GNU libraries * Libc: (libc). C library. direntry This file documents the GNU C library. This is Edition 0.07 DRAFT, last updated 4 Oct 1996, of `The GNU C Library Reference Manual', for Version 2.00 Beta. Copyright (C) 1993, '94, '95, '96, '97 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU Library General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the text of the translation of the section entitled "GNU Library General Public License" must be approved for accuracy by the Foundation. Main Menu ********* This is Edition 0.07 DRAFT, last updated 4 Oct 1996, of `The GNU C Library Reference Manual', for Version 2.00 Beta 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 nonportable 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 IEEE "POSIX" family of standards, known more formally as the "Portable Operating System Interface for Computer Environments". 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 `IEEE Std 1003.1-1990, 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 `IEEE Std 1003.2-1992, 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 some 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.) 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 implementationally 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. - 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. - Macro: _POSIX_C_SOURCE If you define this macro with a value of `1', then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is made available. If you define this macro with a value of `2', then both the functionality from the POSIX.1 standard and the functionality from the POSIX.2 standard (IEEE Standard 1003.2) are made available. This is in addition to the ISO C facilities. - 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 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. - Macro: _GNU_SOURCE If you define this macro, everything is included: ISO C, POSIX.1, POSIX.2, BSD, SVID, X/Open, 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 that the standardization of the thread safe C library interface still is behind. 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 Allocation::, describes the GNU library's facilities for dynamic allocation of storage. If you do not know in advance how much storage 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 Extended Characters::, 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 Process Startup::, 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 Information::, describes functions for 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 the 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. Some functions have commands or options defined that might not be supported in all implementations, and this is the kind of error you get if you request them and they are not supported. - 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 writes the 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::. 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 is a new feature of ISO C. 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; } Memory Allocation ***************** The GNU system provides several methods for allocating memory space under explicit program control. They vary in generality and in efficiency. Dynamic Memory Allocation Concepts ================================== "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 number of memory blocks you need, or how long you continue to need them, depends on the data you are working on. 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 storage 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 and C ======================== 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, 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 length of the automatic storage can be an expression that varies. In other C implementations, it must be a constant. 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 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 Storage 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); memcpy (value, ptr, len); value[len] = '\0'; return value; } 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' 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 storage 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 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 apposed to other versions, the `malloc' in GNU libc 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' or `valloc'. These functions are declared in `stdlib.h'. With the GNU library, you can use `free' to free the blocks that `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: 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); } 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, releaseable 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 storage by using the `mcheck' function. This function is a GNU extension, declared in `malloc.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. - 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. Storage 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 storage allocation, for example. The hook variables are declared in `malloc.h'. - Variable: __malloc_hook The value of this variable is a pointer to 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) - 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) - 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) 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. Here is an example showing how to use `__malloc_hook' properly. It installs a function that prints out information every time `malloc' is called. static void *(*old_malloc_hook) (size_t); static void * my_malloc_hook (size_t size) { void *result; __malloc_hook = old_malloc_hook; result = malloc (size); /* `printf' might call `malloc', so protect it too. */ printf ("malloc (%u) returns %p\n", (unsigned int) size, result); __malloc_hook = my_malloc_hook; return result; } main () { ... old_malloc_hook = __malloc_hook; __malloc_hook = my_malloc_hook; ... } The `mcheck' function (*note Heap Consistency Checking::.) works by installing such hooks. Statistics for Storage Allocation with `malloc' ----------------------------------------------- You can get information about dynamic storage 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 storage 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 storage 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, releaseable chunk that normally borders the end of the heap (i.e. the "brk" of the process). - 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)' A pointer to a function that `malloc' uses whenever it is called. `void *(*__realloc_hook) (void *PTR, size_t SIZE)' A pointer to a function that `realloc' uses whenever it is called. `void (*__free_hook) (void *PTR)' A pointer to a function that `free' uses whenever it is called. `struct mallinfo mallinfo (void)' Return information about the current dynamic memory usage. *Note Statistics of Malloc::. 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 storage 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. It returns 0 if `obstack_chunk_alloc' returns a null pointer, meaning that it is out of memory. Otherwise, it returns 1. If you supply an `obstack_chunk_alloc' function that calls `exit' (*note Program Termination::.) or `longjmp' (*note Non-Local Exits::.) when out of memory, you can safely ignore the value that `obstack_init' returns. 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); 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 returns a null pointer if `obstack_chunk_alloc' returns one. In that case, it has not changed the amount of memory allocated in the obstack. If you supply an `obstack_chunk_alloc' function that calls `exit' (*note Program Termination::.) or `longjmp' (*note Non-Local Exits::.) when out of memory, then `obstack_alloc' will never return a null pointer. 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 can return a null pointer under the same conditions as `obstack_alloc'. - 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 storage 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 storage 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 xmalloc #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 storage, 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 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 storage 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 storage 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 storage 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 storage 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. Relocating Allocator ==================== Any system of dynamic memory allocation has overhead: the amount of space it uses is more than the amount the program asks for. The "relocating memory allocator" achieves very low overhead by moving blocks in memory as necessary, on its own initiative. Concepts of Relocating Allocation --------------------------------- The "relocating memory allocator" achieves very low overhead by moving blocks in memory as necessary, on its own initiative. When you allocate a block with `malloc', the address of the block never changes unless you use `realloc' to change its size. Thus, you can safely store the address in various places, temporarily or permanently, as you like. This is not safe when you use the relocating memory allocator, because any and all relocatable blocks can move whenever you allocate memory in any fashion. Even calling `malloc' or `realloc' can move the relocatable blocks. For each relocatable block, you must make a "handle"--a pointer object in memory, designated to store the address of that block. The relocating allocator knows where each block's handle is, and updates the address stored there whenever it moves the block, so that the handle always points to the block. Each time you access the contents of the block, you should fetch its address anew from the handle. To call any of the relocating allocator functions from a signal handler is almost certainly incorrect, because the signal could happen at any time and relocate all the blocks. The only way to make this safe is to block the signal around any access to the contents of any relocatable block--not a convenient mode of operation. *Note Nonreentrancy::. Allocating and Freeing Relocatable Blocks ----------------------------------------- In the descriptions below, HANDLEPTR designates the address of the handle. All the functions are declared in `malloc.h'; all are GNU extensions. - Function: void * r_alloc (void **HANDLEPTR, size_t SIZE) This function allocates a relocatable block of size SIZE. It stores the block's address in `*HANDLEPTR' and returns a non-null pointer to indicate success. If `r_alloc' can't get the space needed, it stores a null pointer in `*HANDLEPTR', and returns a null pointer. - Function: void r_alloc_free (void **HANDLEPTR) This function is the way to free a relocatable block. It frees the block that `*HANDLEPTR' points to, and stores a null pointer in `*HANDLEPTR' to show it doesn't point to an allocated block any more. - Function: void * r_re_alloc (void **HANDLEPTR, size_t SIZE) The function `r_re_alloc' adjusts the size of the block that `*HANDLEPTR' points to, making it SIZE bytes long. It stores the address of the resized block in `*HANDLEPTR' and returns a non-null pointer to indicate success. If enough memory is not available, this function returns a null pointer and does not modify `*HANDLEPTR'. 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::.) 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. - Function: int isupper (int C) Returns true if C is an upper-case letter. - 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, `tolower' 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. 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. By convention, a "null character", `'\0'', marks the end of a 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 (`"'). In ISO C, string literals can also be formed by "string concatenation": `"a" "b"' is the same as `"ab"'. 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 "allocation 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. 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. Functions that operate on arbitrary blocks of memory have names beginning with `mem' (such as `memcpy') and invariably take an argument which specifies the size (in bytes) of the block of memory to operate on. The array arguments and return values for these functions have type `void *', and as a matter of style, the elements of these arrays 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. In contrast, functions that operate specifically on strings have names beginning with `str' (such as `strcpy') 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 the array elements are referred to as "characters". In many cases, there are both `mem' and `str' 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' functions, unless you already know the length of the string in advance. 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 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 allocation size. You can get the allocation size of the character array that holds a string using the `sizeof' operator: char string[32] = "hello, world"; sizeof (string) => 32 strlen (string) => 12 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. These functions are declared in the header file `string.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 *TO, const void *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: 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. - Function: void * memccpy (void *TO, const void *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: char * strcpy (char *TO, const char *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: char * strncpy (char *TO, const char *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: 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: 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. - Function: char * stpcpy (char *TO, const char *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) 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. - Function: char * stpncpy (char *TO, const char *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 behaviour is rarely useful, but it is implemented to be useful in contexts where this behaviour 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 GNU C Library itself. Its behaviour is undefined if the strings overlap. - Function: char * strdupa (const char *S) This function 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. I.e., you cannot get the address of this function. Despite this limitations 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 illegal. This function is only available if GNU CC is used. - Function: 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 which means one cannot get the address of it. `strndupa' is only available if GNU CC is used. - Function: char * strcat (char *TO, const char *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 *to, const char *from) { strcpy (to + strlen (to), from); return to; } This function has undefined results if the strings overlap. - Function: char * strncat (char *TO, const char *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) { strncpy (to + strlen (to), from, size); return to; } The behavior of `strncat' is undefined if the strings overlap. Here is an example showing the use of `strncpy' and `strncat'. 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 (void *FROM, const 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. - 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'. 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. 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. - Function: int strcasecmp (const char *S1, const char *S2) This function is like `strcmp', except that differences in case are ignored. `strcasecmp' is derived from BSD. - Function: int strncasecmp (const char *S1, const char *S2, size_t N) This function is like `strncmp', except that differences in case are ignored. `strncasecmp' 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 characters are compared. In other words, if the two strings are the same in their first SIZE characters, the return value is zero. Here are some examples showing the use of `strcmp' and `strncmp'. 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 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 header file `string.h') 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'. 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 function `strcoll' performs this translation implicitly, in order to do one comparison. By contrast, `strxfrm' performs 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' to transform all the strings just once, and subsequently compare the transformed strings with `strcmp'. - 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). 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, sizeof (char *), nstrings, compare_elements); } - Function: size_t strxfrm (char *TO, const char *FROM, size_t SIZE) The function `strxfrm' transforms STRING 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 string to allocate. It does not matter what TO is if SIZE is zero; TO 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_lenght; 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); } *Compatibility Note:* The string collation functions are a new feature of ISO C. Older C dialects have no equivalent feature. 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: 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. - Function: char * index (const char *STRING, int C) `index' is another name for `strchr'; they are exactly the same. - 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: char * rindex (const char *STRING, int C) `rindex' is another name for `strrchr'; they are exactly the same. - 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: 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 - 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 - 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" 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 *NEWSTRING, const char *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. *Warning:* Since `strtok' alters the string it is parsing, you always copy the string to a temporary buffer before parsing it with `strtok'. If you allow `strtok' to modify a string that came from another part of your program, you are asking for trouble; that string may be part of a data structure that could be used for other purposes during the parsing, when alteration by `strtok' makes the data structure temporarily inaccurate. The string that you are operating on might even be a constant. Then when `strtok' tries to modify it, your program will get a fatal signal for writing in read-only memory. *Note Program Error Signals::. 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 function `strtok' is 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 ... char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token; ... token = strtok (string, 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. - 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 be successive calls to `strtok_r'. The difference is that the information about the next token is not set up in some internal state information. Instead the caller has to provide another 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 limiting reentrancy. This function was proposed for POSIX.1b and can be found on many systems which support multi-threading. - Function: char * strsep (char **STRING_PTR, const char *DELIMITER) A second reentrant approach is to avoid the additional first argument. The initialization of the moving pointer has to be done by the user. Successive calls of `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. 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 ... char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = 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 => "and" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => NULL */ 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 terminal 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 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::. 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 can 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::. Any other characters in OPENTYPE are simply ignored. They may be meaningful in other systems. If the open fails, `fopen' returns a null pointer. 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::. - 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. Closing Streams =============== When a stream is closed with `fclose', the connection between the stream and the file is cancelled. 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 inputis 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 in only in special situation, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with one stream can be identifier. 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::. Simple Output by Characters or Lines ==================================== This section describes functions for performing character- and line-oriented output. These functions are declared in the header file `stdio.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: 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: int putchar (int C) The `putchar' function is equivalent to `putc' with `stdout' as the value of the STREAM argument. - 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 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."); - 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 functions are declared in the header file `stdio.h'. These functions return an `int' value that is either a character of input, or the special value `EOF' (usually -1). 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: 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: int getchar (void) The `getchar' function is equivalent to `getc' with `stdin' as the value of the STREAM argument. 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)'. 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's 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, `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'. - 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' 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. 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 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. 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 Extended Characters::.) are permitted in a template string. The conversion specifications in a `printf' template string have the general form: % 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: * 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. `%c' Print a single character. *Note Other Output Conversions::. `%s' Print a string. *Note Other Output Conversions::. `%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: `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. `l' Specifies that the argument is a `long int' or `unsigned long int', as appropriate. Two `l' characters is like the `L' modifier, below. `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'. `Z' Specifies that the argument is a `size_t'. This is a GNU extension. 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| 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| 0x0| 0X0|0x00000000| | 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. 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 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 `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: "|%12.4f|%12.4e|%12.4g|\n" Here is the output: | 0.0000| 0.0000e+00| 0| | 1.0000| 1.0000e+00| 1| | -1.0000| -1.0000e+00| -1| | 100.0000| 1.0000e+02| 100| | 1000.0000| 1.0000e+03| 1000| | 10000.0000| 1.0000e+04| 1e+04| | 12345.0000| 1.2345e+04| 1.234e+04| | 100000.0000| 1.0000e+05| 1e+05| | 123456.0000| 1.2346e+05| 1.234e+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. The `int' argument is first converted to an `unsigned char'. 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'. The `%s' conversion prints a string. The corresponding argument must be of type `char *' (or `const char *'). 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 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 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 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 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 stored, not including the terminating null. If this value equals `SIZE - 1', then there was not enough space in S for all the output. 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); while (1) { /* Try to print in the allocated space. */ int nchars = snprintf (buffer, size, "value of %s is %s", name, value); /* If that worked, return the string. */ if (nchars < size) return buffer; /* Else try again with twice as much space. */ size *= 2; buffer = (char *) xrealloc (size, buffer); } } In practice, it is often easier just to use `asprintf', below. 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. 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; asprintf (&result, "value of %s is %s", name, value); 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 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 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 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, count); 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 N argument specifies the number of elements in the array ARGTYPES. This is the most 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 more than that many 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 `'z'', it defines the conversion `%z'. 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. 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 version before 2.0 the ARGINFO-FUNCTION function did not need to be installed unless the user uses the `parse_printf_format' function. This 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 arguments to `register_printf_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_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'. `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 change in the 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 were 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, va_list *app) { Widget *w; char *buffer; int len; /* Format the output into a string. */ w = va_arg (*app, Widget *); 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: || | | | | 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 as is 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 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 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' 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 are GNU extensions. - Function: int vscanf (const char *TEMPLATE, va_list AP) This function is similar to `scanf' except that, 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 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'. 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. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for details. 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. These symbols are declared in the header file `stdio.h'. - Macro: int EOF This macro is an integer value that is returned by a number of 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. - 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: int feof (FILE *STREAM) The `feof' function returns nonzero if and only if the end-of-file indicator for the stream STREAM is set. - 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. 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::. 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: 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. *Portability Note:* In non-POSIX systems, `ftell' and `fseek' 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' 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' 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' 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 begining of the file. It is equivalent to calling `fseek' 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' 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' to move back to the same file position. * In a call to `fseek' on a text stream, either the OFFSET must either be zero; or WHENCE must be `SEEK_SET' 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. 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 equivalent to `off_t' or `long int'. 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'. - 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'. 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 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. *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. 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. 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 (as with `malloc'; *note Unconstrained Allocation::.) an array SIZE bytes long. 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 open. 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' (*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' (*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". 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 ths 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' 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, void *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 void *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. 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. The `open' function is the underlying primitive for the `fopen' and `freopen' functions, that create streams. - 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) - 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. 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. 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 there is no terminating null character 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 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. The `read' function is the underlying primitive for all of the functions that read from streams, such as `fgetc'. - 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.) 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 necessary 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)); The `write' function is the underlying primitive for all of the functions that write to streams, such as `fputc'. 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 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 so; 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.) The `lseek' function is the underlying primitive for the `fseek', `ftell' and `rewind' functions, which operate on streams instead of file descriptors. 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'. 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 affect 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'. 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::. 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 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 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::. 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::. 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. - 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. File System Interface ********************* This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions described in *Note I/O on Streams:: and *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. Here is an example showing how you could implement the behavior of GNU's `getcwd (NULL, 0)' using only the standard behavior of `getcwd': char * gnu_getcwd () { int size = 100; char *buffer = (char *) xmalloc (size); while (1) { char *value = getcwd (buffer, size); if (value != 0) return buffer; size *= 2; free (buffer); buffer = (char *) xmalloc (size); } } *Note Malloc Examples::, for information about `xmalloc', which is not a library function but is a customary name used in most GNU software. - 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: 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. 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. Each value except DT_UNKNOWN corresponds to the file type bits in the `st_mode' member of `struct statbuf'. 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 DIRTYPE) This returns the `st_mode' value corresponding to DIRTYPE. This structure may contain additional members in the future. 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, and the like are part of the file itself, not 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::.). 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 for simultaneously running threads the result is not stored in some internal memory. Instead the argument ENTRY has to point to a place where the result is stored. 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 `-1'. The `errno' variable is set like described for `readdir'. *Portability Note:* On some systems, `readdir_r' may not return a terminated string as the file name even if no `d_reclen' element is available in `struct dirent' and the file name as the maximal 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 `d_reclen' field one could use multiple threads by using external locking. - 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 puts ("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 as the result a list of names. - Function: int scandir (const char *DIR, struct dirent ***NAMELIST, int (*SELECTOR) (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 nonzero value are selected. Finally the entries in the NAMELIST are sorted using the user supplied function CMP. The arguments of the CMP function are of type `struct dirent **'. I.e., one cannot directly use the `strcmp' or `strcoll' function; see the function `alphasort' below. The return value of the function gives the number of entries placed in NAMELIST. If it is `-1' an error occurred and the global variable `errno' contains more information on the error. As said 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 an implementation of a function which is very helpful for this purpose. - Function: int alphasort (const void *A, const void *B) The `alphasort' function behaves like the `strcmp' function (*note String/Array Comparison::.). The difference is that the arguments are not string pointers but instead they are of type `struct dirent **'. Return value of is less than, equal to, or greater than zero depending on the order of the two entries A and B. 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 using the functions which directly work with the directory contents. After the call the found entries are available for direct used. #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; } Please note the simple selector function for this example. Since we want to see all directory entries we always return `1'. 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 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 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. So does `link', the function that makes a hard link--it makes a hard link to the symbolic link, which one rarely wants. Prototypes for 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 (char *filename) { int size = 100; while (1) { char *buffer = (char *) xmalloc (size); int nchars = readlink (filename, buffer, size); 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. Deleting Files ============== You can delete a file with the functions `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 names as well (*note Hard Links::.), it remains accessible under its other 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 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 in which the file name is 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 name OLDNAME with NEWNAME. The file formerly accessible under the name OLDNAME is afterward 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 file (as indicated by 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, POSIX requires that in this case `rename' do nothing and report success--which is inconsistent. We don't know what your operating system will do. If the 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 the 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 the name NEWNAME changes "atomically" from any previously existing file by that name to its new meaning (the file that was called OLDNAME). There is no instant at which NEWNAME is nonexistent "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' The OLDNAME is a directory that contains NEWNAME. `EISDIR' The NEWNAME names a directory, but the OLDNAME doesn't. `EMLINK' The parent directory of NEWNAME would have too many links. `ENOENT' The file named by 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 OLDNAMES 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 whose name is 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. 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, and the like. This kind of information is called the "file attributes"; it is 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 these attributes of files. What the File Attribute Values Mean ----------------------------------- 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 and the like, 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 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 last modification to the attributes of the file. *Note File Times::. `unsigned int 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'.) 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'. - 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'. 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 at 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, 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: `ENOENT' The file named by FILENAME doesn't exist. - 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. - 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::. 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, whether it is a socket, and so on. For information about the access permission, *Note Permission Bits::. There are two predefined ways you can access the file type portion of the file mode. First of all, for each type of file, there is a "predicate macro" which examines a file mode value and returns true or false--is the file of that type, or not. Secondly, you can mask out the rest of the file mode to get just a file type code. You can compare this against various constants for 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 nonzero if the file is a directory. - Macro: int S_ISCHR (mode_t M) This macro returns nonzero if the file is a character special file (a device like a terminal). - Macro: int S_ISBLK (mode_t M) This macro returns nonzero if the file is a block special file (a device like a disk). - Macro: int S_ISREG (mode_t M) This macro returns nonzero if the file is a regular file. - Macro: int S_ISFIFO (mode_t M) This macro returns nonzero 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 nonzero if the file is a symbolic link. *Note Symbolic Links::. - Macro: int S_ISSOCK (mode_t M) This macro returns nonzero if the file is a socket. *Note Sockets::. An alterate non-POSIX method of testing the file type is supported for compatibility with BSD. The mode can be bitwise ANDed with `S_IFMT' to extract the file type code, and compared to the appropriate type code 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 portion of a mode value. These are the symbolic names for the different file type codes: `S_IFDIR' This macro represents the value of the file type code for a directory file. `S_IFCHR' This macro represents the value of the file type code for a character-oriented device file. `S_IFBLK' This macro represents the value of the file type code for a block-oriented device file. `S_IFREG' This macro represents the value of the file type code for a regular file. `S_IFLNK' This macro represents the value of the file type code for a symbolic link. `S_IFSOCK' This macro represents the value of the file type code for a socket. `S_IFIFO' This macro represents the value of the file type code for a FIFO or pipe. 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 user that is the owner, another set that apply to users who belong to the file's group, and a third set of bits that apply 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 from the effective user ID of the process that creates it (*note Process Persona::.). The file's group ID may be set from either 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 rule, 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 bits of the file's permissions. (This is because those bits may not be appropriate for the new owner.) The 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 file with open file 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. On a directory, it gives permission to delete a file in the directory only if you own that file. Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory). The same restriction applies--you must both have 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. In this use the sticky bit also says that the filesystem may fail to record the file's modification time onto disk reliably (the idea being that noone cares for a swap file). 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. It is not only nonportable, 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 file permission bits. As a special case, for a file to be executable even for 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 for the newly created file. But the specified 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 specified to the creation 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 ignores the file creation mask; it uses exactly the specified permission bits. In normal use, the file creation mask is initialized in 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 do automatically what it is supposed to do. When your program should 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) { mask = umask (0); umask (mask); } However, it is better to use `getumask' if you just want to read the mask value, because that 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 the FILENAME names a symbolic link, `chmod' changes the permission 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 permission 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 file currently open via descriptor 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 permission 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 ----------------------------------- When a program runs as a privileged user, this permits it to access files off-limits to ordinary users--for example, to modify `/etc/passwd'. Programs designed to be run by ordinary users but access such files use the setuid bit feature so that they always run with `root' as the effective user ID. Such a program may also access files specified by the user, files which conceptually are being accessed explicitly by the user. Since the program runs as `root', it has permission to access whatever file the user specifies--but usually the desired behavior is to permit only those files which the user could ordinarily access. 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 ID's of the calling process, rather than the *effective* ID's, 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 Argument that means, test for read permission. - Macro: int W_OK Argument that means, test for write permission. - Macro: int X_OK Argument that means, test for execute/search permission. - Macro: int F_OK Argument that means, test for existence of the file. File Times ---------- Each file has three timestamps 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 timestamps 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 attributes of a file (for example, with `chmod') updates its attribute change time field. You can also change some of the timestamps 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 at by TIMES. The attribute modification time for the file is set to the current time in either case (since changing the timestamps 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 timestamp on the file, you must either be the owner of the file, have write permission on 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. This error is used to report the problem. `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 for the file named by 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. 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 make a name for a temporary file and then 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. - Function: char * tmpnam (char *RESULT) This function constructs and returns a file name that is a valid file name and that does not name 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 fixed length of a temporary file name gives room for only a finite number of different names. If `tmpnam' fails, it returns a null pointer. - Function: char * tmpnam_r (char *RESULT) This function is nearly identical to the `tmpnam' function. But it does not allow RESULT to be a null pointer. In the later case a null pointer is returned. This function is reentrant because the non-reentrant situation of `tmpnam' cannot happen here. - Macro: int L_tmpnam The value of this macro is an integer constant expression that represents the minimum allocation size of a string large enough to hold the 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 create the files, you will probably run out of disk space before you run 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 filename. 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'; 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. - 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. - 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 open on that file for reading and writing. If `mkstemp' cannot create a uniquely-named file, it makes TEMPLATE an empty string and returns `-1'. If TEMPLATE does not end with `XXXXXX', `mkstemp' returns `-1' and does not modify TEMPLATE. 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 bit, which says you want to always create a new file, and get an error if the file already exists. 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. */ 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. */ 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 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, "Could not run more.\n"); return EXIT_FAILURE; } write_data (output); pclose (output); 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. But, 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. 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'. This type was on almost all machines of this time 32 bits wide and so a de-factor standard required 32 bit variables. This is important since references to variables of this type are passed to the kernel. But now the POSIX people came and unified the interface with their words "all size values are of type `size_t'". But on 64 bit machines `size_t' is 64 bits wide and so variable references are not anymore possible. A solution provides the Unix98 specification which finally introduces a type `socklen_t'. This type is used in all of the cases in previously changed to use `size_t'. The only requirement of this type is that it is an unsigned type of at least 32 bits. Therefore, implementations which require references to 32 bit variables be passed can be as happy as implementations which right from the start of 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 resend 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 addresses; for example, the rsh and rlogin protocols look at the client's socket address and don't bypass password checking unless it is less than `IPPORT_RESERVED' (*note Ports::.). The details of socket addresses vary depending on what namespace you are using. *Note File 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 in 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 which 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'. - Date 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_FILE' This designates the address format that goes with the file namespace. (`PF_FILE' is the name of that namespace.) *Note File Namespace Details::, for information about this address format. `AF_UNIX' This is a synonym for `AF_FILE', for compatibility. (`PF_UNIX' is likewise a synonym for `PF_FILE'.) `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, all or most 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 File Namespace::, 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 for 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. The File Namespace ================== This section describes the details of the file namespace, whose symbolic name (required when you create a socket) is `PF_FILE'. File Namespace Concepts ----------------------- In the file 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. In order to connect to a socket, you must have read permission for it. It's common to put these files in the `/tmp' directory. One peculiarity of the file namespace is that the name is only used when opening the connection; once that is over with, 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 file namespace, you should delete the file name from the file system. Use `unlink' or `remove' to do this; see *Note Deleting Files::. The file namespace supports just one protocol for any communication style; it is protocol number `0'. Details of File Namespace ------------------------- To create a socket in the file namespace, use the constant `PF_FILE' as the NAMESPACE argument to `socket' or `socketpair'. This constant is defined in `sys/socket.h'. - Macro: int PF_FILE This designates the file namespace, in which socket addresses are file names, and its associated family of protocols. - Macro: int PF_UNIX This is a synonym for `PF_FILE', for compatibility's sake. The structure for specifying socket names in the file namespace is defined in the header file `sys/un.h': - Data Type: struct sockaddr_un This structure is used to specify file 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_FILE' to designate the file 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 example following 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 file 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. Example of File-Namespace Sockets --------------------------------- Here is an example showing how to create and name a socket in the file 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_UNIX, SOCK_DGRAM, 0); if (sock < 0) { perror ("socket"); exit (EXIT_FAILURE); } /* Bind a name to the socket. */ name.sun_family = AF_FILE; strcpy (name.sun_path, filename); /* The size of the address is the offset of the start of the filename, plus its length, plus one for the terminating null byte. */ 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 the protocols and socket naming conventions used in the Internet namespace. To create a socket in the Internet namespace, use the symbolic name `PF_INET' of this namespace as the NAMESPACE argument to `socket' or `socketpair'. This macro is defined in `sys/socket.h'. - Macro: int PF_INET This designates the 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: `short int sin_family' This identifies the address family or format of the socket address. You should store the value of `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 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: `short int 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 seperated 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 `churchy.gnu.ai.mit.edu'. Programs that let the user specify a host typically accept both numeric addresses and host names. But the program needs a numeric address to open a connection; to use a host name, you must convert it 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 Internet host address is a number containing four bytes of data. These are divided into two parts, a "network number" and a "local network address number" within that network. The network number consists of the first one, two or three bytes; the rest of the bytes are the local address. 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 specifies 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. 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. `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, C, 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. Host Address Data Type ...................... Internet host addresses are represented in some contexts as integers (type `unsigned long int'). 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. The following basic definitions for Internet addresses appear in the header file `netinet/in.h': - Data Type: struct in_addr This data type is used in certain contexts to contain an Internet host address. It has just one field, named `s_addr', which records the host address number as an `unsigned long int'. - Macro: unsigned int INADDR_LOOPBACK You can use this constant to stand for "the address of this machine," instead of finding its actual address. It is the 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: unsigned int 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: unsigned int INADDR_BROADCAST This constant is the address you use to send a broadcast message. - Macro: unsigned int 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 initialise 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 initialise your own variables to this value. Host Address Functions ...................... These additional functions for manipulating Internet addresses are declared in `arpa/inet.h'. They represent Internet addresses in network byte order; they represent 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 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: unsigned long int inet_addr (const char *NAME) This function converts the 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: unsigned long int inet_network (const char *NAME) This function extracts the network number from the address NAME, given in the standard numbers-and-dots notation. If the input is not valid, `inet_network' returns `-1'. - Function: char * inet_ntoa (struct in_addr ADDR) This function converts the 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. - Function: struct in_addr inet_makeaddr (int NET, int LOCAL) This function makes an Internet host address by combining the network number NET with the local-address-within-network number LOCAL. - Function: int inet_lnaof (struct in_addr ADDR) This function returns the local-address-within-network part of the Internet host address ADDR. - Function: int inet_netof (struct in_addr ADDR) This function returns the network number part of the Internet host address ADDR. - 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: 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 `128.52.46.32' is also known as `churchy.gnu.ai.mit.edu'; and other machines in the `gnu.ai.mit.edu' domain can refer to it simply as `churchy'. 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 data base 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 this to a `struct in_addr' or an `unsigned long int'. 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') for the result. - Function: struct hostent * gethostbyaddr (const char *ADDR, int LENGTH, int FORMAT) The `gethostbyaddr' function returns information about the host with Internet address ADDR. The LENGTH argument is the size (in bytes) of the address at ADDR. FORMAT specifies the address format; for an Internet address, specify a value of `AF_INET'. 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.) Before using `h_errno', you must declare it like this: extern int h_errno; Here are the error codes that you may find in `h_errno': `HOST_NOT_FOUND' No such host is known in the data base. `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. You can also scan the entire hosts database one entry at a time using `sethostent', `gethostent', and `endhostent'. Be careful in 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 () This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries. - Function: void endhostent () 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 in 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 the "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 the 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 the 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 values for the `sin_addr' member. (Remember, `struct in_addr' is equivalent to `unsigned long int'.) These functions are declared in `netinet/in.h'. - Function: unsigned short int htons (unsigned short int HOSTSHORT) This function converts the `short' integer HOSTSHORT from host byte order to network byte order. - Function: unsigned short int ntohs (unsigned short int NETSHORT) This function converts the `short' integer NETSHORT from network byte order to host byte order. - Function: unsigned long int htonl (unsigned long int HOSTLONG) This function converts the `long' integer HOSTLONG from host byte order to network byte order. - Function: unsigned long int ntohl (unsigned long int NETLONG) This function converts the `long' integer NETLONG from network byte order to host byte order. Protocols Database ------------------ The communications protocol used with a socket controls low-level details of how data is 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 in 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 (unsigned short int 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, unsigned short int 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 on `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_FILE' (*note File 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. `EACCESS' The process does not have 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. But, like pipes, sockets do not support file positioning operations. For examples of how to call the `socket' function, see *Note File Namespace::, or *Note Inet Example::. Closing a Socket ---------------- When you are 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 only 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_FILE' (*note File 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, using 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). 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 connect 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 File 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 a `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. 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, size_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 the 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'. 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 is 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. 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. #include #include #include #include #include #include #include #include #define PORT 5555 #define MESSAGE "Yow!!! Are we having fun yet?!?" #define SERVERHOST "churchy.gnu.ai.mit.edu" 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, unsigned short int 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' and `init_sockaddr' 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 (unsigned short int 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. The way to send out-of-band data is using `send', specifying the flag `MSG_OOB' (*note Sending Data::.). Out-of-band data is 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 the ordinary data. When a socket finds that out-of-band data is on its 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 is 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, &result); 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 result, success; /* If we have reached the mark, return. */ success = ioctl (socket, SIOCATMARK, &result); 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 *buffer; 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 is 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? */ char *buffer = (char *) xmalloc (1024); struct buffer *link; int success; int result; /* Try again to read the out-of-band data. */ success = recv (socket, buffer, sizeof buffer, MSG_OOB); if (success >= 0) { /* We got it, so return it. */ struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buffer = buffer; link->size = success; link->next = list; return link; } /* If we fail, see if we are at the mark. */ success = ioctl (socket, SIOCATMARK, &result); if (success < 0) perror ("ioctl"); if (result) { /* 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, buffer, sizeof buffer); if (success < 0) perror ("read"); /* Save this data in the buffer list. */ { struct buffer *link = (struct buffer *) xmalloc (sizeof (struct buffer)); link->buffer = buffer; 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, then 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 due to a problem related to a previous call. 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 file domain, the address information won't be meaningful, since you can't read the address of such a socket (*note File 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'. You can use plain `recv' (*note Receiving Data::.) instead of `recvfrom' if you know 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 file namespace. Both the client and server programs use the `make_named_socket' function that was presented in *Note File Namespace::, 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; /* 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_UNIX; 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 it 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 resend 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 service for 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. So 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 many as 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, which has 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 socket 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 is 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 data base, 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 (long 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 in 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 both `isatty' and `ttyname' 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. 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 terminal's 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 STOP 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'. 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 RET and LFD 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 RET 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 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 DEL) 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, `INTR' is the index of the element that specifies the INTR character, so storing `'='' in `TERMIOS.c_cc[INTR]' 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 RET 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 DEL. - 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; 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. 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. 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'. All of the functions that operate on floating-point numbers accept arguments and return results of type `double'. In the future, there may be additional functions that operate on `float' and `long double' values. For example, `cosf' and `cosl' would be versions of the `cos' function that operate on `float' and `long double' arguments, respectively. In the meantime, you should avoid using these names yourself. *Note Reserved Names::. Domain and Range Errors ======================= Many of the functions listed in this chapter are defined mathematically over a domain that is only a subset of real numbers. For example, the `acos' function is defined over the domain between `-1' and `1'. If you pass an argument to one of these functions that is outside the domain over which it is defined, the function sets `errno' to `EDOM' to indicate a "domain error". On machines that support IEEE 754 floating point, functions reporting error `EDOM' also return a NaN. Some of these 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 functions in this chapter take only real arguments and return only real values; therefore, if the value ought to be nonreal, this is treated as a domain error. A related problem is that the mathematical result of a function may not be representable as a floating point number. If magnitude of the correct result is too large to be represented, the function sets `errno' to `ERANGE' to indicate a "range error", and returns a particular very large value (named by the macro `HUGE_VAL') or its negation (`- HUGE_VAL'). If the magnitude of the result is too small, a value of zero is returned instead. In this case, `errno' might or might not be set to `ERANGE'. The only completely reliable way to check for domain and range errors is to set `errno' to `0' before you call the mathematical function and test `errno' afterward. As a consequence of this use of `errno', use of the mathematical functions is not reentrant if you check for errors. None of the mathematical functions ever generates signals as a result of domain or range errors. In particular, this means that you won't see `SIGFPE' signals generated within these functions. (*Note Signal Handling::, for more information about signals.) - Macro: double HUGE_VAL An expression representing a particular very large number. On machines that use IEEE 754/IEEE 854 floating point format, the value is "infinity". On other machines, it's typically the largest positive number that can be represented. The value of this macro is used as the return value from various mathematical `double' returning functions in overflow situations. - Macro: float HUGE_VALf This macro is similar to the `HUGE_VAL' macro except that it is used by functions returning `float' values. This macro is a GNU extension. - Macro: long double HUGE_VALl This macro is similar to the `HUGE_VAL' macro except that it is used by functions returning `long double' values. The value is only different from `HUGE_VAL' if the architecture really supports `long double' values. This macro is a GNU extension. For more information about floating-point representations and limits, see *Note Floating Point Parameters::. In particular, the macro `DBL_MAX' might be more appropriate than `HUGE_VAL' for many uses other than testing for an error in a mathematical function. 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 doesn't define a symbolic constant for pi, but you can define your own if you need one: #define PI 3.14159265358979323846264338327 You can also compute the value of pi with the expression `acos (-1.0)'. - Function: double sin (double X) This function returns 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) This function returns 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) This function returns the tangent of X, where X is given in radians. The following `errno' error conditions are defined for this function: `ERANGE' 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' sets `errno' to `ERANGE' and returns either positive or negative `HUGE_VAL'. 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) This function computes 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). `asin' fails, and sets `errno' to `EDOM', if X is out of range. The arc sine function is defined mathematically only over the domain `-1' to `1'. - Function: double acos (double X) This function computes 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). `acos' fails, and sets `errno' to `EDOM', if X is out of range. The arc cosine function is defined mathematically only over the domain `-1' to `1'. - Function: double atan (double X) This function computes 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) This is the two argument arc tangent function. It is similar to computing the arc tangent of Y/X, except that 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::.) The function `atan2' sets `errno' to `EDOM' if both X and Y are zero; the return value is not defined in this case. Exponentiation and Logarithms ============================= - Function: double exp (double X) The `exp' function returns the value of e (the base of natural logarithms) raised to power X. The function fails, and sets `errno' to `ERANGE', if the magnitude of the result is too large to be representable. - Function: double log (double X) This function returns the natural logarithm of X. `exp (log (X))' equals X, exactly in mathematics and approximately in C. The following `errno' error conditions are defined for this function: `EDOM' The argument X is negative. The log function is defined mathematically to return a real result only on positive arguments. `ERANGE' The argument is zero. The log of zero is not defined. - Function: double log10 (double X) This function returns the base-10 logarithm of X. Except for the different base, it is similar to the `log' function. In fact, `log10 (X)' equals `log (X) / log (10)'. - Function: double pow (double BASE, double POWER) This is a general exponentiation function, returning BASE raised to POWER. The following `errno' error conditions are defined for this function: `EDOM' The argument BASE is negative and POWER is not an integral value. Mathematically, the result would be a complex number in this case. `ERANGE' An underflow or overflow condition was detected in the result. - Function: double sqrt (double X) This function returns the nonnegative square root of X. The `sqrt' function fails, and sets `errno' to `EDOM', if X is negative. Mathematically, the square root would be a complex number. - Function: double cbrt (double X) This function returns the cube root of X. This function cannot fail; every representable real value has a representable real cube root. - Function: double hypot (double X, double Y) The `hypot' function returns `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.) See also the function `cabs' in *Note Absolute Value::. - Function: double expm1 (double X) This function returns a value equivalent to `exp (X) - 1'. It is computed in a way that is accurate even if the value of X is near zero--a case where `exp (X) - 1' would be inaccurate due to subtraction of two numbers that are nearly equal. - Function: double log1p (double X) This function returns a value equivalent to `log (1 + X)'. It is computed in a way that is accurate even if the value of X is near zero. Hyperbolic Functions ==================== The functions in this section are related to the exponential functions; see *Note Exponents and Logarithms::. - Function: double sinh (double X) The `sinh' function returns the hyperbolic sine of X, defined mathematically as `exp (X) - exp (-X) / 2'. The function fails, and sets `errno' to `ERANGE', if the value of X is too large; that is, if overflow occurs. - Function: double cosh (double X) The `cosh' function returns the hyperbolic cosine of X, defined mathematically as `exp (X) + exp (-X) / 2'. The function fails, and sets `errno' to `ERANGE', if the value of X is too large; that is, if overflow occurs. - Function: double tanh (double X) This function returns the hyperbolic tangent of X, whose mathematical definition is `sinh (X) / cosh (X)'. - Function: double asinh (double X) This function returns the inverse hyperbolic sine of X--the value whose hyperbolic sine is X. - Function: double acosh (double X) This function returns the inverse hyperbolic cosine of X--the value whose hyperbolic cosine is X. If X is less than `1', `acosh' returns `HUGE_VAL'. - Function: double atanh (double X) This function returns the inverse hyperbolic tangent of X--the value whose hyperbolic tangent is X. If the absolute value of X is greater than or equal to `1', `atanh' returns `HUGE_VAL'. 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 at all times 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 truly random numbers, not just pseudo-random, specify a seed based on the current time. You can get 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 another set derived from BSD. We recommend you use the standard ones, `rand' and `srand'. 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 expression that represents the maximum possible value returned by the `rand' function. In the GNU library, it is `037777777', which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as `32767'. - Function: int rand () The `rand' function returns the next pseudo-random number in the series. The value is in the range 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 truly random numbers (not just pseudo-random), do `srand (time (0))'. 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 () This function returns the next pseudo-random number in the sequence. The range of values returned is from `0' to `RAND_MAX'. - Function: void srandom (unsigned int SEED) The `srandom' function sets the seed for the current random number state 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 truly random numbers (not just pseudo-random), 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. The size must be at least 8 bytes, and optimal sizes are 8, 16, 32, 64, 128, and 256. 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 thise value later as an argument to `setstate' to restore that state. Low-Level Arithmetic Functions ****************************** This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts. These functions are declared in the header file `math.h'. "Not a Number" Values ===================== The IEEE floating point format used by most modern computers supports values that are "not a number". These values are called "NaNs". "Not a number" values result from certain operations which have no meaningful numeric result, such as zero divided by zero or infinity divided by infinity. One noteworthy property of NaNs is that they are not equal to themselves. Thus, `x == x' can be 0 if the value of `x' is a NaN. You can use this to test whether a value is a NaN or not: if it is not equal to itself, then it is a NaN. But the recommended way to test for a NaN is with the `isnan' function (*note Predicates on Floats::.). Almost any arithmetic operation in which one argument is a NaN returns a NaN. - Macro: double NAN An expression representing a value which is "not a number". This macro is a GNU extension, available only on machines that support "not a number" values--that is to say, on all machines that support IEEE floating point. You can use `#ifdef NAN' to test whether the machine supports NaNs. (Of course, you must arrange for GNU extensions to be visible, such as by defining `_GNU_SOURCE', and then you must include `math.h'.) Predicates on Floats ==================== This section describes some miscellaneous test functions on doubles. Prototypes for these functions appear in `math.h'. These are BSD functions, and thus are available if you define `_BSD_SOURCE' or `_GNU_SOURCE'. - Function: int isinf (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) This function returns a nonzero value if X is a "not a number" value, and zero otherwise. (You can just as well use `X != X' to get the same result). - Function: int finite (double X) This function returns a nonzero value if X is finite or a "not a number" value, and zero otherwise. - Function: double infnan (int ERROR) This function is provided for compatibility with BSD. The other mathematical functions use `infnan' to decide what to return on occasion of an error. Its argument is an error code, `EDOM' or `ERANGE'; `infnan' returns a suitable value to indicate this with. `-ERANGE' is also acceptable as an argument, and corresponds to `-HUGE_VAL' as a value. In the BSD library, on certain machines, `infnan' raises a fatal signal in all cases. The GNU library does not do likewise, because that does not fit the ISO C specification. *Portability Note:* The functions listed in this section are BSD extensions. 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 is 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' and `labs' are in `stdlib.h'; `fabs' and `cabs' are declared in `math.h'. - Function: int abs (int NUMBER) This function returns 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. - Function: long int labs (long int NUMBER) This is similar to `abs', except that both the argument and result are of type `long int' rather than `int'. - Function: double fabs (double NUMBER) This function returns the absolute value of the floating-point number NUMBER. - Function: double cabs (struct { double real, imag; } Z) The `cabs' function returns the absolute value of the complex number Z, whose real part is `Z.real' and whose imaginary part is `Z.imag'. (See also the function `hypot' in *Note Exponents and Logarithms::.) The value is: sqrt (Z.real*Z.real + Z.imag*Z.imag) 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) The `frexp' function is 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) This function returns 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': - Function: double scalb (double VALUE, int EXPONENT) The `scalb' function is the BSD name for `ldexp'. - Function: double logb (double X) This BSD function returns the integer part of the base-2 logarithm of X, an integer value represented in type `double'. This is the highest integer power of `2' contained in X. The sign of X is ignored. For example, `logb (3.5)' is `1.0' and `logb (4.0)' is `2.0'. When `2' raised to this power is divided into X, it gives a quotient between `1' (inclusive) and `2' (exclusive). If X is zero, the value is minus infinity (if the machine supports such a value), or else a very small number. If X is infinity, the value is infinity. The value returned by `logb' is one less than the value that `frexp' would store into `*EXPONENT'. - Function: double copysign (double VALUE, double SIGN) The `copysign' function returns a value whose absolute value is the same as that of VALUE, and whose sign matches that of SIGN. This is a BSD function. Rounding and Remainder Functions ================================ The functions listed here perform operations such as rounding, truncation, and remainder in division of floating point numbers. 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) The `ceil' function rounds X upwards to the nearest integer, returning that value as a `double'. Thus, `ceil (1.5)' is `2.0'. - Function: double floor (double X) The `ceil' function rounds 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 rint (double X) This function rounds 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 explicit select another. - Function: double modf (double VALUE, double *INTEGER-PART) This function breaks 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, so the rounding of the integer part is towards 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'. - Function: double fmod (double NUMERATOR, double DENOMINATOR) This function computes 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' fails and sets `errno' to `EDOM'. - Function: double drem (double NUMERATOR, double DENOMINATOR) The function `drem' is like `fmod' except that it 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' fails and sets `errno' to `EDOM'. Integer Division ================ This section describes functions for performing integer division. These functions are redundant in the GNU C library, since 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'. 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 ------------------- These functions are declared in `stdlib.h'. - Function: long int strtol (const char *STRING, char **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 `35'. 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. Because the value `0l' is a correct result for `strtol' the user who is interested in handling errors should set the global variable `errno' to `0' before calling this function, so that the program can later test whether an error occurred. There is an example at the end of this section. - Function: unsigned long int strtoul (const char *STRING, char **TAILPTR, int BASE) The `strtoul' ("string-to-unsigned-long") function is like `strtol' except it deals with unsigned numbers, and returns its value with type `unsigned long int'. No `+' or `-' sign may appear before the number, but the syntax is otherwise the same as described above for `strtol'. The value returned in case of overflow is `ULONG_MAX' (*note Range of Type::.). Like `strtol' this function sets `errno' and returns the value `0ul' in case the value for BASE is not in the legal range. For `strtoul' this can happen in another situation. In case the number to be converted is negative `strtoul' also sets `errno' to `EINVAL' and returns `0ul'. - Function: long long int strtoq (const char *STRING, char **TAILPTR, int BASE) The `strtoq' ("string-to-quad-word") function is like `strtol' except that is deals with extra long numbers and it returns its value with type `long long int'. If the string has valid syntax for an integer but the value is not representable because of overflow, `strtoq' 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. - Function: long long int strtoll (const char *STRING, char **TAILPTR, int BASE) `strtoll' is only an commonly used other name for the `strtoq' function. Everything said for `strtoq' applies to `strtoll' as well. - Function: unsigned long long int strtouq (const char *STRING, char **TAILPTR, int BASE) The `strtouq' ("string-to-unsigned-quad-word") function is like `strtoul' except that is deals with extra long numbers and it returns its value with type `unsigned long long int'. The value returned in case of overflow is `ULONG_LONG_MAX' (*note Range of Type::.). - Function: unsigned long long int strtoull (const char *STRING, char **TAILPTR, int BASE) `strtoull' is only an commonly used other name for the `strtouq' function. Everything said for `strtouq' applies to `strtoull' as well. - 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' value rather than `long int'. The `atoi' function is also considered obsolete; use `strtol' instead. The POSIX locales contain some information about how to format numbers (*note General Numeric::.). This mainly deals with representing numbers for better readability for humans. The functions present so far in this section cannot handle numbers in this form. If this functionality is needed in a program one can use the functions from the `scanf' family which know about the flag `'' for parsing numeric input (*note Numeric Input Conversions::.). Sometimes it is more desirable to have finer control. In these situation one could use the function `__strtoXXX_internal'. XXX here stands for any of the above forms. All numeric conversion functions (including the functions to process floating-point numbers) have such a counterpart. The difference to the normal form is the extra argument at the end of the parameter list. If this value has an non-zero value the handling of number grouping is enabled. The advantage of using these functions is that the TAILPTR parameters allow to determine which part of the input is processed. The `scanf' functions don't provide this information. The drawback of using these functions is that they are not portable. They only exist in the GNU C library. 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 ----------------- These functions are declared in `stdlib.h'. - Function: double strtod (const char *STRING, char **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 nonempty sequence of digits optionally containing a decimal-point character--normally `.', but it depends on the locale (*note Numeric Formatting::.). * An optional exponent part, consisting of a character `e' or `E', 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 not representable because of overflow, `strtod' returns either positive or negative `HUGE_VAL' (*note Mathematics::.), depending on the sign of the value. Similarly, if the value is not representable because of underflow, `strtod' returns zero. It also sets `errno' to `ERANGE' if there was overflow or underflow. Since the value zero which is returned in the error case is also a valid result the user should set the global variable `errno' to zero before calling this function. So one can test for failures after the call since all failures set `errno' to a non-zero value. - Function: float strtof (const char *STRING, char **TAILPTR) This function is similar to the `strtod' function but it returns a `float' value instead of a `double' value. If the precision of a `float' value is sufficient this function should be used since it is much faster than `strtod' on some architectures. The reasons are obvious: IEEE 754 defines `float' to have a mantissa of 23 bits while `double' has 53 bits and every additional bit of precision can require additional computation. If the string has valid syntax for a floating-point number but the value is not representable because of overflow, `strtof' returns either positive or negative `HUGE_VALf' (*note Mathematics::.), depending on the sign of the value. This function is a GNU extension. - Function: long double strtold (const char *STRING, char **TAILPTR) This function is similar to the `strtod' function but it returns a `long double' value instead of a `double' value. It should be used when high precision is needed. On systems which define a `long double' type (i.e., on which it is not the same as `double') running this function might take significantly more time since more bits of precision are required. If the string has valid syntax for a floating-point number but the value is not representable because of overflow, `strtold' returns either positive or negative `HUGE_VALl' (*note Mathematics::.), depending on the sign of the value. This function is a GNU extension. As for the integer parsing functions there are additional functions which will handle numbers represented using the grouping scheme of the current locale (*note Parsing of Integers::.). - 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. 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 double *a, const double *b) { return (int) (*a - *b); } 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 ===================== 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. 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. 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. 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. `gl_pathc' The number of elements in the vector. `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. - 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. 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'. 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; } In practice, since `wordexp' is executed by running a subshell, it would be faster to do this by concatenating the strings with spaces between them and running that as a shell command using `sh -c'. Date and Time ************* This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations. The time functions fall into three main categories: * Functions for measuring elapsed CPU time are discussed in *Note Processor Time::. * Functions for measuring absolute clock or calendar time are discussed in *Note Calendar Time::. * Functions for setting alarms and timers are discussed in *Note Setting an Alarm::. Processor Time ============== If you're trying to optimize your program or measure its efficiency, it's very useful to be able to know how much "processor time" or "CPU time" it has used at any given point. Processor time is different from actual wall clock time because it doesn't include any time spent waiting for I/O or when some other process is running. Processor time is represented by the data type `clock_t', and is given as a number of "clock ticks" relative to an arbitrary base time marking the beginning of a single program invocation. Basic CPU Time Inquiry ---------------------- To get the elapsed CPU time used by a process, 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), like this: #include clock_t start, end; double elapsed; start = clock(); ... /* Do the work. */ end = clock(); elapsed = ((double) (end - start)) / CLOCKS_PER_SEC; Different computers and operating systems vary wildly in how they keep track of processor time. It's common for the internal processor clock to have a resolution somewhere between hundredths and millionths of a second. 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 type of the macro `CLOCKS_PER_SEC' can be either integer or floating-point types. Casting processor 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. - Macro: int CLOCKS_PER_SEC The value of this macro is the number of clock ticks per second measured by the `clock' function. - 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 in units of clock ticks. - Function: clock_t clock (void) This function returns the elapsed processor time. The base time is arbitrary but doesn't change within a single process. If the processor time is not available or cannot be represented, `clock' returns the value `(clock_t)(-1)'. Detailed Elapsed CPU Time Inquiry --------------------------------- The `times' function returns more detailed information about elapsed processor time in a `struct tms' object. 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 CPU time used in executing the instructions of the calling process. `clock_t tms_stime' This is the CPU time used by the system 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 CPU 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 CPU time used by the system on behalf of all the terminated child processes of the calling process. All of the times are given in clock ticks. These are absolute values; in a newly created process, they are all zero. *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 same as the value of `clock()': the elapsed real time relative to an arbitrary base. The base is a constant within a particular process, and typically represents the time since system start-up. A value of `(clock_t)(-1)' is returned to indicate failure. *Portability Note:* The `clock' function described in *Note Basic CPU Time::, is specified by the ISO C standard. The `times' function is a feature of POSIX.1. In the GNU system, the value returned by the `clock' function is 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 dates and times according to the Gregorian calendar. There are three representations for date and time information: * "Calendar time" (the `time_t' data type) is a compact representation, typically giving the number of seconds elapsed since some implementation-specific base time. * There is also a "high-resolution time" representation (the `struct timeval' data type) that includes fractions of a second. Use this time representation instead of ordinary calendar time when you need greater precision. * "Local time" or "broken-down time" (the `struct tm' data type) represents the date and time as a set of components specifying the year, month, and so on, for a specific time zone. This time representation is usually used in conjunction with formatting date and time values. Simple Calendar Time -------------------- This section describes the `time_t' data type for representing calendar time, and the functions which operate on calendar time objects. These facilities are declared in the header file `time.h'. - Data Type: time_t This is the data type used to represent calendar time. When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This date is sometimes referred to as the "epoch".) POSIX requires that this count ignore leap seconds, but on some hosts this count includes leap seconds if you set `TZ' to certain values (*note TZ Variable::.). 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. - Function: double difftime (time_t TIME1, time_t TIME0) The `difftime' function returns the number of seconds elapsed between time TIME1 and 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. - Function: time_t time (time_t *RESULT) The `time' function returns the current time as a value of type `time_t'. If the argument RESULT is not a null pointer, the time value is also stored in `*RESULT'. If the calendar time is not available, the value `(time_t)(-1)' is returned. High-Resolution Calendar ------------------------ The `time_t' data type used to represent calendar 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 timeval The `struct timeval' structure represents a calendar time. It has the following members: `long int tv_sec' This represents the number of seconds since the epoch. It is equivalent to a normal `time_t' value. `long int tv_usec' This is the fractional second value, represented as the number of microseconds. Some times struct timeval values are used for time intervals. Then the `tv_sec' member is the number of seconds in the interval, and `tv_usec' is the number of additional microseconds. - 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::. It is often necessary to subtract two values of type `struct timeval'. 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 = (y->tv_usec - x->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; } - Function: int gettimeofday (struct timeval *TP, struct timezone *TZP) The `gettimeofday' function returns the current date and time in the `struct timeval' structure indicated by TP. 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 date and time according to the arguments. 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'. 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 time 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 gradual adjustments in the current time. This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time. The DELTA argument specifies a relative adjustment to be made to the current time. If negative, the system clock is slowed down for a while until it has lost this much 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. 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. Broken-down Time ---------------- Calendar time is represented as a number of seconds. This is convenient for calculation, but has no resemblance to the way people normally represent dates and times. By contrast, "broken-down time" is a binary representation 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. A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used. 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 seconds after the minute, normally in the range `0' through `59'. (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 minutes after the hour, in the range `0' through `59'. `int tm_hour' This is the number of hours past midnight, in the range `0' through `23'. `int tm_mday' This is the day of the month, in the range `1' through `31'. `int tm_mon' This is the number of months since January, in the range `0' through `11'. `int tm_year' This is the number of years since `1900'. `int tm_wday' This is the number of days since Sunday, in the range `0' through `6'. `int tm_yday' This is the number of days since January 1, 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 calendar 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.) Calling `localtime' has one other effect: it sets the variable `tzname' with information about the current time zone. *Note Time Zone Functions::. - 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)--that is, as Greenwich Mean Time (GMT)--rather than relative to the local time zone. Recall that calendar times are *always* expressed in coordinated universal time. - Function: time_t mktime (struct tm *BROKENTIME) The `mktime' function is used to convert a broken-down time structure to a calendar 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 compute the calendar time; it's permissible for these components to have unnormalized values outside of 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 calendar 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::. Formatting Date and Time ------------------------ The functions described in this section format 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 * ctime (const time_t *TIME) The `ctime' function is similar to `asctime', except that the time value is specified as a `time_t' calendar 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: 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, which are GNU extensions, 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 date and 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. `%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. `%e' The day of the month like with `%d', but padded with blank (range ` 1' through `31'). This format is a POSIX.2 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 is a GNU extension. `%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 is 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. `%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. `%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 is a GNU extension. `%r' The complete time using the AM/PM format of the current locale. This format is a POSIX.2 extension. `%R' The hour and minute in decimal numbers using the format `%H:%M'. This format is 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 second as a decimal number (range `00' through `60'). `%t' A single `\t' (tabulator) character. This format is a POSIX.2 extension. `%T' The time 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. `%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. `%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, but without the time. `%X' The preferred time representation for the current locale, but with no date. `%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 is a GNU extension. `%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, the excess characters are discarded. The return value from `strftime' is the number of characters placed in the array S, not including the terminating null character. If the value equals SIZE, it means that the array S was too small; you should repeat the call, providing a bigger array. 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::. 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 the network from a different time zone, and would like times reported to you in the time zone that local for you, rather than what is local for the computer. In POSIX.1 systems the value of the `TZ' variable can be of 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 or embedded digits, commas, or plus or 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 one 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 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 one 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 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. - 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 savings time rules apply. A nonzero value does not necessarily mean that daylight savings time is now in effect; it means only that daylight savings time is sometimes in effect. Time Functions Example ---------------------- Here is an example program showing the use of some of the local time and 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 at some future time. 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 clock time. This timer sends a `SIGALRM' signal to the process when it expires. * A virtual timer that counts CPU time used by the process. This timer sends a `SIGVTALRM' signal to the process when it expires. * A profiling timer that counts both CPU time used by the process, and CPU 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, and in that case it would be terminated, since that 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 interval between successive timer interrupts. If zero, the alarm will only be sent once. `struct timeval it_value' This is the interval to the first timer interrupt. If zero, the alarm is disabled. The `struct timeval' data type is described in *Note High-Resolution Calendar::. - 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 interval was 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 short periods of time. If your program doesn't use signals (except to terminate), then you can expect `sleep' to wait reliably for the specified amount of time. Otherwise, `sleep' can return sooner if a signal arrives; if you want to wait for a given period 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 time has elapsed, 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 period. 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 time at which the program should stop waiting, and keep trying to wait until that 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 unavoidable additional 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'. Resource Usage ============== The function `getrusage' and the data type `struct rusage' are used for examining the usage figures of a process. They are declared in `sys/resource.h'. - Function: int getrusage (int PROCESSES, struct rusage *RUSAGE) This function reports the 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 terminated already. 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 usage figures 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 records a collection usage amounts for various sorts of resources. 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 that PROCESSES used in real memory 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 in data. `long int ru_isrss' An integral value expressed the same way, which is the amount of unshared memory used in 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 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 the time slice expired, or another process of higher priority became runnable). An additional historical function for examining usage figures, `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 limit. Each process initially inherits its limit values from its parent, but it can subsequently change them. The symbols in this section are defined in `sys/resource.h'. - Function: int getrlimit (int RESOURCE, struct rlimit *RLP) Read the current value and the maximum value of 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'. - Function: int setrlimit (int RESOURCE, struct rlimit *RLP) Store the current value and the maximum value of resource RESOURCE in `*RLP'. The return value is `0' on success and `-1' on failure. The following `errno' error condition is possible: `EPERM' You tried to change the maximum permissible limit value, but you don't have privileges to do so. - Data Type: struct rlimit This structure is used with `getrlimit' to receive limit values, and with `setrlimit' to specify limit values. It has two fields: `rlim_cur' The current value of the limit in question. This is also called the "soft limit". `rlim_max' The maximum permissible value of the limit in question. You cannot set the current value of the limit to a larger number than this maximum. Only the super user can change the maximum permissible value. This is also called the "hard limit". In `getrlimit', the structure is an output; it receives the current values. In `setrlimit', it specifies the new values. Here is a list of resources that you can specify a limit for. Those that are 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 maximum size, 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, it gets error code `EMFILE'. *Note Error Codes::. Not all systems support this limit; GNU does, and 4.4 BSD does. `RLIM_NLIMITS' The number of different resource limits. Any valid RESOURCE operand must be less than `RLIM_NLIMITS'. - Constant: int This constant stands for a value of "infinity" when supplied as the limit value in `setrlimit'. Two historical functions for setting resource limits, `ulimit' and `vlimit', are not documented here. The latter is declared in `sys/vlimit.h' and comes from BSD. Process Priority ================ When several processes try to run, their respective priorities determine what share of the CPU each process gets. This section describes how you can read and set the priority of a process. All these functions and macros are declared in `sys/resource.h'. The range of valid priority values depends on the operating system, but typically it runs from `-20' to `20'. A lower priority value means the process runs more often. These constants describe the range of priority values: `PRIO_MIN' The smallest valid priority value. `PRIO_MAX' The smallest valid priority value. - Function: int getpriority (int CLASS, int ID) Read the priority of a class of processes; CLASS and ID specify which ones (see below). If the processes specified do not all have the same priority, this returns the smallest value that any of them has. The return value is the priority value 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. When the return value is `-1', it could indicate failure, or it could be the priority 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 PRIORITY) Set the priority of a class of processes to PRIORITY; 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 defined 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' You tried to set the priority of some other user's process, and you don't have privileges for that. `EACCES' You tried to lower the priority of a process, and you don't have privileges for that. The arguments CLASS and ID together specify a set of processes you are interested in. These are the possible values for CLASS: `PRIO_PROCESS' Read or set the priority of one process. The argument ID is a process ID. `PRIO_PGRP' Read or set the priority of one process group. The argument ID is a process group ID. `PRIO_USER' Read or set the priority of one user's processes. The argument ID is a user ID. If the argument ID is 0, it stands for the current process, current process group, or the current user, according to CLASS. - Function: int nice (int INCREMENT) Increment the priority of the current process by INCREMENT. The return value is the same as for `setpriority'. Here is an equivalent definition for `nice': int nice (int increment) { int old = getpriority (PRIO_PROCESS, 0); return setpriority (PRIO_PROCESS, 0, old + increment); } Extended Characters ******************* A number of languages use character sets that are larger than the range of values of type `char'. Japanese and Chinese are probably the most familiar examples. The GNU C library includes support for two mechanisms for dealing with extended character sets: multibyte characters and wide characters. This chapter describes how to use these mechanisms, and the functions for converting between them. The behavior of the functions in this chapter is affected by the current locale for character classification--the `LC_CTYPE' category; see *Note Locale Categories::. This choice of locale selects which multibyte code is used, and also controls the meanings and characteristics of wide character codes. Introduction to Extended Characters =================================== You can represent extended characters in either of two ways: * As "multibyte characters" which can be embedded in an ordinary string, an array of `char' objects. Their advantage is that many programs and operating systems can handle occasional multibyte characters scattered among ordinary ASCII characters, without any change. * As "wide characters", which are like ordinary characters except that they occupy more bits. The wide character data type, `wchar_t', has a range large enough to hold extended character codes as well as old-fashioned ASCII codes. An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters. There are a few disadvantages: * Each existing program must be modified and recompiled to make it use wide characters. * Files of wide characters cannot be read by programs that expect ordinary characters. Typically, you use the multibyte character representation as part of the external program interface, such as reading or writing text to files. However, it's usually easier to perform internal manipulations on strings containing extended characters on arrays of `wchar_t' objects, since the uniform representation makes most editing operations easier. If you do use multibyte characters for files and wide characters for internal operations, you need to convert between them when you read and write data. If your system supports extended characters, then it supports them both as multibyte characters and as wide characters. The library includes functions you can use to convert between the two representations. These functions are described in this chapter. Locales and Extended Characters =============================== A computer system can support more than one multibyte character code, and more than one wide character code. The user controls the choice of codes through the current locale for character classification (*note Locales::.). Each locale specifies a particular multibyte character code and a particular wide character code. The choice of locale influences the behavior of the conversion functions in the library. Some locales support neither wide characters nor nontrivial multibyte characters. In these locales, the library conversion functions still work, even though what they do is basically trivial. If you select a new locale for character classification, the internal shift state maintained by these functions can become confused, so it's not a good idea to change the locale while you are in the middle of processing a string. Multibyte Characters ==================== In the ordinary ASCII code, a sequence of characters is a sequence of bytes, and each character is one byte. This is very simple, but allows for only 256 distinct characters. In a "multibyte character code", a sequence of characters is a sequence of bytes, but each character may occupy one or more consecutive bytes of the sequence. There are many different ways of designing a multibyte character code; different systems use different codes. To specify a particular code means designating the "basic" byte sequences--those which represent a single character--and what characters they stand for. A code that a computer can actually use must have a finite number of these basic sequences, and typically none of them is more than a few characters long. These sequences need not all have the same length. In fact, many of them are just one byte long. Because the basic ASCII characters in the range from `0' to `0177' are so important, they stand for themselves in all multibyte character codes. That is to say, a byte whose value is `0' through `0177' is always a character in itself. The characters which are more than one byte must always start with a byte in the range from `0200' through `0377'. The byte value `0' can be used to terminate a string, just as it is often used in a string of ASCII characters. Specifying the basic byte sequences that represent single characters automatically gives meanings to many longer byte sequences, as more than one character. For example, if the two byte sequence `0205 049' stands for the Greek letter alpha, then `0205 049 065' must stand for an alpha followed by an `A' (ASCII code 065), and `0205 049 0205 049' must stand for two alphas in a row. If any byte sequence can have more than one meaning as a sequence of characters, then the multibyte code is ambiguous--and no good. The codes that systems actually use are all unambiguous. In most codes, there are certain sequences of bytes that have no meaning as a character or characters. These are called "invalid". The simplest possible multibyte code is a trivial one: The basic sequences consist of single bytes. This particular code is equivalent to not using multibyte characters at all. It has no invalid sequences. But it can handle only 256 different characters. Here is another possible code which can handle 9376 different characters: The basic sequences consist of * single bytes with values in the range `0' through `0237'. * two-byte sequences, in which both of the bytes have values in the range from `0240' through `0377'. This code or a similar one is used on some systems to represent Japanese characters. The invalid sequences are those which consist of an odd number of consecutive bytes in the range from `0240' through `0377'. Here is another multibyte code which can handle more distinct extended characters--in fact, almost thirty million: The basic sequences consist of * single bytes with values in the range `0' through `0177'. * sequences of up to four bytes in which the first byte is in the range from `0200' through `0237', and the remaining bytes are in the range from `0240' through `0377'. In this code, any sequence that starts with a byte in the range from `0240' through `0377' is invalid. And here is another variant which has the advantage that removing the last byte or bytes from a valid character can never produce another valid character. (This property is convenient when you want to search strings for particular characters.) The basic sequences consist of * single bytes with values in the range `0' through `0177'. * two-byte sequences in which the first byte is in the range from `0200' through `0207', and the second byte is in the range from `0240' through `0377'. * three-byte sequences in which the first byte is in the range from `0210' through `0217', and the other bytes are in the range from `0240' through `0377'. * four-byte sequences in which the first byte is in the range from `0220' through `0227', and the other bytes are in the range from `0240' through `0377'. The list of invalid sequences for this code is long and not worth stating in full; examples of invalid sequences include `0240' and `0220 0300 065'. The number of *possible* multibyte codes is astronomical. But a given computer system will support at most a few different codes. (One of these codes may allow for thousands of different characters.) Another computer system may support a completely different code. The library facilities described in this chapter are helpful because they package up the knowledge of the details of a particular computer system's multibyte code, so your programs need not know them. You can use special standard macros to find out the maximum possible number of bytes in a character in the currently selected multibyte code with `MB_CUR_MAX', and the maximum for *any* multibyte code supported on your computer with `MB_LEN_MAX'. - Macro: int MB_LEN_MAX This is the maximum length of a multibyte character for any supported locale. It is defined in `limits.h'. - Macro: int MB_CUR_MAX This macro expands into a (possibly non-constant) 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'. `MB_CUR_MAX' is defined in `stdlib.h'. Normally, each basic sequence in a particular character code stands for one character, the same character regardless of context. Some multibyte character codes have a concept of "shift state"; certain codes, called "shift sequences", change to a different shift state, and the meaning of some or all basic sequences varies according to the current shift state. In fact, the set of basic sequences might even be different depending on the current shift state. *Note Shift State::, for more information on handling this sort of code. What happens if you try to pass a string containing multibyte characters to a function that doesn't know about them? Normally, such a function treats a string as a sequence of bytes, and interprets certain byte values specially; all other byte values are "ordinary". As long as a multibyte character doesn't contain any of the special byte values, the function should pass it through as if it were several ordinary characters. For example, let's figure out what happens if you use multibyte characters in a file name. The functions such as `open' and `unlink' that operate on file names treat the name as a sequence of byte values, with `/' as the only special value. Any other byte values are copied, or compared, in sequence, and all byte values are treated alike. Thus, you may think of the file name as a sequence of bytes or as a string containing multibyte characters; the same behavior makes sense equally either way, provided no multibyte character contains a `/'. Wide Character Introduction =========================== "Wide characters" are much simpler than multibyte characters. They are simply characters with more than eight bits, so that they have room for more than 256 distinct codes. The wide character data type, `wchar_t', has a range large enough to hold extended character codes as well as old-fashioned ASCII codes. An advantage of wide characters is that each character is a single data object, just like ordinary ASCII characters. Wide characters also have some disadvantages: * A program must be modified and recompiled in order to use wide characters at all. * Files of wide characters cannot be read by programs that expect ordinary characters. Wide character values `0' through `0177' are always identical in meaning to the ASCII character codes. The wide character value zero is often used to terminate a string of wide characters, just as a single byte with value zero often terminates a string of ordinary characters. - Data Type: wchar_t This is the "wide character" type, an integer type whose range is large enough to represent all distinct values in any extended character set in the supported locales. *Note Locales::, for more information about locales. This type is defined in the header file `stddef.h'. If your system supports extended characters, then each extended character has both a wide character code and a corresponding multibyte basic sequence. In this chapter, the term "code" is used to refer to a single extended character object to emphasize the distinction from the `char' data type. Conversion of Extended Strings ============================== The `mbstowcs' function converts a string of multibyte characters to a wide character array. The `wcstombs' function does the reverse. These functions are declared in the header file `stdlib.h'. In most programs, these functions are the only ones you need for conversion between wide strings and multibyte character strings. But they have limitations. If your data is not null-terminated or is not all in core at once, you probably need to use the low-level conversion functions to convert one character at a time. *Note Converting One Char::. - 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, this 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, this 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. Multibyte Character Length ========================== This section describes how to scan a string containing multibyte characters, one character at a time. The difficulty in doing this is to know how many bytes each character contains. Your program can use `mblen' to find this out. - 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 character, 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 nonzero if the multibyte character code in use actually has a shift state. *Note Shift State::. The function `mblen' is declared in `stdlib.h'. Conversion of Extended Characters One by One ============================================ You can convert multibyte characters one at a time to wide characters with the `mbtowc' function. The `wctomb' function does the reverse. These functions are declared in `stdlib.h'. - Function: int mbtowc (wchar_t *RESULT, const char *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 character, 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 `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 `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'. Character-by-Character Conversion Example ========================================= Here is an example that reads multibyte character text from descriptor `input' and writes the corresponding wide characters to descriptor `output'. We need to convert characters one by one for this example because `mbstowcs' is unable to continue past a null character, and cannot cope with an apparently invalid partial character by reading more input. int file_mbstowcs (int input, int output) { char buffer[BUFSIZ + MB_LEN_MAX]; int filled = 0; int eof = 0; while (!eof) { int nread; int 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) { int thislen = mbtowc (outp, inp, filled); /* Stop converting at invalid character; this can mean we have read just the first part of a valid character. */ if (thislen == -1) break; /* Treat null character like any other, but also reset shift state. */ if (thislen == 0) { thislen = 1; mbtowc (NULL, NULL, 0); } /* 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 ("invalid multibyte character"); return 0; } /* If any characters must be carried forward, put them at the beginning of `buffer'. */ if (filled > 0) memcpy (inp, buffer, filled); } } return 1; } Multibyte Codes Using Shift Sequences ===================================== 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 which 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. 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 Extended Characters::.). * 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 Numeric Formatting::.). * Formatting of dates and times (*note Formatting Date and Time::.). * What language to use for output, including error messages. (The C library doesn't yet help you implement this.) * What language to use for user answers to 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. (Eventually, we hope to provide facilities to make this easier.) 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'. 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 Extended Characters::. `LC_MONETARY' This category applies to formatting monetary values; see *Note Numeric Formatting::. `LC_NUMERIC' This category applies to formatting numeric values that are not monetary; see *Note Numeric Formatting::. `LC_TIME' This category applies to formatting date and time values; see *Note Formatting Date and Time::. `LC_MESSAGES' This category applies to selecting the language used in the user interface for message translation. `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. `LANG' If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above. 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 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. If CATEGORY is `LC_ALL', this specifies the locale for all purposes. The other possible values of CATEGORY specify an individual 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'. 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. 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 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 (old_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. 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). Some systems may allow users to create locales, but we don't discuss that here. 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. Numeric Formatting ================== When you want to format a number or a currency amount using the conventions of the current locale, you can use the function `localeconv' to get the data on how to do it. The function `localeconv' is declared in the header file `locale.h'. - 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 shouldn't 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 This is the data type of the value returned by `localeconv'. 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 string is made up of decimal numbers separated by semicolons. Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point). The last number in the string is used over and over for all the remaining groups. If the last integer is `-1', it means that there is no more grouping--or, put another way, any remaining digits form one large group without separators. For example, if `grouping' is `"4;3;2"', 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 `"3"' 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 the 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 found in the appropriate member. `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 found in the appropriate member. `char p_cs_precedes' `char n_cs_precedes' These members are `1' if the `currency_symbol' string should precede the value of a monetary amount, or `0' if the string should follow the value. The `p_cs_precedes' member applies to positive amounts (or zero), and the `n_cs_precedes' member applies to negative amounts. In the standard `C' locale, both 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, but we recommend printing the currency symbol before the amount. That's right for most countries. In other words, treat all nonzero values alike in these members. The POSIX standard says that these two members apply to the `int_curr_symbol' as well as the `currency_symbol'. The ISO C standard seems to imply that they should apply only to the `currency_symbol'--so the `int_curr_symbol' should always precede the amount. We can only guess which of these (if either) matches the usual conventions for printing international currency symbols. Our guess is that they should always precede the amount. If we find out a reliable answer, we will put it here. `char p_sep_by_space' `char n_sep_by_space' These members are `1' if a space should appear between the `currency_symbol' string and the amount, or `0' if no space should appear. The `p_sep_by_space' member applies to positive amounts (or zero), and the `n_sep_by_space' member applies to negative amounts. In the standard `C' locale, both 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 one (print a space). In other words, treat all nonzero values alike in these members. These members apply only to `currency_symbol'. When you use `int_curr_symbol', you never print an additional space, because `int_curr_symbol' itself contains the appropriate separator. The POSIX standard says that these two members apply to the `int_curr_symbol' as well as the `currency_symbol'. But an example in the ISO C standard clearly implies that they should apply only to the `currency_symbol'--that the `int_curr_symbol' contains any appropriate separator, so you should never print an additional space. Based on what we know now, we recommend you ignore these members when printing international currency symbols, and print no extra space. Printing the Sign of an Amount of Money --------------------------------------- These members of the `struct lconv' structure specify how to print the sign (if any) in a monetary value. `char *positive_sign' `char *negative_sign' These are strings used to indicate positive (or zero) and negative (respectively) monetary quantities. 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' These members have values that are small integers indicating 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. It is not clear whether you should let these members apply to the international currency format or not. POSIX says you should, but intuition plus the examples in the ISO C standard suggest you should not. We hope that someone who knows well the conventions for formatting monetary quantities will tell us what we should recommend. 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 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 a 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 `sigsetjmp') 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. 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 possible 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 is 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 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 described in *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'. 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 how 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 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 *ACTION, struct sigaction *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. Since the signal is blocked, it will receive its 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. */ 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 recieved may not equal the number of child processes 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 accessings 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 at 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. The description of a function in this manual always mentions 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. The relocating allocation functions (*note Relocating Allocator::.) are certainly not safe to use in a signal handler. * 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 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 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 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 SIGNUM 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 *SET, sigset_t *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' 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). 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 deliver 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: struct sigaltstack 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: `SA_DISABLE' This tells the system that it should not use the signal stack. `SA_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 struct sigaltstack *STACK, struct sigaltstack *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. Process Startup and Termination ******************************* "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. This chapter explains what your program should do to handle the startup of a process, to terminate its process, and to receive information (arguments and the environment) from the parent process. 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"'. 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' to do the parsing. 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 process'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 Parsing Options::.) makes 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'; see *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 implementation of `getopt' in the GNU C library normally makes 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. It does 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 Options ----------------------- 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 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 `\0'. 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 -------------------- 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 **ARGV, const char *SHORTOPTS, 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. Example of Parsing Long Options ------------------------------- #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", 0, &verbose_flag, 1}, {"brief", 0, &verbose_flag, 0}, /* These options don't set a flag. We distinguish them by their indices. */ {"add", 1, 0, 0}, {"append", 0, 0, 0}, {"delete", 1, 0, 0}, {"create", 0, 0, 0}, {"file", 1, 0, 0}, {0, 0, 0, 0} }; /* `getopt_long' stores the option index here. */ int option_index = 0; c = getopt_long (argc, argv, "abc:d:", 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 '?': /* `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 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 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 }; const char *mount_opts[] = { [RO_OPTION] = "ro", [RW_OPTION] = "rw", [READ_SIZE_OPTION] = "rsize", [WRITE_SIZE_OPTION] = "wsize" }; 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'. - 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 (const 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 GNU library provides this function for compatibility with SVID; it may not be available in other systems. 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; see *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 process; 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_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_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. `_POSIX_OPTION_ORDER' If this environment variable is defined, it suppresses the usual reordering of command line arguments by `getopt'. *Note Argument Syntax::. 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 the program calls `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 terminates the process with status 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. 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'. When a process terminates for any reason--either by an explicit termination call, or termination 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; *Note I/O on Streams::. * The low-order 8 bits of the return status code are saved to be reported back to the parent process via `wait' or `waitpid'; see *Note Process Completion::. * 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. 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 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 exits 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. 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. The return value is normally the process ID of the child process whose status is reported. If the `WNOHANG' option was specified and no child process is waiting to be noticed, the value is zero. 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) - 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; int status; while (1) { pid = waitpid (WAIT_ANY, &status, WNOHANG); if (pid < 0) { perror ("waitpid"); break; } if (pid == 0) break; notice_termination (pid, status); } } 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 the result of the `WTERMSIG' macro. `int w_coredump' The value of this member is the same as the result of the `WCOREDUMP' macro. `int w_retcode' The value of this member is the same as the result of the `WEXITSTATUS' macro. `int w_stopsig' The value of this member is the same as the result 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 been 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. 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. - 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. 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::.. `network' 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.1'. 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 statii 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 `network' 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 ultimative. 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 described how 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.1 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.1' would be on a Solaris 2 system `nss_files.so.1'. 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 itself. This is *not* true in the moment, though. But the different 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 to duplicate this information. The answer is that we want to keep the possibility 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 do not tell this 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 *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. 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 explain 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 adata 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 1 and this manifests in the version number of the shared library object of the NSS modules: they have the extension `.1'. If the interface ever changes in an incompatible way, this number will be increased--hopefully this will never be necessary. 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.1 -Wl,-soname,libnss_NAME.so.1 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 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 program (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 more much we can tell 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)' 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. As explained above this function could also have an additional last argument. This depends on the database used; it happens only for `host' and `network'. The function shall return `NSS_STATUS_SUCCESS' as long as their 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)' 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 honour the STAYOPEN flag set by the `setDBent' function whenever this makes sense. Again, this function takes an additional last argument for the `host' and `network' 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 also belongs to one or more groups, and has one "default group". 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 a single user ID and a group ID which determine the privileges of the process. These are collectively called the "persona" of the process, because they determine "who it is" for purposes of access control. These IDs are also called the "effective user ID" and "effective group ID" of the process. Your login shell starts out with a persona which consists of your user ID and your default group ID. 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::. In addition, a user can belong to multiple groups, so the persona includes "supplementary group IDs" that also contribute to access permission. For details on how a process's effective user IDs and group IDs affect its permission to access files, see *Note Access Permission::. The user ID of a process also controls permissions for sending signals using the `kill' function. *Note Signaling Another Process::. 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 set, executing that file automatically changes the effective user ID to the user that owns the file. Likewise, executing a file whose setgid bit is set changes the effective group ID to the group of the file. *Note Executing a File::. Creating a file that changes to a particular user or group ID thus requires full access to that user or group ID. *Note File Attributes::, for a more general discussion of file modes and accessibility. A process can always change its effective user (or group) ID back to its real ID. Programs do this so as to turn off their special privileges when they are not needed, which makes for more robustness. 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 setuid (uid_t NEWUID) This function sets both the real and effective user ID of the process to NEWUID, provided that the process has appropriate privileges. If the process is not privileged, then NEWUID must either be equal to the real user ID or the saved user ID (if the system supports the `_POSIX_SAVED_IDS' feature). In this case, `setuid' sets only the effective user ID and not the real user ID. The `setuid' 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 does not have the appropriate privileges; you do not have permission to change to the specified ID. - 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 saved 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 saved 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 setgid (gid_t NEWGID) This function sets both the real and effective group ID of the process to NEWGID, provided that the process has appropriate privileges. If the process is not privileged, then NEWGID must either be equal to the real group ID or the saved group ID. In this case, `setgid' sets only the effective group ID and not the real group ID. The return values and error conditions for `setgid' are the same as those for `setuid'. - Function: int setregid (gid_t RGID, fid_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 saved 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 saved 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'. The GNU 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 GID) The `initgroups' function effectively calls `setgroups' to set the process's supplementary group IDs to be the normal default for the user name USER. The group ID GID is also included. 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 saved user ID feature, you can accomplish this with `setuid'. 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 setuid (user_user_id); and turn it on with setuid (game_user_id); Throughout this process, the real user ID remains `jdoe' and the saved user ID remains `games', so the program can always set its effective user ID to either one. On other systems that don't support the saved user ID feature, 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 set-user-ID 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 set-user-ID 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 saved IDs feature if it is supported, and otherwise uses `setreuid' to swap the effective and real user IDs. #include #include #include #include /* Save 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 = setuid (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 = setuid (ruid); #else status = setreuid (euid, ruid); #endif if (status < 0) { fprintf (stderr, "Couldn't set uid.\n"); exit (status); } } /* Main program. */ int main (void) { /* Save 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 original effective 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. * Be cautious about using the `system' and `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. * 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::.). 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 it is already removed in 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; *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::. User Database ============= This section describes all about 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 is returns information about the user whose user ID is UID. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by RESULT_BUF is filled with the information. 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 the return value is `0' the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). In case the return value is non null there is no user in the data base with user ID UID or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable 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 value 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. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by RESULT_BUF is filled with the information. 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 the return value is `0' the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). In case the return value is non null there is no user in the data base with user name NAME or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to `ERANGE'. 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. This stream must correspond to a file in the same format as the standard password database file. This function comes from System V. - 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. This stream must correspond to a file in the same format as the standard password database file. If the function returns null RESULT points to the structure with the wanted data (normally this is in RESULT_BUF). If errors occurred the return value is non-null 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 in case no further entry is 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'. But in contrast to the `getpwent' function this function is reentrant since the result is placed in the user supplied structure pointed to by RESULT_BUF. Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length BUFLEN starting at BUFFER. 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. - 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 all about 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 is returns information about the group whose group ID is GID. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by RESULT_BUF is filled with the information. 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 the return value is `0' the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). If the return value is non-zero there is no group in the data base with group ID GID or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable 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. But the result is not placed in a static buffer. Instead the user supplied structure pointed to by RESULT_BUF is filled with the information. 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 the return value is `0' the pointer returned in RESULT points to the record which contains the wanted data (i.e., RESULT contains the value RESULT_BUF). If the return value is non-zero there is no group in the data base with group name NAME or the buffer BUFFER is too small to contain all the needed information. In the later case the global ERRNO variable is set to `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 rewritten 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 rewritten 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'. But in contrast to the `getgrent' function this function is reentrant since the result is placed in the user supplied structure pointed to by RESULT_BUF. Additional data, normally the strings pointed to by the elements of the result structure, are placed in the additional buffer or length BUFLEN starting at BUFFER. 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. - Function: void endgrent (void) This function closes the internal stream used by `getgrent' or `getgrent_r'. Netgroup Database ================= Netgroup Data ------------- Sometimes it is useful group users according to other criterias like the ones used in the *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 to group hosts, users, and domain freely, giving them individual names. 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 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' iterate over all entries in the netgroup with name NETGROUP. When the call is successful (i.e., when a netgroup with this name exist) 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, 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 unless no 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 free 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 excepted 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. 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 System Information ****************** This chapter describes functions that return information about the particular machine that is in use--the type of hardware, the type of software, and the individual machine's name. Host Identification =================== This section explains how to identify the particular machine that your program is running on. The identification of a machine consists of its Internet host name and Internet address; see *Note Internet Namespace::. The host name should always be a fully qualified domain name, like `crispy-wheats-n-chicken.ai.mit.edu', not a simple name like just `crispy-wheats-n-chicken'. Prototypes for these functions appear in `unistd.h'. The shell commands `hostname' and `hostid' work by calling them. - Function: int gethostname (char *NAME, size_t SIZE) This function returns the name of the host machine in the array NAME. The SIZE argument specifies the size of this array, in bytes. 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 name of the host machine to NAME, a string with length LENGTH. Only privileged processes are allowed to do this. Usually it happens just once, at system boot time. 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: 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 address of that machine, converted to a `long int'. However, some systems it is a meaningless but unique number which is hard-coded for each machine. - Function: int sethostid (long int ID) The `sethostid' function sets the "host ID" of the host machine to ID. Only privileged processes are allowed to do this. Usually it happens just once, at system boot time. 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. `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. Hardware/Software 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'. - 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 nodename[]' This is the network name of this particular computer. In the GNU library, the value is the same as that returned by `gethostname'; see *Note Host Identification::. `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"' - 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. 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 1990 POSIX.1 standard, the value is the integer `199009L'. `_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 Basic CPU Time::.. `_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. 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_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 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. `_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 `255'. `_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'. 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'. 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_SCALE_MAX The largest value of `scale' 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_STRING_MAX The largest number of characters in one string constant 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 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. 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; } 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 should be 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 written. 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 a 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 a just 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' 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. So you can't use `assert' or `assert_perror' to print the error messages for these eventualities. 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, ...) { ... } outlines a definition of 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 and you do not really need to call it. 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 the 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 tell it how many arguments it has, and what kind. 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 write anything special when you call a variadic function. Just write the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual. But you should prepare by declaring the function with a prototype, and you must 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 should get 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. 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; *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' just 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 Wide 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", 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 actually 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. (Currently GCC does not support `long double' as a distinct data type, so 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 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 minimum 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. `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::. `double acosh (double X)' `math.h' (BSD): *Note Hyperbolic Functions::. `double acos (double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `int adjtime (const struct timeval *DELTA, struct timeval *OLDDELTA)' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `AF_FILE' `sys/socket.h' (GNU): *Note Address Formats::. `AF_INET6' `sys/socket.h' (IPv6 Basic API): *Note Address Formats::. `AF_INET' `sys/socket.h' (BSD): *Note Address Formats::. `AF_UNIX' `sys/socket.h' (BSD): *Note Address Formats::. `AF_UNSPEC' `sys/socket.h' (BSD): *Note Address Formats::. `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::. `tcflag_t ALTWERASE' `termios.h' (BSD): *Note Local Modes::. `int ARG_MAX' `limits.h' (POSIX.1): *Note General Limits::. `char * asctime (const struct tm *BROKENTIME)' `time.h' (ISO): *Note Formatting Date and Time::. `double asinh (double X)' `math.h' (BSD): *Note Hyperbolic Functions::. `double asin (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 atan2 (double Y, double X)' `math.h' (ISO): *Note Inverse Trig Functions::. `double atanh (double X)' `math.h' (BSD): *Note Hyperbolic Functions::. `double atan (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::. `B0' `termios.h' (POSIX.1): *Note Line Speed::. `B110' `termios.h' (POSIX.1): *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::. `B2400' `termios.h' (POSIX.1): *Note Line Speed::. `B300' `termios.h' (POSIX.1): *Note Line Speed::. `B38400' `termios.h' (POSIX.1): *Note Line Speed::. `B4800' `termios.h' (POSIX.1): *Note Line Speed::. `B50' `termios.h' (POSIX.1): *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 BC_BASE_MAX' `limits.h' (POSIX.2): *Note Utility Limits::. `int BC_DIM_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 (void *FROM, const 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::. `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::. `int BUFSIZ' `stdio.h' (ISO): *Note Controlling Buffering::. `void * bzero (void *BLOCK, size_t SIZE)' `string.h' (BSD): *Note Copying and Concatenation::. `double cabs (struct { double real, imag; } Z)' `math.h' (BSD): *Note Absolute Value::. `void * calloc (size_t COUNT, size_t ELTSIZE)' `malloc.h', `stdlib.h' (ISO): *Note Allocating Cleared Space::. `double cbrt (double X)' `math.h' (BSD): *Note Exponents and Logarithms::. `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 and Remainders::. `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::. `void clearerr (FILE *STREAM)' `stdio.h' (ISO): *Note EOF and Errors::. `int CLK_TCK' `time.h' (POSIX.1): *Note Basic CPU Time::. `tcflag_t CLOCAL' `termios.h' (POSIX.1): *Note Control Modes::. `clock_t clock (void)' `time.h' (ISO): *Note Basic CPU Time::. `int CLOCKS_PER_SEC' `time.h' (ISO): *Note Basic CPU Time::. `clock_t' `time.h' (ISO): *Note Basic CPU Time::. `int closedir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Reading/Closing Directory::. `int close (int FILEDES)' `unistd.h' (POSIX.1): *Note Opening and Closing Files::. `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::. `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 VALUE, double SIGN)' `math.h' (BSD): *Note Normalization Functions::. `double cosh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `double cos (double X)' `math.h' (ISO): *Note Trig Functions::. `tcflag_t CREAD' `termios.h' (POSIX.1): *Note Control Modes::. `int creat (const char *FILENAME, mode_t MODE)' `fcntl.h' (POSIX.1): *Note Opening and Closing Files::. `tcflag_t CRTS_IFLOW' `termios.h' (BSD): *Note Control Modes::. `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::. `tcflag_t CSIZE' `termios.h' (POSIX.1): *Note Control Modes::. `_CS_PATH' `unistd.h' (POSIX.2): *Note String Parameters::. `tcflag_t CSTOPB' `termios.h' (POSIX.1): *Note Control Modes::. `char * ctermid (char *STRING)' `stdio.h' (POSIX.1): *Note Identifying the Terminal::. `char * ctime (const time_t *TIME)' `time.h' (ISO): *Note Formatting Date and 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_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MAX_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `DBL_MAX' `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::. `DBL_MIN' `float.h' (ISO): *Note Floating Point Parameters::. `dev_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `double difftime (time_t TIME1, time_t TIME0)' `time.h' (ISO): *Note Simple Calendar Time::. `DIR' `dirent.h' (POSIX.1): *Note Opening a Directory::. `div_t div (int NUMERATOR, int DENOMINATOR)' `stdlib.h' (ISO): *Note Integer Division::. `div_t' `stdlib.h' (ISO): *Note Integer Division::. `double drem (double NUMERATOR, double DENOMINATOR)' `math.h' (BSD): *Note Rounding and Remainders::. `int dup2 (int OLD, int NEW)' `unistd.h' (POSIX.1): *Note Duplicating Descriptors::. `int dup (int OLD)' `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 EBADFD' `errno.h' (Linux???: File descriptor in bad state): *Note Error Codes::. `int EBADF' `errno.h' (POSIX.1: Bad file descriptor): *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 ECHILD' `errno.h' (POSIX.1: No child processes): *Note Error Codes::. `tcflag_t ECHOCTL' `termios.h' (BSD): *Note Local Modes::. `tcflag_t ECHOE' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHO' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ECHOKE' `termios.h' (BSD): *Note Local Modes::. `tcflag_t ECHOK' `termios.h' (POSIX.1): *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::. `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 ED' `errno.h' (GNU: ?): *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: Disc 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' (Linux???: Level 2 halted): *Note Error Codes::. `int EL2NSYNC' `errno.h' (Linux???: Level 2 not synchronized): *Note Error Codes::. `int EL3HLT' `errno.h' (Linux???: Level 3 halted): *Note Error Codes::. `int EL3RST' `errno.h' (Linux???: 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::. `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 endgrent (void)' `grp.h' (SVID, BSD): *Note Scanning All Groups::. `void endhostent ()' `netdb.h' (BSD): *Note Host Names::. `void endnetent (void)' `netdb.h' (BSD): *Note Networks Database::. `void endnetgrent (void)' `netdb.h' (netdb.h): *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::. `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: Operation not supported by 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 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::. `int ENXIO' `errno.h' (POSIX.1: Device not configured): *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::. `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 EREMOTEIO' `errno.h' (Linux???: Remote I/O error): *Note Error Codes::. `int EREMOTE' `errno.h' (BSD: Object is remote): *Note Error Codes::. `int ERESTART' `errno.h' (Linux???: Interrupted system call should be restarted): *Note Error Codes::. `int EROFS' `errno.h' (POSIX.1: Read-only file system): *Note Error Codes::. `int ERPCMISMATCH' `errno.h' (BSD: RPC version wrong): *Note Error Codes::. `volatile int errno' `errno.h' (ISO): *Note Checking for Errors::. `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 ETIMEDOUT' `errno.h' (BSD: Connection timed out): *Note Error Codes::. `int ETIME' `errno.h' (XOPEN: Timer expired): *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 execle (const char *FILENAME, const char *ARG0, char *const ENV[], ...)' `unistd.h' (POSIX.1): *Note Executing a File::. `int execl (const char *FILENAME, const char *ARG0, ...)' `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 execve (const char *FILENAME, char *const ARGV[], char *const ENV[])' `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 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::. `int EXIT_FAILURE' `stdlib.h' (ISO): *Note Exit Status::. `void _exit (int STATUS)' `unistd.h' (POSIX.1): *Note Termination Internals::. `void exit (int STATUS)' `stdlib.h' (ISO): *Note Normal Termination::. `int EXIT_SUCCESS' `stdlib.h' (ISO): *Note Exit Status::. `double exp (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double expm1 (double X)' `math.h' (BSD): *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::. `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 fcloseall (void)' `stdio.h' (GNU): *Note Closing Streams::. `int fclose (FILE *STREAM)' `stdio.h' (ISO): *Note Closing Streams::. `int fcntl (int FILEDES, int COMMAND, ...)' `fcntl.h' (POSIX.1): *Note Control Operations::. `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::. `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 feof (FILE *STREAM)' `stdio.h' (ISO): *Note EOF and Errors::. `int ferror (FILE *STREAM)' `stdio.h' (ISO): *Note EOF and Errors::. `int fflush (FILE *STREAM)' `stdio.h' (ISO): *Note Flushing Buffers::. `int fgetc (FILE *STREAM)' `stdio.h' (ISO): *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::. `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::. `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 finite (double X)' `math.h' (BSD): *Note Predicates on Floats::. `double floor (double X)' `math.h' (ISO): *Note Rounding and Remainders::. `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_10_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MAX_EXP' `float.h' (ISO): *Note Floating Point Parameters::. `FLT_MAX' `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_MIN' `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::. `tcflag_t FLUSHO' `termios.h' (BSD): *Note Local Modes::. `FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE)' `stdio.h' (GNU): *Note String Streams::. `double fmod (double NUMERATOR, double DENOMINATOR)' `math.h' (ISO): *Note Rounding and Remainders::. `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_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 * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS)' `stdio.h' (GNU): *Note Streams and Cookies::. `FILE * fopen (const char *FILENAME, const char *OPENTYPE)' `stdio.h' (ISO): *Note Opening Streams::. `int FOPEN_MAX' `stdio.h' (ISO): *Note Opening Streams::. `pid_t fork (void)' `unistd.h' (POSIX.1): *Note Creating a Process::. `long int fpathconf (int FILEDES, int PARAMETER)' `unistd.h' (POSIX.1): *Note Pathconf::. `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::. `FPE_SUBRNG_TRAP' `signal.h' (BSD): *Note Program Error Signals::. `fpos_t' `stdio.h' (ISO): *Note Portable Positioning::. `int fprintf (FILE *STREAM, const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Output Functions::. `int fputc (int C, FILE *STREAM)' `stdio.h' (ISO): *Note Simple Output::. `int fputs (const char *S, FILE *STREAM)' `stdio.h' (ISO): *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::. `__free_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `void free (void *PTR)' `malloc.h', `stdlib.h' (ISO): *Note Freeing after Malloc::. `FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM)' `stdio.h' (ISO): *Note Opening Streams::. `double frexp (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 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 F_SETOWN' `fcntl.h' (BSD): *Note Interrupt Input::. `int fsetpos (FILE *STREAM, const fpos_t POSITION)' `stdio.h' (ISO): *Note Portable Positioning::. `int fstat (int FILEDES, struct stat *BUF)' `sys/stat.h' (POSIX.1): *Note Reading Attributes::. `long int ftell (FILE *STREAM)' `stdio.h' (ISO): *Note File Positioning::. `F_UNLCK' `fcntl.h' (POSIX.1): *Note File Locks::. `size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM)' `stdio.h' (ISO): *Note Block Input/Output::. `F_WRLCK' `fcntl.h' (POSIX.1): *Note File Locks::. `int getchar (void)' `stdio.h' (ISO): *Note Character Input::. `int getc (FILE *STREAM)' `stdio.h' (ISO): *Note Character Input::. `char * getcwd (char *BUFFER, size_t SIZE)' `unistd.h' (POSIX.1): *Note Working Directory::. `ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM)' `stdio.h' (GNU): *Note Line Input::. `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::. `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 getgroups (int COUNT, gid_t *GROUPS)' `unistd.h' (POSIX.1): *Note Reading Persona::. `struct hostent * gethostbyaddr (const char *ADDR, int LENGTH, int FORMAT)' `netdb.h' (BSD): *Note Host Names::. `struct hostent * gethostbyname2 (const char *NAME, int AF)' `netdb.h' (IPv6 Basic API): *Note Host Names::. `struct hostent * gethostbyname (const char *NAME)' `netdb.h' (BSD): *Note Host Names::. `struct hostent * gethostent ()' `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::. `char * getlogin (void)' `unistd.h' (POSIX.1): *Note Who Logged In::. `struct netent * getnetbyaddr (long 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' (netdb.h): *Note Lookup Netgroup::. `int getnetgrent_r (char **HOSTP, char **USERP, char **DOMAINP, char *BUFFER, int BUFLEN)' `netdb.h' (netdb.h): *Note Lookup Netgroup::. `int getopt (int ARGC, char **ARGV, const char *OPTIONS)' `unistd.h' (POSIX.2): *Note Parsing Options::. `int getopt_long (int ARGC, char **ARGV, const char *SHORTOPTS, struct option *LONGOPTS, int *INDEXPTR)' `getopt.h' (GNU): *Note Long Options::. `int getpeername (int SOCKET, struct sockaddr *ADDR, size_t *LENGTH-PTR)' `sys/socket.h' (BSD): *Note Who is Connected::. `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::. `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): *Note Priority::. `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::. `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 getrusage (int PROCESSES, struct rusage *RUSAGE)' `sys/resource.h' (BSD): *Note Resource Usage::. `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::. `char * gets (char *S)' `stdio.h' (ISO): *Note Line Input::. `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::. `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::. `char * getwd (char *BUFFER)' `unistd.h' (BSD): *Note Working Directory::. `int getw (FILE *STREAM)' `stdio.h' (SVID): *Note Character Input::. `gid_t' `sys/types.h' (POSIX.1): *Note Reading Persona::. `GLOB_ABORTED' `glob.h' (POSIX.2): *Note Calling Glob::. `GLOB_APPEND' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_DOOFFS' `glob.h' (POSIX.2): *Note Flags for Globbing::. `GLOB_ERR' `glob.h' (POSIX.2): *Note Flags for Globbing::. `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::. `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_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_t' `glob.h' (POSIX.2): *Note Calling Glob::. `struct tm * gmtime (const time_t *TIME)' `time.h' (ISO): *Note Broken-down Time::. `_GNU_SOURCE' (GNU): *Note Feature Test Macros::. `int gsignal (int SIGNUM)' `signal.h' (SVID): *Note Signaling Yourself::. `HOST_NOT_FOUND' `netdb.h' (BSD): *Note Host Names::. `unsigned long int htonl (unsigned long int HOSTLONG)' `netinet/in.h' (BSD): *Note Byte Order::. `unsigned short int htons (unsigned short int HOSTSHORT)' `netinet/in.h' (BSD): *Note Byte Order::. `float HUGE_VALf' `math.h' (GNU): *Note Domain and Range Errors::. `double HUGE_VAL' `math.h' (ISO): *Note Domain and Range Errors::. `long double HUGE_VALl' `math.h' (GNU): *Note Domain and Range Errors::. `tcflag_t HUPCL' `termios.h' (POSIX.1): *Note Control Modes::. `double hypot (double X, double Y)' `math.h' (BSD): *Note Exponents and Logarithms::. `tcflag_t ICANON' `termios.h' (POSIX.1): *Note Local Modes::. `tcflag_t ICRNL' `termios.h' (POSIX.1): *Note Input Modes::. `tcflag_t IEXTEN' `termios.h' (POSIX.1): *Note Local Modes::. `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::. `tcflag_t IMAXBEL' `termios.h' (BSD): *Note Input Modes::. `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::. `unsigned int INADDR_ANY' `netinet/in.h' (BSD): *Note Host Address Data Type::. `unsigned int INADDR_BROADCAST' `netinet/in.h' (BSD): *Note Host Address Data Type::. `unsigned int INADDR_LOOPBACK' `netinet/in.h' (BSD): *Note Host Address Data Type::. `unsigned int INADDR_NONE' `netinet/in.h' (BSD): *Note Host Address Data Type::. `char * index (const char *STRING, int C)' `string.h' (BSD): *Note Search Functions::. `unsigned long int 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::. `int inet_lnaof (struct in_addr ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `struct in_addr inet_makeaddr (int NET, int LOCAL)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `int inet_netof (struct in_addr ADDR)' `arpa/inet.h' (BSD): *Note Host Address Functions::. `unsigned long int 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::. `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::. `double infnan (int ERROR)' `math.h' (BSD): *Note Predicates on Floats::. `int initgroups (const char *USER, gid_t GID)' `grp.h' (BSD): *Note Setting Groups::. `void * initstate (unsigned int SEED, void *STATE, size_t SIZE)' `stdlib.h' (BSD): *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' (netdb.h): *Note Netgroup Membership::. `ino_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `tcflag_t INPCK' `termios.h' (POSIX.1): *Note Input Modes::. `int RLIM_INFINITY' `sys/resource.h' (BSD): *Note Limits on Resources::. `INT_MAX' `limits.h' (ISO): *Note Range of Type::. `INT_MIN' `limits.h' (ISO): *Note Range of Type::. `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 isgraph (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `tcflag_t ISIG' `termios.h' (POSIX.1): *Note Local Modes::. `int isinf (double X)' `math.h' (BSD): *Note Predicates on Floats::. `int islower (int C)' `ctype.h' (ISO): *Note Classification of Characters::. `int isnan (double X)' `math.h' (BSD): *Note Predicates on Floats::. `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 isupper (int C)' `ctype.h' (ISO): *Note Classification of 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::. `jmp_buf' `setjmp.h' (ISO): *Note Non-Local Details::. `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::. `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::. `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::. `ldiv_t ldiv (long int NUMERATOR, long int DENOMINATOR)' `stdlib.h' (ISO): *Note Integer Division::. `ldiv_t' `stdlib.h' (ISO): *Note Integer Division::. `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 listen (int SOCKET, unsigned int N)' `sys/socket.h' (BSD): *Note Listening::. `struct lconv * localeconv (void)' `locale.h' (ISO): *Note Numeric Formatting::. `struct tm * localtime (const time_t *TIME)' `time.h' (ISO): *Note Broken-down Time::. `double log10 (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `double log1p (double X)' `math.h' (BSD): *Note Exponents and Logarithms::. `double logb (double X)' `math.h' (BSD): *Note Normalization Functions::. `double log (double X)' `math.h' (ISO): *Note Exponents and Logarithms::. `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::. `off_t lseek (int FILEDES, off_t OFFSET, int WHENCE)' `unistd.h' (POSIX.1): *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 L_tmpnam' `stdio.h' (ISO): *Note Temporary Files::. `L_XTND' `sys/file.h' (BSD): *Note File Positioning::. `struct mallinfo mallinfo (void)' `malloc.h' (SVID): *Note Statistics of Malloc::. `__malloc_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `void * malloc (size_t SIZE)' `malloc.h', `stdlib.h' (ISO): *Note Basic Allocation::. `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 MB_CUR_MAX' `stdlib.h' (ISO): *Note Multibyte Char Intro::. `int mblen (const char *STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Length of Char::. `int MB_LEN_MAX' `limits.h' (ISO): *Note Multibyte Char Intro::. `size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Wide String Conversion::. `int mbtowc (wchar_t *RESULT, const char *STRING, size_t SIZE)' `stdlib.h' (ISO): *Note Converting One Char::. `int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))' `malloc.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::. `void * memccpy (void *TO, const void *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 *TO, const void *FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `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 * 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::. `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)' `unistd.h' (BSD): *Note Temporary Files::. `char * mktemp (char *TEMPLATE)' `unistd.h' (Unix): *Note Temporary Files::. `time_t mktime (struct tm *BROKENTIME)' `time.h' (ISO): *Note Broken-down Time::. `mode_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `double modf (double VALUE, double *INTEGER-PART)' `math.h' (ISO): *Note Rounding and Remainders::. `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 NAME_MAX' `limits.h' (POSIX.1): *Note Limits for Files::. `double NAN' `math.h' (GNU): *Note Not a Number::. `int NCCS' `termios.h' (POSIX.1): *Note Mode Data Types::. `int NGROUPS_MAX' `limits.h' (POSIX.1): *Note General Limits::. `int nice (int INCREMENT)' `dunno.h' (dunno.h): *Note Priority::. `nlink_t' `sys/types.h' (POSIX.1): *Note Attribute Meanings::. `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::. `int NSIG' `signal.h' (BSD): *Note Standard Signals::. `unsigned long int ntohl (unsigned long int NETLONG)' `netinet/in.h' (BSD): *Note Byte Order::. `unsigned short int ntohs (unsigned short int NETSHORT)' `netinet/in.h' (BSD): *Note Byte Order::. `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_fast (struct obstack *OBSTACK-PTR, char C)' `obstack.h' (GNU): *Note Extra Fast Growing::. `void obstack_1grow (struct obstack *OBSTACK-PTR, char C)' `obstack.h' (GNU): *Note Growing Objects::. `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::. `void * obstack_base (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Status of an Obstack::. `void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)' `obstack.h' (GNU): *Note Extra Fast Growing::. `void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)' `obstack.h' (GNU): *Note Growing Objects::. `int obstack_chunk_size (struct obstack *OBSTACK-PTR)' `obstack.h' (GNU): *Note Obstack Chunks::. `void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' `obstack.h' (GNU): *Note Allocation in an Obstack::. `void * obstack_copy (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_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE)' `obstack.h' (GNU): *Note Growing Objects::. `void obstack_grow (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_fast (struct obstack *OBSTACK-PTR, int DATA)' `obstack.h' (GNU): *Note Extra Fast Growing::. `void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA)' `obstack.h' (GNU): *Note Growing Objects::. `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_fast (struct obstack *OBSTACK-PTR, void *DATA)' `obstack.h' (GNU): *Note Extra Fast Growing::. `void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA)' `obstack.h' (GNU): *Note Growing Objects::. `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::. `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::. `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::. `DIR * opendir (const char *DIRNAME)' `dirent.h' (POSIX.1): *Note Opening a Directory::. `int open (const char *FILENAME, int FLAGS[, mode_t MODE])' `fcntl.h' (POSIX.1): *Note Opening and Closing Files::. `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::. `tcflag_t OPOST' `termios.h' (POSIX.1): *Note Output Modes::. `char * optarg' `unistd.h' (POSIX.2): *Note Parsing Options::. `int opterr' `unistd.h' (POSIX.2): *Note Parsing Options::. `int optind' `unistd.h' (POSIX.2): *Note Parsing Options::. `int optopt' `unistd.h' (POSIX.2): *Note Parsing Options::. `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_DOUBLE' `printf.h' (GNU): *Note Parsing a Template String::. `PA_FLAG_LONG' `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_CHOWN_RESTRICTED' `unistd.h' (POSIX.1): *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_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 File Namespace Details::. `int PF_INET' `sys/socket.h' (BSD): *Note Internet Namespace::. `int PF_UNIX' `sys/socket.h' (BSD): *Note File Namespace Details::. `pid_t' `sys/types.h' (POSIX.1): *Note Process Identification::. `int PIPE_BUF' `limits.h' (POSIX.1): *Note Limits for Files::. `int pipe (int FILEDES[2])' `unistd.h' (POSIX.1): *Note Creating a Pipe::. `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_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::. `_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::. `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::. `printf_arginfo_function' `printf.h' (GNU): *Note Defining the Output Handler::. `printf_function' `printf.h' (GNU): *Note Defining the Output Handler::. `int printf (const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Output Functions::. `PRIO_MAX' `sys/resource.h' (BSD): *Note Priority::. `PRIO_MIN' `sys/resource.h' (BSD): *Note Priority::. `PRIO_PGRP' `sys/resource.h' (BSD): *Note Priority::. `PRIO_PROCESS' `sys/resource.h' (BSD): *Note Priority::. `PRIO_USER' `sys/resource.h' (BSD): *Note Priority::. `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::. `int putchar (int C)' `stdio.h' (ISO): *Note Simple Output::. `int putc (int C, FILE *STREAM)' `stdio.h' (ISO): *Note Simple Output::. `int putenv (const 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::. `int putw (int W, FILE *STREAM)' `stdio.h' (SVID): *Note Simple Output::. `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::. `void r_alloc_free (void **HANDLEPTR)' `malloc.h' (GNU): *Note Using Relocator::. `void * r_alloc (void **HANDLEPTR, size_t SIZE)' `malloc.h' (GNU): *Note Using Relocator::. `int rand ()' `stdlib.h' (ISO): *Note ISO Random::. `int RAND_MAX' `stdlib.h' (ISO): *Note ISO Random::. `long int random ()' `stdlib.h' (BSD): *Note BSD Random::. `struct dirent * readdir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Reading/Closing Directory::. `int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY, struct dirent **RESULT)' `dirent.h' (GNU): *Note Reading/Closing Directory::. `ssize_t read (int FILEDES, void *BUFFER, size_t SIZE)' `unistd.h' (POSIX.1): *Note I/O Primitives::. `int readlink (const char *FILENAME, char *BUFFER, size_t SIZE)' `unistd.h' (BSD): *Note Symbolic Links::. `__realloc_hook' `malloc.h' (GNU): *Note Hooks for Malloc::. `void * realloc (void *PTR, size_t NEWSIZE)' `malloc.h', `stdlib.h' (ISO): *Note Changing Block Size::. `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 recv (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)' `sys/socket.h' (BSD): *Note Receiving Data::. `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::. `REG_EXTENDED' `regex.h' (POSIX.2): *Note Flags for POSIX Regexps::. `regex_t' `regex.h' (POSIX.2): *Note POSIX Regexp Compilation::. `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::. `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 rewinddir (DIR *DIRSTREAM)' `dirent.h' (POSIX.1): *Note Random Access Directory::. `void rewind (FILE *STREAM)' `stdio.h' (ISO): *Note File Positioning::. `char * rindex (const char *STRING, int C)' `string.h' (BSD): *Note Search Functions::. `double rint (double X)' `math.h' (BSD): *Note Rounding and Remainders::. `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::. `void * r_re_alloc (void **HANDLEPTR, size_t SIZE)' `malloc.h' (GNU): *Note Using Relocator::. `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::. `_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::. `double scalb (double VALUE, int EXPONENT)' `math.h' (BSD): *Note Normalization Functions::. `int scanf (const char *TEMPLATE, ...)' `stdio.h' (ISO): *Note Formatted Input Functions::. `_SC_ARG_MAX' `unistd.h' (POSIX.1): *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_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_EQUIV_CLASS_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_EXPR_NEST_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `SCHAR_MAX' `limits.h' (ISO): *Note Range of Type::. `SCHAR_MIN' `limits.h' (ISO): *Note Range of Type::. `_SC_JOB_CONTROL' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_LINE_MAX' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `_SC_NGROUPS_MAX' `unistd.h' (POSIX.1): *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_SAVED_IDS' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_STREAM_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_TZNAME_MAX' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_VERSION' `unistd.h' (POSIX.1): *Note Constants for Sysconf::. `_SC_VERSION' `unistd.h' (POSIX.2): *Note Constants for Sysconf::. `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 setbuffer (FILE *STREAM, char *BUF, size_t SIZE)' `stdio.h' (BSD): *Note Controlling Buffering::. `void setbuf (FILE *STREAM, char *BUF)' `stdio.h' (ISO): *Note Controlling Buffering::. `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 setlinebuf (FILE *STREAM)' `stdio.h' (BSD): *Note Controlling Buffering::. `char * setlocale (int CATEGORY, const char *LOCALE)' `locale.h' (ISO): *Note Setting the Locale::. `void setnetent (int STAYOPEN)' `netdb.h' (BSD): *Note Networks Database::. `int setnetgrent (const char *NETGROUP)' `netdb.h' (netdb.h): *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 PRIORITY)' `sys/resource.h' (BSD): *Note Priority::. `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, fid_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, struct rlimit *RLP)' `sys/resource.h' (BSD): *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 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::. `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 *ACTION, struct sigaction *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 struct sigaltstack *STACK, struct sigaltstack *OLDSTACK)' `signal.h' (BSD): *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 siginterrupt (int SIGNUM, int FAILFLAG)' `signal.h' (BSD): *Note BSD Handler::. `int SIGINT' `signal.h' (ISO): *Note Termination Signals::. `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 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 *SET, sigset_t *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 sinh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `double sin (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::. `void srand (unsigned int SEED)' `stdlib.h' (ISO): *Note ISO Random::. `void srandom (unsigned int SEED)' `stdlib.h' (BSD): *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::. `int stat (const char *FILENAME, struct stat *BUF)' `sys/stat.h' (POSIX.1): *Note Reading Attributes::. `STDERR_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `FILE * stderr' `stdio.h' (ISO): *Note Standard Streams::. `STDIN_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `FILE * stdin' `stdio.h' (ISO): *Note Standard Streams::. `STDOUT_FILENO' `unistd.h' (POSIX.1): *Note Descriptors and Streams::. `FILE * stdout' `stdio.h' (ISO): *Note Standard Streams::. `char * stpcpy (char *TO, const char *FROM)' `string.h' (Unknown origin): *Note Copying and Concatenation::. `char * stpncpy (char *TO, const char *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 * strcat (char *TO, const char *FROM)' `string.h' (ISO): *Note Copying and Concatenation::. `char * strchr (const char *STRING, int C)' `string.h' (ISO): *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 *TO, const char *FROM)' `string.h' (ISO): *Note Copying and Concatenation::. `size_t strcspn (const char *STRING, const char *STOPSET)' `string.h' (ISO): *Note Search Functions::. `char * strdupa (const char *S)' `string.h' (GNU): *Note Copying and Concatenation::. `char * strdup (const char *S)' `string.h' (SVID): *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::. `size_t strftime (char *S, size_t SIZE, const char *TEMPLATE, const struct tm *BROKENTIME)' `time.h' (POSIX.2): *Note Formatting Date and 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 *TO, const char *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 *TO, const char *FROM, size_t SIZE)' `string.h' (ISO): *Note Copying and Concatenation::. `char * strndupa (const char *S, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `char * strndup (const char *S, size_t SIZE)' `string.h' (GNU): *Note Copying and Concatenation::. `char * strpbrk (const char *STRING, const char *STOPSET)' `string.h' (ISO): *Note Search Functions::. `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 *STRING, char **TAILPTR)' `stdlib.h' (ISO): *Note Parsing of Floats::. `float strtof (const char *STRING, char **TAILPTR)' `stdlib.h' (GNU): *Note Parsing of Floats::. `char * strtok (char *NEWSTRING, const char *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 double strtold (const char *STRING, char **TAILPTR)' `stdlib.h' (GNU): *Note Parsing of Floats::. `long int strtol (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `long long int strtoll (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (GNU): *Note Parsing of Integers::. `long long int strtoq (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (BSD): *Note Parsing of Integers::. `unsigned long int strtoul (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (ISO): *Note Parsing of Integers::. `unsigned long long int strtoull (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (GNU): *Note Parsing of Integers::. `unsigned long long int strtouq (const char *STRING, char **TAILPTR, int BASE)' `stdlib.h' (BSD): *Note Parsing of Integers::. `struct dirent' `dirent.h' (POSIX.1): *Note Directory Entries::. `struct flock' `fcntl.h' (POSIX.1): *Note File Locks::. `struct group' `grp.h' (POSIX.1): *Note Group Data Structure::. `struct hostent' `netdb.h' (BSD): *Note Host Names::. `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 itimerval' `sys/time.h' (BSD): *Note Setting an Alarm::. `struct lconv' `locale.h' (ISO): *Note Numeric Formatting::. `struct linger' `sys/socket.h' (BSD): *Note Socket-Level Options::. `struct mallinfo' `malloc.h' (GNU): *Note Statistics of Malloc::. `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 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 rlimit' `sys/resource.h' (BSD): *Note Limits on Resources::. `struct rusage' `sys/resource.h' (BSD): *Note Resource Usage::. `struct servent' `netdb.h' (BSD): *Note Services Database::. `struct sigaction' `signal.h' (POSIX.1): *Note Advanced Signal Handling::. `struct sigaltstack' `signal.h' (BSD): *Note Signal Stack::. `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 File Namespace Details::. `struct stat' `sys/stat.h' (POSIX.1): *Note Attribute Meanings::. `struct termios' `termios.h' (POSIX.1): *Note Mode Data Types::. `struct timeval' `sys/time.h' (BSD): *Note High-Resolution Calendar::. `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 Detailed CPU Time::. `struct utimbuf' `time.h' (POSIX.1): *Note File Times::. `struct utsname' `sys/utsname.h' (POSIX.1): *Note Hardware/Software Type ID::. `size_t strxfrm (char *TO, const char *FROM, size_t SIZE)' `string.h' (ISO): *Note Collation Functions::. `_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 symlink (const char *OLDNAME, const char *NEWNAME)' `unistd.h' (BSD): *Note Symbolic Links::. `long int sysconf (int PARAMETER)' `unistd.h' (POSIX.1): *Note Sysconf Definition::. `int system (const char *COMMAND)' `stdlib.h' (ISO): *Note Running a Command::. `double tanh (double X)' `math.h' (ISO): *Note Hyperbolic Functions::. `double tan (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::. `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::. `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::. `time_t time (time_t *RESULT)' `time.h' (ISO): *Note Simple Calendar Time::. `clock_t times (struct tms *BUFFER)' `sys/times.h' (POSIX.1): *Note Detailed CPU 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::. `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::. `TRY_AGAIN' `netdb.h' (BSD): *Note Host Names::. `char * ttyname (int FILEDES)' `unistd.h' (POSIX.1): *Note Is It a Terminal::. `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::. `uid_t' `sys/types.h' (POSIX.1): *Note Reading Persona::. `UINT_MAX' `limits.h' (ISO): *Note Range of Type::. `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 uname (struct utsname *INFO)' `sys/utsname.h' (POSIX.1): *Note Hardware/Software Type ID::. `int ungetc (int C, FILE *STREAM)' `stdio.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::. `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::. `va_alist' `varargs.h' (Unix): *Note Old Varargs::. `TYPE va_arg (va_list AP, TYPE)' `stdarg.h' (ISO): *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 VEOL2' `termios.h' (BSD): *Note Editing Characters::. `int VEOL' `termios.h' (POSIX.1): *Note Editing Characters::. `int VERASE' `termios.h' (POSIX.1): *Note Editing Characters::. `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' (GNU): *Note Variable Arguments Input::. `int VINTR' `termios.h' (POSIX.1): *Note Signal Characters::. `int VKILL' `termios.h' (POSIX.1): *Note Editing Characters::. `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' (GNU): *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' (GNU): *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 VTIME' `termios.h' (POSIX.1): *Note Noncanonical Input::. `int VWERASE' `termios.h' (BSD): *Note Editing Characters::. `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 wait (int *STATUS-PTR)' `sys/wait.h' (POSIX.1): *Note Process Completion::. `pid_t waitpid (pid_t PID, int *STATUS-PTR, int OPTIONS)' `sys/wait.h' (POSIX.1): *Note Process Completion::. `WCHAR_MAX' `limits.h' (GNU): *Note Range of Type::. `wchar_t' `stddef.h' (ISO): *Note Wide Char Intro::. `int WCOREDUMP (int STATUS)' `sys/wait.h' (BSD): *Note Process Completion Status::. `size_t wcstombs (char *STRING, const wchar_t WSTRING, size_t SIZE)' `stdlib.h' (ISO): *Note Wide String Conversion::. `int wctomb (char *STRING, wchar_t WCHAR)' `stdlib.h' (ISO): *Note Converting One Char::. `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::. `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::. `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::. `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' (XOPEN): *Note Feature Test Macros::. Library Maintenance ******************* How to Install the GNU C Library ================================ Installation of the GNU C library is relatively simple, but usually requires several GNU tools to be installed already. To configure the GNU C library for your system, run the shell script `configure' with `sh'. Use an argument which is the conventional GNU name for your system configuration--for example, `sparc-sun-sunos4.1', for a Sun 4 running SunOS 4.1. *Note Installation: (gcc.info)Installation, for a full description of standard GNU configuration names. If you omit the configuration name, `configure' will try to guess one for you by inspecting the system it is running on. It may or may not be able to come up with a guess, and the its guess might be wrong. `configure' will tell you the canonical name of the chosen configuration before proceeding. Here are some options that you should specify (if appropriate) when you run `configure': `--with-gnu-ld' Use this option if you plan to use GNU `ld' to link programs with the GNU C Library. (We strongly recommend that you do.) This option enables use of features that exist only in GNU `ld'; so if you configure for GNU `ld' you must use GNU `ld' *every time* you link with the GNU C Library, and when building it. `--with-gnu-as' Use this option if you plan to use the GNU assembler, `gas', when building the GNU C Library. On some systems, the library may not build properly if you do *not* use `gas'. `--with-gnu-binutils' This option implies both `--with-gnu-ld' and `--with-gnu-as'. On systems where GNU tools are the system tools, there is no need to specify this option. These include GNU, GNU/Linux, and free BSD systems. `--without-fp' `--nfp' Use this option if your computer lacks hardware floating-point support. `--prefix=DIRECTORY' Install machine-independent data files in subdirectories of `DIRECTORY'. (You can also set this in `configparms'; see below.) `--exec-prefix=DIRECTORY' Install the library and other machine-dependent files in subdirectories of `DIRECTORY'. (You can also set this in `configparms'; see below.) `--enable-shared' `--disable-shared' Enable or disable building of an ELF shared library on systems that support it. The default is to build the shared library on systems using ELF when the GNU `binutils' are available. `--enable-profile' `--disable-profile' Enable or disable building of the profiled C library, `-lc_p'. The default is to build the profiled library. You may wish to disable it if you don't plan to do profiling, because it doubles the build time of compiling just the unprofiled static library. `--enable-omitfp' Enable building a highly-optimized but possibly undebuggable static C library. This causes the normal static and shared (if enabled) C libraries to be compiled with maximal optimization, including the `-fomit-frame-pointer' switch that makes debugging impossible on many machines, and without debugging information (which makes the binaries substantially smaller). An additional static library is compiled with no optimization and full debugging information, and installed as `-lc_g'. The simplest way to run `configure' is to do it in the directory that contains the library sources. This prepares to build the library in that very directory. You can prepare to build the library in some other directory by going to that other directory to run `configure'. In order to run configure, you will have to specify a directory for it, like this: mkdir sun4 cd sun4 ../configure sparc-sun-sunos4.1 `configure' looks for the sources in whatever directory you specified for finding `configure' itself. It does not matter where in the file system the source and build directories are--as long as you specify the source directory when you run `configure', you will get the proper results. This feature lets you keep sources and binaries in different directories, and that makes it easy to build the library for several different machines from the same set of sources. Simply create a build directory for each target machine, and run `configure' in that directory specifying the target machine's configuration name. The library has a number of special-purpose configuration parameters. These are defined in the file `Makeconfig'; see the comments in that file for the details. But don't edit the file `Makeconfig' yourself--instead, create a file `configparms' in the directory where you are building the library, and define in that file the parameters you want to specify. `configparms' should *not* be an edited copy of `Makeconfig'; specify only the parameters that you want to override. To see how to set these parameters, find the section of `Makeconfig' that says "These are the configuration variables." Then for each parameter that you want to change, copy the definition from `Makeconfig' to your new `configparms' file, and change the value as appropriate for your system. 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. Some of the machine-dependent code for some machines uses extensions in the GNU C compiler, so you may need to compile the library with GCC. (In fact, all of the existing complete ports require GCC.) 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. To build and run some test programs which exercise some of the library facilities, type `make check'. This will produce several files with names like `PROGRAM.out'. To format the `GNU C Library Reference Manual' for printing, type `make dvi'. You need a working TeX installation to do this. To install the library and its header files, and the Info files of the manual, type `make install'. This will build things if necessary, before installing them. If you want to install the files in a different place than the one specified at configuration time you can specify a value for the Makefile variable `install_root' on the command line. This is useful to create chroot'ed environment or to prepare binary releases. Recommended Tools to Install the GNU C Library ---------------------------------------------- We recommend installing the following GNU tools before attempting to build the GNU C library: * `make' 3.75 You need the latest version of GNU `make'. Modifying the GNU C Library to work with other `make' programs would be so hard that we recommend you port GNU `make' instead. *Really.* We recommend version GNU `make' version 3.75. Versions 3.76 and 3.76.1 are known to have bugs which only show up in big projects like GNU `libc'. * GCC 2.7.2 On most platforms, the GNU C library can only be compiled with the GNU C compiler. We recommend GCC version 2.7.2 or later; earlier versions may have problems. * `binutils' 2.8.1 Using the GNU `binutils' (assembler, linker, and related tools) is preferable when possible, and they are required to build an ELF shared C library. We recommend `binutils' version 2.8 or later; earlier versions are known to have problems or to not support all architectures. * `texinfo' 3.11 To correctly translate and install the Texinfo documentation you need this version of the `texinfo' package. Former versions did not understand all the tags used in the document and also the installation mechanisms for the info files was not present or worked differently. On some Debian Linux based systems the used `install-info' program works differently. Here you have to run make like this: make INSTALL_INFO=/path/to/GNU/install-info install Supported Configurations ------------------------ The GNU C Library currently supports configurations that match the following patterns: alpha-ANYTHING-linux iX86-ANYTHING-gnu iX86-ANYTHING-linux m68k-ANYTHING-linux Former releases of this library (version 1.09.1 and perhaps earlier versions) used to run on the following configurations: alpha-dec-osf1 iX86-ANYTHING-bsd4.3 iX86-ANYTHING-isc2.2 iX86-ANYTHING-isc3.N iX86-ANYTHING-sco3.2 iX86-ANYTHING-sco3.2v4 iX86-ANYTHING-sysv iX86-ANYTHING-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 the above configurations, these are not supported at the moment. It's expected that these 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 bug-glibc@prep.ai.mit.edu. Each case of `iX86' can be `i386', `i486', `i586', or `i686'. All of those configurations produce a library that can run on any of these processors. The library will be optimized for the specified processor, but will not use instructions not available on all of them. While no other configurations are supported, there are handy aliases for these few. (These aliases work in other GNU software as well.) decstation hp320-bsd4.3 hp300bsd i486-gnu i586-linux i386-sco i386-sco3.2v4 i386-sequent-dynix i386-svr4 news sun3-sunos4.N sun3 sun4-solaris2.N sun4-sunos5.N sun4-sunos4.N sun4 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. 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. 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. When reporting a bug, send your test case, the results you got, the results you expected, what you think the problem might be (if you've thought of anything), your system type, and the version of the GNU C library which you are using. Also include the files `config.status' and `config.make' which are created by running `configure'; they will be in whatever directory was current when you ran `configure'. 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! Send bug reports to the Internet address bug-glibc@prep.ai.mit.edu or the UUCP path mit-eddie!prep.ai.mit.edu!bug-glibc. If you have other problems with installation or use, please report those as well. 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 bug-glibc-manual@prep.ai.mit.edu or the UUCP path mit-eddie!prep.ai.mit.edu!bug-glibc-manual. 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 two "special" subdirectories, called `generic' and `stub'. These two are always implicitly appended to the list of subdirectories (in that order), so you needn't put them in an `Implies' file, and you should not create any subdirectories under them intended to be new specific categories. `generic' is for things that can be implemented in machine-independent C, using only other machine-independent functions in the C library. `stub' is for "stub" versions of functions which cannot be implemented on a particular machine or operating system. The stub functions always return an error, and set `errno' to `ENOSYS' (Function not implemented). *Note Error Reporting::. A source file is known to be system-dependent by its having a version in `generic' or `stub'; every generally-available function whose implementation is system-dependent in should have either a generic or stub implementation (there is no point in having both). 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' and `stub' directories), 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 `sunos4.1', the base operating system is `unix/bsd'. 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 `sparc-sun-sunos4.1' results in `unix/bsd/sun/sparc/sunos4.1'. `configure' then tries removing each element of the list in turn, so `unix/bsd/sparc' and `sun/sparc' 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 `sunos4.1.1' and `sunos4.1.2' 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 `sparc-sun-sunos4.1' (without the `--nfp' option): sparc/fpu unix/bsd/sun/sunos4.1/sparc unix/bsd/sun/sunos4.1 unix/bsd/sun/sunos4/sparc unix/bsd/sun/sunos4 unix/bsd/sun/sunos/sparc unix/bsd/sun/sunos unix/bsd/sun/sparc unix/bsd/sun unix/bsd/sunos4.1/sparc unix/bsd/sunos4.1 unix/bsd/sunos4/sparc unix/bsd/sunos4 unix/bsd/sunos/sparc unix/bsd/sunos unix/bsd/sparc unix/bsd unix/sun/sunos4.1/sparc unix/sun/sunos4.1 unix/sun/sunos4/sparc unix/sun/sunos4 unix/sun/sunos/sparc unix/sun/sunos unix/sun/sparc unix/sun unix/sunos4.1/sparc unix/sunos4.1 unix/sunos4/sparc unix/sunos4 unix/sunos/sparc unix/sunos unix/sparc unix sun/sunos4.1/sparc sun/sunos4.1 sun/sunos4/sparc sun/sunos4 sun/sunos/sparc sun/sunos sun/sparc sun sunos4.1/sparc sunos4.1 sunos4/sparc sunos4 sunos/sparc sunos sparc 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' `stub' As described above (*note Porting::.), these are the two subdirectories 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'. `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. The `inet' top-level subdirectory is enabled by `unix/inet/Subdirs'. `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 in files in `sysdeps/unix'. These files are named with a suffix of `.S'; for example, `__open.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 (that is, the file `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. Some parts of the library were contributed or worked on by other people. * The `getopt' function and related code were 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. * Fast implementations of many of the string functions (`memcpy', `strlen', etc.) were written by Torbjorn 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' function was contributed by Ian Lance Taylor. * 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 Torbjorn 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. * 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'). * 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. * 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. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 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. Cohen for the University of California at Berkeley and are copyrighted by the Regents of the University of California. 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Concept Index ************* * Menu: * 4.N BSD Unix: Berkeley Unix. * alloca disadvantages: Disadvantages of Alloca. * alloca function: Variable Size Automatic. * exec functions: Executing a File. * fcntl function: Control Operations. * main function: Program Arguments. * malloc function: Unconstrained Allocation. * pause function: Waiting for a Signal. * setuid programs: How Change Persona. * sigaction flags: Flags for Sigaction. * sigaction function: Advanced Signal Handling. * SIGCHLD, handling of: Stopped and Terminated Jobs. * signal function: Basic Signal Handling. * SIGTTIN, from background job: Access to the Terminal. * SIGTTOU, from background job: Access to the Terminal. * volatile declarations: Nonreentrancy. * /etc/nsswitch.conf: NSS Configuration File. * nsswitch.conf: NSS Configuration File. * _POSIX_OPTION_ORDER environment variable.: Standard Environment. * abort signal: Program Error Signals. * aborting a program: Aborting a Program. * absolute file name: File Name Resolution. * absolute value functions: Absolute Value. * accepting connections: Accepting Connections. * access permission for a file: Access Permission. * access, testing for: Testing File Access. * accessing directories: Accessing Directories. * address of socket: Socket Addresses. * alarm signal: Alarm Signals. * alarms, setting: Setting an Alarm. * alignment (in obstacks): Obstacks Data Alignment. * alignment (with malloc): Aligned Memory Blocks. * allocation (obstacks): Allocation in an Obstack. * allocation hooks, for malloc: Hooks for Malloc. * allocation of memory with malloc: Basic Allocation. * allocation size of string: Representation of Strings. * allocation statistics: Statistics of Malloc. * alphabetic character: Classification of Characters. * alphanumeric character: Classification of Characters. * append-access files: File Position. * argc (program argument count): Program Arguments. * argument promotion: Calling Variadics. * arguments (variadic functions): Receiving Arguments. * arguments, how many: How Many Arguments. * arguments, to program: Program Arguments. * argv (program argument vector): Program Arguments. * arithmetic expansion: Expansion Stages. * array comparison functions: String/Array Comparison. * array copy functions: Copying and Concatenation. * array search function: Array Search Function. * array sort function: Array Sort Function. * ASCII character: Classification of Characters. * assertions: Consistency Checking. * attributes of a file: Attribute Meanings. * automatic allocation: Memory Concepts. * automatic freeing: Variable Size Automatic. * automatic storage with variable size: Variable Size Automatic. * background job: Concepts of Job Control. * background job, launching: Foreground and Background. * base (of floating point number): Floating Point Concepts. * basic byte sequence: Multibyte Char Intro. * baud rate: Line Speed. * Berkeley Unix: Berkeley Unix. * bias (of floating point number exponent): Floating Point Concepts. * big-endian: Byte Order. * binary I/O to a stream: Block Input/Output. * binary search function (for arrays): Array Search Function. * binary stream: Binary Streams. * binding a socket address: Socket Addresses. * blank character: Classification of Characters. * block I/O to a stream: Block Input/Output. * blocked signals: Delivery of Signal. * blocked signals, checking for: Checking for Pending Signals. * blocking signals: Blocking Signals. * blocking signals, in a handler: Blocking for Handler. * bootstrapping, and services: Actions in the NSS configuration. * break condition, detecting: Input Modes. * break condition, generating: Line Control. * breaking a string into tokens: Finding Tokens in a String. * broken pipe signal: Operation Error Signals. * broken-down time <1>: Broken-down Time. * broken-down time: Calendar Time. * BSD compatibility library: Process Group Functions. * BSD compatibility library.: Feature Test Macros. * BSD Unix: Berkeley Unix. * buffering of streams: Stream Buffering. * buffering, controlling: Controlling Buffering. * bugs, reporting: Reporting Bugs. * bus error: Program Error Signals. * byte order conversion, for socket: Byte Order. * byte stream: Socket Concepts. * calendar time: Calendar Time. * calendar time and broken-down time: Broken-down Time. * calling variadic functions: Calling Variadics. * canonical input processing: Canonical or Not. * capacity limits, POSIX: General Limits. * carrier detect: Control Modes. * case conversion of characters: Case Conversion. * catching signals: Delivery of Signal. * categories for locales: Locale Categories. * change working directory: Working Directory. * changing the locale: Setting the Locale. * changing the size of a block (malloc): Changing Block Size. * changing the size of a block (obstacks): Growing Objects. * channels: Stream/Descriptor Precautions. * character case conversion: Case Conversion. * character code: Wide Char Intro. * character predicates: Classification of Characters. * character testing: Classification of Characters. * checking for pending signals: Checking for Pending Signals. * child process <1>: Process Creation Concepts. * child process: Processes. * child process signal: Job Control Signals. * chunks: Obstack Chunks. * classification of characters: Classification of Characters. * cleaning up a stream: Linked Channels. * clearing terminal input queue: Line Control. * client: Connections. * clock ticks: Processor Time. * close-on-exec (file descriptor flag): Descriptor Flags. * closing a file descriptor: Opening and Closing Files. * closing a socket: Closing a Socket. * closing a stream: Closing Streams. * code, character: Wide Char Intro. * collating strings: Collation Functions. * combining locales: Choosing Locale. * command argument syntax: Argument Syntax. * command arguments, parsing: Parsing Options. * command line arguments: Program Arguments. * command substitution: Expansion Stages. * communication style (of a socket): Socket Concepts. * comparing strings and arrays: String/Array Comparison. * Comparison Function: Comparison Functions. * concatenating strings: Copying and Concatenation. * configurations, all supported: Supported Configurations. * connecting a socket: Connecting. * connection: Connections. * consistency checking: Consistency Checking. * consistency checking, of heap: Heap Consistency Checking. * continue signal: Job Control Signals. * control character: Classification of Characters. * control operations on files: Control Operations. * controlling process: Controlling Terminal. * controlling terminal: Concepts of Job Control. * controlling terminal, access to: Access to the Terminal. * controlling terminal, determining: Identifying the Terminal. * controlling terminal, setting: Open-time Flags. * conversion specifications (printf): Formatted Output Basics. * conversion specifications (scanf): Formatted Input Basics. * converting byte order: Byte Order. * converting case of characters: Case Conversion. * converting extended characters: Converting One Char. * converting extended strings: Wide String Conversion. * converting file descriptor to stream: Descriptors and Streams. * converting floats to integers: Rounding and Remainders. * converting group ID to group name: Lookup Group. * converting group name to group ID: Lookup Group. * converting host address to name: Host Names. * converting host name to address: Host Names. * converting network name to network number: Networks Database. * converting network number to network name: Networks Database. * converting port number to service name: Services Database. * converting service name to port number: Services Database. * converting string to collation order: Collation Functions. * converting strings to numbers: Parsing of Numbers. * converting user ID to user name: Lookup User. * converting user name to user ID: Lookup User. * cookie, for custom stream: Streams and Cookies. * copying strings and arrays: Copying and Concatenation. * CPU time: Processor Time. * create on open (file status flag): Open-time Flags. * creating a directory: Creating Directories. * creating a FIFO special file: FIFO Special Files. * creating a pipe: Creating a Pipe. * creating a pipe to a subprocess: Pipe to a Subprocess. * creating a process: Process Creation Concepts. * creating a socket: Creating a Socket. * creating a socket pair: Socket Pairs. * creating special files: Making Special Files. * cube root function: Exponents and Logarithms. * currency symbols: Currency Symbol. * current working directory: Working Directory. * custom streams: Custom Streams. * customizing printf: Customizing Printf. * data loss on sockets: Socket Concepts. * databases: Name Service Switch. * datagram socket: Datagrams. * datagrams, transmitting: Sending Datagrams. * date and time: Calendar Time. * Daylight Saving Time: Broken-down Time. * decimal digit character: Classification of Characters. * decimal-point separator: General Numeric. * declaration (compared to definition): Header Files. * declaring variadic functions: Calling Variadics. * default action (for a signal): Delivery of Signal. * default action for a signal: Basic Signal Handling. * default argument promotions: Calling Variadics. * default value, and NSS: Notes on NSS Configuration File. * defining new printf conversions: Customizing Printf. * definition (compared to declaration): Header Files. * delayed suspend character: Signal Characters. * deleting a directory: Deleting Files. * deleting a file: Deleting Files. * delivery of signals: Delivery of Signal. * descriptors and streams: Stream/Descriptor Precautions. * digit character: Classification of Characters. * directories, accessing: Accessing Directories. * directories, creating: Creating Directories. * directories, deleting: Deleting Files. * directory: Directories. * directory entry: Directories. * directory stream: Accessing Directories. * disadvantages of alloca: Disadvantages of Alloca. * DISCARD character: Other Special. * DNS server unavailable: Actions in the NSS configuration. * domain (of socket): Socket Concepts. * domain error: Domain and Range Errors. * dot notation, for Internet addresses: Abstract Host Addresses. * DSUSP character: Signal Characters. * duplicating file descriptors: Duplicating Descriptors. * dynamic allocation: Memory Concepts. * echo of terminal input: Local Modes. * effective group ID: Process Persona. * effective user ID: Process Persona. * efficiency and malloc: Efficiency and Malloc. * efficiency and obstacks: Extra Fast Growing. * efficiency of chunks: Obstack Chunks. * EINTR, and restarting interrupted primitives: Interrupted Primitives. * end of file, on a stream: EOF and Errors. * end-of-file, on a file descriptor: I/O Primitives. * environment: Environment Variables. * environment access: Environment Access. * environment representation: Environment Access. * environment variable: Environment Variables. * EOF character: Editing Characters. * EOL character: Editing Characters. * EOL2 character: Editing Characters. * epoch: Simple Calendar Time. * ERASE character: Editing Characters. * error codes: Error Reporting. * error reporting: Error Reporting. * establishing a handler: Signal Actions. * ethers: NSS Basics. * exception: Program Error Signals. * exclusive lock: File Locks. * executing a file: Executing a File. * exit status: Exit Status. * exit status value: Program Termination. * expansion of shell words: Word Expansion. * exponent (of floating point number): Floating Point Concepts. * exponentiation functions: Exponents and Logarithms. * extended character sets: Extended Characters. * extended characters, converting: Converting One Char. * extended strings, converting representations: Wide String Conversion. * extending printf: Customizing Printf. * extracting file descriptor from stream: Descriptors and Streams. * feature test macros: Feature Test Macros. * field splitting: Expansion Stages. * FIFO special file: Pipes and FIFOs. * file access permission: Access Permission. * file access time: File Times. * file attribute modification time: File Times. * file attributes: Attribute Meanings. * file creation mask: Setting Permissions. * file descriptor flags: Descriptor Flags. * file descriptor sets, for select: Waiting for I/O. * file descriptors, standard: Descriptors and Streams. * file locks: File Locks. * file modification time: File Times. * file name: File Names. * file name component: Directories. * file name errors: File Name Errors. * file name resolution: File Name Resolution. * file name translation flags: Open-time Flags. * file names, multiple: Hard Links. * file namespace, for sockets: File Namespace. * file owner: File Owner. * file permission bits: Permission Bits. * file pointer: Streams. * file position: File Position. * file positioning on a file descriptor: File Position Primitive. * file positioning on a stream: File Positioning. * file status flags: File Status Flags. * filtering i/o through subprocess: Pipe to a Subprocess. * flag character (printf): Output Conversion Syntax. * flag character (scanf): Input Conversion Syntax. * flags for sigaction: Flags for Sigaction. * flags, file name translation: Open-time Flags. * flags, open-time action: Open-time Flags. * floating point, IEEE: IEEE Floating Point. * floating type measurements: Floating Type Macros. * floating-point exception: Program Error Signals. * flow control, terminal: Line Control. * flushing a stream: Flushing Buffers. * flushing terminal output queue: Line Control. * foreground job: Concepts of Job Control. * foreground job, launching: Foreground and Background. * forking a process: Process Creation Concepts. * format string, for printf: Formatted Output. * format string, for scanf: Formatted Input. * formatted input from a stream: Formatted Input. * formatted output to a stream: Formatted Output. * freeing (obstacks): Freeing Obstack Objects. * freeing memory allocated with malloc: Freeing after Malloc. * fully buffered stream: Buffering Concepts. * function prototypes (variadic): Variadic Prototypes. * generation of signals: Signal Generation. * globbing: Globbing. * graphic character: Classification of Characters. * Gregorian calendar: Calendar Time. * group: NSS Basics. * group database: Group Database. * group ID: User and Group IDs. * group name: User and Group IDs. * group owner of a file: File Owner. * grouping of digits: General Numeric. * growing objects (in obstacks): Growing Objects. * handle: Relocator Concepts. * handling multiple signals: Merged Signals. * hangup signal: Termination Signals. * hard limit: Limits on Resources. * hard link: Hard Links. * header files: Header Files. * heap consistency checking: Heap Consistency Checking. * heap, dynamic allocation from: Unconstrained Allocation. * heap, freeing memory from: Freeing after Malloc. * hexadecimal digit character: Classification of Characters. * hidden bit (of floating point number mantissa): Floating Point Concepts. * high-priority data: Out-of-Band Data. * high-resolution time: Calendar Time. * holes in files: File Position Primitive. * home directory: Standard Environment. * HOME environment variable: Standard Environment. * hook functions (of custom streams): Hook Functions. * host address, Internet: Abstract Host Addresses. * hosts: NSS Basics. * hosts database: Host Names. * how many arguments: How Many Arguments. * hyperbolic functions: Hyperbolic Functions. * identifying terminals: Is It a Terminal. * IEEE floating point: Not a Number. * IEEE floating point representation: IEEE Floating Point. * IEEE Std 1003.1: POSIX. * IEEE Std 1003.2: POSIX. * ignore action for a signal: Basic Signal Handling. * illegal instruction: Program Error Signals. * impossible events: Consistency Checking. * independent channels: Independent Channels. * initial signal actions: Initial Signal Actions. * inode number: Attribute Meanings. * input available signal: Asynchronous I/O Signals. * input conversions, for scanf: Table of Input Conversions. * input from multiple files: Waiting for I/O. * installation tools: Tools for Installation. * installing the library: Installation. * integer division functions: Integer Division. * integer type range: Range of Type. * integer type width: Width of Type. * interactive signals, from terminal: Local Modes. * interactive stop signal: Job Control Signals. * internationalization: Locales. * Internet host address: Abstract Host Addresses. * Internet namespace, for sockets: Internet Namespace. * interprocess communication, with FIFO: FIFO Special Files. * interprocess communication, with pipes: Creating a Pipe. * interprocess communication, with signals: Kill Example. * interprocess communication, with sockets: Sockets. * interrupt character: Signal Characters. * interrupt signal: Termination Signals. * interrupt-driven input: Interrupt Input. * interrupting primitives: Interrupted Primitives. * interval timer, setting: Setting an Alarm. * INTR character: Signal Characters. * inverse hyperbolic functions: Hyperbolic Functions. * inverse trigonometric functions: Inverse Trig Functions. * invocation of program: Program Arguments. * ISO C: ISO C. * job: Job Control. * job control: Job Control. * job control functions: Functions for Job Control. * job control is optional: Job Control is Optional. * job control signals: Job Control Signals. * job control, enabling: Initializing the Shell. * Kermit the frog: Search/Sort Example. * KILL character: Editing Characters. * kill signal: Termination Signals. * killing a process: Signaling Another Process. * LANG environment variable: Standard Environment. * launching jobs: Launching Jobs. * LC_COLLATE environment variable: Standard Environment. * LC_CTYPE environment variable: Standard Environment. * LC_MONETARY environment variable: Standard Environment. * LC_NUMERIC environment variable: Standard Environment. * LC_TIME environment variable: Standard Environment. * leap second: Broken-down Time. * length of multibyte character: Length of Char. * length of string: Representation of Strings. * level, for socket options: Socket Options. * library: Introduction. * limits on resource usage: Limits on Resources. * limits, file name length: Limits for Files. * limits, floating types: Floating Type Macros. * limits, integer types: Range of Type. * limits, link count of files: Limits for Files. * limits, number of open files: General Limits. * limits, number of processes: General Limits. * limits, number of supplementary group IDs: General Limits. * limits, pipe buffer size: Limits for Files. * limits, POSIX: General Limits. * limits, program argument size: General Limits. * limits, terminal input queue: Limits for Files. * limits, time zone name length: General Limits. * line buffered stream: Buffering Concepts. * line speed: Line Speed. * lines (in a text file): Binary Streams. * link: Directories. * link, hard: Hard Links. * link, soft: Symbolic Links. * link, symbolic: Symbolic Links. * linked channels: Linked Channels. * listening (sockets): Listening. * little-endian: Byte Order. * LNEXT character: Other Special. * local network address number: Abstract Host Addresses. * local time: Calendar Time. * locale categories: Locale Categories. * locale, changing: Setting the Locale. * locales: Locales. * logarithm functions: Exponents and Logarithms. * login name: User and Group IDs. * login name, determining: Who Logged In. * LOGNAME environment variable: Standard Environment. * long jumps: Non-Local Exits. * long-named options: Argument Syntax. * longjmp: Advantages of Alloca. * loss of data on sockets: Socket Concepts. * lost resource signal: Operation Error Signals. * lower-case character: Classification of Characters. * macros: Obstack Functions. * mantissa (of floating point number): Floating Point Concepts. * matching failure, in scanf: Formatted Input Basics. * maximum field width (scanf): Input Conversion Syntax. * measurements of floating types: Floating Type Macros. * memory allocation: Memory Allocation. * merging of signals: Merged Signals. * MIN termios slot: Noncanonical Input. * minimum field width (printf): Output Conversion Syntax. * mixing descriptors and streams: Stream/Descriptor Precautions. * modem disconnect: Control Modes. * modem status lines: Control Modes. * monetary value formatting: Numeric Formatting. * multibyte character, length of: Length of Char. * multibyte characters: Multibyte Char Intro. * multiple names for one file: Hard Links. * multiplexing input: Waiting for I/O. * name of running program: Error Messages. * name of socket: Socket Addresses. * Name Service Switch: Name Service Switch. * name space: Reserved Names. * names of signals: Standard Signals. * namespace (of socket): Socket Concepts. * NaN: Not a Number. * Netgroup: Netgroup Data. * netgroup: NSS Basics. * network: NSS Basics. * network byte order: Byte Order. * network number: Abstract Host Addresses. * network protocol: Socket Concepts. * networks database: Networks Database. * nisplus, and booting: Actions in the NSS configuration. * nisplus, and completeness: Actions in the NSS configuration. * non-blocking open: Open-time Flags. * non-local exit, from signal handler: Longjmp in Handler. * non-local exits: Non-Local Exits. * noncanonical input processing: Canonical or Not. * normalization functions (floating-point): Normalization Functions. * normalized floating point number: Floating Point Concepts. * not a number: Not a Number. * NSS: Name Service Switch. * null character: Representation of Strings. * null pointer constant: Null Pointer Constant. * number of arguments passed: How Many Arguments. * number syntax, parsing: Parsing of Numbers. * numeric value formatting: Numeric Formatting. * obstack status: Status of an Obstack. * obstacks: Obstacks. * open-time action flags: Open-time Flags. * opening a file: I/O Concepts. * opening a file descriptor: Opening and Closing Files. * opening a pipe: Creating a Pipe. * opening a socket: Creating a Socket. * opening a socket pair: Socket Pairs. * opening a stream: Opening Streams. * optimizing NSS: Notes on NSS Configuration File. * optional arguments: Variadic Functions. * optional POSIX features: System Options. * orphaned process group: Orphaned Process Groups. * out-of-band data: Out-of-Band Data. * output conversions, for printf: Table of Output Conversions. * output possible signal: Asynchronous I/O Signals. * owner of a file: File Owner. * packet: Socket Concepts. * page boundary: Aligned Memory Blocks. * parent directory: File Name Resolution. * parent process <1>: Process Creation Concepts. * parent process: Processes. * parity checking: Input Modes. * parsing a template string: Parsing a Template String. * parsing numbers (in formatted input): Parsing of Numbers. * parsing program arguments: Parsing Options. * parsing tokens from a string: Finding Tokens in a String. * passwd: NSS Basics. * password database: User Database. * PATH environment variable: Standard Environment. * peeking at input: Unreading. * pending signals: Delivery of Signal. * pending signals, checking for: Checking for Pending Signals. * permission to access a file: Access Permission. * persona: Process Persona. * pi (trigonometric constant): Trig Functions. * pipe: Pipes and FIFOs. * pipe signal: Operation Error Signals. * pipe to a subprocess: Pipe to a Subprocess. * port number: Ports. * positioning a file descriptor: File Position Primitive. * positioning a stream: File Positioning. * POSIX: POSIX. * POSIX capacity limits: General Limits. * POSIX optional features: System Options. * POSIX.1: POSIX. * POSIX.2: POSIX. * power functions: Exponents and Logarithms. * precision (printf): Output Conversion Syntax. * precision (of floating point number): Floating Point Concepts. * predicates on arrays: String/Array Comparison. * predicates on characters: Classification of Characters. * predicates on strings: String/Array Comparison. * primitives, interrupting: Interrupted Primitives. * printing character: Classification of Characters. * priority of a process: Priority. * process <1>: Processes. * process: Process Startup. * process completion: Process Completion. * process group functions: Functions for Job Control. * process group ID: Launching Jobs. * process group leader: Launching Jobs. * process groups: Job Control. * process ID: Process Creation Concepts. * process image: Process Creation Concepts. * process lifetime: Process Creation Concepts. * process priority: Priority. * process signal mask: Process Signal Mask. * process termination: Program Termination. * processor time: Processor Time. * profiling alarm signal: Alarm Signals. * profiling timer: Setting an Alarm. * program argument syntax: Argument Syntax. * program arguments: Program Arguments. * program arguments, parsing: Parsing Options. * program error signals: Program Error Signals. * program name: Error Messages. * program startup: Program Arguments. * program termination: Program Termination. * program termination signals: Termination Signals. * programming your own streams: Custom Streams. * protocol (of socket): Socket Concepts. * protocol family: Socket Concepts. * protocols: NSS Basics. * protocols database: Protocols Database. * prototypes for variadic functions: Variadic Prototypes. * pseudo-random numbers: Pseudo-Random Numbers. * punctuation character: Classification of Characters. * pushing input back: Unreading. * quick sort function (for arrays): Array Sort Function. * QUIT character: Signal Characters. * quit signal: Termination Signals. * quote removal: Expansion Stages. * race conditions, relating to job control: Launching Jobs. * race conditions, relating to signals: Signals in Handler. * radix (of floating point number): Floating Point Concepts. * raising signals: Generating Signals. * random numbers: Pseudo-Random Numbers. * random-access files: File Position. * range error: Domain and Range Errors. * range of integer type: Range of Type. * read lock: File Locks. * reading from a directory: Accessing Directories. * reading from a file descriptor: I/O Primitives. * reading from a socket: Transferring Data. * reading from a stream, by blocks: Block Input/Output. * reading from a stream, by characters: Character Input. * reading from a stream, formatted: Formatted Input. * real group ID: Process Persona. * real user ID: Process Persona. * real-time timer: Setting an Alarm. * receiving datagrams: Receiving Datagrams. * record locking: File Locks. * redirecting input and output: Duplicating Descriptors. * reentrant functions: Nonreentrancy. * reentrant NSS functions: NSS Module Names. * relative file name: File Name Resolution. * relocating memory allocator: Relocating Allocator. * remainder functions: Rounding and Remainders. * removal of quotes: Expansion Stages. * removing a file: Deleting Files. * removing macros that shadow functions: Macro Definitions. * renaming a file: Renaming Files. * reporting bugs: Reporting Bugs. * reporting errors: Error Reporting. * REPRINT character: Editing Characters. * reserved names: Reserved Names. * resource limits: Limits on Resources. * restarting interrupted primitives: Interrupted Primitives. * restrictions on signal handler functions: Nonreentrancy. * root directory: File Name Resolution. * rounding functions: Rounding and Remainders. * rpc: NSS Basics. * running a command: Running a Command. * scanning the group list: Scanning All Groups. * scanning the user list: Scanning All Users. * search function (for arrays): Array Search Function. * search functions (for strings): Search Functions. * seed (for random numbers): Pseudo-Random Numbers. * seeking on a file descriptor: File Position Primitive. * seeking on a stream: File Positioning. * segmentation violation: Program Error Signals. * sending a datagram: Sending Datagrams. * sending signals: Generating Signals. * sequential-access files: File Position. * server: Connections. * services: NSS Basics. * services database: Services Database. * session <1>: Concepts of Job Control. * session: Job Control. * session leader: Concepts of Job Control. * setting an alarm: Setting an Alarm. * setuid programs and file access: Testing File Access. * shadow: NSS Basics. * shadowing functions with macros: Macro Definitions. * shared lock: File Locks. * shell: Concepts of Job Control. * shrinking objects: Growing Objects. * shutting down a socket: Closing a Socket. * sign (of floating point number): Floating Point Concepts. * signal: Signal Handling. * signal action: Delivery of Signal. * signal actions: Signal Actions. * signal flags: Flags for Sigaction. * signal handler function: Defining Handlers. * signal mask: Process Signal Mask. * signal messages: Signal Messages. * signal names: Standard Signals. * signal number: Standard Signals. * signal set: Signal Sets. * signals, generating: Generating Signals. * significand (of floating point number): Floating Point Concepts. * size of string: Representation of Strings. * socket: Sockets. * socket address (name) binding: Socket Addresses. * socket domain: Socket Concepts. * socket namespace: Socket Concepts. * socket option level: Socket Options. * socket options: Socket Options. * socket pair: Socket Pairs. * socket protocol: Socket Concepts. * socket shutdown: Closing a Socket. * socket, client actions: Connecting. * socket, closing: Closing a Socket. * socket, connecting: Connecting. * socket, creating: Creating a Socket. * socket, initiating a connection: Connecting. * sockets, accepting connections: Accepting Connections. * sockets, listening: Listening. * sockets, server actions: Listening. * soft limit: Limits on Resources. * soft link: Symbolic Links. * sort function (for arrays): Array Sort Function. * sparse files: File Position Primitive. * special files: Making Special Files. * specified action (for a signal): Delivery of Signal. * square root function: Exponents and Logarithms. * stable sorting: Array Sort Function. * standard dot notation, for Internet addresses: Abstract Host Addresses. * standard environment variables: Standard Environment. * standard error file descriptor: Descriptors and Streams. * standard error stream: Standard Streams. * standard file descriptors: Descriptors and Streams. * standard input file descriptor: Descriptors and Streams. * standard input stream: Standard Streams. * standard output file descriptor: Descriptors and Streams. * standard output stream: Standard Streams. * standard streams: Standard Streams. * standards: Standards and Portability. * START character: Start/Stop Characters. * startup of program: Program Arguments. * static allocation: Memory Concepts. * STATUS character: Other Special. * status codes: Error Reporting. * status of a file: Attribute Meanings. * status of obstack: Status of an Obstack. * sticky bit: Permission Bits. * STOP character: Start/Stop Characters. * stop signal: Job Control Signals. * stopped job: Concepts of Job Control. * stopped jobs, continuing: Continuing Stopped Jobs. * stopped jobs, detecting: Stopped and Terminated Jobs. * storage allocation: Memory Allocation. * stream (sockets): Socket Concepts. * stream, for I/O to a string: String Streams. * streams and descriptors: Stream/Descriptor Precautions. * streams, and file descriptors: Descriptors and Streams. * streams, standard: Standard Streams. * string: Representation of Strings. * string allocation: Representation of Strings. * string collation functions: Collation Functions. * string comparison functions: String/Array Comparison. * string concatenation functions: Copying and Concatenation. * string copy functions: Copying and Concatenation. * string length: Representation of Strings. * string literal: Representation of Strings. * string search functions: Search Functions. * string stream: String Streams. * string, representation of: Representation of Strings. * style of communication (of a socket): Socket Concepts. * subshell: Initializing the Shell. * substitution of variables and commands: Expansion Stages. * successive signals: Merged Signals. * summer time: Broken-down Time. * SunOS: Berkeley Unix. * supplementary group IDs: Process Persona. * SUSP character: Signal Characters. * suspend character: Signal Characters. * SVID: SVID. * symbolic link: Symbolic Links. * symbolic link, opening: Open-time Flags. * syntax, for program arguments: Argument Syntax. * syntax, for reading numbers: Parsing of Numbers. * System V Unix: SVID. * TCP (Internet protocol): Protocols Database. * template, for printf: Formatted Output. * template, for scanf: Formatted Input. * TERM environment variable: Standard Environment. * terminal flow control: Line Control. * terminal identification: Is It a Terminal. * terminal input queue: I/O Queues. * terminal input queue, clearing: Line Control. * terminal input signal: Job Control Signals. * terminal line control functions: Line Control. * terminal line speed: Line Speed. * terminal mode data types: Mode Data Types. * terminal mode functions: Mode Functions. * terminal output queue: I/O Queues. * terminal output queue, flushing: Line Control. * terminal output signal: Job Control Signals. * terminated jobs, detecting: Stopped and Terminated Jobs. * termination signal: Termination Signals. * testing access permission: Testing File Access. * testing exit status of child process: Process Completion. * text stream: Binary Streams. * ticks, clock: Processor Time. * tilde expansion: Expansion Stages. * TIME termios slot: Noncanonical Input. * time zone: TZ Variable. * time zone database: TZ Variable. * time, calendar: Calendar Time. * time, elapsed CPU: Processor Time. * timer, profiling: Setting an Alarm. * timer, real-time: Setting an Alarm. * timer, virtual: Setting an Alarm. * timers, setting: Setting an Alarm. * timing error in signal handling: Remembering a Signal. * TMPDIR environment variable: Temporary Files. * tokenizing strings: Finding Tokens in a String. * tools, for installing library: Tools for Installation. * transmitting datagrams: Sending Datagrams. * trigonometric functions: Trig Functions. * type measurements, floating: Floating Type Macros. * type measurements, integer: Width of Type. * type modifier character (printf): Output Conversion Syntax. * type modifier character (scanf): Input Conversion Syntax. * typeahead buffer: I/O Queues. * TZ environment variable: Standard Environment. * umask: Setting Permissions. * unbuffered stream: Buffering Concepts. * unconstrained storage allocation: Unconstrained Allocation. * undefining macros that shadow functions: Macro Definitions. * Unix, Berkeley: Berkeley Unix. * Unix, System V: SVID. * unlinking a file: Deleting Files. * unreading characters: Unreading. * upper-case character: Classification of Characters. * urgent data signal: Asynchronous I/O Signals. * urgent socket condition: Out-of-Band Data. * usage limits: Limits on Resources. * user database: User Database. * user ID: User and Group IDs. * user ID, determining: Who Logged In. * user name: User and Group IDs. * user signals: Miscellaneous Signals. * usual file name errors: File Name Errors. * variable number of arguments: Variadic Functions. * variable substitution: Expansion Stages. * variable-sized arrays: GNU C Variable-Size Arrays. * variadic function argument access: Receiving Arguments. * variadic function prototypes: Variadic Prototypes. * variadic functions: Variadic Functions. * variadic functions, calling: Calling Variadics. * virtual time alarm signal: Alarm Signals. * virtual timer: Setting an Alarm. * waiting for a signal: Waiting for a Signal. * waiting for completion of child process: Process Completion. * waiting for input or output: Waiting for I/O. * WERASE character: Editing Characters. * whitespace character: Classification of Characters. * wide characters: Extended Char Intro. * width of integer type: Width of Type. * wildcard expansion: Expansion Stages. * word expansion: Word Expansion. * working directory: Working Directory. * write lock: File Locks. * writing to a file descriptor: I/O Primitives. * writing to a socket: Transferring Data. * writing to a stream, by blocks: Block Input/Output. * writing to a stream, by characters: Simple Output. * writing to a stream, formatted: Formatted Output. Type Index ********** * Menu: * cc_t: Mode Data Types. * clock_t: Basic CPU Time. * comparison_fn_t: Comparison Functions. * cookie_close_function: Hook Functions. * cookie_io_functions_t: Streams and Cookies. * cookie_read_function: Hook Functions. * cookie_seek_function: Hook Functions. * cookie_write_function: Hook Functions. * dev_t: Attribute Meanings. * DIR: Opening a Directory. * div_t: Integer Division. * enum mcheck_status: Heap Consistency Checking. * fd_set: Waiting for I/O. * FILE: Streams. * fpos_t: Portable Positioning. * gid_t: Reading Persona. * glob_t: Calling Glob. * ino_t: Attribute Meanings. * jmp_buf: Non-Local Details. * ldiv_t: Integer Division. * mode_t: Attribute Meanings. * nlink_t: Attribute Meanings. * off_t: File Position Primitive. * pid_t: Process Identification. * printf_arginfo_function: Defining the Output Handler. * printf_function: Defining the Output Handler. * ptrdiff_t: Important Data Types. * regex_t: POSIX Regexp Compilation. * regmatch_t: Regexp Subexpressions. * regoff_t: Regexp Subexpressions. * sig_atomic_t: Atomic Types. * sighandler_t: Basic Signal Handling. * sigjmp_buf: Non-Local Exits and Signals. * sigset_t: Signal Sets. * size_t: Important Data Types. * speed_t: Line Speed. * ssize_t: I/O Primitives. * struct dirent: Directory Entries. * struct flock: File Locks. * struct group: Group Data Structure. * struct hostent: Host Names. * struct in6_addr: Host Address Data Type. * struct in_addr: Host Address Data Type. * struct itimerval: Setting an Alarm. * struct lconv: Numeric Formatting. * struct linger: Socket-Level Options. * struct mallinfo: Statistics of Malloc. * struct netent: Networks Database. * struct obstack: Creating Obstacks. * struct option: Long Options. * struct passwd: User Data Structure. * struct printf_info: Conversion Specifier Options. * struct protoent: Protocols Database. * struct rlimit: Limits on Resources. * struct rusage: Resource Usage. * struct servent: Services Database. * struct sigaction: Advanced Signal Handling. * struct sigaltstack: Signal Stack. * struct sigstack: Signal Stack. * struct sigvec: BSD Handler. * struct sockaddr: Address Formats. * struct sockaddr_in: Internet Address Formats. * struct sockaddr_in6: Internet Address Formats. * struct sockaddr_un: File Namespace Details. * struct stat: Attribute Meanings. * struct termios: Mode Data Types. * struct timeval: High-Resolution Calendar. * struct timezone: High-Resolution Calendar. * struct tm: Broken-down Time. * struct tms: Detailed CPU Time. * struct utimbuf: File Times. * struct utsname: Hardware/Software Type ID. * tcflag_t: Mode Data Types. * time_t: Simple Calendar Time. * uid_t: Reading Persona. * union wait: BSD Wait Functions. * va_list: Argument Macros. * wchar_t: Wide Char Intro. * wordexp_t: Calling Wordexp. Function and Macro Index ************************ * Menu: * _exit: Termination Internals. * _tolower: Case Conversion. * _toupper: Case Conversion. * abort: Aborting a Program. * abs: Absolute Value. * accept: Accepting Connections. * access: Testing File Access. * acos: Inverse Trig Functions. * acosh: Hyperbolic Functions. * adjtime: High-Resolution Calendar. * alarm: Setting an Alarm. * alloca: Variable Size Automatic. * alphasort: Scanning Directory Content. * asctime: Formatting Date and Time. * asin: Inverse Trig Functions. * asinh: Hyperbolic Functions. * asprintf: Dynamic Output. * assert: Consistency Checking. * assert_perror: Consistency Checking. * atan: Inverse Trig Functions. * atan2: Inverse Trig Functions. * atanh: Hyperbolic Functions. * atexit: Cleanups on Exit. * atof: Parsing of Floats. * atoi: Parsing of Integers. * atol: Parsing of Integers. * bcmp: String/Array Comparison. * bcopy: Copying and Concatenation. * bind: Setting Address. * bsearch: Array Search Function. * bzero: Copying and Concatenation. * cabs: Absolute Value. * calloc: Allocating Cleared Space. * cbrt: Exponents and Logarithms. * ceil: Rounding and Remainders. * cfgetispeed: Line Speed. * cfgetospeed: Line Speed. * cfmakeraw: Noncanonical Input. * cfree: Freeing after Malloc. * cfsetispeed: Line Speed. * cfsetospeed: Line Speed. * cfsetspeed: Line Speed. * chdir: Working Directory. * chmod: Setting Permissions. * chown: File Owner. * clearerr: EOF and Errors. * clock: Basic CPU Time. * close: Opening and Closing Files. * closedir: Reading/Closing Directory. * confstr: String Parameters. * connect: Connecting. * copysign: Normalization Functions. * cos: Trig Functions. * cosh: Hyperbolic Functions. * creat: Opening and Closing Files. * ctermid: Identifying the Terminal. * ctime: Formatting Date and Time. * cuserid: Who Logged In. * difftime: Simple Calendar Time. * div: Integer Division. * drem: Rounding and Remainders. * DTTOIF: Directory Entries. * dup: Duplicating Descriptors. * dup2: Duplicating Descriptors. * endgrent: Scanning All Groups. * endhostent: Host Names. * endnetent: Networks Database. * endnetgrent: Lookup Netgroup. * endprotoent: Protocols Database. * endpwent: Scanning All Users. * endservent: Services Database. * execl: Executing a File. * execle: Executing a File. * execlp: Executing a File. * execv: Executing a File. * execve: Executing a File. * execvp: Executing a File. * exit: Normal Termination. * exp: Exponents and Logarithms. * expm1: Exponents and Logarithms. * fabs: Absolute Value. * fchmod: Setting Permissions. * fchown: File Owner. * fclean: Cleaning Streams. * fclose: Closing Streams. * fcloseall: Closing Streams. * fcntl: Control Operations. * FD_CLR: Waiting for I/O. * FD_ISSET: Waiting for I/O. * FD_SET: Waiting for I/O. * FD_ZERO: Waiting for I/O. * fdopen: Descriptors and Streams. * feof: EOF and Errors. * ferror: EOF and Errors. * fflush: Flushing Buffers. * fgetc: Character Input. * fgetgrent: Scanning All Groups. * fgetgrent_r: Scanning All Groups. * fgetpos: Portable Positioning. * fgetpwent: Scanning All Users. * fgetpwent_r: Scanning All Users. * fgets: Line Input. * fileno: Descriptors and Streams. * finite: Predicates on Floats. * floor: Rounding and Remainders. * fmemopen: String Streams. * fmod: Rounding and Remainders. * fnmatch: Wildcard Matching. * fopen: Opening Streams. * fopencookie: Streams and Cookies. * fork: Creating a Process. * fpathconf: Pathconf. * fprintf: Formatted Output Functions. * fputc: Simple Output. * fputs: Simple Output. * fread: Block Input/Output. * free: Freeing after Malloc. * freopen: Opening Streams. * frexp: Normalization Functions. * fscanf: Formatted Input Functions. * fseek: File Positioning. * fsetpos: Portable Positioning. * fstat: Reading Attributes. * ftell: File Positioning. * fwrite: Block Input/Output. * getc: Character Input. * getchar: Character Input. * getcwd: Working Directory. * getdelim: Line Input. * getegid: Reading Persona. * getenv: Environment Access. * geteuid: Reading Persona. * getgid: Reading Persona. * getgrent: Scanning All Groups. * getgrent_r: Scanning All Groups. * getgrgid: Lookup Group. * getgrgid_r: Lookup Group. * getgrnam: Lookup Group. * getgrnam_r: Lookup Group. * getgroups: Reading Persona. * gethostbyaddr: Host Names. * gethostbyname: Host Names. * gethostbyname2: Host Names. * gethostent: Host Names. * gethostid: Host Identification. * gethostname: Host Identification. * getitimer: Setting an Alarm. * getline: Line Input. * getlogin: Who Logged In. * getnetbyaddr: Networks Database. * getnetbyname: Networks Database. * getnetent: Networks Database. * getnetgrent: Lookup Netgroup. * getnetgrent_r: Lookup Netgroup. * getopt: Parsing Options. * getopt_long: Long Options. * getpeername: Who is Connected. * getpgrp: Process Group Functions. * getpid: Process Identification. * getppid: Process Identification. * getpriority: Priority. * getprotobyname: Protocols Database. * getprotobynumber: Protocols Database. * getprotoent: Protocols Database. * getpwent: Scanning All Users. * getpwent_r: Scanning All Users. * getpwnam: Lookup User. * getpwnam_r: Lookup User. * getpwuid: Lookup User. * getpwuid_r: Lookup User. * getrlimit: Limits on Resources. * getrusage: Resource Usage. * gets: Line Input. * getservbyname: Services Database. * getservbyport: Services Database. * getservent: Services Database. * getsockname: Reading Address. * getsockopt: Socket Option Functions. * getsubopt: Suboptions. * gettimeofday: High-Resolution Calendar. * getuid: Reading Persona. * getumask: Setting Permissions. * getw: Character Input. * getwd: Working Directory. * glob: Calling Glob. * gmtime: Broken-down Time. * gsignal: Signaling Yourself. * htonl: Byte Order. * htons: Byte Order. * hypot: Exponents and Logarithms. * IFTODT: Directory Entries. * index: Search Functions. * inet_addr: Host Address Functions. * inet_aton: Host Address Functions. * inet_lnaof: Host Address Functions. * inet_makeaddr: Host Address Functions. * inet_netof: Host Address Functions. * inet_network: Host Address Functions. * inet_ntoa: Host Address Functions. * inet_ntop: Host Address Functions. * inet_pton: Host Address Functions. * infnan: Predicates on Floats. * initgroups: Setting Groups. * initstate: BSD Random. * innetgr: Netgroup Membership. * isalnum: Classification of Characters. * isalpha: Classification of Characters. * isascii: Classification of Characters. * isatty: Is It a Terminal. * isblank: Classification of Characters. * iscntrl: Classification of Characters. * isdigit: Classification of Characters. * isgraph: Classification of Characters. * isinf: Predicates on Floats. * islower: Classification of Characters. * isnan: Predicates on Floats. * isprint: Classification of Characters. * ispunct: Classification of Characters. * isspace: Classification of Characters. * isupper: Classification of Characters. * isxdigit: Classification of Characters. * ITIMER_PROF: Setting an Alarm. * ITIMER_REAL: Setting an Alarm. * ITIMER_VIRTUAL: Setting an Alarm. * kill: Signaling Another Process. * killpg: Signaling Another Process. * labs: Absolute Value. * ldexp: Normalization Functions. * ldiv: Integer Division. * link: Hard Links. * listen: Listening. * localeconv: Numeric Formatting. * localtime: Broken-down Time. * log: Exponents and Logarithms. * log10: Exponents and Logarithms. * log1p: Exponents and Logarithms. * logb: Normalization Functions. * longjmp: Non-Local Details. * lseek: File Position Primitive. * lstat: Reading Attributes. * main: Program Arguments. * mallinfo: Statistics of Malloc. * malloc: Basic Allocation. * mallopt: Malloc Tunable Parameters. * mblen: Length of Char. * mbstowcs: Wide String Conversion. * mbtowc: Converting One Char. * mcheck: Heap Consistency Checking. * memalign: Aligned Memory Blocks. * memccpy: Copying and Concatenation. * memchr: Search Functions. * memcmp: String/Array Comparison. * memcpy: Copying and Concatenation. * memmem: Search Functions. * memmove: Copying and Concatenation. * memset: Copying and Concatenation. * mkdir: Creating Directories. * mkfifo: FIFO Special Files. * mknod: Making Special Files. * mkstemp: Temporary Files. * mktemp: Temporary Files. * mktime: Broken-down Time. * modf: Rounding and Remainders. * mprobe: Heap Consistency Checking. * nice: Priority. * notfound: Actions in the NSS configuration. * NSS_STATUS_NOTFOUND: NSS Modules Interface. * NSS_STATUS_SUCCESS: NSS Modules Interface. * NSS_STATUS_TRYAGAIN: NSS Modules Interface. * NSS_STATUS_UNAVAIL: NSS Modules Interface. * ntohl: Byte Order. * ntohs: Byte Order. * obstack_1grow: Growing Objects. * obstack_1grow_fast: Extra Fast Growing. * obstack_alignment_mask: Obstacks Data Alignment. * obstack_alloc: Allocation in an Obstack. * obstack_base: Status of an Obstack. * obstack_blank: Growing Objects. * obstack_blank_fast: Extra Fast Growing. * obstack_chunk_alloc: Preparing for Obstacks. * obstack_chunk_free: Preparing for Obstacks. * obstack_chunk_size: Obstack Chunks. * obstack_copy: Allocation in an Obstack. * obstack_copy0: Allocation in an Obstack. * obstack_finish: Growing Objects. * obstack_free: Freeing Obstack Objects. * obstack_grow: Growing Objects. * obstack_grow0: Growing Objects. * obstack_init: Preparing for Obstacks. * obstack_int_grow: Growing Objects. * obstack_int_grow_fast: Extra Fast Growing. * obstack_next_free: Status of an Obstack. * obstack_object_size <1>: Status of an Obstack. * obstack_object_size: Growing Objects. * obstack_printf: Dynamic Output. * obstack_ptr_grow: Growing Objects. * obstack_ptr_grow_fast: Extra Fast Growing. * obstack_room: Extra Fast Growing. * obstack_vprintf: Variable Arguments Output. * offsetof: Structure Measurement. * on_exit: Cleanups on Exit. * open: Opening and Closing Files. * open_memstream: String Streams. * open_obstack_stream: Obstack Streams. * opendir: Opening a Directory. * parse_printf_format: Parsing a Template String. * pathconf: Pathconf. * pause: Using Pause. * pclose: Pipe to a Subprocess. * perror: Error Messages. * pipe: Creating a Pipe. * popen: Pipe to a Subprocess. * pow: Exponents and Logarithms. * printf: Formatted Output Functions. * psignal: Signal Messages. * putc: Simple Output. * putchar: Simple Output. * putenv: Environment Access. * putpwent: Writing a User Entry. * puts: Simple Output. * putw: Simple Output. * qsort: Array Sort Function. * r_alloc: Using Relocator. * r_alloc_free: Using Relocator. * r_re_alloc: Using Relocator. * raise: Signaling Yourself. * rand: ISO Random. * random: BSD Random. * read: I/O Primitives. * readdir: Reading/Closing Directory. * readdir_r: Reading/Closing Directory. * readlink: Symbolic Links. * realloc: Changing Block Size. * recv: Receiving Data. * recvfrom: Receiving Datagrams. * regcomp: POSIX Regexp Compilation. * regerror: Regexp Cleanup. * regexec: Matching POSIX Regexps. * regfree: Regexp Cleanup. * register_printf_function: Registering New Conversions. * remove: Deleting Files. * rename: Renaming Files. * rewind: File Positioning. * rewinddir: Random Access Directory. * rindex: Search Functions. * rint: Rounding and Remainders. * rmdir: Deleting Files. * S_ISBLK: Testing File Type. * S_ISCHR: Testing File Type. * S_ISDIR: Testing File Type. * S_ISFIFO: Testing File Type. * S_ISLNK: Testing File Type. * S_ISREG: Testing File Type. * S_ISSOCK: Testing File Type. * scalb: Normalization Functions. * scandir: Scanning Directory Content. * scanf: Formatted Input Functions. * seekdir: Random Access Directory. * select: Waiting for I/O. * send: Sending Data. * sendto: Sending Datagrams. * setbuf: Controlling Buffering. * setbuffer: Controlling Buffering. * setgid: Setting Groups. * setgrent: Scanning All Groups. * setgroups: Setting Groups. * sethostent: Host Names. * sethostid: Host Identification. * sethostname: Host Identification. * setitimer: Setting an Alarm. * setjmp: Non-Local Details. * setlinebuf: Controlling Buffering. * setlocale: Setting the Locale. * setnetent: Networks Database. * setnetgrent: Lookup Netgroup. * setpgid: Process Group Functions. * setpgrp: Process Group Functions. * setpriority: Priority. * setprotoent: Protocols Database. * setpwent: Scanning All Users. * setregid: Setting Groups. * setreuid: Setting User ID. * setrlimit: Limits on Resources. * setservent: Services Database. * setsid: Process Group Functions. * setsockopt: Socket Option Functions. * setstate: BSD Random. * settimeofday: High-Resolution Calendar. * setuid: Setting User ID. * setvbuf: Controlling Buffering. * shutdown: Closing a Socket. * sigaction: Advanced Signal Handling. * sigaddset: Signal Sets. * sigaltstack: Signal Stack. * sigblock: Blocking in BSD. * sigdelset: Signal Sets. * sigemptyset: Signal Sets. * sigfillset: Signal Sets. * siginterrupt: BSD Handler. * sigismember: Signal Sets. * siglongjmp: Non-Local Exits and Signals. * sigmask: Blocking in BSD. * signal: Basic Signal Handling. * sigpause: Blocking in BSD. * sigpending: Checking for Pending Signals. * sigprocmask: Process Signal Mask. * sigsetjmp: Non-Local Exits and Signals. * sigsetmask: Blocking in BSD. * sigstack: Signal Stack. * sigsuspend: Sigsuspend. * sigvec: BSD Handler. * sin: Trig Functions. * sinh: Hyperbolic Functions. * sleep: Sleeping. * snprintf: Formatted Output Functions. * socket: Creating a Socket. * socketpair: Socket Pairs. * sprintf: Formatted Output Functions. * sqrt: Exponents and Logarithms. * srand: ISO Random. * srandom: BSD Random. * sscanf: Formatted Input Functions. * ssignal: Basic Signal Handling. * stat: Reading Attributes. * stpcpy: Copying and Concatenation. * stpncpy: Copying and Concatenation. * strcasecmp: String/Array Comparison. * strcat: Copying and Concatenation. * strchr: Search Functions. * strcmp: String/Array Comparison. * strcoll: Collation Functions. * strcpy: Copying and Concatenation. * strcspn: Search Functions. * strdup: Copying and Concatenation. * strdupa: Copying and Concatenation. * strerror: Error Messages. * strerror_r: Error Messages. * strftime: Formatting Date and Time. * strlen: String Length. * strncasecmp: String/Array Comparison. * strncat: Copying and Concatenation. * strncmp: String/Array Comparison. * strncpy: Copying and Concatenation. * strndup: Copying and Concatenation. * strndupa: Copying and Concatenation. * strpbrk: Search Functions. * strrchr: Search Functions. * strsep: Finding Tokens in a String. * strsignal: Signal Messages. * strspn: Search Functions. * strstr: Search Functions. * strtod: Parsing of Floats. * strtof: Parsing of Floats. * strtok: Finding Tokens in a String. * strtok_r: Finding Tokens in a String. * strtol: Parsing of Integers. * strtold: Parsing of Floats. * strtoll: Parsing of Integers. * strtoq: Parsing of Integers. * strtoul: Parsing of Integers. * strtoull: Parsing of Integers. * strtouq: Parsing of Integers. * strxfrm: Collation Functions. * success: Actions in the NSS configuration. * symlink: Symbolic Links. * sysconf: Sysconf Definition. * system: Running a Command. * tan: Trig Functions. * tanh: Hyperbolic Functions. * tcdrain: Line Control. * tcflow: Line Control. * tcflush: Line Control. * tcgetattr: Mode Functions. * tcgetpgrp: Terminal Access Functions. * tcsendbreak: Line Control. * tcsetattr: Mode Functions. * tcsetpgrp: Terminal Access Functions. * telldir: Random Access Directory. * TEMP_FAILURE_RETRY: Interrupted Primitives. * tempnam: Temporary Files. * time: Simple Calendar Time. * times: Detailed CPU Time. * tmpfile: Temporary Files. * tmpnam: Temporary Files. * tmpnam_r: Temporary Files. * toascii: Case Conversion. * tolower: Case Conversion. * toupper: Case Conversion. * tryagain: Actions in the NSS configuration. * ttyname: Is It a Terminal. * tzset: Time Zone Functions. * umask: Setting Permissions. * uname: Hardware/Software Type ID. * unavail: Actions in the NSS configuration. * ungetc: How Unread. * unlink: Deleting Files. * utime: File Times. * utimes: File Times. * va_alist: Old Varargs. * va_arg: Argument Macros. * va_dcl: Old Varargs. * va_end: Argument Macros. * va_start <1>: Old Varargs. * va_start: Argument Macros. * valloc: Aligned Memory Blocks. * vasprintf: Variable Arguments Output. * vfork: Creating a Process. * vfprintf: Variable Arguments Output. * vfscanf: Variable Arguments Input. * vprintf: Variable Arguments Output. * vscanf: Variable Arguments Input. * vsnprintf: Variable Arguments Output. * vsprintf: Variable Arguments Output. * vsscanf: Variable Arguments Input. * wait: Process Completion. * wait3: BSD Wait Functions. * wait4: Process Completion. * waitpid: Process Completion. * WCOREDUMP: Process Completion Status. * wcstombs: Wide String Conversion. * wctomb: Converting One Char. * WEXITSTATUS: Process Completion Status. * WIFEXITED: Process Completion Status. * WIFSIGNALED: Process Completion Status. * WIFSTOPPED: Process Completion Status. * wordexp: Calling Wordexp. * wordfree: Calling Wordexp. * write: I/O Primitives. * WSTOPSIG: Process Completion Status. * WTERMSIG: Process Completion Status. Variable and Constant Macro Index ********************************* * Menu: * __free_hook: Hooks for Malloc. * __malloc_hook: Hooks for Malloc. * __realloc_hook: Hooks for Malloc. * _BSD_SOURCE: Feature Test Macros. * _GNU_SOURCE: Feature Test Macros. * _IOFBF: Controlling Buffering. * _IOLBF: Controlling Buffering. * _IONBF: Controlling Buffering. * _POSIX2_C_DEV: System Options. * _POSIX2_C_VERSION: Version Supported. * _POSIX2_FORT_DEV: System Options. * _POSIX2_FORT_RUN: System Options. * _POSIX2_LOCALEDEF: System Options. * _POSIX2_SW_DEV: System Options. * _POSIX_C_SOURCE: Feature Test Macros. * _POSIX_CHOWN_RESTRICTED: Options for Files. * _POSIX_JOB_CONTROL: System Options. * _POSIX_NO_TRUNC: Options for Files. * _POSIX_SAVED_IDS: System Options. * _POSIX_SOURCE: Feature Test Macros. * _POSIX_VDISABLE <1>: Options for Files. * _POSIX_VDISABLE: Special Characters. * _POSIX_VERSION: Version Supported. * _REENTRANT: Feature Test Macros. * _SVID_SOURCE: Feature Test Macros. * _THREAD_SAFE: Feature Test Macros. * _XOPEN_SOURCE: Feature Test Macros. * AF_FILE: Address Formats. * AF_INET: Address Formats. * AF_UNIX: Address Formats. * AF_UNSPEC: Address Formats. * aliases: NSS Basics. * ALTWERASE: Local Modes. * ARG_MAX: General Limits. * B0: Line Speed. * B110: Line Speed. * B1200: Line Speed. * B134: Line Speed. * B150: Line Speed. * B1800: Line Speed. * B19200: Line Speed. * B200: Line Speed. * B2400: Line Speed. * B300: Line Speed. * B38400: Line Speed. * B4800: Line Speed. * B50: Line Speed. * B600: Line Speed. * B75: Line Speed. * B9600: Line Speed. * BC_BASE_MAX: Utility Limits. * BC_DIM_MAX: Utility Limits. * BC_SCALE_MAX: Utility Limits. * BC_STRING_MAX: Utility Limits. * BRKINT: Input Modes. * BUFSIZ: Controlling Buffering. * CCTS_OFLOW: Control Modes. * CHILD_MAX: General Limits. * CIGNORE: Control Modes. * CLK_TCK: Basic CPU Time. * CLOCAL: Control Modes. * CLOCKS_PER_SEC: Basic CPU Time. * COLL_WEIGHTS_MAX: Utility Limits. * COREFILE: Program Error Signals. * CREAD: Control Modes. * CRTS_IFLOW: Control Modes. * CS5: Control Modes. * CS6: Control Modes. * CS7: Control Modes. * CS8: Control Modes. * CSIZE: Control Modes. * CSTOPB: Control Modes. * daylight: Time Zone Functions. * E2BIG: Error Codes. * EACCES: Error Codes. * EADDRINUSE: Error Codes. * EADDRNOTAVAIL: Error Codes. * EADV: Error Codes. * EAFNOSUPPORT: Error Codes. * EAGAIN: Error Codes. * EALREADY: Error Codes. * EAUTH: Error Codes. * EBACKGROUND: Error Codes. * EBADE: Error Codes. * EBADF <1>: Line Control. * EBADF: Error Codes. * EBADFD: Error Codes. * EBADMSG: Error Codes. * EBADR: Error Codes. * EBADRPC: Error Codes. * EBADRQC: Error Codes. * EBADSLT: Error Codes. * EBFONT: Error Codes. * EBUSY: Error Codes. * ECHILD: Error Codes. * ECHO: Local Modes. * ECHOCTL: Local Modes. * ECHOE: Local Modes. * ECHOK: Local Modes. * ECHOKE: Local Modes. * ECHONL: Local Modes. * ECHOPRT: Local Modes. * ECHRNG: Error Codes. * ECOMM: Error Codes. * ECONNABORTED: Error Codes. * ECONNREFUSED: Error Codes. * ECONNRESET: Error Codes. * ED: Error Codes. * EDEADLK: Error Codes. * EDEADLOCK: Error Codes. * EDESTADDRREQ: Error Codes. * EDIED: Error Codes. * EDOM: Error Codes. * EDOTDOT: Error Codes. * EDQUOT: Error Codes. * EEXIST: Error Codes. * EFAULT: Error Codes. * EFBIG: Error Codes. * EFTYPE: Error Codes. * EGRATUITOUS: Error Codes. * EGREGIOUS: Error Codes. * EHOSTDOWN: Error Codes. * EHOSTUNREACH: Error Codes. * EIDRM: Error Codes. * EIEIO: Error Codes. * EILSEQ: Error Codes. * EINPROGRESS: Error Codes. * EINTR: Error Codes. * EINVAL <1>: Line Control. * EINVAL: Error Codes. * EIO: Error Codes. * EISCONN: Error Codes. * EISDIR: Error Codes. * EISNAM: Error Codes. * EL2HLT: Error Codes. * EL2NSYNC: Error Codes. * EL3HLT: Error Codes. * EL3RST: Error Codes. * ELIBACC: Error Codes. * ELIBBAD: Error Codes. * ELIBEXEC: Error Codes. * ELIBMAX: Error Codes. * ELIBSCN: Error Codes. * ELNRNG: Error Codes. * ELOOP: Error Codes. * EMEDIUMTYPE: Error Codes. * EMFILE: Error Codes. * EMLINK: Error Codes. * EMSGSIZE: Error Codes. * EMULTIHOP: Error Codes. * ENAMETOOLONG: Error Codes. * ENAVAIL: Error Codes. * ENEEDAUTH: Error Codes. * ENETDOWN: Error Codes. * ENETRESET: Error Codes. * ENETUNREACH: Error Codes. * ENFILE: Error Codes. * ENOANO: Error Codes. * ENOBUFS: Error Codes. * ENOCSI: Error Codes. * ENODATA: Error Codes. * ENODEV: Error Codes. * ENOENT: Error Codes. * ENOEXEC: Error Codes. * ENOLCK: Error Codes. * ENOLINK: Error Codes. * ENOMEDIUM: Error Codes. * ENOMEM: Error Codes. * ENOMSG: Error Codes. * ENONET: Error Codes. * ENOPKG: Error Codes. * ENOPROTOOPT: Error Codes. * ENOSPC: Error Codes. * ENOSR: Error Codes. * ENOSTR: Error Codes. * ENOSYS: Error Codes. * ENOTBLK: Error Codes. * ENOTCONN: Error Codes. * ENOTDIR: Error Codes. * ENOTEMPTY: Error Codes. * ENOTNAM: Error Codes. * ENOTSOCK: Error Codes. * ENOTTY <1>: Line Control. * ENOTTY: Error Codes. * ENOTUNIQ: Error Codes. * environ: Environment Access. * ENXIO: Error Codes. * EOF: EOF and Errors. * EOPNOTSUPP: Error Codes. * EOVERFLOW: Error Codes. * EPERM: Error Codes. * EPFNOSUPPORT: Error Codes. * EPIPE: Error Codes. * EPROCLIM: Error Codes. * EPROCUNAVAIL: Error Codes. * EPROGMISMATCH: Error Codes. * EPROGUNAVAIL: Error Codes. * EPROTO: Error Codes. * EPROTONOSUPPORT: Error Codes. * EPROTOTYPE: Error Codes. * EQUIV_CLASS_MAX: Utility Limits. * ERANGE: Error Codes. * EREMCHG: Error Codes. * EREMOTE: Error Codes. * EREMOTEIO: Error Codes. * ERESTART: Error Codes. * EROFS: Error Codes. * ERPCMISMATCH: Error Codes. * errno: Checking for Errors. * ESHUTDOWN: Error Codes. * ESOCKTNOSUPPORT: Error Codes. * ESPIPE: Error Codes. * ESRCH: Error Codes. * ESRMNT: Error Codes. * ESTALE: Error Codes. * ESTRPIPE: Error Codes. * ethers: NSS Basics. * ETIME: Error Codes. * ETIMEDOUT: Error Codes. * ETOOMANYREFS: Error Codes. * ETXTBSY: Error Codes. * EUCLEAN: Error Codes. * EUNATCH: Error Codes. * EUSERS: Error Codes. * EWOULDBLOCK: Error Codes. * EXDEV: Error Codes. * EXFULL: Error Codes. * EXIT_FAILURE: Exit Status. * EXIT_SUCCESS: Exit Status. * EXPR_NEST_MAX: Utility Limits. * EXTA: Line Speed. * EXTB: Line Speed. * F_DUPFD: Duplicating Descriptors. * F_GETFD: Descriptor Flags. * F_GETFL: Getting File Status Flags. * F_GETLK: File Locks. * F_GETOWN: Interrupt Input. * F_OK: Testing File Access. * F_RDLCK: File Locks. * F_SETFD: Descriptor Flags. * F_SETFL: Getting File Status Flags. * F_SETLK: File Locks. * F_SETLKW: File Locks. * F_SETOWN: Interrupt Input. * F_UNLCK: File Locks. * F_WRLCK: File Locks. * FD_CLOEXEC: Descriptor Flags. * FD_SETSIZE: Waiting for I/O. * FILENAME_MAX: Limits for Files. * FLUSHO: Local Modes. * FOPEN_MAX: Opening Streams. * FPE_DECOVF_TRAP: Program Error Signals. * FPE_FLTDIV_TRAP: Program Error Signals. * FPE_FLTOVF_TRAP: Program Error Signals. * FPE_FLTUND_TRAP: Program Error Signals. * FPE_INTDIV_TRAP: Program Error Signals. * FPE_INTOVF_TRAP: Program Error Signals. * FPE_SUBRNG_TRAP: Program Error Signals. * group: NSS Basics. * h_errno: Host Names. * HOST_NOT_FOUND: Host Names. * hosts: NSS Basics. * HUGE_VAL: Domain and Range Errors. * HUGE_VALf: Domain and Range Errors. * HUGE_VALl: Domain and Range Errors. * HUPCL: Control Modes. * ICANON: Local Modes. * ICRNL: Input Modes. * IEXTEN: Local Modes. * IGNBRK: Input Modes. * IGNCR: Input Modes. * IGNPAR: Input Modes. * IMAXBEL: Input Modes. * in6addr_any: Host Address Data Type. * in6addr_loopback.: Host Address Data Type. * INADDR_ANY: Host Address Data Type. * INADDR_BROADCAST: Host Address Data Type. * INADDR_LOOPBACK: Host Address Data Type. * INADDR_NONE: Host Address Data Type. * INLCR: Input Modes. * INPCK: Input Modes. * int: Limits on Resources. * IPPORT_RESERVED: Ports. * IPPORT_USERRESERVED: Ports. * ISIG: Local Modes. * ISTRIP: Input Modes. * IXANY: Input Modes. * IXOFF: Input Modes. * IXON: Input Modes. * L_ctermid: Identifying the Terminal. * L_cuserid: Who Logged In. * L_INCR: File Positioning. * L_SET: File Positioning. * L_tmpnam: Temporary Files. * L_XTND: File Positioning. * LANG: Locale Categories. * LC_ALL: Locale Categories. * LC_COLLATE: Locale Categories. * LC_CTYPE: Locale Categories. * LC_MESSAGES: Locale Categories. * LC_MONETARY: Locale Categories. * LC_NUMERIC: Locale Categories. * LC_TIME: Locale Categories. * LINE_MAX: Utility Limits. * LINK_MAX: Limits for Files. * MAX_CANON: Limits for Files. * MAX_INPUT: Limits for Files. * MAXNAMLEN: Limits for Files. * MB_CUR_MAX: Multibyte Char Intro. * MB_LEN_MAX: Multibyte Char Intro. * MDMBUF: Control Modes. * MINSIGSTKSZ: Signal Stack. * MSG_DONTROUTE: Socket Data Options. * MSG_OOB: Socket Data Options. * MSG_PEEK: Socket Data Options. * NAME_MAX: Limits for Files. * NAN: Not a Number. * NCCS: Mode Data Types. * NDEBUG: Consistency Checking. * netgroup: NSS Basics. * network: NSS Basics. * NGROUPS_MAX: General Limits. * NO_ADDRESS: Host Names. * NO_RECOVERY: Host Names. * NOFLSH: Local Modes. * NOKERNINFO: Local Modes. * NSIG: Standard Signals. * NSS_STATUS_NOTFOUND: NSS Modules Interface. * NSS_STATUS_SUCCESS: NSS Modules Interface. * NSS_STATUS_TRYAGAIN: NSS Modules Interface. * NSS_STATUS_UNAVAIL: NSS Modules Interface. * NULL: Null Pointer Constant. * O_ACCMODE: Access Modes. * O_APPEND: Operating Modes. * O_ASYNC: Operating Modes. * O_CREAT: Open-time Flags. * O_EXCL: Open-time Flags. * O_EXEC: Access Modes. * O_EXLOCK: Open-time Flags. * O_FSYNC: Operating Modes. * O_IGNORE_CTTY: Open-time Flags. * O_NDELAY: Operating Modes. * O_NOATIME: Operating Modes. * O_NOCTTY: Open-time Flags. * O_NOLINK: Open-time Flags. * O_NONBLOCK <1>: Operating Modes. * O_NONBLOCK: Open-time Flags. * O_NOTRANS: Open-time Flags. * O_RDONLY: Access Modes. * O_RDWR: Access Modes. * O_READ: Access Modes. * O_SHLOCK: Open-time Flags. * O_SYNC: Operating Modes. * O_TRUNC: Open-time Flags. * O_WRITE: Access Modes. * O_WRONLY: Access Modes. * ONLCR: Output Modes. * ONOEOT: Output Modes. * OPEN_MAX: General Limits. * OPOST: Output Modes. * optarg: Parsing Options. * opterr: Parsing Options. * optind: Parsing Options. * optopt: Parsing Options. * OXTABS: Output Modes. * P_tmpdir: Temporary Files. * PA_CHAR: Parsing a Template String. * PA_DOUBLE: Parsing a Template String. * PA_FLAG_LONG: Parsing a Template String. * PA_FLAG_LONG_DOUBLE: Parsing a Template String. * PA_FLAG_LONG_LONG: Parsing a Template String. * PA_FLAG_MASK: Parsing a Template String. * PA_FLAG_PTR: Parsing a Template String. * PA_FLAG_SHORT: Parsing a Template String. * PA_FLOAT: Parsing a Template String. * PA_INT: Parsing a Template String. * PA_LAST: Parsing a Template String. * PA_POINTER: Parsing a Template String. * PA_STRING: Parsing a Template String. * PARENB: Control Modes. * PARMRK: Input Modes. * PARODD: Control Modes. * passwd: NSS Basics. * PATH_MAX: Limits for Files. * PENDIN: Local Modes. * PF_CCITT: Misc Namespaces. * PF_FILE: File Namespace Details. * PF_IMPLINK: Misc Namespaces. * PF_INET: Internet Namespace. * PF_ISO: Misc Namespaces. * PF_NS: Misc Namespaces. * PF_ROUTE: Misc Namespaces. * PF_UNIX: File Namespace Details. * PIPE_BUF: Limits for Files. * PRIO_MAX: Priority. * PRIO_MIN: Priority. * PRIO_PGRP: Priority. * PRIO_PROCESS: Priority. * PRIO_USER: Priority. * program_invocation_name: Error Messages. * program_invocation_short_name: Error Messages. * protocols: NSS Basics. * R_OK: Testing File Access. * RAND_MAX: ISO Random. * RE_DUP_MAX: General Limits. * RLIM_NLIMITS: Limits on Resources. * RLIMIT_CORE: Limits on Resources. * RLIMIT_CPU: Limits on Resources. * RLIMIT_DATA: Limits on Resources. * RLIMIT_FSIZE: Limits on Resources. * RLIMIT_NOFILE: Limits on Resources. * RLIMIT_OFILE: Limits on Resources. * RLIMIT_RSS: Limits on Resources. * RLIMIT_STACK: Limits on Resources. * rpc: NSS Basics. * S_IEXEC: Permission Bits. * S_IFBLK: Testing File Type. * S_IFCHR: Testing File Type. * S_IFDIR: Testing File Type. * S_IFIFO: Testing File Type. * S_IFLNK: Testing File Type. * S_IFMT: Testing File Type. * S_IFREG: Testing File Type. * S_IFSOCK: Testing File Type. * S_IREAD: Permission Bits. * S_IRGRP: Permission Bits. * S_IROTH: Permission Bits. * S_IRUSR: Permission Bits. * S_IRWXG: Permission Bits. * S_IRWXO: Permission Bits. * S_IRWXU: Permission Bits. * S_ISGID: Permission Bits. * S_ISUID: Permission Bits. * S_ISVTX: Permission Bits. * S_IWGRP: Permission Bits. * S_IWOTH: Permission Bits. * S_IWRITE: Permission Bits. * S_IWUSR: Permission Bits. * S_IXGRP: Permission Bits. * S_IXOTH: Permission Bits. * S_IXUSR: Permission Bits. * SA_DISABLE: Signal Stack. * SA_NOCLDSTOP: Flags for Sigaction. * SA_ONSTACK <1>: Signal Stack. * SA_ONSTACK: Flags for Sigaction. * SA_RESTART: Flags for Sigaction. * SEEK_CUR: File Positioning. * SEEK_END: File Positioning. * SEEK_SET: File Positioning. * services: NSS Basics. * shadow: NSS Basics. * SIG_BLOCK: Process Signal Mask. * SIG_DFL: Basic Signal Handling. * SIG_ERR: Basic Signal Handling. * SIG_IGN: Basic Signal Handling. * SIG_SETMASK: Process Signal Mask. * SIG_UNBLOCK: Process Signal Mask. * SIGABRT: Program Error Signals. * SIGALRM: Alarm Signals. * SIGBUS: Program Error Signals. * SIGCHLD: Job Control Signals. * SIGCLD: Job Control Signals. * SIGCONT: Job Control Signals. * SIGEMT: Program Error Signals. * SIGFPE: Program Error Signals. * SIGHUP: Termination Signals. * SIGILL: Program Error Signals. * SIGINFO: Miscellaneous Signals. * SIGINT: Termination Signals. * SIGIO: Asynchronous I/O Signals. * SIGIOT: Program Error Signals. * SIGKILL: Termination Signals. * SIGLOST: Operation Error Signals. * SIGPIPE: Operation Error Signals. * SIGPOLL: Asynchronous I/O Signals. * SIGPROF: Alarm Signals. * SIGQUIT: Termination Signals. * SIGSEGV: Program Error Signals. * SIGSTKSZ: Signal Stack. * SIGSTOP: Job Control Signals. * SIGSYS: Program Error Signals. * SIGTERM: Termination Signals. * SIGTRAP: Program Error Signals. * SIGTSTP: Job Control Signals. * SIGTTIN: Job Control Signals. * SIGTTOU: Job Control Signals. * SIGURG: Asynchronous I/O Signals. * SIGUSR1: Miscellaneous Signals. * SIGUSR2: Miscellaneous Signals. * SIGVTALRM: Alarm Signals. * SIGWINCH: Miscellaneous Signals. * SIGXCPU: Operation Error Signals. * SIGXFSZ: Operation Error Signals. * SOCK_DGRAM: Communication Styles. * SOCK_RAW: Communication Styles. * SOCK_STREAM: Communication Styles. * SOL_SOCKET: Socket-Level Options. * SSIZE_MAX: General Limits. * stderr: Standard Streams. * STDERR_FILENO: Descriptors and Streams. * stdin: Standard Streams. * STDIN_FILENO: Descriptors and Streams. * stdout: Standard Streams. * STDOUT_FILENO: Descriptors and Streams. * STREAM_MAX: General Limits. * SV_INTERRUPT: BSD Handler. * SV_ONSTACK: BSD Handler. * SV_RESETHAND: BSD Handler. * sys_siglist: Signal Messages. * TCIFLUSH: Line Control. * TCIOFF: Line Control. * TCIOFLUSH: Line Control. * TCION: Line Control. * TCOFLUSH: Line Control. * TCOOFF: Line Control. * TCOON: Line Control. * TCSADRAIN: Mode Functions. * TCSAFLUSH: Mode Functions. * TCSANOW: Mode Functions. * TCSASOFT: Mode Functions. * timezone: Time Zone Functions. * TMP_MAX: Temporary Files. * TOSTOP: Local Modes. * TRY_AGAIN: Host Names. * tzname: Time Zone Functions. * TZNAME_MAX: General Limits. * VDISCARD: Other Special. * VDSUSP: Signal Characters. * VEOF: Editing Characters. * VEOL: Editing Characters. * VEOL2: Editing Characters. * VERASE: Editing Characters. * VINTR: Signal Characters. * VKILL: Editing Characters. * VLNEXT: Other Special. * VMIN: Noncanonical Input. * VQUIT: Signal Characters. * VREPRINT: Editing Characters. * VSTART: Start/Stop Characters. * VSTATUS: Other Special. * VSTOP: Start/Stop Characters. * VSUSP: Signal Characters. * VTIME: Noncanonical Input. * VWERASE: Editing Characters. * W_OK: Testing File Access. * X_OK: Testing File Access. Program and File Index ********************** * Menu: * -lbsd-compat <1>: Process Group Functions. * -lbsd-compat: Feature Test Macros. * /etc/group: Group Database. * /etc/hosts: Host Names. * /etc/localtime: TZ Variable. * /etc/networks: Networks Database. * /etc/passwd: User Database. * /etc/protocols: Protocols Database. * /etc/services: Services Database. * /share/lib/zoneinfo: TZ Variable. * arpa/inet.h: Host Address Functions. * assert.h: Consistency Checking. * bsd-compat <1>: Process Group Functions. * bsd-compat: Feature Test Macros. * cd: Working Directory. * chgrp: File Owner. * chown: File Owner. * ctype.h <1>: Case Conversion. * ctype.h <2>: Classification of Characters. * ctype.h: Character Handling. * dirent.h <1>: Random Access Directory. * dirent.h <2>: Reading/Closing Directory. * dirent.h <3>: Opening a Directory. * dirent.h <4>: Directory Entries. * dirent.h: Reserved Names. * errno.h <1>: Error Codes. * errno.h <2>: Checking for Errors. * errno.h: Error Reporting. * fcntl.h <1>: Interrupt Input. * fcntl.h <2>: File Locks. * fcntl.h <3>: File Status Flags. * fcntl.h <4>: Descriptor Flags. * fcntl.h <5>: Duplicating Descriptors. * fcntl.h <6>: Control Operations. * fcntl.h <7>: Opening and Closing Files. * fcntl.h: Reserved Names. * float.h: Floating Point Parameters. * fnmatch.h: Wildcard Matching. * gcc: ISO C. * grp.h <1>: Group Data Structure. * grp.h <2>: Setting Groups. * grp.h: Reserved Names. * hostid: Host Identification. * hostname: Host Identification. * kill: Termination Signals. * limits.h <1>: Width of Type. * limits.h <2>: Limits for Files. * limits.h <3>: General Limits. * limits.h <4>: Multibyte Char Intro. * limits.h: Reserved Names. * locale.h <1>: Numeric Formatting. * locale.h: Setting the Locale. * localtime: TZ Variable. * ls: File Attributes. * malloc.h <1>: Using Relocator. * malloc.h <2>: Statistics of Malloc. * malloc.h <3>: Hooks for Malloc. * malloc.h <4>: Heap Consistency Checking. * malloc.h: Malloc Tunable Parameters. * math.h <1>: Rounding and Remainders. * math.h <2>: Normalization Functions. * math.h <3>: Absolute Value. * math.h <4>: Predicates on Floats. * math.h: Mathematics. * mkdir: Creating Directories. * netdb.h <1>: Networks Database. * netdb.h <2>: Protocols Database. * netdb.h <3>: Services Database. * netdb.h: Host Names. * netinet/in.h <1>: Byte Order. * netinet/in.h <2>: Ports. * netinet/in.h <3>: Host Address Data Type. * netinet/in.h: Internet Address Formats. * obstack.h: Creating Obstacks. * printf.h <1>: Conversion Specifier Options. * printf.h: Registering New Conversions. * pwd.h <1>: User Data Structure. * pwd.h: Reserved Names. * setjmp.h <1>: Non-Local Exits and Signals. * setjmp.h: Non-Local Details. * sh: Running a Command. * signal.h <1>: BSD Signal Handling. * signal.h <2>: Checking for Pending Signals. * signal.h <3>: Process Signal Mask. * signal.h <4>: Signal Sets. * signal.h <5>: Signaling Another Process. * signal.h <6>: Signaling Yourself. * signal.h <7>: Flags for Sigaction. * signal.h <8>: Advanced Signal Handling. * signal.h <9>: Basic Signal Handling. * signal.h <10>: Standard Signals. * signal.h: Reserved Names. * stdarg.h <1>: Argument Macros. * stdarg.h: Receiving Arguments. * stddef.h <1>: Important Data Types. * stddef.h: Wide Char Intro. * stdio.h <1>: Who Logged In. * stdio.h <2>: Identifying the Terminal. * stdio.h <3>: Signal Messages. * stdio.h <4>: Temporary Files. * stdio.h <5>: Deleting Files. * stdio.h <6>: Descriptors and Streams. * stdio.h <7>: Streams and Cookies. * stdio.h <8>: String Streams. * stdio.h <9>: Controlling Buffering. * stdio.h <10>: Flushing Buffers. * stdio.h <11>: Portable Positioning. * stdio.h <12>: File Positioning. * stdio.h <13>: EOF and Errors. * stdio.h <14>: Formatted Input Functions. * stdio.h <15>: Variable Arguments Output. * stdio.h <16>: Formatted Output Functions. * stdio.h <17>: Block Input/Output. * stdio.h <18>: Character Input. * stdio.h <19>: Simple Output. * stdio.h <20>: Opening Streams. * stdio.h <21>: Standard Streams. * stdio.h: Streams. * stdlib.h <1>: Running a Command. * stdlib.h <2>: Aborting a Program. * stdlib.h <3>: Exit Status. * stdlib.h <4>: Environment Access. * stdlib.h <5>: Converting One Char. * stdlib.h <6>: Length of Char. * stdlib.h <7>: Wide String Conversion. * stdlib.h <8>: Multibyte Char Intro. * stdlib.h <9>: Array Sort Function. * stdlib.h <10>: Array Search Function. * stdlib.h <11>: Parsing of Floats. * stdlib.h <12>: Parsing of Integers. * stdlib.h <13>: Integer Division. * stdlib.h <14>: Absolute Value. * stdlib.h <15>: BSD Random. * stdlib.h <16>: ISO Random. * stdlib.h <17>: Variable Size Automatic. * stdlib.h <18>: Aligned Memory Blocks. * stdlib.h <19>: Allocating Cleared Space. * stdlib.h <20>: Changing Block Size. * stdlib.h <21>: Freeing after Malloc. * stdlib.h: Basic Allocation. * string.h <1>: Signal Messages. * string.h <2>: Finding Tokens in a String. * string.h <3>: Search Functions. * string.h <4>: Collation Functions. * string.h <5>: String/Array Comparison. * string.h <6>: Copying and Concatenation. * string.h: String Length. * sys/param.h: Host Identification. * sys/resource.h <1>: Priority. * sys/resource.h <2>: Limits on Resources. * sys/resource.h: Resource Usage. * sys/socket.h <1>: Socket-Level Options. * sys/socket.h <2>: Socket Option Functions. * sys/socket.h <3>: Sending Datagrams. * sys/socket.h <4>: Socket Data Options. * sys/socket.h <5>: Receiving Data. * sys/socket.h <6>: Sending Data. * sys/socket.h <7>: Socket Pairs. * sys/socket.h <8>: Closing a Socket. * sys/socket.h <9>: Creating a Socket. * sys/socket.h <10>: Internet Namespace. * sys/socket.h <11>: File Namespace Details. * sys/socket.h <12>: Reading Address. * sys/socket.h <13>: Setting Address. * sys/socket.h <14>: Address Formats. * sys/socket.h: Communication Styles. * sys/stat.h <1>: FIFO Special Files. * sys/stat.h <2>: Making Special Files. * sys/stat.h <3>: Setting Permissions. * sys/stat.h <4>: Permission Bits. * sys/stat.h <5>: Testing File Type. * sys/stat.h <6>: Attribute Meanings. * sys/stat.h <7>: Creating Directories. * sys/stat.h: Reserved Names. * sys/time.h <1>: Setting an Alarm. * sys/time.h <2>: High-Resolution Calendar. * sys/time.h: File Times. * sys/times.h <1>: Detailed CPU Time. * sys/times.h: Reserved Names. * sys/types.h <1>: Setting Groups. * sys/types.h <2>: Setting User ID. * sys/types.h <3>: Reading Persona. * sys/types.h <4>: Terminal Access Functions. * sys/types.h <5>: Process Group Functions. * sys/types.h <6>: Process Identification. * sys/types.h: Waiting for I/O. * sys/un.h: File Namespace Details. * sys/utsname.h: Hardware/Software Type ID. * sys/wait.h <1>: BSD Wait Functions. * sys/wait.h <2>: Process Completion Status. * sys/wait.h: Process Completion. * termios.h <1>: Terminal Modes. * termios.h: Reserved Names. * time.h <1>: TZ Variable. * time.h <2>: Formatting Date and Time. * time.h <3>: Simple Calendar Time. * time.h <4>: Basic CPU Time. * time.h: File Times. * umask: Setting Permissions. * unistd.h <1>: Options for Files. * unistd.h <2>: System Options. * unistd.h <3>: Host Identification. * unistd.h <4>: Who Logged In. * unistd.h <5>: Setting Groups. * unistd.h <6>: Setting User ID. * unistd.h <7>: Reading Persona. * unistd.h <8>: Terminal Access Functions. * unistd.h <9>: Process Group Functions. * unistd.h <10>: Executing a File. * unistd.h <11>: Creating a Process. * unistd.h <12>: Process Identification. * unistd.h <13>: Termination Internals. * unistd.h <14>: Parsing Options. * unistd.h <15>: Setting an Alarm. * unistd.h <16>: Is It a Terminal. * unistd.h <17>: Creating a Pipe. * unistd.h <18>: Testing File Access. * unistd.h <19>: File Owner. * unistd.h <20>: Deleting Files. * unistd.h <21>: Symbolic Links. * unistd.h <22>: Hard Links. * unistd.h <23>: Working Directory. * unistd.h <24>: Duplicating Descriptors. * unistd.h <25>: Descriptors and Streams. * unistd.h <26>: I/O Primitives. * unistd.h: Opening and Closing Files. * utime.h: File Times. * varargs.h: Old Varargs. * zoneinfo: TZ Variable.