e+05', a shorthand notation indicating that the mantissa `1.23456' is multiplied by the base `10' raised to power `5'. More formally, the internal representation of a floating point number can be characterized in terms of the following parameters: * The "sign" is either `-1' or `1'. * The "base" or "radix" for exponentiation, an integer greater than `1'. This is a constant for a particular representation. * The "exponent" to which the base is raised. The upper and lower bounds of the exponent value are constants for a particular representation. Sometimes, in the actual bits representing the floating point number, the exponent is "biased" by adding a constant to it, to make it always be represented as an unsigned quantity. This is only important if you have some reason to pick apart the bit fields making up the floating point number by hand, which is something for which the GNU library provides no support. So this is ignored in the discussion that follows. * The "mantissa" or "significand" is an unsigned integer which is a part of each floating point number. * The "precision" of the mantissa. If the base of the representation is B, then the precision is the number of base-B digits in the mantissa. This is a constant for a particular representation. Many floating point representations have an implicit "hidden bit" in the mantissa. This is a bit which is present virtually in the mantissa, but not stored in memory because its value is always 1 in a normalized number. The precision figure (see above) includes any hidden bits. Again, the GNU library provides no facilities for dealing with such low-level aspects of the representation. The mantissa of a floating point number represents an implicit fraction whose denominator is the base raised to the power of the precision. Since the largest representable mantissa is one less than this denominator, the value of the fraction is always strictly less than `1'. The mathematical value of a floating point number is then the product of this fraction, the sign, and the base raised to the exponent. We say that the floating point number is "normalized" if the fraction is at least `1/B', where B is the base. In other words, the mantissa would be too large to fit if it were multiplied by the base. Non-normalized numbers are sometimes called "denormal"; they contain less precision than the representation normally can hold. If the number is not normalized, then you can subtract `1' from the exponent while multiplying the mantissa by the base, and get another floating point number with the same value. "Normalization" consists of doing this repeatedly until the number is normalized. Two distinct normalized floating point numbers cannot be equal in value. (There is an exception to this rule: if the mantissa is zero, it is considered normalized. Another exception happens on certain machines where the exponent is as small as the representation can hold. Then it is impossible to subtract `1' from the exponent, so a number may be normalized even if its fraction is less than `1/B'.) Floating Point Parameters ......................... These macro definitions can be accessed by including the header file `float.h' in your program. Macro names starting with `FLT_' refer to the `float' type, while names beginning with `DBL_' refer to the `double' type and names beginning with `LDBL_' refer to the `long double' type. (If GCC does not support `long double' as a distinct data type on a target machine then the values for the `LDBL_' constants are equal to the corresponding constants for the `double' type.) Of these macros, only `FLT_RADIX' is guaranteed to be a constant expression. The other macros listed here cannot be reliably used in places that require constant expressions, such as `#if' preprocessing directives or in the dimensions of static arrays. Although the ISO C standard specifies minimum and maximum values for most of these parameters, the GNU C implementation uses whatever values describe the floating point representation of the target machine. So in principle GNU C actually satisfies the ISO C requirements only if the target machine is suitable. In practice, all the machines currently supported are suitable. `FLT_ROUNDS' This value characterizes the rounding mode for floating point addition. The following values indicate standard rounding modes: `-1' The mode is indeterminable. `0' Rounding is towards zero. `1' Rounding is to the nearest number. `2' Rounding is towards positive infinity. `3' Rounding is towards negative infinity. Any other value represents a machine-dependent nonstandard rounding mode. On most machines, the value is `1', in accordance with the IEEE standard for floating point. Here is a table showing how certain values round for each possible value of `FLT_ROUNDS', if the other aspects of the representation match the IEEE single-precision standard. 0 1 2 3 1.00000003 1.0 1.0 1.00000012 1.0 1.00000007 1.0 1.00000012 1.00000012 1.0 -1.00000003 -1.0 -1.0 -1.0 -1.00000012 -1.00000007 -1.0 -1.00000012 -1.0 -1.00000012 `FLT_RADIX' This is the value of the base, or radix, of the exponent representation. This is guaranteed to be a constant expression, unlike the other macros described in this section. The value is 2 on all machines we know of except the IBM 360 and derivatives. `FLT_MANT_DIG' This is the number of base-`FLT_RADIX' digits in the floating point mantissa for the `float' data type. The following expression yields `1.0' (even though mathematically it should not) due to the limited number of mantissa digits: float radix = FLT_RADIX; 1.0f + 1.0f / radix / radix / ... / radix where `radix' appears `FLT_MANT_DIG' times. `DBL_MANT_DIG' `LDBL_MANT_DIG' This is the number of base-`FLT_RADIX' digits in the floating point mantissa for the data types `double' and `long double', respectively. `FLT_DIG' This is the number of decimal digits of precision for the `float' data type. Technically, if P and B are the precision and base (respectively) for the representation, then the decimal precision Q is the maximum number of decimal digits such that any floating point number with Q base 10 digits can be rounded to a floating point number with P base B digits and back again, without change to the Q decimal digits. The value of this macro is supposed to be at least `6', to satisfy ISO C. `DBL_DIG' `LDBL_DIG' These are similar to `FLT_DIG', but for the data types `double' and `long double', respectively. The values of these macros are supposed to be at least `10'. `FLT_MIN_EXP' This is the smallest possible exponent value for type `float'. More precisely, is the minimum negative integer such that the value `FLT_RADIX' raised to this power minus 1 can be represented as a normalized floating point number of type `float'. `DBL_MIN_EXP' `LDBL_MIN_EXP' These are similar to `FLT_MIN_EXP', but for the data types `double' and `long double', respectively. `FLT_MIN_10_EXP' This is the minimum negative integer such that `10' raised to this power minus 1 can be represented as a normalized floating point number of type `float'. This is supposed to be `-37' or even less. `DBL_MIN_10_EXP' `LDBL_MIN_10_EXP' These are similar to `FLT_MIN_10_EXP', but for the data types `double' and `long double', respectively. `FLT_MAX_EXP' This is the largest possible exponent value for type `float'. More precisely, this is the maximum positive integer such that value `FLT_RADIX' raised to this power minus 1 can be represented as a floating point number of type `float'. `DBL_MAX_EXP' `LDBL_MAX_EXP' These are similar to `FLT_MAX_EXP', but for the data types `double' and `long double', respectively. `FLT_MAX_10_EXP' This is the maximum positive integer such that `10' raised to this power minus 1 can be represented as a normalized floating point number of type `float'. This is supposed to be at least `37'. `DBL_MAX_10_EXP' `LDBL_MAX_10_EXP' These are similar to `FLT_MAX_10_EXP', but for the data types `double' and `long double', respectively. `FLT_MAX' The value of this macro is the maximum number representable in type `float'. It is supposed to be at least `1E+37'. The value has type `float'. The smallest representable number is `- FLT_MAX'. `DBL_MAX' `LDBL_MAX' These are similar to `FLT_MAX', but for the data types `double' and `long double', respectively. The type of the macro's value is the same as the type it describes. `FLT_MIN' The value of this macro is the minimum normalized positive floating point number that is representable in type `float'. It is supposed to be no more than `1E-37'. `DBL_MIN' `LDBL_MIN' These are similar to `FLT_MIN', but for the data types `double' and `long double', respectively. The type of the macro's value is the same as the type it describes. `FLT_EPSILON' This is the maximum positive floating point number of type `float' such that `1.0 + FLT_EPSILON != 1.0' is true. It's supposed to be no greater than `1E-5'. `DBL_EPSILON' `LDBL_EPSILON' These are similar to `FLT_EPSILON', but for the data types `double' and `long double', respectively. The type of the macro's value is the same as the type it describes. The values are not supposed to be greater than `1E-9'. IEEE Floating Point ................... Here is an example showing how the floating type measurements come out for the most common floating point representation, specified by the `IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std 754-1985)'. Nearly all computers designed since the 1980s use this format. The IEEE single-precision float representation uses a base of 2. There is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total precision is 24 base-2 digits), and an 8-bit exponent that can represent values in the range -125 to 128, inclusive. So, for an implementation that uses this representation for the `float' data type, appropriate values for the corresponding parameters are: FLT_RADIX 2 FLT_MANT_DIG 24 FLT_DIG 6 FLT_MIN_EXP -125 FLT_MIN_10_EXP -37 FLT_MAX_EXP 128 FLT_MAX_10_EXP +38 FLT_MIN 1.17549435E-38F FLT_MAX 3.40282347E+38F FLT_EPSILON 1.19209290E-07F Here are the values for the `double' data type: DBL_MANT_DIG 53 DBL_DIG 15 DBL_MIN_EXP -1021 DBL_MIN_10_EXP -307 DBL_MAX_EXP 1024 DBL_MAX_10_EXP 308 DBL_MAX 1.7976931348623157E+308 DBL_MIN 2.2250738585072014E-308 DBL_EPSILON 2.2204460492503131E-016 Structure Field Offset Measurement ---------------------------------- You can use `offsetof' to measure the location within a structure type of a particular structure member. - Macro: size_t offsetof (TYPE, MEMBER) This expands to a integer constant expression that is the offset of the structure member named MEMBER in a the structure type TYPE. For example, `offsetof (struct s, elem)' is the offset, in bytes, of the member `elem' in a `struct s'. This macro won't work if MEMBER is a bit field; you get an error from the C compiler in that case. Summary of Library Facilities ***************************** This appendix is a complete list of the facilities declared within the header files supplied with the GNU C library. Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it. `long int a64l (const char *STRING)' `stdlib.h' (XPG): *Note Encode Binary Data::. `void abort (void)' `stdlib.h' (ISO): *Note Aborting a Program::. `int abs (int NUMBER)' `stdlib.h' (ISO): *Note Absolute Value::. `int accept (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH_PTR)' `sys/socket.h' (BSD): *Note Accepting Connections::. `int access (const char *FILENAME, int HOW)' `unistd.h' (POSIX.1): *Note Testing File Access::. `ACCOUNTING' `utmp.h' (SVID): *Note Manipulating the Database::. `double acos (double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `float acosf (float X)' `math.h' (ISO): *Note Inverse Trig Functions::. `double acosh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float acoshf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double acoshl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double acosl (long double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `int addmntent (FILE *STREAM, const struct mntent *MNT)' `mntent.h' (BSD): *Note mtab::. `int adjtime (const struct timeval *DELTA, struct timeval *OLDDELTA)' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `int adjtimex (struct timex *TIMEX)' `sys/timex.h' (GNU): *Note High-Resolution Calendar::. `AF_FILE' `sys/socket.h' (GNU): *Note Address Formats::. `AF_INET' `sys/socket.h' (BSD): *Note Address Formats::. `AF_INET6' `sys/socket.h' (IPv6 Basic API): *Note Address Formats::. `AF_LOCAL' `sys/socket.h' (POSIX): *Note Address Formats::. `AF_UNIX' `sys/socket.h' (BSD, Unix98): *Note Address Formats::. `AF_UNSPEC' `sys/socket.h' (BSD): *Note Address Formats::. `int aio_cancel (int FILDES, struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Cancel AIO Operations::. `int aio_cancel64 (int FILDES, struct aiocb64 *AIOCBP)' `aio.h' (Unix98): *Note Cancel AIO Operations::. `int aio_error (const struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Status of AIO Operations::. `int aio_error64 (const struct aiocb64 *AIOCBP)' `aio.h' (Unix98): *Note Status of AIO Operations::. `int aio_fsync (int OP, struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Synchronizing AIO Operations::. `int aio_fsync64 (int OP, struct aiocb64 *AIOCBP)' `aio.h' (Unix98): *Note Synchronizing AIO Operations::. `void aio_init (const struct aioinit *INIT)' `aio.h' (GNU): *Note Configuration of AIO::. `int aio_read (struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Asynchronous Reads/Writes::. `int aio_read64 (struct aiocb *AIOCBP)' `aio.h' (Unix98): *Note Asynchronous Reads/Writes::. `ssize_t aio_return (const struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Status of AIO Operations::. `int aio_return64 (const struct aiocb64 *AIOCBP)' `aio.h' (Unix98): *Note Status of AIO Operations::. `int aio_suspend (const struct aiocb *const LIST[], int NENT, const struct timespec *TIMEOUT)' `aio.h' (POSIX.1b): *Note Synchronizing AIO Operations::. `int aio_suspend64 (const struct aiocb64 *const LIST[], int NENT, const struct timespec *TIMEOUT)' `aio.h' (Unix98): *Note Synchronizing AIO Operations::. `int aio_write (struct aiocb *AIOCBP)' `aio.h' (POSIX.1b): *Note Asynchronous Reads/Writes::. `int aio_write64 (struct aiocb *AIOCBP)' `aio.h' (Unix98): *Note Asynchronous Reads/Writes::. `unsigned int alarm (unsigned int SECONDS)' `unistd.h' (POSIX.1): *Note Setting an Alarm::. `void * alloca (size_t SIZE);' `stdlib.h' (GNU, BSD): *Note Variable Size Automatic::. `int alphasort (const void *A, const void *B)' `dirent.h' (BSD/SVID): *Note Scanning Directory Content::. `int alphasort64 (const void *A, const void *B)' `dirent.h' (GNU): *Note Scanning Directory Content::. `tcflag_t ALTWERASE' `termios.h' (BSD): *Note Local Modes::. `int ARG_MAX' `limits.h' (POSIX.1): *Note General Limits::. `error_t argp_err_exit_status' `argp.h' (GNU): *Note Argp Global Variables::. `void argp_error (const struct argp_state *STATE, const char *FMT, ...)' `argp.h' (GNU): *Note Argp Helper Functions::. `int ARGP_ERR_UNKNOWN' `argp.h' (GNU): *Note Argp Parser Functions::. `void argp_failure (const struct argp_state *STATE, int STATUS, int ERRNUM, const char *FMT, ...)' `argp.h' (GNU): *Note Argp Helper Functions::. `void argp_help (const struct argp *ARGP, FILE *STREAM, unsigned FLAGS, char *NAME)' `argp.h' (GNU): *Note Argp Help::. `ARGP_IN_ORDER' `argp.h' (GNU): *Note Argp Flags::. `ARGP_KEY_ARG' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_ARGS' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_END' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_ERROR' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_FINI' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_HELP_ARGS_DOC' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_HELP_DUP_ARGS_NOTE' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_HELP_EXTRA' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_HELP_HEADER' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_HELP_POST_DOC' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_HELP_PRE_DOC' `argp.h' (GNU): *Note Argp Help Filter Keys::. `ARGP_KEY_INIT' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_NO_ARGS' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_KEY_SUCCESS' `argp.h' (GNU): *Note Argp Special Keys::. `ARGP_LONG_ONLY' `argp.h' (GNU): *Note Argp Flags::. `ARGP_NO_ARGS' `argp.h' (GNU): *Note Argp Flags::. `ARGP_NO_ERRS' `argp.h' (GNU): *Note Argp Flags::. `ARGP_NO_EXIT' `argp.h' (GNU): *Note Argp Flags::. `ARGP_NO_HELP' `argp.h' (GNU): *Note Argp Flags::. `error_t argp_parse (const struct argp *ARGP, int ARGC, char **ARGV, unsigned FLAGS, int *ARG_INDEX, void *INPUT)' `argp.h' (GNU): *Note Suboptions: Argp. `ARGP_PARSE_ARGV0' `argp.h' (GNU): *Note Argp Flags::. `const char * argp_program_bug_address' `argp.h' (GNU): *Note Argp Global Variables::. `const char * argp_program_version' `argp.h' (GNU): *Note Argp Global Variables::. `argp_program_version_hook' `argp.h' (GNU): *Note Argp Global Variables::. `ARGP_SILENT' `argp.h' (GNU): *Note Argp Flags::. `void argp_state_help (const struct argp_state *STATE, FILE *STREAM, unsigned FLAGS)' `argp.h' (GNU): *Note Argp Helper Functions::. `void argp_usage (const struct argp_state *STATE)' `argp.h' (GNU): *Note Argp Helper Functions::. `error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const char *STR)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const char *BUF, size_t BUF_LEN)' `argz.h' (GNU): *Note Argz Functions::. `size_t argz_count (const char *ARGZ, size_t ARG_LEN)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_create (char *const ARGV[], char **ARGZ, size_t *ARGZ_LEN)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_create_sep (const char *STRING, int SEP, char **ARGZ, size_t *ARGZ_LEN)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY)' `argz.h' (GNU): *Note Argz Functions::. `void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char *BEFORE, const char *ENTRY)' `argz.h' (GNU): *Note Argz Functions::. `char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY)' `argz.h' (GNU): *Note Argz Functions::. `error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT)' `argz.h' (GNU): *Note Argz Functions::. `void argz_stringify (char *ARGZ, size_t LEN, int SEP)' `argz.h' (GNU): *Note Argz Functions::. `char * asctime (const struct tm *BROKENTIME)' `time.h' (ISO): *Note Formatting Calendar Time::. `char * asctime_r (const struct tm *BROKENTIME, char *BUFFER)' `time.h' (POSIX.1c): *Note Formatting Calendar Time::. `double asin (double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `float asinf (float X)' `math.h' (ISO): *Note Inverse Trig Functions::. `double asinh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float asinhf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double asinhl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double asinl (long double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `int asprintf (char **PTR, const char *TEMPLATE, ...)' `stdio.h' (GNU): *Note Dynamic Output::. `void assert (int EXPRESSION)' `assert.h' (ISO): *Note Consistency Checking::. `void assert_perror (int ERRNUM)' `assert.h' (GNU): *Note Consistency Checking::. `double atan (double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `double atan2 (double Y, double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `float atan2f (float Y, float X)' `math.h' (ISO): *Note Inverse Trig Functions::. `long double atan2l (long double Y, long double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `float atanf (float X)' `math.h' (ISO): *Note Inverse Trig Functions::. `double atanh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float atanhf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double atanhl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double atanl (long double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `int atexit (void (*FUNCTION) (void))' `stdlib.h' (ISO): *Note Cleanups on Exit::. `double atof (const char *STRING)' `stdlib.h' (ISO): *Note Parsing of Floats::. `int atoi (const char *STRING)' `stdlib.h' (ISO): *Note Parsing of Integers::. `long int atol (const char *STRING)' `stdlib.h' (ISO): *Note Parsing of Integers::. `long long int atoll (const char *STRING)' `stdlib.h' (ISO): *Note Parsing of Integers::. `B0' `termios.h' (POSIX.1): *Note Line Speed::. `B110' `termios.h' (POSIX.1): *Note Line Speed::. `B115200' `termios.h' (GNU): *Note Line Speed::. `B1200' `termios.h' (POSIX.1): *Note Line Speed::. `B134' `termios.h' (POSIX.1): *Note Line Speed::. `B150' `termios.h' (POSIX.1): *Note Line Speed::. `B1800' `termios.h' (POSIX.1): *Note Line Speed::. `B19200' `termios.h' (POSIX.1): *Note Line Speed::. `B200' `termios.h' (POSIX.1): *Note Line Speed::. `B230400' `termios.h' (GNU): *Note Line Speed::. `B2400' `termios.h' (POSIX.1): *Note Line Speed::. `B300' `termios.h' (POSIX.1): *Note Line Speed::. `B38400' `termios.h' (POSIX.1): *Note Line Speed::. `B460800' `termios.h' (GNU): *Note Line Speed::. `B4800' `termios.h' (POSIX.1): *Note Line Speed::. `B50' `termios.h' (POSIX.1): *Note Line Speed::. `B57600' `termios.h' (GNU): *Note Line Speed::. `B600' `termios.h' (POSIX.1): *Note Line Speed::. `B75' `termios.h' (POSIX.1): *Note Line Speed::. `B9600' `termios.h' (POSIX.1): *Note Line Speed::. `int backtrace (void **BUFFER, int SIZE)' `execinfo.h' (GNU): *Note Backtraces::. `char ** backtrace_symbols (void *const *BUFFER, int SIZE)' `execinfo.h' (GNU): *Note Backtraces::. `void backtrace_symbols_fd (void *const *BUFFER, int SIZE, int FD)' `execinfo.h' (GNU): *Note Backtraces::. `char * basename (char *PATH)' `libgen.h' (XPG): *Note Finding Tokens in a String::. `char * basename (const char *FILENAME)' `string.h' (GNU): *Note Finding Tokens in a String::. `int BC_BASE_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int BC_DIM_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int bcmp (const void *A1, const void *A2, size_t SIZE)' `string.h' (BSD): *Note String/Array Comparison::. `void bcopy (const void *FROM, void *TO, size_t SIZE)' `string.h' (BSD): *Note Copying and Concatenation::. `int BC_SCALE_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int BC_STRING_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int bind (int SOCKET, struct sockaddr *ADDR, socklen_t LENGTH)' `sys/socket.h' (BSD): *Note Setting Address::. `char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME)' `libintl.h' (GNU): *Note Locating gettext catalog::. `char * bind_textdomain_codeset (const char *DOMAINNAME, const char *CODESET)' `libintl.h' (GNU): *Note Charset conversion in gettext::. `blkcnt64_t' `sys/types.h' (Unix98): *Note Attribute Meanings::. `blkcnt_t' `sys/types.h' (Unix98): *Note Attribute Meanings::. `BOOT_TIME' `utmp.h' (SVID): *Note Manipulating the Database::. `BOOT_TIME' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int brk (void *ADDR)' `unistd.h' (BSD): *Note Resizing the Data Segment::. `tcflag_t BRKINT' `termios.h' (POSIX.1): *Note Input Modes::. `_BSD_SOURCE' (GNU): *Note Feature Test Macros::. `void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)' `stdlib.h' (ISO): *Note Array Search Function::. `wint_t btowc (int C)' `wchar.h' (ISO): *Note Converting a Character::. `int BUFSIZ' `stdio.h' (ISO): *Note Controlling Buffering::. `void bzero (void *BLOCK, size_t SIZE)' `string.h' (BSD): *Note Copying and Concatenation::. `double cabs (complex double Z)' `complex.h' (ISO): *Note Absolute Value::. `float cabsf (complex float Z)' `complex.h' (ISO): *Note Absolute Value::. `long double cabsl (complex long double Z)' `complex.h' (ISO): *Note Absolute Value::. `complex double cacos (complex double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex float cacosf (complex float Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex double cacosh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float cacoshf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double cacoshl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double cacosl (complex long double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `void * calloc (size_t COUNT, size_t ELTSIZE)' `malloc.h', `stdlib.h' (ISO): *Note Allocating Cleared Space::. `char * canonicalize_file_name (const char *NAME)' `stdlib.h' (GNU): *Note Symbolic Links::. `double carg (complex double Z)' `complex.h' (ISO): *Note Operations on Complex::. `float cargf (complex float Z)' `complex.h' (ISO): *Note Operations on Complex::. `long double cargl (complex long double Z)' `complex.h' (ISO): *Note Operations on Complex::. `complex double casin (complex double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex float casinf (complex float Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex double casinh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float casinhf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double casinhl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double casinl (complex long double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex double catan (complex double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex float catanf (complex float Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `complex double catanh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float catanhf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double catanhl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double catanl (complex long double Z)' `complex.h' (ISO): *Note Inverse Trig Functions::. `nl_catd catopen (const char *CAT_NAME, int FLAG)' `nl_types.h' (X/Open): *Note The catgets Functions::. `int cbc_crypt (char *KEY, char *BLOCKS, unsigned LEN, unsigned MODE, char *IVEC)' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `double cbrt (double X)' `math.h' (BSD): *Note Exponents and Logarithms::. `float cbrtf (float X)' `math.h' (BSD): *Note Exponents and Logarithms::. `long double cbrtl (long double X)' `math.h' (BSD): *Note Exponents and Logarithms::. `complex double ccos (complex double Z)' `complex.h' (ISO): *Note Trig Functions::. `complex float ccosf (complex float Z)' `complex.h' (ISO): *Note Trig Functions::. `complex double ccosh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float ccoshf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double ccoshl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double ccosl (complex long double Z)' `complex.h' (ISO): *Note Trig Functions::. `cc_t' `termios.h' (POSIX.1): *Note Mode Data Types::. `tcflag_t CCTS_OFLOW' `termios.h' (BSD): *Note Control Modes::. `double ceil (double X)' `math.h' (ISO): *Note Rounding Functions::. `float ceilf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double ceill (long double X)' `math.h' (ISO): *Note Rounding Functions::. `complex double cexp (complex double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex float cexpf (complex float Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex long double cexpl (complex long double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `speed_t cfgetispeed (const struct termios *TERMIOS-P)' `termios.h' (POSIX.1): *Note Line Speed::. `speed_t cfgetospeed (const struct termios *TERMIOS-P)' `termios.h' (POSIX.1): *Note Line Speed::. `int cfmakeraw (struct termios *TERMIOS-P)' `termios.h' (BSD): *Note Noncanonical Input::. `void cfree (void *PTR)' `stdlib.h' (Sun): *Note Freeing after Malloc::. `int cfsetispeed (struct termios *TERMIOS-P, speed_t SPEED)' `termios.h' (POSIX.1): *Note Line Speed::. `int cfsetospeed (struct termios *TERMIOS-P, speed_t SPEED)' `termios.h' (POSIX.1): *Note Line Speed::. `int cfsetspeed (struct termios *TERMIOS-P, speed_t SPEED)' `termios.h' (BSD): *Note Line Speed::. `CHAR_BIT' `limits.h' (ISO): *Note Width of Type::. `CHAR_MAX' `limits.h' (ISO): *Note Range of Type::. `CHAR_MIN' `limits.h' (ISO): *Note Range of Type::. `int chdir (const char *FILENAME)' `unistd.h' (POSIX.1): *Note Working Directory::. `int CHILD_MAX' `limits.h' (POSIX.1): *Note General Limits::. `int chmod (const char *FILENAME, mode_t MODE)' `sys/stat.h' (POSIX.1): *Note Setting Permissions::. `int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP)' `unistd.h' (POSIX.1): *Note File Owner::. `tcflag_t CIGNORE' `termios.h' (BSD): *Note Control Modes::. `double cimag (complex double Z)' `complex.h' (ISO): *Note Operations on Complex::. `float cimagf (complex float Z)' `complex.h' (ISO): *Note Operations on Complex::. `long double cimagl (complex long double Z)' `complex.h' (ISO): *Note Operations on Complex::. `int clearenv (void)' `stdlib.h' (GNU): *Note Environment Access::. `void clearerr (FILE *STREAM)' `stdio.h' (ISO): *Note Error Recovery::. `void clearerr_unlocked (FILE *STREAM)' `stdio.h' (GNU): *Note Error Recovery::. `int CLK_TCK' `time.h' (POSIX.1): *Note CPU Time::. `tcflag_t CLOCAL' `termios.h' (POSIX.1): *Note Control Modes::. `clock_t clock (void)' `time.h' (ISO): *Note CPU Time::. `int CLOCKS_PER_SEC' `time.h' (ISO): *Note CPU Time::. `clock_t' `time.h' (ISO): *Note CPU Time::. `complex double clog (complex double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex double clog10 (complex double Z)' `complex.h' (GNU): *Note Exponents and Logarithms::. `complex float clog10f (complex float Z)' `complex.h' (GNU): *Note Exponents and Logarithms::. `complex long double clog10l (complex long double Z)' `complex.h' (GNU): *Note Exponents and Logarithms::. `complex float clogf (complex float Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex long double clogl (complex long double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `int close (int FILEDES)' `unistd.h' (POSIX.1): *Note Opening and Closing Files::. `int closedir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Reading/Closing Directory::. `void closelog (void)' `syslog.h' (BSD): *Note closelog::. `int COLL_WEIGHTS_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `size_t confstr (int PARAMETER, char *BUF, size_t LEN)' `unistd.h' (POSIX.2): *Note String Parameters::. `complex double conj (complex double Z)' `complex.h' (ISO): *Note Operations on Complex::. `complex float conjf (complex float Z)' `complex.h' (ISO): *Note Operations on Complex::. `complex long double conjl (complex long double Z)' `complex.h' (ISO): *Note Operations on Complex::. `int connect (int SOCKET, struct sockaddr *ADDR, socklen_t LENGTH)' `sys/socket.h' (BSD): *Note Connecting::. `cookie_close_function' `stdio.h' (GNU): *Note Hook Functions::. `cookie_io_functions_t' `stdio.h' (GNU): *Note Streams and Cookies::. `cookie_read_function' `stdio.h' (GNU): *Note Hook Functions::. `cookie_seek_function' `stdio.h' (GNU): *Note Hook Functions::. `cookie_write_function' `stdio.h' (GNU): *Note Hook Functions::. `double copysign (double X, double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `float copysignf (float X, float Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `long double copysignl (long double X, long double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `double cos (double X)' `math.h' (ISO): *Note Trig Functions::. `float cosf (float X)' `math.h' (ISO): *Note Trig Functions::. `double cosh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float coshf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double coshl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double cosl (long double X)' `math.h' (ISO): *Note Trig Functions::. `complex double cpow (complex double BASE, complex double POWER)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex float cpowf (complex float BASE, complex float POWER)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex long double cpowl (complex long double BASE, complex long double POWER)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex double cproj (complex double Z)' `complex.h' (ISO): *Note Operations on Complex::. `complex float cprojf (complex float Z)' `complex.h' (ISO): *Note Operations on Complex::. `complex long double cprojl (complex long double Z)' `complex.h' (ISO): *Note Operations on Complex::. `tcflag_t CREAD' `termios.h' (POSIX.1): *Note Control Modes::. `double creal (complex double Z)' `complex.h' (ISO): *Note Operations on Complex::. `float crealf (complex float Z)' `complex.h' (ISO): *Note Operations on Complex::. `long double creall (complex long double Z)' `complex.h' (ISO): *Note Operations on Complex::. `int creat (const char *FILENAME, mode_t MODE)' `fcntl.h' (POSIX.1): *Note Opening and Closing Files::. `int creat64 (const char *FILENAME, mode_t MODE)' `fcntl.h' (Unix98): *Note Opening and Closing Files::. `tcflag_t CRTS_IFLOW' `termios.h' (BSD): *Note Control Modes::. `char * crypt (const char *KEY, const char *SALT)' `crypt.h' (BSD, SVID): *Note crypt::. `char * crypt_r (const char *KEY, const char *SALT, struct crypt_data * DATA)' `crypt.h' (GNU): *Note crypt::. `tcflag_t CS5' `termios.h' (POSIX.1): *Note Control Modes::. `tcflag_t CS6' `termios.h' (POSIX.1): *Note Control Modes::. `tcflag_t CS7' `termios.h' (POSIX.1): *Note Control Modes::. `tcflag_t CS8' `termios.h' (POSIX.1): *Note Control Modes::. `complex double csin (complex double Z)' `complex.h' (ISO): *Note Trig Functions::. `complex float csinf (complex float Z)' `complex.h' (ISO): *Note Trig Functions::. `complex double csinh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float csinhf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double csinhl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double csinl (complex long double Z)' `complex.h' (ISO): *Note Trig Functions::. `tcflag_t CSIZE' `termios.h' (POSIX.1): *Note Control Modes::. `_CS_LFS64_CFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS64_LDFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS64_LIBS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS64_LINTFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS_CFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS_LDFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS_LIBS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_LFS_LINTFLAGS' `unistd.h' (Unix98): *Note String Parameters::. `_CS_PATH' `unistd.h' (POSIX.2): *Note String Parameters::. `complex double csqrt (complex double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex float csqrtf (complex float Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `complex long double csqrtl (complex long double Z)' `complex.h' (ISO): *Note Exponents and Logarithms::. `tcflag_t CSTOPB' `termios.h' (POSIX.1): *Note Control Modes::. `complex double ctan (complex double Z)' `complex.h' (ISO): *Note Trig Functions::. `complex float ctanf (complex float Z)' `complex.h' (ISO): *Note Trig Functions::. `complex double ctanh (complex double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex float ctanhf (complex float Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double ctanhl (complex long double Z)' `complex.h' (ISO): *Note Hyperbolic Functions::. `complex long double ctanl (complex long double Z)' `complex.h' (ISO): *Note Trig Functions::. `char * ctermid (char *STRING)' `stdio.h' (POSIX.1): *Note Identifying the Terminal::. `char * ctime (const time_t *TIME)' `time.h' (ISO): *Note Formatting Calendar Time::. `char * ctime_r (const time_t *TIME, char *BUFFER)' `time.h' (POSIX.1c): *Note Formatting Calendar Time::. `char * cuserid (char *STRING)' `stdio.h' (POSIX.1): *Note Who Logged In::. `int daylight' `time.h' (SVID): *Note Time Zone Functions::. `DBL_DIG' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_EPSILON' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MANT_DIG' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MAX' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MAX_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MAX_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MIN' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MIN_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MIN_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY)' `libintl.h' (GNU): *Note Translation with gettext::. `char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY)' `libintl.h' (GNU): *Note Advanced gettext functions::. `DEAD_PROCESS' `utmp.h' (SVID): *Note Manipulating the Database::. `DEAD_PROCESS' `utmpx.h' (XPG4.2): *Note XPG Functions::. `DES_DECRYPT' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DES_ENCRYPT' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DESERR_BADPARAM' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DESERR_HWERROR' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DESERR_NOHWDEVICE' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DESERR_NONE' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `int DES_FAILED (int ERR)' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DES_HW' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `void des_setparity (char *KEY)' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `DES_SW' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `dev_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `char * dgettext (const char *DOMAINNAME, const char *MSGID)' `libintl.h' (GNU): *Note Translation with gettext::. `double difftime (time_t TIME1, time_t TIME0)' `time.h' (ISO): *Note Elapsed Time::. `DIR' `dirent.h' (POSIX.1): *Note Opening a Directory::. `int dirfd (DIR *DIRSTREAM)' `dirent.h' (GNU): *Note Opening a Directory::. `char * dirname (char *PATH)' `libgen.h' (XPG): *Note Finding Tokens in a String::. `div_t div (int NUMERATOR, int DENOMINATOR)' `stdlib.h' (ISO): *Note Integer Division::. `div_t' `stdlib.h' (ISO): *Note Integer Division::. `char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N)' `libintl.h' (GNU): *Note Advanced gettext functions::. `double drand48 (void)' `stdlib.h' (SVID): *Note SVID Random::. `int drand48_r (struct drand48_data *BUFFER, double *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `double drem (double NUMERATOR, double DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `float dremf (float NUMERATOR, float DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `long double dreml (long double NUMERATOR, long double DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `mode_t DTTOIF (int DTYPE)' `dirent.h' (BSD): *Note Directory Entries::. `int dup (int OLD)' `unistd.h' (POSIX.1): *Note Duplicating Descriptors::. `int dup2 (int OLD, int NEW)' `unistd.h' (POSIX.1): *Note Duplicating Descriptors::. `int E2BIG' `errno.h' (POSIX.1: Argument list too long): *Note Error Codes::. `int EACCES' `errno.h' (POSIX.1: Permission denied): *Note Error Codes::. `int EADDRINUSE' `errno.h' (BSD: Address already in use): *Note Error Codes::. `int EADDRNOTAVAIL' `errno.h' (BSD: Cannot assign requested address): *Note Error Codes::. `int EADV' `errno.h' (Linux???: Advertise error): *Note Error Codes::. `int EAFNOSUPPORT' `errno.h' (BSD: Address family not supported by protocol): *Note Error Codes::. `int EAGAIN' `errno.h' (POSIX.1: Resource temporarily unavailable): *Note Error Codes::. `int EALREADY' `errno.h' (BSD: Operation already in progress): *Note Error Codes::. `int EAUTH' `errno.h' (BSD: Authentication error): *Note Error Codes::. `int EBACKGROUND' `errno.h' (GNU: Inappropriate operation for background process): *Note Error Codes::. `int EBADE' `errno.h' (Linux???: Invalid exchange): *Note Error Codes::. `int EBADF' `errno.h' (POSIX.1: Bad file descriptor): *Note Error Codes::. `int EBADFD' `errno.h' (Linux???: File descriptor in bad state): *Note Error Codes::. `int EBADMSG' `errno.h' (XOPEN: Bad message): *Note Error Codes::. `int EBADR' `errno.h' (Linux???: Invalid request descriptor): *Note Error Codes::. `int EBADRPC' `errno.h' (BSD: RPC struct is bad): *Note Error Codes::. `int EBADRQC' `errno.h' (Linux???: Invalid request code): *Note Error Codes::. `int EBADSLT' `errno.h' (Linux???: Invalid slot): *Note Error Codes::. `int EBFONT' `errno.h' (Linux???: Bad font file format): *Note Error Codes::. `int EBUSY' `errno.h' (POSIX.1: Device or resource busy): *Note Error Codes::. `int ecb_crypt (char *KEY, char *BLOCKS, unsigned LEN, unsigned MODE)' `rpc/des_crypt.h' (SUNRPC): *Note DES Encryption::. `int ECHILD' `errno.h' (POSIX.1: No child processes): *Note Error Codes::. `tcflag_t ECHO' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHOCTL' `termios.h' (BSD): *Note Local Modes::. `tcflag_t ECHOE' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHOK' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHOKE' `termios.h' (BSD): *Note Local Modes::. `tcflag_t ECHONL' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHOPRT' `termios.h' (BSD): *Note Local Modes::. `int ECHRNG' `errno.h' (Linux???: Channel number out of range): *Note Error Codes::. `int ECOMM' `errno.h' (Linux???: Communication error on send): *Note Error Codes::. `int ECONNABORTED' `errno.h' (BSD: Software caused connection abort): *Note Error Codes::. `int ECONNREFUSED' `errno.h' (BSD: Connection refused): *Note Error Codes::. `int ECONNRESET' `errno.h' (BSD: Connection reset by peer): *Note Error Codes::. `char * ecvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)' `stdlib.h' (SVID, Unix98): *Note System V Number Conversion::. `char * ecvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)' `stdlib.h' (GNU): *Note System V Number Conversion::. `int ED' `errno.h' (GNU: ?): *Note Error Codes::. `int EDEADLK' `errno.h' (POSIX.1: Resource deadlock avoided): *Note Error Codes::. `int EDEADLOCK' `errno.h' (Linux???: File locking deadlock error): *Note Error Codes::. `int EDESTADDRREQ' `errno.h' (BSD: Destination address required): *Note Error Codes::. `int EDIED' `errno.h' (GNU: Translator died): *Note Error Codes::. `int EDOM' `errno.h' (ISO: Numerical argument out of domain): *Note Error Codes::. `int EDOTDOT' `errno.h' (Linux???: RFS specific error): *Note Error Codes::. `int EDQUOT' `errno.h' (BSD: Disk quota exceeded): *Note Error Codes::. `int EEXIST' `errno.h' (POSIX.1: File exists): *Note Error Codes::. `int EFAULT' `errno.h' (POSIX.1: Bad address): *Note Error Codes::. `int EFBIG' `errno.h' (POSIX.1: File too large): *Note Error Codes::. `int EFTYPE' `errno.h' (BSD: Inappropriate file type or format): *Note Error Codes::. `int EGRATUITOUS' `errno.h' (GNU: Gratuitous error): *Note Error Codes::. `int EGREGIOUS' `errno.h' (GNU: You really blew it this time): *Note Error Codes::. `int EHOSTDOWN' `errno.h' (BSD: Host is down): *Note Error Codes::. `int EHOSTUNREACH' `errno.h' (BSD: No route to host): *Note Error Codes::. `int EIDRM' `errno.h' (XOPEN: Identifier removed): *Note Error Codes::. `int EIEIO' `errno.h' (GNU: Computer bought the farm): *Note Error Codes::. `int EILSEQ' `errno.h' (ISO: Invalid or incomplete multibyte or wide character): *Note Error Codes::. `int EINPROGRESS' `errno.h' (BSD: Operation now in progress): *Note Error Codes::. `int EINTR' `errno.h' (POSIX.1: Interrupted system call): *Note Error Codes::. `int EINVAL' `errno.h' (POSIX.1: Invalid argument): *Note Error Codes::. `int EIO' `errno.h' (POSIX.1: Input/output error): *Note Error Codes::. `int EISCONN' `errno.h' (BSD: Transport endpoint is already connected): *Note Error Codes::. `int EISDIR' `errno.h' (POSIX.1: Is a directory): *Note Error Codes::. `int EISNAM' `errno.h' (Linux???: Is a named type file): *Note Error Codes::. `int EL2HLT' `errno.h' (Obsolete: Level 2 halted): *Note Error Codes::. `int EL2NSYNC' `errno.h' (Obsolete: Level 2 not synchronized): *Note Error Codes::. `int EL3HLT' `errno.h' (Obsolete: Level 3 halted): *Note Error Codes::. `int EL3RST' `errno.h' (Obsolete: Level 3 reset): *Note Error Codes::. `int ELIBACC' `errno.h' (Linux???: Can not access a needed shared library): *Note Error Codes::. `int ELIBBAD' `errno.h' (Linux???: Accessing a corrupted shared library): *Note Error Codes::. `int ELIBEXEC' `errno.h' (Linux???: Cannot exec a shared library directly): *Note Error Codes::. `int ELIBMAX' `errno.h' (Linux???: Attempting to link in too many shared libraries): *Note Error Codes::. `int ELIBSCN' `errno.h' (Linux???: .lib section in a.out corrupted): *Note Error Codes::. `int ELNRNG' `errno.h' (Linux???: Link number out of range): *Note Error Codes::. `int ELOOP' `errno.h' (BSD: Too many levels of symbolic links): *Note Error Codes::. `int EMEDIUMTYPE' `errno.h' (Linux???: Wrong medium type): *Note Error Codes::. `int EMFILE' `errno.h' (POSIX.1: Too many open files): *Note Error Codes::. `int EMLINK' `errno.h' (POSIX.1: Too many links): *Note Error Codes::. `EMPTY' `utmp.h' (SVID): *Note Manipulating the Database::. `EMPTY' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int EMSGSIZE' `errno.h' (BSD: Message too long): *Note Error Codes::. `int EMULTIHOP' `errno.h' (XOPEN: Multihop attempted): *Note Error Codes::. `int ENAMETOOLONG' `errno.h' (POSIX.1: File name too long): *Note Error Codes::. `int ENAVAIL' `errno.h' (Linux???: No XENIX semaphores available): *Note Error Codes::. `void encrypt (char *BLOCK, int EDFLAG)' `crypt.h' (BSD, SVID): *Note DES Encryption::. `void encrypt_r (char *BLOCK, int EDFLAG, struct crypt_data * DATA)' `crypt.h' (GNU): *Note DES Encryption::. `void endfsent (void)' `fstab.h' (BSD): *Note fstab::. `void endgrent (void)' `grp.h' (SVID, BSD): *Note Scanning All Groups::. `void endhostent (void)' `netdb.h' (BSD): *Note Host Names::. `int endmntent (FILE *STREAM)' `mntent.h' (BSD): *Note mtab::. `void endnetent (void)' `netdb.h' (BSD): *Note Networks Database::. `void endnetgrent (void)' `netdb.h' (BSD): *Note Lookup Netgroup::. `void endprotoent (void)' `netdb.h' (BSD): *Note Protocols Database::. `void endpwent (void)' `pwd.h' (SVID, BSD): *Note Scanning All Users::. `void endservent (void)' `netdb.h' (BSD): *Note Services Database::. `void endutent (void)' `utmp.h' (SVID): *Note Manipulating the Database::. `void endutxent (void)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int ENEEDAUTH' `errno.h' (BSD: Need authenticator): *Note Error Codes::. `int ENETDOWN' `errno.h' (BSD: Network is down): *Note Error Codes::. `int ENETRESET' `errno.h' (BSD: Network dropped connection on reset): *Note Error Codes::. `int ENETUNREACH' `errno.h' (BSD: Network is unreachable): *Note Error Codes::. `int ENFILE' `errno.h' (POSIX.1: Too many open files in system): *Note Error Codes::. `int ENOANO' `errno.h' (Linux???: No anode): *Note Error Codes::. `int ENOBUFS' `errno.h' (BSD: No buffer space available): *Note Error Codes::. `int ENOCSI' `errno.h' (Linux???: No CSI structure available): *Note Error Codes::. `int ENODATA' `errno.h' (XOPEN: No data available): *Note Error Codes::. `int ENODEV' `errno.h' (POSIX.1: No such device): *Note Error Codes::. `int ENOENT' `errno.h' (POSIX.1: No such file or directory): *Note Error Codes::. `int ENOEXEC' `errno.h' (POSIX.1: Exec format error): *Note Error Codes::. `int ENOLCK' `errno.h' (POSIX.1: No locks available): *Note Error Codes::. `int ENOLINK' `errno.h' (XOPEN: Link has been severed): *Note Error Codes::. `int ENOMEDIUM' `errno.h' (Linux???: No medium found): *Note Error Codes::. `int ENOMEM' `errno.h' (POSIX.1: Cannot allocate memory): *Note Error Codes::. `int ENOMSG' `errno.h' (XOPEN: No message of desired type): *Note Error Codes::. `int ENONET' `errno.h' (Linux???: Machine is not on the network): *Note Error Codes::. `int ENOPKG' `errno.h' (Linux???: Package not installed): *Note Error Codes::. `int ENOPROTOOPT' `errno.h' (BSD: Protocol not available): *Note Error Codes::. `int ENOSPC' `errno.h' (POSIX.1: No space left on device): *Note Error Codes::. `int ENOSR' `errno.h' (XOPEN: Out of streams resources): *Note Error Codes::. `int ENOSTR' `errno.h' (XOPEN: Device not a stream): *Note Error Codes::. `int ENOSYS' `errno.h' (POSIX.1: Function not implemented): *Note Error Codes::. `int ENOTBLK' `errno.h' (BSD: Block device required): *Note Error Codes::. `int ENOTCONN' `errno.h' (BSD: Transport endpoint is not connected): *Note Error Codes::. `int ENOTDIR' `errno.h' (POSIX.1: Not a directory): *Note Error Codes::. `int ENOTEMPTY' `errno.h' (POSIX.1: Directory not empty): *Note Error Codes::. `int ENOTNAM' `errno.h' (Linux???: Not a XENIX named type file): *Note Error Codes::. `int ENOTSOCK' `errno.h' (BSD: Socket operation on non-socket): *Note Error Codes::. `int ENOTSUP' `errno.h' (POSIX.1: Not supported): *Note Error Codes::. `int ENOTTY' `errno.h' (POSIX.1: Inappropriate ioctl for device): *Note Error Codes::. `int ENOTUNIQ' `errno.h' (Linux???: Name not unique on network): *Note Error Codes::. `char ** environ' `unistd.h' (POSIX.1): *Note Environment Access::. `error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE)' `envz.h' (GNU): *Note Envz Functions::. `char * envz_entry (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)' `envz.h' (GNU): *Note Envz Functions::. `char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME)' `envz.h' (GNU): *Note Envz Functions::. `error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE)' `envz.h' (GNU): *Note Envz Functions::. `void envz_strip (char **ENVZ, size_t *ENVZ_LEN)' `envz.h' (GNU): *Note Envz Functions::. `int ENXIO' `errno.h' (POSIX.1: No such device or address): *Note Error Codes::. `int EOF' `stdio.h' (ISO): *Note EOF and Errors::. `int EOPNOTSUPP' `errno.h' (BSD: Operation not supported): *Note Error Codes::. `int EOVERFLOW' `errno.h' (XOPEN: Value too large for defined data type): *Note Error Codes::. `int EPERM' `errno.h' (POSIX.1: Operation not permitted): *Note Error Codes::. `int EPFNOSUPPORT' `errno.h' (BSD: Protocol family not supported): *Note Error Codes::. `int EPIPE' `errno.h' (POSIX.1: Broken pipe): *Note Error Codes::. `int EPROCLIM' `errno.h' (BSD: Too many processes): *Note Error Codes::. `int EPROCUNAVAIL' `errno.h' (BSD: RPC bad procedure for program): *Note Error Codes::. `int EPROGMISMATCH' `errno.h' (BSD: RPC program version wrong): *Note Error Codes::. `int EPROGUNAVAIL' `errno.h' (BSD: RPC program not available): *Note Error Codes::. `int EPROTO' `errno.h' (XOPEN: Protocol error): *Note Error Codes::. `int EPROTONOSUPPORT' `errno.h' (BSD: Protocol not supported): *Note Error Codes::. `int EPROTOTYPE' `errno.h' (BSD: Protocol wrong type for socket): *Note Error Codes::. `int EQUIV_CLASS_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `double erand48 (unsigned short int XSUBI[3])' `stdlib.h' (SVID): *Note SVID Random::. `int erand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, double *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `int ERANGE' `errno.h' (ISO: Numerical result out of range): *Note Error Codes::. `int EREMCHG' `errno.h' (Linux???: Remote address changed): *Note Error Codes::. `int EREMOTE' `errno.h' (BSD: Object is remote): *Note Error Codes::. `int EREMOTEIO' `errno.h' (Linux???: Remote I/O error): *Note Error Codes::. `int ERESTART' `errno.h' (Linux???: Interrupted system call should be restarted): *Note Error Codes::. `double erf (double X)' `math.h' (SVID): *Note Special Functions::. `double erfc (double X)' `math.h' (SVID): *Note Special Functions::. `float erfcf (float X)' `math.h' (SVID): *Note Special Functions::. `long double erfcl (long double X)' `math.h' (SVID): *Note Special Functions::. `float erff (float X)' `math.h' (SVID): *Note Special Functions::. `long double erfl (long double X)' `math.h' (SVID): *Note Special Functions::. `int EROFS' `errno.h' (POSIX.1: Read-only file system): *Note Error Codes::. `int ERPCMISMATCH' `errno.h' (BSD: RPC version wrong): *Note Error Codes::. `void err (int STATUS, const char *FORMAT, ...)' `err.h' (BSD): *Note Error Messages::. `volatile int errno' `errno.h' (ISO): *Note Checking for Errors::. `void error (int STATUS, int ERRNUM, const char *FORMAT, ...)' `error.h' (GNU): *Note Error Messages::. `void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...)' `error.h' (GNU): *Note Error Messages::. `unsigned int error_message_count' `error.h' (GNU): *Note Error Messages::. `int error_one_per_line' `error.h' (GNU): *Note Error Messages::. `void (* error_print_progname ) (void)' `error.h' (GNU): *Note Error Messages::. `void errx (int STATUS, const char *FORMAT, ...)' `err.h' (BSD): *Note Error Messages::. `int ESHUTDOWN' `errno.h' (BSD: Cannot send after transport endpoint shutdown): *Note Error Codes::. `int ESOCKTNOSUPPORT' `errno.h' (BSD: Socket type not supported): *Note Error Codes::. `int ESPIPE' `errno.h' (POSIX.1: Illegal seek): *Note Error Codes::. `int ESRCH' `errno.h' (POSIX.1: No such process): *Note Error Codes::. `int ESRMNT' `errno.h' (Linux???: Srmount error): *Note Error Codes::. `int ESTALE' `errno.h' (BSD: Stale NFS file handle): *Note Error Codes::. `int ESTRPIPE' `errno.h' (Linux???: Streams pipe error): *Note Error Codes::. `int ETIME' `errno.h' (XOPEN: Timer expired): *Note Error Codes::. `int ETIMEDOUT' `errno.h' (BSD: Connection timed out): *Note Error Codes::. `int ETOOMANYREFS' `errno.h' (BSD: Too many references: cannot splice): *Note Error Codes::. `int ETXTBSY' `errno.h' (BSD: Text file busy): *Note Error Codes::. `int EUCLEAN' `errno.h' (Linux???: Structure needs cleaning): *Note Error Codes::. `int EUNATCH' `errno.h' (Linux???: Protocol driver not attached): *Note Error Codes::. `int EUSERS' `errno.h' (BSD: Too many users): *Note Error Codes::. `int EWOULDBLOCK' `errno.h' (BSD: Operation would block): *Note Error Codes::. `int EXDEV' `errno.h' (POSIX.1: Invalid cross-device link): *Note Error Codes::. `int execl (const char *FILENAME, const char *ARG0, ...)' `unistd.h' (POSIX.1): *Note Executing a File::. `int execle (const char *FILENAME, const char *ARG0, char *const ENV[], ...)' `unistd.h' (POSIX.1): *Note Executing a File::. `int execlp (const char *FILENAME, const char *ARG0, ...)' `unistd.h' (POSIX.1): *Note Executing a File::. `int execv (const char *FILENAME, char *const ARGV[])' `unistd.h' (POSIX.1): *Note Executing a File::. `int execve (const char *FILENAME, char *const ARGV[], char *const ENV[])' `unistd.h' (POSIX.1): *Note Executing a File::. `int execvp (const char *FILENAME, char *const ARGV[])' `unistd.h' (POSIX.1): *Note Executing a File::. `int EXFULL' `errno.h' (Linux???: Exchange full): *Note Error Codes::. `void _Exit (int STATUS)' `stdlib.h' (ISO): *Note Termination Internals::. `void _exit (int STATUS)' `unistd.h' (POSIX.1): *Note Termination Internals::. `void exit (int STATUS)' `stdlib.h' (ISO): *Note Normal Termination::. `int EXIT_FAILURE' `stdlib.h' (ISO): *Note Exit Status::. `int EXIT_SUCCESS' `stdlib.h' (ISO): *Note Exit Status::. `double exp (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double exp10 (double X)' `math.h' (GNU): *Note Exponents and Logarithms::. `float exp10f (float X)' `math.h' (GNU): *Note Exponents and Logarithms::. `long double exp10l (long double X)' `math.h' (GNU): *Note Exponents and Logarithms::. `double exp2 (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float exp2f (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double exp2l (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float expf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double expl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double expm1 (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float expm1f (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double expm1l (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `int EXPR_NEST_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `double fabs (double NUMBER)' `math.h' (ISO): *Note Absolute Value::. `float fabsf (float NUMBER)' `math.h' (ISO): *Note Absolute Value::. `long double fabsl (long double NUMBER)' `math.h' (ISO): *Note Absolute Value::. `size_t __fbufsize (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Controlling Buffering::. `int fchdir (int FILEDES)' `unistd.h' (XPG): *Note Working Directory::. `int fchmod (int FILEDES, int MODE)' `sys/stat.h' (BSD): *Note Setting Permissions::. `int fchown (int FILEDES, int OWNER, int GROUP)' `unistd.h' (BSD): *Note File Owner::. `int fclean (FILE *STREAM)' `stdio.h' (GNU): *Note Cleaning Streams::. `int fclose (FILE *STREAM)' `stdio.h' (ISO): *Note Closing Streams::. `int fcloseall (void)' `stdio.h' (GNU): *Note Closing Streams::. `int fcntl (int FILEDES, int COMMAND, ...)' `fcntl.h' (POSIX.1): *Note Control Operations::. `char * fcvt (double VALUE, int NDIGIT, int *DECPT, int *NEG)' `stdlib.h' (SVID, Unix98): *Note System V Number Conversion::. `char * fcvt_r (double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)' `stdlib.h' (SVID, Unix98): *Note System V Number Conversion::. `int fdatasync (int FILDES)' `unistd.h' (POSIX): *Note Synchronizing I/O::. `int FD_CLOEXEC' `fcntl.h' (POSIX.1): *Note Descriptor Flags::. `void FD_CLR (int FILEDES, fd_set *SET)' `sys/types.h' (BSD): *Note Waiting for I/O::. `double fdim (double X, double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `float fdimf (float X, float Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `long double fdiml (long double X, long double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `int FD_ISSET (int FILEDES, fd_set *SET)' `sys/types.h' (BSD): *Note Waiting for I/O::. `FILE * fdopen (int FILEDES, const char *OPENTYPE)' `stdio.h' (POSIX.1): *Note Descriptors and Streams::. `void FD_SET (int FILEDES, fd_set *SET)' `sys/types.h' (BSD): *Note Waiting for I/O::. `fd_set' `sys/types.h' (BSD): *Note Waiting for I/O::. `int FD_SETSIZE' `sys/types.h' (BSD): *Note Waiting for I/O::. `int F_DUPFD' `fcntl.h' (POSIX.1): *Note Duplicating Descriptors::. `void FD_ZERO (fd_set *SET)' `sys/types.h' (BSD): *Note Waiting for I/O::. `int feclearexcept (int EXCEPTS)' `fenv.h' (ISO): *Note Status bit operations::. `int fedisableexcept (int EXCEPTS)' `fenv.h' (GNU): *Note Control Functions::. `FE_DIVBYZERO' `fenv.h' (ISO): *Note Status bit operations::. `FE_DOWNWARD' `fenv.h' (ISO): *Note Rounding::. `int feenableexcept (int EXCEPTS)' `fenv.h' (GNU): *Note Control Functions::. `int fegetenv (fenv_t *ENVP)' `fenv.h' (ISO): *Note Control Functions::. `int fegetexcept (int EXCEPTS)' `fenv.h' (GNU): *Note Control Functions::. `int fegetexceptflag (fexcept_t *FLAGP, int EXCEPTS)' `fenv.h' (ISO): *Note Status bit operations::. `int fegetround (void)' `fenv.h' (ISO): *Note Rounding::. `int feholdexcept (fenv_t *ENVP)' `fenv.h' (ISO): *Note Control Functions::. `FE_INEXACT' `fenv.h' (ISO): *Note Status bit operations::. `FE_INVALID' `fenv.h' (ISO): *Note Status bit operations::. `int feof (FILE *STREAM)' `stdio.h' (ISO): *Note EOF and Errors::. `int feof_unlocked (FILE *STREAM)' `stdio.h' (GNU): *Note EOF and Errors::. `FE_OVERFLOW' `fenv.h' (ISO): *Note Status bit operations::. `int feraiseexcept (int EXCEPTS)' `fenv.h' (ISO): *Note Status bit operations::. `int ferror (FILE *STREAM)' `stdio.h' (ISO): *Note EOF and Errors::. `int ferror_unlocked (FILE *STREAM)' `stdio.h' (GNU): *Note EOF and Errors::. `int fesetenv (const fenv_t *ENVP)' `fenv.h' (ISO): *Note Control Functions::. `int fesetexceptflag (const fexcept_t *FLAGP, int' `fenv.h' (ISO): *Note Status bit operations::. `int fesetround (int ROUND)' `fenv.h' (ISO): *Note Rounding::. `int fetestexcept (int EXCEPTS)' `fenv.h' (ISO): *Note Status bit operations::. `FE_TONEAREST' `fenv.h' (ISO): *Note Rounding::. `FE_TOWARDZERO' `fenv.h' (ISO): *Note Rounding::. `FE_UNDERFLOW' `fenv.h' (ISO): *Note Status bit operations::. `int feupdateenv (const fenv_t *ENVP)' `fenv.h' (ISO): *Note Control Functions::. `FE_UPWARD' `fenv.h' (ISO): *Note Rounding::. `int fflush (FILE *STREAM)' `stdio.h' (ISO): *Note Flushing Buffers::. `int fflush_unlocked (FILE *STREAM)' `stdio.h' (POSIX): *Note Flushing Buffers::. `int fgetc (FILE *STREAM)' `stdio.h' (ISO): *Note Character Input::. `int fgetc_unlocked (FILE *STREAM)' `stdio.h' (POSIX): *Note Character Input::. `int F_GETFD' `fcntl.h' (POSIX.1): *Note Descriptor Flags::. `int F_GETFL' `fcntl.h' (POSIX.1): *Note Getting File Status Flags::. `struct group * fgetgrent (FILE *STREAM)' `grp.h' (SVID): *Note Scanning All Groups::. `int fgetgrent_r (FILE *STREAM, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)' `grp.h' (GNU): *Note Scanning All Groups::. `int F_GETLK' `fcntl.h' (POSIX.1): *Note File Locks::. `int F_GETOWN' `fcntl.h' (BSD): *Note Interrupt Input::. `int fgetpos (FILE *STREAM, fpos_t *POSITION)' `stdio.h' (ISO): *Note Portable Positioning::. `int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)' `stdio.h' (Unix98): *Note Portable Positioning::. `struct passwd * fgetpwent (FILE *STREAM)' `pwd.h' (SVID): *Note Scanning All Users::. `int fgetpwent_r (FILE *STREAM, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)' `pwd.h' (GNU): *Note Scanning All Users::. `char * fgets (char *S, int COUNT, FILE *STREAM)' `stdio.h' (ISO): *Note Line Input::. `char * fgets_unlocked (char *S, int COUNT, FILE *STREAM)' `stdio.h' (GNU): *Note Line Input::. `wint_t fgetwc (FILE *STREAM)' `wchar.h' (ISO): *Note Character Input::. `wint_t fgetwc_unlocked (FILE *STREAM)' `wchar.h' (GNU): *Note Character Input::. `wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM)' `wchar.h' (ISO): *Note Line Input::. `wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM)' `wchar.h' (GNU): *Note Line Input::. `FILE' `stdio.h' (ISO): *Note Streams::. `int FILENAME_MAX' `stdio.h' (ISO): *Note Limits for Files::. `int fileno (FILE *STREAM)' `stdio.h' (POSIX.1): *Note Descriptors and Streams::. `int fileno_unlocked (FILE *STREAM)' `stdio.h' (GNU): *Note Descriptors and Streams::. `int finite (double X)' `math.h' (BSD): *Note Floating Point Classes::. `int finitef (float X)' `math.h' (BSD): *Note Floating Point Classes::. `int finitel (long double X)' `math.h' (BSD): *Note Floating Point Classes::. `int __flbf (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Controlling Buffering::. `void flockfile (FILE *STREAM)' `stdio.h' (POSIX): *Note Streams and Threads::. `double floor (double X)' `math.h' (ISO): *Note Rounding Functions::. `float floorf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double floorl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `FLT_DIG' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_EPSILON' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MANT_DIG' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MAX' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MAX_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MAX_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MIN' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MIN_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MIN_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_RADIX' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_ROUNDS' `float.h' (ISO): *Note Floating Point Parameters::. `void _flushlbf (void)' `stdio_ext.h' (GNU): *Note Flushing Buffers::. `tcflag_t FLUSHO' `termios.h' (BSD): *Note Local Modes::. `double fma (double X, double Y, double Z)' `math.h' (ISO): *Note Misc FP Arithmetic::. `float fmaf (float X, float Y, float Z)' `math.h' (ISO): *Note Misc FP Arithmetic::. `long double fmal (long double X, long double Y, long double Z)' `math.h' (ISO): *Note Misc FP Arithmetic::. `double fmax (double X, double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `float fmaxf (float X, float Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `long double fmaxl (long double X, long double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE)' `stdio.h' (GNU): *Note String Streams::. `double fmin (double X, double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `float fminf (float X, float Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `long double fminl (long double X, long double Y)' `math.h' (ISO): *Note Misc FP Arithmetic::. `double fmod (double NUMERATOR, double DENOMINATOR)' `math.h' (ISO): *Note Remainder Functions::. `float fmodf (float NUMERATOR, float DENOMINATOR)' `math.h' (ISO): *Note Remainder Functions::. `long double fmodl (long double NUMERATOR, long double DENOMINATOR)' `math.h' (ISO): *Note Remainder Functions::. `int fmtmsg (long int CLASSIFICATION, const char *LABEL, int SEVERITY, const char *TEXT, const char *ACTION, const char *TAG)' `fmtmsg.h' (XPG): *Note Printing Formatted Messages::. `int fnmatch (const char *PATTERN, const char *STRING, int FLAGS)' `fnmatch.h' (POSIX.2): *Note Wildcard Matching::. `FNM_CASEFOLD' `fnmatch.h' (GNU): *Note Wildcard Matching::. `FNM_EXTMATCH' `fnmatch.h' (GNU): *Note Wildcard Matching::. `FNM_FILE_NAME' `fnmatch.h' (GNU): *Note Wildcard Matching::. `FNM_LEADING_DIR' `fnmatch.h' (GNU): *Note Wildcard Matching::. `FNM_NOESCAPE' `fnmatch.h' (POSIX.2): *Note Wildcard Matching::. `FNM_PATHNAME' `fnmatch.h' (POSIX.2): *Note Wildcard Matching::. `FNM_PERIOD' `fnmatch.h' (POSIX.2): *Note Wildcard Matching::. `int F_OK' `unistd.h' (POSIX.1): *Note Testing File Access::. `FILE * fopen (const char *FILENAME, const char *OPENTYPE)' `stdio.h' (ISO): *Note Opening Streams::. `FILE * fopen64 (const char *FILENAME, const char *OPENTYPE)' `stdio.h' (Unix98): *Note Opening Streams::. `FILE * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS)' `stdio.h' (GNU): *Note Streams and Cookies::. `int FOPEN_MAX' `stdio.h' (ISO): *Note Opening Streams::. `pid_t fork (void)' `unistd.h' (POSIX.1): *Note Creating a Process::. `int forkpty (int *AMASTER, char *NAME, struct termios *TERMP, struct winsize *WINP)' `pty.h' (BSD): *Note Pseudo-Terminal Pairs::. `long int fpathconf (int FILEDES, int PARAMETER)' `unistd.h' (POSIX.1): *Note Pathconf::. `int fpclassify (_float-type_ X)' `math.h' (ISO): *Note Floating Point Classes::. `FPE_DECOVF_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTDIV_FAULT' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTDIV_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTOVF_FAULT' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTOVF_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTUND_FAULT' `signal.h' (BSD): *Note Program Error Signals::. `FPE_FLTUND_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `FPE_INTDIV_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `FPE_INTOVF_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `size_t __fpending (FILE *STREAM) The `__fpending'' `stdio_ext.h' (GNU): *Note Controlling Buffering::. `FPE_SUBRNG_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `int FP_ILOGB0' `math.h' (ISO): *Note Exponents and Logarithms::. `int FP_ILOGBNAN' `math.h' (ISO): *Note Exponents and Logarithms::. `fpos64_t' `stdio.h' (Unix98): *Note Portable Positioning::. `fpos_t' `stdio.h' (ISO): *Note Portable Positioning::. `int fprintf (FILE *STREAM, const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Output Functions::. `void __fpurge (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Flushing Buffers::. `int fputc (int C, FILE *STREAM)' `stdio.h' (ISO): *Note Simple Output::. `int fputc_unlocked (int C, FILE *STREAM)' `stdio.h' (POSIX): *Note Simple Output::. `int fputs (const char *S, FILE *STREAM)' `stdio.h' (ISO): *Note Simple Output::. `int fputs_unlocked (const char *S, FILE *STREAM)' `stdio.h' (GNU): *Note Simple Output::. `wint_t fputwc (wchar_t WC, FILE *STREAM)' `wchar.h' (ISO): *Note Simple Output::. `wint_t fputwc_unlocked (wint_t WC, FILE *STREAM)' `wchar.h' (POSIX): *Note Simple Output::. `int fputws (const wchar_t *WS, FILE *STREAM)' `wchar.h' (ISO): *Note Simple Output::. `int fputws_unlocked (const wchar_t *WS, FILE *STREAM)' `wchar.h' (GNU): *Note Simple Output::. `F_RDLCK' `fcntl.h' (POSIX.1): *Note File Locks::. `size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)' `stdio.h' (ISO): *Note Block Input/Output::. `int __freadable (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Opening Streams::. `int __freading (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Opening Streams::. `size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)' `stdio.h' (GNU): *Note Block Input/Output::. `void free (void *PTR)' `malloc.h', `stdlib.h' (ISO): *Note Freeing after Malloc::. `__free_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)' `stdio.h' (ISO): *Note Opening Streams::. `FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)' `stdio.h' (Unix98): *Note Opening Streams::. `double frexp (double VALUE, int *EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `float frexpf (float VALUE, int *EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `long double frexpl (long double VALUE, int *EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `int fscanf (FILE *STREAM, const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Input Functions::. `int fseek (FILE *STREAM, long int OFFSET, int WHENCE)' `stdio.h' (ISO): *Note File Positioning::. `int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE)' `stdio.h' (Unix98): *Note File Positioning::. `int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE)' `stdio.h' (Unix98): *Note File Positioning::. `int F_SETFD' `fcntl.h' (POSIX.1): *Note Descriptor Flags::. `int F_SETFL' `fcntl.h' (POSIX.1): *Note Getting File Status Flags::. `int F_SETLK' `fcntl.h' (POSIX.1): *Note File Locks::. `int F_SETLKW' `fcntl.h' (POSIX.1): *Note File Locks::. `int __fsetlocking (FILE *STREAM, int TYPE)' `stdio_ext.h' (GNU): *Note Streams and Threads::. `int F_SETOWN' `fcntl.h' (BSD): *Note Interrupt Input::. `int fsetpos (FILE *STREAM, const fpos_t *POSITION)' `stdio.h' (ISO): *Note Portable Positioning::. `int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)' `stdio.h' (Unix98): *Note Portable Positioning::. `int fstat (int FILEDES, struct stat *BUF)' `sys/stat.h' (POSIX.1): *Note Reading Attributes::. `int fstat64 (int FILEDES, struct stat64 *BUF)' `sys/stat.h' (Unix98): *Note Reading Attributes::. `int fsync (int FILDES)' `unistd.h' (POSIX): *Note Synchronizing I/O::. `long int ftell (FILE *STREAM)' `stdio.h' (ISO): *Note File Positioning::. `off_t ftello (FILE *STREAM)' `stdio.h' (Unix98): *Note File Positioning::. `off64_t ftello64 (FILE *STREAM)' `stdio.h' (Unix98): *Note File Positioning::. `int ftruncate (int FD, off_t LENGTH)' `unistd.h' (POSIX): *Note File Size::. `int ftruncate64 (int ID, off64_t LENGTH)' `unistd.h' (Unix98): *Note File Size::. `int ftrylockfile (FILE *STREAM)' `stdio.h' (POSIX): *Note Streams and Threads::. `int ftw (const char *FILENAME, __ftw_func_t FUNC, int DESCRIPTORS)' `ftw.h' (SVID): *Note Working with Directory Trees::. `int ftw64 (const char *FILENAME, __ftw64_func_t FUNC, int DESCRIPTORS)' `ftw.h' (Unix98): *Note Working with Directory Trees::. `__ftw64_func_t' `ftw.h' (GNU): *Note Working with Directory Trees::. `__ftw_func_t' `ftw.h' (GNU): *Note Working with Directory Trees::. `F_UNLCK' `fcntl.h' (POSIX.1): *Note File Locks::. `void funlockfile (FILE *STREAM)' `stdio.h' (POSIX): *Note Streams and Threads::. `int fwide (FILE *STREAM, int MODE)' `wchar.h' (ISO): *Note Streams and I18N::. `int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...)' `wchar.h' (ISO): *Note Formatted Output Functions::. `int __fwritable (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Opening Streams::. `size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)' `stdio.h' (ISO): *Note Block Input/Output::. `size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)' `stdio.h' (GNU): *Note Block Input/Output::. `int __fwriting (FILE *STREAM)' `stdio_ext.h' (GNU): *Note Opening Streams::. `F_WRLCK' `fcntl.h' (POSIX.1): *Note File Locks::. `int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...)' `wchar.h' (ISO): *Note Formatted Input Functions::. `double gamma (double X)' `math.h' (SVID): *Note Special Functions::. `float gammaf (float X)' `math.h' (SVID): *Note Special Functions::. `long double gammal (long double X)' `math.h' (SVID): *Note Special Functions::. `void (*__gconv_end_fct) (struct gconv_step *)' `gconv.h' (GNU): *Note glibc iconv Implementation::. `int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)' `gconv.h' (GNU): *Note glibc iconv Implementation::. `int (*__gconv_init_fct) (struct __gconv_step *)' `gconv.h' (GNU): *Note glibc iconv Implementation::. `char * gcvt (double VALUE, int NDIGIT, char *BUF)' `stdlib.h' (SVID, Unix98): *Note System V Number Conversion::. `long int get_avphys_pages (void)' `sys/sysinfo.h' (GNU): *Note Query Memory Parameters::. `int getc (FILE *STREAM)' `stdio.h' (ISO): *Note Character Input::. `int getchar (void)' `stdio.h' (ISO): *Note Character Input::. `int getchar_unlocked (void)' `stdio.h' (POSIX): *Note Character Input::. `int getcontext (ucontext_t *UCP)' `ucontext.h' (SVID): *Note System V contexts::. `int getc_unlocked (FILE *STREAM)' `stdio.h' (POSIX): *Note Character Input::. `char * get_current_dir_name (void)' `unistd.h' (GNU): *Note Working Directory::. `char * getcwd (char *BUFFER, size_t SIZE)' `unistd.h' (POSIX.1): *Note Working Directory::. `struct tm * getdate (const char *STRING)' `time.h' (Unix98): *Note General Time String Parsing::. `getdate_err' `time.h' (Unix98): *Note General Time String Parsing::. `int getdate_r (const char *STRING, struct tm *TP)' `time.h' (GNU): *Note General Time String Parsing::. `ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM)' `stdio.h' (GNU): *Note Line Input::. `int getdomainnname (char *NAME, size_t LENGTH)' `unistd.h' (???): *Note Host Identification::. `gid_t getegid (void)' `unistd.h' (POSIX.1): *Note Reading Persona::. `char * getenv (const char *NAME)' `stdlib.h' (ISO): *Note Environment Access::. `uid_t geteuid (void)' `unistd.h' (POSIX.1): *Note Reading Persona::. `struct fstab * getfsent (void)' `fstab.h' (BSD): *Note fstab::. `struct fstab * getfsfile (const char *NAME)' `fstab.h' (BSD): *Note fstab::. `struct fstab * getfsspec (const char *NAME)' `fstab.h' (BSD): *Note fstab::. `gid_t getgid (void)' `unistd.h' (POSIX.1): *Note Reading Persona::. `struct group * getgrent (void)' `grp.h' (SVID, BSD): *Note Scanning All Groups::. `int getgrent_r (struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)' `grp.h' (GNU): *Note Scanning All Groups::. `struct group * getgrgid (gid_t GID)' `grp.h' (POSIX.1): *Note Lookup Group::. `int getgrgid_r (gid_t GID, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)' `grp.h' (POSIX.1c): *Note Lookup Group::. `struct group * getgrnam (const char *NAME)' `grp.h' (SVID, BSD): *Note Lookup Group::. `int getgrnam_r (const char *NAME, struct group *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct group **RESULT)' `grp.h' (POSIX.1c): *Note Lookup Group::. `int getgrouplist (const char *USER, gid_t GROUP, gid_t *GROUPS, int *NGROUPS)' `grp.h' (BSD): *Note Setting Groups::. `int getgroups (int COUNT, gid_t *GROUPS)' `unistd.h' (POSIX.1): *Note Reading Persona::. `struct hostent * gethostbyaddr (const char *ADDR, size_t LENGTH, int FORMAT)' `netdb.h' (BSD): *Note Host Names::. `int gethostbyaddr_r (const char *ADDR, size_t LENGTH, int FORMAT, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP)' `netdb.h' (GNU): *Note Host Names::. `struct hostent * gethostbyname (const char *NAME)' `netdb.h' (BSD): *Note Host Names::. `struct hostent * gethostbyname2 (const char *NAME, int AF)' `netdb.h' (IPv6 Basic API): *Note Host Names::. `int gethostbyname2_r (const char *NAME, int AF, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP)' `netdb.h' (GNU): *Note Host Names::. `int gethostbyname_r (const char *restrict NAME, struct hostent *restrict RESULT_BUF, char *restrict BUF, size_t BUFLEN, struct hostent **restrict RESULT, int *restrict H_ERRNOP)' `netdb.h' (GNU): *Note Host Names::. `struct hostent * gethostent (void)' `netdb.h' (BSD): *Note Host Names::. `long int gethostid (void)' `unistd.h' (BSD): *Note Host Identification::. `int gethostname (char *NAME, size_t SIZE)' `unistd.h' (BSD): *Note Host Identification::. `int getitimer (int WHICH, struct itimerval *OLD)' `sys/time.h' (BSD): *Note Setting an Alarm::. `ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM)' `stdio.h' (GNU): *Note Line Input::. `int getloadavg (double LOADAVG[], int NELEM)' `stdlib.h' (BSD): *Note Processor Resources::. `char * getlogin (void)' `unistd.h' (POSIX.1): *Note Who Logged In::. `struct mntent * getmntent (FILE *STREAM)' `mntent.h' (BSD): *Note mtab::. `struct mntent * getmntent_r (FILE *STREAM, struct mentent *RESULT, char *BUFFER, int BUFSIZE)' `mntent.h' (BSD): *Note mtab::. `struct netent * getnetbyaddr (unsigned long int NET, int TYPE)' `netdb.h' (BSD): *Note Networks Database::. `struct netent * getnetbyname (const char *NAME)' `netdb.h' (BSD): *Note Networks Database::. `struct netent * getnetent (void)' `netdb.h' (BSD): *Note Networks Database::. `int getnetgrent (char **HOSTP, char **USERP, char **DOMAINP)' `netdb.h' (BSD): *Note Lookup Netgroup::. `int getnetgrent_r (char **HOSTP, char **USERP, char **DOMAINP, char *BUFFER, int BUFLEN)' `netdb.h' (GNU): *Note Lookup Netgroup::. `int get_nprocs (void)' `sys/sysinfo.h' (GNU): *Note Processor Resources::. `int get_nprocs_conf (void)' `sys/sysinfo.h' (GNU): *Note Processor Resources::. `int getopt (int ARGC, char **ARGV, const char *OPTIONS)' `unistd.h' (POSIX.2): *Note Using Getopt::. `int getopt_long (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)' `getopt.h' (GNU): *Note Getopt Long Options::. `int getopt_long_only (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR)' `getopt.h' (GNU): *Note Getopt Long Options::. `int getpagesize (void)' `unistd.h' (BSD): *Note Query Memory Parameters::. `char * getpass (const char *PROMPT)' `unistd.h' (BSD): *Note getpass::. `int getpeername (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH-PTR)' `sys/socket.h' (BSD): *Note Who is Connected::. `int getpgid (pid_t PID)' `unistd.h' (SVID): *Note Process Group Functions::. `pid_t getpgrp (pid_t PID)' `unistd.h' (BSD): *Note Process Group Functions::. `pid_t getpgrp (void)' `unistd.h' (POSIX.1): *Note Process Group Functions::. `long int get_phys_pages (void)' `sys/sysinfo.h' (GNU): *Note Query Memory Parameters::. `pid_t getpid (void)' `unistd.h' (POSIX.1): *Note Process Identification::. `pid_t getppid (void)' `unistd.h' (POSIX.1): *Note Process Identification::. `int getpriority (int CLASS, int ID)' `sys/resource.h' (BSD,POSIX): *Note Traditional Scheduling Functions::. `struct protoent * getprotobyname (const char *NAME)' `netdb.h' (BSD): *Note Protocols Database::. `struct protoent * getprotobynumber (int PROTOCOL)' `netdb.h' (BSD): *Note Protocols Database::. `struct protoent * getprotoent (void)' `netdb.h' (BSD): *Note Protocols Database::. `int getpt (void)' `stdlib.h' (GNU): *Note Allocation::. `struct passwd * getpwent (void)' `pwd.h' (POSIX.1): *Note Scanning All Users::. `int getpwent_r (struct passwd *RESULT_BUF, char *BUFFER, int BUFLEN, struct passwd **RESULT)' `pwd.h' (GNU): *Note Scanning All Users::. `struct passwd * getpwnam (const char *NAME)' `pwd.h' (POSIX.1): *Note Lookup User::. `int getpwnam_r (const char *NAME, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)' `pwd.h' (POSIX.1c): *Note Lookup User::. `struct passwd * getpwuid (uid_t UID)' `pwd.h' (POSIX.1): *Note Lookup User::. `int getpwuid_r (uid_t UID, struct passwd *RESULT_BUF, char *BUFFER, size_t BUFLEN, struct passwd **RESULT)' `pwd.h' (POSIX.1c): *Note Lookup User::. `int getrlimit (int RESOURCE, struct rlimit *RLP)' `sys/resource.h' (BSD): *Note Limits on Resources::. `int getrlimit64 (int RESOURCE, struct rlimit64 *RLP)' `sys/resource.h' (Unix98): *Note Limits on Resources::. `int getrusage (int PROCESSES, struct rusage *RUSAGE)' `sys/resource.h' (BSD): *Note Resource Usage::. `char * gets (char *S)' `stdio.h' (ISO): *Note Line Input::. `struct servent * getservbyname (const char *NAME, const char *PROTO)' `netdb.h' (BSD): *Note Services Database::. `struct servent * getservbyport (int PORT, const char *PROTO)' `netdb.h' (BSD): *Note Services Database::. `struct servent * getservent (void)' `netdb.h' (BSD): *Note Services Database::. `pid_t getsid (pid_t PID)' `unistd.h' (SVID): *Note Process Group Functions::. `int getsockname (int SOCKET, struct sockaddr *ADDR, socklen_t *LENGTH-PTR)' `sys/socket.h' (BSD): *Note Reading Address::. `int getsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, socklen_t *OPTLEN-PTR)' `sys/socket.h' (BSD): *Note Socket Option Functions::. `int getsubopt (char **OPTIONP, const char* const *TOKENS, char **VALUEP)' `stdlib.h' (stdlib.h): *Note Suboptions Example: Suboptions. `char * gettext (const char *MSGID)' `libintl.h' (GNU): *Note Translation with gettext::. `int gettimeofday (struct timeval *TP, struct timezone *TZP)' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `uid_t getuid (void)' `unistd.h' (POSIX.1): *Note Reading Persona::. `mode_t getumask (void)' `sys/stat.h' (GNU): *Note Setting Permissions::. `struct utmp * getutent (void)' `utmp.h' (SVID): *Note Manipulating the Database::. `int getutent_r (struct utmp *BUFFER, struct utmp **RESULT)' `utmp.h' (GNU): *Note Manipulating the Database::. `struct utmp * getutid (const struct utmp *ID)' `utmp.h' (SVID): *Note Manipulating the Database::. `int getutid_r (const struct utmp *ID, struct utmp *BUFFER, struct utmp **RESULT)' `utmp.h' (GNU): *Note Manipulating the Database::. `struct utmp * getutline (const struct utmp *LINE)' `utmp.h' (SVID): *Note Manipulating the Database::. `int getutline_r (const struct utmp *LINE, struct utmp *BUFFER, struct utmp **RESULT)' `utmp.h' (GNU): *Note Manipulating the Database::. `int getutmp (const struct utmpx *utmpx, struct utmp *utmp)' `utmp.h' (GNU): *Note XPG Functions::. `int getutmpx (const struct utmp *utmp, struct utmpx *utmpx)' `utmp.h' (GNU): *Note XPG Functions::. `struct utmpx * getutxent (void)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `struct utmpx * getutxid (const struct utmpx *ID)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `struct utmpx * getutxline (const struct utmpx *LINE)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int getw (FILE *STREAM)' `stdio.h' (SVID): *Note Character Input::. `wint_t getwc (FILE *STREAM)' `wchar.h' (ISO): *Note Character Input::. `wint_t getwchar (void)' `wchar.h' (ISO): *Note Character Input::. `wint_t getwchar_unlocked (void)' `wchar.h' (GNU): *Note Character Input::. `wint_t getwc_unlocked (FILE *STREAM)' `wchar.h' (GNU): *Note Character Input::. `char * getwd (char *BUFFER)' `unistd.h' (BSD): *Note Working Directory::. `gid_t' `sys/types.h' (POSIX.1): *Note Reading Persona::. `int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR)' `glob.h' (POSIX.2): *Note Calling Glob::. `int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR)' `glob.h' (GNU): *Note Calling Glob::. `glob64_t' `glob.h' (GNU): *Note Calling Glob::. `GLOB_ABORTED' `glob.h' (POSIX.2): *Note Calling Glob::. `GLOB_ALTDIRFUNC' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_APPEND' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_BRACE' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_DOOFFS' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_ERR' `glob.h' (POSIX.2): *Note Flags for Globbing::. `void globfree (glob_t *PGLOB)' `glob.h' (POSIX.2): *Note More Flags for Globbing::. `void globfree64 (glob64_t *PGLOB)' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_MAGCHAR' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_MARK' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_NOCHECK' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_NOESCAPE' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_NOMAGIC' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_NOMATCH' `glob.h' (POSIX.2): *Note Calling Glob::. `GLOB_NOSORT' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_NOSPACE' `glob.h' (POSIX.2): *Note Calling Glob::. `GLOB_ONLYDIR' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_PERIOD' `glob.h' (GNU): *Note More Flags for Globbing::. `glob_t' `glob.h' (POSIX.2): *Note Calling Glob::. `GLOB_TILDE' `glob.h' (GNU): *Note More Flags for Globbing::. `GLOB_TILDE_CHECK' `glob.h' (GNU): *Note More Flags for Globbing::. `struct tm * gmtime (const time_t *TIME)' `time.h' (ISO): *Note Broken-down Time::. `struct tm * gmtime_r (const time_t *TIME, struct tm *RESULTP)' `time.h' (POSIX.1c): *Note Broken-down Time::. `_GNU_SOURCE' (GNU): *Note Feature Test Macros::. `int grantpt (int FILEDES)' `stdlib.h' (SVID, XPG4.2): *Note Allocation::. `int gsignal (int SIGNUM)' `signal.h' (SVID): *Note Signaling Yourself::. `int gtty (int FILEDES, struct sgttyb *ATTRIBUTES)' `sgtty.h' (BSD): *Note BSD Terminal Modes::. `char * hasmntopt (const struct mntent *MNT, const char *OPT)' `mntent.h' (BSD): *Note mtab::. `int hcreate (size_t NEL)' `search.h' (SVID): *Note Hash Search Function::. `int hcreate_r (size_t NEL, struct hsearch_data *HTAB)' `search.h' (GNU): *Note Hash Search Function::. `void hdestroy (void)' `search.h' (SVID): *Note Hash Search Function::. `void hdestroy_r (struct hsearch_data *HTAB)' `search.h' (GNU): *Note Hash Search Function::. `HOST_NOT_FOUND' `netdb.h' (BSD): *Note Host Names::. `ENTRY * hsearch (ENTRY ITEM, ACTION ACTION)' `search.h' (SVID): *Note Hash Search Function::. `int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB)' `search.h' (GNU): *Note Hash Search Function::. `uint32_t htonl (uint32_t HOSTLONG)' `netinet/in.h' (BSD): *Note Byte Order::. `uint16_t htons (uint16_t HOSTSHORT)' `netinet/in.h' (BSD): *Note Byte Order::. `double HUGE_VAL' `math.h' (ISO): *Note Math Error Reporting::. `float HUGE_VALF' `math.h' (ISO): *Note Math Error Reporting::. `long double HUGE_VALL' `math.h' (ISO): *Note Math Error Reporting::. `tcflag_t HUPCL' `termios.h' (POSIX.1): *Note Control Modes::. `double hypot (double X, double Y)' `math.h' (ISO): *Note Exponents and Logarithms::. `float hypotf (float X, float Y)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double hypotl (long double X, long double Y)' `math.h' (ISO): *Note Exponents and Logarithms::. `tcflag_t ICANON' `termios.h' (POSIX.1): *Note Local Modes::. `size_t iconv (iconv_t CD, char **INBUF, size_t *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT)' `iconv.h' (XPG2): *Note Generic Conversion Interface::. `int iconv_close (iconv_t CD)' `iconv.h' (XPG2): *Note Generic Conversion Interface::. `iconv_t iconv_open (const char *TOCODE, const char *FROMCODE)' `iconv.h' (XPG2): *Note Generic Conversion Interface::. `iconv_t' `iconv.h' (XPG2): *Note Generic Conversion Interface::. `tcflag_t ICRNL' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t IEXTEN' `termios.h' (POSIX.1): *Note Local Modes::. `void if_freenameindex (struct if_nameindex *ptr)' `net/if.h' (IPv6 basic API): *Note Interface Naming::. `char * if_indextoname (unsigned int ifindex, char *ifname)' `net/if.h' (IPv6 basic API): *Note Interface Naming::. `struct if_nameindex * if_nameindex (void)' `net/if.h' (IPv6 basic API): *Note Interface Naming::. `unsigned int if_nametoindex (const char *ifname)' `net/if.h' (IPv6 basic API): *Note Interface Naming::. `size_t IFNAMSIZ' `net/if.h' (net/if.h): *Note Interface Naming::. `int IFTODT (mode_t MODE)' `dirent.h' (BSD): *Note Directory Entries::. `tcflag_t IGNBRK' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t IGNCR' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t IGNPAR' `termios.h' (POSIX.1): *Note Input Modes::. `int ilogb (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `int ilogbf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `int ilogbl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `intmax_t imaxabs (intmax_t NUMBER)' `inttypes.h' (ISO): *Note Absolute Value::. `tcflag_t IMAXBEL' `termios.h' (BSD): *Note Input Modes::. `imaxdiv_t imaxdiv (intmax_t NUMERATOR, intmax_t DENOMINATOR)' `inttypes.h' (ISO): *Note Integer Division::. `imaxdiv_t' `inttypes.h' (ISO): *Note Integer Division::. `struct in6_addr in6addr_any' `netinet/in.h' (IPv6 basic API): *Note Host Address Data Type::. `struct in6_addr in6addr_loopback' `netinet/in.h' (IPv6 basic API): *Note Host Address Data Type::. `uint32_t INADDR_ANY' `netinet/in.h' (BSD): *Note Host Address Data Type::. `uint32_t INADDR_BROADCAST' `netinet/in.h' (BSD): *Note Host Address Data Type::. `uint32_t INADDR_LOOPBACK' `netinet/in.h' (BSD): *Note Host Address Data Type::. `uint32_t INADDR_NONE' `netinet/in.h' (BSD): *Note Host Address Data Type::. `char * index (const char *STRING, int C)' `string.h' (BSD): *Note Search Functions::. `uint32_t inet_addr (const char *NAME)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `int inet_aton (const char *NAME, struct in_addr *ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `uint32_t inet_lnaof (struct in_addr ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `struct in_addr inet_makeaddr (uint32_t NET, uint32_t LOCAL)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `uint32_t inet_netof (struct in_addr ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `uint32_t inet_network (const char *NAME)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `char * inet_ntoa (struct in_addr ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `const char * inet_ntop (int AF, const void *CP, char *BUF, size_t LEN)' `arpa/inet.h' (IPv6 basic API): *Note Host Address Functions::. `int inet_pton (int AF, const char *CP, void *BUF)' `arpa/inet.h' (IPv6 basic API): *Note Host Address Functions::. `float INFINITY' `math.h' (ISO): *Note Infinity and NaN::. `int initgroups (const char *USER, gid_t GROUP)' `grp.h' (BSD): *Note Setting Groups::. `INIT_PROCESS' `utmp.h' (SVID): *Note Manipulating the Database::. `INIT_PROCESS' `utmpx.h' (XPG4.2): *Note XPG Functions::. `void * initstate (unsigned int SEED, void *STATE, size_t SIZE)' `stdlib.h' (BSD): *Note BSD Random::. `int initstate_r (unsigned int SEED, char *restrict STATEBUF, size_t STATELEN, struct random_data *restrict BUF)' `stdlib.h' (GNU): *Note BSD Random::. `tcflag_t INLCR' `termios.h' (POSIX.1): *Note Input Modes::. `int innetgr (const char *NETGROUP, const char *HOST, const char *USER, const char *DOMAIN)' `netdb.h' (BSD): *Note Netgroup Membership::. `ino64_t' `sys/types.h' (Unix98): *Note Attribute Meanings::. `ino_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `tcflag_t INPCK' `termios.h' (POSIX.1): *Note Input Modes::. `INT_MAX' `limits.h' (ISO): *Note Range of Type::. `INT_MIN' `limits.h' (ISO): *Note Range of Type::. `int ioctl (int FILEDES, int COMMAND, ...)' `sys/ioctl.h' (BSD): *Note IOCTLs::. `int _IOFBF' `stdio.h' (ISO): *Note Controlling Buffering::. `int _IOLBF' `stdio.h' (ISO): *Note Controlling Buffering::. `int _IONBF' `stdio.h' (ISO): *Note Controlling Buffering::. `int IPPORT_RESERVED' `netinet/in.h' (BSD): *Note Ports::. `int IPPORT_USERRESERVED' `netinet/in.h' (BSD): *Note Ports::. `int isalnum (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isalpha (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isascii (int C)' `ctype.h' (SVID, BSD): *Note Classification of Characters::. `int isatty (int FILEDES)' `unistd.h' (POSIX.1): *Note Is It a Terminal::. `int isblank (int C)' `ctype.h' (GNU): *Note Classification of Characters::. `int iscntrl (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isdigit (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isfinite (_float-type_ X)' `math.h' (ISO): *Note Floating Point Classes::. `int isgraph (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isgreater (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `int isgreaterequal (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `tcflag_t ISIG' `termios.h' (POSIX.1): *Note Local Modes::. `int isinf (double X)' `math.h' (BSD): *Note Floating Point Classes::. `int isinff (float X)' `math.h' (BSD): *Note Floating Point Classes::. `int isinfl (long double X)' `math.h' (BSD): *Note Floating Point Classes::. `int isless (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `int islessequal (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `int islessgreater (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `int islower (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isnan (_float-type_ X)' `math.h' (ISO): *Note Floating Point Classes::. `int isnan (double X)' `math.h' (BSD): *Note Floating Point Classes::. `int isnanf (float X)' `math.h' (BSD): *Note Floating Point Classes::. `int isnanl (long double X)' `math.h' (BSD): *Note Floating Point Classes::. `int isnormal (_float-type_ X)' `math.h' (ISO): *Note Floating Point Classes::. `_ISOC99_SOURCE' (GNU): *Note Feature Test Macros::. `int isprint (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int ispunct (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isspace (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `tcflag_t ISTRIP' `termios.h' (POSIX.1): *Note Input Modes::. `int isunordered (_real-floating_ X, _real-floating_ Y)' `math.h' (ISO): *Note FP Comparison Functions::. `int isupper (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int iswalnum (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswalpha (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswblank (wint_t WC)' `wctype.h' (GNU): *Note Classification of Wide Characters::. `int iswcntrl (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswctype (wint_t WC, wctype_t DESC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswdigit (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswgraph (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswlower (wint_t WC)' `ctype.h' (ISO): *Note Classification of Wide Characters::. `int iswprint (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswpunct (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswspace (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswupper (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int iswxdigit (wint_t WC)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int isxdigit (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `ITIMER_PROF' `sys/time.h' (BSD): *Note Setting an Alarm::. `ITIMER_REAL' `sys/time.h' (BSD): *Note Setting an Alarm::. `ITIMER_VIRTUAL' `sys/time.h' (BSD): *Note Setting an Alarm::. `tcflag_t IXANY' `termios.h' (BSD): *Note Input Modes::. `tcflag_t IXOFF' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t IXON' `termios.h' (POSIX.1): *Note Input Modes::. `double j0 (double X)' `math.h' (SVID): *Note Special Functions::. `float j0f (float X)' `math.h' (SVID): *Note Special Functions::. `long double j0l (long double X)' `math.h' (SVID): *Note Special Functions::. `double j1 (double X)' `math.h' (SVID): *Note Special Functions::. `float j1f (float X)' `math.h' (SVID): *Note Special Functions::. `long double j1l (long double X)' `math.h' (SVID): *Note Special Functions::. `jmp_buf' `setjmp.h' (ISO): *Note Non-Local Details::. `double jn (int n, double X)' `math.h' (SVID): *Note Special Functions::. `float jnf (int n, float X)' `math.h' (SVID): *Note Special Functions::. `long double jnl (int n, long double X)' `math.h' (SVID): *Note Special Functions::. `long int jrand48 (unsigned short int XSUBI[3])' `stdlib.h' (SVID): *Note SVID Random::. `int jrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `int kill (pid_t PID, int SIGNUM)' `signal.h' (POSIX.1): *Note Signaling Another Process::. `int killpg (int PGID, int SIGNUM)' `signal.h' (BSD): *Note Signaling Another Process::. `char * l64a (long int N)' `stdlib.h' (XPG): *Note Encode Binary Data::. `long int labs (long int NUMBER)' `stdlib.h' (ISO): *Note Absolute Value::. `LANG' `locale.h' (ISO): *Note Locale Categories::. `LC_ALL' `locale.h' (ISO): *Note Locale Categories::. `LC_COLLATE' `locale.h' (ISO): *Note Locale Categories::. `LC_CTYPE' `locale.h' (ISO): *Note Locale Categories::. `LC_MESSAGES' `locale.h' (XOPEN): *Note Locale Categories::. `LC_MONETARY' `locale.h' (ISO): *Note Locale Categories::. `LC_NUMERIC' `locale.h' (ISO): *Note Locale Categories::. `void lcong48 (unsigned short int PARAM[7])' `stdlib.h' (SVID): *Note SVID Random::. `int lcong48_r (unsigned short int PARAM[7], struct drand48_data *BUFFER)' `stdlib.h' (GNU): *Note SVID Random::. `int L_ctermid' `stdio.h' (POSIX.1): *Note Identifying the Terminal::. `LC_TIME' `locale.h' (ISO): *Note Locale Categories::. `int L_cuserid' `stdio.h' (POSIX.1): *Note Who Logged In::. `double ldexp (double VALUE, int EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `float ldexpf (float VALUE, int EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `long double ldexpl (long double VALUE, int EXPONENT)' `math.h' (ISO): *Note Normalization Functions::. `ldiv_t ldiv (long int NUMERATOR, long int DENOMINATOR)' `stdlib.h' (ISO): *Note Integer Division::. `ldiv_t' `stdlib.h' (ISO): *Note Integer Division::. `void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)' `search.h' (SVID): *Note Array Search Function::. `double lgamma (double X)' `math.h' (SVID): *Note Special Functions::. `float lgammaf (float X)' `math.h' (SVID): *Note Special Functions::. `float lgammaf_r (float X, int *SIGNP)' `math.h' (XPG): *Note Special Functions::. `long double lgammal (long double X)' `math.h' (SVID): *Note Special Functions::. `long double lgammal_r (long double X, int *SIGNP)' `math.h' (XPG): *Note Special Functions::. `double lgamma_r (double X, int *SIGNP)' `math.h' (XPG): *Note Special Functions::. `L_INCR' `sys/file.h' (BSD): *Note File Positioning::. `int LINE_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int link (const char *OLDNAME, const char *NEWNAME)' `unistd.h' (POSIX.1): *Note Hard Links::. `int LINK_MAX' `limits.h' (POSIX.1): *Note Limits for Files::. `int lio_listio (int MODE, struct aiocb *const LIST[], int NENT, struct sigevent *SIG)' `aio.h' (POSIX.1b): *Note Asynchronous Reads/Writes::. `int lio_listio64 (int MODE, struct aiocb *const LIST, int NENT, struct sigevent *SIG)' `aio.h' (Unix98): *Note Asynchronous Reads/Writes::. `int listen (int SOCKET, unsigned int N)' `sys/socket.h' (BSD): *Note Listening::. `long long int llabs (long long int NUMBER)' `stdlib.h' (ISO): *Note Absolute Value::. `lldiv_t lldiv (long long int NUMERATOR, long long int DENOMINATOR)' `stdlib.h' (ISO): *Note Integer Division::. `lldiv_t' `stdlib.h' (ISO): *Note Integer Division::. `long long int llrint (double X)' `math.h' (ISO): *Note Rounding Functions::. `long long int llrintf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long long int llrintl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `long long int llround (double X)' `math.h' (ISO): *Note Rounding Functions::. `long long int llroundf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long long int llroundl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `struct lconv * localeconv (void)' `locale.h' (ISO): *Note The Lame Way to Locale Data::. `struct tm * localtime (const time_t *TIME)' `time.h' (ISO): *Note Broken-down Time::. `struct tm * localtime_r (const time_t *TIME, struct tm *RESULTP)' `time.h' (POSIX.1c): *Note Broken-down Time::. `double log (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double log10 (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float log10f (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double log10l (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double log1p (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float log1pf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double log1pl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double log2 (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float log2f (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double log2l (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double logb (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float logbf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double logbl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float logf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `void login (const struct utmp *ENTRY)' `utmp.h' (BSD): *Note Logging In and Out::. `LOGIN_PROCESS' `utmp.h' (SVID): *Note Manipulating the Database::. `LOGIN_PROCESS' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int login_tty (int FILEDES)' `utmp.h' (BSD): *Note Logging In and Out::. `long double logl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `int logout (const char *UT_LINE)' `utmp.h' (BSD): *Note Logging In and Out::. `void logwtmp (const char *UT_LINE, const char *UT_NAME, const char *UT_HOST)' `utmp.h' (BSD): *Note Logging In and Out::. `void longjmp (jmp_buf STATE, int VALUE)' `setjmp.h' (ISO): *Note Non-Local Details::. `LONG_LONG_MAX' `limits.h' (GNU): *Note Range of Type::. `LONG_LONG_MIN' `limits.h' (GNU): *Note Range of Type::. `LONG_MAX' `limits.h' (ISO): *Note Range of Type::. `LONG_MIN' `limits.h' (ISO): *Note Range of Type::. `long int lrand48 (void)' `stdlib.h' (SVID): *Note SVID Random::. `int lrand48_r (struct drand48_data *BUFFER, double *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `long int lrint (double X)' `math.h' (ISO): *Note Rounding Functions::. `long int lrintf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long int lrintl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `long int lround (double X)' `math.h' (ISO): *Note Rounding Functions::. `long int lroundf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long int lroundl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR)' `search.h' (SVID): *Note Array Search Function::. `off_t lseek (int FILEDES, off_t OFFSET, int WHENCE)' `unistd.h' (POSIX.1): *Note File Position Primitive::. `off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE)' `unistd.h' (Unix98): *Note File Position Primitive::. `L_SET' `sys/file.h' (BSD): *Note File Positioning::. `int lstat (const char *FILENAME, struct stat *BUF)' `sys/stat.h' (BSD): *Note Reading Attributes::. `int lstat64 (const char *FILENAME, struct stat64 *BUF)' `sys/stat.h' (Unix98): *Note Reading Attributes::. `int L_tmpnam' `stdio.h' (ISO): *Note Temporary Files::. `L_XTND' `sys/file.h' (BSD): *Note File Positioning::. `int madvise (void *ADDR, size_t LENGTH, int ADVICE)' `sys/mman.h' (POSIX): *Note Memory-mapped I/O::. `void makecontext (ucontext_t *UCP, void (*FUNC) (void), int ARGC, ...)' `ucontext.h' (SVID): *Note System V contexts::. `struct mallinfo mallinfo (void)' `malloc.h' (SVID): *Note Statistics of Malloc::. `void * malloc (size_t SIZE)' `malloc.h', `stdlib.h' (ISO): *Note Basic Allocation::. `__malloc_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `__malloc_initialize_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `int MAX_CANON' `limits.h' (POSIX.1): *Note Limits for Files::. `int MAX_INPUT' `limits.h' (POSIX.1): *Note Limits for Files::. `int MAXNAMLEN' `dirent.h' (BSD): *Note Limits for Files::. `int MAXSYMLINKS' `sys/param.h' (BSD): *Note Symbolic Links::. `int MB_CUR_MAX' `stdlib.h' (ISO): *Note Selecting the Conversion::. `int mblen (const char *STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Non-reentrant Character Conversion::. `int MB_LEN_MAX' `limits.h' (ISO): *Note Selecting the Conversion::. `size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS)' `wchar.h' (ISO): *Note Converting a Character::. `size_t mbrtowc (wchar_t *restrict PWC, const char *restrict S, size_t N, mbstate_t *restrict PS)' `wchar.h' (ISO): *Note Converting a Character::. `int mbsinit (const mbstate_t *PS)' `wchar.h' (ISO): *Note Keeping the state::. `size_t mbsnrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict PS)' `wchar.h' (GNU): *Note Converting Strings::. `size_t mbsrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t LEN, mbstate_t *restrict PS)' `wchar.h' (ISO): *Note Converting Strings::. `mbstate_t' `wchar.h' (ISO): *Note Keeping the state::. `size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Non-reentrant String Conversion::. `int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Non-reentrant Character Conversion::. `int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))' `mcheck.h' (GNU): *Note Heap Consistency Checking::. `tcflag_t MDMBUF' `termios.h' (BSD): *Note Control Modes::. `void * memalign (size_t BOUNDARY, size_t SIZE)' `malloc.h', `stdlib.h' (BSD): *Note Aligned Memory Blocks::. `__memalign_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `void * memccpy (void *restrict TO, const void *restrict FROM, int C, size_t SIZE)' `string.h' (SVID): *Note Copying and Concatenation::. `void * memchr (const void *BLOCK, int C, size_t SIZE)' `string.h' (ISO): *Note Search Functions::. `int memcmp (const void *A1, const void *A2, size_t SIZE)' `string.h' (ISO): *Note String/Array Comparison::. `void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `void * memfrob (void *MEM, size_t LENGTH)' `string.h' (GNU): *Note Trivial Encryption::. `void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN, const void *NEEDLE, size_t NEEDLE-LEN)' `string.h' (GNU): *Note Search Functions::. `void * memmove (void *TO, const void *FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `void * mempcpy (void *restrict TO, const void *restrict FROM, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `void * memrchr (const void *BLOCK, int C, size_t SIZE)' `string.h' (GNU): *Note Search Functions::. `void * memset (void *BLOCK, int C, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `int mkdir (const char *FILENAME, mode_t MODE)' `sys/stat.h' (POSIX.1): *Note Creating Directories::. `char * mkdtemp (char *TEMPLATE)' `stdlib.h' (BSD): *Note Temporary Files::. `int mkfifo (const char *FILENAME, mode_t MODE)' `sys/stat.h' (POSIX.1): *Note FIFO Special Files::. `int mknod (const char *FILENAME, int MODE, int DEV)' `sys/stat.h' (BSD): *Note Making Special Files::. `int mkstemp (char *TEMPLATE)' `stdlib.h' (BSD): *Note Temporary Files::. `char * mktemp (char *TEMPLATE)' `stdlib.h' (Unix): *Note Temporary Files::. `time_t mktime (struct tm *BROKENTIME)' `time.h' (ISO): *Note Broken-down Time::. `int mlock (const void *ADDR, size_t LEN)' `sys/mman.h' (POSIX.1b): *Note Page Lock Functions::. `int mlockall (int FLAGS)' `sys/mman.h' (POSIX.1b): *Note Page Lock Functions::. `void * mmap (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off_t OFFSET)' `sys/mman.h' (POSIX): *Note Memory-mapped I/O::. `void * mmap64 (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off64_t OFFSET)' `sys/mman.h' (LFS): *Note Memory-mapped I/O::. `mode_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `double modf (double VALUE, double *INTEGER-PART)' `math.h' (ISO): *Note Rounding Functions::. `float modff (float VALUE, float *INTEGER-PART)' `math.h' (ISO): *Note Rounding Functions::. `long double modfl (long double VALUE, long double *INTEGER-PART)' `math.h' (ISO): *Note Rounding Functions::. `int mount (const char *SPECIAL_FILE, const char *DIR, const char *FSTYPE, unsigned long int OPTIONS, const void *DATA)' `sys/mount.h' (SVID, BSD): *Note Mount-Unmount-Remount::. `long int mrand48 (void)' `stdlib.h' (SVID): *Note SVID Random::. `int mrand48_r (struct drand48_data *BUFFER, double *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `void * mremap (void *ADDRESS, size_t LENGTH, size_t NEW_LENGTH, int FLAG)' `sys/mman.h' (GNU): *Note Memory-mapped I/O::. `int MSG_DONTROUTE' `sys/socket.h' (BSD): *Note Socket Data Options::. `int MSG_OOB' `sys/socket.h' (BSD): *Note Socket Data Options::. `int MSG_PEEK' `sys/socket.h' (BSD): *Note Socket Data Options::. `int msync (void *ADDRESS, size_t LENGTH, int FLAGS)' `sys/mman.h' (POSIX): *Note Memory-mapped I/O::. `void mtrace (void)' `mcheck.h' (GNU): *Note Tracing malloc::. `int munlock (const void *ADDR, size_t LEN)' `sys/mman.h' (POSIX.1b): *Note Page Lock Functions::. `int munlockall (void)' `sys/mman.h' (POSIX.1b): *Note Page Lock Functions::. `int munmap (void *ADDR, size_t LENGTH)' `sys/mman.h' (POSIX): *Note Memory-mapped I/O::. `void muntrace (void)' `mcheck.h' (GNU): *Note Tracing malloc::. `int NAME_MAX' `limits.h' (POSIX.1): *Note Limits for Files::. `float NAN' `math.h' (GNU): *Note Infinity and NaN::. `double nan (const char *TAGP)' `math.h' (ISO): *Note FP Bit Twiddling::. `float nanf (const char *TAGP)' `math.h' (ISO): *Note FP Bit Twiddling::. `long double nanl (const char *TAGP)' `math.h' (ISO): *Note FP Bit Twiddling::. `int nanosleep (const struct timespec *REQUESTED_TIME, struct timespec *REMAINING)' `time.h' (POSIX.1): *Note Sleeping::. `int NCCS' `termios.h' (POSIX.1): *Note Mode Data Types::. `double nearbyint (double X)' `math.h' (ISO): *Note Rounding Functions::. `float nearbyintf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double nearbyintl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `NEW_TIME' `utmp.h' (SVID): *Note Manipulating the Database::. `NEW_TIME' `utmpx.h' (XPG4.2): *Note XPG Functions::. `double nextafter (double X, double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `float nextafterf (float X, float Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `long double nextafterl (long double X, long double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `double nexttoward (double X, long double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `float nexttowardf (float X, long double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `long double nexttowardl (long double X, long double Y)' `math.h' (ISO): *Note FP Bit Twiddling::. `int nftw (const char *FILENAME, __nftw_func_t FUNC, int DESCRIPTORS, int FLAG)' `ftw.h' (XPG4.2): *Note Working with Directory Trees::. `int nftw64 (const char *FILENAME, __nftw64_func_t FUNC, int DESCRIPTORS, int FLAG)' `ftw.h' (Unix98): *Note Working with Directory Trees::. `__nftw64_func_t' `ftw.h' (GNU): *Note Working with Directory Trees::. `__nftw_func_t' `ftw.h' (GNU): *Note Working with Directory Trees::. `char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N)' `libintl.h' (GNU): *Note Advanced gettext functions::. `int NGROUPS_MAX' `limits.h' (POSIX.1): *Note General Limits::. `int nice (int INCREMENT)' `unistd.h' (BSD): *Note Traditional Scheduling Functions::. `nlink_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `char * nl_langinfo (nl_item ITEM)' `langinfo.h' (XOPEN): *Note The Elegant and Fast Way::. `NO_ADDRESS' `netdb.h' (BSD): *Note Host Names::. `tcflag_t NOFLSH' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t NOKERNINFO' `termios.h' (BSD): *Note Local Modes::. `NO_RECOVERY' `netdb.h' (BSD): *Note Host Names::. `long int nrand48 (unsigned short int XSUBI[3])' `stdlib.h' (SVID): *Note SVID Random::. `int nrand48_r (unsigned short int XSUBI[3], struct drand48_data *BUFFER, long int *RESULT)' `stdlib.h' (GNU): *Note SVID Random::. `int NSIG' `signal.h' (BSD): *Note Standard Signals::. `uint32_t ntohl (uint32_t NETLONG)' `netinet/in.h' (BSD): *Note Byte Order::. `uint16_t ntohs (uint16_t NETSHORT)' `netinet/in.h' (BSD): *Note Byte Order::. `int ntp_adjtime (struct timex *TPTR)' `sys/timex.h' (GNU): *Note High Accuracy Clock::. `int ntp_gettime (struct ntptimeval *TPTR)' `sys/timex.h' (GNU): *Note High Accuracy Clock::. `void * NULL' `stddef.h' (ISO): *Note Null Pointer Constant::. `int O_ACCMODE' `fcntl.h' (POSIX.1): *Note Access Modes::. `int O_APPEND' `fcntl.h' (POSIX.1): *Note Operating Modes::. `int O_ASYNC' `fcntl.h' (BSD): *Note Operating Modes::. `void obstack_1grow (struct obstack *OBSTACK-PTR, char C)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C)' `obstack.h' (GNU): *Note Extra Fast Growing::. `int obstack_alignment_mask (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Obstacks Data Alignment::. `void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)' `obstack.h' (GNU): *Note Allocation in an Obstack::. `obstack_alloc_failed_handler' `obstack.h' (GNU): *Note Preparing for Obstacks::. `void * obstack_base (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Status of an Obstack::. `void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)' `obstack.h' (GNU): *Note Extra Fast Growing::. `int obstack_chunk_size (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Obstack Chunks::. `void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' `obstack.h' (GNU): *Note Allocation in an Obstack::. `void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' `obstack.h' (GNU): *Note Allocation in an Obstack::. `void * obstack_finish (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)' `obstack.h' (GNU): *Note Freeing Obstack Objects::. `void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)' `obstack.h' (GNU): *Note Growing Objects::. `int obstack_init (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Preparing for Obstacks::. `void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA)' `obstack.h' (GNU): *Note Extra Fast Growing::. `void * obstack_next_free (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Status of an Obstack::. `int obstack_object_size (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Growing Objects::. `int obstack_object_size (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Status of an Obstack::. `int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...)' `stdio.h' (GNU): *Note Dynamic Output::. `void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA)' `obstack.h' (GNU): *Note Extra Fast Growing::. `int obstack_room (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Extra Fast Growing::. `int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP)' `stdio.h' (GNU): *Note Variable Arguments Output::. `int O_CREAT' `fcntl.h' (POSIX.1): *Note Open-time Flags::. `int O_EXCL' `fcntl.h' (POSIX.1): *Note Open-time Flags::. `int O_EXEC' `fcntl.h' (GNU): *Note Access Modes::. `int O_EXLOCK' `fcntl.h' (BSD): *Note Open-time Flags::. `off64_t' `sys/types.h' (Unix98): *Note File Position Primitive::. `size_t offsetof (TYPE, MEMBER)' `stddef.h' (ISO): *Note Structure Measurement::. `off_t' `sys/types.h' (POSIX.1): *Note File Position Primitive::. `int O_FSYNC' `fcntl.h' (BSD): *Note Operating Modes::. `int O_IGNORE_CTTY' `fcntl.h' (GNU): *Note Open-time Flags::. `OLD_TIME' `utmp.h' (SVID): *Note Manipulating the Database::. `OLD_TIME' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int O_NDELAY' `fcntl.h' (BSD): *Note Operating Modes::. `int on_exit (void (*FUNCTION)(int STATUS, void *ARG), void *ARG)' `stdlib.h' (SunOS): *Note Cleanups on Exit::. `tcflag_t ONLCR' `termios.h' (BSD): *Note Output Modes::. `int O_NOATIME' `fcntl.h' (GNU): *Note Operating Modes::. `int O_NOCTTY' `fcntl.h' (POSIX.1): *Note Open-time Flags::. `tcflag_t ONOEOT' `termios.h' (BSD): *Note Output Modes::. `int O_NOLINK' `fcntl.h' (GNU): *Note Open-time Flags::. `int O_NONBLOCK' `fcntl.h' (POSIX.1): *Note Open-time Flags::. `int O_NONBLOCK' `fcntl.h' (POSIX.1): *Note Operating Modes::. `int O_NOTRANS' `fcntl.h' (GNU): *Note Open-time Flags::. `int open (const char *FILENAME, int FLAGS[, mode_t MODE])' `fcntl.h' (POSIX.1): *Note Opening and Closing Files::. `int open64 (const char *FILENAME, int FLAGS[, mode_t MODE])' `fcntl.h' (Unix98): *Note Opening and Closing Files::. `DIR * opendir (const char *DIRNAME)' `dirent.h' (POSIX.1): *Note Opening a Directory::. `void openlog (char *IDENT, int OPTION,' `syslog.h' (BSD): *Note openlog::. `int OPEN_MAX' `limits.h' (POSIX.1): *Note General Limits::. `FILE * open_memstream (char **PTR, size_t *SIZELOC)' `stdio.h' (GNU): *Note String Streams::. `FILE * open_obstack_stream (struct obstack *OBSTACK)' `stdio.h' (GNU): *Note Obstack Streams::. `int openpty (int *AMASTER, int *ASLAVE, char *NAME, struct termios *TERMP, struct winsize *WINP)' `pty.h' (BSD): *Note Pseudo-Terminal Pairs::. `tcflag_t OPOST' `termios.h' (POSIX.1): *Note Output Modes::. `char * optarg' `unistd.h' (POSIX.2): *Note Using Getopt::. `int opterr' `unistd.h' (POSIX.2): *Note Using Getopt::. `int optind' `unistd.h' (POSIX.2): *Note Using Getopt::. `OPTION_ALIAS' `argp.h' (GNU): *Note Argp Option Flags::. `OPTION_ARG_OPTIONAL' `argp.h' (GNU): *Note Argp Option Flags::. `OPTION_DOC' `argp.h' (GNU): *Note Argp Option Flags::. `OPTION_HIDDEN' `argp.h' (GNU): *Note Argp Option Flags::. `OPTION_NO_USAGE' `argp.h' (GNU): *Note Argp Option Flags::. `int optopt' `unistd.h' (POSIX.2): *Note Using Getopt::. `int O_RDONLY' `fcntl.h' (POSIX.1): *Note Access Modes::. `int O_RDWR' `fcntl.h' (POSIX.1): *Note Access Modes::. `int O_READ' `fcntl.h' (GNU): *Note Access Modes::. `int O_SHLOCK' `fcntl.h' (BSD): *Note Open-time Flags::. `int O_SYNC' `fcntl.h' (BSD): *Note Operating Modes::. `int O_TRUNC' `fcntl.h' (POSIX.1): *Note Open-time Flags::. `int O_WRITE' `fcntl.h' (GNU): *Note Access Modes::. `int O_WRONLY' `fcntl.h' (POSIX.1): *Note Access Modes::. `tcflag_t OXTABS' `termios.h' (BSD): *Note Output Modes::. `PA_CHAR' `printf.h' (GNU): *Note Parsing a Template String::. `PA_DOUBLE' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_LONG' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_LONG_DOUBLE' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_LONG_LONG' `printf.h' (GNU): *Note Parsing a Template String::. `int PA_FLAG_MASK' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_PTR' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_SHORT' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLOAT' `printf.h' (GNU): *Note Parsing a Template String::. `PA_INT' `printf.h' (GNU): *Note Parsing a Template String::. `PA_LAST' `printf.h' (GNU): *Note Parsing a Template String::. `PA_POINTER' `printf.h' (GNU): *Note Parsing a Template String::. `tcflag_t PARENB' `termios.h' (POSIX.1): *Note Control Modes::. `tcflag_t PARMRK' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t PARODD' `termios.h' (POSIX.1): *Note Control Modes::. `size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES)' `printf.h' (GNU): *Note Parsing a Template String::. `PA_STRING' `printf.h' (GNU): *Note Parsing a Template String::. `long int pathconf (const char *FILENAME, int PARAMETER)' `unistd.h' (POSIX.1): *Note Pathconf::. `int PATH_MAX' `limits.h' (POSIX.1): *Note Limits for Files::. `int pause ()' `unistd.h' (POSIX.1): *Note Using Pause::. `_PC_ASYNC_IO' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_CHOWN_RESTRICTED' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_FILESIZEBITS' `unistd.h' (LFS): *Note Pathconf::. `_PC_LINK_MAX' `unistd.h' (POSIX.1): *Note Pathconf::. `int pclose (FILE *STREAM)' `stdio.h' (POSIX.2, SVID, BSD): *Note Pipe to a Subprocess::. `_PC_MAX_CANON' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_MAX_INPUT' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_NAME_MAX' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_NO_TRUNC' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_PATH_MAX' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_PIPE_BUF' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_PRIO_IO' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_REC_INCR_XFER_SIZE' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_REC_MAX_XFER_SIZE' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_REC_MIN_XFER_SIZE' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_REC_XFER_ALIGN' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_SOCK_MAXBUF' `unistd.h' (POSIX.1g): *Note Pathconf::. `_PC_SYNC_IO' `unistd.h' (POSIX.1): *Note Pathconf::. `_PC_VDISABLE' `unistd.h' (POSIX.1): *Note Pathconf::. `tcflag_t PENDIN' `termios.h' (BSD): *Note Local Modes::. `void perror (const char *MESSAGE)' `stdio.h' (ISO): *Note Error Messages::. `int PF_FILE' `sys/socket.h' (GNU): *Note Local Namespace Details::. `int PF_INET' `sys/socket.h' (BSD): *Note Internet Namespace::. `int PF_INET6' `sys/socket.h' (X/Open): *Note Internet Namespace::. `int PF_LOCAL' `sys/socket.h' (POSIX): *Note Local Namespace Details::. `int PF_UNIX' `sys/socket.h' (BSD): *Note Local Namespace Details::. `pid_t' `sys/types.h' (POSIX.1): *Note Process Identification::. `int pipe (int FILEDES[2])' `unistd.h' (POSIX.1): *Note Creating a Pipe::. `int PIPE_BUF' `limits.h' (POSIX.1): *Note Limits for Files::. `FILE * popen (const char *COMMAND, const char *MODE)' `stdio.h' (POSIX.2, SVID, BSD): *Note Pipe to a Subprocess::. `_POSIX2_BC_BASE_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `_POSIX2_BC_DIM_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `_POSIX2_BC_SCALE_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `_POSIX2_BC_STRING_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `int _POSIX2_C_DEV' `unistd.h' (POSIX.2): *Note System Options::. `_POSIX2_COLL_WEIGHTS_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `long int _POSIX2_C_VERSION' `unistd.h' (POSIX.2): *Note Version Supported::. `_POSIX2_EQUIV_CLASS_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `_POSIX2_EXPR_NEST_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `int _POSIX2_FORT_DEV' `unistd.h' (POSIX.2): *Note System Options::. `int _POSIX2_FORT_RUN' `unistd.h' (POSIX.2): *Note System Options::. `_POSIX2_LINE_MAX' `limits.h' (POSIX.2): *Note Utility Minimums::. `int _POSIX2_LOCALEDEF' `unistd.h' (POSIX.2): *Note System Options::. `_POSIX2_RE_DUP_MAX' `limits.h' (POSIX.2): *Note Minimums::. `int _POSIX2_SW_DEV' `unistd.h' (POSIX.2): *Note System Options::. `_POSIX_AIO_LISTIO_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_AIO_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_ARG_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_CHILD_MAX' `limits.h' (POSIX.1): *Note Minimums::. `int _POSIX_CHOWN_RESTRICTED' `unistd.h' (POSIX.1): *Note Options for Files::. `_POSIX_C_SOURCE' (POSIX.2): *Note Feature Test Macros::. `int _POSIX_JOB_CONTROL' `unistd.h' (POSIX.1): *Note System Options::. `_POSIX_LINK_MAX' `limits.h' (POSIX.1): *Note File Minimums::. `_POSIX_MAX_CANON' `limits.h' (POSIX.1): *Note File Minimums::. `_POSIX_MAX_INPUT' `limits.h' (POSIX.1): *Note File Minimums::. `int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE)' `stdlib.h' (POSIX): *Note Aligned Memory Blocks::. `_POSIX_NAME_MAX' `limits.h' (POSIX.1): *Note File Minimums::. `_POSIX_NGROUPS_MAX' `limits.h' (POSIX.1): *Note Minimums::. `int _POSIX_NO_TRUNC' `unistd.h' (POSIX.1): *Note Options for Files::. `_POSIX_OPEN_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_PATH_MAX' `limits.h' (POSIX.1): *Note File Minimums::. `_POSIX_PIPE_BUF' `limits.h' (POSIX.1): *Note File Minimums::. `POSIX_REC_INCR_XFER_SIZE' `limits.h' (POSIX.1): *Note File Minimums::. `POSIX_REC_MAX_XFER_SIZE' `limits.h' (POSIX.1): *Note File Minimums::. `POSIX_REC_MIN_XFER_SIZE' `limits.h' (POSIX.1): *Note File Minimums::. `POSIX_REC_XFER_ALIGN' `limits.h' (POSIX.1): *Note File Minimums::. `int _POSIX_SAVED_IDS' `unistd.h' (POSIX.1): *Note System Options::. `_POSIX_SOURCE' (POSIX.1): *Note Feature Test Macros::. `_POSIX_SSIZE_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_STREAM_MAX' `limits.h' (POSIX.1): *Note Minimums::. `_POSIX_TZNAME_MAX' `limits.h' (POSIX.1): *Note Minimums::. `unsigned char _POSIX_VDISABLE' `unistd.h' (POSIX.1): *Note Options for Files::. `long int _POSIX_VERSION' `unistd.h' (POSIX.1): *Note Version Supported::. `double pow (double BASE, double POWER)' `math.h' (ISO): *Note Exponents and Logarithms::. `double pow10 (double X)' `math.h' (GNU): *Note Exponents and Logarithms::. `float pow10f (float X)' `math.h' (GNU): *Note Exponents and Logarithms::. `long double pow10l (long double X)' `math.h' (GNU): *Note Exponents and Logarithms::. `float powf (float BASE, float POWER)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double powl (long double BASE, long double POWER)' `math.h' (ISO): *Note Exponents and Logarithms::. `ssize_t pread (int FILEDES, void *BUFFER, size_t SIZE, off_t OFFSET)' `unistd.h' (Unix98): *Note I/O Primitives::. `ssize_t pread64 (int FILEDES, void *BUFFER, size_t SIZE, off64_t OFFSET)' `unistd.h' (Unix98): *Note I/O Primitives::. `int printf (const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Output Functions::. `printf_arginfo_function' `printf.h' (GNU): *Note Defining the Output Handler::. `printf_function' `printf.h' (GNU): *Note Defining the Output Handler::. `int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS)' `printf.h' (GNU): *Note Predefined Printf Handlers::. `int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES)' `printf.h' (GNU): *Note Predefined Printf Handlers::. `PRIO_MAX' `sys/resource.h' (BSD): *Note Traditional Scheduling Functions::. `PRIO_MIN' `sys/resource.h' (BSD): *Note Traditional Scheduling Functions::. `PRIO_PGRP' `sys/resource.h' (BSD): *Note Traditional Scheduling Functions::. `PRIO_PROCESS' `sys/resource.h' (BSD): *Note Traditional Scheduling Functions::. `PRIO_USER' `sys/resource.h' (BSD): *Note Traditional Scheduling Functions::. `char * program_invocation_name' `errno.h' (GNU): *Note Error Messages::. `char * program_invocation_short_name' `errno.h' (GNU): *Note Error Messages::. `void psignal (int SIGNUM, const char *MESSAGE)' `signal.h' (BSD): *Note Signal Messages::. `char * P_tmpdir' `stdio.h' (SVID): *Note Temporary Files::. `ptrdiff_t' `stddef.h' (ISO): *Note Important Data Types::. `char * ptsname (int FILEDES)' `stdlib.h' (SVID, XPG4.2): *Note Allocation::. `int ptsname_r (int FILEDES, char *BUF, size_t LEN)' `stdlib.h' (GNU): *Note Allocation::. `int putc (int C, FILE *STREAM)' `stdio.h' (ISO): *Note Simple Output::. `int putchar (int C)' `stdio.h' (ISO): *Note Simple Output::. `int putchar_unlocked (int C)' `stdio.h' (POSIX): *Note Simple Output::. `int putc_unlocked (int C, FILE *STREAM)' `stdio.h' (POSIX): *Note Simple Output::. `int putenv (char *STRING)' `stdlib.h' (SVID): *Note Environment Access::. `int putpwent (const struct passwd *P, FILE *STREAM)' `pwd.h' (SVID): *Note Writing a User Entry::. `int puts (const char *S)' `stdio.h' (ISO): *Note Simple Output::. `struct utmp * pututline (const struct utmp *UTMP)' `utmp.h' (SVID): *Note Manipulating the Database::. `struct utmpx * pututxline (const struct utmpx *UTMP)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int putw (int W, FILE *STREAM)' `stdio.h' (SVID): *Note Simple Output::. `wint_t putwc (wchar_t WC, FILE *STREAM)' `wchar.h' (ISO): *Note Simple Output::. `wint_t putwchar (wchar_t WC)' `wchar.h' (ISO): *Note Simple Output::. `wint_t putwchar_unlocked (wchar_t WC)' `wchar.h' (GNU): *Note Simple Output::. `wint_t putwc_unlocked (wchar_t WC, FILE *STREAM)' `wchar.h' (GNU): *Note Simple Output::. `ssize_t pwrite (int FILEDES, const void *BUFFER, size_t SIZE, off_t OFFSET)' `unistd.h' (Unix98): *Note I/O Primitives::. `ssize_t pwrite64 (int FILEDES, const void *BUFFER, size_t SIZE, off64_t OFFSET)' `unistd.h' (Unix98): *Note I/O Primitives::. `char * qecvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)' `stdlib.h' (GNU): *Note System V Number Conversion::. `char * qecvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)' `stdlib.h' (GNU): *Note System V Number Conversion::. `char * qfcvt (long double VALUE, int NDIGIT, int *DECPT, int *NEG)' `stdlib.h' (GNU): *Note System V Number Conversion::. `char * qfcvt_r (long double VALUE, int NDIGIT, int *DECPT, int *NEG, char *BUF, size_t LEN)' `stdlib.h' (GNU): *Note System V Number Conversion::. `char * qgcvt (long double VALUE, int NDIGIT, char *BUF)' `stdlib.h' (GNU): *Note System V Number Conversion::. `void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)' `stdlib.h' (ISO): *Note Array Sort Function::. `int raise (int SIGNUM)' `signal.h' (ISO): *Note Signaling Yourself::. `int rand (void)' `stdlib.h' (ISO): *Note ISO Random::. `int RAND_MAX' `stdlib.h' (ISO): *Note ISO Random::. `long int random (void)' `stdlib.h' (BSD): *Note BSD Random::. `int random_r (struct random_data *restrict BUF, int32_t *restrict RESULT)' `stdlib.h' (GNU): *Note BSD Random::. `int rand_r (unsigned int *SEED)' `stdlib.h' (POSIX.1): *Note ISO Random::. `void * rawmemchr (const void *BLOCK, int C)' `string.h' (GNU): *Note Search Functions::. `ssize_t read (int FILEDES, void *BUFFER, size_t SIZE)' `unistd.h' (POSIX.1): *Note I/O Primitives::. `struct dirent * readdir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Reading/Closing Directory::. `struct dirent64 * readdir64 (DIR *DIRSTREAM)' `dirent.h' (LFS): *Note Reading/Closing Directory::. `int readdir64_r (DIR *DIRSTREAM, struct dirent64 *ENTRY, struct dirent64 **RESULT)' `dirent.h' (LFS): *Note Reading/Closing Directory::. `int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY, struct dirent **RESULT)' `dirent.h' (GNU): *Note Reading/Closing Directory::. `int readlink (const char *FILENAME, char *BUFFER, size_t SIZE)' `unistd.h' (BSD): *Note Symbolic Links::. `ssize_t readv (int FILEDES, const struct iovec *VECTOR, int COUNT)' `sys/uio.h' (BSD): *Note Scatter-Gather::. `void * realloc (void *PTR, size_t NEWSIZE)' `malloc.h', `stdlib.h' (ISO): *Note Changing Block Size::. `__realloc_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `char * realpath (const char *restrict NAME, char *restrict RESOLVED)' `stdlib.h' (XPG): *Note Symbolic Links::. `int recv (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)' `sys/socket.h' (BSD): *Note Receiving Data::. `int recvfrom (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS, struct sockaddr *ADDR, socklen_t *LENGTH-PTR)' `sys/socket.h' (BSD): *Note Receiving Datagrams::. `int recvmsg (int SOCKET, struct msghdr *MESSAGE, int FLAGS)' `sys/socket.h' (BSD): *Note Receiving Datagrams::. `int RE_DUP_MAX' `limits.h' (POSIX.2): *Note General Limits::. `_REENTRANT' (GNU): *Note Feature Test Macros::. `REG_BADBR' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_BADPAT' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_BADRPT' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS)' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_EBRACE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_EBRACK' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_ECOLLATE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_ECTYPE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_EESCAPE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_EPAREN' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_ERANGE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH)' `regex.h' (POSIX.2): *Note Regexp Cleanup::. `REG_ESPACE' `regex.h' (POSIX.2): *Note Matching POSIX Regexps::. `REG_ESPACE' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_ESUBREG' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS)' `regex.h' (POSIX.2): *Note Matching POSIX Regexps::. `regex_t' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `REG_EXTENDED' `regex.h' (POSIX.2): *Note Flags for POSIX Regexps::. `void regfree (regex_t *COMPILED)' `regex.h' (POSIX.2): *Note Regexp Cleanup::. `REG_ICASE' `regex.h' (POSIX.2): *Note Flags for POSIX Regexps::. `int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION)' `printf.h' (GNU): *Note Registering New Conversions::. `regmatch_t' `regex.h' (POSIX.2): *Note Regexp Subexpressions::. `REG_NEWLINE' `regex.h' (POSIX.2): *Note Flags for POSIX Regexps::. `REG_NOMATCH' `regex.h' (POSIX.2): *Note Matching POSIX Regexps::. `REG_NOSUB' `regex.h' (POSIX.2): *Note Flags for POSIX Regexps::. `REG_NOTBOL' `regex.h' (POSIX.2): *Note Matching POSIX Regexps::. `REG_NOTEOL' `regex.h' (POSIX.2): *Note Matching POSIX Regexps::. `regoff_t' `regex.h' (POSIX.2): *Note Regexp Subexpressions::. `double remainder (double NUMERATOR, double DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `float remainderf (float NUMERATOR, float DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `long double remainderl (long double NUMERATOR, long double DENOMINATOR)' `math.h' (BSD): *Note Remainder Functions::. `int remove (const char *FILENAME)' `stdio.h' (ISO): *Note Deleting Files::. `int rename (const char *OLDNAME, const char *NEWNAME)' `stdio.h' (ISO): *Note Renaming Files::. `void rewind (FILE *STREAM)' `stdio.h' (ISO): *Note File Positioning::. `void rewinddir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Random Access Directory::. `char * rindex (const char *STRING, int C)' `string.h' (BSD): *Note Search Functions::. `double rint (double X)' `math.h' (ISO): *Note Rounding Functions::. `float rintf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double rintl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `int RLIM_INFINITY' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_AS' `sys/resource.h' (Unix98): *Note Limits on Resources::. `RLIMIT_CORE' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_CPU' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_DATA' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_FSIZE' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_MEMLOCK' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_NOFILE' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_NPROC' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_RSS' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIMIT_STACK' `sys/resource.h' (BSD): *Note Limits on Resources::. `RLIM_NLIMITS' `sys/resource.h' (BSD): *Note Limits on Resources::. `int rmdir (const char *FILENAME)' `unistd.h' (POSIX.1): *Note Deleting Files::. `int R_OK' `unistd.h' (POSIX.1): *Note Testing File Access::. `double round (double X)' `math.h' (ISO): *Note Rounding Functions::. `float roundf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double roundl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `int rpmatch (const char *RESPONSE)' `stdlib.h' (stdlib.h): *Note Yes-or-No Questions::. `RUN_LVL' `utmp.h' (SVID): *Note Manipulating the Database::. `RUN_LVL' `utmpx.h' (XPG4.2): *Note XPG Functions::. `RUSAGE_CHILDREN' `sys/resource.h' (BSD): *Note Resource Usage::. `RUSAGE_SELF' `sys/resource.h' (BSD): *Note Resource Usage::. `int SA_NOCLDSTOP' `signal.h' (POSIX.1): *Note Flags for Sigaction::. `int SA_ONSTACK' `signal.h' (BSD): *Note Flags for Sigaction::. `int SA_RESTART' `signal.h' (BSD): *Note Flags for Sigaction::. `int sbrk (ptrdiff_t DELTA)' `unistd.h' (BSD): *Note Resizing the Data Segment::. `_SC_2_C_DEV' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_2_FORT_DEV' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_2_FORT_RUN' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_2_LOCALEDEF' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_2_SW_DEV' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_2_VERSION' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_AIO_LISTIO_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_AIO_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_AIO_PRIO_DELTA_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `double scalb (double VALUE, int EXPONENT)' `math.h' (BSD): *Note Normalization Functions::. `float scalbf (float VALUE, int EXPONENT)' `math.h' (BSD): *Note Normalization Functions::. `long double scalbl (long double VALUE, int EXPONENT)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalbln (double X, long int n)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalblnf (float X, long int n)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalblnl (long double X, long int n)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalbn (double X, int n)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalbnf (float X, int n)' `math.h' (BSD): *Note Normalization Functions::. `long long int scalbnl (long double X, int n)' `math.h' (BSD): *Note Normalization Functions::. `int scandir (const char *DIR, struct dirent ***NAMELIST, int (*SELECTOR) (const struct dirent *), int (*CMP) (const void *, const void *))' `dirent.h' (BSD/SVID): *Note Scanning Directory Content::. `int scandir64 (const char *DIR, struct dirent64 ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int (*CMP) (const void *, const void *))' `dirent.h' (GNU): *Note Scanning Directory Content::. `int scanf (const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Input Functions::. `_SC_ARG_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_ASYNCHRONOUS_IO' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_ATEXIT_MAX' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_AVPHYS_PAGES' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_BC_BASE_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_BC_DIM_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_BC_SCALE_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_BC_STRING_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_CHAR_BIT' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_CHARCLASS_NAME_MAX' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_CHAR_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_CHAR_MIN' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_CHILD_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_CLK_TCK' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_COLL_WEIGHTS_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_DELAYTIMER_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_EQUIV_CLASS_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_EXPR_NEST_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_FSYNC' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_GETGR_R_SIZE_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_GETPW_R_SIZE_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `SCHAR_MAX' `limits.h' (ISO): *Note Range of Type::. `SCHAR_MIN' `limits.h' (ISO): *Note Range of Type::. `int sched_getparam (pid_t PID, const struct sched_param *PARAM)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_get_priority_max (int *POLICY);' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_get_priority_min (int *POLICY);' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_getscheduler (pid_t PID)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_rr_get_interval (pid_t PID, struct timespec *INTERVAL)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_setparam (pid_t PID, const struct sched_param *PARAM)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_setscheduler (pid_t PID, int POLICY, const struct sched_param *PARAM)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `int sched_yield (void)' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `_SC_INT_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_INT_MIN' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_JOB_CONTROL' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_LINE_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_LOGIN_NAME_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_LONG_BIT' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_MAPPED_FILES' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MB_LEN_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_MEMLOCK' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MEMLOCK_RANGE' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MEMORY_PROTECTION' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MESSAGE_PASSING' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MQ_OPEN_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_MQ_PRIO_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_NGROUPS_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_NL_ARGMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NL_LANGMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NL_MSGMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NL_NMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NL_SETMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NL_TEXTMAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_NPROCESSORS_CONF' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_NPROCESSORS_ONLN' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_NZERO' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_OPEN_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_PAGESIZE' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_PHYS_PAGES' `unistd.h' (GNU): *Note Constants for Sysconf::. `_SC_PII' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_INTERNET' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_INTERNET_DGRAM' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_INTERNET_STREAM' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_OSI' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_OSI_CLTS' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_OSI_COTS' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_OSI_M' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_SOCKET' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PII_XTI' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_PRIORITIZED_IO' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_PRIORITY_SCHEDULING' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_REALTIME_SIGNALS' `unistdh.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_RTSIG_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SAVED_IDS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SCHAR_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_SCHAR_MIN' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_SELECT' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_SEMAPHORES' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SEM_NSEMS_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SEM_VALUE_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SHARED_MEMORY_OBJECTS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SHRT_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_SHRT_MIN' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_SIGQUEUE_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `SC_SSIZE_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_STREAM_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_SYNCHRONIZED_IO' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_ATTR_STACKADDR' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_ATTR_STACKSIZE' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_DESTRUCTOR_ITERATIONS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_KEYS_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_PRIO_INHERIT' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_PRIO_PROTECT' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_PRIORITY_SCHEDULING' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_PROCESS_SHARED' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREADS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_SAFE_FUNCTIONS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_STACK_MIN' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_THREAD_THREADS_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_TIMER_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_TIMERS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_T_IOV_MAX' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_TTY_NAME_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_TZNAME_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_UCHAR_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_UINT_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_UIO_MAXIOV' `unistd.h' (POSIX.1g): *Note Constants for Sysconf::. `_SC_ULONG_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_USHRT_MAX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_VERSION' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_VERSION' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_WORD_BIT' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_CRYPT' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_ENH_I18N' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_LEGACY' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_REALTIME' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_REALTIME_THREADS' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_SHM' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_UNIX' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_VERSION' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_XCU_VERSION' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_XPG2' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_XPG3' `unistd.h' (X/Open): *Note Constants for Sysconf::. `_SC_XOPEN_XPG4' `unistd.h' (X/Open): *Note Constants for Sysconf::. `unsigned short int * seed48 (unsigned short int SEED16V[3])' `stdlib.h' (SVID): *Note SVID Random::. `int seed48_r (unsigned short int SEED16V[3], struct drand48_data *BUFFER)' `stdlib.h' (GNU): *Note SVID Random::. `int SEEK_CUR' `stdio.h' (ISO): *Note File Positioning::. `void seekdir (DIR *DIRSTREAM, off_t POS)' `dirent.h' (BSD): *Note Random Access Directory::. `int SEEK_END' `stdio.h' (ISO): *Note File Positioning::. `int SEEK_SET' `stdio.h' (ISO): *Note File Positioning::. `int select (int NFDS, fd_set *READ-FDS, fd_set *WRITE-FDS, fd_set *EXCEPT-FDS, struct timeval *TIMEOUT)' `sys/types.h' (BSD): *Note Waiting for I/O::. `int send (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)' `sys/socket.h' (BSD): *Note Sending Data::. `int sendmsg (int SOCKET, const struct msghdr *MESSAGE, int FLAGS)' `sys/socket.h' (BSD): *Note Receiving Datagrams::. `int sendto (int SOCKET, void *BUFFER. size_t SIZE, int FLAGS, struct sockaddr *ADDR, socklen_t LENGTH)' `sys/socket.h' (BSD): *Note Sending Datagrams::. `void setbuf (FILE *STREAM, char *BUF)' `stdio.h' (ISO): *Note Controlling Buffering::. `void setbuffer (FILE *STREAM, char *BUF, size_t SIZE)' `stdio.h' (BSD): *Note Controlling Buffering::. `int setcontext (const ucontext_t *UCP)' `ucontext.h' (SVID): *Note System V contexts::. `int setdomainname (const char *NAME, size_t LENGTH)' `unistd.h' (???): *Note Host Identification::. `int setegid (gid_t NEWGID)' `unistd.h' (POSIX.1): *Note Setting Groups::. `int setenv (const char *NAME, const char *VALUE, int REPLACE)' `stdlib.h' (BSD): *Note Environment Access::. `int seteuid (uid_t NEWEUID)' `unistd.h' (POSIX.1): *Note Setting User ID::. `int setfsent (void)' `fstab.h' (BSD): *Note fstab::. `int setgid (gid_t NEWGID)' `unistd.h' (POSIX.1): *Note Setting Groups::. `void setgrent (void)' `grp.h' (SVID, BSD): *Note Scanning All Groups::. `int setgroups (size_t COUNT, gid_t *GROUPS)' `grp.h' (BSD): *Note Setting Groups::. `void sethostent (int STAYOPEN)' `netdb.h' (BSD): *Note Host Names::. `int sethostid (long int ID)' `unistd.h' (BSD): *Note Host Identification::. `int sethostname (const char *NAME, size_t LENGTH)' `unistd.h' (BSD): *Note Host Identification::. `int setitimer (int WHICH, struct itimerval *NEW, struct itimerval *OLD)' `sys/time.h' (BSD): *Note Setting an Alarm::. `int setjmp (jmp_buf STATE)' `setjmp.h' (ISO): *Note Non-Local Details::. `void setkey (const char *KEY)' `crypt.h' (BSD, SVID): *Note DES Encryption::. `void setkey_r (const char *KEY, struct crypt_data * DATA)' `crypt.h' (GNU): *Note DES Encryption::. `void setlinebuf (FILE *STREAM)' `stdio.h' (BSD): *Note Controlling Buffering::. `char * setlocale (int CATEGORY, const char *LOCALE)' `locale.h' (ISO): *Note Setting the Locale::. `int setlogmask (int MASK)' `syslog.h' (BSD): *Note setlogmask::. `FILE * setmntent (const char *FILE, const char *MODE)' `mntent.h' (BSD): *Note mtab::. `void setnetent (int STAYOPEN)' `netdb.h' (BSD): *Note Networks Database::. `int setnetgrent (const char *NETGROUP)' `netdb.h' (BSD): *Note Lookup Netgroup::. `int setpgid (pid_t PID, pid_t PGID)' `unistd.h' (POSIX.1): *Note Process Group Functions::. `int setpgrp (pid_t PID, pid_t PGID)' `unistd.h' (BSD): *Note Process Group Functions::. `int setpriority (int CLASS, int ID, int NICEVAL)' `sys/resource.h' (BSD,POSIX): *Note Traditional Scheduling Functions::. `void setprotoent (int STAYOPEN)' `netdb.h' (BSD): *Note Protocols Database::. `void setpwent (void)' `pwd.h' (SVID, BSD): *Note Scanning All Users::. `int setregid (gid_t RGID, gid_t EGID)' `unistd.h' (BSD): *Note Setting Groups::. `int setreuid (uid_t RUID, uid_t EUID)' `unistd.h' (BSD): *Note Setting User ID::. `int setrlimit (int RESOURCE, const struct rlimit *RLP)' `sys/resource.h' (BSD): *Note Limits on Resources::. `int setrlimit64 (int RESOURCE, const struct rlimit64 *RLP)' `sys/resource.h' (Unix98): *Note Limits on Resources::. `void setservent (int STAYOPEN)' `netdb.h' (BSD): *Note Services Database::. `pid_t setsid (void)' `unistd.h' (POSIX.1): *Note Process Group Functions::. `int setsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, socklen_t OPTLEN)' `sys/socket.h' (BSD): *Note Socket Option Functions::. `void * setstate (void *STATE)' `stdlib.h' (BSD): *Note BSD Random::. `int setstate_r (char *restrict STATEBUF, struct random_data *restrict BUF)' `stdlib.h' (GNU): *Note BSD Random::. `int settimeofday (const struct timeval *TP, const struct timezone *TZP)' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `int setuid (uid_t NEWUID)' `unistd.h' (POSIX.1): *Note Setting User ID::. `void setutent (void)' `utmp.h' (SVID): *Note Manipulating the Database::. `void setutxent (void)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE)' `stdio.h' (ISO): *Note Controlling Buffering::. `SHRT_MAX' `limits.h' (ISO): *Note Range of Type::. `SHRT_MIN' `limits.h' (ISO): *Note Range of Type::. `int shutdown (int SOCKET, int HOW)' `sys/socket.h' (BSD): *Note Closing a Socket::. `S_IEXEC' `sys/stat.h' (BSD): *Note Permission Bits::. `S_IFBLK' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFCHR' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFDIR' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFIFO' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFLNK' `sys/stat.h' (BSD): *Note Testing File Type::. `int S_IFMT' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFREG' `sys/stat.h' (BSD): *Note Testing File Type::. `S_IFSOCK' `sys/stat.h' (BSD): *Note Testing File Type::. `int SIGABRT' `signal.h' (ISO): *Note Program Error Signals::. `int sigaction (int SIGNUM, const struct sigaction *restrict ACTION, struct sigaction *restrict OLD-ACTION)' `signal.h' (POSIX.1): *Note Advanced Signal Handling::. `int sigaddset (sigset_t *SET, int SIGNUM)' `signal.h' (POSIX.1): *Note Signal Sets::. `int SIGALRM' `signal.h' (POSIX.1): *Note Alarm Signals::. `int sigaltstack (const stack_t *restrict STACK, stack_t *restrict OLDSTACK)' `signal.h' (XPG): *Note Signal Stack::. `sig_atomic_t' `signal.h' (ISO): *Note Atomic Types::. `SIG_BLOCK' `signal.h' (POSIX.1): *Note Process Signal Mask::. `int sigblock (int MASK)' `signal.h' (BSD): *Note Blocking in BSD::. `int SIGBUS' `signal.h' (BSD): *Note Program Error Signals::. `int SIGCHLD' `signal.h' (POSIX.1): *Note Job Control Signals::. `int SIGCLD' `signal.h' (SVID): *Note Job Control Signals::. `int SIGCONT' `signal.h' (POSIX.1): *Note Job Control Signals::. `int sigdelset (sigset_t *SET, int SIGNUM)' `signal.h' (POSIX.1): *Note Signal Sets::. `int sigemptyset (sigset_t *SET)' `signal.h' (POSIX.1): *Note Signal Sets::. `int SIGEMT' `signal.h' (BSD): *Note Program Error Signals::. `sighandler_t SIG_ERR' `signal.h' (ISO): *Note Basic Signal Handling::. `int sigfillset (sigset_t *SET)' `signal.h' (POSIX.1): *Note Signal Sets::. `int SIGFPE' `signal.h' (ISO): *Note Program Error Signals::. `sighandler_t' `signal.h' (GNU): *Note Basic Signal Handling::. `int SIGHUP' `signal.h' (POSIX.1): *Note Termination Signals::. `int SIGILL' `signal.h' (ISO): *Note Program Error Signals::. `int SIGINFO' `signal.h' (BSD): *Note Miscellaneous Signals::. `int SIGINT' `signal.h' (ISO): *Note Termination Signals::. `int siginterrupt (int SIGNUM, int FAILFLAG)' `signal.h' (BSD): *Note BSD Handler::. `int SIGIO' `signal.h' (BSD): *Note Asynchronous I/O Signals::. `int SIGIOT' `signal.h' (Unix): *Note Program Error Signals::. `int sigismember (const sigset_t *SET, int SIGNUM)' `signal.h' (POSIX.1): *Note Signal Sets::. `sigjmp_buf' `setjmp.h' (POSIX.1): *Note Non-Local Exits and Signals::. `int SIGKILL' `signal.h' (POSIX.1): *Note Termination Signals::. `void siglongjmp (sigjmp_buf STATE, int VALUE)' `setjmp.h' (POSIX.1): *Note Non-Local Exits and Signals::. `int SIGLOST' `signal.h' (GNU): *Note Operation Error Signals::. `int sigmask (int SIGNUM)' `signal.h' (BSD): *Note Blocking in BSD::. `sighandler_t signal (int SIGNUM, sighandler_t ACTION)' `signal.h' (ISO): *Note Basic Signal Handling::. `int signbit (_float-type_ X)' `math.h' (ISO): *Note FP Bit Twiddling::. `long long int significand (double X)' `math.h' (BSD): *Note Normalization Functions::. `long long int significandf (float X)' `math.h' (BSD): *Note Normalization Functions::. `long long int significandl (long double X)' `math.h' (BSD): *Note Normalization Functions::. `int sigpause (int MASK)' `signal.h' (BSD): *Note Blocking in BSD::. `int sigpending (sigset_t *SET)' `signal.h' (POSIX.1): *Note Checking for Pending Signals::. `int SIGPIPE' `signal.h' (POSIX.1): *Note Operation Error Signals::. `int SIGPOLL' `signal.h' (SVID): *Note Asynchronous I/O Signals::. `int sigprocmask (int HOW, const sigset_t *restrict SET, sigset_t *restrict OLDSET)' `signal.h' (POSIX.1): *Note Process Signal Mask::. `int SIGPROF' `signal.h' (BSD): *Note Alarm Signals::. `int SIGQUIT' `signal.h' (POSIX.1): *Note Termination Signals::. `int SIGSEGV' `signal.h' (ISO): *Note Program Error Signals::. `int sigsetjmp (sigjmp_buf STATE, int SAVESIGS)' `setjmp.h' (POSIX.1): *Note Non-Local Exits and Signals::. `SIG_SETMASK' `signal.h' (POSIX.1): *Note Process Signal Mask::. `int sigsetmask (int MASK)' `signal.h' (BSD): *Note Blocking in BSD::. `sigset_t' `signal.h' (POSIX.1): *Note Signal Sets::. `int sigstack (const struct sigstack *STACK, struct sigstack *OLDSTACK)' `signal.h' (BSD): *Note Signal Stack::. `int SIGSTOP' `signal.h' (POSIX.1): *Note Job Control Signals::. `int sigsuspend (const sigset_t *SET)' `signal.h' (POSIX.1): *Note Sigsuspend::. `int SIGSYS' `signal.h' (Unix): *Note Program Error Signals::. `int SIGTERM' `signal.h' (ISO): *Note Termination Signals::. `int SIGTRAP' `signal.h' (BSD): *Note Program Error Signals::. `int SIGTSTP' `signal.h' (POSIX.1): *Note Job Control Signals::. `int SIGTTIN' `signal.h' (POSIX.1): *Note Job Control Signals::. `int SIGTTOU' `signal.h' (POSIX.1): *Note Job Control Signals::. `SIG_UNBLOCK' `signal.h' (POSIX.1): *Note Process Signal Mask::. `int SIGURG' `signal.h' (BSD): *Note Asynchronous I/O Signals::. `int SIGUSR1' `signal.h' (POSIX.1): *Note Miscellaneous Signals::. `int SIGUSR2' `signal.h' (POSIX.1): *Note Miscellaneous Signals::. `int sigvec (int SIGNUM, const struct sigvec *ACTION,struct sigvec *OLD-ACTION)' `signal.h' (BSD): *Note BSD Handler::. `int SIGVTALRM' `signal.h' (BSD): *Note Alarm Signals::. `int SIGWINCH' `signal.h' (BSD): *Note Miscellaneous Signals::. `int SIGXCPU' `signal.h' (BSD): *Note Operation Error Signals::. `int SIGXFSZ' `signal.h' (BSD): *Note Operation Error Signals::. `double sin (double X)' `math.h' (ISO): *Note Trig Functions::. `void sincos (double X, double *SINX, double *COSX)' `math.h' (GNU): *Note Trig Functions::. `void sincosf (float X, float *SINX, float *COSX)' `math.h' (GNU): *Note Trig Functions::. `void sincosl (long double X, long double *SINX, long double *COSX)' `math.h' (GNU): *Note Trig Functions::. `float sinf (float X)' `math.h' (ISO): *Note Trig Functions::. `double sinh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float sinhf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double sinhl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double sinl (long double X)' `math.h' (ISO): *Note Trig Functions::. `S_IREAD' `sys/stat.h' (BSD): *Note Permission Bits::. `S_IRGRP' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IROTH' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IRUSR' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IRWXG' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IRWXO' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IRWXU' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `int S_ISBLK (mode_t M)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_ISCHR (mode_t M)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_ISDIR (mode_t M)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_ISFIFO (mode_t M)' `sys/stat.h' (POSIX): *Note Testing File Type::. `S_ISGID' `sys/stat.h' (POSIX): *Note Permission Bits::. `int S_ISLNK (mode_t M)' `sys/stat.h' (GNU): *Note Testing File Type::. `int S_ISREG (mode_t M)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_ISSOCK (mode_t M)' `sys/stat.h' (GNU): *Note Testing File Type::. `S_ISUID' `sys/stat.h' (POSIX): *Note Permission Bits::. `S_ISVTX' `sys/stat.h' (BSD): *Note Permission Bits::. `S_IWGRP' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IWOTH' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IWRITE' `sys/stat.h' (BSD): *Note Permission Bits::. `S_IWUSR' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IXGRP' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IXOTH' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `S_IXUSR' `sys/stat.h' (POSIX.1): *Note Permission Bits::. `size_t' `stddef.h' (ISO): *Note Important Data Types::. `unsigned int sleep (unsigned int SECONDS)' `unistd.h' (POSIX.1): *Note Sleeping::. `int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...)' `stdio.h' (GNU): *Note Formatted Output Functions::. `SO_BROADCAST' `sys/socket.h' (BSD): *Note Socket-Level Options::. `int SOCK_DGRAM' `sys/socket.h' (BSD): *Note Communication Styles::. `int socket (int NAMESPACE, int STYLE, int PROTOCOL)' `sys/socket.h' (BSD): *Note Creating a Socket::. `int socketpair (int NAMESPACE, int STYLE, int PROTOCOL, int FILEDES[2])' `sys/socket.h' (BSD): *Note Socket Pairs::. `int SOCK_RAW' `sys/socket.h' (BSD): *Note Communication Styles::. `int SOCK_RDM' `sys/socket.h' (BSD): *Note Communication Styles::. `int SOCK_SEQPACKET' `sys/socket.h' (BSD): *Note Communication Styles::. `int SOCK_STREAM' `sys/socket.h' (BSD): *Note Communication Styles::. `SO_DEBUG' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_DONTROUTE' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_ERROR' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_KEEPALIVE' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_LINGER' `sys/socket.h' (BSD): *Note Socket-Level Options::. `int SOL_SOCKET' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_OOBINLINE' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_RCVBUF' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_REUSEADDR' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_SNDBUF' `sys/socket.h' (BSD): *Note Socket-Level Options::. `SO_STYLE' `sys/socket.h' (GNU): *Note Socket-Level Options::. `SO_TYPE' `sys/socket.h' (BSD): *Note Socket-Level Options::. `speed_t' `termios.h' (POSIX.1): *Note Line Speed::. `int sprintf (char *S, const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Output Functions::. `double sqrt (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `float sqrtf (float X)' `math.h' (ISO): *Note Exponents and Logarithms::. `long double sqrtl (long double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `void srand (unsigned int SEED)' `stdlib.h' (ISO): *Note ISO Random::. `void srand48 (long int SEEDVAL)' `stdlib.h' (SVID): *Note SVID Random::. `int srand48_r (long int SEEDVAL, struct drand48_data *BUFFER)' `stdlib.h' (GNU): *Note SVID Random::. `void srandom (unsigned int SEED)' `stdlib.h' (BSD): *Note BSD Random::. `int srandom_r (unsigned int SEED, struct random_data *BUF)' `stdlib.h' (GNU): *Note BSD Random::. `int sscanf (const char *S, const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Input Functions::. `sighandler_t ssignal (int SIGNUM, sighandler_t ACTION)' `signal.h' (SVID): *Note Basic Signal Handling::. `int SSIZE_MAX' `limits.h' (POSIX.1): *Note General Limits::. `ssize_t' `unistd.h' (POSIX.1): *Note I/O Primitives::. `stack_t' `signal.h' (XPG): *Note Signal Stack::. `int stat (const char *FILENAME, struct stat *BUF)' `sys/stat.h' (POSIX.1): *Note Reading Attributes::. `int stat64 (const char *FILENAME, struct stat64 *BUF)' `sys/stat.h' (Unix98): *Note Reading Attributes::. `FILE * stderr' `stdio.h' (ISO): *Note Standard Streams::. `STDERR_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `FILE * stdin' `stdio.h' (ISO): *Note Standard Streams::. `STDIN_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `FILE * stdout' `stdio.h' (ISO): *Note Standard Streams::. `STDOUT_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `int stime (time_t *NEWTIME)' `time.h' (SVID, XPG): *Note Simple Calendar Time::. `char * stpcpy (char *restrict TO, const char *restrict FROM)' `string.h' (Unknown origin): *Note Copying and Concatenation::. `char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `int strcasecmp (const char *S1, const char *S2)' `string.h' (BSD): *Note String/Array Comparison::. `char * strcasestr (const char *HAYSTACK, const char *NEEDLE)' `string.h' (GNU): *Note Search Functions::. `char * strcat (char *restrict TO, const char *restrict FROM)' `string.h' (ISO): *Note Copying and Concatenation::. `char * strchr (const char *STRING, int C)' `string.h' (ISO): *Note Search Functions::. `char * strchrnul (const char *STRING, int C)' `string.h' (GNU): *Note Search Functions::. `int strcmp (const char *S1, const char *S2)' `string.h' (ISO): *Note String/Array Comparison::. `int strcoll (const char *S1, const char *S2)' `string.h' (ISO): *Note Collation Functions::. `char * strcpy (char *restrict TO, const char *restrict FROM)' `string.h' (ISO): *Note Copying and Concatenation::. `size_t strcspn (const char *STRING, const char *STOPSET)' `string.h' (ISO): *Note Search Functions::. `char * strdup (const char *S)' `string.h' (SVID): *Note Copying and Concatenation::. `char * strdupa (const char *S)' `string.h' (GNU): *Note Copying and Concatenation::. `int STREAM_MAX' `limits.h' (POSIX.1): *Note General Limits::. `char * strerror (int ERRNUM)' `string.h' (ISO): *Note Error Messages::. `char * strerror_r (int ERRNUM, char *BUF, size_t N)' `string.h' (GNU): *Note Error Messages::. `char * strfry (char *STRING)' `string.h' (GNU): *Note strfry::. `size_t strftime (char *S, size_t SIZE, const char *TEMPLATE, const struct tm *BROKENTIME)' `time.h' (ISO): *Note Formatting Calendar Time::. `size_t strlen (const char *S)' `string.h' (ISO): *Note String Length::. `int strncasecmp (const char *S1, const char *S2, size_t N)' `string.h' (BSD): *Note String/Array Comparison::. `char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `int strncmp (const char *S1, const char *S2, size_t SIZE)' `string.h' (ISO): *Note String/Array Comparison::. `char * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `char * strndup (const char *S, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `char * strndupa (const char *S, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `size_t strnlen (const char *S, size_t MAXLEN)' `string.h' (GNU): *Note String Length::. `char * strpbrk (const char *STRING, const char *STOPSET)' `string.h' (ISO): *Note Search Functions::. `char * strptime (const char *S, const char *FMT, struct tm *TP)' `time.h' (XPG4): *Note Low-Level Time String Parsing::. `char * strrchr (const char *STRING, int C)' `string.h' (ISO): *Note Search Functions::. `char * strsep (char **STRING_PTR, const char *DELIMITER)' `string.h' (BSD): *Note Finding Tokens in a String::. `char * strsignal (int SIGNUM)' `string.h' (GNU): *Note Signal Messages::. `size_t strspn (const char *STRING, const char *SKIPSET)' `string.h' (ISO): *Note Search Functions::. `char * strstr (const char *HAYSTACK, const char *NEEDLE)' `string.h' (ISO): *Note Search Functions::. `double strtod (const char *restrict STRING, char **restrict TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `float strtof (const char *STRING, char **TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `intmax_t strtoimax (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `inttypes.h' (ISO): *Note Parsing of Integers::. `char * strtok (char *restrict NEWSTRING, const char *restrict DELIMITERS)' `string.h' (ISO): *Note Finding Tokens in a String::. `char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR)' `string.h' (POSIX): *Note Finding Tokens in a String::. `long int strtol (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `long double strtold (const char *STRING, char **TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `long long int strtoll (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `long long int strtoq (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (BSD): *Note Parsing of Integers::. `unsigned long int strtoul (const char *retrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `unsigned long long int strtoull (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `uintmax_t strtoumax (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `inttypes.h' (ISO): *Note Parsing of Integers::. `unsigned long long int strtouq (const char *restrict STRING, char **restrict TAILPTR, int BASE)' `stdlib.h' (BSD): *Note Parsing of Integers::. `struct aiocb' `aio.h' (POSIX.1b): *Note Asynchronous I/O::. `struct aiocb64' `aio.h' (POSIX.1b): *Note Asynchronous I/O::. `struct aioinit' `aio.h' (GNU): *Note Configuration of AIO::. `struct argp' `argp.h' (GNU): *Note Argp Parsers::. `struct argp_child' `argp.h' (GNU): *Note Argp Children::. `struct argp_option' `argp.h' (GNU): *Note Argp Option Vectors::. `struct argp_state' `argp.h' (GNU): *Note Argp Parsing State::. `struct dirent' `dirent.h' (POSIX.1): *Note Directory Entries::. `struct exit_status' `utmp.h' (SVID): *Note Manipulating the Database::. `struct flock' `fcntl.h' (POSIX.1): *Note File Locks::. `struct fstab' `fstab.h' (BSD): *Note fstab::. `struct FTW' `ftw.h' (XPG4.2): *Note Working with Directory Trees::. `struct __gconv_step' `gconv.h' (GNU): *Note glibc iconv Implementation::. `struct __gconv_step_data' `gconv.h' (GNU): *Note glibc iconv Implementation::. `struct group' `grp.h' (POSIX.1): *Note Group Data Structure::. `struct hostent' `netdb.h' (BSD): *Note Host Names::. `struct if_nameindex' `net/if.h' (IPv6 basic API): *Note Interface Naming::. `struct in6_addr' `netinet/in.h' (IPv6 basic API): *Note Host Address Data Type::. `struct in_addr' `netinet/in.h' (BSD): *Note Host Address Data Type::. `struct iovec' `sys/uio.h' (BSD): *Note Scatter-Gather::. `struct itimerval' `sys/time.h' (BSD): *Note Setting an Alarm::. `struct lconv' `locale.h' (ISO): *Note The Lame Way to Locale Data::. `struct linger' `sys/socket.h' (BSD): *Note Socket-Level Options::. `struct mallinfo' `malloc.h' (GNU): *Note Statistics of Malloc::. `struct mntent' `mntent.h' (BSD): *Note mtab::. `struct msghdr' `sys/socket.h' (BSD): *Note Receiving Datagrams::. `struct netent' `netdb.h' (BSD): *Note Networks Database::. `struct obstack' `obstack.h' (GNU): *Note Creating Obstacks::. `struct option' `getopt.h' (GNU): *Note Getopt Long Options::. `struct passwd' `pwd.h' (POSIX.1): *Note User Data Structure::. `struct printf_info' `printf.h' (GNU): *Note Conversion Specifier Options::. `struct protoent' `netdb.h' (BSD): *Note Protocols Database::. `struct random_data' `stdlib.h' (GNU): *Note BSD Random::. `struct rlimit' `sys/resource.h' (BSD): *Note Limits on Resources::. `struct rlimit64' `sys/resource.h' (Unix98): *Note Limits on Resources::. `struct rusage' `sys/resource.h' (BSD): *Note Resource Usage::. `struct sched_param' `sched.h' (POSIX): *Note Basic Scheduling Functions::. `struct servent' `netdb.h' (BSD): *Note Services Database::. `struct sgttyb' `termios.h' (BSD): *Note BSD Terminal Modes::. `struct sigaction' `signal.h' (POSIX.1): *Note Advanced Signal Handling::. `struct sigstack' `signal.h' (BSD): *Note Signal Stack::. `struct sigvec' `signal.h' (BSD): *Note BSD Handler::. `struct sockaddr' `sys/socket.h' (BSD): *Note Address Formats::. `struct sockaddr_in' `netinet/in.h' (BSD): *Note Internet Address Formats::. `struct sockaddr_un' `sys/un.h' (BSD): *Note Local Namespace Details::. `struct stat' `sys/stat.h' (POSIX.1): *Note Attribute Meanings::. `struct stat64' `sys/stat.h' (LFS): *Note Attribute Meanings::. `struct termios' `termios.h' (POSIX.1): *Note Mode Data Types::. `struct timespec' `sys/time.h' (POSIX.1): *Note Elapsed Time::. `struct timeval' `sys/time.h' (BSD): *Note Elapsed Time::. `struct timezone' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `struct tm' `time.h' (ISO): *Note Broken-down Time::. `struct tms' `sys/times.h' (POSIX.1): *Note Processor Time::. `struct utimbuf' `time.h' (POSIX.1): *Note File Times::. `struct utsname' `sys/utsname.h' (POSIX.1): *Note Platform Type::. `int strverscmp (const char *S1, const char *S2)' `string.h' (GNU): *Note String/Array Comparison::. `size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE)' `string.h' (ISO): *Note Collation Functions::. `int stty (int FILEDES, struct sgttyb * attributes)' `sgtty.h' (BSD): *Note BSD Terminal Modes::. `int S_TYPEISMQ (struct stat *S)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_TYPEISSEM (struct stat *S)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int S_TYPEISSHM (struct stat *S)' `sys/stat.h' (POSIX): *Note Testing File Type::. `int SUN_LEN (_struct sockaddr_un *_ PTR)' `sys/un.h' (BSD): *Note Local Namespace Details::. `_SVID_SOURCE' (GNU): *Note Feature Test Macros::. `int SV_INTERRUPT' `signal.h' (BSD): *Note BSD Handler::. `int SV_ONSTACK' `signal.h' (BSD): *Note BSD Handler::. `int SV_RESETHAND' `signal.h' (Sun): *Note BSD Handler::. `int swapcontext (ucontext_t *restrict OUCP, const ucontext_t *restrict UCP)' `ucontext.h' (SVID): *Note System V contexts::. `int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...)' `wchar.h' (GNU): *Note Formatted Output Functions::. `int swscanf (const wchar_t *WS, const char *TEMPLATE, ...)' `wchar.h' (ISO): *Note Formatted Input Functions::. `int symlink (const char *OLDNAME, const char *NEWNAME)' `unistd.h' (BSD): *Note Symbolic Links::. `SYMLINK_MAX' `limits.h' (POSIX.1): *Note File Minimums::. `int sync (void)' `unistd.h' (X/Open): *Note Synchronizing I/O::. `long int syscall (long int SYSNO, ...)' `unistd.h' (???): *Note System Calls::. `long int sysconf (int PARAMETER)' `unistd.h' (POSIX.1): *Note Sysconf Definition::. `int sysctl (int *NAMES, int NLEN, void *OLDVAL,' `sysctl.h' (BSD): *Note System Parameters::. `void syslog (int FACILITY_PRIORITY, char *FORMAT, ...)' `syslog.h' (BSD): *Note syslog; vsyslog::. `int system (const char *COMMAND)' `stdlib.h' (ISO): *Note Running a Command::. `sighandler_t sysv_signal (int SIGNUM, sighandler_t ACTION)' `signal.h' (GNU): *Note Basic Signal Handling::. `double tan (double X)' `math.h' (ISO): *Note Trig Functions::. `float tanf (float X)' `math.h' (ISO): *Note Trig Functions::. `double tanh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `float tanhf (float X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double tanhl (long double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `long double tanl (long double X)' `math.h' (ISO): *Note Trig Functions::. `int tcdrain (int FILEDES)' `termios.h' (POSIX.1): *Note Line Control::. `tcflag_t' `termios.h' (POSIX.1): *Note Mode Data Types::. `int tcflow (int FILEDES, int ACTION)' `termios.h' (POSIX.1): *Note Line Control::. `int tcflush (int FILEDES, int QUEUE)' `termios.h' (POSIX.1): *Note Line Control::. `int tcgetattr (int FILEDES, struct termios *TERMIOS-P)' `termios.h' (POSIX.1): *Note Mode Functions::. `pid_t tcgetpgrp (int FILEDES)' `unistd.h' (POSIX.1): *Note Terminal Access Functions::. `pid_t tcgetsid (int FILDES)' `termios.h' (Unix98): *Note Terminal Access Functions::. `TCSADRAIN' `termios.h' (POSIX.1): *Note Mode Functions::. `TCSAFLUSH' `termios.h' (POSIX.1): *Note Mode Functions::. `TCSANOW' `termios.h' (POSIX.1): *Note Mode Functions::. `TCSASOFT' `termios.h' (BSD): *Note Mode Functions::. `int tcsendbreak (int FILEDES, int DURATION)' `termios.h' (POSIX.1): *Note Line Control::. `int tcsetattr (int FILEDES, int WHEN, const struct termios *TERMIOS-P)' `termios.h' (POSIX.1): *Note Mode Functions::. `int tcsetpgrp (int FILEDES, pid_t PGID)' `unistd.h' (POSIX.1): *Note Terminal Access Functions::. `void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)' `search.h' (SVID): *Note Tree Search Function::. `void tdestroy (void *VROOT, __free_fn_t FREEFCT)' `search.h' (GNU): *Note Tree Search Function::. `off_t telldir (DIR *DIRSTREAM)' `dirent.h' (BSD): *Note Random Access Directory::. `TEMP_FAILURE_RETRY (EXPRESSION)' `unistd.h' (GNU): *Note Interrupted Primitives::. `char * tempnam (const char *DIR, const char *PREFIX)' `stdio.h' (SVID): *Note Temporary Files::. `char * textdomain (const char *DOMAINNAME)' `libintl.h' (GNU): *Note Locating gettext catalog::. `void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR)' `search.h' (SVID): *Note Tree Search Function::. `double tgamma (double X)' `math.h' (XPG, ISO): *Note Special Functions::. `float tgammaf (float X)' `math.h' (XPG, ISO): *Note Special Functions::. `long double tgammal (long double X)' `math.h' (XPG, ISO): *Note Special Functions::. `time_t time (time_t *RESULT)' `time.h' (ISO): *Note Simple Calendar Time::. `time_t timegm (struct tm *BROKENTIME)' `time.h' (???): *Note Broken-down Time::. `time_t timelocal (struct tm *BROKENTIME)' `time.h' (???): *Note Broken-down Time::. `clock_t times (struct tms *BUFFER)' `sys/times.h' (POSIX.1): *Note Processor Time::. `time_t' `time.h' (ISO): *Note Simple Calendar Time::. `long int timezone' `time.h' (SVID): *Note Time Zone Functions::. `FILE * tmpfile (void)' `stdio.h' (ISO): *Note Temporary Files::. `FILE * tmpfile64 (void)' `stdio.h' (Unix98): *Note Temporary Files::. `int TMP_MAX' `stdio.h' (ISO): *Note Temporary Files::. `char * tmpnam (char *RESULT)' `stdio.h' (ISO): *Note Temporary Files::. `char * tmpnam_r (char *RESULT)' `stdio.h' (GNU): *Note Temporary Files::. `int toascii (int C)' `ctype.h' (SVID, BSD): *Note Case Conversion::. `int _tolower (int C)' `ctype.h' (SVID): *Note Case Conversion::. `int tolower (int C)' `ctype.h' (ISO): *Note Case Conversion::. `tcflag_t TOSTOP' `termios.h' (POSIX.1): *Note Local Modes::. `int _toupper (int C)' `ctype.h' (SVID): *Note Case Conversion::. `int toupper (int C)' `ctype.h' (ISO): *Note Case Conversion::. `wint_t towctrans (wint_t WC, wctrans_t DESC)' `wctype.h' (ISO): *Note Wide Character Case Conversion::. `wint_t towlower (wint_t WC)' `wctype.h' (ISO): *Note Wide Character Case Conversion::. `wint_t towupper (wint_t WC)' `wctype.h' (ISO): *Note Wide Character Case Conversion::. `double trunc (double X)' `math.h' (ISO): *Note Rounding Functions::. `int truncate (const char *FILENAME, off_t LENGTH)' `unistd.h' (X/Open): *Note File Size::. `int truncate64 (const char *NAME, off64_t LENGTH)' `unistd.h' (Unix98): *Note File Size::. `float truncf (float X)' `math.h' (ISO): *Note Rounding Functions::. `long double truncl (long double X)' `math.h' (ISO): *Note Rounding Functions::. `TRY_AGAIN' `netdb.h' (BSD): *Note Host Names::. `void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR)' `search.h' (SVID): *Note Tree Search Function::. `char * ttyname (int FILEDES)' `unistd.h' (POSIX.1): *Note Is It a Terminal::. `int ttyname_r (int FILEDES, char *BUF, size_t LEN)' `unistd.h' (POSIX.1): *Note Is It a Terminal::. `void twalk (const void *ROOT, __action_fn_t ACTION)' `search.h' (SVID): *Note Tree Search Function::. `char * tzname [2]' `time.h' (POSIX.1): *Note Time Zone Functions::. `int TZNAME_MAX' `limits.h' (POSIX.1): *Note General Limits::. `void tzset (void)' `time.h' (POSIX.1): *Note Time Zone Functions::. `UCHAR_MAX' `limits.h' (ISO): *Note Range of Type::. `ucontext_t' `ucontext.h' (SVID): *Note System V contexts::. `uid_t' `sys/types.h' (POSIX.1): *Note Reading Persona::. `UINT_MAX' `limits.h' (ISO): *Note Range of Type::. `int ulimit (int CMD, ...)' `ulimit.h' (BSD): *Note Limits on Resources::. `ULONG_LONG_MAX' `limits.h' (ISO): *Note Range of Type::. `ULONG_MAX' `limits.h' (ISO): *Note Range of Type::. `mode_t umask (mode_t MASK)' `sys/stat.h' (POSIX.1): *Note Setting Permissions::. `int umount (const char *FILE)' `sys/mount.h' (SVID, GNU): *Note Mount-Unmount-Remount::. `int umount2 (const char *FILE, int FLAGS)' `sys/mount.h' (GNU): *Note Mount-Unmount-Remount::. `int uname (struct utsname *INFO)' `sys/utsname.h' (POSIX.1): *Note Platform Type::. `int ungetc (int C, FILE *STREAM)' `stdio.h' (ISO): *Note How Unread::. `wint_t ungetwc (wint_t WC, FILE *STREAM)' `wchar.h' (ISO): *Note How Unread::. `union wait' `sys/wait.h' (BSD): *Note BSD Wait Functions::. `int unlink (const char *FILENAME)' `unistd.h' (POSIX.1): *Note Deleting Files::. `int unlockpt (int FILEDES)' `stdlib.h' (SVID, XPG4.2): *Note Allocation::. `int unsetenv (const char *NAME)' `stdlib.h' (BSD): *Note Environment Access::. `void updwtmp (const char *WTMP_FILE, const struct utmp *UTMP)' `utmp.h' (SVID): *Note Manipulating the Database::. `USER_PROCESS' `utmp.h' (SVID): *Note Manipulating the Database::. `USER_PROCESS' `utmpx.h' (XPG4.2): *Note XPG Functions::. `USHRT_MAX' `limits.h' (ISO): *Note Range of Type::. `int utime (const char *FILENAME, const struct utimbuf *TIMES)' `time.h' (POSIX.1): *Note File Times::. `int utimes (const char *FILENAME, struct timeval TVP[2])' `sys/time.h' (BSD): *Note File Times::. `int utmpname (const char *FILE)' `utmp.h' (SVID): *Note Manipulating the Database::. `int utmpxname (const char *FILE)' `utmpx.h' (XPG4.2): *Note XPG Functions::. `va_alist' `varargs.h' (Unix): *Note Old Varargs::. `TYPE va_arg (va_list AP, TYPE)' `stdarg.h' (ISO): *Note Argument Macros::. `void __va_copy (va_list DEST, va_list SRC)' `stdarg.h' (GNU): *Note Argument Macros::. `va_dcl' `varargs.h' (Unix): *Note Old Varargs::. `void va_end (va_list AP)' `stdarg.h' (ISO): *Note Argument Macros::. `va_list' `stdarg.h' (ISO): *Note Argument Macros::. `void * valloc (size_t SIZE)' `malloc.h', `stdlib.h' (BSD): *Note Aligned Memory Blocks::. `int vasprintf (char **PTR, const char *TEMPLATE, va_list AP)' `stdio.h' (GNU): *Note Variable Arguments Output::. `void va_start (va_list AP)' `varargs.h' (Unix): *Note Old Varargs::. `void va_start (va_list AP, LAST-REQUIRED)' `stdarg.h' (ISO): *Note Argument Macros::. `int VDISCARD' `termios.h' (BSD): *Note Other Special::. `int VDSUSP' `termios.h' (BSD): *Note Signal Characters::. `int VEOF' `termios.h' (POSIX.1): *Note Editing Characters::. `int VEOL' `termios.h' (POSIX.1): *Note Editing Characters::. `int VEOL2' `termios.h' (BSD): *Note Editing Characters::. `int VERASE' `termios.h' (POSIX.1): *Note Editing Characters::. `void verr (int STATUS, const char *FORMAT, va_list)' `err.h' (BSD): *Note Error Messages::. `void verrx (int STATUS, const char *FORMAT, va_list)' `err.h' (BSD): *Note Error Messages::. `int versionsort (const void *A, const void *B)' `dirent.h' (GNU): *Note Scanning Directory Content::. `int versionsort64 (const void *A, const void *B)' `dirent.h' (GNU): *Note Scanning Directory Content::. `pid_t vfork (void)' `unistd.h' (BSD): *Note Creating a Process::. `int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Output::. `int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Input::. `int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (ISO): *Note Variable Arguments Output::. `int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (ISO): *Note Variable Arguments Input::. `int VINTR' `termios.h' (POSIX.1): *Note Signal Characters::. `int VKILL' `termios.h' (POSIX.1): *Note Editing Characters::. `int vlimit (int RESOURCE, int LIMIT)' `sys/vlimit.h' (BSD): *Note Limits on Resources::. `int VLNEXT' `termios.h' (BSD): *Note Other Special::. `int VMIN' `termios.h' (POSIX.1): *Note Noncanonical Input::. `int vprintf (const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Output::. `int VQUIT' `termios.h' (POSIX.1): *Note Signal Characters::. `int VREPRINT' `termios.h' (BSD): *Note Editing Characters::. `int vscanf (const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Input::. `int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP)' `stdio.h' (GNU): *Note Variable Arguments Output::. `int vsprintf (char *S, const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Output::. `int vsscanf (const char *S, const char *TEMPLATE, va_list AP)' `stdio.h' (ISO): *Note Variable Arguments Input::. `int VSTART' `termios.h' (POSIX.1): *Note Start/Stop Characters::. `int VSTATUS' `termios.h' (BSD): *Note Other Special::. `int VSTOP' `termios.h' (POSIX.1): *Note Start/Stop Characters::. `int VSUSP' `termios.h' (POSIX.1): *Note Signal Characters::. `int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (GNU): *Note Variable Arguments Output::. `int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (ISO): *Note Variable Arguments Input::. `void vsyslog (int FACILITY_PRIORITY, char *FORMAT, va_list arglist)' `syslog.h' (BSD): *Note syslog; vsyslog::. `int VTIME' `termios.h' (POSIX.1): *Note Noncanonical Input::. `int vtimes (struct vtimes CURRENT, struct vtimes CHILD)' `vtimes.h' (vtimes.h): *Note Resource Usage::. `void vwarn (const char *FORMAT, va_list)' `err.h' (BSD): *Note Error Messages::. `void vwarnx (const char *FORMAT, va_list)' `err.h' (BSD): *Note Error Messages::. `int VWERASE' `termios.h' (BSD): *Note Editing Characters::. `int vwprintf (const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (ISO): *Note Variable Arguments Output::. `int vwscanf (const wchar_t *TEMPLATE, va_list AP)' `wchar.h' (ISO): *Note Variable Arguments Input::. `pid_t wait (int *STATUS-PTR)' `sys/wait.h' (POSIX.1): *Note Process Completion::. `pid_t wait3 (union wait *STATUS-PTR, int OPTIONS, struct rusage *USAGE)' `sys/wait.h' (BSD): *Note BSD Wait Functions::. `pid_t wait4 (pid_t PID, int *STATUS-PTR, int OPTIONS, struct rusage *USAGE)' `sys/wait.h' (BSD): *Note Process Completion::. `pid_t waitpid (pid_t PID, int *STATUS-PTR, int OPTIONS)' `sys/wait.h' (POSIX.1): *Note Process Completion::. `void warn (const char *FORMAT, ...)' `err.h' (BSD): *Note Error Messages::. `void warnx (const char *FORMAT, ...)' `err.h' (BSD): *Note Error Messages::. `WCHAR_MAX' `limits.h' (GNU): *Note Range of Type::. `wint_t WCHAR_MAX' `wchar.h' (ISO): *Note Extended Char Intro::. `wint_t WCHAR_MIN' `wchar.h' (ISO): *Note Extended Char Intro::. `wchar_t' `stddef.h' (ISO): *Note Extended Char Intro::. `int WCOREDUMP (int STATUS)' `sys/wait.h' (BSD): *Note Process Completion Status::. `wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)' `wchar.h' (GNU): *Note Copying and Concatenation::. `wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)' `wchar.h' (GNU): *Note Copying and Concatenation::. `size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t *restrict PS)' `wchar.h' (ISO): *Note Converting a Character::. `int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2)' `wchar.h' (GNU): *Note String/Array Comparison::. `wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM)' `wchar.h' (ISO): *Note Copying and Concatenation::. `wchar_t * wcschr (const wchar_t *WSTRING, int WC)' `wchar.h' (ISO): *Note Search Functions::. `wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC)' `wchar.h' (GNU): *Note Search Functions::. `int wcscmp (const wchar_t *WS1, const wchar_t *WS2)' `wchar.h' (ISO): *Note String/Array Comparison::. `int wcscoll (const wchar_t *WS1, const wchar_t *WS2)' `wchar.h' (ISO): *Note Collation Functions::. `wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM)' `wchar.h' (ISO): *Note Copying and Concatenation::. `size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET)' `wchar.h' (ISO): *Note Search Functions::. `wchar_t * wcsdup (const wchar_t *WS)' `wchar.h' (GNU): *Note Copying and Concatenation::. `size_t wcsftime (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, const struct tm *BROKENTIME)' `time.h' (ISO/Amend1): *Note Formatting Calendar Time::. `size_t wcslen (const wchar_t *WS)' `wchar.h' (ISO): *Note String Length::. `int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2, size_t N)' `wchar.h' (GNU): *Note String/Array Comparison::. `wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)' `wchar.h' (ISO): *Note Copying and Concatenation::. `int wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE)' `wchar.h' (ISO): *Note String/Array Comparison::. `wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)' `wchar.h' (ISO): *Note Copying and Concatenation::. `size_t wcsnlen (const wchar_t *WS, size_t MAXLEN)' `wchar.h' (GNU): *Note String Length::. `size_t wcsnrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict PS)' `wchar.h' (GNU): *Note Converting Strings::. `wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET)' `wchar.h' (ISO): *Note Search Functions::. `wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C)' `wchar.h' (ISO): *Note Search Functions::. `size_t wcsrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t LEN, mbstate_t *restrict PS)' `wchar.h' (ISO): *Note Converting Strings::. `size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET)' `wchar.h' (ISO): *Note Search Functions::. `wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)' `wchar.h' (ISO): *Note Search Functions::. `double wcstod (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR)' `wchar.h' (ISO): *Note Parsing of Floats::. `float wcstof (const wchar_t *STRING, wchar_t **TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `intmax_t wcstoimax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS)' `wchar.h' (ISO): *Note Finding Tokens in a String::. `long int wcstol (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `long double wcstold (const wchar_t *STRING, wchar_t **TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `long long int wcstoll (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `size_t wcstombs (char *STRING, const wchar_t *WSTRING, size_t SIZE)' `stdlib.h' (ISO): *Note Non-reentrant String Conversion::. `long long int wcstoq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (GNU): *Note Parsing of Integers::. `unsigned long int wcstoul (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `unsigned long long int wcstoull (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `uintmax_t wcstoumax (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (ISO): *Note Parsing of Integers::. `unsigned long long int wcstouq (const wchar_t *restrict STRING, wchar_t **restrict TAILPTR, int BASE)' `wchar.h' (GNU): *Note Parsing of Integers::. `wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t *NEEDLE)' `wchar.h' (XPG): *Note Search Functions::. `size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t *WFROM, size_t SIZE)' `wchar.h' (ISO): *Note Collation Functions::. `int wctob (wint_t C)' `wchar.h' (ISO): *Note Converting a Character::. `int wctomb (char *STRING, wchar_t WCHAR)' `stdlib.h' (ISO): *Note Non-reentrant Character Conversion::. `wctrans_t wctrans (const char *PROPERTY)' `wctype.h' (ISO): *Note Wide Character Case Conversion::. `wctrans_t' `wctype.h' (ISO): *Note Wide Character Case Conversion::. `wctype_t wctype (const char *PROPERTY)' `wctype.h' (ISO): *Note Classification of Wide Characters::. `wctype_t' `wctype.h' (ISO): *Note Classification of Wide Characters::. `int WEOF' `wchar.h' (ISO): *Note EOF and Errors::. `wint_t WEOF' `wchar.h' (ISO): *Note Extended Char Intro::. `int WEXITSTATUS (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `int WIFEXITED (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `int WIFSIGNALED (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `int WIFSTOPPED (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `wint_t' `wchar.h' (ISO): *Note Extended Char Intro::. `wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC, size_t SIZE)' `wchar.h' (ISO): *Note Search Functions::. `int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE)' `wcjar.h' (ISO): *Note String/Array Comparison::. `wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t *restruct WFROM, size_t SIZE)' `wchar.h' (ISO): *Note Copying and Concatenation::. `wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM, size_t SIZE)' `wchar.h' (ISO): *Note Copying and Concatenation::. `wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE)' `wchar.h' (GNU): *Note Copying and Concatenation::. `wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t SIZE)' `wchar.h' (ISO): *Note Copying and Concatenation::. `int W_OK' `unistd.h' (POSIX.1): *Note Testing File Access::. `int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS)' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `wordexp_t' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `void wordfree (wordexp_t *WORD-VECTOR-PTR)' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `int wprintf (const wchar_t *TEMPLATE, ...)' `wchar.h' (ISO): *Note Formatted Output Functions::. `WRDE_APPEND' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `WRDE_BADCHAR' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `WRDE_BADVAL' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `WRDE_CMDSUB' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `WRDE_DOOFFS' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `WRDE_NOCMD' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `WRDE_NOSPACE' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `WRDE_REUSE' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `WRDE_SHOWERR' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `WRDE_SYNTAX' `wordexp.h' (POSIX.2): *Note Calling Wordexp::. `WRDE_UNDEF' `wordexp.h' (POSIX.2): *Note Flags for Wordexp::. `ssize_t write (int FILEDES, const void *BUFFER, size_t SIZE)' `unistd.h' (POSIX.1): *Note I/O Primitives::. `ssize_t writev (int FILEDES, const struct iovec *VECTOR, int COUNT)' `sys/uio.h' (BSD): *Note Scatter-Gather::. `int wscanf (const wchar_t *TEMPLATE, ...)' `wchar.h' (ISO): *Note Formatted Input Functions::. `int WSTOPSIG (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `int WTERMSIG (int STATUS)' `sys/wait.h' (POSIX.1): *Note Process Completion Status::. `int X_OK' `unistd.h' (POSIX.1): *Note Testing File Access::. `_XOPEN_SOURCE' (X/Open): *Note Feature Test Macros::. `_XOPEN_SOURCE_EXTENDED' (X/Open): *Note Feature Test Macros::. `double y0 (double X)' `math.h' (SVID): *Note Special Functions::. `float y0f (float X)' `math.h' (SVID): *Note Special Functions::. `long double y0l (long double X)' `math.h' (SVID): *Note Special Functions::. `double y1 (double X)' `math.h' (SVID): *Note Special Functions::. `float y1f (float X)' `math.h' (SVID): *Note Special Functions::. `long double y1l (long double X)' `math.h' (SVID): *Note Special Functions::. `double yn (int n, double X)' `math.h' (SVID): *Note Special Functions::. `float ynf (int n, float X)' `math.h' (SVID): *Note Special Functions::. `long double ynl (int n, long double X)' `math.h' (SVID): *Note Special Functions::. Installing the GNU C Library **************************** Before you do anything else, you should read the file `FAQ' found at the top level of the source tree. This file answers common questions and describes problems you may experience with compilation and installation. It is updated more frequently than this manual. Features can be added to GNU Libc via "add-on" bundles. These are separate tarfiles which you unpack into the top level of the source tree. Then you give `configure' the `--enable-add-ons' option to activate them, and they will be compiled into the library. As of the 2.2 release, one important component of glibc is distributed as "official" add-ons: the linuxthreads add-on. Unless you are doing an unusual installation, you should get this. Support for POSIX threads is maintained by someone else, so it's in a separate package. It is only available for Linux systems, but this will change in the future. Get it from the same place you got the main bundle; the file is `glibc-linuxthreads-VERSION.tar.gz'. You will need recent versions of several GNU tools: definitely GCC and GNU Make, and possibly others. *Note Tools for Compilation::, below. Configuring and compiling GNU Libc ================================== GNU libc can be compiled in the source directory, but we strongly advise to build it in a separate build directory. For example, if you have unpacked the glibc sources in `/src/gnu/glibc-2.2.0', create a directory `/src/gnu/glibc-build' to put the object files in. This allows removing the whole build directory in case an error occurs, which is the safest way to get a fresh start and should always be done. From your object directory, run the shell script `configure' found at the top level of the source tree. In the scenario above, you'd type $ ../glibc-2.2.0/configure ARGS... Please note that even if you're building in a separate build directory, the compilation needs to modify a few files in the source directory, especially some files in the manual subdirectory. `configure' takes many options, but you can get away with knowing only two: `--prefix' and `--enable-add-ons'. The `--prefix' option tells configure where you want glibc installed. This defaults to `/usr/local'. The `--enable-add-ons' option tells configure to use all the add-on bundles it finds in the source directory. Since important functionality is provided in add-ons, you should always specify this option. It may also be useful to set the CC and CFLAGS variables in the environment when running `configure'. CC selects the C compiler that will be used, and CFLAGS sets optimization options for the compiler. The following list describes all of the available options for `configure': `--prefix=DIRECTORY' Install machine-independent data files in subdirectories of `DIRECTORY'. The default is to install in `/usr/local'. `--exec-prefix=DIRECTORY' Install the library and other machine-dependent files in subdirectories of `DIRECTORY'. The default is to the `--prefix' directory if that option is specified, or `/usr/local' otherwise. `--with-headers=DIRECTORY' Look for kernel header files in DIRECTORY, not `/usr/include'. Glibc needs information from the kernel's private header files. It will normally look in `/usr/include' for them, but if you specify this option, it will look in DIRECTORY instead. This option is primarily of use on a system where the headers in `/usr/include' come from an older version of glibc. Conflicts can occasionally happen in this case. Note that Linux libc5 qualifies as an older version of glibc. You can also use this option if you want to compile glibc with a newer set of kernel headers than the ones found in `/usr/include'. `--enable-add-ons[=LIST]' Enable add-on packages in your source tree. If this option is specified with no list, it enables all the add-on packages it finds. If you do not wish to use some add-on package that you have present in your source tree, give this option a list of the add-ons that you _do_ want used, like this: `--enable-add-ons=linuxthreads' `--enable-kernel=VERSION' This option is currently only useful on Linux systems. The VERSION parameter should have the form X.Y.Z and describes the smallest version of the Linux kernel the generated library is expected to support. The higher the VERSION number is, the less compatibility code is added, and the faster the code gets. `--with-binutils=DIRECTORY' Use the binutils (assembler and linker) in `DIRECTORY', not the ones the C compiler would default to. You could use this option if the default binutils on your system cannot deal with all the constructs in the GNU C library. In that case, `configure' will detect the problem and suppress these constructs, so that the library will still be usable, but functionality may be lost--for example, you can't build a shared libc with old binutils. `--without-fp' Use this option if your computer lacks hardware floating-point support and your operating system does not emulate an FPU. `--disable-shared' Don't build shared libraries even if it is possible. Not all systems support shared libraries; you need ELF support and (currently) the GNU linker. `--disable-profile' Don't build libraries with profiling information. You may want to use this option if you don't plan to do profiling. `--enable-omitfp' Use maximum optimization for the normal (static and shared) libraries, and compile separate static libraries with debugging information and no optimization. We recommend against this. The extra optimization doesn't gain you much, it may provoke compiler bugs, and you won't be able to trace bugs through the C library. `--disable-versioning' Don't compile the shared libraries with symbol version information. Doing this will make the resulting library incompatible with old binaries, so it's not recommended. `--enable-static-nss' Compile static versions of the NSS (Name Service Switch) libraries. This is not recommended because it defeats the purpose of NSS; a program linked statically with the NSS libraries cannot be dynamically reconfigured to use a different name database. `--build=BUILD-SYSTEM' `--host=HOST-SYSTEM' These options are for cross-compiling. If you specify both options and BUILD-SYSTEM is different from HOST-SYSTEM, `configure' will prepare to cross-compile glibc from BUILD-SYSTEM to be used on HOST-SYSTEM. You'll probably need the `--with-headers' option too, and you may have to override CONFIGURE's selection of the compiler and/or binutils. If you only specify `--host', configure will prepare for a native compile but use what you specify instead of guessing what your system is. This is most useful to change the CPU submodel. For example, if configure guesses your machine as `i586-pc-linux-gnu' but you want to compile a library for 386es, give `--host=i386-pc-linux-gnu' or just `--host=i386-linux' and add the appropriate compiler flags (`-mcpu=i386' will do the trick) to CFLAGS. If you specify just `--build', configure will get confused. To build the library and related programs, type `make'. This will produce a lot of output, some of which may look like errors from `make' but isn't. Look for error messages from `make' containing `***'. Those indicate that something is really wrong. The compilation process takes several hours even on fast hardware. Expect at least two hours for the default configuration on i586 for Linux. For Hurd times are much longer. Except for EGCS 1.1 and GCC 2.95 (and later versions of GCC), all supported versions of GCC have a problem which causes them to take several minutes to compile certain files in the iconvdata directory. Do not panic if the compiler appears to hang. If you want to run a parallel make, you can't just give `make' the `-j' option, because it won't be passed down to the sub-makes. Instead, edit the generated `Makefile' and uncomment the line # PARALLELMFLAGS = -j 4 You can change the `4' to some other number as appropriate for your system. Instead of changing the `Makefile', you could give this option directly to `make' and call it as, for example, `make PARALLELMFLAGS=-j4'. If you're building in the source directory, you must use the latter approach since in this case no new `Makefile' is generated for you to change. To build and run test programs which exercise some of the library facilities, type `make check'. If it does not complete successfully, do not use the built library, and report a bug after verifying that the problem is not already known. *Note Reporting Bugs::, for instructions on reporting bugs. Note that some of the tests assume they are not being run by `root'. We recommend you compile and test glibc as an unprivileged user. To format the `GNU C Library Reference Manual' for printing, type `make dvi'. You need a working TeX installation to do this. The distribution already includes the on-line formatted version of the manual, as Info files. You can regenerate those with `make info', but it shouldn't be necessary. The library has a number of special-purpose configuration parameters which you can find in `Makeconfig'. These can be overwritten with the file `configparms'. To change them, create a `configparms' in your build directory and add values as appropriate for your system. The file is included and parsed by `make' and has to follow the conventions for makefiles. It is easy to configure the GNU C library for cross-compilation by setting a few variables in `configparms'. Set `CC' to the cross-compiler for the target you configured the library for; it is important to use this same `CC' value when running `configure', like this: `CC=TARGET-gcc configure TARGET'. Set `BUILD_CC' to the compiler to use for for programs run on the build system as part of compiling the library. You may need to set `AR' and `RANLIB' to cross-compiling versions of `ar' and `ranlib' if the native tools are not configured to work with object files for the target you configured for. Installing the C Library ======================== To install the library and its header files, and the Info files of the manual, type `env LANGUAGE=C LC_ALL=C make install'. This will build things if necessary, before installing them. However, you should still compile everything first. If you are installing glibc as your primary C library, we recommend that you shut the system down to single-user mode first, and reboot afterward. This minimizes the risk of breaking things when the library changes out from underneath. If you're upgrading from Linux libc5 or some other C library, you need to replace the `/usr/include' with a fresh directory before installing it. The new `/usr/include' should contain the Linux headers, but nothing else. You must first build the library (`make'), optionally check it (`make check'), switch the include directories and then install (`make install'). The steps must be done in this order. Not moving the directory before install will result in an unusable mixture of header files from both libraries, but configuring, building, and checking the library requires the ability to compile and run programs against the old library. If you are upgrading from a previous installation of glibc 2.0 or 2.1, `make install' will do the entire job. You do not need to remove the old includes - if you want to do so anyway you must then follow the order given above. You may also need to reconfigure GCC to work with the new library. The easiest way to do that is to figure out the compiler switches to make it work again (`-Wl,--dynamic-linker=/lib/ld-linux.so.2' should work on Linux systems) and use them to recompile gcc. You can also edit the specs file (`/usr/lib/gcc-lib/TARGET/VERSION/specs'), but that is a bit of a black art. You can install glibc somewhere other than where you configured it to go by setting the `install_root' variable on the command line for `make install'. The value of this variable is prepended to all the paths for installation. This is useful when setting up a chroot environment or preparing a binary distribution. The directory should be specified with an absolute file name. Glibc 2.2 includes a daemon called `nscd', which you may or may not want to run. `nscd' caches name service lookups; it can dramatically improve performance with NIS+, and may help with DNS as well. One auxiliary program, `/usr/libexec/pt_chown', is installed setuid `root'. This program is invoked by the `grantpt' function; it sets the permissions on a pseudoterminal so it can be used by the calling process. This means programs like `xterm' and `screen' do not have to be setuid to get a pty. (There may be other reasons why they need privileges.) If you are using a 2.1 or newer Linux kernel with the `devptsfs' or `devfs' filesystems providing pty slaves, you don't need this program; otherwise you do. The source for `pt_chown' is in `login/programs/pt_chown.c'. After installation you might want to configure the timezone and locale installation of your system. The GNU C library comes with a locale database which gets configured with `localedef'. For example, to set up a German locale with name `de_DE', simply issue the command `localedef -i de_DE -f ISO-8859-1 de_DE'. To configure all locales that are supported by glibc, you can issue from your build directory the command `make localedata/install-locales'. To configure the locally used timezone, you can either set the `TZ' environment variable. The script `tzselect' helps you to select the right value. As an example for Germany, tzselect would tell you to use `TZ='Europe/Berlin''. For a system wide installation (the given paths are for an installation with `--prefix=/usr'), link the timezone file which is in `/usr/share/zoneinfo' to the file `/etc/localtime'. For Germany, you might execute `ln -s /usr/share/zoneinfo/Europe/Berlin /etc/localtime'. Recommended Tools for Compilation ================================= We recommend installing the following GNU tools before attempting to build the GNU C library: * GNU `make' 3.79 or newer You need the latest version of GNU `make'. Modifying the GNU C Library to work with other `make' programs would be so difficult that we recommend you port GNU `make' instead. *Really.* We recommend version GNU `make' version 3.79. All earlier versions have severe bugs or lack features. * GCC 2.95 or newer The GNU C library can only be compiled with the GNU C compiler family. As of the 2.2 release, GCC 2.95.2 or higher is required. As of this writing, GCC 2.95.3 is the compiler we advise to use. You can use whatever compiler you like to compile programs that use GNU libc, but be aware that both GCC 2.7 and 2.8 have bugs in their floating-point support that may be triggered by the math library. For PPC you might need some patches even on top of the last GCC version. See the FAQ. * GNU `binutils' 2.10.1 or later You must use GNU binutils (as and ld) if you want to build a shared library. Even if you don't, we recommend you use them anyway. No one has tested compilation with non-GNU binutils in a long time. The quality of binutils releases has varied a bit recently. The bugs are in obscure features, but glibc uses quite a few of those. 2.10.1 and later releases are known to work. Versions after 2.8.1.0.23 may or may not work. Older versions definitely don't. For PPC you might need some patches even on top of the last binutils version. See the FAQ. * GNU `texinfo' 3.12f To correctly translate and install the Texinfo documentation you need this version of the `texinfo' package. Earlier versions do not understand all the tags used in the document, and the installation mechanism for the info files is not present or works differently. * GNU `awk' 3.0, or some other POSIX awk Awk is used in several places to generate files. The scripts should work with any POSIX-compliant awk implementation; `gawk' 3.0 and `mawk' 1.3 are known to work. * Perl 5 Perl is not required, but it is used if present to test the installation. We may decide to use it elsewhere in the future. * GNU `sed' 3.02 or newer Sed is used in several places to generate files. Most scripts work with any version of `sed'. The known exception is the script `po2test.sed' in the `intl' subdirectory which is used to generate `msgs.h' for the testsuite. This script works correctly only with GNU `sed' 3.02. If you like to run the testsuite, you should definitely upgrade `sed'. If you change any of the `configure.in' files you will also need * GNU `autoconf' 2.12 or higher and if you change any of the message translation files you will need * GNU `gettext' 0.10.36 or later You may also need these packages if you upgrade your source tree using patches, although we try to avoid this. Supported Configurations ======================== The GNU C Library currently supports configurations that match the following patterns: alpha*-*-linux arm-*-linux cris-*-linux hppa-*-linux iX86-*-gnu iX86-*-linux ia64-*-linux m68k-*-linux mips*-*-linux powerpc-*-linux s390-*-linux s390x-*-linux sparc-*-linux sparc64-*-linux Former releases of this library (version 2.1 and/or 2.0) used to run on the following configurations: arm-*-linuxaout arm-*-none Very early releases (version 1.09.1 and perhaps earlier versions) used to run on the following configurations: alpha-dec-osf1 alpha-*-linuxecoff iX86-*-bsd4.3 iX86-*-isc2.2 iX86-*-isc3.N iX86-*-sco3.2 iX86-*-sco3.2v4 iX86-*-sysv iX86-*-sysv4 iX86-force_cpu386-none iX86-sequent-bsd i960-nindy960-none m68k-hp-bsd4.3 m68k-mvme135-none m68k-mvme136-none m68k-sony-newsos3 m68k-sony-newsos4 m68k-sun-sunos4.N mips-dec-ultrix4.N mips-sgi-irix4.N sparc-sun-solaris2.N sparc-sun-sunos4.N Since no one has volunteered to test and fix these configurations, they are not supported at the moment. They probably don't compile; they definitely don't work anymore. Porting the library is not hard. If you are interested in doing a port, please contact the glibc maintainers by sending electronic mail to . Valid cases of `iX86' include `i386', `i486', `i586', and `i686'. All of those configurations produce a library that can run on this processor and newer processors. The GCC compiler by default generates code that's optimized for the machine it's configured for and will use the instructions available on that machine. For example if your GCC is configured for `i686', gcc will optimize for `i686' and might issue some `i686' specific instructions. To generate code for other models, you have to configure for that model and give GCC the appropriate `-march=' and `-mcpu=' compiler switches via CFLAGS. Specific advice for Linux systems ================================= If you are installing GNU libc on a Linux system, you need to have the header files from a 2.2 or newer kernel around for reference. For some architectures, like ia64, sh and hppa, you need at least headers from kernel 2.3.99 (sh and hppa) or 2.4.0 (ia64). You do not need to use that kernel, just have its headers where glibc can access at them. The easiest way to do this is to unpack it in a directory such as `/usr/src/linux-2.2.1'. In that directory, run `make config' and accept all the defaults. Then run `make include/linux/version.h'. Finally, configure glibc with the option `--with-headers=/usr/src/linux-2.2.1/include'. Use the most recent kernel you can get your hands on. An alternate tactic is to unpack the 2.2 kernel and run `make config' as above. Then rename or delete `/usr/include', create a new `/usr/include', and make the usual symbolic links of `/usr/include/linux' and `/usr/include/asm' into the 2.2 kernel sources. You can then configure glibc with no special options. This tactic is recommended if you are upgrading from libc5, since you need to get rid of the old header files anyway. Note that `/usr/include/net' and `/usr/include/scsi' should *not* be symlinks into the kernel sources. GNU libc provides its own versions of these files. Linux expects some components of the libc installation to be in `/lib' and some in `/usr/lib'. This is handled automatically if you configure glibc with `--prefix=/usr'. If you set some other prefix or allow it to default to `/usr/local', then all the components are installed there. If you are upgrading from libc5, you need to recompile every shared library on your system against the new library for the sake of new code, but keep the old libraries around for old binaries to use. This is complicated and difficult. Consult the Glibc2 HOWTO at for details. You cannot use `nscd' with 2.0 kernels, due to bugs in the kernel-side thread support. `nscd' happens to hit these bugs particularly hard, but you might have problems with any threaded program. Reporting Bugs ============== There are probably bugs in the GNU C library. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer. It is a good idea to verify that the problem has not already been reported. Bugs are documented in two places: The file `BUGS' describes a number of well known bugs and the bug tracking system has a WWW interface at . The WWW interface gives you access to open and closed reports. The closed reports normally include a patch or a hint on solving the problem. To report a bug, first you must find it. Hopefully, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong. It might not be the GNU library. Many historical Unix C libraries permit things that we don't, such as closing a file twice. If you think you have found some way in which the GNU C library does not conform to the ISO and POSIX standards (*note Standards and Portability::), that is definitely a bug. Report it! Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult. The final step when you have a simple test case is to report the bug. Do this using the `glibcbug' script. It is installed with libc, or if you haven't installed it, will be in your build directory. Send your test case, the results you got, the results you expected, and what you think the problem might be (if you've thought of anything). `glibcbug' will insert the configuration information we need to see, and ship the report off to . Don't send a message there directly; it is fed to a program that expects mail to be formatted in a particular way. Use the script. If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual. Report that too! If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement. If you find any errors or omissions in this manual, please report them to the Internet address . If you refer to specific sections of the manual, please include the section names for easier identification. Library Maintenance ******************* Adding New Functions ==================== The process of building the library is driven by the makefiles, which make heavy use of special features of GNU `make'. The makefiles are very complex, and you probably don't want to try to understand them. But what they do is fairly straightforward, and only requires that you define a few variables in the right places. The library sources are divided into subdirectories, grouped by topic. The `string' subdirectory has all the string-manipulation functions, `math' has all the mathematical functions, etc. Each subdirectory contains a simple makefile, called `Makefile', which defines a few `make' variables and then includes the global makefile `Rules' with a line like: include ../Rules The basic variables that a subdirectory makefile defines are: `subdir' The name of the subdirectory, for example `stdio'. This variable *must* be defined. `headers' The names of the header files in this section of the library, such as `stdio.h'. `routines' `aux' The names of the modules (source files) in this section of the library. These should be simple names, such as `strlen' (rather than complete file names, such as `strlen.c'). Use `routines' for modules that define functions in the library, and `aux' for auxiliary modules containing things like data definitions. But the values of `routines' and `aux' are just concatenated, so there really is no practical difference. `tests' The names of test programs for this section of the library. These should be simple names, such as `tester' (rather than complete file names, such as `tester.c'). `make tests' will build and run all the test programs. If a test program needs input, put the test data in a file called `TEST-PROGRAM.input'; it will be given to the test program on its standard input. If a test program wants to be run with arguments, put the arguments (all on a single line) in a file called `TEST-PROGRAM.args'. Test programs should exit with zero status when the test passes, and nonzero status when the test indicates a bug in the library or error in building. `others' The names of "other" programs associated with this section of the library. These are programs which are not tests per se, but are other small programs included with the library. They are built by `make others'. `install-lib' `install-data' `install' Files to be installed by `make install'. Files listed in `install-lib' are installed in the directory specified by `libdir' in `configparms' or `Makeconfig' (*note Installation::). Files listed in `install-data' are installed in the directory specified by `datadir' in `configparms' or `Makeconfig'. Files listed in `install' are installed in the directory specified by `bindir' in `configparms' or `Makeconfig'. `distribute' Other files from this subdirectory which should be put into a distribution tar file. You need not list here the makefile itself or the source and header files listed in the other standard variables. Only define `distribute' if there are files used in an unusual way that should go into the distribution. `generated' Files which are generated by `Makefile' in this subdirectory. These files will be removed by `make clean', and they will never go into a distribution. `extra-objs' Extra object files which are built by `Makefile' in this subdirectory. This should be a list of file names like `foo.o'; the files will actually be found in whatever directory object files are being built in. These files will be removed by `make clean'. This variable is used for secondary object files needed to build `others' or `tests'. Porting the GNU C Library ========================= The GNU C library is written to be easily portable to a variety of machines and operating systems. Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems. This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use. All the machine-dependent and operating system-dependent files in the library are in the subdirectory `sysdeps' under the top-level library source directory. This directory contains a hierarchy of subdirectories (*note Hierarchy Conventions::). Each subdirectory of `sysdeps' contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format). A configuration specifies an ordered list of these subdirectories. Each subdirectory implicitly appends its parent directory to the list. For example, specifying the list `unix/bsd/vax' is equivalent to specifying the list `unix/bsd/vax unix/bsd unix'. A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy. If the file `Implies' exists in a subdirectory, it lists other subdirectories of `sysdeps' which are appended to the list, appearing after the subdirectory containing the `Implies' file. Lines in an `Implies' file that begin with a `#' character are ignored as comments. For example, `unix/bsd/Implies' contains: # BSD has Internet-related things. unix/inet and `unix/Implies' contains: posix So the final list is `unix/bsd/vax unix/bsd unix/inet unix posix'. `sysdeps' has a "special" subdirectory called `generic'. It is always implicitly appended to the list of subdirectories, so you needn't put it in an `Implies' file, and you should not create any subdirectories under it intended to be new specific categories. `generic' serves two purposes. First, the makefiles do not bother to look for a system-dependent version of a file that's not in `generic'. This means that any system-dependent source file must have an analogue in `generic', even if the routines defined by that file are not implemented on other platforms. Second. the `generic' version of a system-dependent file is used if the makefiles do not find a version specific to the system you're compiling for. If it is possible to implement the routines in a `generic' file in machine-independent C, using only other machine-independent functions in the C library, then you should do so. Otherwise, make them stubs. A "stub" function is a function which cannot be implemented on a particular machine or operating system. Stub functions always return an error, and set `errno' to `ENOSYS' (Function not implemented). *Note Error Reporting::. If you define a stub function, you must place the statement `stub_warning(FUNCTION)', where FUNCTION is the name of your function, after its definition; also, you must include the file `' into your file. This causes the function to be listed in the installed `', and makes GNU ld warn when the function is used. Some rare functions are only useful on specific systems and aren't defined at all on others; these do not appear anywhere in the system-independent source code or makefiles (including the `generic' directory), only in the system-dependent `Makefile' in the specific system's subdirectory. If you come across a file that is in one of the main source directories (`string', `stdio', etc.), and you want to write a machine- or operating system-dependent version of it, move the file into `sysdeps/generic' and write your new implementation in the appropriate system-specific subdirectory. Note that if a file is to be system-dependent, it *must not* appear in one of the main source directories. There are a few special files that may exist in each subdirectory of `sysdeps': `Makefile' A makefile for this machine or operating system, or class of machine or operating system. This file is included by the library makefile `Makerules', which is used by the top-level makefile and the subdirectory makefiles. It can change the variables set in the including makefile or add new rules. It can use GNU `make' conditional directives based on the variable `subdir' (see above) to select different sets of variables and rules for different sections of the library. It can also set the `make' variable `sysdep-routines', to specify extra modules to be included in the library. You should use `sysdep-routines' rather than adding modules to `routines' because the latter is used in determining what to distribute for each subdirectory of the main source tree. Each makefile in a subdirectory in the ordered list of subdirectories to be searched is included in order. Since several system-dependent makefiles may be included, each should append to `sysdep-routines' rather than simply setting it: sysdep-routines := $(sysdep-routines) foo bar `Subdirs' This file contains the names of new whole subdirectories under the top-level library source tree that should be included for this system. These subdirectories are treated just like the system-independent subdirectories in the library source tree, such as `stdio' and `math'. Use this when there are completely new sets of functions and header files that should go into the library for the system this subdirectory of `sysdeps' implements. For example, `sysdeps/unix/inet/Subdirs' contains `inet'; the `inet' directory contains various network-oriented operations which only make sense to put in the library on systems that support the Internet. `Dist' This file contains the names of files (relative to the subdirectory of `sysdeps' in which it appears) which should be included in the distribution. List any new files used by rules in the `Makefile' in the same directory, or header files used by the source files in that directory. You don't need to list files that are implementations (either C or assembly source) of routines whose names are given in the machine-independent makefiles in the main source tree. `configure' This file is a shell script fragment to be run at configuration time. The top-level `configure' script uses the shell `.' command to read the `configure' file in each system-dependent directory chosen, in order. The `configure' files are often generated from `configure.in' files using Autoconf. A system-dependent `configure' script will usually add things to the shell variables `DEFS' and `config_vars'; see the top-level `configure' script for details. The script can check for `--with-PACKAGE' options that were passed to the top-level `configure'. For an option `--with-PACKAGE=VALUE' `configure' sets the shell variable `with_PACKAGE' (with any dashes in PACKAGE converted to underscores) to VALUE; if the option is just `--with-PACKAGE' (no argument), then it sets `with_PACKAGE' to `yes'. `configure.in' This file is an Autoconf input fragment to be processed into the file `configure' in this subdirectory. *Note Introduction: (autoconf.info)Introduction, for a description of Autoconf. You should write either `configure' or `configure.in', but not both. The first line of `configure.in' should invoke the `m4' macro `GLIBC_PROVIDES'. This macro does several `AC_PROVIDE' calls for Autoconf macros which are used by the top-level `configure' script; without this, those macros might be invoked again unnecessarily by Autoconf. That is the general system for how system-dependencies are isolated. Layout of the `sysdeps' Directory Hierarchy ------------------------------------------- A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. `configure' uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is _not_ passed to `configure', the directory `MACHINE/fpu' is also used. The operating system often has a "base operating system"; for example, if the operating system is `Linux', the base operating system is `unix/sysv'. The algorithm used to pick the list of directories is simple: `configure' makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `i686-linux-gnu' results in `unix/sysv/linux/i386/i686'. `configure' then tries removing each element of the list in turn, so `unix/sysv/linux' and `unix/sysv' are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical `irix6.2' and `irix6.3' directories, `configure' tries successively less specific operating system names by removing trailing suffixes starting with a period. As an example, here is the complete list of directories that would be tried for the configuration `i686-linux-gnu' (with the `crypt' and `linuxthreads' add-on): sysdeps/i386/elf crypt/sysdeps/unix linuxthreads/sysdeps/unix/sysv/linux linuxthreads/sysdeps/pthread linuxthreads/sysdeps/unix/sysv linuxthreads/sysdeps/unix linuxthreads/sysdeps/i386/i686 linuxthreads/sysdeps/i386 linuxthreads/sysdeps/pthread/no-cmpxchg sysdeps/unix/sysv/linux/i386 sysdeps/unix/sysv/linux sysdeps/gnu sysdeps/unix/common sysdeps/unix/mman sysdeps/unix/inet sysdeps/unix/sysv/i386/i686 sysdeps/unix/sysv/i386 sysdeps/unix/sysv sysdeps/unix/i386 sysdeps/unix sysdeps/posix sysdeps/i386/i686 sysdeps/i386/i486 sysdeps/libm-i387/i686 sysdeps/i386/fpu sysdeps/libm-i387 sysdeps/i386 sysdeps/wordsize-32 sysdeps/ieee754 sysdeps/libm-ieee754 sysdeps/generic Different machine architectures are conventionally subdirectories at the top level of the `sysdeps' directory tree. For example, `sysdeps/sparc' and `sysdeps/m68k'. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as `sysdeps/m68k/68020'. Code which is specific to the floating-point coprocessor used with a particular machine should go in `sysdeps/MACHINE/fpu'. There are a few directories at the top level of the `sysdeps' hierarchy that are not for particular machine architectures. `generic' As described above (*note Porting::), this is the subdirectory that every configuration implicitly uses after all others. `ieee754' This directory is for code using the IEEE 754 floating-point format, where the C type `float' is IEEE 754 single-precision format, and `double' is IEEE 754 double-precision format. Usually this directory is referred to in the `Implies' file in a machine architecture-specific directory, such as `m68k/Implies'. `libm-ieee754' This directory contains an implementation of a mathematical library usable on platforms which use IEEE 754 conformant floating-point arithmetic. `libm-i387' This is a special case. Ideally the code should be in `sysdeps/i386/fpu' but for various reasons it is kept aside. `posix' This directory contains implementations of things in the library in terms of POSIX.1 functions. This includes some of the POSIX.1 functions themselves. Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just `posix' cannot be complete. `unix' This is the directory for Unix-like things. *Note Porting to Unix::. `unix' implies `posix'. There are some special-purpose subdirectories of `unix': `unix/common' This directory is for things common to both BSD and System V release 4. Both `unix/bsd' and `unix/sysv/sysv4' imply `unix/common'. `unix/inet' This directory is for `socket' and related functions on Unix systems. `unix/inet/Subdirs' enables the `inet' top-level subdirectory. `unix/common' implies `unix/inet'. `mach' This is the directory for things based on the Mach microkernel from CMU (including the GNU operating system). Other basic operating systems (VMS, for example) would have their own directories at the top level of the `sysdeps' hierarchy, parallel to `unix' and `mach'. Porting the GNU C Library to Unix Systems ----------------------------------------- Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple. The code for Unix systems is in the directory `unix', at the top level of the `sysdeps' hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants. The functions which are system calls in most Unix systems are implemented in assembly code, which is generated automatically from specifications in files named `syscalls.list'. There are several such files, one in `sysdeps/unix' and others in its subdirectories. Some special system calls are implemented in files that are named with a suffix of `.S'; for example, `_exit.S'. Files ending in `.S' are run through the C preprocessor before being fed to the assembler. These files all use a set of macros that should be defined in `sysdep.h'. The `sysdep.h' file in `sysdeps/unix' partially defines them; a `sysdep.h' file in another directory must finish defining them for the particular machine and operating system variant. See `sysdeps/unix/sysdep.h' and the machine-specific `sysdep.h' implementations to see what these macros are and what they should do. The system-specific makefile for the `unix' directory (`sysdeps/unix/Makefile') gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library _for_). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are `ioctls.h', `errnos.h', `sys/param.h', and `errlist.c' (for the `stdio' section of the library). Contributors to the GNU C Library ********************************* The GNU C library was written originally by Roland McGrath, and is currently maintained by Ulrich Drepper. Some parts of the library were contributed or worked on by other people. * The `getopt' function and related code was written by Richard Stallman, David J. MacKenzie, and Roland McGrath. * The merge sort function `qsort' was written by Michael J. Haertel. * The quick sort function used as a fallback by `qsort' was written by Douglas C. Schmidt. * The memory allocation functions `malloc', `realloc' and `free' and related code were written by Michael J. Haertel, Wolfram Gloger, and Doug Lea. * Fast implementations of many of the string functions (`memcpy', `strlen', etc.) were written by Torbjo"rn Granlund. * The `tar.h' header file was written by David J. MacKenzie. * The port to the MIPS DECStation running Ultrix 4 (`mips-dec-ultrix4') was contributed by Brendan Kehoe and Ian Lance Taylor. * The DES encryption function `crypt' and related functions were contributed by Michael Glad. * The `ftw' and `nftw' functions were contributed by Ulrich Drepper. * The startup code to support SunOS shared libraries was contributed by Tom Quinn. * The `mktime' function was contributed by Paul Eggert. * The port to the Sequent Symmetry running Dynix version 3 (`i386-sequent-bsd') was contributed by Jason Merrill. * The timezone support code is derived from the public-domain timezone package by Arthur David Olson and his many contributors. * The port to the DEC Alpha running OSF/1 (`alpha-dec-osf1') was contributed by Brendan Kehoe, using some code written by Roland McGrath. * The port to SGI machines running Irix 4 (`mips-sgi-irix4') was contributed by Tom Quinn. * The port of the Mach and Hurd code to the MIPS architecture (`mips-ANYTHING-gnu') was contributed by Kazumoto Kojima. * The floating-point printing function used by `printf' and friends and the floating-point reading function used by `scanf', `strtod' and friends were written by Ulrich Drepper. The multi-precision integer functions used in those functions are taken from GNU MP, which was contributed by Torbjo"rn Granlund. * The internationalization support in the library, and the support programs `locale' and `localedef', were written by Ulrich Drepper. Ulrich Drepper adapted the support code for message catalogs (`libintl.h', etc.) from the GNU `gettext' package, which he also wrote. He also contributed the `catgets' support and the entire suite of multi-byte and wide-character support functions (`wctype.h', `wchar.h', etc.). * The implementations of the `nsswitch.conf' mechanism and the files and DNS backends for it were designed and written by Ulrich Drepper and Roland McGrath, based on a backend interface defined by Peter Eriksson. * The port to Linux i386/ELF (`i386-ANYTHING-linux') was contributed by Ulrich Drepper, based in large part on work done in Hongjiu Lu's Linux version of the GNU C Library. * The port to Linux/m68k (`m68k-ANYTHING-linux') was contributed by Andreas Schwab. * The ports to Linux/ARM (`arm-ANYTHING-linuxaout') and ARM standalone (`arm-ANYTHING-none'), as well as parts of the IPv6 support code, were contributed by Philip Blundell. * Richard Henderson contributed the ELF dynamic linking code and other support for the Alpha processor. * David Mosberger-Tang contributed the port to Linux/Alpha (`alpha-ANYTHING-linux'). * The port to Linux on PowerPC (`powerpc-ANYTHING-linux') was contributed by Geoffrey Keating. * Miles Bader wrote the argp argument-parsing package, and the argz/envz interfaces. * Stephen R. van den Berg contributed a highly-optimized `strstr' function. * Ulrich Drepper contributed the `hsearch' and `drand48' families of functions; reentrant `...`_r'' versions of the `random' family; System V shared memory and IPC support code; and several highly-optimized string functions for iX86 processors. * The math functions are taken from `fdlibm-5.1' by Sun Microsystems, as modified by J.T. Conklin, Ian Lance Taylor, Ulrich Drepper, Andreas Schwab, and Roland McGrath. * The `libio' library used to implement `stdio' functions on some platforms was written by Per Bothner and modified by Ulrich Drepper. * Eric Youngdale and Ulrich Drepper implemented versioning of objects on symbol level. * Thorsten Kukuk provided an implementation for NIS (YP) and NIS+, securelevel 0, 1 and 2. * Andreas Jaeger provided a test suite for the math library. * Mark Kettenis implemented the utmpx interface and an utmp daemon. * Ulrich Drepper added character conversion functions (`iconv'). * Thorsten Kukuk provided an implementation for a caching daemon for NSS (nscd). * Tim Waugh provided an implementation of the POSIX.2 wordexp function family. * Mark Kettenis provided a Hesiod NSS module. * The Internet-related code (most of the `inet' subdirectory) and several other miscellaneous functions and header files have been included from 4.4 BSD with little or no modification. All code incorporated from 4.4 BSD is under the following copyright: Copyright (C) 1991 Regents of the University of California. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. [This condition was removed.] 4. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. * The random number generation functions `random', `srandom', `setstate' and `initstate', which are also the basis for the `rand' and `srand' functions, were written by Earl T. 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Concept Index ************* /etc/hostname: See ``Host Identification''. /etc/nsswitch.conf: See ``The NSS Configuration File''. 4.N BSD Unix: See ``Berkeley Unix''. __va_copy: See ``Copying and Concatenation''. _POSIX_OPTION_ORDER environment variable.: See ``Standard Environment Variables''. _POSIX_SAVED_IDS: See ``How an Application Can Change Persona''. abort signal: See ``Program Error Signals''. aborting a program: See ``Aborting a Program''. absolute file name: See ``File Name Resolution''. absolute priority: See ``Absolute Priority''. absolute value functions: See ``Absolute Value''. accepting connections: See ``Accepting Connections''. access permission for a file: See ``How Your Access to a File is Decided''. access, testing for: See ``Testing Permission to Access a File''. accessing directories: See ``Accessing Directories''. address of socket: See ``Socket Addresses''. address space <1>: See ``The Basic Program/System Interface''. address space: See ``Overview about traditional Unix memory handling''. alarm signal: See ``Alarm Signals''. alarms, setting: See ``Setting an Alarm''. alignment (in obstacks): See ``Alignment of Data in Obstacks''. alignment (with malloc): See ``Allocating Aligned Memory Blocks''. alloca disadvantages: See ``Disadvantages of `alloca'''. alloca function: See ``Automatic Storage with Variable Size''. allocating pseudo-terminals: See ``Allocating Pseudo-Terminals''. allocation (obstacks): See ``Allocation in an Obstack''. allocation debugging: See ``Allocation Debugging''. allocation hooks, for malloc: See ``Memory Allocation Hooks''. allocation of memory with malloc: See ``Basic Memory Allocation''. allocation size of string: See ``Representation of Strings''. allocation statistics: See ``Statistics for Memory Allocation with `malloc'''. alphabetic character <1>: See ``Character class determination for wide characters''. alphabetic character: See ``Classification of Characters''. alphanumeric character <1>: See ``Character class determination for wide characters''. alphanumeric character: See ``Classification of Characters''. append-access files: See ``File Position''. argc (program argument count): See ``Program Arguments''. argp (program argument parser): See ``Parsing Program Options with Argp''. argp parser functions: See ``Argp Parser Functions''. ARGP_HELP_FMT environment variable: See ``Argp User Customization''. argument parsing with argp: See ``Parsing Program Options with Argp''. argument promotion: See ``Calling Variadic Functions''. argument vectors, null-character separated: See ``Argz and Envz Vectors''. arguments (variadic functions): See ``Receiving the Argument Values''. arguments, how many: See ``How Many Arguments Were Supplied''. arguments, to program: See ``Program Arguments''. argv (program argument vector): See ``Program Arguments''. argz vectors (string vectors): See ``Argz and Envz Vectors''. arithmetic expansion: See ``The Stages of Word Expansion''. array comparison functions: See ``String/Array Comparison''. array copy functions: See ``Copying and Concatenation''. array search function: See ``Array Search Function''. array sort function: See ``Array Sort Function''. ASCII character: See ``Classification of Characters''. assertions: See ``Explicitly Checking Internal Consistency''. attributes of a file: See ``The meaning of the File Attributes''. automatic freeing: See ``Automatic Storage with Variable Size''. automatic memory allocation: See ``Memory Allocation in C Programs''. automatic storage class: See ``Memory Allocation in C Programs''. automatic storage with variable size: See ``Automatic Storage with Variable Size''. background job: See ``Concepts of Job Control''. background job, launching: See ``Foreground and Background''. backtrace: See ``Backtraces''. backtrace_fd: See ``Backtraces''. backtrace_symbols: See ``Backtraces''. base (of floating point number): See ``Floating Point Representation Concepts''. baud rate: See ``Line Speed''. Berkeley Unix: See ``Berkeley Unix''. Bessel functions: See ``Special Functions''. bias (of floating point number exponent): See ``Floating Point Representation Concepts''. big-endian: See ``Byte Order Conversion''. binary I/O to a stream: See ``Block Input/Output''. binary search function (for arrays): See ``Array Search Function''. binary stream: See ``Text and Binary Streams''. binding a socket address: See ``Socket Addresses''. blank character <1>: See ``Character class determination for wide characters''. blank character: See ``Classification of Characters''. block I/O to a stream: See ``Block Input/Output''. blocked signals: See ``How Signals Are Delivered''. blocked signals, checking for: See ``Checking for Pending Signals''. blocking signals: See ``Blocking Signals''. blocking signals, in a handler: See ``Blocking Signals for a Handler''. bootstrapping, and services: See ``Actions in the NSS configuration''. break condition, detecting: See ``Input Modes''. break condition, generating: See ``Line Control Functions''. breaking a string into tokens: See ``Finding Tokens in a String''. broken pipe signal: See ``Operation Error Signals''. broken-down time <1>: See ``Broken-down Time''. broken-down time: See ``Calendar Time''. BSD compatibility library: See ``Process Group Functions''. BSD compatibility library.: See ``Feature Test Macros''. BSD Unix: See ``Berkeley Unix''. buffering of streams: See ``Stream Buffering''. buffering, controlling: See ``Controlling Which Kind of Buffering''. bugs, reporting: See ``Reporting Bugs''. bus error: See ``Program Error Signals''. butterfly: See ``Miscellaneous FP arithmetic functions''. byte order conversion, for socket: See ``Byte Order Conversion''. byte stream: See ``Socket Concepts''. C++ streams: See ``Streams in Internationalized Applications''. calendar time: See ``Time Basics''. calendar time and broken-down time: See ``Broken-down Time''. calendar, Gregorian: See ``Calendar Time''. calling variadic functions: See ``Calling Variadic Functions''. canonical input processing: See ``Two Styles of Input: Canonical or Not''. capacity limits, POSIX: See ``General Capacity Limits''. carrier detect: See ``Control Modes''. case conversion of characters: See ``Case Conversion''. catching signals: See ``How Signals Are Delivered''. categories for locales: See ``Categories of Activities that Locales Affect''. change working directory: See ``Working Directory''. changing the locale: See ``How Programs Set the Locale''. changing the size of a block (malloc): See ``Changing the Size of a Block''. changing the size of a block (obstacks): See ``Growing Objects''. channels: See ``Dangers of Mixing Streams and Descriptors''. character case conversion: See ``Case Conversion''. character predicates: See ``Classification of Characters''. character testing: See ``Classification of Characters''. checking for pending signals: See ``Checking for Pending Signals''. child process <1>: See ``Process Creation Concepts''. child process <2>: See ``Processes''. child process: See ``Processor Time Inquiry''. child process signal: See ``Job Control Signals''. chunks: See ``Obstack Chunks''. classes, floating-point: See ``Floating-Point Number Classification Functions''. classification of characters: See ``Classification of Characters''. cleaning up a stream: See ``Linked Channels''. clearing terminal input queue: See ``Line Control Functions''. client: See ``Using Sockets with Connections''. clock ticks: See ``Processor And CPU Time''. clock, high accuracy: See ``High Accuracy Clock''. close-on-exec (file descriptor flag): See ``File Descriptor Flags''. closing a file descriptor: See ``Opening and Closing Files''. closing a socket: See ``Closing a Socket''. closing a stream: See ``Closing Streams''. collating strings: See ``Collation Functions''. combining locales: See ``Choosing a Locale''. command argument syntax: See ``Program Argument Syntax Conventions''. command arguments, parsing: See ``Parsing Program Arguments''. command line arguments: See ``Program Arguments''. command substitution: See ``The Stages of Word Expansion''. communication style (of a socket): See ``Socket Concepts''. comparing strings and arrays: See ``String/Array Comparison''. Comparison Function: See ``Defining the Comparison Function''. compiling: See ``Configuring and compiling GNU Libc''. complex exponentiation functions: See ``Exponentiation and Logarithms''. complex logarithm functions: See ``Exponentiation and Logarithms''. complex numbers: See ``Complex Numbers''. complex trigonometric functions: See ``Trigonometric Functions''. concatenating strings: See ``Copying and Concatenation''. configurations, all supported: See ``Supported Configurations''. configuring: See ``Configuring and compiling GNU Libc''. conjugate complex numbers: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. connecting a socket: See ``Making a Connection''. connection: See ``Using Sockets with Connections''. consistency checking: See ``Explicitly Checking Internal Consistency''. consistency checking, of heap: See ``Heap Consistency Checking''. constants <1>: See ``Predefined Mathematical Constants''. constants: See ``Process Memory Concepts''. continue signal: See ``Job Control Signals''. control character <1>: See ``Character class determination for wide characters''. control character: See ``Classification of Characters''. control operations on files: See ``Control Operations on Files''. controlling process: See ``Controlling Terminal of a Process''. controlling terminal: See ``Concepts of Job Control''. controlling terminal, access to: See ``Access to the Controlling Terminal''. controlling terminal, determining: See ``Identifying the Controlling Terminal''. controlling terminal, setting: See ``Open-time Flags''. conversion specifications (printf): See ``Formatted Output Basics''. conversion specifications (scanf): See ``Formatted Input Basics''. converting byte order: See ``Byte Order Conversion''. converting case of characters: See ``Case Conversion''. converting file descriptor to stream: See ``Descriptors and Streams''. converting floats to integers: See ``Rounding Functions''. converting group ID to group name: See ``Looking Up One Group''. converting group name to group ID: See ``Looking Up One Group''. converting host address to name: See ``Host Names''. converting host name to address: See ``Host Names''. converting network name to network number: See ``Networks Database''. converting network number to network name: See ``Networks Database''. converting port number to service name: See ``The Services Database''. converting service name to port number: See ``The Services Database''. converting string to collation order: See ``Collation Functions''. converting strings to numbers: See ``Parsing of Numbers''. converting user ID to user name: See ``Looking Up One User''. converting user name to user ID: See ``Looking Up One User''. cookie, for custom stream: See ``Custom Streams and Cookies''. copy-on-write page fault: See ``Locked Memory Details''. copying strings and arrays: See ``Copying and Concatenation''. cpu priority: See ``Process CPU Priority And Scheduling''. CPU time <1>: See ``Processor Time Inquiry''. CPU time <2>: See ``Processor And CPU Time''. CPU time: See ``Time Basics''. create on open (file status flag): See ``Open-time Flags''. creating a directory: See ``Creating Directories''. creating a FIFO special file: See ``FIFO Special Files''. creating a pipe: See ``Creating a Pipe''. creating a pipe to a subprocess: See ``Pipe to a Subprocess''. creating a process: See ``Process Creation Concepts''. creating a socket: See ``Creating a Socket''. creating a socket pair: See ``Socket Pairs''. creating special files: See ``Making Special Files''. cube root function: See ``Exponentiation and Logarithms''. currency symbols: See ``Printing the Currency Symbol''. current limit: See ``Limiting Resource Usage''. current working directory: See ``Working Directory''. custom streams: See ``Programming Your Own Custom Streams''. customizing printf: See ``Customizing `printf'''. data loss on sockets: See ``Socket Concepts''. databases: See ``System Databases and Name Service Switch''. datagram socket: See ``Datagram Socket Operations''. datagrams, transmitting: See ``Sending Datagrams''. date: See ``Time Basics''. Daylight Saving Time: See ``Broken-down Time''. decimal digit character: See ``Classification of Characters''. decimal-point separator: See ``Generic Numeric Formatting Parameters''. declaration (compared to definition): See ``Header Files''. declaring variadic functions: See ``Calling Variadic Functions''. decompose complex numbers: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. default action (for a signal): See ``How Signals Are Delivered''. default action for a signal: See ``Basic Signal Handling''. default argument promotions: See ``Calling Variadic Functions''. default value, and NSS: See ``Notes on the NSS Configuration File''. defining new printf conversions: See ``Customizing `printf'''. definition (compared to declaration): See ``Header Files''. delayed suspend character: See ``Characters that Cause Signals''. deleting a directory: See ``Deleting Files''. deleting a file: See ``Deleting Files''. delivery of signals: See ``How Signals Are Delivered''. descriptors and streams: See ``Dangers of Mixing Streams and Descriptors''. digit character <1>: See ``Character class determination for wide characters''. digit character: See ``Classification of Characters''. directories, accessing: See ``Accessing Directories''. directories, creating: See ``Creating Directories''. directories, deleting: See ``Deleting Files''. directory: See ``Directories''. directory entry: See ``Directories''. directory hierarchy: See ``Working with Directory Trees''. directory stream: See ``Accessing Directories''. disadvantages of alloca: See ``Disadvantages of `alloca'''. DISCARD character: See ``Other Special Characters''. division by zero: See ``FP Exceptions''. DNS: See ``Host Identification''. DNS server unavailable: See ``Actions in the NSS configuration''. domain (of socket): See ``Socket Concepts''. domain error: See ``Error Reporting by Mathematical Functions''. domain name: See ``Host Identification''. Domain Name System: See ``Host Identification''. dot notation, for Internet addresses: See ``Internet Host Addresses''. DSUSP character: See ``Characters that Cause Signals''. duplicating file descriptors: See ``Duplicating Descriptors''. dynamic memory allocation: See ``Memory Allocation in C Programs''. EBCDIC: See ``Introduction to Extended Characters''. echo of terminal input: See ``Local Modes''. effective group ID: See ``The Persona of a Process''. effective user ID: See ``The Persona of a Process''. efficiency and malloc: See ``Efficiency Considerations for `malloc'''. efficiency and obstacks: See ``Extra Fast Growing Objects''. efficiency of chunks: See ``Obstack Chunks''. EINTR, and restarting interrupted primitives: See ``Primitives Interrupted by Signals''. elapsed time <1>: See ``Elapsed Time''. elapsed time: See ``Time Basics''. encryption: See ``Trivial Encryption''. end of file, on a stream: See ``End-Of-File and Errors''. end-of-file, on a file descriptor: See ``Input and Output Primitives''. environment: See ``Environment Variables''. environment access: See ``Environment Access''. environment representation: See ``Environment Access''. environment variable: See ``Environment Variables''. environment vectors, null-character separated: See ``Argz and Envz Vectors''. envz vectors (environment vectors): See ``Argz and Envz Vectors''. EOF character: See ``Characters for Input Editing''. EOL character: See ``Characters for Input Editing''. EOL2 character: See ``Characters for Input Editing''. epoch: See ``Simple Calendar Time''. ERASE character: See ``Characters for Input Editing''. errno: See ``System Calls''. error codes: See ``Error Reporting''. error messages, in argp: See ``Functions For Use in Argp Parsers''. error reporting: See ``Error Reporting''. errors, mathematical: See ``Error Reporting by Mathematical Functions''. establishing a handler: See ``Specifying Signal Actions''. ethers: See ``NSS Basics''. EUC: See ``Introduction to Extended Characters''. EUC-JP: See ``The `iconv' Implementation in the GNU C library''. exception <1>: See ``Program Error Signals''. exception: See ``FP Exceptions''. exclusive lock: See ``File Locks''. exec functions: See ``Executing a File''. execing a program: See ``Process Memory Concepts''. executable: See ``Process Memory Concepts''. executing a file: See ``Executing a File''. exit status: See ``Exit Status''. exit status value: See ``Program Termination''. exiting a program: See ``Process Memory Concepts''. expansion of shell words: See ``Shell-Style Word Expansion''. exponent (of floating point number): See ``Floating Point Representation Concepts''. exponentiation functions: See ``Exponentiation and Logarithms''. extending printf: See ``Customizing `printf'''. extracting file descriptor from stream: See ``Descriptors and Streams''. fcntl function: See ``Control Operations on Files''. FDL, GNU Free Documentation License: See ``GNU Free Documentation License''. feature test macros: See ``Feature Test Macros''. field splitting: See ``The Stages of Word Expansion''. FIFO special file: See ``Pipes and FIFOs''. file access permission: See ``How Your Access to a File is Decided''. file access time: See ``File Times''. file attribute modification time: See ``File Times''. file attributes: See ``The meaning of the File Attributes''. file creation mask: See ``Assigning File Permissions''. file descriptor flags: See ``File Descriptor Flags''. file descriptor sets, for select: See ``Waiting for Input or Output''. file descriptors, standard: See ``Descriptors and Streams''. file locks: See ``File Locks''. file modification time: See ``File Times''. file name: See ``File Names''. file name component: See ``Directories''. file name errors: See ``File Name Errors''. file name resolution: See ``File Name Resolution''. file name translation flags: See ``Open-time Flags''. file names, multiple: See ``Hard Links''. file owner: See ``File Owner''. file permission bits: See ``The Mode Bits for Access Permission''. file pointer: See ``Streams''. file position: See ``File Position''. file positioning on a file descriptor: See ``Setting the File Position of a Descriptor''. file positioning on a stream: See ``File Positioning''. file status flags: See ``File Status Flags''. files, accessing: See ``Process Memory Concepts''. filtering i/o through subprocess: See ``Pipe to a Subprocess''. flag character (printf): See ``Output Conversion Syntax''. flag character (scanf): See ``Input Conversion Syntax''. flags for sigaction: See ``Flags for `sigaction'''. flags, file name translation: See ``Open-time Flags''. flags, open-time action: See ``Open-time Flags''. floating point: See ``Floating Point Numbers''. floating point, IEEE: See ``IEEE Floating Point''. floating type measurements: See ``Floating Type Macros''. floating-point classes: See ``Floating-Point Number Classification Functions''. floating-point exception: See ``Program Error Signals''. flow control, terminal: See ``Line Control Functions''. flushing a stream: See ``Flushing Buffers''. flushing terminal output queue: See ``Line Control Functions''. foreground job: See ``Concepts of Job Control''. foreground job, launching: See ``Foreground and Background''. forking a process: See ``Process Creation Concepts''. format string, for printf: See ``Formatted Output''. format string, for scanf: See ``Formatted Input''. formatted input from a stream: See ``Formatted Input''. formatted messages: See ``Formatted Messages''. formatted output to a stream: See ``Formatted Output''. FP arithmetic: See ``Setting and modifying single bits of FP values''. FQDN: See ``Host Identification''. frame, real memory: See ``Process Memory Concepts''. free documentation: See ``Free Software Needs Free Documentation''. freeing (obstacks): See ``Freeing Objects in an Obstack''. freeing memory: See ``Process Memory Concepts''. freeing memory allocated with malloc: See ``Freeing Memory Allocated with `malloc'''. fully buffered stream: See ``Buffering Concepts''. function prototypes (variadic): See ``Syntax for Variable Arguments''. gamma function: See ``Special Functions''. gcvt_r: See ``Old-fashioned System V number-to-string functions''. gencat: See ``Generate Message Catalogs files''. generation of signals: See ``Concepts of Signal Generation''. generic i/o control operations: See ``Generic I/O Control operations''. globbing: See ``Globbing''. graphic character <1>: See ``Character class determination for wide characters''. graphic character: See ``Classification of Characters''. Gregorian calendar: See ``Calendar Time''. group: See ``NSS Basics''. group database: See ``Group Database''. group ID: See ``User and Group IDs''. group name: See ``User and Group IDs''. group owner of a file: See ``File Owner''. grouping of digits: See ``Generic Numeric Formatting Parameters''. growing objects (in obstacks): See ``Growing Objects''. handling multiple signals: See ``Signals Close Together Merge into One''. hangup signal: See ``Termination Signals''. hard limit: See ``Limiting Resource Usage''. hard link: See ``Hard Links''. header files: See ``Header Files''. heap consistency checking: See ``Heap Consistency Checking''. heap, dynamic allocation from: See ``Unconstrained Allocation''. heap, freeing memory from: See ``Freeing Memory Allocated with `malloc'''. hexadecimal digit character <1>: See ``Character class determination for wide characters''. hexadecimal digit character: See ``Classification of Characters''. hidden bit (of floating point number mantissa): See ``Floating Point Representation Concepts''. hierarchy, directory: See ``Working with Directory Trees''. high-priority data: See ``Out-of-Band Data''. high-resolution time: See ``Calendar Time''. holes in files: See ``Setting the File Position of a Descriptor''. home directory: See ``Standard Environment Variables''. HOME environment variable: See ``Standard Environment Variables''. hook functions (of custom streams): See ``Custom Stream Hook Functions''. host address, Internet: See ``Internet Host Addresses''. host name: See ``Host Identification''. hostname: See ``Host Identification''. hosts: See ``NSS Basics''. hosts database: See ``Host Names''. how many arguments: See ``How Many Arguments Were Supplied''. hyperbolic functions: See ``Hyperbolic Functions''. identifying terminals: See ``Identifying Terminals''. IEEE 754: See ``Floating Point Numbers''. IEEE floating point: See ``Floating Point Numbers''. IEEE floating point representation: See ``IEEE Floating Point''. IEEE Std 1003.1: See ``POSIX (The Portable Operating System Interface)''. IEEE Std 1003.2: See ``POSIX (The Portable Operating System Interface)''. ignore action for a signal: See ``Basic Signal Handling''. illegal instruction: See ``Program Error Signals''. impossible events: See ``Explicitly Checking Internal Consistency''. independent channels: See ``Independent Channels''. inexact exception: See ``FP Exceptions''. infinity: See ``Infinity and NaN''. initial signal actions: See ``Initial Signal Actions''. inode number: See ``The meaning of the File Attributes''. input available signal: See ``Asynchronous I/O Signals''. input conversions, for scanf: See ``Table of Input Conversions''. input from multiple files: See ``Waiting for Input or Output''. installation tools: See ``Recommended Tools for Compilation''. installing: See ``Installing the C Library''. integer: See ``Integers''. integer division functions: See ``Integer Division''. integer type range: See ``Range of an Integer Type''. integer type width: See ``Computing the Width of an Integer Data Type''. interactive signals, from terminal: See ``Local Modes''. interactive stop signal: See ``Job Control Signals''. internal representation: See ``Introduction to Extended Characters''. internationalization: See ``Locales and Internationalization''. Internet host address: See ``Internet Host Addresses''. Internet namespace, for sockets: See ``The Internet Namespace''. interprocess communication, with FIFO: See ``FIFO Special Files''. interprocess communication, with pipes: See ``Creating a Pipe''. interprocess communication, with signals: See ``Using `kill' for Communication''. interprocess communication, with sockets: See ``Sockets''. interrupt character: See ``Characters that Cause Signals''. interrupt signal: See ``Termination Signals''. interrupt-driven input: See ``Interrupt-Driven Input''. interrupting primitives: See ``Primitives Interrupted by Signals''. interval: See ``Time Basics''. interval timer, setting: See ``Setting an Alarm''. INTR character: See ``Characters that Cause Signals''. invalid exception: See ``FP Exceptions''. inverse complex hyperbolic functions: See ``Hyperbolic Functions''. inverse complex trigonometric functions: See ``Inverse Trigonometric Functions''. inverse hyperbolic functions: See ``Hyperbolic Functions''. inverse trigonometric functions: See ``Inverse Trigonometric Functions''. invocation of program: See ``Program Arguments''. IOCTLs: See ``Generic I/O Control operations''. ISO 10646: See ``Introduction to Extended Characters''. ISO 2022: See ``Introduction to Extended Characters''. ISO 6937: See ``Introduction to Extended Characters''. ISO C: See ``ISO C''. ISO-2022-JP: See ``The `iconv' Implementation in the GNU C library''. ISO/IEC 9945-1: See ``POSIX (The Portable Operating System Interface)''. ISO/IEC 9945-2: See ``POSIX (The Portable Operating System Interface)''. job: See ``Job Control''. job control: See ``Job Control''. job control functions: See ``Functions for Job Control''. job control is optional: See ``Job Control is Optional''. job control signals: See ``Job Control Signals''. job control, enabling: See ``Initializing the Shell''. Kermit the frog: See ``Searching and Sorting Example''. kernel call: See ``System Calls''. kernel header files: See ``Specific advice for Linux systems''. KILL character: See ``Characters for Input Editing''. kill signal: See ``Termination Signals''. killing a process: See ``Signaling Another Process''. Korn Shell: See ``Wildcard Matching''. LANG environment variable <1>: See ``Standard Environment Variables''. LANG environment variable: See ``The `catgets' function family''. launching jobs: See ``Launching Jobs''. LC_ALL environment variable <1>: See ``Standard Environment Variables''. LC_ALL environment variable: See ``The `catgets' function family''. LC_COLLATE environment variable: See ``Standard Environment Variables''. LC_CTYPE environment variable: See ``Standard Environment Variables''. LC_MESSAGES environment variable <1>: See ``Standard Environment Variables''. LC_MESSAGES environment variable: See ``The `catgets' function family''. LC_MONETARY environment variable: See ``Standard Environment Variables''. LC_NUMERIC environment variable: See ``Standard Environment Variables''. LC_TIME environment variable: See ``Standard Environment Variables''. leap second: See ``Broken-down Time''. length of string: See ``Representation of Strings''. level, for socket options: See ``Socket Options''. LGPL, Lesser General Public License: See ``GNU Lesser General Public License''. library: See ``Introduction''. limit: See ``Limiting Resource Usage''. limits on resource usage: See ``Limiting Resource Usage''. limits, file name length: See ``Limits on File System Capacity''. limits, floating types: See ``Floating Type Macros''. limits, integer types: See ``Range of an Integer Type''. limits, link count of files: See ``Limits on File System Capacity''. limits, number of open files: See ``General Capacity Limits''. limits, number of processes: See ``General Capacity Limits''. limits, number of supplementary group IDs: See ``General Capacity Limits''. limits, pipe buffer size: See ``Limits on File System Capacity''. limits, POSIX: See ``General Capacity Limits''. limits, program argument size: See ``General Capacity Limits''. limits, terminal input queue: See ``Limits on File System Capacity''. limits, time zone name length: See ``General Capacity Limits''. line buffered stream: See ``Buffering Concepts''. line speed: See ``Line Speed''. lines (in a text file): See ``Text and Binary Streams''. link: See ``Directories''. link, hard: See ``Hard Links''. link, soft: See ``Symbolic Links''. link, symbolic: See ``Symbolic Links''. linked channels: See ``Linked Channels''. listening (sockets): See ``Listening for Connections''. literals: See ``Process Memory Concepts''. little-endian: See ``Byte Order Conversion''. LNEXT character: See ``Other Special Characters''. load average: See ``Learn about the processors available''. local namespace, for sockets: See ``The Local Namespace''. local network address number: See ``Internet Host Addresses''. local time: See ``Calendar Time''. locale categories: See ``Categories of Activities that Locales Affect''. locale, changing: See ``How Programs Set the Locale''. locales: See ``Locales and Internationalization''. locking pages: See ``Locking Pages''. logarithm functions: See ``Exponentiation and Logarithms''. login name: See ``User and Group IDs''. login name, determining: See ``Identifying Who Logged In''. LOGNAME environment variable: See ``Standard Environment Variables''. long jumps: See ``Non-Local Exits''. long-named options: See ``Program Argument Syntax Conventions''. longjmp: See ``Advantages of `alloca'''. loss of data on sockets: See ``Socket Concepts''. lost resource signal: See ``Operation Error Signals''. lower-case character <1>: See ``Character class determination for wide characters''. lower-case character: See ``Classification of Characters''. macros: See ``Obstack Functions and Macros''. main function: See ``Program Arguments''. malloc debugger: See ``Allocation Debugging''. malloc function: See ``Unconstrained Allocation''. mantissa (of floating point number): See ``Floating Point Representation Concepts''. matching failure, in scanf: See ``Formatted Input Basics''. math errors: See ``Known Maximum Errors in Math Functions''. mathematical constants: See ``Predefined Mathematical Constants''. maximum: See ``Miscellaneous FP arithmetic functions''. maximum field width (scanf): See ``Input Conversion Syntax''. maximum limit: See ``Limiting Resource Usage''. maximum possible integer: See ``Integers''. measurements of floating types: See ``Floating Type Macros''. memory allocation: See ``Virtual Memory Allocation And Paging''. memory lock: See ``Locking Pages''. memory mapped file: See ``Process Memory Concepts''. memory mapped I/O: See ``Process Memory Concepts''. memory page: See ``Overview about traditional Unix memory handling''. merging of signals: See ``Signals Close Together Merge into One''. MIN termios slot: See ``Noncanonical Input''. minimum: See ``Miscellaneous FP arithmetic functions''. minimum field width (printf): See ``Output Conversion Syntax''. minimum possible integer: See ``Integers''. mixing descriptors and streams: See ``Dangers of Mixing Streams and Descriptors''. modem disconnect: See ``Control Modes''. modem status lines: See ``Control Modes''. monetary value formatting: See ```localeconv': It is portable but ...''. multi-threaded application: See ``Streams and Threads''. multibyte character: See ``Introduction to Extended Characters''. multibyte character string: See ``Representation of Strings''. multibyte string: See ``Representation of Strings''. multiple names for one file: See ``Hard Links''. multiplexing input: See ``Waiting for Input or Output''. multiply-add: See ``Miscellaneous FP arithmetic functions''. name of running program: See ``Error Messages''. name of socket: See ``Socket Addresses''. Name Service Switch: See ``System Databases and Name Service Switch''. name space: See ``Reserved Names''. names of signals: See ``Standard Signals''. namespace (of socket): See ``Socket Concepts''. NaN <1>: See ``Setting and modifying single bits of FP values''. NaN: See ``Infinity and NaN''. Netgroup: See ``Netgroup Data''. netgroup: See ``NSS Basics''. network byte order: See ``Byte Order Conversion''. network number: See ``Internet Host Addresses''. network protocol: See ``Socket Concepts''. networks: See ``NSS Basics''. networks database: See ``Networks Database''. NIS: See ``Host Identification''. NIS domain name: See ``Host Identification''. nisplus, and booting: See ``Actions in the NSS configuration''. nisplus, and completeness: See ``Actions in the NSS configuration''. NLSPATH environment variable <1>: See ``Standard Environment Variables''. NLSPATH environment variable: See ``The `catgets' function family''. non-blocking open: See ``Open-time Flags''. non-local exit, from signal handler: See ``Nonlocal Control Transfer in Handlers''. non-local exits: See ``Non-Local Exits''. noncanonical input processing: See ``Two Styles of Input: Canonical or Not''. normalization functions (floating-point): See ``Normalization Functions''. normalized floating point number: See ``Floating Point Representation Concepts''. not a number: See ``Infinity and NaN''. NSS: See ``System Databases and Name Service Switch''. nsswitch.conf: See ``The NSS Configuration File''. null character: See ``Representation of Strings''. null pointer constant: See ``Null Pointer Constant''. null wide character: See ``Representation of Strings''. number of arguments passed: See ``How Many Arguments Were Supplied''. number syntax, parsing: See ``Parsing of Numbers''. numeric value formatting: See ```localeconv': It is portable but ...''. obstack status: See ``Status of an Obstack''. obstacks: See ``Obstacks''. open-time action flags: See ``Open-time Flags''. opening a file: See ``Input/Output Concepts''. opening a file descriptor: See ``Opening and Closing Files''. opening a pipe: See ``Creating a Pipe''. opening a pseudo-terminal pair: See ``Opening a Pseudo-Terminal Pair''. opening a socket: See ``Creating a Socket''. opening a socket pair: See ``Socket Pairs''. opening a stream: See ``Opening Streams''. Optimization: See ``Is Fast Code or Small Code preferred?''. optimizing NSS: See ``Notes on the NSS Configuration File''. option parsing with argp: See ``Parsing Program Options with Argp''. optional arguments: See ``Variadic Functions''. optional POSIX features: See ``Overall System Options''. orientation, stream <1>: See ``Streams in Internationalized Applications''. orientation, stream: See ``Opening Streams''. orphaned process group: See ``Orphaned Process Groups''. out-of-band data: See ``Out-of-Band Data''. output conversions, for printf: See ``Table of Output Conversions''. output possible signal: See ``Asynchronous I/O Signals''. overflow exception: See ``FP Exceptions''. owner of a file: See ``File Owner''. packet: See ``Socket Concepts''. page boundary: See ``Allocating Aligned Memory Blocks''. page fault: See ``Process Memory Concepts''. page fault, copy-on-write: See ``Locked Memory Details''. page frame: See ``Process Memory Concepts''. page, memory: See ``Overview about traditional Unix memory handling''. page, virtual memory: See ``Process Memory Concepts''. paging <1>: See ``Locking Pages''. paging: See ``Process Memory Concepts''. parameter promotion: See ``String and Array Conventions''. parent directory: See ``File Name Resolution''. parent process <1>: See ``Process Creation Concepts''. parent process: See ``Processes''. parity checking: See ``Input Modes''. parsing a template string: See ``Parsing a Template String''. parsing numbers (in formatted input): See ``Parsing of Numbers''. parsing program arguments: See ``Parsing Program Arguments''. parsing tokens from a string: See ``Finding Tokens in a String''. passwd: See ``NSS Basics''. password database: See ``User Database''. PATH environment variable: See ``Standard Environment Variables''. pause function: See ``Waiting for a Signal''. peeking at input: See ``Unreading''. pending signals: See ``How Signals Are Delivered''. pending signals, checking for: See ``Checking for Pending Signals''. period of time: See ``Time Basics''. permission to access a file: See ``How Your Access to a File is Decided''. persona: See ``The Persona of a Process''. physical address: See ``Overview about traditional Unix memory handling''. physical memory: See ``Overview about traditional Unix memory handling''. pi (trigonometric constant): See ``Trigonometric Functions''. pipe: See ``Pipes and FIFOs''. pipe signal: See ``Operation Error Signals''. pipe to a subprocess: See ``Pipe to a Subprocess''. port number: See ``Internet Ports''. positioning a file descriptor: See ``Setting the File Position of a Descriptor''. positioning a stream: See ``File Positioning''. positive difference: See ``Miscellaneous FP arithmetic functions''. POSIX: See ``POSIX (The Portable Operating System Interface)''. POSIX capacity limits: See ``General Capacity Limits''. POSIX optional features: See ``Overall System Options''. POSIX.1: See ``POSIX (The Portable Operating System Interface)''. POSIX.2: See ``POSIX (The Portable Operating System Interface)''. power functions: See ``Exponentiation and Logarithms''. precision (of floating point number): See ``Floating Point Representation Concepts''. precision (printf): See ``Output Conversion Syntax''. predicates on arrays: See ``String/Array Comparison''. predicates on characters: See ``Classification of Characters''. predicates on strings: See ``String/Array Comparison''. preemptive scheduling: See ``Absolute Priority''. primitives, interrupting: See ``Primitives Interrupted by Signals''. printing character <1>: See ``Character class determination for wide characters''. printing character: See ``Classification of Characters''. priority of a process: See ``Process CPU Priority And Scheduling''. priority, absolute: See ``Absolute Priority''. process <1>: See ``Processes''. process: See ``The Basic Program/System Interface''. process completion: See ``Process Completion''. process group functions: See ``Functions for Job Control''. process group ID: See ``Launching Jobs''. process group leader: See ``Launching Jobs''. process groups: See ``Job Control''. process ID: See ``Process Creation Concepts''. process image: See ``Process Creation Concepts''. process lifetime: See ``Process Creation Concepts''. process priority: See ``Process CPU Priority And Scheduling''. process signal mask: See ``Process Signal Mask''. process termination: See ``Program Termination''. processor time <1>: See ``Processor Time Inquiry''. processor time: See ``Time Basics''. profiling alarm signal: See ``Alarm Signals''. profiling timer: See ``Setting an Alarm''. program: See ``The Basic Program/System Interface''. program argument syntax: See ``Program Argument Syntax Conventions''. program arguments: See ``Program Arguments''. program arguments, parsing: See ``Parsing Program Arguments''. program error signals: See ``Program Error Signals''. program name: See ``Error Messages''. program startup: See ``Program Arguments''. program termination: See ``Program Termination''. program termination signals: See ``Termination Signals''. programming your own streams: See ``Programming Your Own Custom Streams''. project complex numbers: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. protocol (of socket): See ``Socket Concepts''. protocol family: See ``Socket Concepts''. protocols: See ``NSS Basics''. protocols database: See ``Protocols Database''. prototypes for variadic functions: See ``Syntax for Variable Arguments''. pseudo-random numbers: See ``Pseudo-Random Numbers''. pseudo-terminals: See ``Pseudo-Terminals''. punctuation character <1>: See ``Character class determination for wide characters''. punctuation character: See ``Classification of Characters''. pushing input back: See ``Unreading''. quick sort function (for arrays): See ``Array Sort Function''. QUIT character: See ``Characters that Cause Signals''. quit signal: See ``Termination Signals''. quote removal: See ``The Stages of Word Expansion''. race conditions, relating to job control: See ``Launching Jobs''. race conditions, relating to signals: See ``Signals Arriving While a Handler Runs''. radix (of floating point number): See ``Floating Point Representation Concepts''. raising signals: See ``Generating Signals''. random numbers: See ``Pseudo-Random Numbers''. random-access files: See ``File Position''. range error: See ``Error Reporting by Mathematical Functions''. range of integer type: See ``Range of an Integer Type''. read lock: See ``File Locks''. reading from a directory: See ``Accessing Directories''. reading from a file descriptor: See ``Input and Output Primitives''. reading from a socket: See ``Transferring Data''. reading from a stream, by blocks: See ``Block Input/Output''. reading from a stream, by characters: See ``Character Input''. reading from a stream, formatted: See ``Formatted Input''. ready to run: See ``Absolute Priority''. real group ID: See ``The Persona of a Process''. real user ID: See ``The Persona of a Process''. real-time timer: See ``Setting an Alarm''. realtime CPU scheduling: See ``Absolute Priority''. realtime processing: See ``Why Lock Pages''. realtime scheduling: See ``Realtime Scheduling''. receiving datagrams: See ``Receiving Datagrams''. record locking: See ``File Locks''. redirecting input and output: See ``Duplicating Descriptors''. reentrant functions: See ``Signal Handling and Nonreentrant Functions''. reentrant NSS functions: See ``The Naming Scheme of the NSS Modules''. relative file name: See ``File Name Resolution''. removal of quotes: See ``The Stages of Word Expansion''. removing a file: See ``Deleting Files''. removing macros that shadow functions: See ``Macro Definitions of Functions''. renaming a file: See ``Renaming Files''. reporting bugs: See ``Reporting Bugs''. reporting errors: See ``Error Reporting''. REPRINT character: See ``Characters for Input Editing''. reserved names: See ``Reserved Names''. resource limits: See ``Limiting Resource Usage''. restarting interrupted primitives: See ``Primitives Interrupted by Signals''. restrictions on signal handler functions: See ``Signal Handling and Nonreentrant Functions''. root directory: See ``File Name Resolution''. Rot13: See ``Trivial Encryption''. rpc: See ``NSS Basics''. runnable process: See ``Absolute Priority''. running a command: See ``Running a Command''. saved set-group-ID: See ``How an Application Can Change Persona''. saved set-user-ID: See ``How an Application Can Change Persona''. scanning the group list: See ``Scanning the List of All Groups''. scanning the user list: See ``Scanning the List of All Users''. scatter-gather: See ``Fast Scatter-Gather I/O''. scheduling, traditional: See ``Traditional Scheduling''. search function (for arrays): See ``Array Search Function''. search functions (for strings): See ``Search Functions''. seed (for random numbers): See ``Pseudo-Random Numbers''. seeking on a file descriptor: See ``Setting the File Position of a Descriptor''. seeking on a stream: See ``File Positioning''. segmentation violation: See ``Program Error Signals''. sending a datagram: See ``Sending Datagrams''. sending signals: See ``Generating Signals''. sequential-access files: See ``File Position''. server: See ``Using Sockets with Connections''. services: See ``NSS Basics''. services database: See ``The Services Database''. session <1>: See ``Concepts of Job Control''. session: See ``Job Control''. session leader: See ``Concepts of Job Control''. setting an alarm: See ``Setting an Alarm''. setuid programs: See ``How an Application Can Change Persona''. setuid programs and file access: See ``Testing Permission to Access a File''. severity class <1>: See ``Adding Severity Classes''. severity class: See ``Printing Formatted Messages''. sgettext: See ``How to use `gettext' in GUI programs''. shadow: See ``NSS Basics''. shadowing functions with macros: See ``Macro Definitions of Functions''. shared lock: See ``File Locks''. shared memory: See ``Overview about traditional Unix memory handling''. shell: See ``Concepts of Job Control''. shift state: See ``Representing the state of the conversion''. Shift_JIS: See ``Introduction to Extended Characters''. shrinking objects: See ``Growing Objects''. shutting down a socket: See ``Closing a Socket''. sigaction flags: See ``Flags for `sigaction'''. sigaction function: See ``Advanced Signal Handling''. SIGCHLD, handling of: See ``Stopped and Terminated Jobs''. sign (of floating point number): See ``Floating Point Representation Concepts''. signal <1>: See ``Signal Handling''. signal: See ``FP Exceptions''. signal action: See ``How Signals Are Delivered''. signal actions: See ``Specifying Signal Actions''. signal flags: See ``Flags for `sigaction'''. signal function: See ``Basic Signal Handling''. signal handler function: See ``Defining Signal Handlers''. signal mask: See ``Process Signal Mask''. signal messages: See ``Signal Messages''. signal names: See ``Standard Signals''. signal number: See ``Standard Signals''. signal set: See ``Signal Sets''. signals, generating: See ``Generating Signals''. signedness: See ``Integers''. significand (of floating point number): See ``Floating Point Representation Concepts''. SIGTTIN, from background job: See ``Access to the Controlling Terminal''. SIGTTOU, from background job: See ``Access to the Controlling Terminal''. simple time: See ``Calendar Time''. single-byte string: See ``Representation of Strings''. size of string: See ``Representation of Strings''. SJIS: See ``Introduction to Extended Characters''. socket: See ``Sockets''. socket address (name) binding: See ``Socket Addresses''. socket domain: See ``Socket Concepts''. socket namespace: See ``Socket Concepts''. socket option level: See ``Socket Options''. socket options: See ``Socket Options''. socket pair: See ``Socket Pairs''. socket protocol: See ``Socket Concepts''. socket shutdown: See ``Closing a Socket''. socket, client actions: See ``Making a Connection''. socket, closing: See ``Closing a Socket''. socket, connecting: See ``Making a Connection''. socket, creating: See ``Creating a Socket''. socket, initiating a connection: See ``Making a Connection''. sockets, accepting connections: See ``Accepting Connections''. sockets, listening: See ``Listening for Connections''. sockets, server actions: See ``Listening for Connections''. soft limit: See ``Limiting Resource Usage''. soft link: See ``Symbolic Links''. sort function (for arrays): See ``Array Sort Function''. sparse files: See ``Setting the File Position of a Descriptor''. special files: See ``Making Special Files''. special functions: See ``Special Functions''. specified action (for a signal): See ``How Signals Are Delivered''. speed of execution: See ``Why Lock Pages''. square root function: See ``Exponentiation and Logarithms''. stable sorting: See ``Array Sort Function''. standard dot notation, for Internet addresses: See ``Internet Host Addresses''. standard environment variables: See ``Standard Environment Variables''. standard error file descriptor: See ``Descriptors and Streams''. standard error stream: See ``Standard Streams''. standard file descriptors: See ``Descriptors and Streams''. standard input file descriptor: See ``Descriptors and Streams''. standard input stream: See ``Standard Streams''. standard output file descriptor: See ``Descriptors and Streams''. standard output stream: See ``Standard Streams''. standard streams: See ``Standard Streams''. standards: See ``Standards and Portability''. START character: See ``Special Characters for Flow Control''. startup of program: See ``Program Arguments''. stateful <1>: See ``The `iconv' Implementation in the GNU C library''. stateful <2>: See ``A complete `iconv' example''. stateful <3>: See ``Generic Character Set Conversion Interface''. stateful <4>: See ``Converting Multibyte and Wide Character Strings''. stateful <5>: See ``Converting Single Characters''. stateful: See ``Representing the state of the conversion''. static memory allocation: See ``Memory Allocation in C Programs''. static storage class: See ``Memory Allocation in C Programs''. STATUS character: See ``Other Special Characters''. status codes: See ``Error Reporting''. status of a file: See ``The meaning of the File Attributes''. status of obstack: See ``Status of an Obstack''. sticky bit: See ``The Mode Bits for Access Permission''. STOP character: See ``Special Characters for Flow Control''. stop signal: See ``Job Control Signals''. stopped job: See ``Concepts of Job Control''. stopped jobs, continuing: See ``Continuing Stopped Jobs''. stopped jobs, detecting: See ``Stopped and Terminated Jobs''. storage allocation: See ``Virtual Memory Allocation And Paging''. stream (sockets): See ``Socket Concepts''. stream orientation <1>: See ``Streams in Internationalized Applications''. stream orientation: See ``Opening Streams''. stream, for I/O to a string: See ``String Streams''. streams and descriptors: See ``Dangers of Mixing Streams and Descriptors''. streams, and file descriptors: See ``Descriptors and Streams''. streams, C++: See ``Streams in Internationalized Applications''. streams, standard: See ``Standard Streams''. string: See ``Representation of Strings''. string allocation: See ``Representation of Strings''. string collation functions: See ``Collation Functions''. string comparison functions: See ``String/Array Comparison''. string concatenation functions: See ``Copying and Concatenation''. string copy functions: See ``Copying and Concatenation''. string length: See ``Representation of Strings''. string literal: See ``Representation of Strings''. string search functions: See ``Search Functions''. string stream: See ``String Streams''. string vectors, null-character separated: See ``Argz and Envz Vectors''. string, representation of: See ``Representation of Strings''. style of communication (of a socket): See ``Socket Concepts''. subshell: See ``Initializing the Shell''. substitution of variables and commands: See ``The Stages of Word Expansion''. successive signals: See ``Signals Close Together Merge into One''. summer time: See ``Broken-down Time''. SunOS: See ``Berkeley Unix''. supplementary group IDs: See ``The Persona of a Process''. SUSP character: See ``Characters that Cause Signals''. suspend character: See ``Characters that Cause Signals''. SVID: See ``SVID (The System V Interface Description)''. swap space: See ``Process Memory Concepts''. symbolic link: See ``Symbolic Links''. symbolic link, opening: See ``Open-time Flags''. synchronizing <1>: See ``Getting into a Consistent State''. synchronizing: See ``Synchronizing I/O operations''. syntax error messages, in argp: See ``Functions For Use in Argp Parsers''. syntax, for program arguments: See ``Program Argument Syntax Conventions''. syntax, for reading numbers: See ``Parsing of Numbers''. sysconf <1>: See ``Learn about the processors available''. sysconf: See ``How to get information about the memory subsystem?''. system call: See ``System Calls''. system call number: See ``System Calls''. System V Unix: See ``SVID (The System V Interface Description)''. TCP (Internet protocol): See ``Protocols Database''. template, for printf: See ``Formatted Output''. template, for scanf: See ``Formatted Input''. TERM environment variable: See ``Standard Environment Variables''. terminal flow control: See ``Line Control Functions''. terminal identification: See ``Identifying Terminals''. terminal input queue: See ``I/O Queues''. terminal input queue, clearing: See ``Line Control Functions''. terminal input signal: See ``Job Control Signals''. terminal line control functions: See ``Line Control Functions''. terminal line speed: See ``Line Speed''. terminal mode data types: See ``Terminal Mode Data Types''. terminal mode functions: See ``Terminal Mode Functions''. terminal modes, BSD: See ``BSD Terminal Modes''. terminal output queue: See ``I/O Queues''. terminal output queue, flushing: See ``Line Control Functions''. terminal output signal: See ``Job Control Signals''. terminated jobs, detecting: See ``Stopped and Terminated Jobs''. termination signal: See ``Termination Signals''. testing access permission: See ``Testing Permission to Access a File''. testing exit status of child process: See ``Process Completion''. text stream: See ``Text and Binary Streams''. thrashing: See ``Overview about traditional Unix memory handling''. thread of control: See ``The Basic Program/System Interface''. threads: See ``Streams and Threads''. ticks, clock: See ``Processor And CPU Time''. tilde expansion: See ``The Stages of Word Expansion''. time: See ``Time Basics''. TIME termios slot: See ``Noncanonical Input''. time zone: See ``Specifying the Time Zone with `TZ'''. time zone database: See ``Specifying the Time Zone with `TZ'''. time, elapsed: See ``Time Basics''. time, high precision: See ``High Accuracy Clock''. timer, profiling: See ``Setting an Alarm''. timer, real-time: See ``Setting an Alarm''. timer, virtual: See ``Setting an Alarm''. timers, setting: See ``Setting an Alarm''. timespec: See ``Elapsed Time''. timeval: See ``Elapsed Time''. timing error in signal handling: See ``Remembering a Signal to Act On Later''. TMPDIR environment variable: See ``Temporary Files''. tokenizing strings: See ``Finding Tokens in a String''. tools, for installing library: See ``Recommended Tools for Compilation''. transmitting datagrams: See ``Sending Datagrams''. tree, directory: See ``Working with Directory Trees''. triangulation: See ``The `iconv' Implementation in the GNU C library''. trigonometric functions: See ``Trigonometric Functions''. type measurements, floating: See ``Floating Type Macros''. type measurements, integer: See ``Computing the Width of an Integer Data Type''. type modifier character (printf): See ``Output Conversion Syntax''. type modifier character (scanf): See ``Input Conversion Syntax''. typeahead buffer: See ``I/O Queues''. TZ environment variable: See ``Standard Environment Variables''. UCS-2: See ``Introduction to Extended Characters''. UCS-4: See ``Introduction to Extended Characters''. ulps: See ``Known Maximum Errors in Math Functions''. umask: See ``Assigning File Permissions''. unbuffered stream: See ``Buffering Concepts''. unconstrained memory allocation: See ``Unconstrained Allocation''. undefining macros that shadow functions: See ``Macro Definitions of Functions''. underflow exception: See ``FP Exceptions''. Unicode: See ``Introduction to Extended Characters''. Unix, Berkeley: See ``Berkeley Unix''. Unix, System V: See ``SVID (The System V Interface Description)''. unlinking a file: See ``Deleting Files''. unordered comparison: See ``Floating-Point Comparison Functions''. unreading characters: See ``Unreading''. upgrading from libc5: See ``Specific advice for Linux systems''. upper-case character <1>: See ``Character class determination for wide characters''. upper-case character: See ``Classification of Characters''. urgent data signal: See ``Asynchronous I/O Signals''. urgent socket condition: See ``Out-of-Band Data''. usage limits: See ``Limiting Resource Usage''. usage messages, in argp: See ``Functions For Use in Argp Parsers''. user accounting database: See ``The User Accounting Database''. user database: See ``User Database''. user ID: See ``User and Group IDs''. user ID, determining: See ``Identifying Who Logged In''. user name: See ``User and Group IDs''. user signals: See ``Miscellaneous Signals''. usual file name errors: See ``File Name Errors''. UTF-16: See ``Introduction to Extended Characters''. UTF-7: See ``Introduction to Extended Characters''. UTF-8: See ``Introduction to Extended Characters''. va_copy: See ``Copying and Concatenation''. variable number of arguments: See ``Variadic Functions''. variable substitution: See ``The Stages of Word Expansion''. variable-sized arrays: See ``GNU C Variable-Size Arrays''. variadic function argument access: See ``Receiving the Argument Values''. variadic function prototypes: See ``Syntax for Variable Arguments''. variadic functions: See ``Variadic Functions''. variadic functions, calling: See ``Calling Variadic Functions''. virtual time alarm signal: See ``Alarm Signals''. virtual timer: See ``Setting an Alarm''. volatile declarations: See ``Signal Handling and Nonreentrant Functions''. waiting for a signal: See ``Waiting for a Signal''. waiting for completion of child process: See ``Process Completion''. waiting for input or output: See ``Waiting for Input or Output''. WERASE character: See ``Characters for Input Editing''. whitespace character <1>: See ``Character class determination for wide characters''. whitespace character: See ``Classification of Characters''. wide character: See ``Introduction to Extended Characters''. wide character string: See ``Representation of Strings''. width of integer type: See ``Computing the Width of an Integer Data Type''. wildcard expansion: See ``The Stages of Word Expansion''. wint_t: See ``String and Array Conventions''. word expansion: See ``Shell-Style Word Expansion''. working directory: See ``Working Directory''. write lock: See ``File Locks''. writing to a file descriptor: See ``Input and Output Primitives''. writing to a socket: See ``Transferring Data''. writing to a stream, by blocks: See ``Block Input/Output''. writing to a stream, by characters: See ``Simple Output by Characters or Lines''. writing to a stream, formatted: See ``Formatted Output''. YP: See ``Host Identification''. YP domain name: See ``Host Identification''. zero divide: See ``FP Exceptions''. Type Index ********** __ftw64_func_t: See ``Working with Directory Trees''. __ftw_func_t: See ``Working with Directory Trees''. __nftw64_func_t: See ``Working with Directory Trees''. __nftw_func_t: See ``Working with Directory Trees''. blkcnt64_t: See ``The meaning of the File Attributes''. blkcnt_t: See ``The meaning of the File Attributes''. cc_t: See ``Terminal Mode Data Types''. clock_t: See ``CPU Time Inquiry''. comparison_fn_t: See ``Defining the Comparison Function''. cookie_close_function: See ``Custom Stream Hook Functions''. cookie_io_functions_t: See ``Custom Streams and Cookies''. cookie_read_function: See ``Custom Stream Hook Functions''. cookie_seek_function: See ``Custom Stream Hook Functions''. cookie_write_function: See ``Custom Stream Hook Functions''. dev_t: See ``The meaning of the File Attributes''. DIR: See ``Opening a Directory Stream''. div_t: See ``Integer Division''. enum mcheck_status: See ``Heap Consistency Checking''. fd_set: See ``Waiting for Input or Output''. FILE: See ``Streams''. fpos64_t: See ``Portable File-Position Functions''. fpos_t: See ``Portable File-Position Functions''. gid_t: See ``Reading the Persona of a Process''. glob64_t: See ``Calling `glob'''. glob_t: See ``Calling `glob'''. iconv_t: See ``Generic Character Set Conversion Interface''. imaxdiv_t: See ``Integer Division''. ino64_t: See ``The meaning of the File Attributes''. ino_t: See ``The meaning of the File Attributes''. jmp_buf: See ``Details of Non-Local Exits''. ldiv_t: See ``Integer Division''. lldiv_t: See ``Integer Division''. mbstate_t: See ``Representing the state of the conversion''. mode_t: See ``The meaning of the File Attributes''. nlink_t: See ``The meaning of the File Attributes''. off64_t: See ``Setting the File Position of a Descriptor''. off_t: See ``Setting the File Position of a Descriptor''. pid_t: See ``Process Identification''. printf_arginfo_function: See ``Defining the Output Handler''. printf_function: See ``Defining the Output Handler''. ptrdiff_t: See ``Important Data Types''. regex_t: See ``POSIX Regular Expression Compilation''. regmatch_t: See ``Match Results with Subexpressions''. regoff_t: See ``Match Results with Subexpressions''. sig_atomic_t: See ``Atomic Types''. sighandler_t: See ``Basic Signal Handling''. sigjmp_buf: See ``Non-Local Exits and Signals''. sigset_t: See ``Signal Sets''. size_t: See ``Important Data Types''. speed_t: See ``Line Speed''. ssize_t: See ``Input and Output Primitives''. stack_t: See ``Using a Separate Signal Stack''. struct __gconv_step: See ``The `iconv' Implementation in the GNU C library''. struct __gconv_step_data: See ``The `iconv' Implementation in the GNU C library''. struct aiocb: See ``Perform I/O Operations in Parallel''. struct aiocb64: See ``Perform I/O Operations in Parallel''. struct aioinit: See ``How to optimize the AIO implementation''. struct argp: See ``Specifying Argp Parsers''. struct argp_child: See ``Combining Multiple Argp Parsers''. struct argp_option: See ``Specifying Options in an Argp Parser''. struct argp_state: See ``Argp Parsing State''. struct dirent: See ``Format of a Directory Entry''. struct ENTRY: See ``The `hsearch' function.''. struct exit_status: See ``Manipulating the User Accounting Database''. struct flock: See ``File Locks''. struct fstab: See ``The `fstab' file''. struct FTW: See ``Working with Directory Trees''. struct group: See ``The Data Structure for a Group''. struct hostent: See ``Host Names''. struct if_nameindex: See ``Interface Naming''. struct in6_addr: See ``Host Address Data Type''. struct in_addr: See ``Host Address Data Type''. struct iovec: See ``Fast Scatter-Gather I/O''. struct itimerval: See ``Setting an Alarm''. struct lconv: See ```localeconv': It is portable but ...''. struct linger: See ``Socket-Level Options''. struct mallinfo: See ``Statistics for Memory Allocation with `malloc'''. struct mntent: See ``The `mtab' file''. struct netent: See ``Networks Database''. struct ntptimeval: See ``High Accuracy Clock''. struct obstack: See ``Creating Obstacks''. struct option: See ``Parsing Long Options with `getopt_long'''. struct passwd: See ``The Data Structure that Describes a User''. struct printf_info: See ``Conversion Specifier Options''. struct protoent: See ``Protocols Database''. struct random_data: See ``BSD Random Number Functions''. struct rlimit: See ``Limiting Resource Usage''. struct rlimit64: See ``Limiting Resource Usage''. struct rusage: See ``Resource Usage''. struct sched_param: See ``Basic Scheduling Functions''. struct servent: See ``The Services Database''. struct sgttyb: See ``BSD Terminal Modes''. struct sigaction: See ``Advanced Signal Handling''. struct sigstack: See ``Using a Separate Signal Stack''. struct sigvec: See ``BSD Function to Establish a Handler''. struct sockaddr: See ``Address Formats''. struct sockaddr_in: See ``Internet Socket Address Formats''. struct sockaddr_in6: See ``Internet Socket Address Formats''. struct sockaddr_un: See ``Details of Local Namespace''. struct stat: See ``The meaning of the File Attributes''. struct stat64: See ``The meaning of the File Attributes''. struct termios: See ``Terminal Mode Data Types''. struct timespec: See ``Elapsed Time''. struct timeval: See ``Elapsed Time''. struct timex: See ``High Accuracy Clock''. struct timezone: See ``High-Resolution Calendar''. struct tm: See ``Broken-down Time''. struct tms: See ``Processor Time Inquiry''. struct utimbuf: See ``File Times''. struct utmp: See ``Manipulating the User Accounting Database''. struct utmpx: See ``XPG User Accounting Database Functions''. struct utsname: See ``Platform Type Identification''. struct vtimes: See ``Resource Usage''. tcflag_t: See ``Terminal Mode Data Types''. time_t: See ``Simple Calendar Time''. ucontext_t: See ``Complete Context Control''. uid_t: See ``Reading the Persona of a Process''. union wait: See ``BSD Process Wait Functions''. va_list: See ``Argument Access Macros''. VISIT: See ``The `tsearch' function.''. wchar_t: See ``Introduction to Extended Characters''. wctrans_t: See ``Mapping of wide characters.''. wctype_t: See ``Character class determination for wide characters''. wint_t: See ``Introduction to Extended Characters''. wordexp_t: See ``Calling `wordexp'''. Function and Macro Index ************************ __fbufsize: See ``Controlling Which Kind of Buffering''. __flbf: See ``Controlling Which Kind of Buffering''. __fpending: See ``Controlling Which Kind of Buffering''. __fpurge: See ``Flushing Buffers''. __freadable: See ``Opening Streams''. __freading: See ``Opening Streams''. __fsetlocking: See ``Streams and Threads''. __fwritable: See ``Opening Streams''. __fwriting: See ``Opening Streams''. __va_copy: See ``Argument Access Macros''. _Exit: See ``Termination Internals''. _exit: See ``Termination Internals''. _flushlbf: See ``Flushing Buffers''. _tolower: See ``Case Conversion''. _toupper: See ``Case Conversion''. a64l: See ``Encode Binary Data''. abort: See ``Aborting a Program''. abs: See ``Absolute Value''. accept: See ``Accepting Connections''. access: See ``Testing Permission to Access a File''. acos: See ``Inverse Trigonometric Functions''. acosf: See ``Inverse Trigonometric Functions''. acosh: See ``Hyperbolic Functions''. acoshf: See ``Hyperbolic Functions''. acoshl: See ``Hyperbolic Functions''. acosl: See ``Inverse Trigonometric Functions''. addmntent: See ``The `mtab' file''. addseverity: See ``Adding Severity Classes''. adjtime: See ``High-Resolution Calendar''. adjtimex: See ``High-Resolution Calendar''. aio_cancel: See ``Cancellation of AIO Operations''. aio_cancel64: See ``Cancellation of AIO Operations''. aio_error: See ``Getting the Status of AIO Operations''. aio_error64: See ``Getting the Status of AIO Operations''. aio_fsync: See ``Getting into a Consistent State''. aio_fsync64: See ``Getting into a Consistent State''. aio_init: See ``How to optimize the AIO implementation''. aio_read: See ``Asynchronous Read and Write Operations''. aio_read64: See ``Asynchronous Read and Write Operations''. aio_return: See ``Getting the Status of AIO Operations''. aio_return64: See ``Getting the Status of AIO Operations''. aio_suspend: See ``Getting into a Consistent State''. aio_suspend64: See ``Getting into a Consistent State''. aio_write: See ``Asynchronous Read and Write Operations''. aio_write64: See ``Asynchronous Read and Write Operations''. alarm: See ``Setting an Alarm''. alloca: See ``Automatic Storage with Variable Size''. alphasort: See ``Scanning the Content of a Directory''. alphasort64: See ``Scanning the Content of a Directory''. argp_error: See ``Functions For Use in Argp Parsers''. argp_failure: See ``Functions For Use in Argp Parsers''. argp_help: See ``The `argp_help' Function''. argp_parse: See ``Parsing Program Options with Argp''. argp_state_help: See ``Functions For Use in Argp Parsers''. argp_usage: See ``Functions For Use in Argp Parsers''. argz_add: See ``Argz Functions''. argz_add_sep: See ``Argz Functions''. argz_append: See ``Argz Functions''. argz_count: See ``Argz Functions''. argz_create: See ``Argz Functions''. argz_create_sep: See ``Argz Functions''. argz_delete: See ``Argz Functions''. argz_extract: See ``Argz Functions''. argz_insert: See ``Argz Functions''. argz_next: See ``Argz Functions''. argz_replace: See ``Argz Functions''. argz_stringify: See ``Argz Functions''. asctime: See ``Formatting Calendar Time''. asctime_r: See ``Formatting Calendar Time''. asin: See ``Inverse Trigonometric Functions''. asinf: See ``Inverse Trigonometric Functions''. asinh: See ``Hyperbolic Functions''. asinhf: See ``Hyperbolic Functions''. asinhl: See ``Hyperbolic Functions''. asinl: See ``Inverse Trigonometric Functions''. asprintf: See ``Dynamically Allocating Formatted Output''. assert: See ``Explicitly Checking Internal Consistency''. assert_perror: See ``Explicitly Checking Internal Consistency''. atan: See ``Inverse Trigonometric Functions''. atan2: See ``Inverse Trigonometric Functions''. atan2f: See ``Inverse Trigonometric Functions''. atan2l: See ``Inverse Trigonometric Functions''. atanf: See ``Inverse Trigonometric Functions''. atanh: See ``Hyperbolic Functions''. atanhf: See ``Hyperbolic Functions''. atanhl: See ``Hyperbolic Functions''. atanl: See ``Inverse Trigonometric Functions''. atexit: See ``Cleanups on Exit''. atof: See ``Parsing of Floats''. atoi: See ``Parsing of Integers''. atol: See ``Parsing of Integers''. atoll: See ``Parsing of Integers''. backtrace: See ``Backtraces''. backtrace_symbols: See ``Backtraces''. backtrace_symbols_fd: See ``Backtraces''. basename: See ``Finding Tokens in a String''. bcmp: See ``String/Array Comparison''. bcopy: See ``Copying and Concatenation''. bind: See ``Setting the Address of a Socket''. bind_textdomain_codeset: See ``How to specify the output character set `gettext' uses''. bindtextdomain: See ``How to determine which catalog to be used''. brk: See ``Resizing the Data Segment''. bsearch: See ``Array Search Function''. btowc: See ``Converting Single Characters''. bzero: See ``Copying and Concatenation''. cabs: See ``Absolute Value''. cabsf: See ``Absolute Value''. cabsl: See ``Absolute Value''. cacos: See ``Inverse Trigonometric Functions''. cacosf: See ``Inverse Trigonometric Functions''. cacosh: See ``Hyperbolic Functions''. cacoshf: See ``Hyperbolic Functions''. cacoshl: See ``Hyperbolic Functions''. cacosl: See ``Inverse Trigonometric Functions''. calloc: See ``Allocating Cleared Space''. canonicalize_file_name: See ``Symbolic Links''. carg: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cargf: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cargl: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. casin: See ``Inverse Trigonometric Functions''. casinf: See ``Inverse Trigonometric Functions''. casinh: See ``Hyperbolic Functions''. casinhf: See ``Hyperbolic Functions''. casinhl: See ``Hyperbolic Functions''. casinl: See ``Inverse Trigonometric Functions''. catan: See ``Inverse Trigonometric Functions''. catanf: See ``Inverse Trigonometric Functions''. catanh: See ``Hyperbolic Functions''. catanhf: See ``Hyperbolic Functions''. catanhl: See ``Hyperbolic Functions''. catanl: See ``Inverse Trigonometric Functions''. catclose: See ``The `catgets' function family''. catgets: See ``The `catgets' function family''. catopen: See ``The `catgets' function family''. cbc_crypt: See ``DES Encryption''. cbrt: See ``Exponentiation and Logarithms''. cbrtf: See ``Exponentiation and Logarithms''. cbrtl: See ``Exponentiation and Logarithms''. ccos: See ``Trigonometric Functions''. ccosf: See ``Trigonometric Functions''. ccosh: See ``Hyperbolic Functions''. ccoshf: See ``Hyperbolic Functions''. ccoshl: See ``Hyperbolic Functions''. ccosl: See ``Trigonometric Functions''. ceil: See ``Rounding Functions''. ceilf: See ``Rounding Functions''. ceill: See ``Rounding Functions''. cexp: See ``Exponentiation and Logarithms''. cexpf: See ``Exponentiation and Logarithms''. cexpl: See ``Exponentiation and Logarithms''. cfgetispeed: See ``Line Speed''. cfgetospeed: See ``Line Speed''. cfmakeraw: See ``Noncanonical Input''. cfree: See ``Freeing Memory Allocated with `malloc'''. cfsetispeed: See ``Line Speed''. cfsetospeed: See ``Line Speed''. cfsetspeed: See ``Line Speed''. chdir: See ``Working Directory''. chmod: See ``Assigning File Permissions''. chown: See ``File Owner''. cimag: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cimagf: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cimagl: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. clearenv: See ``Environment Access''. clearerr: See ``Recovering from errors''. clearerr_unlocked: See ``Recovering from errors''. clock: See ``CPU Time Inquiry''. clog: See ``Exponentiation and Logarithms''. clog10: See ``Exponentiation and Logarithms''. clog10f: See ``Exponentiation and Logarithms''. clog10l: See ``Exponentiation and Logarithms''. clogf: See ``Exponentiation and Logarithms''. clogl: See ``Exponentiation and Logarithms''. close: See ``Opening and Closing Files''. closedir: See ``Reading and Closing a Directory Stream''. closelog: See ``closelog''. confstr: See ``String-Valued Parameters''. conj: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. conjf: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. conjl: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. connect: See ``Making a Connection''. copysign: See ``Setting and modifying single bits of FP values''. copysignf: See ``Setting and modifying single bits of FP values''. copysignl: See ``Setting and modifying single bits of FP values''. cos: See ``Trigonometric Functions''. cosf: See ``Trigonometric Functions''. cosh: See ``Hyperbolic Functions''. coshf: See ``Hyperbolic Functions''. coshl: See ``Hyperbolic Functions''. cosl: See ``Trigonometric Functions''. cpow: See ``Exponentiation and Logarithms''. cpowf: See ``Exponentiation and Logarithms''. cpowl: See ``Exponentiation and Logarithms''. cproj: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cprojf: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. cprojl: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. creal: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. crealf: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. creall: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. creat: See ``Opening and Closing Files''. creat64: See ``Opening and Closing Files''. crypt: See ``Encrypting Passwords''. crypt_r: See ``Encrypting Passwords''. csin: See ``Trigonometric Functions''. csinf: See ``Trigonometric Functions''. csinh: See ``Hyperbolic Functions''. csinhf: See ``Hyperbolic Functions''. csinhl: See ``Hyperbolic Functions''. csinl: See ``Trigonometric Functions''. csqrt: See ``Exponentiation and Logarithms''. csqrtf: See ``Exponentiation and Logarithms''. csqrtl: See ``Exponentiation and Logarithms''. ctan: See ``Trigonometric Functions''. ctanf: See ``Trigonometric Functions''. ctanh: See ``Hyperbolic Functions''. ctanhf: See ``Hyperbolic Functions''. ctanhl: See ``Hyperbolic Functions''. ctanl: See ``Trigonometric Functions''. ctermid: See ``Identifying the Controlling Terminal''. ctime: See ``Formatting Calendar Time''. ctime_r: See ``Formatting Calendar Time''. cuserid: See ``Identifying Who Logged In''. dcgettext: See ``What has to be done to translate a message?''. dcngettext: See ``Additional functions for more complicated situations''. DES_FAILED: See ``DES Encryption''. des_setparity: See ``DES Encryption''. dgettext: See ``What has to be done to translate a message?''. difftime: See ``Elapsed Time''. dirfd: See ``Opening a Directory Stream''. dirname: See ``Finding Tokens in a String''. div: See ``Integer Division''. dngettext: See ``Additional functions for more complicated situations''. drand48: See ``SVID Random Number Function''. drand48_r: See ``SVID Random Number Function''. drem: See ``Remainder Functions''. dremf: See ``Remainder Functions''. dreml: See ``Remainder Functions''. DTTOIF: See ``Format of a Directory Entry''. dup: See ``Duplicating Descriptors''. dup2: See ``Duplicating Descriptors''. ecb_crypt: See ``DES Encryption''. ecvt: See ``Old-fashioned System V number-to-string functions''. ecvt_r: See ``Old-fashioned System V number-to-string functions''. encrypt: See ``DES Encryption''. encrypt_r: See ``DES Encryption''. endfsent: See ``The `fstab' file''. endgrent: See ``Scanning the List of All Groups''. endhostent: See ``Host Names''. endmntent: See ``The `mtab' file''. endnetent: See ``Networks Database''. endnetgrent: See ``Looking up one Netgroup''. endprotoent: See ``Protocols Database''. endpwent: See ``Scanning the List of All Users''. endservent: See ``The Services Database''. endutent: See ``Manipulating the User Accounting Database''. endutxent: See ``XPG User Accounting Database Functions''. envz_add: See ``Envz Functions''. envz_entry: See ``Envz Functions''. envz_get: See ``Envz Functions''. envz_merge: See ``Envz Functions''. envz_strip: See ``Envz Functions''. erand48: See ``SVID Random Number Function''. erand48_r: See ``SVID Random Number Function''. erf: See ``Special Functions''. erfc: See ``Special Functions''. erfcf: See ``Special Functions''. erfcl: See ``Special Functions''. erff: See ``Special Functions''. erfl: See ``Special Functions''. err: See ``Error Messages''. error: See ``Error Messages''. error_at_line: See ``Error Messages''. errx: See ``Error Messages''. execl: See ``Executing a File''. execle: See ``Executing a File''. execlp: See ``Executing a File''. execv: See ``Executing a File''. execve: See ``Executing a File''. execvp: See ``Executing a File''. exit: See ``Normal Termination''. exp: See ``Exponentiation and Logarithms''. exp10: See ``Exponentiation and Logarithms''. exp10f: See ``Exponentiation and Logarithms''. exp10l: See ``Exponentiation and Logarithms''. exp2: See ``Exponentiation and Logarithms''. exp2f: See ``Exponentiation and Logarithms''. exp2l: See ``Exponentiation and Logarithms''. expf: See ``Exponentiation and Logarithms''. expl: See ``Exponentiation and Logarithms''. expm1: See ``Exponentiation and Logarithms''. expm1f: See ``Exponentiation and Logarithms''. expm1l: See ``Exponentiation and Logarithms''. fabs: See ``Absolute Value''. fabsf: See ``Absolute Value''. fabsl: See ``Absolute Value''. fchdir: See ``Working Directory''. fchmod: See ``Assigning File Permissions''. fchown: See ``File Owner''. fclean: See ``Cleaning Streams''. fclose: See ``Closing Streams''. fcloseall: See ``Closing Streams''. fcntl: See ``Control Operations on Files''. fcvt: See ``Old-fashioned System V number-to-string functions''. fcvt_r: See ``Old-fashioned System V number-to-string functions''. FD_CLR: See ``Waiting for Input or Output''. FD_ISSET: See ``Waiting for Input or Output''. FD_SET: See ``Waiting for Input or Output''. FD_ZERO: See ``Waiting for Input or Output''. fdatasync: See ``Synchronizing I/O operations''. fdim: See ``Miscellaneous FP arithmetic functions''. fdimf: See ``Miscellaneous FP arithmetic functions''. fdiml: See ``Miscellaneous FP arithmetic functions''. fdopen: See ``Descriptors and Streams''. feclearexcept: See ``Examining the FPU status word''. fedisableexcept: See ``Floating-Point Control Functions''. feenableexcept: See ``Floating-Point Control Functions''. fegetenv: See ``Floating-Point Control Functions''. fegetexcept: See ``Floating-Point Control Functions''. fegetexceptflag: See ``Examining the FPU status word''. fegetround: See ``Rounding Modes''. feholdexcept: See ``Floating-Point Control Functions''. feof: See ``End-Of-File and Errors''. feof_unlocked: See ``End-Of-File and Errors''. feraiseexcept: See ``Examining the FPU status word''. ferror: See ``End-Of-File and Errors''. ferror_unlocked: See ``End-Of-File and Errors''. fesetenv: See ``Floating-Point Control Functions''. fesetexceptflag: See ``Examining the FPU status word''. fesetround: See ``Rounding Modes''. fetestexcept: See ``Examining the FPU status word''. feupdateenv: See ``Floating-Point Control Functions''. fflush: See ``Flushing Buffers''. fflush_unlocked: See ``Flushing Buffers''. fgetc: See ``Character Input''. fgetc_unlocked: See ``Character Input''. fgetgrent: See ``Scanning the List of All Groups''. fgetgrent_r: See ``Scanning the List of All Groups''. fgetpos: See ``Portable File-Position Functions''. fgetpos64: See ``Portable File-Position Functions''. fgetpwent: See ``Scanning the List of All Users''. fgetpwent_r: See ``Scanning the List of All Users''. fgets: See ``Line-Oriented Input''. fgets_unlocked: See ``Line-Oriented Input''. fgetwc: See ``Character Input''. fgetwc_unlocked: See ``Character Input''. fgetws: See ``Line-Oriented Input''. fgetws_unlocked: See ``Line-Oriented Input''. fileno: See ``Descriptors and Streams''. fileno_unlocked: See ``Descriptors and Streams''. finite: See ``Floating-Point Number Classification Functions''. finitef: See ``Floating-Point Number Classification Functions''. finitel: See ``Floating-Point Number Classification Functions''. flockfile: See ``Streams and Threads''. floor: See ``Rounding Functions''. floorf: See ``Rounding Functions''. floorl: See ``Rounding Functions''. fma: See ``Miscellaneous FP arithmetic functions''. fmaf: See ``Miscellaneous FP arithmetic functions''. fmal: See ``Miscellaneous FP arithmetic functions''. fmax: See ``Miscellaneous FP arithmetic functions''. fmaxf: See ``Miscellaneous FP arithmetic functions''. fmaxl: See ``Miscellaneous FP arithmetic functions''. fmemopen: See ``String Streams''. fmin: See ``Miscellaneous FP arithmetic functions''. fminf: See ``Miscellaneous FP arithmetic functions''. fminl: See ``Miscellaneous FP arithmetic functions''. fmod: See ``Remainder Functions''. fmodf: See ``Remainder Functions''. fmodl: See ``Remainder Functions''. fmtmsg: See ``Printing Formatted Messages''. fnmatch: See ``Wildcard Matching''. fopen: See ``Opening Streams''. fopen64: See ``Opening Streams''. fopencookie: See ``Custom Streams and Cookies''. fork: See ``Creating a Process''. forkpty: See ``Opening a Pseudo-Terminal Pair''. fpathconf: See ``Using `pathconf'''. fpclassify: See ``Floating-Point Number Classification Functions''. fprintf: See ``Formatted Output Functions''. fputc: See ``Simple Output by Characters or Lines''. fputc_unlocked: See ``Simple Output by Characters or Lines''. fputs: See ``Simple Output by Characters or Lines''. fputs_unlocked: See ``Simple Output by Characters or Lines''. fputwc: See ``Simple Output by Characters or Lines''. fputwc_unlocked: See ``Simple Output by Characters or Lines''. fputws: See ``Simple Output by Characters or Lines''. fputws_unlocked: See ``Simple Output by Characters or Lines''. fread: See ``Block Input/Output''. fread_unlocked: See ``Block Input/Output''. free: See ``Freeing Memory Allocated with `malloc'''. freopen: See ``Opening Streams''. freopen64: See ``Opening Streams''. frexp: See ``Normalization Functions''. frexpf: See ``Normalization Functions''. frexpl: See ``Normalization Functions''. fscanf: See ``Formatted Input Functions''. fseek: See ``File Positioning''. fseeko: See ``File Positioning''. fseeko64: See ``File Positioning''. fsetpos: See ``Portable File-Position Functions''. fsetpos64: See ``Portable File-Position Functions''. fstat: See ``Reading the Attributes of a File''. fstat64: See ``Reading the Attributes of a File''. fsync: See ``Synchronizing I/O operations''. ftell: See ``File Positioning''. ftello: See ``File Positioning''. ftello64: See ``File Positioning''. ftruncate: See ``File Size''. ftruncate64: See ``File Size''. ftrylockfile: See ``Streams and Threads''. ftw: See ``Working with Directory Trees''. ftw64: See ``Working with Directory Trees''. funlockfile: See ``Streams and Threads''. fwide: See ``Streams in Internationalized Applications''. fwprintf: See ``Formatted Output Functions''. fwrite: See ``Block Input/Output''. fwrite_unlocked: See ``Block Input/Output''. fwscanf: See ``Formatted Input Functions''. gamma: See ``Special Functions''. gammaf: See ``Special Functions''. gammal: See ``Special Functions''. gcvt: See ``Old-fashioned System V number-to-string functions''. get_avphys_pages: See ``How to get information about the memory subsystem?''. get_current_dir_name: See ``Working Directory''. get_nprocs: See ``Learn about the processors available''. get_nprocs_conf: See ``Learn about the processors available''. get_phys_pages: See ``How to get information about the memory subsystem?''. getc: See ``Character Input''. getc_unlocked: See ``Character Input''. getchar: See ``Character Input''. getchar_unlocked: See ``Character Input''. getcontext: See ``Complete Context Control''. getcwd: See ``Working Directory''. getdate: See ``A More User-friendly Way to Parse Times and Dates''. getdate_r: See ``A More User-friendly Way to Parse Times and Dates''. getdelim: See ``Line-Oriented Input''. getdomainnname: See ``Host Identification''. getegid: See ``Reading the Persona of a Process''. getenv: See ``Environment Access''. geteuid: See ``Reading the Persona of a Process''. getfsent: See ``The `fstab' file''. getfsfile: See ``The `fstab' file''. getfsspec: See ``The `fstab' file''. getgid: See ``Reading the Persona of a Process''. getgrent: See ``Scanning the List of All Groups''. getgrent_r: See ``Scanning the List of All Groups''. getgrgid: See ``Looking Up One Group''. getgrgid_r: See ``Looking Up One Group''. getgrnam: See ``Looking Up One Group''. getgrnam_r: See ``Looking Up One Group''. getgrouplist: See ``Setting the Group IDs''. getgroups: See ``Reading the Persona of a Process''. gethostbyaddr: See ``Host Names''. gethostbyaddr_r: See ``Host Names''. gethostbyname: See ``Host Names''. gethostbyname2: See ``Host Names''. gethostbyname2_r: See ``Host Names''. gethostbyname_r: See ``Host Names''. gethostent: See ``Host Names''. gethostid: See ``Host Identification''. gethostname: See ``Host Identification''. getitimer: See ``Setting an Alarm''. getline: See ``Line-Oriented Input''. getloadavg: See ``Learn about the processors available''. getlogin: See ``Identifying Who Logged In''. getmntent: See ``The `mtab' file''. getmntent_r: See ``The `mtab' file''. getnetbyaddr: See ``Networks Database''. getnetbyname: See ``Networks Database''. getnetent: See ``Networks Database''. getnetgrent: See ``Looking up one Netgroup''. getnetgrent_r: See ``Looking up one Netgroup''. getopt: See ``Using the `getopt' function''. getopt_long: See ``Parsing Long Options with `getopt_long'''. getopt_long_only: See ``Parsing Long Options with `getopt_long'''. getpagesize: See ``How to get information about the memory subsystem?''. getpass: See ``Reading Passwords''. getpeername: See ``Who is Connected to Me?''. getpgid: See ``Process Group Functions''. getpgrp: See ``Process Group Functions''. getpid: See ``Process Identification''. getppid: See ``Process Identification''. getpriority: See ``Functions For Traditional Scheduling''. getprotobyname: See ``Protocols Database''. getprotobynumber: See ``Protocols Database''. getprotoent: See ``Protocols Database''. getpt: See ``Allocating Pseudo-Terminals''. getpwent: See ``Scanning the List of All Users''. getpwent_r: See ``Scanning the List of All Users''. getpwnam: See ``Looking Up One User''. getpwnam_r: See ``Looking Up One User''. getpwuid: See ``Looking Up One User''. getpwuid_r: See ``Looking Up One User''. getrlimit: See ``Limiting Resource Usage''. getrlimit64: See ``Limiting Resource Usage''. getrusage: See ``Resource Usage''. gets: See ``Line-Oriented Input''. getservbyname: See ``The Services Database''. getservbyport: See ``The Services Database''. getservent: See ``The Services Database''. getsid: See ``Process Group Functions''. getsockname: See ``Reading the Address of a Socket''. getsockopt: See ``Socket Option Functions''. getsubopt: See ``Parsing of Suboptions''. gettext: See ``What has to be done to translate a message?''. gettimeofday: See ``High-Resolution Calendar''. getuid: See ``Reading the Persona of a Process''. getumask: See ``Assigning File Permissions''. getutent: See ``Manipulating the User Accounting Database''. getutent_r: See ``Manipulating the User Accounting Database''. getutid: See ``Manipulating the User Accounting Database''. getutid_r: See ``Manipulating the User Accounting Database''. getutline: See ``Manipulating the User Accounting Database''. getutline_r: See ``Manipulating the User Accounting Database''. getutmp: See ``XPG User Accounting Database Functions''. getutmpx: See ``XPG User Accounting Database Functions''. getutxent: See ``XPG User Accounting Database Functions''. getutxid: See ``XPG User Accounting Database Functions''. getutxline: See ``XPG User Accounting Database Functions''. getw: See ``Character Input''. getwc: See ``Character Input''. getwc_unlocked: See ``Character Input''. getwchar: See ``Character Input''. getwchar_unlocked: See ``Character Input''. getwd: See ``Working Directory''. glob: See ``Calling `glob'''. glob64: See ``Calling `glob'''. globfree: See ``More Flags for Globbing''. globfree64: See ``More Flags for Globbing''. gmtime: See ``Broken-down Time''. gmtime_r: See ``Broken-down Time''. grantpt: See ``Allocating Pseudo-Terminals''. gsignal: See ``Signaling Yourself''. gtty: See ``BSD Terminal Modes''. hasmntopt: See ``The `mtab' file''. hcreate: See ``The `hsearch' function.''. hcreate_r: See ``The `hsearch' function.''. hdestroy: See ``The `hsearch' function.''. hdestroy_r: See ``The `hsearch' function.''. hsearch: See ``The `hsearch' function.''. hsearch_r: See ``The `hsearch' function.''. htonl: See ``Byte Order Conversion''. htons: See ``Byte Order Conversion''. hypot: See ``Exponentiation and Logarithms''. hypotf: See ``Exponentiation and Logarithms''. hypotl: See ``Exponentiation and Logarithms''. iconv: See ``Generic Character Set Conversion Interface''. iconv_close: See ``Generic Character Set Conversion Interface''. iconv_open: See ``Generic Character Set Conversion Interface''. if_freenameindex: See ``Interface Naming''. if_indextoname: See ``Interface Naming''. if_nameindex: See ``Interface Naming''. if_nametoindex: See ``Interface Naming''. IFTODT: See ``Format of a Directory Entry''. ilogb: See ``Exponentiation and Logarithms''. ilogbf: See ``Exponentiation and Logarithms''. ilogbl: See ``Exponentiation and Logarithms''. imaxabs: See ``Absolute Value''. imaxdiv: See ``Integer Division''. index: See ``Search Functions''. inet_addr: See ``Host Address Functions''. inet_aton: See ``Host Address Functions''. inet_lnaof: See ``Host Address Functions''. inet_makeaddr: See ``Host Address Functions''. inet_netof: See ``Host Address Functions''. inet_network: See ``Host Address Functions''. inet_ntoa: See ``Host Address Functions''. inet_ntop: See ``Host Address Functions''. inet_pton: See ``Host Address Functions''. initgroups: See ``Setting the Group IDs''. initstate: See ``BSD Random Number Functions''. initstate_r: See ``BSD Random Number Functions''. innetgr: See ``Testing for Netgroup Membership''. ioctl: See ``Generic I/O Control operations''. isalnum: See ``Classification of Characters''. isalpha: See ``Classification of Characters''. isascii: See ``Classification of Characters''. isatty: See ``Identifying Terminals''. isblank: See ``Classification of Characters''. iscntrl: See ``Classification of Characters''. isdigit: See ``Classification of Characters''. isfinite: See ``Floating-Point Number Classification Functions''. isgraph: See ``Classification of Characters''. isgreater: See ``Floating-Point Comparison Functions''. isgreaterequal: See ``Floating-Point Comparison Functions''. isinf: See ``Floating-Point Number Classification Functions''. isinff: See ``Floating-Point Number Classification Functions''. isinfl: See ``Floating-Point Number Classification Functions''. isless: See ``Floating-Point Comparison Functions''. islessequal: See ``Floating-Point Comparison Functions''. islessgreater: See ``Floating-Point Comparison Functions''. islower: See ``Classification of Characters''. isnan: See ``Floating-Point Number Classification Functions''. isnanf: See ``Floating-Point Number Classification Functions''. isnanl: See ``Floating-Point Number Classification Functions''. isnormal: See ``Floating-Point Number Classification Functions''. isprint: See ``Classification of Characters''. ispunct: See ``Classification of Characters''. isspace: See ``Classification of Characters''. isunordered: See ``Floating-Point Comparison Functions''. isupper: See ``Classification of Characters''. iswalnum: See ``Character class determination for wide characters''. iswalpha: See ``Character class determination for wide characters''. iswblank: See ``Character class determination for wide characters''. iswcntrl: See ``Character class determination for wide characters''. iswctype: See ``Character class determination for wide characters''. iswdigit: See ``Character class determination for wide characters''. iswgraph: See ``Character class determination for wide characters''. iswlower: See ``Character class determination for wide characters''. iswprint: See ``Character class determination for wide characters''. iswpunct: See ``Character class determination for wide characters''. iswspace: See ``Character class determination for wide characters''. iswupper: See ``Character class determination for wide characters''. iswxdigit: See ``Character class determination for wide characters''. isxdigit: See ``Classification of Characters''. j0: See ``Special Functions''. j0f: See ``Special Functions''. j0l: See ``Special Functions''. j1: See ``Special Functions''. j1f: See ``Special Functions''. j1l: See ``Special Functions''. jn: See ``Special Functions''. jnf: See ``Special Functions''. jnl: See ``Special Functions''. jrand48: See ``SVID Random Number Function''. jrand48_r: See ``SVID Random Number Function''. kill: See ``Signaling Another Process''. killpg: See ``Signaling Another Process''. l64a: See ``Encode Binary Data''. labs: See ``Absolute Value''. lcong48: See ``SVID Random Number Function''. lcong48_r: See ``SVID Random Number Function''. ldexp: See ``Normalization Functions''. ldexpf: See ``Normalization Functions''. ldexpl: See ``Normalization Functions''. ldiv: See ``Integer Division''. lfind: See ``Array Search Function''. lgamma: See ``Special Functions''. lgamma_r: See ``Special Functions''. lgammaf: See ``Special Functions''. lgammaf_r: See ``Special Functions''. lgammal: See ``Special Functions''. lgammal_r: See ``Special Functions''. link: See ``Hard Links''. lio_listio: See ``Asynchronous Read and Write Operations''. lio_listio64: See ``Asynchronous Read and Write Operations''. listen: See ``Listening for Connections''. llabs: See ``Absolute Value''. lldiv: See ``Integer Division''. llrint: See ``Rounding Functions''. llrintf: See ``Rounding Functions''. llrintl: See ``Rounding Functions''. llround: See ``Rounding Functions''. llroundf: See ``Rounding Functions''. llroundl: See ``Rounding Functions''. localeconv: See ```localeconv': It is portable but ...''. localtime: See ``Broken-down Time''. localtime_r: See ``Broken-down Time''. log: See ``Exponentiation and Logarithms''. log10: See ``Exponentiation and Logarithms''. log10f: See ``Exponentiation and Logarithms''. log10l: See ``Exponentiation and Logarithms''. log1p: See ``Exponentiation and Logarithms''. log1pf: See ``Exponentiation and Logarithms''. log1pl: See ``Exponentiation and Logarithms''. log2: See ``Exponentiation and Logarithms''. log2f: See ``Exponentiation and Logarithms''. log2l: See ``Exponentiation and Logarithms''. logb: See ``Exponentiation and Logarithms''. logbf: See ``Exponentiation and Logarithms''. logbl: See ``Exponentiation and Logarithms''. logf: See ``Exponentiation and Logarithms''. login: See ``Logging In and Out''. login_tty: See ``Logging In and Out''. logl: See ``Exponentiation and Logarithms''. logout: See ``Logging In and Out''. logwtmp: See ``Logging In and Out''. longjmp: See ``Details of Non-Local Exits''. lrand48: See ``SVID Random Number Function''. lrand48_r: See ``SVID Random Number Function''. lrint: See ``Rounding Functions''. lrintf: See ``Rounding Functions''. lrintl: See ``Rounding Functions''. lround: See ``Rounding Functions''. lroundf: See ``Rounding Functions''. lroundl: See ``Rounding Functions''. lsearch: See ``Array Search Function''. lseek: See ``Setting the File Position of a Descriptor''. lseek64: See ``Setting the File Position of a Descriptor''. lstat: See ``Reading the Attributes of a File''. lstat64: See ``Reading the Attributes of a File''. madvise: See ``Memory-mapped I/O''. main: See ``Program Arguments''. makecontext: See ``Complete Context Control''. mallinfo: See ``Statistics for Memory Allocation with `malloc'''. malloc: See ``Basic Memory Allocation''. mallopt: See ``Malloc Tunable Parameters''. matherr: See ``FP Exceptions''. mblen: See ``Non-reentrant Conversion of Single Characters''. mbrlen: See ``Converting Single Characters''. mbrtowc: See ``Converting Single Characters''. mbsinit: See ``Representing the state of the conversion''. mbsnrtowcs: See ``Converting Multibyte and Wide Character Strings''. mbsrtowcs: See ``Converting Multibyte and Wide Character Strings''. mbstowcs: See ``Non-reentrant Conversion of Strings''. mbtowc: See ``Non-reentrant Conversion of Single Characters''. mcheck: See ``Heap Consistency Checking''. memalign: See ``Allocating Aligned Memory Blocks''. memccpy: See ``Copying and Concatenation''. memchr: See ``Search Functions''. memcmp: See ``String/Array Comparison''. memcpy: See ``Copying and Concatenation''. memfrob: See ``Trivial Encryption''. memmem: See ``Search Functions''. memmove: See ``Copying and Concatenation''. mempcpy: See ``Copying and Concatenation''. memrchr: See ``Search Functions''. memset: See ``Copying and Concatenation''. mkdir: See ``Creating Directories''. mkdtemp: See ``Temporary Files''. mkfifo: See ``FIFO Special Files''. mknod: See ``Making Special Files''. mkstemp: See ``Temporary Files''. mktemp: See ``Temporary Files''. mktime: See ``Broken-down Time''. mlock: See ``Functions To Lock And Unlock Pages''. mlockall: See ``Functions To Lock And Unlock Pages''. mmap: See ``Memory-mapped I/O''. mmap64: See ``Memory-mapped I/O''. modf: See ``Rounding Functions''. modff: See ``Rounding Functions''. modfl: See ``Rounding Functions''. mount: See ``Mount, Unmount, Remount''. mprobe: See ``Heap Consistency Checking''. mrand48: See ``SVID Random Number Function''. mrand48_r: See ``SVID Random Number Function''. mremap: See ``Memory-mapped I/O''. msync: See ``Memory-mapped I/O''. mtrace: See ``How to install the tracing functionality''. munlock: See ``Functions To Lock And Unlock Pages''. munlockall: See ``Functions To Lock And Unlock Pages''. munmap: See ``Memory-mapped I/O''. muntrace: See ``How to install the tracing functionality''. nan: See ``Setting and modifying single bits of FP values''. nanf: See ``Setting and modifying single bits of FP values''. nanl: See ``Setting and modifying single bits of FP values''. nanosleep: See ``Sleeping''. nearbyint: See ``Rounding Functions''. nearbyintf: See ``Rounding Functions''. nearbyintl: See ``Rounding Functions''. nextafter: See ``Setting and modifying single bits of FP values''. nextafterf: See ``Setting and modifying single bits of FP values''. nextafterl: See ``Setting and modifying single bits of FP values''. nexttoward: See ``Setting and modifying single bits of FP values''. nexttowardf: See ``Setting and modifying single bits of FP values''. nexttowardl: See ``Setting and modifying single bits of FP values''. nftw: See ``Working with Directory Trees''. nftw64: See ``Working with Directory Trees''. ngettext: See ``Additional functions for more complicated situations''. nice: See ``Functions For Traditional Scheduling''. nl_langinfo: See ``Pinpoint Access to Locale Data''. notfound: See ``Actions in the NSS configuration''. nrand48: See ``SVID Random Number Function''. nrand48_r: See ``SVID Random Number Function''. ntohl: See ``Byte Order Conversion''. ntohs: See ``Byte Order Conversion''. ntp_adjtime: See ``High Accuracy Clock''. ntp_gettime: See ``High Accuracy Clock''. obstack_1grow: See ``Growing Objects''. obstack_1grow_fast: See ``Extra Fast Growing Objects''. obstack_alignment_mask: See ``Alignment of Data in Obstacks''. obstack_alloc: See ``Allocation in an Obstack''. obstack_base: See ``Status of an Obstack''. obstack_blank: See ``Growing Objects''. obstack_blank_fast: See ``Extra Fast Growing Objects''. obstack_chunk_alloc: See ``Preparing for Using Obstacks''. obstack_chunk_free: See ``Preparing for Using Obstacks''. obstack_chunk_size: See ``Obstack Chunks''. obstack_copy: See ``Allocation in an Obstack''. obstack_copy0: See ``Allocation in an Obstack''. obstack_finish: See ``Growing Objects''. obstack_free: See ``Freeing Objects in an Obstack''. obstack_grow: See ``Growing Objects''. obstack_grow0: See ``Growing Objects''. obstack_init: See ``Preparing for Using Obstacks''. obstack_int_grow: See ``Growing Objects''. obstack_int_grow_fast: See ``Extra Fast Growing Objects''. obstack_next_free: See ``Status of an Obstack''. obstack_object_size <1>: See ``Status of an Obstack''. obstack_object_size: See ``Growing Objects''. obstack_printf: See ``Dynamically Allocating Formatted Output''. obstack_ptr_grow: See ``Growing Objects''. obstack_ptr_grow_fast: See ``Extra Fast Growing Objects''. obstack_room: See ``Extra Fast Growing Objects''. obstack_vprintf: See ``Variable Arguments Output Functions''. offsetof: See ``Structure Field Offset Measurement''. on_exit: See ``Cleanups on Exit''. open: See ``Opening and Closing Files''. open64: See ``Opening and Closing Files''. open_memstream: See ``String Streams''. open_obstack_stream: See ``Obstack Streams''. opendir: See ``Opening a Directory Stream''. openlog: See ``openlog''. openpty: See ``Opening a Pseudo-Terminal Pair''. parse_printf_format: See ``Parsing a Template String''. pathconf: See ``Using `pathconf'''. pause: See ``Using `pause'''. pclose: See ``Pipe to a Subprocess''. perror: See ``Error Messages''. pipe: See ``Creating a Pipe''. popen: See ``Pipe to a Subprocess''. posix_memalign: See ``Allocating Aligned Memory Blocks''. pow: See ``Exponentiation and Logarithms''. pow10: See ``Exponentiation and Logarithms''. pow10f: See ``Exponentiation and Logarithms''. pow10l: See ``Exponentiation and Logarithms''. powf: See ``Exponentiation and Logarithms''. powl: See ``Exponentiation and Logarithms''. pread: See ``Input and Output Primitives''. pread64: See ``Input and Output Primitives''. printf: See ``Formatted Output Functions''. printf_size: See ``Predefined `printf' Handlers''. printf_size_info: See ``Predefined `printf' Handlers''. psignal: See ``Signal Messages''. ptsname: See ``Allocating Pseudo-Terminals''. ptsname_r: See ``Allocating Pseudo-Terminals''. putc: See ``Simple Output by Characters or Lines''. putc_unlocked: See ``Simple Output by Characters or Lines''. putchar: See ``Simple Output by Characters or Lines''. putchar_unlocked: See ``Simple Output by Characters or Lines''. putenv: See ``Environment Access''. putpwent: See ``Writing a User Entry''. puts: See ``Simple Output by Characters or Lines''. pututline: See ``Manipulating the User Accounting Database''. pututxline: See ``XPG User Accounting Database Functions''. putw: See ``Simple Output by Characters or Lines''. putwc: See ``Simple Output by Characters or Lines''. putwc_unlocked: See ``Simple Output by Characters or Lines''. putwchar: See ``Simple Output by Characters or Lines''. putwchar_unlocked: See ``Simple Output by Characters or Lines''. pwrite: See ``Input and Output Primitives''. pwrite64: See ``Input and Output Primitives''. qecvt: See ``Old-fashioned System V number-to-string functions''. qecvt_r: See ``Old-fashioned System V number-to-string functions''. qfcvt: See ``Old-fashioned System V number-to-string functions''. qfcvt_r: See ``Old-fashioned System V number-to-string functions''. qgcvt: See ``Old-fashioned System V number-to-string functions''. qsort: See ``Array Sort Function''. raise: See ``Signaling Yourself''. rand: See ``ISO C Random Number Functions''. rand_r: See ``ISO C Random Number Functions''. random: See ``BSD Random Number Functions''. random_r: See ``BSD Random Number Functions''. rawmemchr: See ``Search Functions''. read: See ``Input and Output Primitives''. readdir: See ``Reading and Closing a Directory Stream''. readdir64: See ``Reading and Closing a Directory Stream''. readdir64_r: See ``Reading and Closing a Directory Stream''. readdir_r: See ``Reading and Closing a Directory Stream''. readlink: See ``Symbolic Links''. readv: See ``Fast Scatter-Gather I/O''. realloc: See ``Changing the Size of a Block''. realpath: See ``Symbolic Links''. recv: See ``Receiving Data''. recvfrom: See ``Receiving Datagrams''. regcomp: See ``POSIX Regular Expression Compilation''. regerror: See ``POSIX Regexp Matching Cleanup''. regexec: See ``Matching a Compiled POSIX Regular Expression''. regfree: See ``POSIX Regexp Matching Cleanup''. register_printf_function: See ``Registering New Conversions''. remainder: See ``Remainder Functions''. remainderf: See ``Remainder Functions''. remainderl: See ``Remainder Functions''. remove: See ``Deleting Files''. rename: See ``Renaming Files''. rewind: See ``File Positioning''. rewinddir: See ``Random Access in a Directory Stream''. rindex: See ``Search Functions''. rint: See ``Rounding Functions''. rintf: See ``Rounding Functions''. rintl: See ``Rounding Functions''. rmdir: See ``Deleting Files''. round: See ``Rounding Functions''. roundf: See ``Rounding Functions''. roundl: See ``Rounding Functions''. rpmatch: See ``Yes-or-No Questions''. S_ISBLK: See ``Testing the Type of a File''. S_ISCHR: See ``Testing the Type of a File''. S_ISDIR: See ``Testing the Type of a File''. S_ISFIFO: See ``Testing the Type of a File''. S_ISLNK: See ``Testing the Type of a File''. S_ISREG: See ``Testing the Type of a File''. S_ISSOCK: See ``Testing the Type of a File''. S_TYPEISMQ: See ``Testing the Type of a File''. S_TYPEISSEM: See ``Testing the Type of a File''. S_TYPEISSHM: See ``Testing the Type of a File''. sbrk: See ``Resizing the Data Segment''. scalb: See ``Normalization Functions''. scalbf: See ``Normalization Functions''. scalbl: See ``Normalization Functions''. scalbln: See ``Normalization Functions''. scalblnf: See ``Normalization Functions''. scalblnl: See ``Normalization Functions''. scalbn: See ``Normalization Functions''. scalbnf: See ``Normalization Functions''. scalbnl: See ``Normalization Functions''. scandir: See ``Scanning the Content of a Directory''. scandir64: See ``Scanning the Content of a Directory''. scanf: See ``Formatted Input Functions''. sched_get_priority_max: See ``Basic Scheduling Functions''. sched_get_priority_min: See ``Basic Scheduling Functions''. sched_getparam: See ``Basic Scheduling Functions''. sched_getscheduler: See ``Basic Scheduling Functions''. sched_rr_get_interval: See ``Basic Scheduling Functions''. sched_setparam: See ``Basic Scheduling Functions''. sched_setscheduler: See ``Basic Scheduling Functions''. sched_yield: See ``Basic Scheduling Functions''. seed48: See ``SVID Random Number Function''. seed48_r: See ``SVID Random Number Function''. seekdir: See ``Random Access in a Directory Stream''. select: See ``Waiting for Input or Output''. send: See ``Sending Data''. sendto: See ``Sending Datagrams''. setbuf: See ``Controlling Which Kind of Buffering''. setbuffer: See ``Controlling Which Kind of Buffering''. setcontext: See ``Complete Context Control''. setdomainname: See ``Host Identification''. setegid: See ``Setting the Group IDs''. setenv: See ``Environment Access''. seteuid: See ``Setting the User ID''. setfsent: See ``The `fstab' file''. setgid: See ``Setting the Group IDs''. setgrent: See ``Scanning the List of All Groups''. setgroups: See ``Setting the Group IDs''. sethostent: See ``Host Names''. sethostid: See ``Host Identification''. sethostname: See ``Host Identification''. setitimer: See ``Setting an Alarm''. setjmp: See ``Details of Non-Local Exits''. setkey: See ``DES Encryption''. setkey_r: See ``DES Encryption''. setlinebuf: See ``Controlling Which Kind of Buffering''. setlocale: See ``How Programs Set the Locale''. setlogmask: See ``setlogmask''. setmntent: See ``The `mtab' file''. setnetent: See ``Networks Database''. setnetgrent: See ``Looking up one Netgroup''. setpgid: See ``Process Group Functions''. setpgrp: See ``Process Group Functions''. setpriority: See ``Functions For Traditional Scheduling''. setprotoent: See ``Protocols Database''. setpwent: See ``Scanning the List of All Users''. setregid: See ``Setting the Group IDs''. setreuid: See ``Setting the User ID''. setrlimit: See ``Limiting Resource Usage''. setrlimit64: See ``Limiting Resource Usage''. setservent: See ``The Services Database''. setsid: See ``Process Group Functions''. setsockopt: See ``Socket Option Functions''. setstate: See ``BSD Random Number Functions''. setstate_r: See ``BSD Random Number Functions''. settimeofday: See ``High-Resolution Calendar''. setuid: See ``Setting the User ID''. setutent: See ``Manipulating the User Accounting Database''. setutxent: See ``XPG User Accounting Database Functions''. setvbuf: See ``Controlling Which Kind of Buffering''. shutdown: See ``Closing a Socket''. sigaction: See ``Advanced Signal Handling''. sigaddset: See ``Signal Sets''. sigaltstack: See ``Using a Separate Signal Stack''. sigblock: See ``BSD Functions for Blocking Signals''. sigdelset: See ``Signal Sets''. sigemptyset: See ``Signal Sets''. sigfillset: See ``Signal Sets''. siginterrupt: See ``BSD Function to Establish a Handler''. sigismember: See ``Signal Sets''. siglongjmp: See ``Non-Local Exits and Signals''. sigmask: See ``BSD Functions for Blocking Signals''. signal: See ``Basic Signal Handling''. signbit: See ``Setting and modifying single bits of FP values''. significand: See ``Normalization Functions''. significandf: See ``Normalization Functions''. significandl: See ``Normalization Functions''. sigpause: See ``BSD Functions for Blocking Signals''. sigpending: See ``Checking for Pending Signals''. sigprocmask: See ``Process Signal Mask''. sigsetjmp: See ``Non-Local Exits and Signals''. sigsetmask: See ``BSD Functions for Blocking Signals''. sigstack: See ``Using a Separate Signal Stack''. sigsuspend: See ``Using `sigsuspend'''. sigvec: See ``BSD Function to Establish a Handler''. sin: See ``Trigonometric Functions''. sincos: See ``Trigonometric Functions''. sincosf: See ``Trigonometric Functions''. sincosl: See ``Trigonometric Functions''. sinf: See ``Trigonometric Functions''. sinh: See ``Hyperbolic Functions''. sinhf: See ``Hyperbolic Functions''. sinhl: See ``Hyperbolic Functions''. sinl: See ``Trigonometric Functions''. sleep: See ``Sleeping''. snprintf: See ``Formatted Output Functions''. socket: See ``Creating a Socket''. socketpair: See ``Socket Pairs''. sprintf: See ``Formatted Output Functions''. sqrt: See ``Exponentiation and Logarithms''. sqrtf: See ``Exponentiation and Logarithms''. sqrtl: See ``Exponentiation and Logarithms''. srand: See ``ISO C Random Number Functions''. srand48: See ``SVID Random Number Function''. srand48_r: See ``SVID Random Number Function''. srandom: See ``BSD Random Number Functions''. srandom_r: See ``BSD Random Number Functions''. sscanf: See ``Formatted Input Functions''. ssignal: See ``Basic Signal Handling''. stat: See ``Reading the Attributes of a File''. stat64: See ``Reading the Attributes of a File''. stime: See ``Simple Calendar Time''. stpcpy: See ``Copying and Concatenation''. stpncpy: See ``Copying and Concatenation''. strcasecmp: See ``String/Array Comparison''. strcasestr: See ``Search Functions''. strcat: See ``Copying and Concatenation''. strchr: See ``Search Functions''. strchrnul: See ``Search Functions''. strcmp: See ``String/Array Comparison''. strcoll: See ``Collation Functions''. strcpy: See ``Copying and Concatenation''. strcspn: See ``Search Functions''. strdup: See ``Copying and Concatenation''. strdupa: See ``Copying and Concatenation''. strerror: See ``Error Messages''. strerror_r: See ``Error Messages''. strfmon: See ``A dedicated function to format numbers''. strfry: See ``strfry''. strftime: See ``Formatting Calendar Time''. strlen: See ``String Length''. strncasecmp: See ``String/Array Comparison''. strncat: See ``Copying and Concatenation''. strncmp: See ``String/Array Comparison''. strncpy: See ``Copying and Concatenation''. strndup: See ``Copying and Concatenation''. strndupa: See ``Copying and Concatenation''. strnlen: See ``String Length''. strpbrk: See ``Search Functions''. strptime: See ``Interpret string according to given format''. strrchr: See ``Search Functions''. strsep: See ``Finding Tokens in a String''. strsignal: See ``Signal Messages''. strspn: See ``Search Functions''. strstr: See ``Search Functions''. strtod: See ``Parsing of Floats''. strtof: See ``Parsing of Floats''. strtoimax: See ``Parsing of Integers''. strtok: See ``Finding Tokens in a String''. strtok_r: See ``Finding Tokens in a String''. strtol: See ``Parsing of Integers''. strtold: See ``Parsing of Floats''. strtoll: See ``Parsing of Integers''. strtoq: See ``Parsing of Integers''. strtoul: See ``Parsing of Integers''. strtoull: See ``Parsing of Integers''. strtoumax: See ``Parsing of Integers''. strtouq: See ``Parsing of Integers''. strverscmp: See ``String/Array Comparison''. strxfrm: See ``Collation Functions''. stty: See ``BSD Terminal Modes''. success: See ``Actions in the NSS configuration''. SUN_LEN: See ``Details of Local Namespace''. swapcontext: See ``Complete Context Control''. swprintf: See ``Formatted Output Functions''. swscanf: See ``Formatted Input Functions''. symlink: See ``Symbolic Links''. sync: See ``Synchronizing I/O operations''. syscall: See ``System Calls''. sysconf: See ``Definition of `sysconf'''. sysctl: See ``System Parameters''. syslog: See ``syslog, vsyslog''. system: See ``Running a Command''. sysv_signal: See ``Basic Signal Handling''. tan: See ``Trigonometric Functions''. tanf: See ``Trigonometric Functions''. tanh: See ``Hyperbolic Functions''. tanhf: See ``Hyperbolic Functions''. tanhl: See ``Hyperbolic Functions''. tanl: See ``Trigonometric Functions''. tcdrain: See ``Line Control Functions''. tcflow: See ``Line Control Functions''. tcflush: See ``Line Control Functions''. tcgetattr: See ``Terminal Mode Functions''. tcgetpgrp: See ``Functions for Controlling Terminal Access''. tcgetsid: See ``Functions for Controlling Terminal Access''. tcsendbreak: See ``Line Control Functions''. tcsetattr: See ``Terminal Mode Functions''. tcsetpgrp: See ``Functions for Controlling Terminal Access''. tdelete: See ``The `tsearch' function.''. tdestroy: See ``The `tsearch' function.''. telldir: See ``Random Access in a Directory Stream''. TEMP_FAILURE_RETRY: See ``Primitives Interrupted by Signals''. tempnam: See ``Temporary Files''. textdomain: See ``How to determine which catalog to be used''. tfind: See ``The `tsearch' function.''. tgamma: See ``Special Functions''. tgammaf: See ``Special Functions''. tgammal: See ``Special Functions''. time: See ``Simple Calendar Time''. timegm: See ``Broken-down Time''. timelocal: See ``Broken-down Time''. times: See ``Processor Time Inquiry''. tmpfile: See ``Temporary Files''. tmpfile64: See ``Temporary Files''. tmpnam: See ``Temporary Files''. tmpnam_r: See ``Temporary Files''. toascii: See ``Case Conversion''. tolower: See ``Case Conversion''. toupper: See ``Case Conversion''. towctrans: See ``Mapping of wide characters.''. towlower: See ``Mapping of wide characters.''. towupper: See ``Mapping of wide characters.''. trunc: See ``Rounding Functions''. truncate: See ``File Size''. truncate64: See ``File Size''. truncf: See ``Rounding Functions''. truncl: See ``Rounding Functions''. tryagain: See ``Actions in the NSS configuration''. tsearch: See ``The `tsearch' function.''. ttyname: See ``Identifying Terminals''. ttyname_r: See ``Identifying Terminals''. twalk: See ``The `tsearch' function.''. tzset: See ``Functions and Variables for Time Zones''. ulimit: See ``Limiting Resource Usage''. umask: See ``Assigning File Permissions''. umount: See ``Mount, Unmount, Remount''. umount2: See ``Mount, Unmount, Remount''. uname: See ``Platform Type Identification''. unavail: See ``Actions in the NSS configuration''. ungetc: See ``Using `ungetc' To Do Unreading''. ungetwc: See ``Using `ungetc' To Do Unreading''. unlink: See ``Deleting Files''. unlockpt: See ``Allocating Pseudo-Terminals''. unsetenv: See ``Environment Access''. updwtmp: See ``Manipulating the User Accounting Database''. utime: See ``File Times''. utimes: See ``File Times''. utmpname: See ``Manipulating the User Accounting Database''. utmpxname: See ``XPG User Accounting Database Functions''. va_alist: See ``Old-Style Variadic Functions''. va_arg: See ``Argument Access Macros''. va_dcl: See ``Old-Style Variadic Functions''. va_end: See ``Argument Access Macros''. va_start <1>: See ``Old-Style Variadic Functions''. va_start: See ``Argument Access Macros''. valloc: See ``Allocating Aligned Memory Blocks''. vasprintf: See ``Variable Arguments Output Functions''. verr: See ``Error Messages''. verrx: See ``Error Messages''. versionsort: See ``Scanning the Content of a Directory''. versionsort64: See ``Scanning the Content of a Directory''. vfork: See ``Creating a Process''. vfprintf: See ``Variable Arguments Output Functions''. vfscanf: See ``Variable Arguments Input Functions''. vfwprintf: See ``Variable Arguments Output Functions''. vfwscanf: See ``Variable Arguments Input Functions''. vlimit: See ``Limiting Resource Usage''. vprintf: See ``Variable Arguments Output Functions''. vscanf: See ``Variable Arguments Input Functions''. vsnprintf: See ``Variable Arguments Output Functions''. vsprintf: See ``Variable Arguments Output Functions''. vsscanf: See ``Variable Arguments Input Functions''. vswprintf: See ``Variable Arguments Output Functions''. vswscanf: See ``Variable Arguments Input Functions''. vsyslog: See ``syslog, vsyslog''. vtimes: See ``Resource Usage''. vwarn: See ``Error Messages''. vwarnx: See ``Error Messages''. vwprintf: See ``Variable Arguments Output Functions''. vwscanf: See ``Variable Arguments Input Functions''. wait: See ``Process Completion''. wait3: See ``BSD Process Wait Functions''. wait4: See ``Process Completion''. waitpid: See ``Process Completion''. warn: See ``Error Messages''. warnx: See ``Error Messages''. WCOREDUMP: See ``Process Completion Status''. wcpcpy: See ``Copying and Concatenation''. wcpncpy: See ``Copying and Concatenation''. wcrtomb: See ``Converting Single Characters''. wcscasecmp: See ``String/Array Comparison''. wcscat: See ``Copying and Concatenation''. wcschr: See ``Search Functions''. wcschrnul: See ``Search Functions''. wcscmp: See ``String/Array Comparison''. wcscoll: See ``Collation Functions''. wcscpy: See ``Copying and Concatenation''. wcscspn: See ``Search Functions''. wcsdup: See ``Copying and Concatenation''. wcsftime: See ``Formatting Calendar Time''. wcslen: See ``String Length''. wcsncasecmp: See ``String/Array Comparison''. wcsncat: See ``Copying and Concatenation''. wcsncmp: See ``String/Array Comparison''. wcsncpy: See ``Copying and Concatenation''. wcsnlen: See ``String Length''. wcsnrtombs: See ``Converting Multibyte and Wide Character Strings''. wcspbrk: See ``Search Functions''. wcsrchr: See ``Search Functions''. wcsrtombs: See ``Converting Multibyte and Wide Character Strings''. wcsspn: See ``Search Functions''. wcsstr: See ``Search Functions''. wcstod: See ``Parsing of Floats''. wcstof: See ``Parsing of Floats''. wcstoimax: See ``Parsing of Integers''. wcstok: See ``Finding Tokens in a String''. wcstol: See ``Parsing of Integers''. wcstold: See ``Parsing of Floats''. wcstoll: See ``Parsing of Integers''. wcstombs: See ``Non-reentrant Conversion of Strings''. wcstoq: See ``Parsing of Integers''. wcstoul: See ``Parsing of Integers''. wcstoull: See ``Parsing of Integers''. wcstoumax: See ``Parsing of Integers''. wcstouq: See ``Parsing of Integers''. wcswcs: See ``Search Functions''. wcsxfrm: See ``Collation Functions''. wctob: See ``Converting Single Characters''. wctomb: See ``Non-reentrant Conversion of Single Characters''. wctrans: See ``Mapping of wide characters.''. wctype: See ``Character class determination for wide characters''. WEXITSTATUS: See ``Process Completion Status''. WIFEXITED: See ``Process Completion Status''. WIFSIGNALED: See ``Process Completion Status''. WIFSTOPPED: See ``Process Completion Status''. wmemchr: See ``Search Functions''. wmemcmp: See ``String/Array Comparison''. wmemcpy: See ``Copying and Concatenation''. wmemmove: See ``Copying and Concatenation''. wmempcpy: See ``Copying and Concatenation''. wmemset: See ``Copying and Concatenation''. wordexp: See ``Calling `wordexp'''. wordfree: See ``Calling `wordexp'''. wprintf: See ``Formatted Output Functions''. write: See ``Input and Output Primitives''. writev: See ``Fast Scatter-Gather I/O''. wscanf: See ``Formatted Input Functions''. WSTOPSIG: See ``Process Completion Status''. WTERMSIG: See ``Process Completion Status''. y0: See ``Special Functions''. y0f: See ``Special Functions''. y0l: See ``Special Functions''. y1: See ``Special Functions''. y1f: See ``Special Functions''. y1l: See ``Special Functions''. yn: See ``Special Functions''. ynf: See ``Special Functions''. ynl: See ``Special Functions''. Variable and Constant Macro Index ********************************* (*__gconv_end_fct): See ``The `iconv' Implementation in the GNU C library''. (*__gconv_fct): See ``The `iconv' Implementation in the GNU C library''. (*__gconv_init_fct): See ``The `iconv' Implementation in the GNU C library''. __free_hook: See ``Memory Allocation Hooks''. __malloc_hook: See ``Memory Allocation Hooks''. __malloc_initialize_hook: See ``Memory Allocation Hooks''. __memalign_hook: See ``Memory Allocation Hooks''. __realloc_hook: See ``Memory Allocation Hooks''. _BSD_SOURCE: See ``Feature Test Macros''. _Complex_I: See ``Complex Numbers''. _FILE_OFFSET_BITS: See ``Feature Test Macros''. _GNU_SOURCE: See ``Feature Test Macros''. _IOFBF: See ``Controlling Which Kind of Buffering''. _IOLBF: See ``Controlling Which Kind of Buffering''. _IONBF: See ``Controlling Which Kind of Buffering''. _ISOC99_SOURCE: See ``Feature Test Macros''. _LARGEFILE64_SOURCE: See ``Feature Test Macros''. _LARGEFILE_SOURCE: See ``Feature Test Macros''. _PATH_FSTAB: See ``Mount Information''. _PATH_MNTTAB: See ``Mount Information''. _PATH_MOUNTED: See ``Mount Information''. _PATH_UTMP: See ``Manipulating the User Accounting Database''. _PATH_WTMP: See ``Manipulating the User Accounting Database''. _POSIX2_C_DEV: See ``Overall System Options''. _POSIX2_C_VERSION: See ``Which Version of POSIX is Supported''. _POSIX2_FORT_DEV: See ``Overall System Options''. _POSIX2_FORT_RUN: See ``Overall System Options''. _POSIX2_LOCALEDEF: See ``Overall System Options''. _POSIX2_SW_DEV: See ``Overall System Options''. _POSIX_C_SOURCE: See ``Feature Test Macros''. _POSIX_CHOWN_RESTRICTED: See ``Optional Features in File Support''. _POSIX_JOB_CONTROL: See ``Overall System Options''. _POSIX_NO_TRUNC: See ``Optional Features in File Support''. _POSIX_SAVED_IDS: See ``Overall System Options''. _POSIX_SOURCE: See ``Feature Test Macros''. _POSIX_VDISABLE <1>: See ``Optional Features in File Support''. _POSIX_VDISABLE: See ``Special Characters''. _POSIX_VERSION: See ``Which Version of POSIX is Supported''. _REENTRANT: See ``Feature Test Macros''. _SC_2_C_DEV: See ``Constants for `sysconf' Parameters''. _SC_2_FORT_DEV: See ``Constants for `sysconf' Parameters''. _SC_2_FORT_RUN: See ``Constants for `sysconf' Parameters''. _SC_2_LOCALEDEF: See ``Constants for `sysconf' Parameters''. _SC_2_SW_DEV: See ``Constants for `sysconf' Parameters''. _SC_2_VERSION: See ``Constants for `sysconf' Parameters''. _SC_AIO_LISTIO_MAX: See ``Constants for `sysconf' Parameters''. _SC_AIO_MAX: See ``Constants for `sysconf' Parameters''. _SC_AIO_PRIO_DELTA_MAX: See ``Constants for `sysconf' Parameters''. _SC_ARG_MAX: See ``Constants for `sysconf' Parameters''. _SC_ASYNCHRONOUS_IO: See ``Constants for `sysconf' Parameters''. _SC_ATEXIT_MAX: See ``Constants for `sysconf' Parameters''. _SC_AVPHYS_PAGES <1>: See ``Constants for `sysconf' Parameters''. _SC_AVPHYS_PAGES: See ``How to get information about the memory subsystem?''. _SC_BC_BASE_MAX: See ``Constants for `sysconf' Parameters''. _SC_BC_DIM_MAX: See ``Constants for `sysconf' Parameters''. _SC_BC_SCALE_MAX: See ``Constants for `sysconf' Parameters''. _SC_BC_STRING_MAX: See ``Constants for `sysconf' Parameters''. _SC_CHAR_BIT: See ``Constants for `sysconf' Parameters''. _SC_CHAR_MAX: See ``Constants for `sysconf' Parameters''. _SC_CHAR_MIN: See ``Constants for `sysconf' Parameters''. _SC_CHARCLASS_NAME_MAX: See ``Constants for `sysconf' Parameters''. _SC_CHILD_MAX: See ``Constants for `sysconf' Parameters''. _SC_CLK_TCK: See ``Constants for `sysconf' Parameters''. _SC_COLL_WEIGHTS_MAX: See ``Constants for `sysconf' Parameters''. _SC_DELAYTIMER_MAX: See ``Constants for `sysconf' Parameters''. _SC_EQUIV_CLASS_MAX: See ``Constants for `sysconf' Parameters''. _SC_EXPR_NEST_MAX: See ``Constants for `sysconf' Parameters''. _SC_FSYNC: See ``Constants for `sysconf' Parameters''. _SC_GETGR_R_SIZE_MAX: See ``Constants for `sysconf' Parameters''. _SC_GETPW_R_SIZE_MAX: See ``Constants for `sysconf' Parameters''. _SC_INT_MAX: See ``Constants for `sysconf' Parameters''. _SC_INT_MIN: See ``Constants for `sysconf' Parameters''. _SC_JOB_CONTROL: See ``Constants for `sysconf' Parameters''. _SC_LINE_MAX: See ``Constants for `sysconf' Parameters''. _SC_LOGIN_NAME_MAX: See ``Constants for `sysconf' Parameters''. _SC_LONG_BIT: See ``Constants for `sysconf' Parameters''. _SC_MAPPED_FILES: See ``Constants for `sysconf' Parameters''. _SC_MB_LEN_MAX: See ``Constants for `sysconf' Parameters''. _SC_MEMLOCK: See ``Constants for `sysconf' Parameters''. _SC_MEMLOCK_RANGE: See ``Constants for `sysconf' Parameters''. _SC_MEMORY_PROTECTION: See ``Constants for `sysconf' Parameters''. _SC_MESSAGE_PASSING: See ``Constants for `sysconf' Parameters''. _SC_MQ_OPEN_MAX: See ``Constants for `sysconf' Parameters''. _SC_MQ_PRIO_MAX: See ``Constants for `sysconf' Parameters''. _SC_NGROUPS_MAX: See ``Constants for `sysconf' Parameters''. _SC_NL_ARGMAX: See ``Constants for `sysconf' Parameters''. _SC_NL_LANGMAX: See ``Constants for `sysconf' Parameters''. _SC_NL_MSGMAX: See ``Constants for `sysconf' Parameters''. _SC_NL_NMAX: See ``Constants for `sysconf' Parameters''. _SC_NL_SETMAX: See ``Constants for `sysconf' Parameters''. _SC_NL_TEXTMAX: See ``Constants for `sysconf' Parameters''. _SC_NPROCESSORS_CONF <1>: See ``Constants for `sysconf' Parameters''. _SC_NPROCESSORS_CONF: See ``Learn about the processors available''. _SC_NPROCESSORS_ONLN <1>: See ``Constants for `sysconf' Parameters''. _SC_NPROCESSORS_ONLN: See ``Learn about the processors available''. _SC_NZERO: See ``Constants for `sysconf' Parameters''. _SC_OPEN_MAX: See ``Constants for `sysconf' Parameters''. _SC_PAGESIZE <1>: See ``Constants for `sysconf' Parameters''. _SC_PAGESIZE <2>: See ``How to get information about the memory subsystem?''. _SC_PAGESIZE: See ``Memory-mapped I/O''. _SC_PHYS_PAGES <1>: See ``Constants for `sysconf' Parameters''. _SC_PHYS_PAGES: See ``How to get information about the memory subsystem?''. _SC_PII: See ``Constants for `sysconf' Parameters''. _SC_PII_INTERNET: See ``Constants for `sysconf' Parameters''. _SC_PII_INTERNET_DGRAM: See ``Constants for `sysconf' Parameters''. _SC_PII_INTERNET_STREAM: See ``Constants for `sysconf' Parameters''. _SC_PII_OSI: See ``Constants for `sysconf' Parameters''. _SC_PII_OSI_CLTS: See ``Constants for `sysconf' Parameters''. _SC_PII_OSI_COTS: See ``Constants for `sysconf' Parameters''. _SC_PII_OSI_M: See ``Constants for `sysconf' Parameters''. _SC_PII_SOCKET: See ``Constants for `sysconf' Parameters''. _SC_PII_XTI: See ``Constants for `sysconf' Parameters''. _SC_PRIORITIZED_IO: See ``Constants for `sysconf' Parameters''. _SC_PRIORITY_SCHEDULING: See ``Constants for `sysconf' Parameters''. _SC_REALTIME_SIGNALS: See ``Constants for `sysconf' Parameters''. _SC_RTSIG_MAX: See ``Constants for `sysconf' Parameters''. _SC_SAVED_IDS: See ``Constants for `sysconf' Parameters''. _SC_SCHAR_MAX: See ``Constants for `sysconf' Parameters''. _SC_SCHAR_MIN: See ``Constants for `sysconf' Parameters''. _SC_SELECT: See ``Constants for `sysconf' Parameters''. _SC_SEM_NSEMS_MAX: See ``Constants for `sysconf' Parameters''. _SC_SEM_VALUE_MAX: See ``Constants for `sysconf' Parameters''. _SC_SEMAPHORES: See ``Constants for `sysconf' Parameters''. _SC_SHARED_MEMORY_OBJECTS: See ``Constants for `sysconf' Parameters''. _SC_SHRT_MAX: See ``Constants for `sysconf' Parameters''. _SC_SHRT_MIN: See ``Constants for `sysconf' Parameters''. _SC_SIGQUEUE_MAX: See ``Constants for `sysconf' Parameters''. _SC_STREAM_MAX: See ``Constants for `sysconf' Parameters''. _SC_SYNCHRONIZED_IO: See ``Constants for `sysconf' Parameters''. _SC_T_IOV_MAX: See ``Constants for `sysconf' Parameters''. _SC_THREAD_ATTR_STACKADDR: See ``Constants for `sysconf' Parameters''. _SC_THREAD_ATTR_STACKSIZE: See ``Constants for `sysconf' Parameters''. _SC_THREAD_DESTRUCTOR_ITERATIONS: See ``Constants for `sysconf' Parameters''. _SC_THREAD_KEYS_MAX: See ``Constants for `sysconf' Parameters''. _SC_THREAD_PRIO_INHERIT: See ``Constants for `sysconf' Parameters''. _SC_THREAD_PRIO_PROTECT: See ``Constants for `sysconf' Parameters''. _SC_THREAD_PRIORITY_SCHEDULING: See ``Constants for `sysconf' Parameters''. _SC_THREAD_PROCESS_SHARED: See ``Constants for `sysconf' Parameters''. _SC_THREAD_SAFE_FUNCTIONS: See ``Constants for `sysconf' Parameters''. _SC_THREAD_STACK_MIN: See ``Constants for `sysconf' Parameters''. _SC_THREAD_THREADS_MAX: See ``Constants for `sysconf' Parameters''. _SC_THREADS: See ``Constants for `sysconf' Parameters''. _SC_TIMER_MAX: See ``Constants for `sysconf' Parameters''. _SC_TIMERS: See ``Constants for `sysconf' Parameters''. _SC_TTY_NAME_MAX: See ``Constants for `sysconf' Parameters''. _SC_TZNAME_MAX: See ``Constants for `sysconf' Parameters''. _SC_UCHAR_MAX: See ``Constants for `sysconf' Parameters''. _SC_UINT_MAX: See ``Constants for `sysconf' Parameters''. _SC_UIO_MAXIOV: See ``Constants for `sysconf' Parameters''. _SC_ULONG_MAX: See ``Constants for `sysconf' Parameters''. _SC_USHRT_MAX: See ``Constants for `sysconf' Parameters''. _SC_VERSION: See ``Constants for `sysconf' Parameters''. _SC_WORD_BIT: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_CRYPT: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_ENH_I18N: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_LEGACY: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_REALTIME: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_REALTIME_THREADS: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_SHM: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_UNIX: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_VERSION: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_XCU_VERSION: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_XPG2: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_XPG3: See ``Constants for `sysconf' Parameters''. _SC_XOPEN_XPG4: See ``Constants for `sysconf' Parameters''. _SVID_SOURCE: See ``Feature Test Macros''. _THREAD_SAFE: See ``Feature Test Macros''. _XOPEN_SOURCE: See ``Feature Test Macros''. _XOPEN_SOURCE_EXTENDED: See ``Feature Test Macros''. ABDAY_1: See ``Pinpoint Access to Locale Data''. ABDAY_2: See ``Pinpoint Access to Locale Data''. ABDAY_3: See ``Pinpoint Access to Locale Data''. ABDAY_4: See ``Pinpoint Access to Locale Data''. ABDAY_5: See ``Pinpoint Access to Locale Data''. ABDAY_6: See ``Pinpoint Access to Locale Data''. ABDAY_7: See ``Pinpoint Access to Locale Data''. ABMON_1: See ``Pinpoint Access to Locale Data''. ABMON_10: See ``Pinpoint Access to Locale Data''. ABMON_11: See ``Pinpoint Access to Locale Data''. ABMON_12: See ``Pinpoint Access to Locale Data''. ABMON_2: See ``Pinpoint Access to Locale Data''. ABMON_3: See ``Pinpoint Access to Locale Data''. ABMON_4: See ``Pinpoint Access to Locale Data''. ABMON_5: See ``Pinpoint Access to Locale Data''. ABMON_6: See ``Pinpoint Access to Locale Data''. ABMON_7: See ``Pinpoint Access to Locale Data''. ABMON_8: See ``Pinpoint Access to Locale Data''. ABMON_9: See ``Pinpoint Access to Locale Data''. ACCOUNTING: See ``Manipulating the User Accounting Database''. AF_FILE: See ``Address Formats''. AF_INET: See ``Address Formats''. AF_LOCAL: See ``Address Formats''. AF_UNIX: See ``Address Formats''. AF_UNSPEC: See ``Address Formats''. aliases: See ``NSS Basics''. ALT_DIGITS: See ``Pinpoint Access to Locale Data''. ALTWERASE: See ``Local Modes''. AM_STR: See ``Pinpoint Access to Locale Data''. ARG_MAX: See ``General Capacity Limits''. argp_err_exit_status: See ``Argp Global Variables''. ARGP_ERR_UNKNOWN: See ``Argp Parser Functions''. ARGP_HELP_BUG_ADDR: See ``Flags for the `argp_help' Function''. ARGP_HELP_DOC: See ``Flags for the `argp_help' Function''. ARGP_HELP_EXIT_ERR: See ``Flags for the `argp_help' Function''. ARGP_HELP_EXIT_OK: See ``Flags for the `argp_help' Function''. ARGP_HELP_LONG: See ``Flags for the `argp_help' Function''. ARGP_HELP_LONG_ONLY: See ``Flags for the `argp_help' Function''. ARGP_HELP_POST_DOC: See ``Flags for the `argp_help' Function''. ARGP_HELP_PRE_DOC: See ``Flags for the `argp_help' Function''. ARGP_HELP_SEE: See ``Flags for the `argp_help' Function''. ARGP_HELP_SHORT_USAGE: See ``Flags for the `argp_help' Function''. ARGP_HELP_STD_ERR: See ``Flags for the `argp_help' Function''. ARGP_HELP_STD_HELP: See ``Flags for the `argp_help' Function''. ARGP_HELP_STD_USAGE: See ``Flags for the `argp_help' Function''. ARGP_HELP_USAGE: See ``Flags for the `argp_help' Function''. ARGP_IN_ORDER: See ``Flags for `argp_parse'''. ARGP_KEY_ARG: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_ARGS: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_END: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_ERROR: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_FINI: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_HELP_ARGS_DOC: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_HELP_DUP_ARGS_NOTE: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_HELP_EXTRA: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_HELP_HEADER: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_HELP_POST_DOC: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_HELP_PRE_DOC: See ``Special Keys for Argp Help Filter Functions''. ARGP_KEY_INIT: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_NO_ARGS: See ``Special Keys for Argp Parser Functions''. ARGP_KEY_SUCCESS: See ``Special Keys for Argp Parser Functions''. ARGP_LONG_ONLY: See ``Flags for `argp_parse'''. ARGP_NO_ARGS: See ``Flags for `argp_parse'''. ARGP_NO_ERRS: See ``Flags for `argp_parse'''. ARGP_NO_EXIT: See ``Flags for `argp_parse'''. ARGP_NO_HELP: See ``Flags for `argp_parse'''. ARGP_PARSE_ARGV0: See ``Flags for `argp_parse'''. argp_program_bug_address: See ``Argp Global Variables''. argp_program_version: See ``Argp Global Variables''. argp_program_version_hook: See ``Argp Global Variables''. ARGP_SILENT: See ``Flags for `argp_parse'''. B0: See ``Line Speed''. B110: See ``Line Speed''. B115200: See ``Line Speed''. B1200: See ``Line Speed''. B134: See ``Line Speed''. B150: See ``Line Speed''. B1800: See ``Line Speed''. B19200: See ``Line Speed''. B200: See ``Line Speed''. B230400: See ``Line Speed''. B2400: See ``Line Speed''. B300: See ``Line Speed''. B38400: See ``Line Speed''. B460800: See ``Line Speed''. B4800: See ``Line Speed''. B50: See ``Line Speed''. B57600: See ``Line Speed''. B600: See ``Line Speed''. B75: See ``Line Speed''. B9600: See ``Line Speed''. BC_BASE_MAX: See ``Utility Program Capacity Limits''. BC_DIM_MAX: See ``Utility Program Capacity Limits''. BC_SCALE_MAX: See ``Utility Program Capacity Limits''. BC_STRING_MAX: See ``Utility Program Capacity Limits''. BOOT_TIME <1>: See ``XPG User Accounting Database Functions''. BOOT_TIME: See ``Manipulating the User Accounting Database''. BRKINT: See ``Input Modes''. BUFSIZ: See ``Controlling Which Kind of Buffering''. CCTS_OFLOW: See ``Control Modes''. CHAR_MAX: See ``Range of an Integer Type''. CHAR_MIN: See ``Range of an Integer Type''. CHILD_MAX: See ``General Capacity Limits''. CIGNORE: See ``Control Modes''. CLK_TCK: See ``CPU Time Inquiry''. CLOCAL: See ``Control Modes''. CLOCKS_PER_SEC: See ``CPU Time Inquiry''. CODESET: See ``Pinpoint Access to Locale Data''. COLL_WEIGHTS_MAX: See ``Utility Program Capacity Limits''. COREFILE: See ``Program Error Signals''. CREAD: See ``Control Modes''. CRNCYSTR: See ``Pinpoint Access to Locale Data''. CRTS_IFLOW: See ``Control Modes''. CS5: See ``Control Modes''. CS6: See ``Control Modes''. CS7: See ``Control Modes''. CS8: See ``Control Modes''. CSIZE: See ``Control Modes''. CSTOPB: See ``Control Modes''. CURRENCY_SYMBOL: See ``Pinpoint Access to Locale Data''. D_FMT: See ``Pinpoint Access to Locale Data''. D_T_FMT: See ``Pinpoint Access to Locale Data''. DAY_1: See ``Pinpoint Access to Locale Data''. DAY_2: See ``Pinpoint Access to Locale Data''. DAY_3: See ``Pinpoint Access to Locale Data''. DAY_4: See ``Pinpoint Access to Locale Data''. DAY_5: See ``Pinpoint Access to Locale Data''. DAY_6: See ``Pinpoint Access to Locale Data''. DAY_7: See ``Pinpoint Access to Locale Data''. daylight: See ``Functions and Variables for Time Zones''. DBL_DIG: See ``Floating Point Parameters''. DBL_EPSILON: See ``Floating Point Parameters''. DBL_MANT_DIG: See ``Floating Point Parameters''. DBL_MAX: See ``Floating Point Parameters''. DBL_MAX_10_EXP: See ``Floating Point Parameters''. DBL_MAX_EXP: See ``Floating Point Parameters''. DBL_MIN: See ``Floating Point Parameters''. DBL_MIN_10_EXP: See ``Floating Point Parameters''. DBL_MIN_EXP: See ``Floating Point Parameters''. DEAD_PROCESS <1>: See ``XPG User Accounting Database Functions''. DEAD_PROCESS: See ``Manipulating the User Accounting Database''. DECIMAL_POINT: See ``Pinpoint Access to Locale Data''. DES_DECRYPT: See ``DES Encryption''. DES_ENCRYPT: See ``DES Encryption''. DES_HW: See ``DES Encryption''. DES_SW: See ``DES Encryption''. DESERR_BADPARAM: See ``DES Encryption''. DESERR_HWERROR: See ``DES Encryption''. DESERR_NOHWDEVICE: See ``DES Encryption''. DESERR_NONE: See ``DES Encryption''. DT_BLK: See ``Format of a Directory Entry''. DT_CHR: See ``Format of a Directory Entry''. DT_DIR: See ``Format of a Directory Entry''. DT_FIFO: See ``Format of a Directory Entry''. DT_REG: See ``Format of a Directory Entry''. DT_SOCK: See ``Format of a Directory Entry''. DT_UNKNOWN: See ``Format of a Directory Entry''. E2BIG: See ``Error Codes''. EACCES: See ``Error Codes''. EADDRINUSE: See ``Error Codes''. EADDRNOTAVAIL: See ``Error Codes''. EADV: See ``Error Codes''. EAFNOSUPPORT: See ``Error Codes''. EAGAIN: See ``Error Codes''. EALREADY: See ``Error Codes''. EAUTH: See ``Error Codes''. EBACKGROUND: See ``Error Codes''. EBADE: See ``Error Codes''. EBADF <1>: See ``Line Control Functions''. EBADF: See ``Error Codes''. EBADFD: See ``Error Codes''. EBADMSG: See ``Error Codes''. EBADR: See ``Error Codes''. EBADRPC: See ``Error Codes''. EBADRQC: See ``Error Codes''. EBADSLT: See ``Error Codes''. EBFONT: See ``Error Codes''. EBUSY: See ``Error Codes''. ECHILD: See ``Error Codes''. ECHO: See ``Local Modes''. ECHOCTL: See ``Local Modes''. ECHOE: See ``Local Modes''. ECHOK: See ``Local Modes''. ECHOKE: See ``Local Modes''. ECHONL: See ``Local Modes''. ECHOPRT: See ``Local Modes''. ECHRNG: See ``Error Codes''. ECOMM: See ``Error Codes''. ECONNABORTED: See ``Error Codes''. ECONNREFUSED: See ``Error Codes''. ECONNRESET: See ``Error Codes''. ED: See ``Error Codes''. EDEADLK: See ``Error Codes''. EDEADLOCK: See ``Error Codes''. EDESTADDRREQ: See ``Error Codes''. EDIED: See ``Error Codes''. EDOM: See ``Error Codes''. EDOTDOT: See ``Error Codes''. EDQUOT: See ``Error Codes''. EEXIST: See ``Error Codes''. EFAULT: See ``Error Codes''. EFBIG: See ``Error Codes''. EFTYPE: See ``Error Codes''. EGRATUITOUS: See ``Error Codes''. EGREGIOUS: See ``Error Codes''. EHOSTDOWN: See ``Error Codes''. EHOSTUNREACH: See ``Error Codes''. EIDRM: See ``Error Codes''. EIEIO: See ``Error Codes''. EILSEQ: See ``Error Codes''. EINPROGRESS: See ``Error Codes''. EINTR: See ``Error Codes''. EINVAL <1>: See ``Line Control Functions''. EINVAL: See ``Error Codes''. EIO: See ``Error Codes''. EISCONN: See ``Error Codes''. EISDIR: See ``Error Codes''. EISNAM: See ``Error Codes''. EL2HLT: See ``Error Codes''. EL2NSYNC: See ``Error Codes''. EL3HLT: See ``Error Codes''. EL3RST: See ``Error Codes''. ELIBACC: See ``Error Codes''. ELIBBAD: See ``Error Codes''. ELIBEXEC: See ``Error Codes''. ELIBMAX: See ``Error Codes''. ELIBSCN: See ``Error Codes''. ELNRNG: See ``Error Codes''. ELOOP: See ``Error Codes''. EMEDIUMTYPE: See ``Error Codes''. EMFILE: See ``Error Codes''. EMLINK: See ``Error Codes''. EMPTY <1>: See ``XPG User Accounting Database Functions''. EMPTY: See ``Manipulating the User Accounting Database''. EMSGSIZE: See ``Error Codes''. EMULTIHOP: See ``Error Codes''. ENAMETOOLONG: See ``Error Codes''. ENAVAIL: See ``Error Codes''. ENEEDAUTH: See ``Error Codes''. ENETDOWN: See ``Error Codes''. ENETRESET: See ``Error Codes''. ENETUNREACH: See ``Error Codes''. ENFILE: See ``Error Codes''. ENOANO: See ``Error Codes''. ENOBUFS: See ``Error Codes''. ENOCSI: See ``Error Codes''. ENODATA: See ``Error Codes''. ENODEV: See ``Error Codes''. ENOENT: See ``Error Codes''. ENOEXEC: See ``Error Codes''. ENOLCK: See ``Error Codes''. ENOLINK: See ``Error Codes''. ENOMEDIUM: See ``Error Codes''. ENOMEM: See ``Error Codes''. ENOMSG: See ``Error Codes''. ENONET: See ``Error Codes''. ENOPKG: See ``Error Codes''. ENOPROTOOPT: See ``Error Codes''. ENOSPC: See ``Error Codes''. ENOSR: See ``Error Codes''. ENOSTR: See ``Error Codes''. ENOSYS: See ``Error Codes''. ENOTBLK: See ``Error Codes''. ENOTCONN: See ``Error Codes''. ENOTDIR: See ``Error Codes''. ENOTEMPTY: See ``Error Codes''. ENOTNAM: See ``Error Codes''. ENOTSOCK: See ``Error Codes''. ENOTSUP: See ``Error Codes''. ENOTTY <1>: See ``Line Control Functions''. ENOTTY: See ``Error Codes''. ENOTUNIQ: See ``Error Codes''. environ: See ``Environment Access''. ENXIO: See ``Error Codes''. EOF: See ``End-Of-File and Errors''. EOPNOTSUPP: See ``Error Codes''. EOVERFLOW: See ``Error Codes''. EPERM: See ``Error Codes''. EPFNOSUPPORT: See ``Error Codes''. EPIPE: See ``Error Codes''. EPROCLIM: See ``Error Codes''. EPROCUNAVAIL: See ``Error Codes''. EPROGMISMATCH: See ``Error Codes''. EPROGUNAVAIL: See ``Error Codes''. EPROTO: See ``Error Codes''. EPROTONOSUPPORT: See ``Error Codes''. EPROTOTYPE: See ``Error Codes''. EQUIV_CLASS_MAX: See ``Utility Program Capacity Limits''. ERA: See ``Pinpoint Access to Locale Data''. ERA_D_FMT: See ``Pinpoint Access to Locale Data''. ERA_D_T_FMT: See ``Pinpoint Access to Locale Data''. ERA_T_FMT: See ``Pinpoint Access to Locale Data''. ERA_YEAR: See ``Pinpoint Access to Locale Data''. ERANGE: See ``Error Codes''. EREMCHG: See ``Error Codes''. EREMOTE: See ``Error Codes''. EREMOTEIO: See ``Error Codes''. ERESTART: See ``Error Codes''. EROFS: See ``Error Codes''. ERPCMISMATCH: See ``Error Codes''. errno: See ``Checking for Errors''. error_message_count: See ``Error Messages''. error_one_per_line: See ``Error Messages''. error_print_progname: See ``Error Messages''. ESHUTDOWN: See ``Error Codes''. ESOCKTNOSUPPORT: See ``Error Codes''. ESPIPE: See ``Error Codes''. ESRCH: See ``Error Codes''. ESRMNT: See ``Error Codes''. ESTALE: See ``Error Codes''. ESTRPIPE: See ``Error Codes''. ethers: See ``NSS Basics''. ETIME: See ``Error Codes''. ETIMEDOUT: See ``Error Codes''. ETOOMANYREFS: See ``Error Codes''. ETXTBSY: See ``Error Codes''. EUCLEAN: See ``Error Codes''. EUNATCH: See ``Error Codes''. EUSERS: See ``Error Codes''. EWOULDBLOCK: See ``Error Codes''. EXDEV: See ``Error Codes''. EXFULL: See ``Error Codes''. EXIT_FAILURE: See ``Exit Status''. EXIT_SUCCESS: See ``Exit Status''. EXPR_NEST_MAX: See ``Utility Program Capacity Limits''. EXTA: See ``Line Speed''. EXTB: See ``Line Speed''. F_DUPFD: See ``Duplicating Descriptors''. F_GETFD: See ``File Descriptor Flags''. F_GETFL: See ``Getting and Setting File Status Flags''. F_GETLK: See ``File Locks''. F_GETOWN: See ``Interrupt-Driven Input''. F_OK: See ``Testing Permission to Access a File''. F_RDLCK: See ``File Locks''. F_SETFD: See ``File Descriptor Flags''. F_SETFL: See ``Getting and Setting File Status Flags''. F_SETLK: See ``File Locks''. F_SETLKW: See ``File Locks''. F_SETOWN: See ``Interrupt-Driven Input''. F_UNLCK: See ``File Locks''. F_WRLCK: See ``File Locks''. FD_CLOEXEC: See ``File Descriptor Flags''. FD_SETSIZE: See ``Waiting for Input or Output''. FE_DFL_ENV: See ``Floating-Point Control Functions''. FE_DIVBYZERO: See ``Examining the FPU status word''. FE_DOWNWARD: See ``Rounding Modes''. FE_INEXACT: See ``Examining the FPU status word''. FE_INVALID: See ``Examining the FPU status word''. FE_NOMASK_ENV: See ``Floating-Point Control Functions''. FE_OVERFLOW: See ``Examining the FPU status word''. FE_TONEAREST: See ``Rounding Modes''. FE_TOWARDZERO: See ``Rounding Modes''. FE_UNDERFLOW: See ``Examining the FPU status word''. FE_UPWARD: See ``Rounding Modes''. FILENAME_MAX: See ``Limits on File System Capacity''. FLT_DIG: See ``Floating Point Parameters''. FLT_EPSILON: See ``Floating Point Parameters''. FLT_MANT_DIG: See ``Floating Point Parameters''. FLT_MAX: See ``Floating Point Parameters''. FLT_MAX_10_EXP: See ``Floating Point Parameters''. FLT_MAX_EXP: See ``Floating Point Parameters''. FLT_MIN: See ``Floating Point Parameters''. FLT_MIN_10_EXP: See ``Floating Point Parameters''. FLT_MIN_EXP: See ``Floating Point Parameters''. FLT_RADIX: See ``Floating Point Parameters''. FLT_ROUNDS: See ``Floating Point Parameters''. FLUSHO: See ``Local Modes''. FOPEN_MAX: See ``Opening Streams''. FP_FAST_FMA: See ``Miscellaneous FP arithmetic functions''. FP_ILOGB0: See ``Exponentiation and Logarithms''. FP_ILOGBNAN: See ``Exponentiation and Logarithms''. FP_INFINITE: See ``Floating-Point Number Classification Functions''. FP_NAN: See ``Floating-Point Number Classification Functions''. FP_NORMAL: See ``Floating-Point Number Classification Functions''. FP_SUBNORMAL: See ``Floating-Point Number Classification Functions''. FP_ZERO: See ``Floating-Point Number Classification Functions''. FPE_DECOVF_TRAP: See ``Program Error Signals''. FPE_FLTDIV_TRAP: See ``Program Error Signals''. FPE_FLTOVF_TRAP: See ``Program Error Signals''. FPE_FLTUND_TRAP: See ``Program Error Signals''. FPE_INTDIV_TRAP: See ``Program Error Signals''. FPE_INTOVF_TRAP: See ``Program Error Signals''. FPE_SUBRNG_TRAP: See ``Program Error Signals''. FRAC_DIGITS: See ``Pinpoint Access to Locale Data''. FSETLOCKING_BYCALLER: See ``Streams and Threads''. FSETLOCKING_INTERNAL: See ``Streams and Threads''. FSETLOCKING_QUERY: See ``Streams and Threads''. FSTAB: See ``Mount Information''. FSTAB_RO: See ``The `fstab' file''. FSTAB_RQ: See ``The `fstab' file''. FSTAB_RW: See ``The `fstab' file''. FSTAB_SW: See ``The `fstab' file''. FSTAB_XX: See ``The `fstab' file''. FTW_CHDIR: See ``Working with Directory Trees''. FTW_D: See ``Working with Directory Trees''. FTW_DEPTH: See ``Working with Directory Trees''. FTW_DNR: See ``Working with Directory Trees''. FTW_DP: See ``Working with Directory Trees''. FTW_F: See ``Working with Directory Trees''. FTW_MOUNT: See ``Working with Directory Trees''. FTW_NS: See ``Working with Directory Trees''. FTW_PHYS: See ``Working with Directory Trees''. FTW_SL: See ``Working with Directory Trees''. FTW_SLN: See ``Working with Directory Trees''. getdate_err: See ``A More User-friendly Way to Parse Times and Dates''. GLOB_ABORTED: See ``Calling `glob'''. GLOB_ALTDIRFUNC: See ``More Flags for Globbing''. GLOB_APPEND: See ``Flags for Globbing''. GLOB_BRACE: See ``More Flags for Globbing''. GLOB_DOOFFS: See ``Flags for Globbing''. GLOB_ERR: See ``Flags for Globbing''. GLOB_MAGCHAR: See ``More Flags for Globbing''. GLOB_MARK: See ``Flags for Globbing''. GLOB_NOCHECK: See ``Flags for Globbing''. GLOB_NOESCAPE: See ``Flags for Globbing''. GLOB_NOMAGIC: See ``More Flags for Globbing''. GLOB_NOMATCH: See ``Calling `glob'''. GLOB_NOSORT: See ``Flags for Globbing''. GLOB_NOSPACE: See ``Calling `glob'''. GLOB_ONLYDIR: See ``More Flags for Globbing''. GLOB_PERIOD: See ``More Flags for Globbing''. GLOB_TILDE: See ``More Flags for Globbing''. GLOB_TILDE_CHECK: See ``More Flags for Globbing''. group: See ``NSS Basics''. GROUPING: See ``Pinpoint Access to Locale Data''. h_errno: See ``Host Names''. HOST_NOT_FOUND: See ``Host Names''. hosts: See ``NSS Basics''. HUGE_VAL: See ``Error Reporting by Mathematical Functions''. HUGE_VALF: See ``Error Reporting by Mathematical Functions''. HUGE_VALL: See ``Error Reporting by Mathematical Functions''. HUPCL: See ``Control Modes''. I: See ``Complex Numbers''. ICANON: See ``Local Modes''. ICRNL: See ``Input Modes''. IEXTEN: See ``Local Modes''. IFNAMSIZ: See ``Interface Naming''. IGNBRK: See ``Input Modes''. IGNCR: See ``Input Modes''. IGNPAR: See ``Input Modes''. IMAXBEL: See ``Input Modes''. in6addr_any: See ``Host Address Data Type''. in6addr_loopback: See ``Host Address Data Type''. INADDR_ANY: See ``Host Address Data Type''. INADDR_BROADCAST: See ``Host Address Data Type''. INADDR_LOOPBACK: See ``Host Address Data Type''. INADDR_NONE: See ``Host Address Data Type''. INFINITY: See ``Infinity and NaN''. INIT_PROCESS <1>: See ``XPG User Accounting Database Functions''. INIT_PROCESS: See ``Manipulating the User Accounting Database''. INLCR: See ``Input Modes''. INPCK: See ``Input Modes''. INT_CURR_SYMBOL: See ``Pinpoint Access to Locale Data''. INT_FRAC_DIGITS: See ``Pinpoint Access to Locale Data''. INT_MAX: See ``Range of an Integer Type''. INT_MIN: See ``Range of an Integer Type''. INT_N_CS_PRECEDES: See ``Pinpoint Access to Locale Data''. INT_N_SEP_BY_SPACE: See ``Pinpoint Access to Locale Data''. INT_N_SIGN_POSN: See ``Pinpoint Access to Locale Data''. INT_P_CS_PRECEDES: See ``Pinpoint Access to Locale Data''. INT_P_SEP_BY_SPACE: See ``Pinpoint Access to Locale Data''. INT_P_SIGN_POSN: See ``Pinpoint Access to Locale Data''. IPPORT_RESERVED: See ``Internet Ports''. IPPORT_USERRESERVED: See ``Internet Ports''. ISIG: See ``Local Modes''. ISTRIP: See ``Input Modes''. ITIMER_PROF: See ``Setting an Alarm''. ITIMER_REAL: See ``Setting an Alarm''. ITIMER_VIRTUAL: See ``Setting an Alarm''. IXANY: See ``Input Modes''. IXOFF: See ``Input Modes''. IXON: See ``Input Modes''. L_ctermid: See ``Identifying the Controlling Terminal''. L_cuserid: See ``Identifying Who Logged In''. L_INCR: See ``File Positioning''. L_SET: See ``File Positioning''. L_tmpnam: See ``Temporary Files''. L_XTND: See ``File Positioning''. LANG: See ``Categories of Activities that Locales Affect''. LANGUAGE: See ``Categories of Activities that Locales Affect''. LC_ALL: See ``Categories of Activities that Locales Affect''. LC_COLLATE: See ``Categories of Activities that Locales Affect''. LC_CTYPE: See ``Categories of Activities that Locales Affect''. LC_MESSAGES: See ``Categories of Activities that Locales Affect''. LC_MONETARY: See ``Categories of Activities that Locales Affect''. LC_NUMERIC: See ``Categories of Activities that Locales Affect''. LC_TIME: See ``Categories of Activities that Locales Affect''. LDBL_DIG: See ``Floating Point Parameters''. LDBL_EPSILON: See ``Floating Point Parameters''. LDBL_MANT_DIG: See ``Floating Point Parameters''. LDBL_MAX: See ``Floating Point Parameters''. LDBL_MAX_10_EXP: See ``Floating Point Parameters''. LDBL_MAX_EXP: See ``Floating Point Parameters''. LDBL_MIN: See ``Floating Point Parameters''. LDBL_MIN_10_EXP: See ``Floating Point Parameters''. LDBL_MIN_EXP: See ``Floating Point Parameters''. LINE_MAX: See ``Utility Program Capacity Limits''. LINK_MAX: See ``Limits on File System Capacity''. LIO_NOP: See ``Perform I/O Operations in Parallel''. LIO_READ: See ``Perform I/O Operations in Parallel''. LIO_WRITE: See ``Perform I/O Operations in Parallel''. LOG_ALERT: See ``syslog, vsyslog''. LOG_AUTH: See ``syslog, vsyslog''. LOG_AUTHPRIV: See ``syslog, vsyslog''. LOG_CRIT: See ``syslog, vsyslog''. LOG_CRON: See ``syslog, vsyslog''. LOG_DAEMON: See ``syslog, vsyslog''. LOG_DEBUG: See ``syslog, vsyslog''. LOG_EMERG: See ``syslog, vsyslog''. LOG_ERR: See ``syslog, vsyslog''. LOG_FTP: See ``syslog, vsyslog''. LOG_INFO: See ``syslog, vsyslog''. LOG_LOCAL0: See ``syslog, vsyslog''. LOG_LOCAL1: See ``syslog, vsyslog''. LOG_LOCAL2: See ``syslog, vsyslog''. LOG_LOCAL3: See ``syslog, vsyslog''. LOG_LOCAL4: See ``syslog, vsyslog''. LOG_LOCAL5: See ``syslog, vsyslog''. LOG_LOCAL6: See ``syslog, vsyslog''. LOG_LOCAL7: See ``syslog, vsyslog''. LOG_LPR: See ``syslog, vsyslog''. LOG_MAIL: See ``syslog, vsyslog''. LOG_NEWS: See ``syslog, vsyslog''. LOG_NOTICE: See ``syslog, vsyslog''. LOG_SYSLOG: See ``syslog, vsyslog''. LOG_USER: See ``syslog, vsyslog''. LOG_UUCP: See ``syslog, vsyslog''. LOG_WARNING: See ``syslog, vsyslog''. LOGIN_PROCESS <1>: See ``XPG User Accounting Database Functions''. LOGIN_PROCESS: See ``Manipulating the User Accounting Database''. LONG_LONG_MAX: See ``Range of an Integer Type''. LONG_LONG_MIN: See ``Range of an Integer Type''. LONG_MAX: See ``Range of an Integer Type''. LONG_MIN: See ``Range of an Integer Type''. M_1_PI: See ``Predefined Mathematical Constants''. M_2_PI: See ``Predefined Mathematical Constants''. M_2_SQRTPI: See ``Predefined Mathematical Constants''. M_E: See ``Predefined Mathematical Constants''. M_LN10: See ``Predefined Mathematical Constants''. M_LN2: See ``Predefined Mathematical Constants''. M_LOG10E: See ``Predefined Mathematical Constants''. M_LOG2E: See ``Predefined Mathematical Constants''. M_PI: See ``Predefined Mathematical Constants''. M_PI_2: See ``Predefined Mathematical Constants''. M_PI_4: See ``Predefined Mathematical Constants''. M_SQRT1_2: See ``Predefined Mathematical Constants''. M_SQRT2: See ``Predefined Mathematical Constants''. MAP_ANON: See ``Memory-mapped I/O''. MAP_ANONYMOUS: See ``Memory-mapped I/O''. MAP_FIXED: See ``Memory-mapped I/O''. MAP_PRIVATE: See ``Memory-mapped I/O''. MAP_SHARED: See ``Memory-mapped I/O''. MAX_CANON: See ``Limits on File System Capacity''. MAX_INPUT: See ``Limits on File System Capacity''. MAXNAMLEN: See ``Limits on File System Capacity''. MAXSYMLINKS: See ``Symbolic Links''. MB_CUR_MAX: See ``Selecting the conversion and its properties''. MB_LEN_MAX: See ``Selecting the conversion and its properties''. MDMBUF: See ``Control Modes''. MINSIGSTKSZ: See ``Using a Separate Signal Stack''. MM_APPL: See ``Printing Formatted Messages''. MM_CONSOLE: See ``Printing Formatted Messages''. MM_ERROR: See ``Printing Formatted Messages''. MM_FIRM: See ``Printing Formatted Messages''. MM_HALT: See ``Printing Formatted Messages''. MM_HARD: See ``Printing Formatted Messages''. MM_INFO: See ``Printing Formatted Messages''. MM_NOSEV: See ``Printing Formatted Messages''. MM_NRECOV: See ``Printing Formatted Messages''. MM_NULLACT: See ``Printing Formatted Messages''. MM_NULLLBL: See ``Printing Formatted Messages''. MM_NULLMC: See ``Printing Formatted Messages''. MM_NULLSEV: See ``Printing Formatted Messages''. MM_NULLTAG: See ``Printing Formatted Messages''. MM_NULLTXT: See ``Printing Formatted Messages''. MM_OPSYS: See ``Printing Formatted Messages''. MM_PRINT: See ``Printing Formatted Messages''. MM_RECOVER: See ``Printing Formatted Messages''. MM_SOFT: See ``Printing Formatted Messages''. MM_UTIL: See ``Printing Formatted Messages''. MM_WARNING: See ``Printing Formatted Messages''. MNTOPT_DEFAULTS: See ``The `mtab' file''. MNTOPT_NOAUTO: See ``The `mtab' file''. MNTOPT_NOSUID: See ``The `mtab' file''. MNTOPT_RO: See ``The `mtab' file''. MNTOPT_RW: See ``The `mtab' file''. MNTOPT_SUID: See ``The `mtab' file''. MNTTYPE_IGNORE: See ``The `mtab' file''. MNTTYPE_NFS: See ``The `mtab' file''. MNTTYPE_SWAP: See ``The `mtab' file''. MON_1: See ``Pinpoint Access to Locale Data''. MON_10: See ``Pinpoint Access to Locale Data''. MON_11: See ``Pinpoint Access to Locale Data''. MON_12: See ``Pinpoint Access to Locale Data''. MON_2: See ``Pinpoint Access to Locale Data''. MON_3: See ``Pinpoint Access to Locale Data''. MON_4: See ``Pinpoint Access to Locale Data''. MON_5: See ``Pinpoint Access to Locale Data''. MON_6: See ``Pinpoint Access to Locale Data''. MON_7: See ``Pinpoint Access to Locale Data''. MON_8: See ``Pinpoint Access to Locale Data''. MON_9: See ``Pinpoint Access to Locale Data''. MON_DECIMAL_POINT: See ``Pinpoint Access to Locale Data''. MON_GROUPING: See ``Pinpoint Access to Locale Data''. MON_THOUSANDS_SEP: See ``Pinpoint Access to Locale Data''. MS_ASYNC: See ``Memory-mapped I/O''. MS_SYNC: See ``Memory-mapped I/O''. MSG_DONTROUTE: See ``Socket Data Options''. MSG_OOB: See ``Socket Data Options''. MSG_PEEK: See ``Socket Data Options''. N_CS_PRECEDES: See ``Pinpoint Access to Locale Data''. N_SEP_BY_SPACE: See ``Pinpoint Access to Locale Data''. N_SIGN_POSN: See ``Pinpoint Access to Locale Data''. NAME_MAX: See ``Limits on File System Capacity''. NAN: See ``Infinity and NaN''. NCCS: See ``Terminal Mode Data Types''. NDEBUG: See ``Explicitly Checking Internal Consistency''. NEGATIVE_SIGN: See ``Pinpoint Access to Locale Data''. netgroup: See ``NSS Basics''. networks: See ``NSS Basics''. NEW_TIME <1>: See ``XPG User Accounting Database Functions''. NEW_TIME: See ``Manipulating the User Accounting Database''. NGROUPS_MAX: See ``General Capacity Limits''. NL_ARGMAX: See ``Output Conversion Syntax''. NO_ADDRESS: See ``Host Names''. NO_RECOVERY: See ``Host Names''. NOEXPR: See ``Pinpoint Access to Locale Data''. NOFLSH: See ``Local Modes''. NOKERNINFO: See ``Local Modes''. NOSTR: See ``Pinpoint Access to Locale Data''. NSIG: See ``Standard Signals''. NSS_STATUS_NOTFOUND: See ``The Interface of the Function in NSS Modules''. NSS_STATUS_SUCCESS: See ``The Interface of the Function in NSS Modules''. NSS_STATUS_TRYAGAIN: See ``The Interface of the Function in NSS Modules''. NSS_STATUS_UNAVAIL: See ``The Interface of the Function in NSS Modules''. NULL: See ``Null Pointer Constant''. O_ACCMODE: See ``File Access Modes''. O_APPEND: See ``I/O Operating Modes''. O_ASYNC: See ``I/O Operating Modes''. O_CREAT: See ``Open-time Flags''. O_EXCL: See ``Open-time Flags''. O_EXEC: See ``File Access Modes''. O_EXLOCK: See ``Open-time Flags''. O_FSYNC: See ``I/O Operating Modes''. O_IGNORE_CTTY: See ``Open-time Flags''. O_NDELAY: See ``I/O Operating Modes''. O_NOATIME: See ``I/O Operating Modes''. O_NOCTTY: See ``Open-time Flags''. O_NOLINK: See ``Open-time Flags''. O_NONBLOCK <1>: See ``I/O Operating Modes''. O_NONBLOCK: See ``Open-time Flags''. O_NOTRANS: See ``Open-time Flags''. O_RDONLY: See ``File Access Modes''. O_RDWR: See ``File Access Modes''. O_READ: See ``File Access Modes''. O_SHLOCK: See ``Open-time Flags''. O_SYNC: See ``I/O Operating Modes''. O_TRUNC: See ``Open-time Flags''. O_WRITE: See ``File Access Modes''. O_WRONLY: See ``File Access Modes''. obstack_alloc_failed_handler: See ``Preparing for Using Obstacks''. OLD_TIME <1>: See ``XPG User Accounting Database Functions''. OLD_TIME: See ``Manipulating the User Accounting Database''. ONLCR: See ``Output Modes''. ONOEOT: See ``Output Modes''. OPEN_MAX: See ``General Capacity Limits''. OPOST: See ``Output Modes''. optarg: See ``Using the `getopt' function''. opterr: See ``Using the `getopt' function''. optind: See ``Using the `getopt' function''. OPTION_ALIAS: See ``Flags for Argp Options''. OPTION_ARG_OPTIONAL: See ``Flags for Argp Options''. OPTION_DOC: See ``Flags for Argp Options''. OPTION_HIDDEN: See ``Flags for Argp Options''. OPTION_NO_USAGE: See ``Flags for Argp Options''. optopt: See ``Using the `getopt' function''. OXTABS: See ``Output Modes''. P_CS_PRECEDES: See ``Pinpoint Access to Locale Data''. P_SEP_BY_SPACE: See ``Pinpoint Access to Locale Data''. P_SIGN_POSN: See ``Pinpoint Access to Locale Data''. P_tmpdir: See ``Temporary Files''. PA_CHAR: See ``Parsing a Template String''. PA_DOUBLE: See ``Parsing a Template String''. PA_FLAG_LONG: See ``Parsing a Template String''. PA_FLAG_LONG_DOUBLE: See ``Parsing a Template String''. PA_FLAG_LONG_LONG: See ``Parsing a Template String''. PA_FLAG_MASK: See ``Parsing a Template String''. PA_FLAG_PTR: See ``Parsing a Template String''. PA_FLAG_SHORT: See ``Parsing a Template String''. PA_FLOAT: See ``Parsing a Template String''. PA_INT: See ``Parsing a Template String''. PA_LAST: See ``Parsing a Template String''. PA_POINTER: See ``Parsing a Template String''. PA_STRING: See ``Parsing a Template String''. PARENB: See ``Control Modes''. PARMRK: See ``Input Modes''. PARODD: See ``Control Modes''. passwd: See ``NSS Basics''. PATH_MAX: See ``Limits on File System Capacity''. PENDIN: See ``Local Modes''. PF_CCITT: See ``Other Namespaces''. PF_FILE: See ``Details of Local Namespace''. PF_IMPLINK: See ``Other Namespaces''. PF_INET: See ``The Internet Namespace''. PF_INET6: See ``The Internet Namespace''. PF_ISO: See ``Other Namespaces''. PF_LOCAL: See ``Details of Local Namespace''. PF_NS: See ``Other Namespaces''. PF_ROUTE: See ``Other Namespaces''. PF_UNIX: See ``Details of Local Namespace''. PI: See ``Predefined Mathematical Constants''. PIPE_BUF: See ``Limits on File System Capacity''. PM_STR: See ``Pinpoint Access to Locale Data''. POSITIVE_SIGN: See ``Pinpoint Access to Locale Data''. PRIO_MAX: See ``Functions For Traditional Scheduling''. PRIO_MIN: See ``Functions For Traditional Scheduling''. PRIO_PGRP: See ``Functions For Traditional Scheduling''. PRIO_PROCESS: See ``Functions For Traditional Scheduling''. PRIO_USER: See ``Functions For Traditional Scheduling''. program_invocation_name: See ``Error Messages''. program_invocation_short_name: See ``Error Messages''. PROT_EXEC: See ``Memory-mapped I/O''. PROT_READ: See ``Memory-mapped I/O''. PROT_WRITE: See ``Memory-mapped I/O''. protocols: See ``NSS Basics''. PWD: See ``Working Directory''. R_OK: See ``Testing Permission to Access a File''. RADIXCHAR: See ``Pinpoint Access to Locale Data''. RAND_MAX: See ``ISO C Random Number Functions''. RE_DUP_MAX: See ``General Capacity Limits''. RLIM_INFINITY: See ``Limiting Resource Usage''. RLIM_NLIMITS: See ``Limiting Resource Usage''. RLIMIT_AS: See ``Limiting Resource Usage''. RLIMIT_CORE: See ``Limiting Resource Usage''. RLIMIT_CPU: See ``Limiting Resource Usage''. RLIMIT_DATA: See ``Limiting Resource Usage''. RLIMIT_FSIZE: See ``Limiting Resource Usage''. RLIMIT_NOFILE: See ``Limiting Resource Usage''. RLIMIT_OFILE: See ``Limiting Resource Usage''. RLIMIT_RSS: See ``Limiting Resource Usage''. RLIMIT_STACK: See ``Limiting Resource Usage''. rpc: See ``NSS Basics''. RUN_LVL <1>: See ``XPG User Accounting Database Functions''. RUN_LVL: See ``Manipulating the User Accounting Database''. S_IEXEC: See ``The Mode Bits for Access Permission''. S_IFBLK: See ``Testing the Type of a File''. S_IFCHR: See ``Testing the Type of a File''. S_IFDIR: See ``Testing the Type of a File''. S_IFIFO: See ``Testing the Type of a File''. S_IFLNK: See ``Testing the Type of a File''. S_IFMT: See ``Testing the Type of a File''. S_IFREG: See ``Testing the Type of a File''. S_IFSOCK: See ``Testing the Type of a File''. S_IREAD: See ``The Mode Bits for Access Permission''. S_IRGRP: See ``The Mode Bits for Access Permission''. S_IROTH: See ``The Mode Bits for Access Permission''. S_IRUSR: See ``The Mode Bits for Access Permission''. S_IRWXG: See ``The Mode Bits for Access Permission''. S_IRWXO: See ``The Mode Bits for Access Permission''. S_IRWXU: See ``The Mode Bits for Access Permission''. S_ISGID: See ``The Mode Bits for Access Permission''. S_ISUID: See ``The Mode Bits for Access Permission''. S_ISVTX: See ``The Mode Bits for Access Permission''. S_IWGRP: See ``The Mode Bits for Access Permission''. S_IWOTH: See ``The Mode Bits for Access Permission''. S_IWRITE: See ``The Mode Bits for Access Permission''. S_IWUSR: See ``The Mode Bits for Access Permission''. S_IXGRP: See ``The Mode Bits for Access Permission''. S_IXOTH: See ``The Mode Bits for Access Permission''. S_IXUSR: See ``The Mode Bits for Access Permission''. SA_NOCLDSTOP: See ``Flags for `sigaction'''. SA_ONSTACK: See ``Flags for `sigaction'''. SA_RESTART: See ``Flags for `sigaction'''. SC_SSIZE_MAX: See ``Constants for `sysconf' Parameters''. SCHAR_MAX: See ``Range of an Integer Type''. SCHAR_MIN: See ``Range of an Integer Type''. SEEK_CUR: See ``File Positioning''. SEEK_END: See ``File Positioning''. SEEK_SET: See ``File Positioning''. services: See ``NSS Basics''. shadow: See ``NSS Basics''. SHRT_MAX: See ``Range of an Integer Type''. SHRT_MIN: See ``Range of an Integer Type''. SIG_BLOCK: See ``Process Signal Mask''. SIG_DFL: See ``Basic Signal Handling''. SIG_ERR: See ``Basic Signal Handling''. SIG_IGN: See ``Basic Signal Handling''. SIG_SETMASK: See ``Process Signal Mask''. SIG_UNBLOCK: See ``Process Signal Mask''. SIGABRT: See ``Program Error Signals''. SIGALRM: See ``Alarm Signals''. SIGBUS: See ``Program Error Signals''. SIGCHLD: See ``Job Control Signals''. SIGCLD: See ``Job Control Signals''. SIGCONT: See ``Job Control Signals''. SIGEMT: See ``Program Error Signals''. SIGFPE: See ``Program Error Signals''. SIGHUP: See ``Termination Signals''. SIGILL: See ``Program Error Signals''. SIGINFO: See ``Miscellaneous Signals''. SIGINT: See ``Termination Signals''. SIGIO: See ``Asynchronous I/O Signals''. SIGIOT: See ``Program Error Signals''. SIGKILL: See ``Termination Signals''. SIGLOST: See ``Operation Error Signals''. signgam: See ``Special Functions''. SIGPIPE: See ``Operation Error Signals''. SIGPOLL: See ``Asynchronous I/O Signals''. SIGPROF: See ``Alarm Signals''. SIGQUIT: See ``Termination Signals''. SIGSEGV: See ``Program Error Signals''. SIGSTKSZ: See ``Using a Separate Signal Stack''. SIGSTOP: See ``Job Control Signals''. SIGSYS: See ``Program Error Signals''. SIGTERM: See ``Termination Signals''. SIGTRAP: See ``Program Error Signals''. SIGTSTP: See ``Job Control Signals''. SIGTTIN: See ``Job Control Signals''. SIGTTOU: See ``Job Control Signals''. SIGURG: See ``Asynchronous I/O Signals''. SIGUSR1: See ``Miscellaneous Signals''. SIGUSR2: See ``Miscellaneous Signals''. SIGVTALRM: See ``Alarm Signals''. SIGWINCH: See ``Miscellaneous Signals''. SIGXCPU: See ``Operation Error Signals''. SIGXFSZ: See ``Operation Error Signals''. SOCK_DGRAM: See ``Communication Styles''. SOCK_RAW: See ``Communication Styles''. SOCK_STREAM: See ``Communication Styles''. SOL_SOCKET: See ``Socket-Level Options''. SS_DISABLE: See ``Using a Separate Signal Stack''. SS_ONSTACK: See ``Using a Separate Signal Stack''. SSIZE_MAX: See ``General Capacity Limits''. stderr: See ``Standard Streams''. STDERR_FILENO: See ``Descriptors and Streams''. stdin: See ``Standard Streams''. STDIN_FILENO: See ``Descriptors and Streams''. stdout: See ``Standard Streams''. STDOUT_FILENO: See ``Descriptors and Streams''. STREAM_MAX: See ``General Capacity Limits''. SV_INTERRUPT: See ``BSD Function to Establish a Handler''. SV_ONSTACK: See ``BSD Function to Establish a Handler''. SV_RESETHAND: See ``BSD Function to Establish a Handler''. sys_siglist: See ``Signal Messages''. T_FMT: See ``Pinpoint Access to Locale Data''. T_FMT_AMPM: See ``Pinpoint Access to Locale Data''. TCIFLUSH: See ``Line Control Functions''. TCIOFF: See ``Line Control Functions''. TCIOFLUSH: See ``Line Control Functions''. TCION: See ``Line Control Functions''. TCOFLUSH: See ``Line Control Functions''. TCOOFF: See ``Line Control Functions''. TCOON: See ``Line Control Functions''. TCSADRAIN: See ``Terminal Mode Functions''. TCSAFLUSH: See ``Terminal Mode Functions''. TCSANOW: See ``Terminal Mode Functions''. TCSASOFT: See ``Terminal Mode Functions''. THOUSANDS_SEP: See ``Pinpoint Access to Locale Data''. THOUSEP: See ``Pinpoint Access to Locale Data''. timezone: See ``Functions and Variables for Time Zones''. TMP_MAX: See ``Temporary Files''. TOSTOP: See ``Local Modes''. TRY_AGAIN: See ``Host Names''. tzname: See ``Functions and Variables for Time Zones''. TZNAME_MAX: See ``General Capacity Limits''. UCHAR_MAX: See ``Range of an Integer Type''. UINT_MAX: See ``Range of an Integer Type''. ULONG_LONG_MAX: See ``Range of an Integer Type''. ULONG_MAX: See ``Range of an Integer Type''. USER_PROCESS <1>: See ``XPG User Accounting Database Functions''. USER_PROCESS: See ``Manipulating the User Accounting Database''. USHRT_MAX: See ``Range of an Integer Type''. VDISCARD: See ``Other Special Characters''. VDSUSP: See ``Characters that Cause Signals''. VEOF: See ``Characters for Input Editing''. VEOL: See ``Characters for Input Editing''. VEOL2: See ``Characters for Input Editing''. VERASE: See ``Characters for Input Editing''. VINTR: See ``Characters that Cause Signals''. VKILL: See ``Characters for Input Editing''. VLNEXT: See ``Other Special Characters''. VMIN: See ``Noncanonical Input''. VQUIT: See ``Characters that Cause Signals''. VREPRINT: See ``Characters for Input Editing''. VSTART: See ``Special Characters for Flow Control''. VSTATUS: See ``Other Special Characters''. VSTOP: See ``Special Characters for Flow Control''. VSUSP: See ``Characters that Cause Signals''. VTIME: See ``Noncanonical Input''. VWERASE: See ``Characters for Input Editing''. W_OK: See ``Testing Permission to Access a File''. WCHAR_MAX <1>: See ``Range of an Integer Type''. WCHAR_MAX: See ``Introduction to Extended Characters''. WCHAR_MIN: See ``Introduction to Extended Characters''. WEOF <1>: See ``End-Of-File and Errors''. WEOF: See ``Introduction to Extended Characters''. X_OK: See ``Testing Permission to Access a File''. YESEXPR: See ``Pinpoint Access to Locale Data''. YESSTR: See ``Pinpoint Access to Locale Data''. Program and File Index ********************** -lbsd-compat <1>: See ``Process Group Functions''. -lbsd-compat: See ``Feature Test Macros''. /etc/group: See ``Group Database''. /etc/hosts: See ``Host Names''. /etc/localtime: See ``Specifying the Time Zone with `TZ'''. /etc/networks: See ``Networks Database''. /etc/passwd: See ``User Database''. /etc/protocols: See ``Protocols Database''. /etc/services: See ``The Services Database''. /share/lib/zoneinfo: See ``Specifying the Time Zone with `TZ'''. argp.h: See ``Parsing Program Options with Argp''. argz.h: See ``Argz Functions''. arpa/inet.h: See ``Host Address Functions''. assert.h: See ``Explicitly Checking Internal Consistency''. bsd-compat <1>: See ``Process Group Functions''. bsd-compat: See ``Feature Test Macros''. cd: See ``Working Directory''. chgrp: See ``File Owner''. chown: See ``File Owner''. complex.h <1>: See ``Projections, Conjugates, and Decomposing of Complex Numbers''. complex.h <2>: See ``Complex Numbers''. complex.h: See ``Mathematics''. ctype.h <1>: See ``Case Conversion''. ctype.h <2>: See ``Classification of Characters''. ctype.h: See ``Character Handling''. dirent.h <1>: See ``Random Access in a Directory Stream''. dirent.h <2>: See ``Reading and Closing a Directory Stream''. dirent.h <3>: See ``Opening a Directory Stream''. dirent.h <4>: See ``Format of a Directory Entry''. dirent.h: See ``Reserved Names''. envz.h: See ``Envz Functions''. errno.h <1>: See ``Error Codes''. errno.h <2>: See ``Checking for Errors''. errno.h: See ``Error Reporting''. execinfo.h: See ``Backtraces''. fcntl.h <1>: See ``Interrupt-Driven Input''. fcntl.h <2>: See ``File Locks''. fcntl.h <3>: See ``File Status Flags''. fcntl.h <4>: See ``File Descriptor Flags''. fcntl.h <5>: See ``Duplicating Descriptors''. fcntl.h <6>: See ``Control Operations on Files''. fcntl.h <7>: See ``Opening and Closing Files''. fcntl.h: See ``Reserved Names''. float.h: See ``Floating Point Parameters''. fnmatch.h: See ``Wildcard Matching''. gcc: See ``ISO C''. gconv.h: See ``The `iconv' Implementation in the GNU C library''. grp.h <1>: See ``The Data Structure for a Group''. grp.h <2>: See ``Setting the Group IDs''. grp.h: See ``Reserved Names''. hostid: See ``Host Identification''. hostname: See ``Host Identification''. iconv.h: See ``Generic Character Set Conversion Interface''. kill: See ``Termination Signals''. ksh: See ``Wildcard Matching''. langinfo.h: See ``Pinpoint Access to Locale Data''. limits.h <1>: See ``Computing the Width of an Integer Data Type''. limits.h <2>: See ``Limits on File System Capacity''. limits.h <3>: See ``General Capacity Limits''. limits.h <4>: See ``Selecting the conversion and its properties''. limits.h: See ``Reserved Names''. locale: See ``How Programs Set the Locale''. locale.h <1>: See ```localeconv': It is portable but ...''. locale.h: See ``How Programs Set the Locale''. localtime: See ``Specifying the Time Zone with `TZ'''. ls: See ``File Attributes''. malloc.h <1>: See ``Statistics for Memory Allocation with `malloc'''. malloc.h <2>: See ``Memory Allocation Hooks''. malloc.h: See ``Malloc Tunable Parameters''. math.h <1>: See ``Rounding Functions''. math.h <2>: See ``Normalization Functions''. math.h <3>: See ``Absolute Value''. math.h <4>: See ``Floating-Point Number Classification Functions''. math.h: See ``Mathematics''. mcheck.h: See ``Heap Consistency Checking''. mkdir: See ``Creating Directories''. netdb.h <1>: See ``Networks Database''. netdb.h <2>: See ``Protocols Database''. netdb.h <3>: See ``The Services Database''. netdb.h: See ``Host Names''. netinet/in.h <1>: See ``Byte Order Conversion''. netinet/in.h <2>: See ``Internet Ports''. netinet/in.h <3>: See ``Host Address Data Type''. netinet/in.h: See ``Internet Socket Address Formats''. obstack.h: See ``Creating Obstacks''. printf.h <1>: See ``Conversion Specifier Options''. printf.h: See ``Registering New Conversions''. pwd.h <1>: See ``The Data Structure that Describes a User''. pwd.h: See ``Reserved Names''. setjmp.h <1>: See ``Non-Local Exits and Signals''. setjmp.h: See ``Details of Non-Local Exits''. sh: See ``Running a Command''. signal.h <1>: See ``BSD Signal Handling''. signal.h <2>: See ``Checking for Pending Signals''. signal.h <3>: See ``Process Signal Mask''. signal.h <4>: See ``Signal Sets''. signal.h <5>: See ``Signaling Another Process''. signal.h <6>: See ``Signaling Yourself''. signal.h <7>: See ``Flags for `sigaction'''. signal.h <8>: See ``Advanced Signal Handling''. signal.h <9>: See ``Basic Signal Handling''. signal.h <10>: See ``Standard Signals''. signal.h: See ``Reserved Names''. stdarg.h <1>: See ``Argument Access Macros''. stdarg.h: See ``Receiving the Argument Values''. stddef.h: See ``Important Data Types''. stdint.h: See ``Integers''. stdio.h <1>: See ``Identifying Who Logged In''. stdio.h <2>: See ``Identifying the Controlling Terminal''. stdio.h <3>: See ``Signal Messages''. stdio.h <4>: See ``Temporary Files''. stdio.h <5>: See ``Deleting Files''. stdio.h <6>: See ``Descriptors and Streams''. stdio.h <7>: See ``Custom Streams and Cookies''. stdio.h <8>: See ``String Streams''. stdio.h <9>: See ``Controlling Which Kind of Buffering''. stdio.h <10>: See ``Flushing Buffers''. stdio.h <11>: See ``Portable File-Position Functions''. stdio.h <12>: See ``File Positioning''. stdio.h <13>: See ``Formatted Input Functions''. stdio.h <14>: See ``Variable Arguments Output Functions''. stdio.h <15>: See ``Formatted Output Functions''. stdio.h <16>: See ``Block Input/Output''. stdio.h <17>: See ``Character Input''. stdio.h <18>: See ``Simple Output by Characters or Lines''. stdio.h <19>: See ``Opening Streams''. stdio.h <20>: See ``Standard Streams''. stdio.h: See ``Streams''. stdlib.h <1>: See ``Running a Command''. stdlib.h <2>: See ``Aborting a Program''. stdlib.h <3>: See ``Exit Status''. stdlib.h <4>: See ``Environment Access''. stdlib.h <5>: See ``Parsing of Floats''. stdlib.h <6>: See ``Parsing of Integers''. stdlib.h <7>: See ``Absolute Value''. stdlib.h <8>: See ``Integer Division''. stdlib.h <9>: See ``SVID Random Number Function''. stdlib.h <10>: See ``BSD Random Number Functions''. stdlib.h <11>: See ``ISO C Random Number Functions''. stdlib.h <12>: See ``Allocating Pseudo-Terminals''. stdlib.h <13>: See ``Array Sort Function''. stdlib.h <14>: See ``Array Search Function''. stdlib.h <15>: See ``Non-reentrant Conversion of Single Characters''. stdlib.h <16>: See ``Selecting the conversion and its properties''. stdlib.h <17>: See ``Automatic Storage with Variable Size''. stdlib.h <18>: See ``Allocating Aligned Memory Blocks''. stdlib.h <19>: See ``Allocating Cleared Space''. stdlib.h <20>: See ``Changing the Size of a Block''. stdlib.h <21>: See ``Freeing Memory Allocated with `malloc'''. stdlib.h: See ``Basic Memory Allocation''. string.h <1>: See ``Signal Messages''. string.h <2>: See ``Trivial Encryption''. string.h <3>: See ``Finding Tokens in a String''. string.h <4>: See ``Search Functions''. string.h <5>: See ``Collation Functions''. string.h <6>: See ``String/Array Comparison''. string.h <7>: See ``Copying and Concatenation''. string.h: See ``String Length''. sys/param.h: See ``Host Identification''. sys/resource.h <1>: See ``Functions For Traditional Scheduling''. sys/resource.h <2>: See ``Limiting Resource Usage''. sys/resource.h: See ``Resource Usage''. sys/socket.h <1>: See ``Socket-Level Options''. sys/socket.h <2>: See ``Socket Option Functions''. sys/socket.h <3>: See ``Sending Datagrams''. sys/socket.h <4>: See ``Socket Data Options''. sys/socket.h <5>: See ``Receiving Data''. sys/socket.h <6>: See ``Sending Data''. sys/socket.h <7>: See ``Socket Pairs''. sys/socket.h <8>: See ``Closing a Socket''. sys/socket.h <9>: See ``Creating a Socket''. sys/socket.h <10>: See ``The Internet Namespace''. sys/socket.h <11>: See ``Details of Local Namespace''. sys/socket.h <12>: See ``Reading the Address of a Socket''. sys/socket.h <13>: See ``Setting the Address of a Socket''. sys/socket.h <14>: See ``Address Formats''. sys/socket.h: See ``Communication Styles''. sys/stat.h <1>: See ``FIFO Special Files''. sys/stat.h <2>: See ``Making Special Files''. sys/stat.h <3>: See ``Assigning File Permissions''. sys/stat.h <4>: See ``The Mode Bits for Access Permission''. sys/stat.h <5>: See ``Testing the Type of a File''. sys/stat.h <6>: See ``The meaning of the File Attributes''. sys/stat.h <7>: See ``Creating Directories''. sys/stat.h: See ``Reserved Names''. sys/time.h <1>: See ``Setting an Alarm''. sys/time.h <2>: See ``High-Resolution Calendar''. sys/time.h: See ``File Times''. sys/times.h <1>: See ``Processor Time Inquiry''. sys/times.h: See ``Reserved Names''. sys/timex.h: See ``High Accuracy Clock''. sys/types.h <1>: See ``Setting the Group IDs''. sys/types.h <2>: See ``Setting the User ID''. sys/types.h <3>: See ``Reading the Persona of a Process''. sys/types.h <4>: See ``Functions for Controlling Terminal Access''. sys/types.h <5>: See ``Process Group Functions''. sys/types.h <6>: See ``Process Identification''. sys/types.h: See ``Waiting for Input or Output''. sys/un.h: See ``Details of Local Namespace''. sys/utsname.h: See ``Platform Type Identification''. sys/vlimit.h: See ``Limiting Resource Usage''. sys/vtimes.h: See ``Resource Usage''. sys/wait.h <1>: See ``BSD Process Wait Functions''. sys/wait.h <2>: See ``Process Completion Status''. sys/wait.h: See ``Process Completion''. termios.h <1>: See ``Terminal Modes''. termios.h: See ``Reserved Names''. time.h <1>: See ``Specifying the Time Zone with `TZ'''. time.h <2>: See ``Formatting Calendar Time''. time.h <3>: See ``Simple Calendar Time''. time.h <4>: See ``CPU Time Inquiry''. time.h: See ``File Times''. ulimit.h: See ``Limiting Resource Usage''. umask: See ``Assigning File Permissions''. unistd.h <1>: See ``Optional Features in File Support''. unistd.h <2>: See ``Overall System Options''. unistd.h <3>: See ``Host Identification''. unistd.h <4>: See ``Identifying Who Logged In''. unistd.h <5>: See ``Setting the Group IDs''. unistd.h <6>: See ``Setting the User ID''. unistd.h <7>: See ``Reading the Persona of a Process''. unistd.h <8>: See ``Functions for Controlling Terminal Access''. unistd.h <9>: See ``Process Group Functions''. unistd.h <10>: See ``Executing a File''. unistd.h <11>: See ``Creating a Process''. unistd.h <12>: See ``Process Identification''. unistd.h <13>: See ``Termination Internals''. unistd.h <14>: See ``Using the `getopt' function''. unistd.h <15>: See ``Setting an Alarm''. unistd.h <16>: See ``Identifying Terminals''. unistd.h <17>: See ``Creating a Pipe''. unistd.h <18>: See ``Testing Permission to Access a File''. unistd.h <19>: See ``File Owner''. unistd.h <20>: See ``Deleting Files''. unistd.h <21>: See ``Symbolic Links''. unistd.h <22>: See ``Hard Links''. unistd.h <23>: See ``Working Directory''. unistd.h <24>: See ``Duplicating Descriptors''. unistd.h <25>: See ``Descriptors and Streams''. unistd.h <26>: See ``Input and Output Primitives''. unistd.h: See ``Opening and Closing Files''. utime.h: See ``File Times''. utmp.h <1>: See ``Logging In and Out''. utmp.h: See ``Manipulating the User Accounting Database''. utmpx.h: See ``XPG User Accounting Database Functions''. varargs.h: See ``Old-Style Variadic Functions''. wchar.h <1>: See ``Parsing of Integers''. wchar.h <2>: See ``Character Input''. wchar.h <3>: See ``Simple Output by Characters or Lines''. wchar.h <4>: See ``Converting Multibyte and Wide Character Strings''. wchar.h <5>: See ``Converting Single Characters''. wchar.h <6>: See ``Representing the state of the conversion''. wchar.h <7>: See ``Introduction to Extended Characters''. wchar.h <8>: See ``Collation Functions''. wchar.h: See ``Copying and Concatenation''. wctype.h <1>: See ``Mapping of wide characters.''. wctype.h: See ``Character class determination for wide characters''. zoneinfo: See ``Specifying the Time Zone with `TZ'''. ...Short Contents... ...Table of 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