START-INFO-DIR-ENTRY * Libc: (libc). C library. END-INFO-DIR-ENTRY This file documents the GNU C library. This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library Reference Manual', for Version 2.2.x. Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software Needs Free Documentation" and "GNU Lesser General Public License", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development. Main Menu ********* This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library Reference Manual', for Version 2.2.x of the GNU C Library. Introduction ************ The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard "library", which you compile and link with your programs. The GNU C library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system. The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability. Getting Started =============== This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (*note ISO C::), rather than "traditional" pre-ISO C dialects, is assumed. The GNU C library includes several "header files", each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file `stdio.h' declares facilities for performing input and output, and the header file `string.h' declares string processing utilities. The organization of this manual generally follows the same division as the header files. If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a _lot_ of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly _how_ to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize _when_ to make use of library functions, and _where_ in this manual you can find more specific information about them. Standards and Portability ========================= This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations. The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual. *Note Library Summary::, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from. ISO C ----- The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): `American National Standard X3.159-1989--"ANSI C"' and later by the International Standardization Organization (ISO): `ISO/IEC 9899:1990, "Programming languages--C"'. We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard. If you are concerned about strict adherence to the ISO C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define _only_ ISO standard features from the library header files, unless you explicitly ask for additional features. *Note Feature Test Macros::, for information on how to do this. Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. *Note Reserved Names::, for more information about these restrictions. This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness. POSIX (The Portable Operating System Interface) ----------------------------------------------- The GNU library is also compatible with the ISO "POSIX" family of standards, known more formally as the "Portable Operating System Interface for Computer Environments" (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system. The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments. The GNU C library implements all of the functions specified in `ISO/IEC 9945-1:1996, the POSIX System Application Program Interface', commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (*note File System Interface::), device-specific terminal control functions (*note Low-Level Terminal Interface::), and process control functions (*note Processes::). Some facilities from `ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard' (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (*note Pattern Matching::). Berkeley Unix ------------- The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as "Berkeley Unix") and from "SunOS" (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all. The BSD facilities include symbolic links (*note Symbolic Links::), the `select' function (*note Waiting for I/O::), the BSD signal functions (*note BSD Signal Handling::), and sockets (*note Sockets::). SVID (The System V Interface Description) ----------------------------------------- The "System V Interface Description" (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (*note POSIX::). The GNU C library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.) The supported facilities from System V include the methods for inter-process communication and shared memory, the `hsearch' and `drand48' families of functions, `fmtmsg' and several of the mathematical functions. XPG (The X/Open Portability Guide) ---------------------------------- The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system. The GNU C library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions. The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems. Using the Library ================= This section describes some of the practical issues involved in using the GNU C library. Header Files ------------ Libraries for use by C programs really consist of two parts: "header files" that define types and macros and declare variables and functions; and the actual library or "archive" that contains the definitions of the variables and functions. (Recall that in C, a "declaration" merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A "definition", on the other hand, actually allocates storage for a variable or says what a function does.) In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file. Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first, #include "HEADER" is typically used to include a header file HEADER that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast, #include is typically used to include a header file `file.h' that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files. Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (*note Feature Test Macros::). For more information about the use of header files and `#include' directives, *note Header Files: (cpp.info)Header Files.. The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using. Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter. *Compatibility Note:* Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations. Strictly speaking, you don't _have to_ include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration. Macro Definitions of Functions ------------------------------ If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs--the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call. Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call. You might occasionally want to avoid using the macro definition of a function--perhaps to make your program easier to debug. There are two ways you can do this: * You can avoid a macro definition in a specific use by enclosing the name of the function in parentheses. This works because the name of the function doesn't appear in a syntactic context where it is recognizable as a macro call. * You can suppress any macro definition for a whole source file by using the `#undef' preprocessor directive, unless otherwise stated explicitly in the description of that facility. For example, suppose the header file `stdlib.h' declares a function named `abs' with extern int abs (int); and also provides a macro definition for `abs'. Then, in: #include int f (int *i) { return abs (++*i); } the reference to `abs' might refer to either a macro or a function. On the other hand, in each of the following examples the reference is to a function and not a macro. #include int g (int *i) { return (abs) (++*i); } #undef abs int h (int *i) { return abs (++*i); } Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower. Reserved Names -------------- The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program *may not* redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions: * Other people reading your code could get very confused if you were using a function named `exit' to do something completely different from what the standard `exit' function does, for example. Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability. * It avoids the possibility of a user accidentally redefining a library function that is called by other library functions. If redefinition were allowed, those other functions would not work properly. * It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user. Some library facilities, such as those for dealing with variadic arguments (*note Variadic Functions::) and non-local exits (*note Non-Local Exits::), actually require a considerable amount of cooperation on the part of the C compiler, and with respect to the implementation, it might be easier for the compiler to treat these as built-in parts of the language. In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs. Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names. * Names beginning with a capital `E' followed a digit or uppercase letter may be used for additional error code names. *Note Error Reporting::. * Names that begin with either `is' or `to' followed by a lowercase letter may be used for additional character testing and conversion functions. *Note Character Handling::. * Names that begin with `LC_' followed by an uppercase letter may be used for additional macros specifying locale attributes. *Note Locales::. * Names of all existing mathematics functions (*note Mathematics::) suffixed with `f' or `l' are reserved for corresponding functions that operate on `float' and `long double' arguments, respectively. * Names that begin with `SIG' followed by an uppercase letter are reserved for additional signal names. *Note Standard Signals::. * Names that begin with `SIG_' followed by an uppercase letter are reserved for additional signal actions. *Note Basic Signal Handling::. * Names beginning with `str', `mem', or `wcs' followed by a lowercase letter are reserved for additional string and array functions. *Note String and Array Utilities::. * Names that end with `_t' are reserved for additional type names. In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file. * The header file `dirent.h' reserves names prefixed with `d_'. * The header file `fcntl.h' reserves names prefixed with `l_', `F_', `O_', and `S_'. * The header file `grp.h' reserves names prefixed with `gr_'. * The header file `limits.h' reserves names suffixed with `_MAX'. * The header file `pwd.h' reserves names prefixed with `pw_'. * The header file `signal.h' reserves names prefixed with `sa_' and `SA_'. * The header file `sys/stat.h' reserves names prefixed with `st_' and `S_'. * The header file `sys/times.h' reserves names prefixed with `tms_'. * The header file `termios.h' reserves names prefixed with `c_', `V', `I', `O', and `TC'; and names prefixed with `B' followed by a digit. Feature Test Macros ------------------- The exact set of features available when you compile a source file is controlled by which "feature test macros" you define. If you compile your programs using `gcc -ansi', you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. *Note GNU CC Command Options: (gcc.info)Invoking GCC, for more information about GCC options. You should define these macros by using `#define' preprocessor directives at the top of your source code files. These directives _must_ come before any `#include' of a system header file. It is best to make them the very first thing in the file, preceded only by comments. You could also use the `-D' option to GCC, but it's better if you make the source files indicate their own meaning in a self-contained way. This system exists to allow the library to conform to multiple standards. Although the different standards are often described as supersets of each other, they are usually incompatible because larger standards require functions with names that smaller ones reserve to the user program. This is not mere pedantry -- it has been a problem in practice. For instance, some non-GNU programs define functions named `getline' that have nothing to do with this library's `getline'. They would not be compilable if all features were enabled indiscriminately. This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard. - Macro: _POSIX_SOURCE If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities. The state of `_POSIX_SOURCE' is irrelevant if you define the macro `_POSIX_C_SOURCE' to a positive integer. - Macro: _POSIX_C_SOURCE Define this macro to a positive integer to control which POSIX functionality is made available. The greater the value of this macro, the more functionality is made available. If you define this macro to a value greater than or equal to `1', then the functionality from the 1990 edition of the POSIX.1 standard (IEEE Standard 1003.1-1990) is made available. If you define this macro to a value greater than or equal to `2', then the functionality from the 1992 edition of the POSIX.2 standard (IEEE Standard 1003.2-1992) is made available. If you define this macro to a value greater than or equal to `199309L', then the functionality from the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available. Greater values for `_POSIX_C_SOURCE' will enable future extensions. The POSIX standards process will define these values as necessary, and the GNU C Library should support them some time after they become standardized. The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that if you define `_POSIX_C_SOURCE' to a value greater than or equal to `199506L', then the functionality from the 1996 edition is made available. - Macro: _BSD_SOURCE If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material. Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions. Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special "BSD compatibility library" when linking programs compiled for BSD compatibility. This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library. If your program defines `_BSD_SOURCE', you must give the option `-lbsd-compat' to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library. - Macro: _SVID_SOURCE If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material. - Macro: _XOPEN_SOURCE - Macro: _XOPEN_SOURCE_EXTENDED If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact `_POSIX_SOURCE' and `_POSIX_C_SOURCE' are automatically defined. As the unification of all Unices, functionality only available in BSD and SVID is also included. If the macro `_XOPEN_SOURCE_EXTENDED' is also defined, even more functionality is available. The extra functions will make all functions available which are necessary for the X/Open Unix brand. If the macro `_XOPEN_SOURCE' has the value 500 this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2. - Macro: _LARGEFILE_SOURCE If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions `fseeko' and `ftello' are available. Without these functions the difference between the ISO C interface (`fseek', `ftell') and the low-level POSIX interface (`lseek') would lead to problems. This macro was introduced as part of the Large File Support extension (LFS). - Macro: _LARGEFILE64_SOURCE If you define this macro an additional set of functions is made available which enables 32 bit systems to use files of sizes beyond the usual limit of 2GB. This interface is not available if the system does not support files that large. On systems where the natural file size limit is greater than 2GB (i.e., on 64 bit systems) the new functions are identical to the replaced functions. The new functionality is made available by a new set of types and functions which replace the existing ones. The names of these new objects contain `64' to indicate the intention, e.g., `off_t' vs. `off64_t' and `fseeko' vs. `fseeko64'. This macro was introduced as part of the Large File Support extension (LFS). It is a transition interface for the period when 64 bit offsets are not generally used (see `_FILE_OFFSET_BITS'). - Macro: _FILE_OFFSET_BITS This macro determines which file system interface shall be used, one replacing the other. Whereas `_LARGEFILE64_SOURCE' makes the 64 bit interface available as an additional interface, `_FILE_OFFSET_BITS' allows the 64 bit interface to replace the old interface. If `_FILE_OFFSET_BITS' is undefined, or if it is defined to the value `32', nothing changes. The 32 bit interface is used and types like `off_t' have a size of 32 bits on 32 bit systems. If the macro is defined to the value `64', the large file interface replaces the old interface. I.e., the functions are not made available under different names (as they are with `_LARGEFILE64_SOURCE'). Instead the old function names now reference the new functions, e.g., a call to `fseeko' now indeed calls `fseeko64'. This macro should only be selected if the system provides mechanisms for handling large files. On 64 bit systems this macro has no effect since the `*64' functions are identical to the normal functions. This macro was introduced as part of the Large File Support extension (LFS). - Macro: _ISOC99_SOURCE Until the revised ISO C standard is widely adopted the new features are not automatically enabled. The GNU libc nevertheless has a complete implementation of the new standard and to enable the new features the macro `_ISOC99_SOURCE' should be defined. - Macro: _GNU_SOURCE If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence. If you want to get the full effect of `_GNU_SOURCE' but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions: #define _GNU_SOURCE #define _BSD_SOURCE #define _SVID_SOURCE Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. *Note:* If you forget to do this, you may get very strange errors at run time. - Macro: _REENTRANT - Macro: _THREAD_SAFE If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface. Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe. We recommend you use `_GNU_SOURCE' in new programs. If you don't specify the `-ansi' option to GCC and don't define any of these macros explicitly, the effect is the same as defining `_POSIX_C_SOURCE' to 2 and `_POSIX_SOURCE', `_SVID_SOURCE', and `_BSD_SOURCE' to 1. When you define a feature test macro to request a larger class of features, it is harmless to define in addition a feature test macro for a subset of those features. For example, if you define `_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no effect. Likewise, if you define `_GNU_SOURCE', then defining either `_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no effect. Note, however, that the features of `_BSD_SOURCE' are not a subset of any of the other feature test macros supported. This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros. For this reason, defining `_BSD_SOURCE' in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features. Roadmap to the Manual ===================== Here is an overview of the contents of the remaining chapters of this manual. * *Note Error Reporting::, describes how errors detected by the library are reported. * *Note Language Features::, contains information about library support for standard parts of the C language, including things like the `sizeof' operator and the symbolic constant `NULL', how to write functions accepting variable numbers of arguments, and constants describing the ranges and other properties of the numerical types. There is also a simple debugging mechanism which allows you to put assertions in your code, and have diagnostic messages printed if the tests fail. * *Note Memory::, describes the GNU library's facilities for managing and using virtual and real memory, including dynamic allocation of virtual memory. If you do not know in advance how much memory your program needs, you can allocate it dynamically instead, and manipulate it via pointers. * *Note Character Handling::, contains information about character classification functions (such as `isspace') and functions for performing case conversion. * *Note String and Array Utilities::, has descriptions of functions for manipulating strings (null-terminated character arrays) and general byte arrays, including operations such as copying and comparison. * *Note I/O Overview::, gives an overall look at the input and output facilities in the library, and contains information about basic concepts such as file names. * *Note I/O on Streams::, describes I/O operations involving streams (or `FILE *' objects). These are the normal C library functions from `stdio.h'. * *Note Low-Level I/O::, contains information about I/O operations on file descriptors. File descriptors are a lower-level mechanism specific to the Unix family of operating systems. * *Note File System Interface::, has descriptions of operations on entire files, such as functions for deleting and renaming them and for creating new directories. This chapter also contains information about how you can access the attributes of a file, such as its owner and file protection modes. * *Note Pipes and FIFOs::, contains information about simple interprocess communication mechanisms. Pipes allow communication between two related processes (such as between a parent and child), while FIFOs allow communication between processes sharing a common file system on the same machine. * *Note Sockets::, describes a more complicated interprocess communication mechanism that allows processes running on different machines to communicate over a network. This chapter also contains information about Internet host addressing and how to use the system network databases. * *Note Low-Level Terminal Interface::, describes how you can change the attributes of a terminal device. If you want to disable echo of characters typed by the user, for example, read this chapter. * *Note Mathematics::, contains information about the math library functions. These include things like random-number generators and remainder functions on integers as well as the usual trigonometric and exponential functions on floating-point numbers. * *Note Low-Level Arithmetic Functions: Arithmetic, describes functions for simple arithmetic, analysis of floating-point values, and reading numbers from strings. * *Note Searching and Sorting::, contains information about functions for searching and sorting arrays. You can use these functions on any kind of array by providing an appropriate comparison function. * *Note Pattern Matching::, presents functions for matching regular expressions and shell file name patterns, and for expanding words as the shell does. * *Note Date and Time::, describes functions for measuring both calendar time and CPU time, as well as functions for setting alarms and timers. * *Note Character Set Handling::, contains information about manipulating characters and strings using character sets larger than will fit in the usual `char' data type. * *Note Locales::, describes how selecting a particular country or language affects the behavior of the library. For example, the locale affects collation sequences for strings and how monetary values are formatted. * *Note Non-Local Exits::, contains descriptions of the `setjmp' and `longjmp' functions. These functions provide a facility for `goto'-like jumps which can jump from one function to another. * *Note Signal Handling::, tells you all about signals--what they are, how to establish a handler that is called when a particular kind of signal is delivered, and how to prevent signals from arriving during critical sections of your program. * *Note Program Basics::, tells how your programs can access their command-line arguments and environment variables. * *Note Processes::, contains information about how to start new processes and run programs. * *Note Job Control::, describes functions for manipulating process groups and the controlling terminal. This material is probably only of interest if you are writing a shell or other program which handles job control specially. * *Note Name Service Switch::, describes the services which are available for looking up names in the system databases, how to determine which service is used for which database, and how these services are implemented so that contributors can design their own services. * *Note User Database::, and *Note Group Database::, tell you how to access the system user and group databases. * *Note System Management::, describes functions for controlling and getting information about the hardware and software configuration your program is executing under. * *Note System Configuration::, tells you how you can get information about various operating system limits. Most of these parameters are provided for compatibility with POSIX. * *Note Library Summary::, gives a summary of all the functions, variables, and macros in the library, with complete data types and function prototypes, and says what standard or system each is derived from. * *Note Maintenance::, explains how to build and install the GNU C library on your system, how to report any bugs you might find, and how to add new functions or port the library to a new system. If you already know the name of the facility you are interested in, you can look it up in *Note Library Summary::. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from. Error Reporting *************** Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed. This chapter describes how the error reporting facility works. Your program should include the header file `errno.h' to use this facility. Checking for Errors =================== Most library functions return a special value to indicate that they have failed. The special value is typically `-1', a null pointer, or a constant such as `EOF' that is defined for that purpose. But this return value tells you only that an error has occurred. To find out what kind of error it was, you need to look at the error code stored in the variable `errno'. This variable is declared in the header file `errno.h'. - Variable: volatile int errno The variable `errno' contains the system error number. You can change the value of `errno'. Since `errno' is declared `volatile', it might be changed asynchronously by a signal handler; see *Note Defining Handlers::. However, a properly written signal handler saves and restores the value of `errno', so you generally do not need to worry about this possibility except when writing signal handlers. The initial value of `errno' at program startup is zero. Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors. These error conditions are listed for each function. These functions do not change `errno' when they succeed; thus, the value of `errno' after a successful call is not necessarily zero, and you should not use `errno' to determine _whether_ a call failed. The proper way to do that is documented for each function. _If_ the call failed, you can examine `errno'. Many library functions can set `errno' to a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter `errno' when the function returns an error. *Portability Note:* ISO C specifies `errno' as a "modifiable lvalue" rather than as a variable, permitting it to be implemented as a macro. For example, its expansion might involve a function call, like `*_errno ()'. In fact, that is what it is on the GNU system itself. The GNU library, on non-GNU systems, does whatever is right for the particular system. There are a few library functions, like `sqrt' and `atan', that return a perfectly legitimate value in case of an error, but also set `errno'. For these functions, if you want to check to see whether an error occurred, the recommended method is to set `errno' to zero before calling the function, and then check its value afterward. All the error codes have symbolic names; they are macros defined in `errno.h'. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. *Note Reserved Names::. The error code values are all positive integers and are all distinct, with one exception: `EWOULDBLOCK' and `EAGAIN' are the same. Since the values are distinct, you can use them as labels in a `switch' statement; just don't use both `EWOULDBLOCK' and `EAGAIN'. Your program should not make any other assumptions about the specific values of these symbolic constants. The value of `errno' doesn't necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations. The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function. On non-GNU systems, almost any system call can return `EFAULT' if it is given an invalid pointer as an argument. Since this could only happen as a result of a bug in your program, and since it will not happen on the GNU system, we have saved space by not mentioning `EFAULT' in the descriptions of individual functions. In some Unix systems, many system calls can also return `EFAULT' if given as an argument a pointer into the stack, and the kernel for some obscure reason fails in its attempt to extend the stack. If this ever happens, you should probably try using statically or dynamically allocated memory instead of stack memory on that system. Error Codes =========== The error code macros are defined in the header file `errno.h'. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems. - Macro: int EPERM Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation. - Macro: int ENOENT No such file or directory. This is a "file doesn't exist" error for ordinary files that are referenced in contexts where they are expected to already exist. - Macro: int ESRCH No process matches the specified process ID. - Macro: int EINTR Interrupted function call; an asynchronous signal occurred and prevented completion of the call. When this happens, you should try the call again. You can choose to have functions resume after a signal that is handled, rather than failing with `EINTR'; see *Note Interrupted Primitives::. - Macro: int EIO Input/output error; usually used for physical read or write errors. - Macro: int ENXIO No such device or address. The system tried to use the device represented by a file you specified, and it couldn't find the device. This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer. - Macro: int E2BIG Argument list too long; used when the arguments passed to a new program being executed with one of the `exec' functions (*note Executing a File::) occupy too much memory space. This condition never arises in the GNU system. - Macro: int ENOEXEC Invalid executable file format. This condition is detected by the `exec' functions; see *Note Executing a File::. - Macro: int EBADF Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa). - Macro: int ECHILD There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate. - Macro: int EDEADLK Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. The system does not guarantee that it will notice all such situations. This error means you got lucky and the system noticed; it might just hang. *Note File Locks::, for an example. - Macro: int ENOMEM No memory available. The system cannot allocate more virtual memory because its capacity is full. - Macro: int EACCES Permission denied; the file permissions do not allow the attempted operation. - Macro: int EFAULT Bad address; an invalid pointer was detected. In the GNU system, this error never happens; you get a signal instead. - Macro: int ENOTBLK A file that isn't a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error. - Macro: int EBUSY Resource busy; a system resource that can't be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error. - Macro: int EEXIST File exists; an existing file was specified in a context where it only makes sense to specify a new file. - Macro: int EXDEV An attempt to make an improper link across file systems was detected. This happens not only when you use `link' (*note Hard Links::) but also when you rename a file with `rename' (*note Renaming Files::). - Macro: int ENODEV The wrong type of device was given to a function that expects a particular sort of device. - Macro: int ENOTDIR A file that isn't a directory was specified when a directory is required. - Macro: int EISDIR File is a directory; you cannot open a directory for writing, or create or remove hard links to it. - Macro: int EINVAL Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function. - Macro: int EMFILE The current process has too many files open and can't open any more. Duplicate descriptors do count toward this limit. In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased. If you get this error, you might want to increase the `RLIMIT_NOFILE' limit or make it unlimited; *note Limits on Resources::. - Macro: int ENFILE There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see *Note Linked Channels::. This error never occurs in the GNU system. - Macro: int ENOTTY Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file. - Macro: int ETXTBSY An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. Often using a debugger to run a program is considered having it open for writing and will cause this error. (The name stands for "text file busy".) This is not an error in the GNU system; the text is copied as necessary. - Macro: int EFBIG File too big; the size of a file would be larger than allowed by the system. - Macro: int ENOSPC No space left on device; write operation on a file failed because the disk is full. - Macro: int ESPIPE Invalid seek operation (such as on a pipe). - Macro: int EROFS An attempt was made to modify something on a read-only file system. - Macro: int EMLINK Too many links; the link count of a single file would become too large. `rename' can cause this error if the file being renamed already has as many links as it can take (*note Renaming Files::). - Macro: int EPIPE Broken pipe; there is no process reading from the other end of a pipe. Every library function that returns this error code also generates a `SIGPIPE' signal; this signal terminates the program if not handled or blocked. Thus, your program will never actually see `EPIPE' unless it has handled or blocked `SIGPIPE'. - Macro: int EDOM Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined. - Macro: int ERANGE Range error; used by mathematical functions when the result value is not representable because of overflow or underflow. - Macro: int EAGAIN Resource temporarily unavailable; the call might work if you try again later. The macro `EWOULDBLOCK' is another name for `EAGAIN'; they are always the same in the GNU C library. This error can happen in a few different situations: * An operation that would block was attempted on an object that has non-blocking mode selected. Trying the same operation again will block until some external condition makes it possible to read, write, or connect (whatever the operation). You can use `select' to find out when the operation will be possible; *note Waiting for I/O::. *Portability Note:* In many older Unix systems, this condition was indicated by `EWOULDBLOCK', which was a distinct error code different from `EAGAIN'. To make your program portable, you should check for both codes and treat them the same. * A temporary resource shortage made an operation impossible. `fork' can return this error. It indicates that the shortage is expected to pass, so your program can try the call again later and it may succeed. It is probably a good idea to delay for a few seconds before trying it again, to allow time for other processes to release scarce resources. Such shortages are usually fairly serious and affect the whole system, so usually an interactive program should report the error to the user and return to its command loop. - Macro: int EWOULDBLOCK In the GNU C library, this is another name for `EAGAIN' (above). The values are always the same, on every operating system. C libraries in many older Unix systems have `EWOULDBLOCK' as a separate error code. - Macro: int EINPROGRESS An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected. Some functions that must always block (such as `connect'; *note Connecting::) never return `EAGAIN'. Instead, they return `EINPROGRESS' to indicate that the operation has begun and will take some time. Attempts to manipulate the object before the call completes return `EALREADY'. You can use the `select' function to find out when the pending operation has completed; *note Waiting for I/O::. - Macro: int EALREADY An operation is already in progress on an object that has non-blocking mode selected. - Macro: int ENOTSOCK A file that isn't a socket was specified when a socket is required. - Macro: int EMSGSIZE The size of a message sent on a socket was larger than the supported maximum size. - Macro: int EPROTOTYPE The socket type does not support the requested communications protocol. - Macro: int ENOPROTOOPT You specified a socket option that doesn't make sense for the particular protocol being used by the socket. *Note Socket Options::. - Macro: int EPROTONOSUPPORT The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). *Note Creating a Socket::. - Macro: int ESOCKTNOSUPPORT The socket type is not supported. - Macro: int EOPNOTSUPP The operation you requested is not supported. Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols. In the GNU system, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call. - Macro: int EPFNOSUPPORT The socket communications protocol family you requested is not supported. - Macro: int EAFNOSUPPORT The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. *Note Sockets::. - Macro: int EADDRINUSE The requested socket address is already in use. *Note Socket Addresses::. - Macro: int EADDRNOTAVAIL The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. *Note Socket Addresses::. - Macro: int ENETDOWN A socket operation failed because the network was down. - Macro: int ENETUNREACH A socket operation failed because the subnet containing the remote host was unreachable. - Macro: int ENETRESET A network connection was reset because the remote host crashed. - Macro: int ECONNABORTED A network connection was aborted locally. - Macro: int ECONNRESET A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation. - Macro: int ENOBUFS The kernel's buffers for I/O operations are all in use. In GNU, this error is always synonymous with `ENOMEM'; you may get one or the other from network operations. - Macro: int EISCONN You tried to connect a socket that is already connected. *Note Connecting::. - Macro: int ENOTCONN The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data. For a connectionless socket (for datagram protocols, such as UDP), you get `EDESTADDRREQ' instead. - Macro: int EDESTADDRREQ No default destination address was set for the socket. You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with `connect'. - Macro: int ESHUTDOWN The socket has already been shut down. - Macro: int ETOOMANYREFS ??? - Macro: int ETIMEDOUT A socket operation with a specified timeout received no response during the timeout period. - Macro: int ECONNREFUSED A remote host refused to allow the network connection (typically because it is not running the requested service). - Macro: int ELOOP Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links. - Macro: int ENAMETOOLONG Filename too long (longer than `PATH_MAX'; *note Limits for Files::) or host name too long (in `gethostname' or `sethostname'; *note Host Identification::). - Macro: int EHOSTDOWN The remote host for a requested network connection is down. - Macro: int EHOSTUNREACH The remote host for a requested network connection is not reachable. - Macro: int ENOTEMPTY Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory. - Macro: int EPROCLIM This means that the per-user limit on new process would be exceeded by an attempted `fork'. *Note Limits on Resources::, for details on the `RLIMIT_NPROC' limit. - Macro: int EUSERS The file quota system is confused because there are too many users. - Macro: int EDQUOT The user's disk quota was exceeded. - Macro: int ESTALE Stale NFS file handle. This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host. Repairing this condition usually requires unmounting and remounting the NFS file system on the local host. - Macro: int EREMOTE An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.) - Macro: int EBADRPC ??? - Macro: int ERPCMISMATCH ??? - Macro: int EPROGUNAVAIL ??? - Macro: int EPROGMISMATCH ??? - Macro: int EPROCUNAVAIL ??? - Macro: int ENOLCK No locks available. This is used by the file locking facilities; see *Note File Locks::. This error is never generated by the GNU system, but it can result from an operation to an NFS server running another operating system. - Macro: int EFTYPE Inappropriate file type or format. The file was the wrong type for the operation, or a data file had the wrong format. On some systems `chmod' returns this error if you try to set the sticky bit on a non-directory file; *note Setting Permissions::. - Macro: int EAUTH ??? - Macro: int ENEEDAUTH ??? - Macro: int ENOSYS Function not implemented. This indicates that the function called is not implemented at all, either in the C library itself or in the operating system. When you get this error, you can be sure that this particular function will always fail with `ENOSYS' unless you install a new version of the C library or the operating system. - Macro: int ENOTSUP Not supported. A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all. For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only _that specific object_ (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values. If the entire function is not available at all in the implementation, it returns `ENOSYS' instead. - Macro: int EILSEQ While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid. - Macro: int EBACKGROUND In the GNU system, servers supporting the `term' protocol return this error for certain operations when the caller is not in the foreground process group of the terminal. Users do not usually see this error because functions such as `read' and `write' translate it into a `SIGTTIN' or `SIGTTOU' signal. *Note Job Control::, for information on process groups and these signals. - Macro: int EDIED In the GNU system, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file. - Macro: int ED The experienced user will know what is wrong. - Macro: int EGREGIOUS You did *what*? - Macro: int EIEIO Go home and have a glass of warm, dairy-fresh milk. - Macro: int EGRATUITOUS This error code has no purpose. - Macro: int EBADMSG - Macro: int EIDRM - Macro: int EMULTIHOP - Macro: int ENODATA - Macro: int ENOLINK - Macro: int ENOMSG - Macro: int ENOSR - Macro: int ENOSTR - Macro: int EOVERFLOW - Macro: int EPROTO - Macro: int ETIME _The following error codes are defined by the Linux/i386 kernel. They are not yet documented._ - Macro: int ERESTART - Macro: int ECHRNG - Macro: int EL2NSYNC - Macro: int EL3HLT - Macro: int EL3RST - Macro: int ELNRNG - Macro: int EUNATCH - Macro: int ENOCSI - Macro: int EL2HLT - Macro: int EBADE - Macro: int EBADR - Macro: int EXFULL - Macro: int ENOANO - Macro: int EBADRQC - Macro: int EBADSLT - Macro: int EDEADLOCK - Macro: int EBFONT - Macro: int ENONET - Macro: int ENOPKG - Macro: int EADV - Macro: int ESRMNT - Macro: int ECOMM - Macro: int EDOTDOT - Macro: int ENOTUNIQ - Macro: int EBADFD - Macro: int EREMCHG - Macro: int ELIBACC - Macro: int ELIBBAD - Macro: int ELIBSCN - Macro: int ELIBMAX - Macro: int ELIBEXEC - Macro: int ESTRPIPE - Macro: int EUCLEAN - Macro: int ENOTNAM - Macro: int ENAVAIL - Macro: int EISNAM - Macro: int EREMOTEIO - Macro: int ENOMEDIUM - Macro: int EMEDIUMTYPE Error Messages ============== The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call. The functions `strerror' and `perror' give you the standard error message for a given error code; the variable `program_invocation_short_name' gives you convenient access to the name of the program that encountered the error. - Function: char * strerror (int ERRNUM) The `strerror' function maps the error code (*note Checking for Errors::) specified by the ERRNUM argument to a descriptive error message string. The return value is a pointer to this string. The value ERRNUM normally comes from the variable `errno'. You should not modify the string returned by `strerror'. Also, if you make subsequent calls to `strerror', the string might be overwritten. (But it's guaranteed that no library function ever calls `strerror' behind your back.) The function `strerror' is declared in `string.h'. - Function: char * strerror_r (int ERRNUM, char *BUF, size_t N) The `strerror_r' function works like `strerror' but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user supplied buffer starting at BUF with the length of N bytes. At most N characters are written (including the NUL byte) so it is up to the user to select the buffer large enough. This function should always be used in multi-threaded programs since there is no way to guarantee the string returned by `strerror' really belongs to the last call of the current thread. This function `strerror_r' is a GNU extension and it is declared in `string.h'. - Function: void perror (const char *MESSAGE) This function prints an error message to the stream `stderr'; see *Note Standard Streams::. The orientation of `stderr' is not changed. If you call `perror' with a MESSAGE that is either a null pointer or an empty string, `perror' just prints the error message corresponding to `errno', adding a trailing newline. If you supply a non-null MESSAGE argument, then `perror' prefixes its output with this string. It adds a colon and a space character to separate the MESSAGE from the error string corresponding to `errno'. The function `perror' is declared in `stdio.h'. `strerror' and `perror' produce the exact same message for any given error code; the precise text varies from system to system. On the GNU system, the messages are fairly short; there are no multi-line messages or embedded newlines. Each error message begins with a capital letter and does not include any terminating punctuation. *Compatibility Note:* The `strerror' function was introduced in ISO C89. Many older C systems do not support this function yet. Many programs that don't read input from the terminal are designed to exit if any system call fails. By convention, the error message from such a program should start with the program's name, sans directories. You can find that name in the variable `program_invocation_short_name'; the full file name is stored the variable `program_invocation_name'. - Variable: char * program_invocation_name This variable's value is the name that was used to invoke the program running in the current process. It is the same as `argv[0]'. Note that this is not necessarily a useful file name; often it contains no directory names. *Note Program Arguments::. - Variable: char * program_invocation_short_name This variable's value is the name that was used to invoke the program running in the current process, with directory names removed. (That is to say, it is the same as `program_invocation_name' minus everything up to the last slash, if any.) The library initialization code sets up both of these variables before calling `main'. *Portability Note:* These two variables are GNU extensions. If you want your program to work with non-GNU libraries, you must save the value of `argv[0]' in `main', and then strip off the directory names yourself. We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from `main'. Here is an example showing how to handle failure to open a file correctly. The function `open_sesame' tries to open the named file for reading and returns a stream if successful. The `fopen' library function returns a null pointer if it couldn't open the file for some reason. In that situation, `open_sesame' constructs an appropriate error message using the `strerror' function, and terminates the program. If we were going to make some other library calls before passing the error code to `strerror', we'd have to save it in a local variable instead, because those other library functions might overwrite `errno' in the meantime. #include #include #include #include FILE * open_sesame (char *name) { FILE *stream; errno = 0; stream = fopen (name, "r"); if (stream == NULL) { fprintf (stderr, "%s: Couldn't open file %s; %s\n", program_invocation_short_name, name, strerror (errno)); exit (EXIT_FAILURE); } else return stream; } Using `perror' has the advantage that the function is portable and available on all systems implementing ISO C. But often the text `perror' generates is not what is wanted and there is no way to extend or change what `perror' does. The GNU coding standard, for instance, requires error messages to be preceded by the program name and programs which read some input files should should provide information about the input file name and the line number in case an error is encountered while reading the file. For these occasions there are two functions available which are widely used throughout the GNU project. These functions are declared in `error.h'. - Function: void error (int STATUS, int ERRNUM, const char *FORMAT, ...) The `error' function can be used to report general problems during program execution. The FORMAT argument is a format string just like those given to the `printf' family of functions. The arguments required for the format can follow the FORMAT parameter. Just like `perror', `error' also can report an error code in textual form. But unlike `perror' the error value is explicitly passed to the function in the ERRNUM parameter. This elimintates the problem mentioned above that the error reporting function must be called immediately after the function causing the error since otherwise `errno' might have a different value. The `error' prints first the program name. If the application defined a global variable `error_print_progname' and points it to a function this function will be called to print the program name. Otherwise the string from the global variable `program_name' is used. The program name is followed by a colon and a space which in turn is followed by the output produced by the format string. If the ERRNUM parameter is non-zero the format string output is followed by a colon and a space, followed by the error message for the error code ERRNUM. In any case is the output terminated with a newline. The output is directed to the `stderr' stream. If the `stderr' wasn't oriented before the call it will be narrow-oriented afterwards. The function will return unless the STATUS parameter has a non-zero value. In this case the function will call `exit' with the STATUS value for its parameter and therefore never return. If `error' returns the global variable `error_message_count' is incremented by one to keep track of the number of errors reported. - Function: void error_at_line (int STATUS, int ERRNUM, const char *FNAME, unsigned int LINENO, const char *FORMAT, ...) The `error_at_line' function is very similar to the `error' function. The only difference are the additional parameters FNAME and LINENO. The handling of the other parameters is identical to that of `error' except that between the program name and the string generated by the format string additional text is inserted. Directly following the program name a colon, followed by the file name pointer to by FNAME, another colon, and a value of LINENO is printed. This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file etc). If the global variable `error_one_per_line' is set to a non-zero value `error_at_line' will avoid printing consecutive messages for the same file anem line. Repetition which are not directly following each other are not caught. Just like `error' this function only returned if STATUS is zero. Otherwise `exit' is called with the non-zero value. If `error' returns the global variable `error_message_count' is incremented by one to keep track of the number of errors reported. As mentioned above the `error' and `error_at_line' functions can be customized by defining a variable named `error_print_progname'. - Variable: void (* error_print_progname ) (void) If the `error_print_progname' variable is defined to a non-zero value the function pointed to is called by `error' or `error_at_line'. It is expected to print the program name or do something similarly useful. The function is expected to be print to the `stderr' stream and must be able to handle whatever orientation the stream has. The variable is global and shared by all threads. - Variable: unsigned int error_message_count The `error_message_count' variable is incremented whenever one of the functions `error' or `error_at_line' returns. The variable is global and shared by all threads. - Variable: int error_one_per_line The `error_one_per_line' variable influences only `error_at_line'. Normally the `error_at_line' function creates output for every invocation. If `error_one_per_line' is set to a non-zero value `error_at_line' keeps track of the last file name and line number for which an error was reported and avoid directly following messages for the same file and line. This variable is global and shared by all threads. A program which read some input file and reports errors in it could look like this: { char *line = NULL; size_t len = 0; unsigned int lineno = 0; error_message_count = 0; while (! feof_unlocked (fp)) { ssize_t n = getline (&line, &len, fp); if (n <= 0) /* End of file or error. */ break; ++lineno; /* Process the line. */ ... if (Detect error in line) error_at_line (0, errval, filename, lineno, "some error text %s", some_variable); } if (error_message_count != 0) error (EXIT_FAILURE, 0, "%u errors found", error_message_count); } `error' and `error_at_line' are clearly the functions of choice and enable the programmer to write applications which follow the GNU coding standard. The GNU libc additionally contains functions which are used in BSD for the same purpose. These functions are declared in `err.h'. It is generally advised to not use these functions. They are included only for compatibility. - Function: void warn (const char *FORMAT, ...) The `warn' function is roughly equivalent to a call like error (0, errno, format, the parameters) except that the global variables `error' respects and modifies are not used. - Function: void vwarn (const char *FORMAT, va_list) The `vwarn' function is just like `warn' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void warnx (const char *FORMAT, ...) The `warnx' function is roughly equivalent to a call like error (0, 0, format, the parameters) except that the global variables `error' respects and modifies are not used. The difference to `warn' is that no error number string is printed. - Function: void vwarnx (const char *FORMAT, va_list) The `vwarnx' function is just like `warnx' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void err (int STATUS, const char *FORMAT, ...) The `err' function is roughly equivalent to a call like error (status, errno, format, the parameters) except that the global variables `error' respects and modifies are not used and that the program is exited even if STATUS is zero. - Function: void verr (int STATUS, const char *FORMAT, va_list) The `verr' function is just like `err' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. - Function: void errx (int STATUS, const char *FORMAT, ...) The `errx' function is roughly equivalent to a call like error (status, 0, format, the parameters) except that the global variables `error' respects and modifies are not used and that the program is exited even if STATUS is zero. The difference to `err' is that no error number string is printed. - Function: void verrx (int STATUS, const char *FORMAT, va_list) The `verrx' function is just like `errx' except that the parameters for the handling of the format string FORMAT are passed in as an value of type `va_list'. Virtual Memory Allocation And Paging ************************************ This chapter describes how processes manage and use memory in a system that uses the GNU C library. The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory. Memory mapped I/O is not discussed in this chapter. *Note Memory-mapped I/O::. Process Memory Concepts ======================= One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e. not all of these addresses actually can be used to store data. The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a "frame") or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it - there's just a flag saying it is all zeroes. The same frame of real memory or backing store can back multiple virtual pages belonging to multiple processes. This is normally the case, for example, with virtual memory occupied by GNU C library code. The same real memory frame containing the `printf' function backs a virtual memory page in each of the existing processes that has a `printf' call in its program. In order for a program to access any part of a virtual page, the page must at that moment be backed by ("connected to") a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called "paging". When a program attempts to access a page which is not at that moment backed by real memory, this is known as a "page fault". When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called "paging in" or "faulting in"), then resumes the process so that from the process' point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in *Note Locking Pages:: can control it. Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things. Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it's not very interesting. *Note Creating a Process::. Exec is the operation of creating a virtual address space for a process, loading its basic program into it, and executing the program. It is done by the "exec" family of functions (e.g. `execl'). The operation takes a program file (an executable), it allocates space to load all the data in the executable, loads it, and transfers control to it. That data is most notably the instructions of the program (the "text"), but also literals and constants in the program and even some variables: C variables with the static storage class (*note Memory Allocation and C::). Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C library, there are two kinds of programmatic allocation: automatic and dynamic. *Note Memory Allocation and C::. Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. *Note Memory-mapped I/O::. Just as it programmatically allocates memory, the program can programmatically deallocate ("free") it. You can't free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. *Note Program Termination::. A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are: * The "text segment" contains a program's instructions and literals and static constants. It is allocated by exec and stays the same size for the life of the virtual address space. * The "data segment" is working storage for the program. It can be preallocated and preloaded by exec and the process can extend or shrink it by calling functions as described in *Note Resizing the Data Segment::. Its lower end is fixed. * The "stack segment" contains a program stack. It grows as the stack grows, but doesn't shrink when the stack shrinks. Allocating Storage For Program Data =================================== This section covers how ordinary programs manage storage for their data, including the famous `malloc' function and some fancier facilities special the GNU C library and GNU Compiler. Memory Allocation in C Programs ------------------------------- The C language supports two kinds of memory allocation through the variables in C programs: * "Static allocation" is what happens when you declare a static or global variable. Each static or global variable defines one block of space, of a fixed size. The space is allocated once, when your program is started (part of the exec operation), and is never freed. * "Automatic allocation" happens when you declare an automatic variable, such as a function argument or a local variable. The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited. In GNU C, the size of the automatic storage can be an expression that varies. In other C implementations, it must be a constant. A third important kind of memory allocation, "dynamic allocation", is not supported by C variables but is available via GNU C library functions. Dynamic Memory Allocation ......................... "Dynamic memory allocation" is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs. For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line. Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it. When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want. Dynamic allocation is not supported by C variables; there is no storage class "dynamic", and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve. For example, if you want to allocate dynamically some space to hold a `struct foobar', you cannot declare a variable of type `struct foobar' whose contents are the dynamically allocated space. But you can declare a variable of pointer type `struct foobar *' and assign it the address of the space. Then you can use the operators `*' and `->' on this pointer variable to refer to the contents of the space: { struct foobar *ptr = (struct foobar *) malloc (sizeof (struct foobar)); ptr->name = x; ptr->next = current_foobar; current_foobar = ptr; } Unconstrained Allocation ------------------------ The most general dynamic allocation facility is `malloc'. It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never). Basic Memory Allocation ....................... To allocate a block of memory, call `malloc'. The prototype for this function is in `stdlib.h'. - Function: void * malloc (size_t SIZE) This function returns a pointer to a newly allocated block SIZE bytes long, or a null pointer if the block could not be allocated. The contents of the block are undefined; you must initialize it yourself (or use `calloc' instead; *note Allocating Cleared Space::). Normally you would cast the value as a pointer to the kind of object that you want to store in the block. Here we show an example of doing so, and of initializing the space with zeros using the library function `memset' (*note Copying and Concatenation::): struct foo *ptr; ... ptr = (struct foo *) malloc (sizeof (struct foo)); if (ptr == 0) abort (); memset (ptr, 0, sizeof (struct foo)); You can store the result of `malloc' into any pointer variable without a cast, because ISO C automatically converts the type `void *' to another type of pointer when necessary. But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C. Remember that when allocating space for a string, the argument to `malloc' must be one plus the length of the string. This is because a string is terminated with a null character that doesn't count in the "length" of the string but does need space. For example: char *ptr; ... ptr = (char *) malloc (length + 1); *Note Representation of Strings::, for more information about this. Examples of `malloc' .................... If no more space is available, `malloc' returns a null pointer. You should check the value of _every_ call to `malloc'. It is useful to write a subroutine that calls `malloc' and reports an error if the value is a null pointer, returning only if the value is nonzero. This function is conventionally called `xmalloc'. Here it is: void * xmalloc (size_t size) { register void *value = malloc (size); if (value == 0) fatal ("virtual memory exhausted"); return value; } Here is a real example of using `malloc' (by way of `xmalloc'). The function `savestring' will copy a sequence of characters into a newly allocated null-terminated string: char * savestring (const char *ptr, size_t len) { register char *value = (char *) xmalloc (len + 1); value[len] = '\0'; return (char *) memcpy (value, ptr, len); } The block that `malloc' gives you is guaranteed to be aligned so that it can hold any type of data. In the GNU system, the address is always a multiple of eight on most systems, and a multiple of 16 on 64-bit systems. Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use `memalign', `posix_memalign' or `valloc' (*note Aligned Memory Blocks::). Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to `malloc'. If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that `malloc' uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use `realloc' (*note Changing Block Size::). Freeing Memory Allocated with `malloc' ...................................... When you no longer need a block that you got with `malloc', use the function `free' to make the block available to be allocated again. The prototype for this function is in `stdlib.h'. - Function: void free (void *PTR) The `free' function deallocates the block of memory pointed at by PTR. - Function: void cfree (void *PTR) This function does the same thing as `free'. It's provided for backward compatibility with SunOS; you should use `free' instead. Freeing a block alters the contents of the block. *Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it.* Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to: struct chain { struct chain *next; char *name; } void free_chain (struct chain *chain) { while (chain != 0) { struct chain *next = chain->next; free (chain->name); free (chain); chain = next; } } Occasionally, `free' can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to `malloc' to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by `malloc'. There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates. Changing the Size of a Block ............................ Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer. You can make the block longer by calling `realloc'. This function is declared in `stdlib.h'. - Function: void * realloc (void *PTR, size_t NEWSIZE) The `realloc' function changes the size of the block whose address is PTR to be NEWSIZE. Since the space after the end of the block may be in use, `realloc' may find it necessary to copy the block to a new address where more free space is available. The value of `realloc' is the new address of the block. If the block needs to be moved, `realloc' copies the old contents. If you pass a null pointer for PTR, `realloc' behaves just like `malloc (NEWSIZE)'. This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when `realloc' is passed a null pointer. Like `malloc', `realloc' may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated. In most cases it makes no difference what happens to the original block when `realloc' fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use a subroutine, conventionally called `xrealloc', that takes care of the error message as `xmalloc' does for `malloc': void * xrealloc (void *ptr, size_t size) { register void *value = realloc (ptr, size); if (value == 0) fatal ("Virtual memory exhausted"); return value; } You can also use `realloc' to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available. If the new size you specify is the same as the old size, `realloc' is guaranteed to change nothing and return the same address that you gave. Allocating Cleared Space ........................ The function `calloc' allocates memory and clears it to zero. It is declared in `stdlib.h'. - Function: void * calloc (size_t COUNT, size_t ELTSIZE) This function allocates a block long enough to contain a vector of COUNT elements, each of size ELTSIZE. Its contents are cleared to zero before `calloc' returns. You could define `calloc' as follows: void * calloc (size_t count, size_t eltsize) { size_t size = count * eltsize; void *value = malloc (size); if (value != 0) memset (value, 0, size); return value; } But in general, it is not guaranteed that `calloc' calls `malloc' internally. Therefore, if an application provides its own `malloc'/`realloc'/`free' outside the C library, it should always define `calloc', too. Efficiency Considerations for `malloc' ...................................... As opposed to other versions, the `malloc' in the GNU C Library does not round up block sizes to powers of two, neither for large nor for small sizes. Neighboring chunks can be coalesced on a `free' no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation. Very large blocks (much larger than a page) are allocated with `mmap' (anonymous or via `/dev/zero') by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed. Therefore, it cannot happen that a large chunk becomes "locked" in between smaller ones and even after calling `free' wastes memory. The size threshold for `mmap' to be used can be adjusted with `mallopt'. The use of `mmap' can also be disabled completely. Allocating Aligned Memory Blocks ................................ The address of a block returned by `malloc' or `realloc' in the GNU system is always a multiple of eight (or sixteen on 64-bit systems). If you need a block whose address is a multiple of a higher power of two than that, use `memalign', `posix_memalign', or `valloc'. These functions are declared in `stdlib.h'. With the GNU library, you can use `free' to free the blocks that `memalign', `posix_memalign', and `valloc' return. That does not work in BSD, however--BSD does not provide any way to free such blocks. - Function: void * memalign (size_t BOUNDARY, size_t SIZE) The `memalign' function allocates a block of SIZE bytes whose address is a multiple of BOUNDARY. The BOUNDARY must be a power of two! The function `memalign' works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary. - Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT, size_t SIZE) The `posix_memalign' function is similar to the `memalign' function in that it returns a buffer of SIZE bytes aligned to a multiple of ALIGNMENT. But it adds one requirement to the parameter ALIGNMENT: the value must be a power of two multiple of `sizeof (void *)'. If the function succeeds in allocation memory a pointer to the allocated memory is returned in `*MEMPTR' and the return value is zero. Otherwise the function returns an error value indicating the problem. This function was introduced in POSIX 1003.1d. - Function: void * valloc (size_t SIZE) Using `valloc' is like using `memalign' and passing the page size as the value of the second argument. It is implemented like this: void * valloc (size_t size) { return memalign (getpagesize (), size); } *Note Query Memory Parameters:: for more information about the memory subsystem. Malloc Tunable Parameters ......................... You can adjust some parameters for dynamic memory allocation with the `mallopt' function. This function is the general SVID/XPG interface, defined in `malloc.h'. - Function: int mallopt (int PARAM, int VALUE) When calling `mallopt', the PARAM argument specifies the parameter to be set, and VALUE the new value to be set. Possible choices for PARAM, as defined in `malloc.h', are: `M_TRIM_THRESHOLD' This is the minimum size (in bytes) of the top-most, releasable chunk that will cause `sbrk' to be called with a negative argument in order to return memory to the system. `M_TOP_PAD' This parameter determines the amount of extra memory to obtain from the system when a call to `sbrk' is required. It also specifies the number of bytes to retain when shrinking the heap by calling `sbrk' with a negative argument. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided. `M_MMAP_THRESHOLD' All chunks larger than this value are allocated outside the normal heap, using the `mmap' system call. This way it is guaranteed that the memory for these chunks can be returned to the system on `free'. `M_MMAP_MAX' The maximum number of chunks to allocate with `mmap'. Setting this to zero disables all use of `mmap'. Heap Consistency Checking ......................... You can ask `malloc' to check the consistency of dynamic memory by using the `mcheck' function. This function is a GNU extension, declared in `mcheck.h'. - Function: int mcheck (void (*ABORTFN) (enum mcheck_status STATUS)) Calling `mcheck' tells `malloc' to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with `malloc'. The ABORTFN argument is the function to call when an inconsistency is found. If you supply a null pointer, then `mcheck' uses a default function which prints a message and calls `abort' (*note Aborting a Program::). The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below. It is too late to begin allocation checking once you have allocated anything with `malloc'. So `mcheck' does nothing in that case. The function returns `-1' if you call it too late, and `0' otherwise (when it is successful). The easiest way to arrange to call `mcheck' early enough is to use the option `-lmcheck' when you link your program; then you don't need to modify your program source at all. Alternatively you might use a debugger to insert a call to `mcheck' whenever the program is started, for example these gdb commands will automatically call `mcheck' whenever the program starts: (gdb) break main Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10 (gdb) command 1 Type commands for when breakpoint 1 is hit, one per line. End with a line saying just "end". >call mcheck(0) >continue >end (gdb) ... This will however only work if no initialization function of any object involved calls any of the `malloc' functions since `mcheck' must be called before the first such function. - Function: enum mcheck_status mprobe (void *POINTER) The `mprobe' function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called `mcheck' at the beginning of the program, to do its occasional checks; calling `mprobe' requests an additional consistency check to be done at the time of the call. The argument POINTER must be a pointer returned by `malloc' or `realloc'. `mprobe' returns a value that says what inconsistency, if any, was found. The values are described below. - Data Type: enum mcheck_status This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values: `MCHECK_DISABLED' `mcheck' was not called before the first allocation. No consistency checking can be done. `MCHECK_OK' No inconsistency detected. `MCHECK_HEAD' The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far. `MCHECK_TAIL' The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far. `MCHECK_FREE' The block was already freed. Another possibility to check for and guard against bugs in the use of `malloc', `realloc' and `free' is to set the environment variable `MALLOC_CHECK_'. When `MALLOC_CHECK_' is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of `free' with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If `MALLOC_CHECK_' is set to `0', any detected heap corruption is silently ignored; if set to `1', a diagnostic is printed on `stderr'; if set to `2', `abort' is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down. There is one problem with `MALLOC_CHECK_': in SUID or SGID binaries it could possibly be exploited since diverging from the normal programs behavior it now writes something to the standard error descriptor. Therefore the use of `MALLOC_CHECK_' is disabled by default for SUID and SGID binaries. It can be enabled again by the system administrator by adding a file `/etc/suid-debug' (the content is not important it could be empty). So, what's the difference between using `MALLOC_CHECK_' and linking with `-lmcheck'? `MALLOC_CHECK_' is orthogonal with respect to `-lmcheck'. `-lmcheck' has been added for backward compatibility. Both `MALLOC_CHECK_' and `-lmcheck' should uncover the same bugs - but using `MALLOC_CHECK_' you don't need to recompile your application. Memory Allocation Hooks ....................... The GNU C library lets you modify the behavior of `malloc', `realloc', and `free' by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example. The hook variables are declared in `malloc.h'. - Variable: __malloc_hook The value of this variable is a pointer to the function that `malloc' uses whenever it is called. You should define this function to look like `malloc'; that is, like: void *FUNCTION (size_t SIZE, const void *CALLER) The value of CALLER is the return address found on the stack when the `malloc' function was called. This value allows you to trace the memory consumption of the program. - Variable: __realloc_hook The value of this variable is a pointer to function that `realloc' uses whenever it is called. You should define this function to look like `realloc'; that is, like: void *FUNCTION (void *PTR, size_t SIZE, const void *CALLER) The value of CALLER is the return address found on the stack when the `realloc' function was called. This value allows you to trace the memory consumption of the program. - Variable: __free_hook The value of this variable is a pointer to function that `free' uses whenever it is called. You should define this function to look like `free'; that is, like: void FUNCTION (void *PTR, const void *CALLER) The value of CALLER is the return address found on the stack when the `free' function was called. This value allows you to trace the memory consumption of the program. - Variable: __memalign_hook The value of this variable is a pointer to function that `memalign' uses whenever it is called. You should define this function to look like `memalign'; that is, like: void *FUNCTION (size_t SIZE, size_t ALIGNMENT, const void *CALLER) The value of CALLER is the return address found on the stack when the `memalign' function was called. This value allows you to trace the memory consumption of the program. You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself. - Variable: __malloc_initialize_hook The value of this variable is a pointer to a function that is called once when the malloc implementation is initialized. This is a weak variable, so it can be overridden in the application with a definition like the following: void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook; An issue to look out for is the time at which the malloc hook functions can be safely installed. If the hook functions call the malloc-related functions recursively, it is necessary that malloc has already properly initialized itself at the time when `__malloc_hook' etc. is assigned to. On the other hand, if the hook functions provide a complete malloc implementation of their own, it is vital that the hooks are assigned to _before_ the very first `malloc' call has completed, because otherwise a chunk obtained from the ordinary, un-hooked malloc may later be handed to `__free_hook', for example. In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by `__malloc_initialize_hook'--then the hooks will be set up safely at the right time. Here is an example showing how to use `__malloc_hook' and `__free_hook' properly. It installs a function that prints out information every time `malloc' or `free' is called. We just assume here that `realloc' and `memalign' are not used in our program. /* Prototypes for __malloc_hook, __free_hook */ #include /* Prototypes for our hooks. */ static void *my_init_hook (void); static void *my_malloc_hook (size_t, const void *); static void my_free_hook (void*, const void *); /* Override initializing hook from the C library. */ void (*__malloc_initialize_hook) (void) = my_init_hook; static void my_init_hook (void) { old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; } static void * my_malloc_hook (size_t size, const void *caller) { void *result; /* Restore all old hooks */ __malloc_hook = old_malloc_hook; __free_hook = old_free_hook; /* Call recursively */ result = malloc (size); /* Save underlying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `malloc', so protect it too. */ printf ("malloc (%u) returns %p\n", (unsigned int) size, result); /* Restore our own hooks */ __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; return result; } static void * my_free_hook (void *ptr, const void *caller) { /* Restore all old hooks */ __malloc_hook = old_malloc_hook; __free_hook = old_free_hook; /* Call recursively */ free (ptr); /* Save underlying hooks */ old_malloc_hook = __malloc_hook; old_free_hook = __free_hook; /* `printf' might call `free', so protect it too. */ printf ("freed pointer %p\n", ptr); /* Restore our own hooks */ __malloc_hook = my_malloc_hook; __free_hook = my_free_hook; } main () { ... } The `mcheck' function (*note Heap Consistency Checking::) works by installing such hooks. Statistics for Memory Allocation with `malloc' .............................................. You can get information about dynamic memory allocation by calling the `mallinfo' function. This function and its associated data type are declared in `malloc.h'; they are an extension of the standard SVID/XPG version. - Data Type: struct mallinfo This structure type is used to return information about the dynamic memory allocator. It contains the following members: `int arena' This is the total size of memory allocated with `sbrk' by `malloc', in bytes. `int ordblks' This is the number of chunks not in use. (The memory allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual `malloc' requests; see *Note Efficiency and Malloc::.) `int smblks' This field is unused. `int hblks' This is the total number of chunks allocated with `mmap'. `int hblkhd' This is the total size of memory allocated with `mmap', in bytes. `int usmblks' This field is unused. `int fsmblks' This field is unused. `int uordblks' This is the total size of memory occupied by chunks handed out by `malloc'. `int fordblks' This is the total size of memory occupied by free (not in use) chunks. `int keepcost' This is the size of the top-most releasable chunk that normally borders the end of the heap (i.e. the high end of the virtual address space's data segment). - Function: struct mallinfo mallinfo (void) This function returns information about the current dynamic memory usage in a structure of type `struct mallinfo'. Summary of `malloc'-Related Functions ..................................... Here is a summary of the functions that work with `malloc': `void *malloc (size_t SIZE)' Allocate a block of SIZE bytes. *Note Basic Allocation::. `void free (void *ADDR)' Free a block previously allocated by `malloc'. *Note Freeing after Malloc::. `void *realloc (void *ADDR, size_t SIZE)' Make a block previously allocated by `malloc' larger or smaller, possibly by copying it to a new location. *Note Changing Block Size::. `void *calloc (size_t COUNT, size_t ELTSIZE)' Allocate a block of COUNT * ELTSIZE bytes using `malloc', and set its contents to zero. *Note Allocating Cleared Space::. `void *valloc (size_t SIZE)' Allocate a block of SIZE bytes, starting on a page boundary. *Note Aligned Memory Blocks::. `void *memalign (size_t SIZE, size_t BOUNDARY)' Allocate a block of SIZE bytes, starting on an address that is a multiple of BOUNDARY. *Note Aligned Memory Blocks::. `int mallopt (int PARAM, int VALUE)' Adjust a tunable parameter. *Note Malloc Tunable Parameters::. `int mcheck (void (*ABORTFN) (void))' Tell `malloc' to perform occasional consistency checks on dynamically allocated memory, and to call ABORTFN when an inconsistency is found. *Note Heap Consistency Checking::. `void *(*__malloc_hook) (size_t SIZE, const void *CALLER)' A pointer to a function that `malloc' uses whenever it is called. `void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)' A pointer to a function that `realloc' uses whenever it is called. `void (*__free_hook) (void *PTR, const void *CALLER)' A pointer to a function that `free' uses whenever it is called. `void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)' A pointer to a function that `memalign' uses whenever it is called. `struct mallinfo mallinfo (void)' Return information about the current dynamic memory usage. *Note Statistics of Malloc::. Allocation Debugging -------------------- A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later. The `malloc' implementation in the GNU C library provides some simple means to detect such leaks and obtain some information to find the location. To do this the application must be started in a special mode which is enabled by an environment variable. There are no speed penalties for the program if the debugging mode is not enabled. How to install the tracing functionality ........................................ - Function: void mtrace (void) When the `mtrace' function is called it looks for an environment variable named `MALLOC_TRACE'. This variable is supposed to contain a valid file name. The user must have write access. If the file already exists it is truncated. If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done. The behavior of `malloc' etc. is not changed. For obvious reasons this also happens if the application is installed with the SUID or SGID bit set. If the named file is successfully opened, `mtrace' installs special handlers for the functions `malloc', `realloc', and `free' (*note Hooks for Malloc::). From then on, all uses of these functions are traced and protocolled into the file. There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during normal use. This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'. - Function: void muntrace (void) The `muntrace' function can be called after `mtrace' was used to enable tracing the `malloc' calls. If no (successful) call of `mtrace' was made `muntrace' does nothing. Otherwise it deinstalls the handlers for `malloc', `realloc', and `free' and then closes the protocol file. No calls are protocolled anymore and the program runs again at full speed. This function is a GNU extension and generally not available on other systems. The prototype can be found in `mcheck.h'. Example program excerpts ........................ Even though the tracing functionality does not influence the runtime behavior of the program it is not a good idea to call `mtrace' in all programs. Just imagine that you debug a program using `mtrace' and all other programs used in the debugging session also trace their `malloc' calls. The output file would be the same for all programs and thus is unusable. Therefore one should call `mtrace' only if compiled for debugging. A program could therefore start like this: #include int main (int argc, char *argv[]) { #ifdef DEBUGGING mtrace (); #endif ... } This is all what is needed if you want to trace the calls during the whole runtime of the program. Alternatively you can stop the tracing at any time with a call to `muntrace'. It is even possible to restart the tracing again with a new call to `mtrace'. But this can cause unreliable results since there may be calls of the functions which are not called. Please note that not only the application uses the traced functions, also libraries (including the C library itself) use these functions. This last point is also why it is no good idea to call `muntrace' before the program terminated. The libraries are informed about the termination of the program only after the program returns from `main' or calls `exit' and so cannot free the memory they use before this time. So the best thing one can do is to call `mtrace' as the very first function in the program and never call `muntrace'. So the program traces almost all uses of the `malloc' functions (except those calls which are executed by constructors of the program or used libraries). Some more or less clever ideas .............................. You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program: #include #include static void enable (int sig) { mtrace (); signal (SIGUSR1, enable); } static void disable (int sig) { muntrace (); signal (SIGUSR2, disable); } int main (int argc, char *argv[]) { ... signal (SIGUSR1, enable); signal (SIGUSR2, disable); ... } I.e., the user can start the memory debugger any time s/he wants if the program was started with `MALLOC_TRACE' set in the environment. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless. Interpreting the traces ....................... If you take a look at the output it will look similar to this: = Start [0x8048209] - 0x8064cc8 [0x8048209] - 0x8064ce0 [0x8048209] - 0x8064cf8 [0x80481eb] + 0x8064c48 0x14 [0x80481eb] + 0x8064c60 0x14 [0x80481eb] + 0x8064c78 0x14 [0x80481eb] + 0x8064c90 0x14 = End What this all means is not really important since the trace file is not meant to be read by a human. Therefore no attention is given to readability. Instead there is a program which comes with the GNU C library which interprets the traces and outputs a summary in an user-friendly way. The program is called `mtrace' (it is in fact a Perl script) and it takes one or two arguments. In any case the name of the file with the trace output must be specified. If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace. drepper$ mtrace tst-mtrace log No memory leaks. In this case the program `tst-mtrace' was run and it produced a trace file `log'. The message printed by `mtrace' shows there are no problems with the code, all allocated memory was freed afterwards. If we call `mtrace' on the example trace given above we would get a different outout: drepper$ mtrace errlog - 0x08064cc8 Free 2 was never alloc'd 0x8048209 - 0x08064ce0 Free 3 was never alloc'd 0x8048209 - 0x08064cf8 Free 4 was never alloc'd 0x8048209 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at 0x80481eb 0x08064c60 0x14 at 0x80481eb 0x08064c78 0x14 at 0x80481eb 0x08064c90 0x14 at 0x80481eb We have called `mtrace' with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace. We can do better: drepper$ mtrace tst errlog - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39 - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39 - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39 Memory not freed: ----------------- Address Size Caller 0x08064c48 0x14 at /home/drepper/tst.c:33 0x08064c60 0x14 at /home/drepper/tst.c:33 0x08064c78 0x14 at /home/drepper/tst.c:33 0x08064c90 0x14 at /home/drepper/tst.c:33 Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found. Interpreting this output is not complicated. There are at most two different situations being detected. First, `free' was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output. Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes. The other situation which is much harder to detect are memory leaks. As you can see in the output the `mtrace' function collects all this information and so can say that the program calls an allocation function from line 33 in the source file `/home/drepper/tst-mtrace.c' four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated. Obstacks -------- An "obstack" is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other. Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary. Creating Obstacks ................. The utilities for manipulating obstacks are declared in the header file `obstack.h'. - Data Type: struct obstack An obstack is represented by a data structure of type `struct obstack'. This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves. You should not try to access the contents of the structure directly; use only the functions described in this chapter. You can declare variables of type `struct obstack' and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object. Dynamic allocation of obstacks allows your program to have a variable number of different stacks. (You can even allocate an obstack structure in another obstack, but this is rarely useful.) All the functions that work with obstacks require you to specify which obstack to use. You do this with a pointer of type `struct obstack *'. In the following, we often say "an obstack" when strictly speaking the object at hand is such a pointer. The objects in the obstack are packed into large blocks called "chunks". The `struct obstack' structure points to a chain of the chunks currently in use. The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk. Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk. Usually you supply a function which uses `malloc' directly or indirectly. You must also supply a function to free a chunk. These matters are described in the following section. Preparing for Using Obstacks ............................ Each source file in which you plan to use the obstack functions must include the header file `obstack.h', like this: #include Also, if the source file uses the macro `obstack_init', it must declare or define two functions or macros that will be called by the obstack library. One, `obstack_chunk_alloc', is used to allocate the chunks of memory into which objects are packed. The other, `obstack_chunk_free', is used to return chunks when the objects in them are freed. These macros should appear before any use of obstacks in the source file. Usually these are defined to use `malloc' via the intermediary `xmalloc' (*note Unconstrained Allocation::). This is done with the following pair of macro definitions: #define obstack_chunk_alloc xmalloc #define obstack_chunk_free free Though the memory you get using obstacks really comes from `malloc', using obstacks is faster because `malloc' is called less often, for larger blocks of memory. *Note Obstack Chunks::, for full details. At run time, before the program can use a `struct obstack' object as an obstack, it must initialize the obstack by calling `obstack_init'. - Function: int obstack_init (struct obstack *OBSTACK-PTR) Initialize obstack OBSTACK-PTR for allocation of objects. This function calls the obstack's `obstack_chunk_alloc' function. If allocation of memory fails, the function pointed to by `obstack_alloc_failed_handler' is called. The `obstack_init' function always returns 1 (Compatibility notice: Former versions of obstack returned 0 if allocation failed). Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable: static struct obstack myobstack; ... obstack_init (&myobstack); Second, an obstack that is itself dynamically allocated: struct obstack *myobstack_ptr = (struct obstack *) xmalloc (sizeof (struct obstack)); obstack_init (myobstack_ptr); - Variable: obstack_alloc_failed_handler The value of this variable is a pointer to a function that `obstack' uses when `obstack_chunk_alloc' fails to allocate memory. The default action is to print a message and abort. You should supply a function that either calls `exit' (*note Program Termination::) or `longjmp' (*note Non-Local Exits::) and doesn't return. void my_obstack_alloc_failed (void) ... obstack_alloc_failed_handler = &my_obstack_alloc_failed; Allocation in an Obstack ........................ The most direct way to allocate an object in an obstack is with `obstack_alloc', which is invoked almost like `malloc'. - Function: void * obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE) This allocates an uninitialized block of SIZE bytes in an obstack and returns its address. Here OBSTACK-PTR specifies which obstack to allocate the block in; it is the address of the `struct obstack' object which represents the obstack. Each obstack function or macro requires you to specify an OBSTACK-PTR as the first argument. This function calls the obstack's `obstack_chunk_alloc' function if it needs to allocate a new chunk of memory; it calls `obstack_alloc_failed_handler' if allocation of memory by `obstack_chunk_alloc' failed. For example, here is a function that allocates a copy of a string STR in a specific obstack, which is in the variable `string_obstack': struct obstack string_obstack; char * copystring (char *string) { size_t len = strlen (string) + 1; char *s = (char *) obstack_alloc (&string_obstack, len); memcpy (s, string, len); return s; } To allocate a block with specified contents, use the function `obstack_copy', declared like this: - Function: void * obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE) This allocates a block and initializes it by copying SIZE bytes of data starting at ADDRESS. It calls `obstack_alloc_failed_handler' if allocation of memory by `obstack_chunk_alloc' failed. - Function: void * obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE) Like `obstack_copy', but appends an extra byte containing a null character. This extra byte is not counted in the argument SIZE. The `obstack_copy0' function is convenient for copying a sequence of characters into an obstack as a null-terminated string. Here is an example of its use: char * obstack_savestring (char *addr, int size) { return obstack_copy0 (&myobstack, addr, size); } Contrast this with the previous example of `savestring' using `malloc' (*note Basic Allocation::). Freeing Objects in an Obstack ............................. To free an object allocated in an obstack, use the function `obstack_free'. Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack. - Function: void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT) If OBJECT is a null pointer, everything allocated in the obstack is freed. Otherwise, OBJECT must be the address of an object allocated in the obstack. Then OBJECT is freed, along with everything allocated in OBSTACK since OBJECT. Note that if OBJECT is a null pointer, the result is an uninitialized obstack. To free all memory in an obstack but leave it valid for further allocation, call `obstack_free' with the address of the first object allocated on the obstack: obstack_free (obstack_ptr, first_object_allocated_ptr); Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (*note Preparing for Obstacks::). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk. Obstack Functions and Macros ............................ The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C. If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address). Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this: obstack_alloc (get_obstack (), 4); you will find that `get_obstack' may be called several times. If you use `*obstack_list_ptr++' as the obstack pointer argument, you will get very strange results since the incrementation may occur several times. In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here: char *x; void *(*funcp) (); /* Use the macro. */ x = (char *) obstack_alloc (obptr, size); /* Call the function. */ x = (char *) (obstack_alloc) (obptr, size); /* Take the address of the function. */ funcp = obstack_alloc; This is the same situation that exists in ISO C for the standard library functions. *Note Macro Definitions::. *Warning:* When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C. If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once. Growing Objects ............... Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of "growing objects". The special functions for adding data to the growing object are described in this section. You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it. However, it is necessary to say explicitly when the object is finished. This is done with the function `obstack_finish'. The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk. While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object. - Function: void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE) The most basic function for adding to a growing object is `obstack_blank', which adds space without initializing it. - Function: void obstack_grow (struct obstack *OBSTACK-PTR, void *DATA, int SIZE) To add a block of initialized space, use `obstack_grow', which is the growing-object analogue of `obstack_copy'. It adds SIZE bytes of data to the growing object, copying the contents from DATA. - Function: void obstack_grow0 (struct obstack *OBSTACK-PTR, void *DATA, int SIZE) This is the growing-object analogue of `obstack_copy0'. It adds SIZE bytes copied from DATA, followed by an additional null character. - Function: void obstack_1grow (struct obstack *OBSTACK-PTR, char C) To add one character at a time, use the function `obstack_1grow'. It adds a single byte containing C to the growing object. - Function: void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void *DATA) Adding the value of a pointer one can use the function `obstack_ptr_grow'. It adds `sizeof (void *)' bytes containing the value of DATA. - Function: void obstack_int_grow (struct obstack *OBSTACK-PTR, int DATA) A single value of type `int' can be added by using the `obstack_int_grow' function. It adds `sizeof (int)' bytes to the growing object and initializes them with the value of DATA. - Function: void * obstack_finish (struct obstack *OBSTACK-PTR) When you are finished growing the object, use the function `obstack_finish' to close it off and return its final address. Once you have finished the object, the obstack is available for ordinary allocation or for growing another object. This function can return a null pointer under the same conditions as `obstack_alloc' (*note Allocation in an Obstack::). When you build an object by growing it, you will probably need to know afterward how long it became. You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function `obstack_object_size', declared as follows: - Function: int obstack_object_size (struct obstack *OBSTACK-PTR) This function returns the current size of the growing object, in bytes. Remember to call this function _before_ finishing the object. After it is finished, `obstack_object_size' will return zero. If you have started growing an object and wish to cancel it, you should finish it and then free it, like this: obstack_free (obstack_ptr, obstack_finish (obstack_ptr)); This has no effect if no object was growing. You can use `obstack_blank' with a negative size argument to make the current object smaller. Just don't try to shrink it beyond zero length--there's no telling what will happen if you do that. Extra Fast Growing Objects .......................... The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant. You can reduce the overhead by using special "fast growth" functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster. The function `obstack_room' returns the amount of room available in the current chunk. It is declared as follows: - Function: int obstack_room (struct obstack *OBSTACK-PTR) This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack OBSTACK using the fast growth functions. While you know there is room, you can use these fast growth functions for adding data to a growing object: - Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char C) The function `obstack_1grow_fast' adds one byte containing the character C to the growing object in obstack OBSTACK-PTR. - Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR, void *DATA) The function `obstack_ptr_grow_fast' adds `sizeof (void *)' bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR. - Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR, int DATA) The function `obstack_int_grow_fast' adds `sizeof (int)' bytes containing the value of DATA to the growing object in obstack OBSTACK-PTR. - Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE) The function `obstack_blank_fast' adds SIZE bytes to the growing object in obstack OBSTACK-PTR without initializing them. When you check for space using `obstack_room' and there is not enough room for what you want to add, the fast growth functions are not safe. In this case, simply use the corresponding ordinary growth function instead. Very soon this will copy the object to a new chunk; then there will be lots of room available again. So, each time you use an ordinary growth function, check afterward for sufficient space using `obstack_room'. Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again. Here is an example: void add_string (struct obstack *obstack, const char *ptr, int len) { while (len > 0) { int room = obstack_room (obstack); if (room == 0) { /* Not enough room. Add one character slowly, which may copy to a new chunk and make room. */ obstack_1grow (obstack, *ptr++); len--; } else { if (room > len) room = len; /* Add fast as much as we have room for. */ len -= room; while (room-- > 0) obstack_1grow_fast (obstack, *ptr++); } } } Status of an Obstack .................... Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it. - Function: void * obstack_base (struct obstack *OBSTACK-PTR) This function returns the tentative address of the beginning of the currently growing object in OBSTACK-PTR. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk--then its address will change! If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk). - Function: void * obstack_next_free (struct obstack *OBSTACK-PTR) This function returns the address of the first free byte in the current chunk of obstack OBSTACK-PTR. This is the end of the currently growing object. If no object is growing, `obstack_next_free' returns the same value as `obstack_base'. - Function: int obstack_object_size (struct obstack *OBSTACK-PTR) This function returns the size in bytes of the currently growing object. This is equivalent to obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR) Alignment of Data in Obstacks ............................. Each obstack has an "alignment boundary"; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is 4 bytes. To access an obstack's alignment boundary, use the macro `obstack_alignment_mask', whose function prototype looks like this: - Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR) The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0. The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2. The default value of the mask is 3, so that addresses are multiples of 4. A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required). The expansion of the macro `obstack_alignment_mask' is an lvalue, so you can alter the mask by assignment. For example, this statement: obstack_alignment_mask (obstack_ptr) = 0; has the effect of turning off alignment processing in the specified obstack. Note that a change in alignment mask does not take effect until _after_ the next time an object is allocated or finished in the obstack. If you are not growing an object, you can make the new alignment mask take effect immediately by calling `obstack_finish'. This will finish a zero-length object and then do proper alignment for the next object. Obstack Chunks .............. Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects. The obstack library allocates chunks by calling the function `obstack_chunk_alloc', which you must define. When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling `obstack_chunk_free', which you must also define. These two must be defined (as macros) or declared (as functions) in each source file that uses `obstack_init' (*note Creating Obstacks::). Most often they are defined as macros like this: #define obstack_chunk_alloc malloc #define obstack_chunk_free free Note that these are simple macros (no arguments). Macro definitions with arguments will not work! It is necessary that `obstack_chunk_alloc' or `obstack_chunk_free', alone, expand into a function name if it is not itself a function name. If you allocate chunks with `malloc', the chunk size should be a power of 2. The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used. - Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR) This returns the chunk size of the given obstack. Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly: if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE) obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE; Summary of Obstack Functions ............................ Here is a summary of all the functions associated with obstacks. Each takes the address of an obstack (`struct obstack *') as its first argument. `void obstack_init (struct obstack *OBSTACK-PTR)' Initialize use of an obstack. *Note Creating Obstacks::. `void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)' Allocate an object of SIZE uninitialized bytes. *Note Allocation in an Obstack::. `void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE bytes, with contents copied from ADDRESS. *Note Allocation in an Obstack::. `void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Allocate an object of SIZE+1 bytes, with SIZE of them copied from ADDRESS, followed by a null character at the end. *Note Allocation in an Obstack::. `void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)' Free OBJECT (and everything allocated in the specified obstack more recently than OBJECT). *Note Freeing Obstack Objects::. `void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object. *Note Growing Objects::. `void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object. *Note Growing Objects::. `void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)' Add SIZE bytes, copied from ADDRESS, to a growing object, and then add another byte containing a null character. *Note Growing Objects::. `void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object. *Note Growing Objects::. `void *obstack_finish (struct obstack *OBSTACK-PTR)' Finalize the object that is growing and return its permanent address. *Note Growing Objects::. `int obstack_object_size (struct obstack *OBSTACK-PTR)' Get the current size of the currently growing object. *Note Growing Objects::. `void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)' Add SIZE uninitialized bytes to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)' Add one byte containing DATA-CHAR to a growing object without checking that there is enough room. *Note Extra Fast Growing::. `int obstack_room (struct obstack *OBSTACK-PTR)' Get the amount of room now available for growing the current object. *Note Extra Fast Growing::. `int obstack_alignment_mask (struct obstack *OBSTACK-PTR)' The mask used for aligning the beginning of an object. This is an lvalue. *Note Obstacks Data Alignment::. `int obstack_chunk_size (struct obstack *OBSTACK-PTR)' The size for allocating chunks. This is an lvalue. *Note Obstack Chunks::. `void *obstack_base (struct obstack *OBSTACK-PTR)' Tentative starting address of the currently growing object. *Note Status of an Obstack::. `void *obstack_next_free (struct obstack *OBSTACK-PTR)' Address just after the end of the currently growing object. *Note Status of an Obstack::. Automatic Storage with Variable Size ------------------------------------ The function `alloca' supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically. Allocating a block with `alloca' is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time. But all the blocks are freed when you exit the function that `alloca' was called from, just as if they were automatic variables declared in that function. There is no way to free the space explicitly. The prototype for `alloca' is in `stdlib.h'. This function is a BSD extension. - Function: void * alloca (size_t SIZE); The return value of `alloca' is the address of a block of SIZE bytes of memory, allocated in the stack frame of the calling function. Do not use `alloca' inside the arguments of a function call--you will get unpredictable results, because the stack space for the `alloca' would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is `foo (x, alloca (4), y)'. `alloca' Example ................ As an example of the use of `alloca', here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure: int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1); stpcpy (stpcpy (name, str1), str2); return open (name, flags, mode); } Here is how you would get the same results with `malloc' and `free': int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1); int desc; if (name == 0) fatal ("virtual memory exceeded"); stpcpy (stpcpy (name, str1), str2); desc = open (name, flags, mode); free (name); return desc; } As you can see, it is simpler with `alloca'. But `alloca' has other, more important advantages, and some disadvantages. Advantages of `alloca' ...................... Here are the reasons why `alloca' may be preferable to `malloc': * Using `alloca' wastes very little space and is very fast. (It is open-coded by the GNU C compiler.) * Since `alloca' does not have separate pools for different sizes of block, space used for any size block can be reused for any other size. `alloca' does not cause memory fragmentation. * Nonlocal exits done with `longjmp' (*note Non-Local Exits::) automatically free the space allocated with `alloca' when they exit through the function that called `alloca'. This is the most important reason to use `alloca'. To illustrate this, suppose you have a function `open_or_report_error' which returns a descriptor, like `open', if it succeeds, but does not return to its caller if it fails. If the file cannot be opened, it prints an error message and jumps out to the command level of your program using `longjmp'. Let's change `open2' (*note Alloca Example::) to use this subroutine: int open2 (char *str1, char *str2, int flags, int mode) { char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1); stpcpy (stpcpy (name, str1), str2); return open_or_report_error (name, flags, mode); } Because of the way `alloca' works, the memory it allocates is freed even when an error occurs, with no special effort required. By contrast, the previous definition of `open2' (which uses `malloc' and `free') would develop a memory leak if it were changed in this way. Even if you are willing to make more changes to fix it, there is no easy way to do so. Disadvantages of `alloca' ......................... These are the disadvantages of `alloca' in comparison with `malloc': * If you try to allocate more memory than the machine can provide, you don't get a clean error message. Instead you get a fatal signal like the one you would get from an infinite recursion; probably a segmentation violation (*note Program Error Signals::). * Some non-GNU systems fail to support `alloca', so it is less portable. However, a slower emulation of `alloca' written in C is available for use on systems with this deficiency. GNU C Variable-Size Arrays .......................... In GNU C, you can replace most uses of `alloca' with an array of variable size. Here is how `open2' would look then: int open2 (char *str1, char *str2, int flags, int mode) { char name[strlen (str1) + strlen (str2) + 1]; stpcpy (stpcpy (name, str1), str2); return open (name, flags, mode); } But `alloca' is not always equivalent to a variable-sized array, for several reasons: * A variable size array's space is freed at the end of the scope of the name of the array. The space allocated with `alloca' remains until the end of the function. * It is possible to use `alloca' within a loop, allocating an additional block on each iteration. This is impossible with variable-sized arrays. *Note:* If you mix use of `alloca' and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with `alloca' during the execution of that scope. Resizing the Data Segment ========================= The symbols in this section are declared in `unistd.h'. You will not normally use the functions in this section, because the functions described in *Note Memory Allocation:: are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls. - Function: int brk (void *ADDR) `brk' sets the high end of the calling process' data segment to ADDR. The address of the end of a segment is defined to be the address of the last byte in the segment plus 1. The function has no effect if ADDR is lower than the low end of the data segment. (This is considered success, by the way). The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit (*note Limits on Resources::). The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the "break". The return value is zero on success. On failure, the return value is `-1' and `errno' is set accordingly. The following `errno' values are specific to this function: `ENOMEM' The request would cause the data segment to overlap another segment or exceed the process' data storage limit. - Function: int sbrk (ptrdiff_t DELTA) This function is the same as `brk' except that you specify the new end of the data segment as an offset DELTA from the current end and on success the return value is the address of the resulting end of the data segment instead of zero. This means you can use `sbrk(0)' to find out what the current end of the data segment is. Locking Pages ============= You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way -- i.e. cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault. This is called "locking" a page. The functions in this chapter lock and unlock the calling process' pages. Why Lock Pages -------------- Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are: * Speed. A page fault is transparent only insofar as the process is not sensitive to how long it takes to do a simple memory access. Time-critical processes, especially realtime processes, may not be able to wait or may not be able to tolerate variance in execution speed. A process that needs to lock pages for this reason probably also needs priority among other processes for use of the CPU. *Note Priority::. In some cases, the programmer knows better than the system's demand paging allocator which pages should remain in real memory to optimize system performance. In this case, locking pages can help. * Privacy. If you keep secrets in virtual memory and that virtual memory gets paged out, that increases the chance that the secrets will get out. If a password gets written out to disk swap space, for example, it might still be there long after virtual and real memory have been wiped clean. Be aware that when you lock a page, that's one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory. Locked Memory Details --------------------- A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don't page it out. Memory locks do not stack. I.e. you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn't. A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more). Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). *Note Creating a Process::. Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page. The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. *Note Limits on Resources::. In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked. But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data. This is known as a "copy-on-write page fault". It takes a small amount of time and in a pathological case, getting that frame may require I/O. To make sure this doesn't happen to your program, don't just lock the pages. Write to them as well, unless you know you won't write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope. Functions To Lock And Unlock Pages ---------------------------------- The symbols in this section are declared in `sys/mman.h'. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They _are_ available with a Linux kernel. *Portability Note:* POSIX.1b requires that when the `mlock' and `munlock' functions are available, the file `unistd.h' define the macro `_POSIX_MEMLOCK_RANGE' and the file `limits.h' define the macro `PAGESIZE' to be the size of a memory page in bytes. It requires that when the `mlockall' and `munlockall' functions are available, the `unistd.h' file define the macro `_POSIX_MEMLOCK'. The GNU C library conforms to this requirement. - Function: int mlock (const void *ADDR, size_t LEN) `mlock' locks a range of the calling process' virtual pages. The range of memory starts at address ADDR and is LEN bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range. When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them. When the function fails, it does not affect the lock status of any pages. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. `errno' values specific to this function are: `ENOMEM' * At least some of the specified address range does not exist in the calling process' virtual address space. * The locking would cause the process to exceed its locked page limit. `EPERM' The calling process is not superuser. `EINVAL' LEN is not positive. `ENOSYS' The kernel does not provide `mlock' capability. You can lock _all_ a process' memory with `mlockall'. You unlock memory with `munlock' or `munlockall'. To avoid all page faults in a C program, you have to use `mlockall', because some of the memory a program uses is hidden from the C code, e.g. the stack and automatic variables, and you wouldn't know what address to tell `mlock'. - Function: int munlock (const void *ADDR, size_t LEN) `mlock' unlocks a range of the calling process' virtual pages. `munlock' is the inverse of `mlock' and functions completely analogously to `mlock', except that there is no `EPERM' failure. - Function: int mlockall (int FLAGS) `mlockall' locks all the pages in a process' virtual memory address space, and/or any that are added to it in the future. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory, and memory mapped files. FLAGS is a string of single bit flags represented by the following macros. They tell `mlockall' which of its functions you want. All other bits must be zero. `MCL_CURRENT' Lock all pages which currently exist in the calling process' virtual address space. `MCL_FUTURE' Set a mode such that any pages added to the process' virtual address space in the future will be locked from birth. This mode does not affect future address spaces owned by the same process so exec, which replaces a process' address space, wipes out `MCL_FUTURE'. *Note Executing a File::. When the function returns successfully, and you specified `MCL_CURRENT', all of the process' pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them. When the process is in `MCL_FUTURE' mode because it successfully executed this function and specified `MCL_CURRENT', any system call by the process that requires space be added to its virtual address space fails with `errno' = `ENOMEM' if locking the additional space would cause the process to exceed its locked page limit. In the case that the address space addition that can't be accommodated is stack expansion, the stack expansion fails and the kernel sends a `SIGSEGV' signal to the process. When the function fails, it does not affect the lock status of any pages or the future locking mode. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. `errno' values specific to this function are: `ENOMEM' * At least some of the specified address range does not exist in the calling process' virtual address space. * The locking would cause the process to exceed its locked page limit. `EPERM' The calling process is not superuser. `EINVAL' Undefined bits in FLAGS are not zero. `ENOSYS' The kernel does not provide `mlockall' capability. You can lock just specific pages with `mlock'. You unlock pages with `munlockall' and `munlock'. - Function: int munlockall (void) `munlockall' unlocks every page in the calling process' virtual address space and turn off `MCL_FUTURE' future locking mode. The return value is zero if the function succeeds. Otherwise, it is `-1' and `errno' is set accordingly. The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific `errno' values. Character Handling ****************** Programs that work with characters and strings often need to classify a character--is it alphabetic, is it a digit, is it whitespace, and so on--and perform case conversion operations on characters. The functions in the header file `ctype.h' are provided for this purpose. Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale. (More precisely, they are affected by the locale currently selected for character classification--the `LC_CTYPE' category; see *Note Locale Categories::.) The ISO C standard specifies two different sets of functions. The one set works on `char' type characters, the other one on `wchar_t' wide characters (*note Extended Char Intro::). Classification of Characters ============================ This section explains the library functions for classifying characters. For example, `isalpha' is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this: if (isalpha (c)) printf ("The character `%c' is alphabetic.\n", c); Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with `is'. Each of them takes one argument, which is a character to test, and returns an `int' which is treated as a boolean value. The character argument is passed as an `int', and it may be the constant value `EOF' instead of a real character. The attributes of any given character can vary between locales. *Note Locales::, for more information on locales. These functions are declared in the header file `ctype.h'. - Function: int islower (int C) Returns true if C is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. - Function: int isupper (int C) Returns true if C is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. - Function: int isalpha (int C) Returns true if C is an alphabetic character (a letter). If `islower' or `isupper' is true of a character, then `isalpha' is also true. In some locales, there may be additional characters for which `isalpha' is true--letters which are neither upper case nor lower case. But in the standard `"C"' locale, there are no such additional characters. - Function: int isdigit (int C) Returns true if C is a decimal digit (`0' through `9'). - Function: int isalnum (int C) Returns true if C is an alphanumeric character (a letter or number); in other words, if either `isalpha' or `isdigit' is true of a character, then `isalnum' is also true. - Function: int isxdigit (int C) Returns true if C is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'. - Function: int ispunct (int C) Returns true if C is a punctuation character. This means any printing character that is not alphanumeric or a space character. - Function: int isspace (int C) Returns true if C is a "whitespace" character. In the standard `"C"' locale, `isspace' returns true for only the standard whitespace characters: `' '' space `'\f'' formfeed `'\n'' newline `'\r'' carriage return `'\t'' horizontal tab `'\v'' vertical tab - Function: int isblank (int C) Returns true if C is a blank character; that is, a space or a tab. This function is a GNU extension. - Function: int isgraph (int C) Returns true if C is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic. - Function: int isprint (int C) Returns true if C is a printing character. Printing characters include all the graphic characters, plus the space (` ') character. - Function: int iscntrl (int C) Returns true if C is a control character (that is, a character that is not a printing character). - Function: int isascii (int C) Returns true if C is a 7-bit `unsigned char' value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension. Case Conversion =============== This section explains the library functions for performing conversions such as case mappings on characters. For example, `toupper' converts any character to upper case if possible. If the character can't be converted, `toupper' returns it unchanged. These functions take one argument of type `int', which is the character to convert, and return the converted character as an `int'. If the conversion is not applicable to the argument given, the argument is returned unchanged. *Compatibility Note:* In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion. Thus for portability, you may need to write `islower(c) ? toupper(c) : c' rather than just `toupper(c)'. These functions are declared in the header file `ctype.h'. - Function: int tolower (int C) If C is an upper-case letter, `tolower' returns the corresponding lower-case letter. If C is not an upper-case letter, C is returned unchanged. - Function: int toupper (int C) If C is a lower-case letter, `toupper' returns the corresponding upper-case letter. Otherwise C is returned unchanged. - Function: int toascii (int C) This function converts C to a 7-bit `unsigned char' value that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension. - Function: int _tolower (int C) This is identical to `tolower', and is provided for compatibility with the SVID. *Note SVID::. - Function: int _toupper (int C) This is identical to `toupper', and is provided for compatibility with the SVID. Character class determination for wide characters ================================================= Amendment 1 to ISO C90 defines functions to classify wide characters. Although the original ISO C90 standard already defined the type `wchar_t', no functions operating on them were defined. The general design of the classification functions for wide characters is more general. It allows extensions to the set of available classifications, beyond those which are always available. The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C library implementation of the `localedef' program. The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class. For the wide character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value. On top of this the normal character classification functions as used for `char' objects can be defined. - Data type: wctype_t The `wctype_t' can hold a value which represents a character class. The only defined way to generate such a value is by using the `wctype' function. This type is defined in `wctype.h'. - Function: wctype_t wctype (const char *PROPERTY) The `wctype' returns a value representing a class of wide characters which is identified by the string PROPERTY. Beside some standard properties each locale can define its own ones. In case no property with the given name is known for the current locale selected for the `LC_CTYPE' category, the function returns zero. The properties known in every locale are: `"alnum"' `"alpha"' `"cntrl"' `"digit"' `"graph"' `"lower"' `"print"' `"punct"' `"space"' `"upper"' `"xdigit"' This function is declared in `wctype.h'. To test the membership of a character to one of the non-standard classes the ISO C standard defines a completely new function. - Function: int iswctype (wint_t WC, wctype_t DESC) This function returns a nonzero value if WC is in the character class specified by DESC. DESC must previously be returned by a successful call to `wctype'. This function is declared in `wctype.h'. To make it easier to use the commonly-used classification functions, they are defined in the C library. There is no need to use `wctype' if the property string is one of the known character classes. In some situations it is desirable to construct the property strings, and then it is important that `wctype' can also handle the standard classes. - Function: int iswalnum (wint_t WC) This function returns a nonzero value if WC is an alphanumeric character (a letter or number); in other words, if either `iswalpha' or `iswdigit' is true of a character, then `iswalnum' is also true. This function can be implemented using iswctype (wc, wctype ("alnum")) It is declared in `wctype.h'. - Function: int iswalpha (wint_t WC) Returns true if WC is an alphabetic character (a letter). If `iswlower' or `iswupper' is true of a character, then `iswalpha' is also true. In some locales, there may be additional characters for which `iswalpha' is true--letters which are neither upper case nor lower case. But in the standard `"C"' locale, there are no such additional characters. This function can be implemented using iswctype (wc, wctype ("alpha")) It is declared in `wctype.h'. - Function: int iswcntrl (wint_t WC) Returns true if WC is a control character (that is, a character that is not a printing character). This function can be implemented using iswctype (wc, wctype ("cntrl")) It is declared in `wctype.h'. - Function: int iswdigit (wint_t WC) Returns true if WC is a digit (e.g., `0' through `9'). Please note that this function does not only return a nonzero value for _decimal_ digits, but for all kinds of digits. A consequence is that code like the following will *not* work unconditionally for wide characters: n = 0; while (iswdigit (*wc)) { n *= 10; n += *wc++ - L'0'; } This function can be implemented using iswctype (wc, wctype ("digit")) It is declared in `wctype.h'. - Function: int iswgraph (wint_t WC) Returns true if WC is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic. This function can be implemented using iswctype (wc, wctype ("graph")) It is declared in `wctype.h'. - Function: int iswlower (wint_t WC) Returns true if WC is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. This function can be implemented using iswctype (wc, wctype ("lower")) It is declared in `wctype.h'. - Function: int iswprint (wint_t WC) Returns true if WC is a printing character. Printing characters include all the graphic characters, plus the space (` ') character. This function can be implemented using iswctype (wc, wctype ("print")) It is declared in `wctype.h'. - Function: int iswpunct (wint_t WC) Returns true if WC is a punctuation character. This means any printing character that is not alphanumeric or a space character. This function can be implemented using iswctype (wc, wctype ("punct")) It is declared in `wctype.h'. - Function: int iswspace (wint_t WC) Returns true if WC is a "whitespace" character. In the standard `"C"' locale, `iswspace' returns true for only the standard whitespace characters: `L' '' space `L'\f'' formfeed `L'\n'' newline `L'\r'' carriage return `L'\t'' horizontal tab `L'\v'' vertical tab This function can be implemented using iswctype (wc, wctype ("space")) It is declared in `wctype.h'. - Function: int iswupper (wint_t WC) Returns true if WC is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid. This function can be implemented using iswctype (wc, wctype ("upper")) It is declared in `wctype.h'. - Function: int iswxdigit (wint_t WC) Returns true if WC is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'. This function can be implemented using iswctype (wc, wctype ("xdigit")) It is declared in `wctype.h'. The GNU C library also provides a function which is not defined in the ISO C standard but which is available as a version for single byte characters as well. - Function: int iswblank (wint_t WC) Returns true if WC is a blank character; that is, a space or a tab. This function is a GNU extension. It is declared in `wchar.h'. Notes on using the wide character classes ========================================= The first note is probably not astonishing but still occasionally a cause of problems. The `iswXXX' functions can be implemented using macros and in fact, the GNU C library does this. They are still available as real functions but when the `wctype.h' header is included the macros will be used. This is the same as the `char' type versions of these functions. The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear. int is_in_class (int c, const char *class) { if (strcmp (class, "alnum") == 0) return isalnum (c); if (strcmp (class, "alpha") == 0) return isalpha (c); if (strcmp (class, "cntrl") == 0) return iscntrl (c); ... return 0; } Now, with the `wctype' and `iswctype' you can avoid the `if' cascades, but rewriting the code as follows is wrong: int is_in_class (int c, const char *class) { wctype_t desc = wctype (class); return desc ? iswctype ((wint_t) c, desc) : 0; } The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows: int is_in_class (int c, const char *class) { wctype_t desc = wctype (class); return desc ? iswctype (btowc (c), desc) : 0; } *Note Converting a Character::, for more information on `btowc'. Note that this change probably does not improve the performance of the program a lot since the `wctype' function still has to make the string comparisons. It gets really interesting if the `is_in_class' function is called more than once for the same class name. In this case the variable DESC could be computed once and reused for all the calls. Therefore the above form of the function is probably not the final one. Mapping of wide characters. =========================== The classification functions are also generalized by the ISO C standard. Instead of just allowing the two standard mappings, a locale can contain others. Again, the `localedef' program already supports generating such locale data files. - Data Type: wctrans_t This data type is defined as a scalar type which can hold a value representing the locale-dependent character mapping. There is no way to construct such a value apart from using the return value of the `wctrans' function. This type is defined in `wctype.h'. - Function: wctrans_t wctrans (const char *PROPERTY) The `wctrans' function has to be used to find out whether a named mapping is defined in the current locale selected for the `LC_CTYPE' category. If the returned value is non-zero, you can use it afterwards in calls to `towctrans'. If the return value is zero no such mapping is known in the current locale. Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale: `"tolower"' `"toupper"' These functions are declared in `wctype.h'. - Function: wint_t towctrans (wint_t WC, wctrans_t DESC) `towctrans' maps the input character WC according to the rules of the mapping for which DESC is a descriptor, and returns the value it finds. DESC must be obtained by a successful call to `wctrans'. This function is declared in `wctype.h'. For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call `wctrans' for them. - Function: wint_t towlower (wint_t WC) If WC is an upper-case letter, `towlower' returns the corresponding lower-case letter. If WC is not an upper-case letter, WC is returned unchanged. `towlower' can be implemented using towctrans (wc, wctrans ("tolower")) This function is declared in `wctype.h'. - Function: wint_t towupper (wint_t WC) If WC is a lower-case letter, `towupper' returns the corresponding upper-case letter. Otherwise WC is returned unchanged. `towupper' can be implemented using towctrans (wc, wctrans ("toupper")) This function is declared in `wctype.h'. The same warnings given in the last section for the use of the wide character classification functions apply here. It is not possible to simply cast a `char' type value to a `wint_t' and use it as an argument to `towctrans' calls. String and Array Utilities ************************** Operations on strings (or arrays of characters) are an important part of many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the `memcpy' function can be used to copy the contents of any kind of array. It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability. For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in `strcmp' function, you're less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too. Representation of Strings ========================= This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section. A "string" is an array of `char' objects. But string-valued variables are usually declared to be pointers of type `char *'. Such variables do not include space for the text of a string; that has to be stored somewhere else--in an array variable, a string constant, or dynamically allocated memory (*note Memory Allocation::). It's up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a "null pointer" in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error. "string" normally refers to multibyte character strings as opposed to wide character strings. Wide character strings are arrays of type `wchar_t' and as for multibyte character strings usually pointers of type `wchar_t *' are used. By convention, a "null character", `'\0'', marks the end of a multibyte character string and the "null wide character", `L'\0'', marks the end of a wide character string. For example, in testing to see whether the `char *' variable P points to a null character marking the end of a string, you can write `!*P' or `*P == '\0''. A null character is quite different conceptually from a null pointer, although both are represented by the integer `0'. "String literals" appear in C program source as strings of characters between double-quote characters (`"') where the initial double-quote character is immediately preceded by a capital `L' (ell) character (as in `L"foo"'). In ISO C, string literals can also be formed by "string concatenation": `"a" "b"' is the same as `"ab"'. For wide character strings one can either use `L"a" L"b"' or `L"a" "b"'. Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage. Character arrays that are declared `const' cannot be modified either. It's generally good style to declare non-modifiable string pointers to be of type `const char *', since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string. The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term "allocated size" is always used to refer to the total amount of memory allocated for the string, while the term "length" refers to the number of characters up to (but not including) the terminating null character. A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions _do not_ do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string. Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see *Note Extended Char Intro::). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly. But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes a call to `strncpy' could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte sequence in the target buffer. To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on "wide characters" (*note Extended Char Intro::). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied. The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available. String and Array Conventions ============================ This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters. Functions that operate on arbitrary blocks of memory have names beginning with `mem' and `wmem' (such as `memcpy' and `wmemcpy') and invariably take an argument which specifies the size (in bytes and wide characters respectively) of the block of memory to operate on. The array arguments and return values for these functions have type `void *' or `wchar_t'. As a matter of style, the elements of the arrays used with the `mem' functions are referred to as "bytes". You can pass any kind of pointer to these functions, and the `sizeof' operator is useful in computing the value for the size argument. Parameters to the `wmem' functions must be of type `wchar_t *'. These functions are not really usable with anything but arrays of this type. In contrast, functions that operate specifically on strings and wide character strings have names beginning with `str' and `wcs' respectively (such as `strcpy' and `wcscpy') and look for a null character to terminate the string instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.) The array arguments and return values for these functions have type `char *' and `wchar_t *' respectively, and the array elements are referred to as "characters" and "wide characters". In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size. Some of the memory and string functions take single characters as arguments. Since a value of type `char' is automatically promoted into an value of type `int' when used as a parameter, the functions are declared with `int' as the type of the parameter in question. In case of the wide character function the situation is similarly: the parameter type for a single wide character is `wint_t' and not `wchar_t'. This would for many implementations not be necessary since the `wchar_t' is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types the `wint_t' type is used. String Length ============= You can get the length of a string using the `strlen' function. This function is declared in the header file `string.h'. - Function: size_t strlen (const char *S) The `strlen' function returns the length of the null-terminated string S in bytes. (In other words, it returns the offset of the terminating null character within the array.) For example, strlen ("hello, world") => 12 When applied to a character array, the `strlen' function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character array that holds a string using the `sizeof' operator: char string[32] = "hello, world"; sizeof (string) => 32 strlen (string) => 12 But beware, this will not work unless STRING is the character array itself, not a pointer to it. For example: char string[32] = "hello, world"; char *ptr = string; sizeof (string) => 32 sizeof (ptr) => 4 /* (on a machine with 4 byte pointers) */ This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays. It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and `wcslen' can be used or something like the following code can be used: /* The input is in `string'. The length is expected in `n'. */ { mbstate_t t; char *scopy = string; /* In initial state. */ memset (&t, '\0', sizeof (t)); /* Determine number of characters. */ n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t); } This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters. The wide character equivalent is declared in `wchar.h'. - Function: size_t wcslen (const wchar_t *WS) The `wcslen' function is the wide character equivalent to `strlen'. The return value is the number of wide characters in the wide character string pointed to by WS (this is also the offset of the terminating null wide character of WS). Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters. This function was introduced in Amendment 1 to ISO C90. - Function: size_t strnlen (const char *S, size_t MAXLEN) The `strnlen' function returns the length of the string S in bytes if this length is smaller than MAXLEN bytes. Otherwise it returns MAXLEN. Therefore this function is equivalent to `(strlen (S) < n ? strlen (S) : MAXLEN)' but it is more efficient and works even if the string S is not null-terminated. char string[32] = "hello, world"; strnlen (string, 32) => 12 strnlen (string, 5) => 5 This function is a GNU extension and is declared in `string.h'. - Function: size_t wcsnlen (const wchar_t *WS, size_t MAXLEN) `wcsnlen' is the wide character equivalent to `strnlen'. The MAXLEN parameter specifies the maximum number of wide characters. This function is a GNU extension and is declared in `wchar.h'. Copying and Concatenation ========================= You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `wchar.h'. A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array. Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program. All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like `sprintf' (*note Formatted Output Functions::) and `scanf' (*note Formatted Input Functions::). - Function: void * memcpy (void *restrict TO, const void *restrict FROM, size_t SIZE) The `memcpy' function copies SIZE bytes from the object beginning at FROM into the object beginning at TO. The behavior of this function is undefined if the two arrays TO and FROM overlap; use `memmove' instead if overlapping is possible. The value returned by `memcpy' is the value of TO. Here is an example of how you might use `memcpy' to copy the contents of an array: struct foo *oldarray, *newarray; int arraysize; ... memcpy (new, old, arraysize * sizeof (struct foo)); - Function: wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t *restruct WFROM, size_t SIZE) The `wmemcpy' function copies SIZE wide characters from the object beginning at WFROM into the object beginning at WTO. The behavior of this function is undefined if the two arrays WTO and WFROM overlap; use `wmemmove' instead if overlapping is possible. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by `wmemcpy' is the value of WTO. This function was introduced in Amendment 1 to ISO C90. - Function: void * mempcpy (void *restrict TO, const void *restrict FROM, size_t SIZE) The `mempcpy' function is nearly identical to the `memcpy' function. It copies SIZE bytes from the object beginning at `from' into the object pointed to by TO. But instead of returning the value of TO it returns a pointer to the byte following the last written byte in the object beginning at TO. I.e., the value is `((void *) ((char *) TO + SIZE))'. This function is useful in situations where a number of objects shall be copied to consecutive memory positions. void * combine (void *o1, size_t s1, void *o2, size_t s2) { void *result = malloc (s1 + s2); if (result != NULL) mempcpy (mempcpy (result, o1, s1), o2, s2); return result; } This function is a GNU extension. - Function: wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) The `wmempcpy' function is nearly identical to the `wmemcpy' function. It copies SIZE wide characters from the object beginning at `wfrom' into the object pointed to by WTO. But instead of returning the value of WTO it returns a pointer to the wide character following the last written wide character in the object beginning at WTO. I.e., the value is `WTO + SIZE'. This function is useful in situations where a number of objects shall be copied to consecutive memory positions. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } This function is a GNU extension. - Function: void * memmove (void *TO, const void *FROM, size_t SIZE) `memmove' copies the SIZE bytes at FROM into the SIZE bytes at TO, even if those two blocks of space overlap. In the case of overlap, `memmove' is careful to copy the original values of the bytes in the block at FROM, including those bytes which also belong to the block at TO. The value returned by `memmove' is the value of TO. - Function: wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM, size_t SIZE) `wmemmove' copies the SIZE wide characters at WFROM into the SIZE wide characters at WTO, even if those two blocks of space overlap. In the case of overlap, `memmove' is careful to copy the original values of the wide characters in the block at WFROM, including those wide characters which also belong to the block at WTO. The following is a possible implementation of `wmemcpy' but there are more optimizations possible. wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t)); } The value returned by `wmemmove' is the value of WTO. This function is a GNU extension. - Function: void * memccpy (void *restrict TO, const void *restrict FROM, int C, size_t SIZE) This function copies no more than SIZE bytes from FROM to TO, stopping if a byte matching C is found. The return value is a pointer into TO one byte past where C was copied, or a null pointer if no byte matching C appeared in the first SIZE bytes of FROM. - Function: void * memset (void *BLOCK, int C, size_t SIZE) This function copies the value of C (converted to an `unsigned char') into each of the first SIZE bytes of the object beginning at BLOCK. It returns the value of BLOCK. - Function: wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t SIZE) This function copies the value of WC into each of the first SIZE wide characters of the object beginning at BLOCK. It returns the value of BLOCK. - Function: char * strcpy (char *restrict TO, const char *restrict FROM) This copies characters from the string FROM (up to and including the terminating null character) into the string TO. Like `memcpy', this function has undefined results if the strings overlap. The return value is the value of TO. - Function: wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM) This copies wide characters from the string WFROM (up to and including the terminating null wide character) into the string WTO. Like `wmemcpy', this function has undefined results if the strings overlap. The return value is the value of WTO. - Function: char * strncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `strcpy' but always copies exactly SIZE characters into TO. If the length of FROM is more than SIZE, then `strncpy' copies just the first SIZE characters. Note that in this case there is no null terminator written into TO. If the length of FROM is less than SIZE, then `strncpy' copies all of FROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is specified by the ISO C standard. The behavior of `strncpy' is undefined if the strings overlap. Using `strncpy' as opposed to `strcpy' is a way to avoid bugs relating to writing past the end of the allocated space for TO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `strncpy' will waste a considerable amount of time copying null characters. - Function: wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcscpy' but always copies exactly SIZE wide characters into WTO. If the length of WFROM is more than SIZE, then `wcsncpy' copies just the first SIZE wide characters. Note that in this case there is no null terminator written into WTO. If the length of WFROM is less than SIZE, then `wcsncpy' copies all of WFROM, followed by enough null wide characters to add up to SIZE wide characters in all. This behavior is rarely useful, but it is specified by the ISO C standard. The behavior of `wcsncpy' is undefined if the strings overlap. Using `wcsncpy' as opposed to `wcscpy' is a way to avoid bugs relating to writing past the end of the allocated space for WTO. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, SIZE may be large, and when it is, `wcsncpy' will waste a considerable amount of time copying null wide characters. - Function: char * strdup (const char *S) This function copies the null-terminated string S into a newly allocated string. The string is allocated using `malloc'; see *Note Unconstrained Allocation::. If `malloc' cannot allocate space for the new string, `strdup' returns a null pointer. Otherwise it returns a pointer to the new string. - Function: wchar_t * wcsdup (const wchar_t *WS) This function copies the null-terminated wide character string WS into a newly allocated string. The string is allocated using `malloc'; see *Note Unconstrained Allocation::. If `malloc' cannot allocate space for the new string, `wcsdup' returns a null pointer. Otherwise it returns a pointer to the new wide character string. This function is a GNU extension. - Function: char * strndup (const char *S, size_t SIZE) This function is similar to `strdup' but always copies at most SIZE characters into the newly allocated string. If the length of S is more than SIZE, then `strndup' copies just the first SIZE characters and adds a closing null terminator. Otherwise all characters are copied and the string is terminated. This function is different to `strncpy' in that it always terminates the destination string. `strndup' is a GNU extension. - Function: char * stpcpy (char *restrict TO, const char *restrict FROM) This function is like `strcpy', except that it returns a pointer to the end of the string TO (that is, the address of the terminating null character `to + strlen (from)') rather than the beginning. For example, this program uses `stpcpy' to concatenate `foo' and `bar' to produce `foobar', which it then prints. #include #include int main (void) { char buffer[10]; char *to = buffer; to = stpcpy (to, "foo"); to = stpcpy (to, "bar"); puts (buffer); return 0; } This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG. Its behavior is undefined if the strings overlap. The function is declared in `string.h'. - Function: wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM) This function is like `wcscpy', except that it returns a pointer to the end of the string WTO (that is, the address of the terminating null character `wto + strlen (wfrom)') rather than the beginning. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. The behavior of `wcpcpy' is undefined if the strings overlap. `wcpcpy' is a GNU extension and is declared in `wchar.h'. - Function: char * stpncpy (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is similar to `stpcpy' but copies always exactly SIZE characters into TO. If the length of FROM is more then SIZE, then `stpncpy' copies just the first SIZE characters and returns a pointer to the character directly following the one which was copied last. Note that in this case there is no null terminator written into TO. If the length of FROM is less than SIZE, then `stpncpy' copies all of FROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the `strncpy' is used. `stpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap. The function is declared in `string.h'. - Function: wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is similar to `wcpcpy' but copies always exactly WSIZE characters into WTO. If the length of WFROM is more then SIZE, then `wcpncpy' copies just the first SIZE wide characters and returns a pointer to the wide character directly following the one which was copied last. Note that in this case there is no null terminator written into WTO. If the length of WFROM is less than SIZE, then `wcpncpy' copies all of WFROM, followed by enough null characters to add up to SIZE characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the `wcsncpy' is used. `wcpncpy' returns a pointer to the _first_ written null character. This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself. Its behavior is undefined if the strings overlap. `wcpncpy' is a GNU extension and is declared in `wchar.h'. - Macro: char * strdupa (const char *S) This macro is similar to `strdup' but allocates the new string using `alloca' instead of `malloc' (*note Variable Size Automatic::). This means of course the returned string has the same limitations as any block of memory allocated using `alloca'. For obvious reasons `strdupa' is implemented only as a macro; you cannot get the address of this function. Despite this limitation it is a useful function. The following code shows a situation where using `malloc' would be a lot more expensive. #include #include #include const char path[] = _PATH_STDPATH; int main (void) { char *wr_path = strdupa (path); char *cp = strtok (wr_path, ":"); while (cp != NULL) { puts (cp); cp = strtok (NULL, ":"); } return 0; } Please note that calling `strtok' using PATH directly is invalid. It is also not allowed to call `strdupa' in the argument list of `strtok' since `strdupa' uses `alloca' (*note Variable Size Automatic::) can interfere with the parameter passing. This function is only available if GNU CC is used. - Macro: char * strndupa (const char *S, size_t SIZE) This function is similar to `strndup' but like `strdupa' it allocates the new string using `alloca' *note Variable Size Automatic::. The same advantages and limitations of `strdupa' are valid for `strndupa', too. This function is implemented only as a macro, just like `strdupa'. Just as `strdupa' this macro also must not be used inside the parameter list in a function call. `strndupa' is only available if GNU CC is used. - Function: char * strcat (char *restrict TO, const char *restrict FROM) The `strcat' function is similar to `strcpy', except that the characters from FROM are concatenated or appended to the end of TO, instead of overwriting it. That is, the first character from FROM overwrites the null character marking the end of TO. An equivalent definition for `strcat' would be: char * strcat (char *restrict to, const char *restrict from) { strcpy (to + strlen (to), from); return to; } This function has undefined results if the strings overlap. - Function: wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t *restrict WFROM) The `wcscat' function is similar to `wcscpy', except that the characters from WFROM are concatenated or appended to the end of WTO, instead of overwriting it. That is, the first character from WFROM overwrites the null character marking the end of WTO. An equivalent definition for `wcscat' would be: wchar_t * wcscat (wchar_t *wto, const wchar_t *wfrom) { wcscpy (wto + wcslen (wto), wfrom); return wto; } This function has undefined results if the strings overlap. Programmers using the `strcat' or `wcscat' function (or the following `strncat' or `wcsncar' functions for that matter) can easily be recognized as lazy and reckless. In almost all situations the lengths of the participating strings are known (it better should be since how can one otherwise ensure the allocated size of the buffer is sufficient?) Or at least, one could know them if one keeps track of the results of the various function calls. But then it is very inefficient to use `strcat'/`wcscat'. A lot of time is wasted finding the end of the destination string so that the actual copying can start. This is a common example: /* This function concatenates arbitrarily many strings. The last parameter must be `NULL'. */ char * concat (const char *str, ...) { va_list ap, ap2; size_t total = 1; const char *s; char *result; va_start (ap, str); /* Actually `va_copy', but this is the name more gcc versions understand. */ __va_copy (ap2, ap); /* Determine how much space we need. */ for (s = str; s != NULL; s = va_arg (ap, const char *)) total += strlen (s); va_end (ap); result = (char *) malloc (total); if (result != NULL) { result[0] = '\0'; /* Copy the strings. */ for (s = str; s != NULL; s = va_arg (ap2, const char *)) strcat (result, s); } va_end (ap2); return result; } This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient: char * concat (const char *str, ...) { va_list ap; size_t allocated = 100; char *result = (char *) malloc (allocated); char *wp; if (allocated != NULL) { char *newp; va_start (ap, atr); wp = result; for (s = str; s != NULL; s = va_arg (ap, const char *)) { size_t len = strlen (s); /* Resize the allocated memory if necessary. */ if (wp + len + 1 > result + allocated) { allocated = (allocated + len) * 2; newp = (char *) realloc (result, allocated); if (newp == NULL) { free (result); return NULL; } wp = newp + (wp - result); result = newp; } wp = mempcpy (wp, s, len); } /* Terminate the result string. */ *wp++ = '\0'; /* Resize memory to the optimal size. */ newp = realloc (result, wp - result); if (newp != NULL) result = newp; va_end (ap); } return result; } With a bit more knowledge about the input strings one could fine-tune the memory allocation. The difference we are pointing to here is that we don't use `strcat' anymore. We always keep track of the length of the current intermediate result so we can safe us the search for the end of the string and use `mempcpy'. Please note that we also don't use `stpcpy' which might seem more natural since we handle with strings. But this is not necessary since we already know the length of the string and therefore can use the faster memory copying function. The example would work for wide characters the same way. Whenever a programmer feels the need to use `strcat' she or he should think twice and look through the program whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use `strcat'. - Function: char * strncat (char *restrict TO, const char *restrict FROM, size_t SIZE) This function is like `strcat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 1' bytes longer than its initial length. The `strncat' function could be implemented like this: char * strncat (char *to, const char *from, size_t size) { to[strlen (to) + size] = '\0'; strncpy (to + strlen (to), from, size); return to; } The behavior of `strncat' is undefined if the strings overlap. - Function: wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t *restrict WFROM, size_t SIZE) This function is like `wcscat' except that not more than SIZE characters from FROM are appended to the end of TO. A single null character is also always appended to TO, so the total allocated size of TO must be at least `SIZE + 1' bytes longer than its initial length. The `wcsncat' function could be implemented like this: wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size) { wto[wcslen (to) + size] = L'\0'; wcsncpy (wto + wcslen (wto), wfrom, size); return wto; } The behavior of `wcsncat' is undefined if the strings overlap. Here is an example showing the use of `strncpy' and `strncat' (the wide character version is equivalent). Notice how, in the call to `strncat', the SIZE parameter is computed to avoid overflowing the character array `buffer'. #include #include #define SIZE 10 static char buffer[SIZE]; main () { strncpy (buffer, "hello", SIZE); puts (buffer); strncat (buffer, ", world", SIZE - strlen (buffer) - 1); puts (buffer); } The output produced by this program looks like: hello hello, wo - Function: void bcopy (const void *FROM, void *TO, size_t SIZE) This is a partially obsolete alternative for `memmove', derived from BSD. Note that it is not quite equivalent to `memmove', because the arguments are not in the same order and there is no return value. - Function: void bzero (void *BLOCK, size_t SIZE) This is a partially obsolete alternative for `memset', derived from BSD. Note that it is not as general as `memset', because the only value it can store is zero. String/Array Comparison ======================= You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. *Note Searching and Sorting::, for an example of this. Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are _not_ equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater". The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'. All of these functions are declared in the header file `string.h'. - Function: int memcmp (const void *A1, const void *A2, size_t SIZE) The function `memcmp' compares the SIZE bytes of memory beginning at A1 against the SIZE bytes of memory beginning at A2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as `unsigned char' objects, then promoted to `int'). If the contents of the two blocks are equal, `memcmp' returns `0'. - Function: int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t SIZE) The function `wmemcmp' compares the SIZE wide characters beginning at A1 against the SIZE wide characters beginning at A2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is A1 is smaller or larger than the corresponding character in A2. If the contents of the two blocks are equal, `wmemcmp' returns `0'. On arbitrary arrays, the `memcmp' function is mostly useful for testing equality. It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers. `wmemcmp' is really only useful to compare arrays of type `wchar_t' since the function looks at `sizeof (wchar_t)' bytes at a time and this number of bytes is system dependent. You should also be careful about using `memcmp' to compare objects that can contain "holes", such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size. The contents of these "holes" are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison. For example, given a structure type definition like: struct foo { unsigned char tag; union { double f; long i; char *p; } value; }; you are better off writing a specialized comparison function to compare `struct foo' objects instead of comparing them with `memcmp'. - Function: int strcmp (const char *S1, const char *S2) The `strcmp' function compares the string S1 against S2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as `unsigned char' objects, then promoted to `int'). If the two strings are equal, `strcmp' returns `0'. A consequence of the ordering used by `strcmp' is that if S1 is an initial substring of S2, then S1 is considered to be "less than" S2. `strcmp' does not take sorting conventions of the language the strings are written in into account. To get that one has to use `strcoll'. - Function: int wcscmp (const wchar_t *WS1, const wchar_t *WS2) The `wcscmp' function compares the wide character string WS1 against WS2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is WS1 is smaller or larger than the corresponding character in WS2. If the two strings are equal, `wcscmp' returns `0'. A consequence of the ordering used by `wcscmp' is that if WS1 is an initial substring of WS2, then WS1 is considered to be "less than" WS2. `wcscmp' does not take sorting conventions of the language the strings are written in into account. To get that one has to use `wcscoll'. - Function: int strcasecmp (const char *S1, const char *S2) This function is like `strcmp', except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard `"C"' locale the characters A" and a" do not match but in a locale which regards these characters as parts of the alphabet they do match. `strcasecmp' is derived from BSD. - Function: int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2) This function is like `wcscmp', except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard `"C"' locale the characters A" and a" do not match but in a locale which regards these characters as parts of the alphabet they do match. `wcscasecmp' is a GNU extension. - Function: int strncmp (const char *S1, const char *S2, size_t SIZE) This function is the similar to `strcmp', except that no more than SIZE wide characters are compared. In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero. - Function: int wcsncmp (const wchar_t *WS1, const wchar_t *WS2, size_t SIZE) This function is the similar to `wcscmp', except that no more than SIZE wide characters are compared. In other words, if the two strings are the same in their first SIZE wide characters, the return value is zero. - Function: int strncasecmp (const char *S1, const char *S2, size_t N) This function is like `strncmp', except that differences in case are ignored. Like `strcasecmp', it is locale dependent how uppercase and lowercase characters are related. `strncasecmp' is a GNU extension. - Function: int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2, size_t N) This function is like `wcsncmp', except that differences in case are ignored. Like `wcscasecmp', it is locale dependent how uppercase and lowercase characters are related. `wcsncasecmp' is a GNU extension. Here are some examples showing the use of `strcmp' and `strncmp' (equivalent examples can be constructed for the wide character functions). These examples assume the use of the ASCII character set. (If some other character set--say, EBCDIC--is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.) strcmp ("hello", "hello") => 0 /* These two strings are the same. */ strcmp ("hello", "Hello") => 32 /* Comparisons are case-sensitive. */ strcmp ("hello", "world") => -15 /* The character `'h'' comes before `'w''. */ strcmp ("hello", "hello, world") => -44 /* Comparing a null character against a comma. */ strncmp ("hello", "hello, world", 5) => 0 /* The initial 5 characters are the same. */ strncmp ("hello, world", "hello, stupid world!!!", 5) => 0 /* The initial 5 characters are the same. */ - Function: int strverscmp (const char *S1, const char *S2) The `strverscmp' function compares the string S1 against S2, considering them as holding indices/version numbers. Return value follows the same conventions as found in the `strverscmp' function. In fact, if S1 and S2 contain no digits, `strverscmp' behaves like `strcmp'. Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them: * integral/integral: we compare values as you would expect. * fractional/integral: the fractional part is less than the integral one. Again, no surprise. * fractional/fractional: the things become a bit more complex. If the common prefix contains only leading zeroes, the longest part is less than the other one; else the comparison behaves normally. strverscmp ("no digit", "no digit") => 0 /* same behavior as strcmp. */ strverscmp ("item#99", "item#100") => <0 /* same prefix, but 99 < 100. */ strverscmp ("alpha1", "alpha001") => >0 /* fractional part inferior to integral one. */ strverscmp ("part1_f012", "part1_f01") => >0 /* two fractional parts. */ strverscmp ("foo.009", "foo.0") => <0 /* idem, but with leading zeroes only. */ This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers. `strverscmp' is a GNU extension. - Function: int bcmp (const void *A1, const void *A2, size_t SIZE) This is an obsolete alias for `memcmp', derived from BSD. Collation Functions =================== In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'. You can use the functions `strcoll' and `strxfrm' (declared in the headers file `string.h') and `wcscoll' and `wcsxfrm' (declared in the headers file `wchar') to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the `LC_COLLATE' category; see *Note Locales::. In the standard C locale, the collation sequence for `strcoll' is the same as that for `strcmp'. Similarly, `wcscoll' and `wcscmp' are the same in this situation. Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence. The functions `strcoll' and `wcscoll' perform this translation implicitly, in order to do one comparison. By contrast, `strxfrm' and `wcsxfrm' perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use `strxfrm' or `wcsxfrm' to transform all the strings just once, and subsequently compare the transformed strings with `strcmp' or `wcscmp'. - Function: int strcoll (const char *S1, const char *S2) The `strcoll' function is similar to `strcmp' but uses the collating sequence of the current locale for collation (the `LC_COLLATE' locale). - Function: int wcscoll (const wchar_t *WS1, const wchar_t *WS2) The `wcscoll' function is similar to `wcscmp' but uses the collating sequence of the current locale for collation (the `LC_COLLATE' locale). Here is an example of sorting an array of strings, using `strcoll' to compare them. The actual sort algorithm is not written here; it comes from `qsort' (*note Array Sort Function::). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using `strxfrm'.) /* This is the comparison function used with `qsort'. */ int compare_elements (char **p1, char **p2) { return strcoll (*p1, *p2); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings (char **array, int nstrings) { /* Sort `temp_array' by comparing the strings. */ qsort (array, nstrings, sizeof (char *), compare_elements); } - Function: size_t strxfrm (char *restrict TO, const char *restrict FROM, size_t SIZE) The function `strxfrm' transforms the string FROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array TO. Up to SIZE characters (including a terminating null character) are stored. The behavior is undefined if the strings TO and FROM overlap; see *Note Copying and Concatenation::. The return value is the length of the entire transformed string. This value is not affected by the value of SIZE, but if it is greater or equal than SIZE, it means that the transformed string did not entirely fit in the array TO. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call `strxfrm' again with a bigger output array. The transformed string may be longer than the original string, and it may also be shorter. If SIZE is zero, no characters are stored in TO. In this case, `strxfrm' simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what TO is if SIZE is zero; TO may even be a null pointer. - Function: size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t *WFROM, size_t SIZE) The function `wcsxfrm' transforms wide character string WFROM using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array WTO. Up to SIZE wide characters (including a terminating null character) are stored. The behavior is undefined if the strings WTO and WFROM overlap; see *Note Copying and Concatenation::. The return value is the length of the entire transformed wide character string. This value is not affected by the value of SIZE, but if it is greater or equal than SIZE, it means that the transformed wide character string did not entirely fit in the array WTO. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call `wcsxfrm' again with a bigger output array. The transformed wide character string may be longer than the original wide character string, and it may also be shorter. If SIZE is zero, no characters are stored in TO. In this case, `wcsxfrm' simply returns the number of wide characters that would be the length of the transformed wide character string. This is useful for determining what size the allocated array should be (remember to multiply with `sizeof (wchar_t)'). It does not matter what WTO is if SIZE is zero; WTO may even be a null pointer. Here is an example of how you can use `strxfrm' when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings. struct sorter { char *input; char *transformed; }; /* This is the comparison function used with `qsort' to sort an array of `struct sorter'. */ int compare_elements (struct sorter *p1, struct sorter *p2) { return strcmp (p1->transformed, p2->transformed); } /* This is the entry point--the function to sort strings using the locale's collating sequence. */ void sort_strings_fast (char **array, int nstrings) { struct sorter temp_array[nstrings]; int i; /* Set up `temp_array'. Each element contains one input string and its transformed string. */ for (i = 0; i < nstrings; i++) { size_t length = strlen (array[i]) * 2; char *transformed; size_t transformed_length; temp_array[i].input = array[i]; /* First try a buffer perhaps big enough. */ transformed = (char *) xmalloc (length); /* Transform `array[i]'. */ transformed_length = strxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (char *) xrealloc (transformed, transformed_length + 1); /* The return value is not interesting because we know how long the transformed string is. */ (void) strxfrm (transformed, array[i], transformed_length + 1); } temp_array[i].transformed = transformed; } /* Sort `temp_array' by comparing transformed strings. */ qsort (temp_array, sizeof (struct sorter), nstrings, compare_elements); /* Put the elements back in the permanent array in their sorted order. */ for (i = 0; i < nstrings; i++) array[i] = temp_array[i].input; /* Free the strings we allocated. */ for (i = 0; i < nstrings; i++) free (temp_array[i].transformed); } The interesting part of this code for the wide character version would look like this: void sort_strings_fast (wchar_t **array, int nstrings) { ... /* Transform `array[i]'. */ transformed_length = wcsxfrm (transformed, array[i], length); /* If the buffer was not large enough, resize it and try again. */ if (transformed_length >= length) { /* Allocate the needed space. +1 for terminating `NUL' character. */ transformed = (wchar_t *) xrealloc (transformed, (transformed_length + 1) * sizeof (wchar_t)); /* The return value is not interesting because we know how long the transformed string is. */ (void) wcsxfrm (transformed, array[i], transformed_length + 1); } ... Note the additional multiplication with `sizeof (wchar_t)' in the `realloc' call. *Compatibility Note:* The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90. Search Functions ================ This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'. - Function: void * memchr (const void *BLOCK, int C, size_t SIZE) This function finds the first occurrence of the byte C (converted to an `unsigned char') in the initial SIZE bytes of the object beginning at BLOCK. The return value is a pointer to the located byte, or a null pointer if no match was found. - Function: wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC, size_t SIZE) This function finds the first occurrence of the wide character WC in the initial SIZE wide characters of the object beginning at BLOCK. The return value is a pointer to the located wide character, or a null pointer if no match was found. - Function: void * rawmemchr (const void *BLOCK, int C) Often the `memchr' function is used with the knowledge that the byte C is available in the memory block specified by the parameters. But this means that the SIZE parameter is not really needed and that the tests performed with it at runtime (to check whether the end of the block is reached) are not needed. The `rawmemchr' function exists for just this situation which is surprisingly frequent. The interface is similar to `memchr' except that the SIZE parameter is missing. The function will look beyond the end of the block pointed to by BLOCK in case the programmer made an error in assuming that the byte C is present in the block. In this case the result is unspecified. Otherwise the return value is a pointer to the located byte. This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like rawmemchr (str, '\0') will never go beyond the end of the string. This function is a GNU extension. - Function: void * memrchr (const void *BLOCK, int C, size_t SIZE) The function `memrchr' is like `memchr', except that it searches backwards from the end of the block defined by BLOCK and SIZE (instead of forwards from the front). - Function: char * strchr (const char *STRING, int C) The `strchr' function finds the first occurrence of the character C (converted to a `char') in the null-terminated string beginning at STRING. The return value is a pointer to the located character, or a null pointer if no match was found. For example, strchr ("hello, world", 'l') => "llo, world" strchr ("hello, world", '?') => NULL The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the C argument. It would be better (but less portable) to use `strchrnul' in this case, though. - Function: wchar_t * wcschr (const wchar_t *WSTRING, int WC) The `wcschr' function finds the first occurrence of the wide character WC in the null-terminated wide character string beginning at WSTRING. The return value is a pointer to the located wide character, or a null pointer if no match was found. The terminating null character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide character string by specifying a null wude character as the value of the WC argument. It would be better (but less portable) to use `wcschrnul' in this case, though. - Function: char * strchrnul (const char *STRING, int C) `strchrnul' is the same as `strchr' except that if it does not find the character, it returns a pointer to string's terminating null character rather than a null pointer. This function is a GNU extension. - Function: wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC) `wcschrnul' is the same as `wcschr' except that if it does not find the wide character, it returns a pointer to wide character string's terminating null wide character rather than a null pointer. This function is a GNU extension. One useful, but unusual, use of the `strchr' function is when one wants to have a pointer pointing to the NUL byte terminating a string. This is often written in this way: s += strlen (s); This is almost optimal but the addition operation duplicated a bit of the work already done in the `strlen' function. A better solution is this: s = strchr (s, '\0'); There is no restriction on the second parameter of `strchr' so it could very well also be the NUL character. Those readers thinking very hard about this might now point out that the `strchr' function is more expensive than the `strlen' function since we have two abort criteria. This is right. But in the GNU C library the implementation of `strchr' is optimized in a special way so that `strchr' actually is faster. - Function: char * strrchr (const char *STRING, int C) The function `strrchr' is like `strchr', except that it searches backwards from the end of the string STRING (instead of forwards from the front). For example, strrchr ("hello, world", 'l') => "ld" - Function: wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C) The function `wcsrchr' is like `wcschr', except that it searches backwards from the end of the string WSTRING (instead of forwards from the front). - Function: char * strstr (const char *HAYSTACK, const char *NEEDLE) This is like `strchr', except that it searches HAYSTACK for a substring NEEDLE rather than just a single character. It returns a pointer into the string HAYSTACK that is the first character of the substring, or a null pointer if no match was found. If NEEDLE is an empty string, the function returns HAYSTACK. For example, strstr ("hello, world", "l") => "llo, world" strstr ("hello, world", "wo") => "world" - Function: wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t *NEEDLE) This is like `wcschr', except that it searches HAYSTACK for a substring NEEDLE rather than just a single wide character. It returns a pointer into the string HAYSTACK that is the first wide character of the substring, or a null pointer if no match was found. If NEEDLE is an empty string, the function returns HAYSTACK. - Function: wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t *NEEDLE) `wcsstr' is an depricated alias for `wcsstr'. This is the name originally used in the X/Open Portability Guide before the Amendment 1 to ISO C90 was published. - Function: char * strcasestr (const char *HAYSTACK, const char *NEEDLE) This is like `strstr', except that it ignores case in searching for the substring. Like `strcasecmp', it is locale dependent how uppercase and lowercase characters are related. For example, strstr ("hello, world", "L") => "llo, world" strstr ("hello, World", "wo") => "World" - Function: void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN, const void *NEEDLE, size_t NEEDLE-LEN) This is like `strstr', but NEEDLE and HAYSTACK are byte arrays rather than null-terminated strings. NEEDLE-LEN is the length of NEEDLE and HAYSTACK-LEN is the length of HAYSTACK. This function is a GNU extension. - Function: size_t strspn (const char *STRING, const char *SKIPSET) The `strspn' ("string span") function returns the length of the initial substring of STRING that consists entirely of characters that are members of the set specified by the string SKIPSET. The order of the characters in SKIPSET is not important. For example, strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcsspn (const wchar_t *WSTRING, const wchar_t *SKIPSET) The `wcsspn' ("wide character string span") function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are members of the set specified by the string SKIPSET. The order of the wide characters in SKIPSET is not important. - Function: size_t strcspn (const char *STRING, const char *STOPSET) The `strcspn' ("string complement span") function returns the length of the initial substring of STRING that consists entirely of characters that are _not_ members of the set specified by the string STOPSET. (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.) For example, strcspn ("hello, world", " \t\n,.;!?") => 5 Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: size_t wcscspn (const wchar_t *WSTRING, const wchar_t *STOPSET) The `wcscspn' ("wide character string complement span") function returns the length of the initial substring of WSTRING that consists entirely of wide characters that are _not_ members of the set specified by the string STOPSET. (In other words, it returns the offset of the first character in STRING that is a member of the set STOPSET.) - Function: char * strpbrk (const char *STRING, const char *STOPSET) The `strpbrk' ("string pointer break") function is related to `strcspn', except that it returns a pointer to the first character in STRING that is a member of the set STOPSET instead of the length of the initial substring. It returns a null pointer if no such character from STOPSET is found. For example, strpbrk ("hello, world", " \t\n,.;!?") => ", world" Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t *STOPSET) The `wcspbrk' ("wide character string pointer break") function is related to `wcscspn', except that it returns a pointer to the first wide character in WSTRING that is a member of the set STOPSET instead of the length of the initial substring. It returns a null pointer if no such character from STOPSET is found. Compatibility String Search Functions ------------------------------------- - Function: char * index (const char *STRING, int C) `index' is another name for `strchr'; they are exactly the same. New code should always use `strchr' since this name is defined in ISO C while `index' is a BSD invention which never was available on System V derived systems. - Function: char * rindex (const char *STRING, int C) `rindex' is another name for `strrchr'; they are exactly the same. New code should always use `strrchr' since this name is defined in ISO C while `rindex' is a BSD invention which never was available on System V derived systems. Finding Tokens in a String ========================== It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the `strtok' function, declared in the header file `string.h'. - Function: char * strtok (char *restrict NEWSTRING, const char *restrict DELIMITERS) A string can be split into tokens by making a series of calls to the function `strtok'. The string to be split up is passed as the NEWSTRING argument on the first call only. The `strtok' function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the NEWSTRING argument. Calling `strtok' with another non-null NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls `strtok' behind your back (which would mess up this internal state information). The DELIMITERS argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is _not_ a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string NEWSTRING is overwritten by a null character, and the pointer to the beginning of the token in NEWSTRING is returned. On the next call to `strtok', the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to `strtok'. If the end of the string NEWSTRING is reached, or if the remainder of string consists only of delimiter characters, `strtok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. - Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char *DELIMITERS) A string can be split into tokens by making a series of calls to the function `wcstok'. The string to be split up is passed as the NEWSTRING argument on the first call only. The `wcstok' function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same wide character string are indicated by passing a null pointer as the NEWSTRING argument. Calling `wcstok' with another non-null NEWSTRING argument reinitializes the state information. It is guaranteed that no other library function ever calls `wcstok' behind your back (which would mess up this internal state information). The DELIMITERS argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is _not_ a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string NEWSTRING is overwritten by a null wide character, and the pointer to the beginning of the token in NEWSTRING is returned. On the next call to `wcstok', the searching begins at the next wide character beyond the one that marked the end of the previous token. Note that the set of delimiters DELIMITERS do not have to be the same on every call in a series of calls to `wcstok'. If the end of the wide character string NEWSTRING is reached, or if the remainder of string consists only of delimiter wide characters, `wcstok' returns a null pointer. Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent. *Warning:* Since `strtok' and `wcstok' alter the string they is parsing, you should always copy the string to a temporary buffer before parsing it with `strtok'/`wcstok' (*note Copying and Concatenation::). If you allow `strtok' or `wcstok' to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after `strtok' or `wcstok' has modified it, and it would not have the expected value. The string that you are operating on might even be a constant. Then when `strtok' or `wcstok' tries to modify it, your program will get a fatal signal for writing in read-only memory. *Note Program Error Signals::. Even if the operation of `strtok' or `wcstok' would not require a modification of the string (e.g., if there is exactly one token) the string can (and in the GNU libc case will) be modified. This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily. The functions `strtok' and `wcstok' are not reentrant. *Note Nonreentrancy::, for a discussion of where and why reentrancy is important. Here is a simple example showing the use of `strtok'. #include #include ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *token, *cp; ... cp = strdupa (string); /* Make writable copy. */ token = strtok (cp, delimiters); /* token => "words" */ token = strtok (NULL, delimiters); /* token => "separated" */ token = strtok (NULL, delimiters); /* token => "by" */ token = strtok (NULL, delimiters); /* token => "spaces" */ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */ The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings. - Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS, char **SAVE_PTR) Just like `strtok', this function splits the string into several tokens which can be accessed by successive calls to `strtok_r'. The difference is that the information about the next token is stored in the space pointed to by the third argument, SAVE_PTR, which is a pointer to a string pointer. Calling `strtok_r' with a null pointer for NEWSTRING and leaving SAVE_PTR between the calls unchanged does the job without hindering reentrancy. This function is defined in POSIX.1 and can be found on many systems which support multi-threading. - Function: char * strsep (char **STRING_PTR, const char *DELIMITER) This function has a similar functionality as `strtok_r' with the NEWSTRING argument replaced by the SAVE_PTR argument. The initialization of the moving pointer has to be done by the user. Successive calls to `strsep' move the pointer along the tokens separated by DELIMITER, returning the address of the next token and updating STRING_PTR to point to the beginning of the next token. One difference between `strsep' and `strtok_r' is that if the input string contains more than one character from DELIMITER in a row `strsep' returns an empty string for each pair of characters from DELIMITER. This means that a program normally should test for `strsep' returning an empty string before processing it. This function was introduced in 4.3BSD and therefore is widely available. Here is how the above example looks like when `strsep' is used. #include #include ... const char string[] = "words separated by spaces -- and, punctuation!"; const char delimiters[] = " .,;:!-"; char *running; char *token; ... running = strdupa (string); token = strsep (&running, delimiters); /* token => "words" */ token = strsep (&running, delimiters); /* token => "separated" */ token = strsep (&running, delimiters); /* token => "by" */ token = strsep (&running, delimiters); /* token => "spaces" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "and" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => "punctuation" */ token = strsep (&running, delimiters); /* token => "" */ token = strsep (&running, delimiters); /* token => NULL */ - Function: char * basename (const char *FILENAME) The GNU version of the `basename' function returns the last component of the path in FILENAME. This function is the preferred usage, since it does not modify the argument, FILENAME, and respects trailing slashes. The prototype for `basename' can be found in `string.h'. Note, this function is overriden by the XPG version, if `libgen.h' is included. Example of using GNU `basename': #include int main (int argc, char *argv[]) { char *prog = basename (argv[0]); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } *Portability Note:* This function may produce different results on different systems. - Function: char * basename (char *PATH) This is the standard XPG defined `basename'. It is similar in spirit to the GNU version, but may modify the PATH by removing trailing '/' characters. If the PATH is made up entirely of '/' characters, then "/" will be returned. Also, if PATH is `NULL' or an empty string, then "." is returned. The prototype for the XPG version can be found in `libgen.h'. Example of using XPG `basename': #include int main (int argc, char *argv[]) { char *prog; char *path = strdupa (argv[0]); prog = basename (path); if (argc < 2) { fprintf (stderr, "Usage %s \n", prog); exit (1); } ... } - Function: char * dirname (char *PATH) The `dirname' function is the compliment to the XPG version of `basename'. It returns the parent directory of the file specified by PATH. If PATH is `NULL', an empty string, or contains no '/' characters, then "." is returned. The prototype for this function can be found in `libgen.h'. strfry ====== The function below addresses the perennial programming quandary: "How do I take good data in string form and painlessly turn it into garbage?" This is actually a fairly simple task for C programmers who do not use the GNU C library string functions, but for programs based on the GNU C library, the `strfry' function is the preferred method for destroying string data. The prototype for this function is in `string.h'. - Function: char * strfry (char *STRING) `strfry' creates a pseudorandom anagram of a string, replacing the input with the anagram in place. For each position in the string, `strfry' swaps it with a position in the string selected at random (from a uniform distribution). The two positions may be the same. The return value of `strfry' is always STRING. *Portability Note:* This function is unique to the GNU C library. Trivial Encryption ================== The `memfrob' function converts an array of data to something unrecognizable and back again. It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text. The transformation is analogous to Usenet's "Rot13" encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, `memfrob' works on arbitrary binary data, not just text. For true encryption, *Note Cryptographic Functions::. This function is declared in `string.h'. - Function: void * memfrob (void *MEM, size_t LENGTH) `memfrob' transforms (frobnicates) each byte of the data structure at MEM, which is LENGTH bytes long, by bitwise exclusive oring it with binary 00101010. It does the transformation in place and its return value is always MEM. Note that `memfrob' a second time on the same data structure returns it to its original state. This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in *Note Cryptographic Functions::. *Portability Note:* This function is unique to the GNU C library. Encode Binary Data ================== To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task. - Function: char * l64a (long int N) This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 6 character buffer which contains an encoded version of N. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if N is zero, which is somewhat bizarre but mandated by the standard. *Warning:* Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library. *Compatibility Note:* The XPG standard states that the return value of `l64a' is undefined if N is negative. In the GNU implementation, `l64a' treats its argument as unsigned, so it will return a sensible encoding for any nonzero N; however, portable programs should not rely on this. To encode a large buffer `l64a' must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this: char * encode (const void *buf, size_t len) { /* We know in advance how long the buffer has to be. */ unsigned char *in = (unsigned char *) buf; char *out = malloc (6 + ((len + 3) / 4) * 6 + 1); char *cp = out; /* Encode the length. */ /* Using `htonl' is necessary so that the data can be decoded even on machines with different byte order. */ cp = mempcpy (cp, l64a (htonl (len)), 6); while (len > 3) { unsigned long int n = *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; n = (n << 8) | *in++; len -= 4; if (n) cp = mempcpy (cp, l64a (htonl (n)), 6); else /* `l64a' returns the empty string for n==0, so we must generate its encoding ("......") by hand. */ cp = stpcpy (cp, "......"); } if (len > 0) { unsigned long int n = *in++; if (--len > 0) { n = (n << 8) | *in++; if (--len > 0) n = (n << 8) | *in; } memcpy (cp, l64a (htonl (n)), 6); cp += 6; } *cp = '\0'; return out; } It is strange that the library does not provide the complete functionality needed but so be it. To decode data produced with `l64a' the following function should be used. - Function: long int a64l (const char *STRING) The parameter STRING should contain a string which was produced by a call to `l64a'. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character not in the table, rather like `atoi'; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line characters. The decoded number is returned as a `long int' value. The `l64a' and `a64l' functions use a base 64 encoding, in which each character of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits: 0 1 2 3 4 5 6 7 0 `.' `/' `0' `1' `2' `3' `4' `5' 8 `6' `7' `8' `9' `A' `B' `C' `D' 16 `E' `F' `G' `H' `I' `J' `K' `L' 24 `M' `N' `O' `P' `Q' `R' `S' `T' 32 `U' `V' `W' `X' `Y' `Z' `a' `b' 40 `c' `d' `e' `f' `g' `h' `i' `j' 48 `k' `l' `m' `n' `o' `p' `q' `r' 56 `s' `t' `u' `v' `w' `x' `y' `z' This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings. Argz and Envz Vectors ===================== "argz vectors" are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters (`'\0''). "Envz vectors" are an extension of argz vectors where each element is a name-value pair, separated by a `'='' character (as in a Unix environment). Argz Functions -------------- Each argz vector is represented by a pointer to the first element, of type `char *', and a size, of type `size_t', both of which can be initialized to `0' to represent an empty argz vector. All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified. The argz functions use `malloc'/`realloc' to allocate/grow argz vectors, and so any argz vector creating using these functions may be freed by using `free'; conversely, any argz function that may grow a string expects that string to have been allocated using `malloc' (those argz functions that only examine their arguments or modify them in place will work on any sort of memory). *Note Unconstrained Allocation::. All argz functions that do memory allocation have a return type of `error_t', and return `0' for success, and `ENOMEM' if an allocation error occurs. These functions are declared in the standard include file `argz.h'. - Function: error_t argz_create (char *const ARGV[], char **ARGZ, size_t *ARGZ_LEN) The `argz_create' function converts the Unix-style argument vector ARGV (a vector of pointers to normal C strings, terminated by `(char *)0'; *note Program Arguments::) into an argz vector with the same elements, which is returned in ARGZ and ARGZ_LEN. - Function: error_t argz_create_sep (const char *STRING, int SEP, char **ARGZ, size_t *ARGZ_LEN) The `argz_create_sep' function converts the null-terminated string STRING into an argz vector (returned in ARGZ and ARGZ_LEN) by splitting it into elements at every occurrence of the character SEP. - Function: size_t argz_count (const char *ARGZ, size_t ARG_LEN) Returns the number of elements in the argz vector ARGZ and ARGZ_LEN. - Function: void argz_extract (char *ARGZ, size_t ARGZ_LEN, char **ARGV) The `argz_extract' function converts the argz vector ARGZ and ARGZ_LEN into a Unix-style argument vector stored in ARGV, by putting pointers to every element in ARGZ into successive positions in ARGV, followed by a terminator of `0'. ARGV must be pre-allocated with enough space to hold all the elements in ARGZ plus the terminating `(char *)0' (`(argz_count (ARGZ, ARGZ_LEN) + 1) * sizeof (char *)' bytes should be enough). Note that the string pointers stored into ARGV point into ARGZ--they are not copies--and so ARGZ must be copied if it will be changed while ARGV is still active. This function is useful for passing the elements in ARGZ to an exec function (*note Executing a File::). - Function: void argz_stringify (char *ARGZ, size_t LEN, int SEP) The `argz_stringify' converts ARGZ into a normal string with the elements separated by the character SEP, by replacing each `'\0'' inside ARGZ (except the last one, which terminates the string) with SEP. This is handy for printing ARGZ in a readable manner. - Function: error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const char *STR) The `argz_add' function adds the string STR to the end of the argz vector `*ARGZ', and updates `*ARGZ' and `*ARGZ_LEN' accordingly. - Function: error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN, const char *STR, int DELIM) The `argz_add_sep' function is similar to `argz_add', but STR is split into separate elements in the result at occurrences of the character DELIM. This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of `':'' for DELIM. - Function: error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const char *BUF, size_t BUF_LEN) The `argz_append' function appends BUF_LEN bytes starting at BUF to the argz vector `*ARGZ', reallocating `*ARGZ' to accommodate it, and adding BUF_LEN to `*ARGZ_LEN'. - Function: error_t argz_delete (char **ARGZ, size_t *ARGZ_LEN, char *ENTRY) If ENTRY points to the beginning of one of the elements in the argz vector `*ARGZ', the `argz_delete' function will remove this entry and reallocate `*ARGZ', modifying `*ARGZ' and `*ARGZ_LEN' accordingly. Note that as destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as ENTRY will then become invalid. - Function: error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char *BEFORE, const char *ENTRY) The `argz_insert' function inserts the string ENTRY into the argz vector `*ARGZ' at a point just before the existing element pointed to by BEFORE, reallocating `*ARGZ' and updating `*ARGZ' and `*ARGZ_LEN'. If BEFORE is `0', ENTRY is added to the end instead (as if by `argz_add'). Since the first element is in fact the same as `*ARGZ', passing in `*ARGZ' as the value of BEFORE will result in ENTRY being inserted at the beginning. - Function: char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char *ENTRY) The `argz_next' function provides a convenient way of iterating over the elements in the argz vector ARGZ. It returns a pointer to the next element in ARGZ after the element ENTRY, or `0' if there are no elements following ENTRY. If ENTRY is `0', the first element of ARGZ is returned. This behavior suggests two styles of iteration: char *entry = 0; while ((entry = argz_next (ARGZ, ARGZ_LEN, entry))) ACTION; (the double parentheses are necessary to make some C compilers shut up about what they consider a questionable `while'-test) and: char *entry; for (entry = ARGZ; entry; entry = argz_next (ARGZ, ARGZ_LEN, entry)) ACTION; Note that the latter depends on ARGZ having a value of `0' if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here. - Function: error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN, const char *STR, const char *WITH, unsigned *REPLACE_COUNT) Replace any occurrences of the string STR in ARGZ with WITH, reallocating ARGZ as necessary. If REPLACE_COUNT is non-zero, `*REPLACE_COUNT' will be incremented by number of replacements performed. Envz Functions -------------- Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense. Each element in an envz vector is a name-value pair, separated by a `'='' character; if multiple `'='' characters are present in an element, those after the first are considered part of the value, and treated like all other non-`'\0'' characters. If _no_ `'='' characters are present in an element, that element is considered the name of a "null" entry, as distinct from an entry with an empty value: `envz_get' will return `0' if given the name of null entry, whereas an entry with an empty value would result in a value of `""'; `envz_entry' will still find such entries, however. Null entries can be removed with `envz_strip' function. As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of `error_t', and return either `0' or `ENOMEM'. These functions are declared in the standard include file `envz.h'. - Function: char * envz_entry (const char *ENVZ, size_t ENVZ_LEN, const char *NAME) The `envz_entry' function finds the entry in ENVZ with the name NAME, and returns a pointer to the whole entry--that is, the argz element which begins with NAME followed by a `'='' character. If there is no entry with that name, `0' is returned. - Function: char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const char *NAME) The `envz_get' function finds the entry in ENVZ with the name NAME (like `envz_entry'), and returns a pointer to the value portion of that entry (following the `'=''). If there is no entry with that name (or only a null entry), `0' is returned. - Function: error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const char *NAME, const char *VALUE) The `envz_add' function adds an entry to `*ENVZ' (updating `*ENVZ' and `*ENVZ_LEN') with the name NAME, and value VALUE. If an entry with the same name already exists in ENVZ, it is removed first. If VALUE is `0', then the new entry will the special null type of entry (mentioned above). - Function: error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE) The `envz_merge' function adds each entry in ENVZ2 to ENVZ, as if with `envz_add', updating `*ENVZ' and `*ENVZ_LEN'. If OVERRIDE is true, then values in ENVZ2 will supersede those with the same name in ENVZ, otherwise not. Null entries are treated just like other entries in this respect, so a null entry in ENVZ can prevent an entry of the same name in ENVZ2 from being added to ENVZ, if OVERRIDE is false. - Function: void envz_strip (char **ENVZ, size_t *ENVZ_LEN) The `envz_strip' function removes any null entries from ENVZ, updating `*ENVZ' and `*ENVZ_LEN'. Character Set Handling ********************** Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by 2^8 choices. This chapter shows the functionality that was added to the C library to support multiple character sets. Introduction to Extended Characters =================================== A variety of solutions is available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library. A distinction we have to make right away is between internal and external representation. "Internal representation" means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed. Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets. One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format. For such a common format (= character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: "wide characters" will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary). As shown in some other part of this manual, a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, ISO 10646 was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first `0x10000' code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to `0x10ffff'. To represent wide characters the `char' type is not suitable. For this reason the ISO C standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to `int' for those functions that take a single wide character. - Data type: wchar_t This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of `char[]' for multibyte character strings. The type is defined in `stddef.h'. The ISO C90 standard, where `wchar_t' was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define `wchar_t' as `char', which might make sense for embedded systems. But for GNU systems `wchar_t' is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of ISO 10646. Some Unix systems define `wchar_t' as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646 one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the `wchar_t' type. - Data type: wint_t `wint_t' is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of `int' when using the normal `char' strings. The types `wchar_t' and `wint_t' often have the same representation if their size is 32 bits wide but if `wchar_t' is defined as `char' the type `wint_t' must be defined as `int' due to the parameter promotion. This type is defined in `wchar.h' and was introduced in Amendment 1 to ISO C90. As there are for the `char' data type macros are available for specifying the minimum and maximum value representable in an object of type `wchar_t'. - Macro: wint_t WCHAR_MIN The macro `WCHAR_MIN' evaluates to the minimum value representable by an object of type `wint_t'. This macro was introduced in Amendment 1 to ISO C90. - Macro: wint_t WCHAR_MAX The macro `WCHAR_MAX' evaluates to the maximum value representable by an object of type `wint_t'. This macro was introduced in Amendment 1 to ISO C90. Another special wide character value is the equivalent to `EOF'. - Macro: wint_t WEOF The macro `WEOF' evaluates to a constant expression of type `wint_t' whose value is different from any member of the extended character set. `WEOF' need not be the same value as `EOF' and unlike `EOF' it also need _not_ be negative. In other words, sloppy code like { int c; ... while ((c = getc (fp)) < 0) ... } has to be rewritten to use `WEOF' explicitly when wide characters are used: { wint_t c; ... while ((c = wgetc (fp)) != WEOF) ... } This macro was introduced in Amendment 1 to ISO C90 and is defined in `wchar.h'. These internal representations present problems when it comes to storing and transmittal. Because each single wide character consists of more than one byte, they are effected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and, thereforet require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set. For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here-instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character `'/'' is used in the encoding _only_ to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operation system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow. * The simplest character sets are single-byte character sets. There can be only up to 256 characters (for 8 bit character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text. Handling of a 8 bit character sets is simple. This is not true for other kinds presented later, and therefore, the application one uses might require the use of 8 bit character sets. * The ISO 2022 standard defines a mechanism for extended character sets where one character _can_ be represented by more than one byte. This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text. Each byte in the text might have a different interpretation in each state. The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes. In most uses of ISO 2022 the defined character sets do not allow state changes that cover more than the next character. This has the big advantage that whenever one can identify the beginning of the byte sequence of a character one can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun's operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding). But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN. * Early attempts to fix 8 bit character sets for other languages using the Roman alphabet lead to character sets like ISO 6937. Here bytes representing characters like the acute accent do not produce output themselves: one has to combine them with other characters to get the desired result. For example, the byte sequence `0xc2 0x61' (non-spacing acute accent, followed by lower-case `a') to get the "small a with acute" character. To get the acute accent character on its own, one has to write `0xc2 0x20' (the non-spacing acute followed by a space). Character sets like ISO 6937 are used in some embedded systems such as teletex. * Instead of converting the Unicode or ISO 10646 text used internally, it is often also sufficient to simply use an encoding different than UCS-2/UCS-4. The Unicode and ISO 10646 standards even specify such an encoding: UTF-8. This encoding is able to represent all of ISO 10646 31 bits in a byte string of length one to six. There were a few other attempts to encode ISO 10646 such as UTF-7, but UTF-8 is today the only encoding that should be used. In fact, with any luck UTF-8 will soon be the only external encoding that has to be supported. It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts. Methods like the Unicode compression scheme can alleviate these problems. The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate. The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past. One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or ISO 10646. This is not required, but at least encouraged, by the ISO C standard. The standard defines at least a macro `__STDC_ISO_10646__' that is only defined on systems where the `wchar_t' type encodes ISO 10646 characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems. Overview about Character Handling Functions =========================================== A Unix C library contains three different sets of functions in two families to handle character set conversion. One of the function families (the most commonly used) is specified in the ISO C90 standard and, therefore, is portable even beyond the Unix world. Unfortunately this family is the least useful one. These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications). The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90. Restartable Multibyte Conversion Functions ========================================== The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities: * The character set assumed for the multibyte encoding is not specified as an argument to the functions. Instead the character set specified by the `LC_CTYPE' category of the current locale is used; see *Note Locale Categories::. * The functions handling more than one character at a time require NUL terminated strings as the argument (i.e., converting blocks of text does not work unless one can add a NUL byte at an appropriate place). The GNU C library contains some extensions to the standard that allow specifying a size, but basically they also expect terminated strings. Despite these limitations the ISO C functions can be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide character string if the text is not simple ASCII. The text itself might come from a file with translations and the user should decide about the current locale, which determines the translation and therefore also the external encoding used. In such a situation (and many others) the functions described here are perfect. If more freedom while performing the conversion is necessary take a look at the `iconv' functions (*note Generic Charset Conversion::). Selecting the conversion and its properties ------------------------------------------- We already said above that the currently selected locale for the `LC_CTYPE' category decides about the conversion that is performed by the functions we are about to describe. Each locale uses its own character set (given as an argument to `localedef') and this is the one assumed as the external multibyte encoding. The wide character character set always is UCS-4, at least on GNU systems. A characteristic of each multibyte character set is the maximum number of bytes that can be necessary to represent one character. This information is quite important when writing code that uses the conversion functions (as shown in the examples below). The ISO C standard defines two macros that provide this information. - Macro: int MB_LEN_MAX `MB_LEN_MAX' specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales. It is a compile-time constant and is defined in `limits.h'. - Macro: int MB_CUR_MAX `MB_CUR_MAX' expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than `MB_LEN_MAX'. Unlike `MB_LEN_MAX' this macro need not be a compile-time constant, and in the GNU C library it is not. `MB_CUR_MAX' is defined in `stdlib.h'. Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions, but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem: { char buf[MB_LEN_MAX]; ssize_t len = 0; while (! feof (fp)) { fread (&buf[len], 1, MB_CUR_MAX - len, fp); /* ... process buf */ len -= used; } } The code in the inner loop is expected to have always enough bytes in the array BUF to convert one multibyte character. The array BUF has to be sized statically since many compilers do not allow a variable size. The `fread' call makes sure that `MB_CUR_MAX' bytes are always available in BUF. Note that it isn't a problem if `MB_CUR_MAX' is not a compile-time constant. Representing the state of the conversion ---------------------------------------- In the introduction of this chapter it was said that certain character sets use a "stateful" encoding. That is, the encoded values depend in some way on the previous bytes in the text. Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another. - Data type: mbstate_t A variable of type `mbstate_t' can contain all the information about the "shift state" needed from one call to a conversion function to another. `mbstate_t' is defined in `wchar.h'. It was introduced in Amendment 1 to ISO C90. To use objects of type `mbstate_t' the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful. There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as follows: { mbstate_t state; memset (&state, '\0', sizeof (state)); /* from now on STATE can be used. */ ... } When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this. - Function: int mbsinit (const mbstate_t *PS) The `mbsinit' function determines whether the state object pointed to by PS is in the initial state. If PS is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero. `mbsinit' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Code using `mbsinit' often looks similar to this: { mbstate_t state; memset (&state, '\0', sizeof (state)); /* Use STATE. */ ... if (! mbsinit (&state)) { /* Emit code to return to initial state. */ const wchar_t empty[] = L""; const wchar_t *srcp = empty; wcsrtombs (outbuf, &srcp, outbuflen, &state); } ... } The code to emit the escape sequence to get back to the initial state is interesting. The `wcsrtombs' function can be used to determine the necessary output code (*note Converting Strings::). Please note that on GNU systems it is not necessary to perform this extra action for the conversion from multibyte text to wide character text since the wide character encoding is not stateful. But there is nothing mentioned in any standard that prohibits making `wchar_t' using a stateful encoding. Converting Single Characters ---------------------------- The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subpart of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127. - Function: wint_t btowc (int C) The `btowc' function ("byte to wide character") converts a valid single byte character C in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the `LC_CTYPE' category. If `(unsigned char) C' is no valid single byte multibyte character or if C is `EOF', the function returns `WEOF'. Please note the restriction of C being tested for validity only in the initial shift state. No `mbstate_t' object is used from which the state information is taken, and the function also does not use any static state. The `btowc' function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Despite the limitation that the single byte value always is interpreted in the initial state this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful): wchar_t * itow (unsigned long int val) { static wchar_t buf[30]; wchar_t *wcp = &buf[29]; *wcp = L'\0'; while (val != 0) { *--wcp = btowc ('0' + val % 10); val /= 10; } if (wcp == &buf[29]) *--wcp = L'0'; return wcp; } Why is it necessary to use such a complicated implementation and not simply cast `'0' + val % 10' to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for `wchar_t' representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this it is necessary `btowc'. There also is a function for the conversion in the other direction. - Function: int wctob (wint_t C) The `wctob' function ("wide character to byte") takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is `EOF'. `wctob' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state. - Function: size_t mbrtowc (wchar_t *restrict PWC, const char *restrict S, size_t N, mbstate_t *restrict PS) The `mbrtowc' function ("multibyte restartable to wide character") converts the next multibyte character in the string pointed to by S into a wide character and stores it in the wide character string pointed to by PWC. The conversion is performed according to the locale currently selected for the `LC_CTYPE' category. If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by PS. If PS is a null pointer, a static, internal state variable used only by the `mbrtowc' function is used. If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state. If the next N or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from S that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the `L'\0'' or the one found in the conversion) is stored in the string pointed to by PWC if PWC is not null. If the first N bytes of the multibyte string possibly form a valid multibyte character but there are more than N bytes needed to complete it, the return value of the function is `(size_t) -2' and no value is stored. Please note that this can happen even if N has a value greater than or equal to `MB_CUR_MAX' since the input might contain redundant shift sequences. If the first `n' bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable `errno' is set to the value `EILSEQ', and the function returns `(size_t) -1'. The conversion state is afterwards undefined. `mbrtowc' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Use of `mbrtowc' is straightforward. A function that copies a multibyte string into a wide character string while at the same time converting all lowercase characters into uppercase could look like this (this is not the final version, just an example; it has no error checking, and sometimes leaks memory): wchar_t * mbstouwcs (const char *s) { size_t len = strlen (s); wchar_t *result = malloc ((len + 1) * sizeof (wchar_t)); wchar_t *wcp = result; wchar_t tmp[1]; mbstate_t state; size_t nbytes; memset (&state, '\0', sizeof (state)); while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0) { if (nbytes >= (size_t) -2) /* Invalid input string. */ return NULL; *result++ = towupper (tmp[0]); len -= nbytes; s += nbytes; } return result; } The use of `mbrtowc' should be clear. A single wide character is stored in `TMP[0]', and the number of consumed bytes is stored in the variable NBYTES. If the conversion is successful, the uppercase variant of the wide character is stored in the RESULT array and the pointer to the input string and the number of available bytes is adjusted. The only non-obvious thing about `mbrtowc' might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string. This method yields a pessimistic guess about the size of the result, and if many wide character strings have to be constructed this way or if the strings are long, the extra memory required to be allocated because the input string contains multibyte characters might be significant. The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. There is, however, a function that does part of the work. - Function: size_t mbrlen (const char *restrict S, size_t N, mbstate_t *PS) The `mbrlen' function ("multibyte restartable length") computes the number of at most N bytes starting at S, which form the next valid and complete multibyte character. If the next multibyte character corresponds to the NUL wide character, the return value is 0. If the next N bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned. If the the first N bytes possibly form a valid multibyte character but the character is incomplete, the return value is `(size_t) -2'. Otherwise the multibyte character sequence is invalid and the return value is `(size_t) -1'. The multibyte sequence is interpreted in the state represented by the object pointed to by PS. If PS is a null pointer, a state object local to `mbrlen' is used. `mbrlen' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The attentive reader now will note that `mbrlen' can be implemented as mbrtowc (NULL, s, n, ps != NULL ? ps : &internal) This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function `mbslen' using it: size_t mbslen (const char *s) { mbstate_t state; size_t result = 0; size_t nbytes; memset (&state, '\0', sizeof (state)); while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0) { if (nbytes >= (size_t) -2) /* Something is wrong. */ return (size_t) -1; s += nbytes; ++result; } return result; } This function simply calls `mbrlen' for each multibyte character in the string and counts the number of function calls. Please note that we here use `MB_LEN_MAX' as the size argument in the `mbrlen' call. This is acceptable since a) this value is larger then the length of the longest multibyte character sequence and b) we know that the string S ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the `mbrlen' function will never read invalid memory. Now that this function is available (just to make this clear, this function is _not_ part of the GNU C library) we can compute the number of wide character required to store the converted multibyte character string S using wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t); Please note that the `mbslen' function is quite inefficient. The implementation of `mbstouwcs' with `mbslen' would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice. - Function: size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t *restrict PS) The `wcrtomb' function ("wide character restartable to multibyte") converts a single wide character into a multibyte string corresponding to that wide character. If S is a null pointer, the function resets the state stored in the objects pointed to by PS (or the internal `mbstate_t' object) to the initial state. This can also be achieved by a call like this: wcrtombs (temp_buf, L'\0', ps) since, if S is a null pointer, `wcrtomb' performs as if it writes into an internal buffer, which is guaranteed to be large enough. If WC is the NUL wide character, `wcrtomb' emits, if necessary, a shift sequence to get the state PS into the initial state followed by a single NUL byte, which is stored in the string S. Otherwise a byte sequence (possibly including shift sequences) is written into the string S. This only happens if WC is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the `LC_CTYPE' category). If WC is no valid wide character, nothing is stored in the strings S, `errno' is set to `EILSEQ', the conversion state in PS is undefined and the return value is `(size_t) -1'. If no error occurred the function returns the number of bytes stored in the string S. This includes all bytes representing shift sequences. One word about the interface of the function: there is no parameter specifying the length of the array S. Instead the function assumes that there are at least `MB_CUR_MAX' bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur. `wcrtomb' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. Using `wcrtomb' is as easy as using `mbrtowc'. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems. char * mbscatwcs (char *s, size_t len, const wchar_t *ws) { mbstate_t state; /* Find the end of the existing string. */ char *wp = strchr (s, '\0'); len -= wp - s; memset (&state, '\0', sizeof (state)); do { size_t nbytes; if (len < MB_CUR_LEN) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } nbytes = wcrtomb (wp, *ws, &state); if (nbytes == (size_t) -1) /* Error in the conversion. */ return NULL; len -= nbytes; wp += nbytes; } while (*ws++ != L'\0'); return s; } First the function has to find the end of the string currently in the array S. The `strchr' call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string). After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array S. We abort if there are not at least `MB_CUR_LEN' bytes available. This is not always optimal but we have no other choice. We might have less than `MB_CUR_LEN' bytes available but the next multibyte character might also be only one byte long. At the time the `wcrtomb' call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution. ... if (len < MB_CUR_LEN) { mbstate_t temp_state; memcpy (&temp_state, &state, sizeof (state)); if (wcrtomb (NULL, *ws, &temp_state) > len) { /* We cannot guarantee that the next character fits into the buffer, so return an error. */ errno = E2BIG; return NULL; } } ... Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the `NULL' argument for the destination buffer in the new `wcrtomb' call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information. There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes. Converting Multibyte and Wide Character Strings ----------------------------------------------- The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, the GNU C library contains a few extensions that can help in some important situations. - Function: size_t mbsrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t LEN, mbstate_t *restrict PS) The `mbsrtowcs' function ("multibyte string restartable to wide character string") converts an NUL-terminated multibyte character string at `*SRC' into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by PS or from an internal object of `mbsrtowcs' if PS is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted. If DST is not a null pointer, the result is stored in the array pointed to by DST; otherwise, the conversion result is not available since it is stored in an internal buffer. If LEN wide characters are stored in the array DST before reaching the end of the input string, the conversion stops and LEN is returned. If DST is a null pointer, LEN is never checked. Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable `errno' is set to `EILSEQ' and the function returns `(size_t) -1'. In all other cases the function returns the number of wide characters converted during this call. If DST is not null, `mbsrtowcs' stores in the pointer pointed to by SRC either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character. `mbsrtowcs' was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The definition of the `mbsrtowcs' function has one important limitation. The requirement that DST has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is normally no collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the `mbsrtowcs' function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter LEN to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess. There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the `mbsrtowcs' function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state-but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code. The generic conversion interface (*note Generic Charset Conversion::) does not have this limitation (it simply works on buffers, not strings), and the GNU C library contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of `mbsrtowcs''s example above could be solved by determining the line length and passing this length to the function. - Function: size_t wcsrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t LEN, mbstate_t *restrict PS) The `wcsrtombs' function ("wide character string restartable to multibyte string") converts the NUL-terminated wide character string at `*SRC' into an equivalent multibyte character string and stores the result in the array pointed to by DST. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by PS or by a state object locally to `wcsrtombs' in case PS is a null pointer. If DST is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if DST is not a null pointer, the pointer pointed to by SRC gets assigned a null pointer. If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable `errno' to `EILSEQ', and returns `(size_t) -1'. Another reason for a premature stop is if DST is not a null pointer and the next converted character would require more than LEN bytes in total to the array DST. In this case (and if DEST is not a null pointer) the pointer pointed to by SRC is assigned a value pointing to the wide character right after the last one successfully converted. Except in the case of an encoding error the return value of the `wcsrtombs' function is the number of bytes in all the multibyte character sequences stored in DST. Before returning the state in the object pointed to by PS (or the internal object in case PS is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted. The `wcsrtombs' function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. The restriction mentioned above for the `mbsrtowcs' function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the LEN parameter). - Function: size_t mbsnrtowcs (wchar_t *restrict DST, const char **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict PS) The `mbsnrtowcs' function is very similar to the `mbsrtowcs' function. All the parameters are the same except for NMC, which is new. The return value is the same as for `mbsrtowcs'. This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string `*SRC' need not be NUL-terminated. But if a NUL byte is found within the NMC first bytes of the string, the conversion stops here. This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state. A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example): void showmbs (const char *src, FILE *fp) { mbstate_t state; int cnt = 0; memset (&state, '\0', sizeof (state)); while (1) { wchar_t linebuf[100]; const char *endp = strchr (src, '\n'); size_t n; /* Exit if there is no more line. */ if (endp == NULL) break; n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state); linebuf[n] = L'\0'; fprintf (fp, "line %d: \"%S\"\n", linebuf); } } There is no problem with the state after a call to `mbsnrtowcs'. Since we don't insert characters in the strings that were not in there right from the beginning and we use STATE only for the conversion of the given buffer, there is no problem with altering the state. - Function: size_t wcsnrtombs (char *restrict DST, const wchar_t **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict PS) The `wcsnrtombs' function implements the conversion from wide character strings to multibyte character strings. It is similar to `wcsrtombs' but, just like `mbsnrtowcs', it takes an extra parameter, which specifies the length of the input string. No more than NWC wide characters from the input string `*SRC' are converted. If the input string contains a NUL wide character in the first NWC characters, the conversion stops at this place. The `wcsnrtombs' function is a GNU extension and just like `mbsnrtowcs' helps in situations where no NUL-terminated input strings are available. A Complete Multibyte Conversion Example --------------------------------------- The example programs given in the last sections are only brief and do not contain all the error checking, etc. Presented here is a complete and documented example. It features the `mbrtowc' function but it should be easy to derive versions using the other functions. int file_mbsrtowcs (int input, int output) { /* Note the use of `MB_LEN_MAX'. `MB_CUR_MAX' cannot portably be used here. */ char buffer[BUFSIZ + MB_LEN_MAX]; mbstate_t state; int filled = 0; int eof = 0; /* Initialize the state. */ memset (&state, '\0', sizeof (state)); while (!eof) { ssize_t nread; ssize_t nwrite; char *inp = buffer; wchar_t outbuf[BUFSIZ]; wchar_t *outp = outbuf; /* Fill up the buffer from the input file. */ nread = read (input, buffer + filled, BUFSIZ); if (nread < 0) { perror ("read"); return 0; } /* If we reach end of file, make a note to read no more. */ if (nread == 0) eof = 1; /* `filled' is now the number of bytes in `buffer'. */ filled += nread; /* Convert those bytes to wide characters-as many as we can. */ while (1) { size_t thislen = mbrtowc (outp, inp, filled, &state); /* Stop converting at invalid character; this can mean we have read just the first part of a valid character. */ if (thislen == (size_t) -1) break; /* We want to handle embedded NUL bytes but the return value is 0. Correct this. */ if (thislen == 0) thislen = 1; /* Advance past this character. */ inp += thislen; filled -= thislen; ++outp; } /* Write the wide characters we just made. */ nwrite = write (output, outbuf, (outp - outbuf) * sizeof (wchar_t)); if (nwrite < 0) { perror ("write"); return 0; } /* See if we have a _real_ invalid character. */ if ((eof && filled > 0) || filled >= MB_CUR_MAX) { error (0, 0, "invalid multibyte character"); return 0; } /* If any characters must be carried forward, put them at the beginning of `buffer'. */ if (filled > 0) memmove (inp, buffer, filled); } return 1; } Non-reentrant Conversion Function ================================= The functions described in the previous chapter are defined in Amendment 1 to ISO C90, but the original ISO C90 standard also contained functions for character set conversion. The reason that these original functions are not described first is that they are almost entirely useless. The problem is that all the conversion functions described in the original ISO C90 use a local state. Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use. These original functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one, and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). *For the above reasons it is highly requested that the functions described in the previous section be used in place of non-reentrant conversion functions.* Non-reentrant Conversion of Single Characters --------------------------------------------- - Function: int mbtowc (wchar_t *restrict RESULT, const char *restrict STRING, size_t SIZE) The `mbtowc' ("multibyte to wide character") function when called with non-null STRING converts the first multibyte character beginning at STRING to its corresponding wide character code. It stores the result in `*RESULT'. `mbtowc' never examines more than SIZE bytes. (The idea is to supply for SIZE the number of bytes of data you have in hand.) `mbtowc' with non-null STRING distinguishes three possibilities: the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or STRING points to an empty string (a null character). For a valid multibyte character, `mbtowc' converts it to a wide character and stores that in `*RESULT', and returns the number of bytes in that character (always at least 1 and never more than SIZE). For an invalid byte sequence, `mbtowc' returns -1. For an empty string, it returns 0, also storing `'\0'' in `*RESULT'. If the multibyte character code uses shift characters, then `mbtowc' maintains and updates a shift state as it scans. If you call `mbtowc' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. *Note Shift State::. - Function: int wctomb (char *STRING, wchar_t WCHAR) The `wctomb' ("wide character to multibyte") function converts the wide character code WCHAR to its corresponding multibyte character sequence, and stores the result in bytes starting at STRING. At most `MB_CUR_MAX' characters are stored. `wctomb' with non-null STRING distinguishes three possibilities for WCHAR: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and `L'\0''. Given a valid code, `wctomb' converts it to a multibyte character, storing the bytes starting at STRING. Then it returns the number of bytes in that character (always at least 1 and never more than `MB_CUR_MAX'). If WCHAR is an invalid wide character code, `wctomb' returns -1. If WCHAR is `L'\0'', it returns `0', also storing `'\0'' in `*STRING'. If the multibyte character code uses shift characters, then `wctomb' maintains and updates a shift state as it scans. If you call `wctomb' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. *Note Shift State::. Calling this function with a WCHAR argument of zero when STRING is not null has the side-effect of reinitializing the stored shift state _as well as_ storing the multibyte character `'\0'' and returning 0. Similar to `mbrlen' there is also a non-reentrant function that computes the length of a multibyte character. It can be defined in terms of `mbtowc'. - Function: int mblen (const char *STRING, size_t SIZE) The `mblen' function with a non-null STRING argument returns the number of bytes that make up the multibyte character beginning at STRING, never examining more than SIZE bytes. (The idea is to supply for SIZE the number of bytes of data you have in hand.) The return value of `mblen' distinguishes three possibilities: the first SIZE bytes at STRING start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or STRING points to an empty string (a null character). For a valid multibyte character, `mblen' returns the number of bytes in that character (always at least `1' and never more than SIZE). For an invalid byte sequence, `mblen' returns -1. For an empty string, it returns 0. If the multibyte character code uses shift characters, then `mblen' maintains and updates a shift state as it scans. If you call `mblen' with a null pointer for STRING, that initializes the shift state to its standard initial value. It also returns a nonzero value if the multibyte character code in use actually has a shift state. *Note Shift State::. The function `mblen' is declared in `stdlib.h'. Non-reentrant Conversion of Strings ----------------------------------- For convenience the ISO C90 standard also defines functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see *Note Converting Strings::. - Function: size_t mbstowcs (wchar_t *WSTRING, const char *STRING, size_t SIZE) The `mbstowcs' ("multibyte string to wide character string") function converts the null-terminated string of multibyte characters STRING to an array of wide character codes, storing not more than SIZE wide characters into the array beginning at WSTRING. The terminating null character counts towards the size, so if SIZE is less than the actual number of wide characters resulting from STRING, no terminating null character is stored. The conversion of characters from STRING begins in the initial shift state. If an invalid multibyte character sequence is found, the `mbstowcs' function returns a value of -1. Otherwise, it returns the number of wide characters stored in the array WSTRING. This number does not include the terminating null character, which is present if the number is less than SIZE. Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result. wchar_t * mbstowcs_alloc (const char *string) { size_t size = strlen (string) + 1; wchar_t *buf = xmalloc (size * sizeof (wchar_t)); size = mbstowcs (buf, string, size); if (size == (size_t) -1) return NULL; buf = xrealloc (buf, (size + 1) * sizeof (wchar_t)); return buf; } - Function: size_t wcstombs (char *STRING, const wchar_t *WSTRING, size_t SIZE) The `wcstombs' ("wide character string to multibyte string") function converts the null-terminated wide character array WSTRING into a string containing multibyte characters, storing not more than SIZE bytes starting at STRING, followed by a terminating null character if there is room. The conversion of characters begins in the initial shift state. The terminating null character counts towards the size, so if SIZE is less than or equal to the number of bytes needed in WSTRING, no terminating null character is stored. If a code that does not correspond to a valid multibyte character is found, the `wcstombs' function returns a value of -1. Otherwise, the return value is the number of bytes stored in the array STRING. This number does not include the terminating null character, which is present if the number is less than SIZE. States in Non-reentrant Functions --------------------------------- In some multibyte character codes, the _meaning_ of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called "shift sequences" and we say that they set the "shift state" for other sequences that follow. To illustrate shift state and shift sequences, suppose we decide that the sequence `0200' (just one byte) enters Japanese mode, in which pairs of bytes in the range from `0240' to `0377' are single characters, while `0201' enters Latin-1 mode, in which single bytes in the range from `0240' to `0377' are characters, and interpreted according to the ISO Latin-1 character set. This is a multibyte code that has two alternative shift states ("Japanese mode" and "Latin-1 mode"), and two shift sequences that specify particular shift states. When the multibyte character code in use has shift states, then `mblen', `mbtowc', and `wctomb' must maintain and update the current shift state as they scan the string. To make this work properly, you must follow these rules: * Before starting to scan a string, call the function with a null pointer for the multibyte character address--for example, `mblen (NULL, 0)'. This initializes the shift state to its standard initial value. * Scan the string one character at a time, in order. Do not "back up" and rescan characters already scanned, and do not intersperse the processing of different strings. Here is an example of using `mblen' following these rules: void scan_string (char *s) { int length = strlen (s); /* Initialize shift state. */ mblen (NULL, 0); while (1) { int thischar = mblen (s, length); /* Deal with end of string and invalid characters. */ if (thischar == 0) break; if (thischar == -1) { error ("invalid multibyte character"); break; } /* Advance past this character. */ s += thischar; length -= thischar; } } The functions `mblen', `mbtowc' and `wctomb' are not reentrant when using a multibyte code that uses a shift state. However, no other library functions call these functions, so you don't have to worry that the shift state will be changed mysteriously. Generic Charset Conversion ========================== The conversion functions mentioned so far in this chapter all had in common that they operate on character sets that are not directly specified by the functions. The multibyte encoding used is specified by the currently selected locale for the `LC_CTYPE' category. The wide character set is fixed by the implementation (in the case of GNU C library it is always UCS-4 encoded ISO 10646. This has of course several problems when it comes to general character conversion: * For every conversion where neither the source nor the destination character set is the character set of the locale for the `LC_CTYPE' category, one has to change the `LC_CTYPE' locale using `setlocale'. Changing the `LC_TYPE' locale introduces major problems for the rest of the programs since several more functions (e.g., the character classification functions, *note Classification of Characters::) use the `LC_CTYPE' category. * Parallel conversions to and from different character sets are not possible since the `LC_CTYPE' selection is global and shared by all threads. * If neither the source nor the destination character set is the character set used for `wchar_t' representation, there is at least a two-step process necessary to convert a text using the functions above. One would have to select the source character set as the multibyte encoding, convert the text into a `wchar_t' text, select the destination character set as the multibyte encoding, and convert the wide character text to the multibyte (= destination) character set. Even if this is possible (which is not guaranteed) it is a very tiring work. Plus it suffers from the other two raised points even more due to the steady changing of the locale. The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation. In the following text first the interface to `iconv' and then the conversion function, will be described. Comparisons with other implementations will show what obstacles stand in the way of portable applications. Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities. Generic Character Set Conversion Interface ------------------------------------------ This set of functions follows the traditional cycle of using a resource: open-use-close. The interface consists of three functions, each of which implements one step. Before the interfaces are described it is necessary to introduce a data type. Just like other open-use-close interfaces the functions introduced here work using handles and the `iconv.h' header defines a special type for the handles used. - Data Type: iconv_t This data type is an abstract type defined in `iconv.h'. The user must not assume anything about the definition of this type; it must be completely opaque. Objects of this type can get assigned handles for the conversions using the `iconv' functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to. The first step is the function to create a handle. - Function: iconv_t iconv_open (const char *TOCODE, const char *FROMCODE) The `iconv_open' function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle. If the wanted conversion is not available, the `iconv_open' function returns `(iconv_t) -1'. In this case the global variable `errno' can have the following values: `EMFILE' The process already has `OPEN_MAX' file descriptors open. `ENFILE' The system limit of open file is reached. `ENOMEM' Not enough memory to carry out the operation. `EINVAL' The conversion from FROMCODE to TOCODE is not supported. It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions. An `iconv' descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from FROMSET to TOSET. The GNU C library implementation of `iconv_open' has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (*note glibc iconv Implementation::). Here it is only important to say that all directories mentioned in the `GCONV_PATH' environment variable are considered only if they contain a file `gconv-modules'. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is `PREFIX/lib/gconv'. The `GCONV_PATH' environment variable is examined exactly once at the first call of the `iconv_open' function. Later modifications of the variable have no effect. The `iconv_open' function was introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The `iconv_open' function is declared in `iconv.h'. The `iconv' implementation can associate large data structure with the handle returned by `iconv_open'. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore. - Function: int iconv_close (iconv_t CD) The `iconv_close' function frees all resources associated with the handle CD, which must have been returned by a successful call to the `iconv_open' function. If the function call was successful the return value is 0. Otherwise it is -1 and `errno' is set appropriately. Defined error are: `EBADF' The conversion descriptor is invalid. The `iconv_close' function was introduced together with the rest of the `iconv' functions in XPG2 and is declared in `iconv.h'. The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it. - Function: size_t iconv (iconv_t CD, char **INBUF, size_t *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT) The `iconv' function converts the text in the input buffer according to the rules associated with the descriptor CD and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor. The input buffer is specified by `*INBUF' and it contains `*INBYTESLEFT' bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type `char' and the length is measured in bytes even if the input text is encoded in wide characters. The output buffer is specified in a similar way. `*OUTBUF' points to the beginning of the buffer with at least `*OUTBYTESLEFT' bytes room for the result. The buffer pointer again is of type `char' and the length is measured in bytes. If OUTBUF or `*OUTBUF' is a null pointer, the conversion is performed but no output is available. If INBUF is a null pointer, the `iconv' function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with INBUF not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for the GNU C library as it is described below. Therefore an `iconv' call to reset the state should always be performed if some protocol requires this for the output text. The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters. In all of these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in INBUF and OUTBUF, and the available room in each buffer is stored in INBYTESLEFT and OUTBYTESLEFT. Since the character sets selected in the `iconv_open' call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The _current_ behavior of the GNU C library in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished. If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is `(size_t) -1' and `errno' is set appropriately. In such cases the value pointed to by INBYTESLEFT is nonzero. `EILSEQ' The conversion stopped because of an invalid byte sequence in the input. After the call, `*INBUF' points at the first byte of the invalid byte sequence. `E2BIG' The conversion stopped because it ran out of space in the output buffer. `EINVAL' The conversion stopped because of an incomplete byte sequence at the end of the input buffer. `EBADF' The CD argument is invalid. The `iconv' function was introduced in the XPG2 standard and is declared in the `iconv.h' header. The definition of the `iconv' function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below. A complete `iconv' example -------------------------- The example below features a solution for a common problem. Given that one knows the internal encoding used by the system for `wchar_t' strings, one often is in the position to read text from a file and store it in wide character buffers. One can do this using `mbsrtowcs', but then we run into the problems discussed above. int file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail) { char inbuf[BUFSIZ]; size_t insize = 0; char *wrptr = (char *) outbuf; int result = 0; iconv_t cd; cd = iconv_open ("WCHAR_T", charset); if (cd == (iconv_t) -1) { /* Something went wrong. */ if (errno == EINVAL) error (0, 0, "conversion from '%s' to wchar_t not available", charset); else perror ("iconv_open"); /* Terminate the output string. */ *outbuf = L'\0'; return -1; } while (avail > 0) { size_t nread; size_t nconv; char *inptr = inbuf; /* Read more input. */ nread = read (fd, inbuf + insize, sizeof (inbuf) - insize); if (nread == 0) { /* When we come here the file is completely read. This still could mean there are some unused characters in the `inbuf'. Put them back. */ if (lseek (fd, -insize, SEEK_CUR) == -1) result = -1; /* Now write out the byte sequence to get into the initial state if this is necessary. */ iconv (cd, NULL, NULL, &wrptr, &avail); break; } insize += nread; /* Do the conversion. */ nconv = iconv (cd, &inptr, &insize, &wrptr, &avail); if (nconv == (size_t) -1) { /* Not everything went right. It might only be an unfinished byte sequence at the end of the buffer. Or it is a real problem. */ if (errno == EINVAL) /* This is harmless. Simply move the unused bytes to the beginning of the buffer so that they can be used in the next round. */ memmove (inbuf, inptr, insize); else { /* It is a real problem. Maybe we ran out of space in the output buffer or we have invalid input. In any case back the file pointer to the position of the last processed byte. */ lseek (fd, -insize, SEEK_CUR); result = -1; break; } } } /* Terminate the output string. */ if (avail >= sizeof (wchar_t)) *((wchar_t *) wrptr) = L'\0'; if (iconv_close (cd) != 0) perror ("iconv_close"); return (wchar_t *) wrptr - outbuf; } This example shows the most important aspects of using the `iconv' functions. It shows how successive calls to `iconv' can be used to convert large amounts of text. The user does not have to care about stateful encodings as the functions take care of everything. An interesting point is the case where `iconv' returns an error and `errno' is set to `EINVAL'. This is not really an error in the transformation. It can happen whenever the input character set contains byte sequences of more than one byte for some character and texts are not processed in one piece. In this case there is a chance that a multibyte sequence is cut. The caller can then simply read the remainder of the takes and feed the offending bytes together with new character from the input to `iconv' and continue the work. The internal state kept in the descriptor is _not_ unspecified after such an event as is the case with the conversion functions from the ISO C standard. The example also shows the problem of using wide character strings with `iconv'. As explained in the description of the `iconv' function above, the function always takes a pointer to a `char' array and the available space is measured in bytes. In the example, the output buffer is a wide character buffer; therefore, we use a local variable WRPTR of type `char *', which is used in the `iconv' calls. This looks rather innocent but can lead to problems on platforms that have tight restriction on alignment. Therefore the caller of `iconv' has to make sure that the pointers passed are suitable for access of characters from the appropriate character set. Since, in the above case, the input parameter to the function is a `wchar_t' pointer, this is the case (unless the user violates alignment when computing the parameter). But in other situations, especially when writing generic functions where one does not know what type of character set one uses and, therefore, treats text as a sequence of bytes, it might become tricky. Some Details about other `iconv' Implementations ------------------------------------------------ This is not really the place to discuss the `iconv' implementation of other systems but it is necessary to know a bit about them to write portable programs. The above mentioned problems with the specification of the `iconv' functions can lead to portability issues. The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library. Therefore, the conversion information must come from files outside the C library. This is usually done in one or both of the following ways: * The C library contains a set of generic conversion functions that can read the needed conversion tables and other information from data files. These files get loaded when necessary. This solution is problematic as it requires a great deal of effort to apply to all character sets (potentially an infinite set). The differences in the structure of the different character sets is so large that many different variants of the table-processing functions must be developed. In addition, the generic nature of these functions make them slower than specifically implemented functions. * The C library only contains a framework that can dynamically load object files and execute the conversion functions contained therein. This solution provides much more flexibility. The C library itself contains only very little code and therefore reduces the general memory footprint. Also, with a documented interface between the C library and the loadable modules it is possible for third parties to extend the set of available conversion modules. A drawback of this solution is that dynamic loading must be available. Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without this capability it is therefore not possible to use this interface in statically linked programs. The GNU C library has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot. The danger is that one gets acquainted with this situation and forgets about the restrictions on other systems. A second thing to know about other `iconv' implementations is that the number of available conversions is often very limited. Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities. This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of 10 others might count as 10 conversions. Together with the other direction this makes 20 conversion possibilities used up by one character set. One can imagine the thin coverage these platform provide. Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses. This directly leads to a third and probably the most problematic point. The way the `iconv' conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set A to B and the conversion from B to C does _not_ imply that the conversion from A to C is available. This might not seem unreasonable and problematic at first, but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program that has to convert from A to C. A call like cd = iconv_open ("C", "A"); fails according to the assumption above. But what does the program do now? The conversion is necessary; therefore, simply giving up is not an option. This is a nuisance. The `iconv' function should take care of this. But how should the program proceed from here on? If it tries to convert to character set B, first the two `iconv_open' calls cd1 = iconv_open ("B", "A"); and cd2 = iconv_open ("C", "B"); will succeed, but how to find B? Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one _could_ try to find them and determine from the available file which conversions are available and whether there is an indirect route from A to C. This example shows one of the design errors of `iconv' mentioned above. It should at least be possible to determine the list of available conversion programmatically so that if `iconv_open' says there is no such conversion, one could make sure this also is true for indirect routes. The `iconv' Implementation in the GNU C library ----------------------------------------------- After reading about the problems of `iconv' implementations in the last section it is certainly good to note that the implementation in the GNU C library has none of the problems mentioned above. What follows is a step-by-step analysis of the points raised above. The evaluation is based on the current state of the development (as of January 1999). The development of the `iconv' functions is not complete, but basic functionality has solidified. The GNU C library's `iconv' implementation uses shared loadable modules to implement the conversions. A very small number of conversions are built into the library itself but these are only rather trivial conversions. All the benefits of loadable modules are available in the GNU C library implementation. This is especially appealing since the interface is well documented (see below), and it, therefore, is easy to write new conversion modules. The drawback of using loadable objects is not a problem in the GNU C library, at least on ELF systems. Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use `iconv'. The second mentioned problem is the number of supported conversions. Currently, the GNU C library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (150 times 149). If any conversion from or to a character set is missing, it can be added easily. Particularly impressive as it may be, this high number is due to the fact that the GNU C library implementation of `iconv' does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set A to B and from B to C it is always possible to convert from A to C directly). If the `iconv_open' returns an error and sets `errno' to `EINVAL', there is no known way, directly or indirectly, to perform the wanted conversion. Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to "triangulate" (i.e., convert with an intermediate representation). There is no inherent requirement to provide a conversion to ISO 10646 for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The existing set of conversions is simply meant to cover all conversions that might be of interest. All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646. In such a situation one easily can write a new conversion and provide it as a better alternative. The GNU C library `iconv' implementation would automatically use the module implementing the conversion if it is specified to be more efficient. Format of `gconv-modules' files ............................... All information about the available conversions comes from a file named `gconv-modules', which can be found in any of the directories along the `GCONV_PATH'. The `gconv-modules' files are line-oriented text files, where each of the lines has one of the following formats: * If the first non-whitespace character is a `#' the line contains only comments and is ignored. * Lines starting with `alias' define an alias name for a character set. Two more words are expected on the line. The first word defines the alias name, and the second defines the original name of the character set. The effect is that it is possible to use the alias name in the FROMSET or TOSET parameters of `iconv_open' and achieve the same result as when using the real character set name. This is quite important as a character set has often many different names. There is normally an official name but this need not correspond to the most popular name. Beside this many character sets have special names that are somehow constructed. For example, all character sets specified by the ISO have an alias of the form `ISO-IR-NNN' where NNN is the registration number. This allows programs that know about the registration number to construct character set names and use them in `iconv_open' calls. More on the available names and aliases follows below. * Lines starting with `module' introduce an available conversion module. These lines must contain three or four more words. The first word specifies the source character set, the second word the destination character set of conversion implemented in this module, and the third word is the name of the loadable module. The filename is constructed by appending the usual shared object suffix (normally `.so') and this file is then supposed to be found in the same directory the `gconv-modules' file is in. The last word on the line, which is optional, is a numeric value representing the cost of the conversion. If this word is missing, a cost of 1 is assumed. The numeric value itself does not matter that much; what counts are the relative values of the sums of costs for all possible conversion paths. Below is a more precise description of the use of the cost value. Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All that has to be done is to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory and add a file `gconv-modules' with the following content in the same directory: module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1 module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1 To see why this is sufficient, it is necessary to understand how the conversion used by `iconv' (and described in the descriptor) is selected. The approach to this problem is quite simple. At the first call of the `iconv_open' function the program reads all available `gconv-modules' files and builds up two tables: one containing all the known aliases and another that contains the information about the conversions and which shared object implements them. Finding the conversion path in `iconv' ...................................... The set of available conversions form a directed graph with weighted edges. The weights on the edges are the costs specified in the `gconv-modules' files. The `iconv_open' function uses an algorithm suitable for search for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set. Explaining why the above `gconv-modules' files allows the `iconv' implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward. Since the latter conversion takes two steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost is 1+1 = 2. The above `gconv-modules' file, however, specifies that the new conversion modules can perform this conversion with only the cost of 1. A mysterious item about the `gconv-modules' file above (and also the file coming with the GNU C library) are the names of the character sets specified in the `module' lines. Why do almost all the names end in `//'? And this is not all: the names can actually be regular expressions. At this point in time this mystery should not be revealed, unless you have the relevant spell-casting materials: ashes from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed by St. Emacs, assorted herbal roots from Central America, sand from Cebu, etc. Sorry! *The part of the implementation where this is used is not yet finished. For now please simply follow the existing examples. It'll become clearer once it is. -drepper* A last remark about the `gconv-modules' is about the names not ending with `//'. A character set named `INTERNAL' is often mentioned. From the discussion above and the chosen name it should have become clear that this is the name for the representation used in the intermediate step of the triangulation. We have said that this is UCS-4 but actually that is not quite right. The UCS-4 specification also includes the specification of the byte ordering used. Since a UCS-4 value consists of four bytes, a stored value is effected by byte ordering. The internal representation is _not_ the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4. This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4. To avoid trouble with endianess, the internal representation consistently is named `INTERNAL' even on big-endian systems where the representations are identical. `iconv' module data structures .............................. So far this section has described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change a bit in the future but, with luck, only in an upwardly compatible way. The definitions necessary to write new modules are publicly available in the non-standard header `gconv.h'. The following text, therefore, describes the definitions from this header file. First, however, it is necessary to get an overview. From the perspective of the user of `iconv' the interface is quite simple: the `iconv_open' function returns a handle that can be used in calls to `iconv', and finally the handle is freed with a call to `iconv_close'. The problem is that the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion since the handle is all that is passed to the `iconv' function. Therefore, the data structures are really the elements necessary to understanding the implementation. We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in `gconv.h'. - Data type: struct __gconv_step This data structure describes one conversion a module can perform. For each function in a loaded module with conversion functions there is exactly one object of this type. This object is shared by all users of the conversion (i.e., this object does not contain any information corresponding to an actual conversion; it only describes the conversion itself). `struct __gconv_loaded_object *__shlib_handle' `const char *__modname' `int __counter' All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared. One must not expect any of the other elements to be available or initialized. `const char *__from_name' `const char *__to_name' `__from_name' and `__to_name' contain the names of the source and destination character sets. They can be used to identify the actual conversion to be carried out since one module might implement conversions for more than one character set and/or direction. `gconv_fct __fct' `gconv_init_fct __init_fct' `gconv_end_fct __end_fct' These elements contain pointers to the functions in the loadable module. The interface will be explained below. `int __min_needed_from' `int __max_needed_from' `int __min_needed_to' `int __max_needed_to;' These values have to be supplied in the init function of the module. The `__min_needed_from' value specifies how many bytes a character of the source character set at least needs. The `__max_needed_from' specifies the maximum value that also includes possible shift sequences. The `__min_needed_to' and `__max_needed_to' values serve the same purpose as `__min_needed_from' and `__max_needed_from' but this time for the destination character set. It is crucial that these values be accurate since otherwise the conversion functions will have problems or not work at all. `int __stateful' This element must also be initialized by the init function. `int __stateful' is nonzero if the source character set is stateful. Otherwise it is zero. `void *__data' This element can be used freely by the conversion functions in the module. `void *__data' can be used to communicate extra information from one call to another. `void *__data' need not be initialized if not needed at all. If `void *__data' element is assigned a pointer to dynamically allocated memory (presumably in the init function) it has to be made sure that the end function deallocates the memory. Otherwise the application will leak memory. It is important to be aware that this data structure is shared by all users of this specification conversion and therefore the `__data' element must not contain data specific to one specific use of the conversion function. - Data type: struct __gconv_step_data This is the data structure that contains the information specific to each use of the conversion functions. `char *__outbuf' `char *__outbufend' These elements specify the output buffer for the conversion step. The `__outbuf' element points to the beginning of the buffer, and `__outbufend' points to the byte following the last byte in the buffer. The conversion function must not assume anything about the size of the buffer but it can be safely assumed the there is room for at least one complete character in the output buffer. Once the conversion is finished, if the conversion is the last step, the `__outbuf' element must be modified to point after the last byte written into the buffer to signal how much output is available. If this conversion step is not the last one, the element must not be modified. The `__outbufend' element must not be modified. `int __is_last' This element is nonzero if this conversion step is the last one. This information is necessary for the recursion. See the description of the conversion function internals below. This element must never be modified. `int __invocation_counter' The conversion function can use this element to see how many calls of the conversion function already happened. Some character sets require a certain prolog when generating output, and by comparing this value with zero, one can find out whether it is the first call and whether, therefore, the prolog should be emitted. This element must never be modified. `int __internal_use' This element is another one rarely used but needed in certain situations. It is assigned a nonzero value in case the conversion functions are used to implement `mbsrtowcs' et.al. (i.e., the function is not used directly through the `iconv' interface). This sometimes makes a difference as it is expected that the `iconv' functions are used to translate entire texts while the `mbsrtowcs' functions are normally used only to convert single strings and might be used multiple times to convert entire texts. But in this situation we would have problem complying with some rules of the character set specification. Some character sets require a prolog, which must appear exactly once for an entire text. If a number of `mbsrtowcs' calls are used to convert the text, only the first call must add the prolog. However, because there is no communication between the different calls of `mbsrtowcs', the conversion functions have no possibility to find this out. The situation is different for sequences of `iconv' calls since the handle allows access to the needed information. The `int __internal_use' element is mostly used together with `__invocation_counter' as follows: if (!data->__internal_use && data->__invocation_counter == 0) /* Emit prolog. */ ... This element must never be modified. `mbstate_t *__statep' The `__statep' element points to an object of type `mbstate_t' (*note Keeping the state::). The conversion of a stateful character set must use the object pointed to by `__statep' to store information about the conversion state. The `__statep' element itself must never be modified. `mbstate_t __state' This element must _never_ be used directly. It is only part of this structure to have the needed space allocated. `iconv' module interfaces ......................... With the knowledge about the data structures we now can describe the conversion function itself. To understand the interface a bit of knowledge is necessary about the functionality in the C library that loads the objects with the conversions. It is often the case that one conversion is used more than once (i.e., there are several `iconv_open' calls for the same set of character sets during one program run). The `mbsrtowcs' et.al. functions in the GNU C library also use the `iconv' functionality, which increases the number of uses of the same functions even more. Because of this multiple use of conversions, the modules do not get loaded exclusively for one conversion. Instead a module once loaded can be used by an arbitrary number of `iconv' or `mbsrtowcs' calls at the same time. The splitting of the information between conversion- function-specific information and conversion data makes this possible. The last section showed the two data structures used to do this. This is of course also reflected in the interface and semantics of the functions that the modules must provide. There are three functions that must have the following names: `gconv_init' The `gconv_init' function initializes the conversion function specific data structure. This very same object is shared by all conversions that use this conversion and, therefore, no state information about the conversion itself must be stored in here. If a module implements more than one conversion, the `gconv_init' function will be called multiple times. `gconv_end' The `gconv_end' function is responsible for freeing all resources allocated by the `gconv_init' function. If there is nothing to do, this function can be missing. Special care must be taken if the module implements more than one conversion and the `gconv_init' function does not allocate the same resources for all conversions. `gconv' This is the actual conversion function. It is called to convert one block of text. It gets passed the conversion step information initialized by `gconv_init' and the conversion data, specific to this use of the conversion functions. There are three data types defined for the three module interface functions and these define the interface. - Data type: int (*__gconv_init_fct) (struct __gconv_step *) This specifies the interface of the initialization function of the module. It is called exactly once for each conversion the module implements. As explained in the description of the `struct __gconv_step' data structure above the initialization function has to initialize parts of it. `__min_needed_from' `__max_needed_from' `__min_needed_to' `__max_needed_to' These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character sets, respectively. If the characters all have the same size, the minimum and maximum values are the same. `__stateful' This element must be initialized to an nonzero value if the source character set is stateful. Otherwise it must be zero. If the initialization function needs to communicate some information to the conversion function, this communication can happen using the `__data' element of the `__gconv_step' structure. But since this data is shared by all the conversions, it must not be modified by the conversion function. The example below shows how this can be used. #define MIN_NEEDED_FROM 1 #define MAX_NEEDED_FROM 4 #define MIN_NEEDED_TO 4 #define MAX_NEEDED_TO 4 int gconv_init (struct __gconv_step *step) { /* Determine which direction. */ struct iso2022jp_data *new_data; enum direction dir = illegal_dir; enum variant var = illegal_var; int result; if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0) { dir = from_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0) { dir = to_iso2022jp; var = iso2022jp; } else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0) { dir = from_iso2022jp; var = iso2022jp2; } else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0) { dir = to_iso2022jp; var = iso2022jp2; } result = __GCONV_NOCONV; if (dir != illegal_dir) { new_data = (struct iso2022jp_data *) malloc (sizeof (struct iso2022jp_data)); result = __GCONV_NOMEM; if (new_data != NULL) { new_data->dir = dir; new_data->var = var; step->__data = new_data; if (dir == from_iso2022jp) { step->__min_needed_from = MIN_NEEDED_FROM; step->__max_needed_from = MAX_NEEDED_FROM; step->__min_needed_to = MIN_NEEDED_TO; step->__max_needed_to = MAX_NEEDED_TO; } else { step->__min_needed_from = MIN_NEEDED_TO; step->__max_needed_from = MAX_NEEDED_TO; step->__min_needed_to = MIN_NEEDED_FROM; step->__max_needed_to = MAX_NEEDED_FROM + 2; } /* Yes, this is a stateful encoding. */ step->__stateful = 1; result = __GCONV_OK; } } return result; } The function first checks which conversion is wanted. The module from which this function is taken implements four different conversions; which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case. Next, a data structure, which contains the necessary information about which conversion is selected, is allocated. The data structure `struct iso2022jp_data' is locally defined since, outside the module, this data is not used at all. Please note that if all four conversions this modules supports are requested there are four data blocks. One interesting thing is the initialization of the `__min_' and `__max_' elements of the step data object. A single ISO-2022-JP character can consist of one to four bytes. Therefore the `MIN_NEEDED_FROM' and `MAX_NEEDED_FROM' macros are defined this way. The output is always the `INTERNAL' character set (aka UCS-4) and therefore each character consists of exactly four bytes. For the conversion from `INTERNAL' to ISO-2022-JP we have to take into account that escape sequences might be necessary to switch the character sets. Therefore the `__max_needed_to' element for this direction gets assigned `MAX_NEEDED_FROM + 2'. This takes into account the two bytes needed for the escape sequences to single the switching. The asymmetry in the maximum values for the two directions can be explained easily: when reading ISO-2022-JP text, escape sequences can be handled alone (i.e., it is not necessary to process a real character since the effect of the escape sequence can be recorded in the state information). The situation is different for the other direction. Since it is in general not known which character comes next, one cannot emit escape sequences to change the state in advance. This means the escape sequences that have to be emitted together with the next character. Therefore one needs more room than only for the character itself. The possible return values of the initialization function are: `__GCONV_OK' The initialization succeeded `__GCONV_NOCONV' The requested conversion is not supported in the module. This can happen if the `gconv-modules' file has errors. `__GCONV_NOMEM' Memory required to store additional information could not be allocated. The function called before the module is unloaded is significantly easier. It often has nothing at all to do; in which case it can be left out completely. - Data type: void (*__gconv_end_fct) (struct gconv_step *) The task of this function is to free all resources allocated in the initialization function. Therefore only the `__data' element of the object pointed to by the argument is of interest. Continuing the example from the initialization function, the finalization function looks like this: void gconv_end (struct __gconv_step *data) { free (data->__data); } The most important function is the conversion function itself, which can get quite complicated for complex character sets. But since this is not of interest here, we will only describe a possible skeleton for the conversion function. - Data type: int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int) The conversion function can be called for two basic reason: to convert text or to reset the state. From the description of the `iconv' function it can be seen why the flushing mode is necessary. What mode is selected is determined by the sixth argument, an integer. This argument being nonzero means that flushing is selected. Common to both modes is where the output buffer can be found. The information about this buffer is stored in the conversion step data. A pointer to this information is passed as the second argument to this function. The description of the `struct __gconv_step_data' structure has more information on the conversion step data. What has to be done for flushing depends on the source character set. If the source character set is not stateful, nothing has to be done. Otherwise the function has to emit a byte sequence to bring the state object into the initial state. Once this all happened the other conversion modules in the chain of conversions have to get the same chance. Whether another step follows can be determined from the `__is_last' element of the step data structure to which the first parameter points. The more interesting mode is when actual text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument, which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer. The conversion has to be performed according to the current state if the character set is stateful. The state is stored in an object pointed to by the `__statep' element of the step data (second argument). Once either the input buffer is empty or the output buffer is full the conversion stops. At this point, the pointer variable referenced by the third parameter must point to the byte following the last processed byte (i.e., if all of the input is consumed, this pointer and the fourth parameter have the same value). What now happens depends on whether this step is the last one. If it is the last step, the only thing that has to be done is to update the `__outbuf' element of the step data structure to point after the last written byte. This update gives the caller the information on how much text is available in the output buffer. In addition, the variable pointed to by the fifth parameter, which is of type `size_t', must be incremented by the number of characters (_not bytes_) that were converted in a non-reversible way. Then, the function can return. In case the step is not the last one, the later conversion functions have to get a chance to do their work. Therefore, the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays, so the next element in both cases can be found by simple pointer arithmetic: int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) { struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; ... The `next_step' pointer references the next step information and `next_data' the next data record. The call of the next function therefore will look similar to this: next_step->__fct (next_step, next_data, &outerr, outbuf, written, 0) But this is not yet all. Once the function call returns the conversion function might have some more to do. If the return value of the function is `__GCONV_EMPTY_INPUT', more room is available in the output buffer. Unless the input buffer is empty the conversion, functions start all over again and process the rest of the input buffer. If the return value is not `__GCONV_EMPTY_INPUT', something went wrong and we have to recover from this. A requirement for the conversion function is that the input buffer pointer (the third argument) always point to the last character that was put in converted form into the output buffer. This is trivially true after the conversion performed in the current step, but if the conversion functions deeper downstream stop prematurely, not all characters from the output buffer are consumed and, therefore, the input buffer pointers must be backed off to the right position. Correcting the input buffers is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and, therefore, can correct the input buffer pointer appropriately with a similar computation. Things are getting tricky if either character set has characters represented with variable length byte sequences, and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion (i.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again). The difference now is that it is known how much input must be created, and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return. One final thing should be mentioned. If it is necessary for the conversion to know whether it is the first invocation (in case a prolog has to be emitted), the conversion function should increment the `__invocation_counter' element of the step data structure just before returning to the caller. See the description of the `struct __gconv_step_data' structure above for more information on how this can be used. The return value must be one of the following values: `__GCONV_EMPTY_INPUT' All input was consumed and there is room left in the output buffer. `__GCONV_FULL_OUTPUT' No more room in the output buffer. In case this is not the last step this value is propagated down from the call of the next conversion function in the chain. `__GCONV_INCOMPLETE_INPUT' The input buffer is not entirely empty since it contains an incomplete character sequence. The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it. int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) { struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; gconv_fct fct = next_step->__fct; int status; /* If the function is called with no input this means we have to reset to the initial state. The possibly partly converted input is dropped. */ if (do_flush) { status = __GCONV_OK; /* Possible emit a byte sequence which put the state object into the initial state. */ /* Call the steps down the chain if there are any but only if we successfully emitted the escape sequence. */ if (status == __GCONV_OK && ! data->__is_last) status = fct (next_step, next_data, NULL, NULL, written, 1); } else { /* We preserve the initial values of the pointer variables. */ const char *inptr = *inbuf; char *outbuf = data->__outbuf; char *outend = data->__outbufend; char *outptr; do { /* Remember the start value for this round. */ inptr = *inbuf; /* The outbuf buffer is empty. */ outptr = outbuf; /* For stateful encodings the state must be safe here. */ /* Run the conversion loop. `status' is set appropriately afterwards. */ /* If this is the last step, leave the loop. There is nothing we can do. */ if (data->__is_last) { /* Store information about how many bytes are available. */ data->__outbuf = outbuf; /* If any non-reversible conversions were performed, add the number to `*written'. */ break; } /* Write out all output that was produced. */ if (outbuf > outptr) { const char *outerr = data->__outbuf; int result; result = fct (next_step, next_data, &outerr, outbuf, written, 0); if (result != __GCONV_EMPTY_INPUT) { if (outerr != outbuf) { /* Reset the input buffer pointer. We document here the complex case. */ size_t nstatus; /* Reload the pointers. */ *inbuf = inptr; outbuf = outptr; /* Possibly reset the state. */ /* Redo the conversion, but this time the end of the output buffer is at `outerr'. */ } /* Change the status. */ status = result; } else /* All the output is consumed, we can make another run if everything was ok. */ if (status == __GCONV_FULL_OUTPUT) status = __GCONV_OK; } } while (status == __GCONV_OK); /* We finished one use of this step. */ ++data->__invocation_counter; } return status; } This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the GNU C library sources. It contains many examples of working and optimized modules. Locales and Internationalization ******************************** Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken. "Internationalization" of software means programming it to be able to adapt to the user's favorite conventions. In ISO C, internationalization works by means of "locales". Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables). All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user. What Effects a Locale Has ========================= Each locale specifies conventions for several purposes, including the following: * What multibyte character sequences are valid, and how they are interpreted (*note Character Set Handling::). * Classification of which characters in the local character set are considered alphabetic, and upper- and lower-case conversion conventions (*note Character Handling::). * The collating sequence for the local language and character set (*note Collation Functions::). * Formatting of numbers and currency amounts (*note General Numeric::). * Formatting of dates and times (*note Formatting Calendar Time::). * What language to use for output, including error messages (*note Message Translation::). * What language to use for user answers to yes-or-no questions (*note Yes-or-No Questions::). * What language to use for more complex user input. (The C library doesn't yet help you implement this.) Some aspects of adapting to the specified locale are handled automatically by the library subroutines. For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use `strcoll' or `strxfrm' to compare strings. Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. The C library provides functions to handle translations for multiple languages easily. This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions. Choosing a Locale ================= The simplest way for the user to choose a locale is to set the environment variable `LANG'. This specifies a single locale to use for all purposes. For example, a user could specify a hypothetical locale named `espana-castellano' to use the standard conventions of most of Spain. The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'. Later we will describe how to construct locales. A user also has the option of specifying different locales for different purposes--in effect, choosing a mixture of multiple locales. For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars. Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes. Categories of Activities that Locales Affect ============================================ The purposes that locales serve are grouped into "categories", so that a user or a program can choose the locale for each category independently. Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to `setlocale'. `LC_COLLATE' This category applies to collation of strings (functions `strcoll' and `strxfrm'); see *Note Collation Functions::. `LC_CTYPE' This category applies to classification and conversion of characters, and to multibyte and wide characters; see *Note Character Handling::, and *Note Character Set Handling::. `LC_MONETARY' This category applies to formatting monetary values; see *Note General Numeric::. `LC_NUMERIC' This category applies to formatting numeric values that are not monetary; see *Note General Numeric::. `LC_TIME' This category applies to formatting date and time values; see *Note Formatting Calendar Time::. `LC_MESSAGES' This category applies to selecting the language used in the user interface for message translation (*note The Uniforum approach::; *note Message catalogs a la X/Open::) and contains regular expressions for affirmative and negative responses. `LC_ALL' This is not an environment variable; it is only a macro that you can use with `setlocale' to set a single locale for all purposes. Setting this environment variable overwrites all selections by the other `LC_*' variables or `LANG'. `LANG' If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above. When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient. For example, it should be possible to specify more than one locale name. Take a Swedish user who better speaks German than English, and a program whose messages are output in English by default. It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails to use English. This is possible with the variable `LANGUAGE'. For further description of this GNU extension see *Note Using gettextized software::. How Programs Set the Locale =========================== A C program inherits its locale environment variables when it starts up. This happens automatically. However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard `C' locale. To use the locales specified by the environment, you must call `setlocale'. Call it as follows: setlocale (LC_ALL, ""); to select a locale based on the user choice of the appropriate environment variables. You can also use `setlocale' to specify a particular locale, for general use or for a specific category. The symbols in this section are defined in the header file `locale.h'. - Function: char * setlocale (int CATEGORY, const char *LOCALE) The function `setlocale' sets the current locale for category CATEGORY to LOCALE. A list of all the locales the system provides can be created by running locale -a If CATEGORY is `LC_ALL', this specifies the locale for all purposes. The other possible values of CATEGORY specify an single purpose (*note Locale Categories::). You can also use this function to find out the current locale by passing a null pointer as the LOCALE argument. In this case, `setlocale' returns a string that is the name of the locale currently selected for category CATEGORY. The string returned by `setlocale' can be overwritten by subsequent calls, so you should make a copy of the string (*note Copying and Concatenation::) if you want to save it past any further calls to `setlocale'. (The standard library is guaranteed never to call `setlocale' itself.) You should not modify the string returned by `setlocale'. It might be the same string that was passed as an argument in a previous call to `setlocale'. One requirement is that the CATEGORY must be the same in the call the string was returned and the one when the string is passed in as LOCALE parameter. When you read the current locale for category `LC_ALL', the value encodes the entire combination of selected locales for all categories. In this case, the value is not just a single locale name. In fact, we don't make any promises about what it looks like. But if you specify the same "locale name" with `LC_ALL' in a subsequent call to `setlocale', it restores the same combination of locale selections. To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string. It is not guaranteed that the returned pointer remains valid over time. When the LOCALE argument is not a null pointer, the string returned by `setlocale' reflects the newly-modified locale. If you specify an empty string for LOCALE, this means to read the appropriate environment variable and use its value to select the locale for CATEGORY. If a nonempty string is given for LOCALE, then the locale of that name is used if possible. If you specify an invalid locale name, `setlocale' returns a null pointer and leaves the current locale unchanged. Here is an example showing how you might use `setlocale' to temporarily switch to a new locale. #include #include #include #include void with_other_locale (char *new_locale, void (*subroutine) (int), int argument) { char *old_locale, *saved_locale; /* Get the name of the current locale. */ old_locale = setlocale (LC_ALL, NULL); /* Copy the name so it won't be clobbered by `setlocale'. */ saved_locale = strdup (old_locale); if (saved_locale == NULL) fatal ("Out of memory"); /* Now change the locale and do some stuff with it. */ setlocale (LC_ALL, new_locale); (*subroutine) (argument); /* Restore the original locale. */ setlocale (LC_ALL, saved_locale); free (saved_locale); } *Portability Note:* Some ISO C systems may define additional locale categories, and future versions of the library will do so. For portability, assume that any symbol beginning with `LC_' might be defined in `locale.h'. Standard Locales ================ The only locale names you can count on finding on all operating systems are these three standard ones: `"C"' This is the standard C locale. The attributes and behavior it provides are specified in the ISO C standard. When your program starts up, it initially uses this locale by default. `"POSIX"' This is the standard POSIX locale. Currently, it is an alias for the standard C locale. `""' The empty name says to select a locale based on environment variables. *Note Locale Categories::. Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). It is also possible for the user to create private locales. All this will be discussed later when describing the tool to do so. If your program needs to use something other than the `C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed. Accessing Locale Information ============================ There are several ways to access locale information. The simplest way is to let the C library itself do the work. Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale. This is how the locale model is meant to work normally. As an example take the `strftime' function, which is meant to nicely format date and time information (*note Formatting Calendar Time::). Part of the standard information contained in the `LC_TIME' category is the names of the months. Instead of requiring the programmer to take care of providing the translations the `strftime' function does this all by itself. `%A' in the format string is replaced by the appropriate weekday name of the locale currently selected by `LC_TIME'. This is an easy example, and wherever possible functions do things automatically in this way. But there are quite often situations when there is simply no function to perform the task, or it is simply not possible to do the work automatically. For these cases it is necessary to access the information in the locale directly. To do this the C library provides two functions: `localeconv' and `nl_langinfo'. The former is part of ISO C and therefore portable, but has a brain-damaged interface. The second is part of the Unix interface and is portable in as far as the system follows the Unix standards. `localeconv': It is portable but ... ------------------------------------ Together with the `setlocale' function the ISO C people invented the `localeconv' function. It is a masterpiece of poor design. It is expensive to use, not extendable, and not generally usable as it provides access to only `LC_MONETARY' and `LC_NUMERIC' related information. Nevertheless, if it is applicable to a given situation it should be used since it is very portable. The function `strfmon' formats monetary amounts according to the selected locale using this information. - Function: struct lconv * localeconv (void) The `localeconv' function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale. You should not modify the structure or its contents. The structure might be overwritten by subsequent calls to `localeconv', or by calls to `setlocale', but no other function in the library overwrites this value. - Data Type: struct lconv `localeconv''s return value is of this data type. Its elements are described in the following subsections. If a member of the structure `struct lconv' has type `char', and the value is `CHAR_MAX', it means that the current locale has no value for that parameter. Generic Numeric Formatting Parameters ..................................... These are the standard members of `struct lconv'; there may be others. `char *decimal_point' `char *mon_decimal_point' These are the decimal-point separators used in formatting non-monetary and monetary quantities, respectively. In the `C' locale, the value of `decimal_point' is `"."', and the value of `mon_decimal_point' is `""'. `char *thousands_sep' `char *mon_thousands_sep' These are the separators used to delimit groups of digits to the left of the decimal point in formatting non-monetary and monetary quantities, respectively. In the `C' locale, both members have a value of `""' (the empty string). `char *grouping' `char *mon_grouping' These are strings that specify how to group the digits to the left of the decimal point. `grouping' applies to non-monetary quantities and `mon_grouping' applies to monetary quantities. Use either `thousands_sep' or `mon_thousands_sep' to separate the digit groups. Each member of these strings is to be interpreted as an integer value of type `char'. Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point.) The last member is either `0', in which case the previous member is used over and over again for all the remaining groups, or `CHAR_MAX', in which case there is no more grouping--or, put another way, any remaining digits form one large group without separators. For example, if `grouping' is `"\04\03\02"', the correct grouping for the number `123456787654321' is `12', `34', `56', `78', `765', `4321'. This uses a group of 4 digits at the end, preceded by a group of 3 digits, preceded by groups of 2 digits (as many as needed). With a separator of `,', the number would be printed as `12,34,56,78,765,4321'. A value of `"\03"' indicates repeated groups of three digits, as normally used in the U.S. In the standard `C' locale, both `grouping' and `mon_grouping' have a value of `""'. This value specifies no grouping at all. `char int_frac_digits' `char frac_digits' These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively. (Most often, both members have the same value.) In the standard `C' locale, both of these members have the value `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend printing no fractional digits. (This locale also specifies the empty string for `mon_decimal_point', so printing any fractional digits would be confusing!) Printing the Currency Symbol ............................ These members of the `struct lconv' structure specify how to print the symbol to identify a monetary value--the international analog of `$' for US dollars. Each country has two standard currency symbols. The "local currency symbol" is used commonly within the country, while the "international currency symbol" is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously. For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit--dollar amounts are implicitly assumed to be in Canadian dollars. `char *currency_symbol' The local currency symbol for the selected locale. In the standard `C' locale, this member has a value of `""' (the empty string), meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string pointed to by this variable. `char *int_curr_symbol' The international currency symbol for the selected locale. The value of `int_curr_symbol' should normally consist of a three-letter abbreviation determined by the international standard `ISO 4217 Codes for the Representation of Currency and Funds', followed by a one-character separator (often a space). In the standard `C' locale, this member has a value of `""' (the empty string), meaning "unspecified". We recommend you simply print the empty string as you would print any other string pointed to by this variable. `char p_cs_precedes' `char n_cs_precedes' `char int_p_cs_precedes' `char int_n_cs_precedes' These members are `1' if the `currency_symbol' or `int_curr_symbol' strings should precede the value of a monetary amount, or `0' if the strings should follow the value. The `p_cs_precedes' and `int_p_cs_precedes' members apply to positive amounts (or zero), and the `n_cs_precedes' and `int_n_cs_precedes' members apply to negative amounts. In the standard `C' locale, all of these members have a value of `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what to do when you find this value. We recommend printing the currency symbol before the amount, which is right for most countries. In other words, treat all nonzero values alike in these members. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. `char p_sep_by_space' `char n_sep_by_space' `char int_p_sep_by_space' `char int_n_sep_by_space' These members are `1' if a space should appear between the `currency_symbol' or `int_curr_symbol' strings and the amount, or `0' if no space should appear. The `p_sep_by_space' and `int_p_sep_by_space' members apply to positive amounts (or zero), and the `n_sep_by_space' and `int_n_sep_by_space' members apply to negative amounts. In the standard `C' locale, all of these members have a value of `CHAR_MAX', meaning "unspecified". The ISO standard doesn't say what you should do when you find this value; we suggest you treat it as 1 (print a space). In other words, treat all nonzero values alike in these members. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. There is one specialty with the `int_curr_symbol', though. Since all legal values contain a space at the end the string one either printf this space (if the currency symbol must appear in front and must be separated) or one has to avoid printing this character at all (especially when at the end of the string). Printing the Sign of a Monetary Amount ...................................... These members of the `struct lconv' structure specify how to print the sign (if any) of a monetary value. `char *positive_sign' `char *negative_sign' These are strings used to indicate positive (or zero) and negative monetary quantities, respectively. In the standard `C' locale, both of these members have a value of `""' (the empty string), meaning "unspecified". The ISO standard doesn't say what to do when you find this value; we recommend printing `positive_sign' as you find it, even if it is empty. For a negative value, print `negative_sign' as you find it unless both it and `positive_sign' are empty, in which case print `-' instead. (Failing to indicate the sign at all seems rather unreasonable.) `char p_sign_posn' `char n_sign_posn' `char int_p_sign_posn' `char int_n_sign_posn' These members are small integers that indicate how to position the sign for nonnegative and negative monetary quantities, respectively. (The string used by the sign is what was specified with `positive_sign' or `negative_sign'.) The possible values are as follows: `0' The currency symbol and quantity should be surrounded by parentheses. `1' Print the sign string before the quantity and currency symbol. `2' Print the sign string after the quantity and currency symbol. `3' Print the sign string right before the currency symbol. `4' Print the sign string right after the currency symbol. `CHAR_MAX' "Unspecified". Both members have this value in the standard `C' locale. The ISO standard doesn't say what you should do when the value is `CHAR_MAX'. We recommend you print the sign after the currency symbol. The members with the `int_' prefix apply to the `int_curr_symbol' while the other two apply to `currency_symbol'. Pinpoint Access to Locale Data ------------------------------ When writing the X/Open Portability Guide the authors realized that the `localeconv' function is not enough to provide reasonable access to locale information. The information which was meant to be available in the locale (as later specified in the POSIX.1 standard) requires more ways to access it. Therefore the `nl_langinfo' function was introduced. - Function: char * nl_langinfo (nl_item ITEM) The `nl_langinfo' function can be used to access individual elements of the locale categories. Unlike the `localeconv' function, which returns all the information, `nl_langinfo' lets the caller select what information it requires. This is very fast and it is not a problem to call this function multiple times. A second advantage is that in addition to the numeric and monetary formatting information, information from the `LC_TIME' and `LC_MESSAGES' categories is available. The type `nl_type' is defined in `nl_types.h'. The argument ITEM is a numeric value defined in the header `langinfo.h'. The X/Open standard defines the following values: `CODESET' `nl_langinfo' returns a string with the name of the coded character set used in the selected locale. `ABDAY_1' `ABDAY_2' `ABDAY_3' `ABDAY_4' `ABDAY_5' `ABDAY_6' `ABDAY_7' `nl_langinfo' returns the abbreviated weekday name. `ABDAY_1' corresponds to Sunday. `DAY_1' `DAY_2' `DAY_3' `DAY_4' `DAY_5' `DAY_6' `DAY_7' Similar to `ABDAY_1' etc., but here the return value is the unabbreviated weekday name. `ABMON_1' `ABMON_2' `ABMON_3' `ABMON_4' `ABMON_5' `ABMON_6' `ABMON_7' `ABMON_8' `ABMON_9' `ABMON_10' `ABMON_11' `ABMON_12' The return value is abbreviated name of the month. `ABMON_1' corresponds to January. `MON_1' `MON_2' `MON_3' `MON_4' `MON_5' `MON_6' `MON_7' `MON_8' `MON_9' `MON_10' `MON_11' `MON_12' Similar to `ABMON_1' etc., but here the month names are not abbreviated. Here the first value `MON_1' also corresponds to January. `AM_STR' `PM_STR' The return values are strings which can be used in the representation of time as an hour from 1 to 12 plus an am/pm specifier. Note that in locales which do not use this time representation these strings might be empty, in which case the am/pm format cannot be used at all. `D_T_FMT' The return value can be used as a format string for `strftime' to represent time and date in a locale-specific way. `D_FMT' The return value can be used as a format string for `strftime' to represent a date in a locale-specific way. `T_FMT' The return value can be used as a format string for `strftime' to represent time in a locale-specific way. `T_FMT_AMPM' The return value can be used as a format string for `strftime' to represent time in the am/pm format. Note that if the am/pm format does not make any sense for the selected locale, the return value might be the same as the one for `T_FMT'. `ERA' The return value represents the era used in the current locale. Most locales do not define this value. An example of a locale which does define this value is the Japanese one. In Japan, the traditional representation of dates includes the name of the era corresponding to the then-emperor's reign. Normally it should not be necessary to use this value directly. Specifying the `E' modifier in their format strings causes the `strftime' functions to use this information. The format of the returned string is not specified, and therefore you should not assume knowledge of it on different systems. `ERA_YEAR' The return value gives the year in the relevant era of the locale. As for `ERA' it should not be necessary to use this value directly. `ERA_D_T_FMT' This return value can be used as a format string for `strftime' to represent dates and times in a locale-specific era-based way. `ERA_D_FMT' This return value can be used as a format string for `strftime' to represent a date in a locale-specific era-based way. `ERA_T_FMT' This return value can be used as a format string for `strftime' to represent time in a locale-specific era-based way. `ALT_DIGITS' The return value is a representation of up to 100 values used to represent the values 0 to 99. As for `ERA' this value is not intended to be used directly, but instead indirectly through the `strftime' function. When the modifier `O' is used in a format which would otherwise use numerals to represent hours, minutes, seconds, weekdays, months, or weeks, the appropriate value for the locale is used instead. `INT_CURR_SYMBOL' The same as the value returned by `localeconv' in the `int_curr_symbol' element of the `struct lconv'. `CURRENCY_SYMBOL' `CRNCYSTR' The same as the value returned by `localeconv' in the `currency_symbol' element of the `struct lconv'. `CRNCYSTR' is a deprecated alias still required by Unix98. `MON_DECIMAL_POINT' The same as the value returned by `localeconv' in the `mon_decimal_point' element of the `struct lconv'. `MON_THOUSANDS_SEP' The same as the value returned by `localeconv' in the `mon_thousands_sep' element of the `struct lconv'. `MON_GROUPING' The same as the value returned by `localeconv' in the `mon_grouping' element of the `struct lconv'. `POSITIVE_SIGN' The same as the value returned by `localeconv' in the `positive_sign' element of the `struct lconv'. `NEGATIVE_SIGN' The same as the value returned by `localeconv' in the `negative_sign' element of the `struct lconv'. `INT_FRAC_DIGITS' The same as the value returned by `localeconv' in the `int_frac_digits' element of the `struct lconv'. `FRAC_DIGITS' The same as the value returned by `localeconv' in the `frac_digits' element of the `struct lconv'. `P_CS_PRECEDES' The same as the value returned by `localeconv' in the `p_cs_precedes' element of the `struct lconv'. `P_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `p_sep_by_space' element of the `struct lconv'. `N_CS_PRECEDES' The same as the value returned by `localeconv' in the `n_cs_precedes' element of the `struct lconv'. `N_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `n_sep_by_space' element of the `struct lconv'. `P_SIGN_POSN' The same as the value returned by `localeconv' in the `p_sign_posn' element of the `struct lconv'. `N_SIGN_POSN' The same as the value returned by `localeconv' in the `n_sign_posn' element of the `struct lconv'. `INT_P_CS_PRECEDES' The same as the value returned by `localeconv' in the `int_p_cs_precedes' element of the `struct lconv'. `INT_P_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `int_p_sep_by_space' element of the `struct lconv'. `INT_N_CS_PRECEDES' The same as the value returned by `localeconv' in the `int_n_cs_precedes' element of the `struct lconv'. `INT_N_SEP_BY_SPACE' The same as the value returned by `localeconv' in the `int_n_sep_by_space' element of the `struct lconv'. `INT_P_SIGN_POSN' The same as the value returned by `localeconv' in the `int_p_sign_posn' element of the `struct lconv'. `INT_N_SIGN_POSN' The same as the value returned by `localeconv' in the `int_n_sign_posn' element of the `struct lconv'. `DECIMAL_POINT' `RADIXCHAR' The same as the value returned by `localeconv' in the `decimal_point' element of the `struct lconv'. The name `RADIXCHAR' is a deprecated alias still used in Unix98. `THOUSANDS_SEP' `THOUSEP' The same as the value returned by `localeconv' in the `thousands_sep' element of the `struct lconv'. The name `THOUSEP' is a deprecated alias still used in Unix98. `GROUPING' The same as the value returned by `localeconv' in the `grouping' element of the `struct lconv'. `YESEXPR' The return value is a regular expression which can be used with the `regex' function to recognize a positive response to a yes/no question. The GNU C library provides the `rpmatch' function for easier handling in applications. `NOEXPR' The return value is a regular expression which can be used with the `regex' function to recognize a negative response to a yes/no question. `YESSTR' The return value is a locale-specific translation of the positive response to a yes/no question. Using this value is deprecated since it is a very special case of message translation, and is better handled by the message translation functions (*note Message Translation::). The use of this symbol is deprecated. Instead message translation should be used. `NOSTR' The return value is a locale-specific translation of the negative response to a yes/no question. What is said for `YESSTR' is also true here. The use of this symbol is deprecated. Instead message translation should be used. The file `langinfo.h' defines a lot more symbols but none of them is official. Using them is not portable, and the format of the return values might change. Therefore we recommended you not use them. Note that the return value for any valid argument can be used for in all situations (with the possible exception of the am/pm time formatting codes). If the user has not selected any locale for the appropriate category, `nl_langinfo' returns the information from the `"C"' locale. It is therefore possible to use this function as shown in the example below. If the argument ITEM is not valid, a pointer to an empty string is returned. An example of `nl_langinfo' usage is a function which has to print a given date and time in a locale-specific way. At first one might think that, since `strftime' internally uses the locale information, writing something like the following is enough: size_t i18n_time_n_data (char *s, size_t len, const struct tm *tp) { return strftime (s, len, "%X %D", tp); } The format contains no weekday or month names and therefore is internationally usable. Wrong! The output produced is something like `"hh:mm:ss MM/DD/YY"'. This format is only recognizable in the USA. Other countries use different formats. Therefore the function should be rewritten like this: size_t i18n_time_n_data (char *s, size_t len, const struct tm *tp) { return strftime (s, len, nl_langinfo (D_T_FMT), tp); } Now it uses the date and time format of the locale selected when the program runs. If the user selects the locale correctly there should never be a misunderstanding over the time and date format. A dedicated function to format numbers ====================================== We have seen that the structure returned by `localeconv' as well as the values given to `nl_langinfo' allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts. We have also seen that the underlying rules are quite complex. Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules. - Function: ssize_t strfmon (char *S, size_t MAXSIZE, const char *FORMAT, ...) The `strfmon' function is similar to the `strftime' function in that it takes a buffer, its size, a format string, and values to write into the buffer as text in a form specified by the format string. Like `strftime', the function also returns the number of bytes written into the buffer. There are two differences: `strfmon' can take more than one argument, and, of course, the format specification is different. Like `strftime', the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a `%'. Immediately after the `%', you can optionally specify various flags and formatting information before the main formatting character, in a similar way to `printf': * Immediately following the `%' there can be one or more of the following flags: `=F' The single byte character F is used for this field as the numeric fill character. By default this character is a space character. Filling with this character is only performed if a left precision is specified. It is not just to fill to the given field width. `^' The number is printed without grouping the digits according to the rules of the current locale. By default grouping is enabled. `+', `(' At most one of these flags can be used. They select which format to represent the sign of a currency amount. By default, and if `+' is given, the locale equivalent of +/- is used. If `(' is given, negative amounts are enclosed in parentheses. The exact format is determined by the values of the `LC_MONETARY' category of the locale selected at program runtime. `!' The output will not contain the currency symbol. `-' The output will be formatted left-justified instead of right-justified if it does not fill the entire field width. The next part of a specification is an optional field width. If no width is specified 0 is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the `-' flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa. So far the format looks familiar, being similar to the `printf' and `strftime' formats. However, the next two optional fields introduce something new. The first one is a `#' character followed by a decimal digit string. The value of the digit string specifies the number of _digit_ positions to the left of the decimal point (or equivalent). This does _not_ include the grouping character when the `^' flag is not given. If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the `=' flag and by default is a space. For example, if the field width is selected as 6 and the number is 123, the fill character is `*' the result will be `***123'. The second optional field starts with a `.' (period) and consists of another decimal digit string. Its value describes the number of characters printed after the decimal point. The default is selected from the current locale (`frac_digits', `int_frac_digits', see *note General Numeric::). If the exact representation needs more digits than given by the field width, the displayed value is rounded. If the number of fractional digits is selected to be zero, no decimal point is printed. As a GNU extension, the `strfmon' implementation in the GNU libc allows an optional `L' next as a format modifier. If this modifier is given, the argument is expected to be a `long double' instead of a `double' value. Finally, the last component is a format specifier. There are three specifiers defined: `i' Use the locale's rules for formatting an international currency value. `n' Use the locale's rules for formatting a national currency value. `%' Place a `%' in the output. There must be no flag, width specifier or modifier given, only `%%' is allowed. As for `printf', the function reads the format string from left to right and uses the values passed to the function following the format string. The values are expected to be either of type `double' or `long double', depending on the presence of the modifier `L'. The result is stored in the buffer pointed to by S. At most MAXSIZE characters are stored. The return value of the function is the number of characters stored in S, including the terminating `NULL' byte. If the number of characters stored would exceed MAXSIZE, the function returns -1 and the content of the buffer S is unspecified. In this case `errno' is set to `E2BIG'. A few examples should make clear how the function works. It is assumed that all the following pieces of code are executed in a program which uses the USA locale (`en_US'). The simplest form of the format is this: strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678); The output produced is "@$123.45@-$567.89@$12,345.68@" We can notice several things here. First, the widths of the output numbers are different. We have not specified a width in the format string, and so this is no wonder. Second, the third number is printed using thousands separators. The thousands separator for the `en_US' locale is a comma. The number is also rounded. .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2. Finally, note that the national currency symbol is printed since `%n' was used, not `i'. The next example shows how we can align the output. strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678); The output this time is: "@ $123.45@ -$567.89@ $12,345.68@" Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a `#' modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification. strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@", 123.45, -567.89, 12345.678); The output is "@ $***123.45@-$***567.89@ $12,456.68@" Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be 5 and 123.45 has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality. strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@", 123.45, -567.89, 12345.678); This rather complex format string produces the following output: "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @" The most noticeable change is the alternative way of representing negative numbers. In financial circles this is often done using parentheses, and this is what the `(' flag selected. The fill character is now `0'. Note that this `0' character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator. Since we used the format specifier `i' instead of `n', the international form of the currency symbol is used. This is a four letter string, in this case `"USD "'. The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding. Yes-or-No Questions =================== Some non GUI programs ask a yes-or-no question. If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too. It would be very bad habit to ask a question in one language and request the answer in another, often English. The GNU C library contains `rpmatch' to give applications easy access to the corresponding locale definitions. - Function: int rpmatch (const char *RESPONSE) The function `rpmatch' checks the string in RESPONSE whether or not it is a correct yes-or-no answer and if yes, which one. The check uses the `YESEXPR' and `NOEXPR' data in the `LC_MESSAGES' category of the currently selected locale. The return value is as follows: `1' The user entered an affirmative answer. `0' The user entered a negative answer. `-1' The answer matched neither the `YESEXPR' nor the `NOEXPR' regular expression. This function is not standardized but available beside in GNU libc at least also in the IBM AIX library. This function would normally be used like this: ... /* Use a safe default. */ _Bool doit = false; fputs (gettext ("Do you really want to do this? "), stdout); fflush (stdout); /* Prepare the `getline' call. */ line = NULL; len = 0; while (getline (&line, &len, stdout) >= 0) { /* Check the response. */ int res = rpmatch (line); if (res >= 0) { /* We got a definitive answer. */ if (res > 0) doit = true; break; } } /* Free what `getline' allocated. */ free (line); Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read. Message Translation ******************* The program's interface with the human should be designed in a way to ease the human the task. One of the possibilities is to use messages in whatever language the user prefers. Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and add among the variants every time a message has to be printed. This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets. A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user. The GNU C Library provides two different sets of functions to support message translation. The problem is that neither of the interfaces is officially defined by the POSIX standard. The `catgets' family of functions is defined in the X/Open standard but this is derived from industry decisions and therefore not necessarily based on reasonable decisions. As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are * locate the external data file with the appropriate translations. * load the data and make it possible to address the messages * map a given key to the translated message The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest. X/Open Message Catalog Handling =============================== The `catgets' functions are based on the simple scheme: Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used. This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same. Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind. All the types, constants and functions for the `catgets' functions are defined/declared in the `nl_types.h' header file. The `catgets' function family ----------------------------- - Function: nl_catd catopen (const char *CAT_NAME, int FLAG) The `catgets' function tries to locate the message data file names CAT_NAME and loads it when found. The return value is of an opaque type and can be used in calls to the other functions to refer to this loaded catalog. The return value is `(nl_catd) -1' in case the function failed and no catalog was loaded. The global variable ERRNO contains a code for the error causing the failure. But even if the function call succeeded this does not mean that all messages can be translated. Locating the catalog file must happen in a way which lets the user of the program influence the decision. It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files. All this can be specified by the user by setting some environment variables. The first problem is to find out where all the message catalogs are stored. Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place. To tell the `catopen' function where the catalog for the program can be found the user can set the environment variable `NLSPATH' to a value which describes her/his choice. Since this value must be usable for different languages and locales it cannot be a simple string. Instead it is a format string (similar to `printf''s). An example is /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N First one can see that more than one directory can be specified (with the usual syntax of separating them by colons). The next things to observe are the format string, `%L' and `%N' in this case. The `catopen' function knows about several of them and the replacement for all of them is of course different. `%N' This format element is substituted with the name of the catalog file. This is the value of the CAT_NAME argument given to `catgets'. `%L' This format element is substituted with the name of the currently selected locale for translating messages. How this is determined is explained below. `%l' (This is the lowercase ell.) This format element is substituted with the language element of the locale name. The string describing the selected locale is expected to have the form `LANG[_TERR[.CODESET]]' and this format uses the first part LANG. `%t' This format element is substituted by the territory part TERR of the name of the currently selected locale. See the explanation of the format above. `%c' This format element is substituted by the codeset part CODESET of the name of the currently selected locale. See the explanation of the format above. `%%' Since `%' is used in a meta character there must be a way to express the `%' character in the result itself. Using `%%' does this just like it works for `printf'. Using `NLSPATH' allows arbitrary directories to be searched for message catalogs while still allowing different languages to be used. If the `NLSPATH' environment variable is not set, the default value is PREFIX/share/locale/%L/%N:PREFIX/share/locale/%L/LC_MESSAGES/%N where PREFIX is given to `configure' while installing the GNU C Library (this value is in many cases `/usr' or the empty string). The remaining problem is to decide which must be used. The value decides about the substitution of the format elements mentioned above. First of all the user can specify a path in the message catalog name (i.e., the name contains a slash character). In this situation the `NLSPATH' environment variable is not used. The catalog must exist as specified in the program, perhaps relative to the current working directory. This situation in not desirable and catalogs names never should be written this way. Beside this, this behavior is not portable to all other platforms providing the `catgets' interface. Otherwise the values of environment variables from the standard environment are examined (*note Standard Environment::). Which variables are examined is decided by the FLAG parameter of `catopen'. If the value is `NL_CAT_LOCALE' (which is defined in `nl_types.h') then the `catopen' function use the name of the locale currently selected for the `LC_MESSAGES' category. If FLAG is zero the `LANG' environment variable is examined. This is a left-over from the early days where the concept of the locales had not even reached the level of POSIX locales. The environment variable and the locale name should have a value of the form `LANG[_TERR[.CODESET]]' as explained above. If no environment variable is set the `"C"' locale is used which prevents any translation. The return value of the function is in any case a valid string. Either it is a translation from a message catalog or it is the same as the STRING parameter. So a piece of code to decide whether a translation actually happened must look like this: { char *trans = catgets (desc, set, msg, input_string); if (trans == input_string) { /* Something went wrong. */ } } When an error occurred the global variable ERRNO is set to EBADF The catalog does not exist. ENOMSG The set/message tuple does not name an existing element in the message catalog. While it sometimes can be useful to test for errors programs normally will avoid any test. If the translation is not available it is no big problem if the original, untranslated message is printed. Either the user understands this as well or s/he will look for the reason why the messages are not translated. Please note that the currently selected locale does not depend on a call to the `setlocale' function. It is not necessary that the locale data files for this locale exist and calling `setlocale' succeeds. The `catopen' function directly reads the values of the environment variables. - Function: char * catgets (nl_catd CATALOG_DESC, int SET, int MESSAGE, const char *STRING) The function `catgets' has to be used to access the massage catalog previously opened using the `catopen' function. The CATALOG_DESC parameter must be a value previously returned by `catopen'. The next two parameters, SET and MESSAGE, reflect the internal organization of the message catalog files. This will be explained in detail below. For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers. Neither the set number nor the message number must be consecutive. They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number. Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter STRING helps to handle this case gracefully. If no matching string can be found STRING is returned. This means for the programmer that * the STRING parameters should contain reasonable text (this also helps to understand the program seems otherwise there would be no hint on the string which is expected to be returned. * all STRING arguments should be written in the same language. It is somewhat uncomfortable to write a program using the `catgets' functions if no supporting functionality is available. Since each set/message number tuple must be unique the programmer must keep lists of the messages at the same time the code is written. And the work between several people working on the same project must be coordinated. We will see some how these problems can be relaxed a bit (*note Common Usage::). - Function: int catclose (nl_catd CATALOG_DESC) The `catclose' function can be used to free the resources associated with a message catalog which previously was opened by a call to `catopen'. If the resources can be successfully freed the function returns `0'. Otherwise it return `-1' and the global variable ERRNO is set. Errors can occur if the catalog descriptor CATALOG_DESC is not valid in which case ERRNO is set to `EBADF'. Format of the message catalog files ----------------------------------- The only reasonable way the translate all the messages of a function and store the result in a message catalog file which can be read by the `catopen' function is to write all the message text to the translator and let her/him translate them all. I.e., we must have a file with entries which associate the set/message tuple with a specific translation. This file format is specified in the X/Open standard and is as follows: * Lines containing only whitespace characters or empty lines are ignored. * Lines which contain as the first non-whitespace character a `$' followed by a whitespace character are comment and are also ignored. * If a line contains as the first non-whitespace characters the sequence `$set' followed by a whitespace character an additional argument is required to follow. This argument can either be: - a number. In this case the value of this number determines the set to which the following messages are added. - an identifier consisting of alphanumeric characters plus the underscore character. In this case the set get automatically a number assigned. This value is one added to the largest set number which so far appeared. How to use the symbolic names is explained in section *Note Common Usage::. It is an error if a symbol name appears more than once. All following messages are placed in a set with this number. * If a line contains as the first non-whitespace characters the sequence `$delset' followed by a whitespace character an additional argument is required to follow. This argument can either be: - a number. In this case the value of this number determines the set which will be deleted. - an identifier consisting of alphanumeric characters plus the underscore character. This symbolic identifier must match a name for a set which previously was defined. It is an error if the name is unknown. In both cases all messages in the specified set will be removed. They will not appear in the output. But if this set is later again selected with a `$set' command again messages could be added and these messages will appear in the output. * If a line contains after leading whitespaces the sequence `$quote', the quoting character used for this input file is changed to the first non-whitespace character following the `$quote'. If no non-whitespace character is present before the line ends quoting is disable. By default no quoting character is used. In this mode strings are terminated with the first unescaped line break. If there is a `$quote' sequence present newline need not be escaped. Instead a string is terminated with the first unescaped appearance of the quote character. A common usage of this feature would be to set the quote character to `"'. Then any appearance of the `"' in the strings must be escaped using the backslash (i.e., `\"' must be written). * Any other line must start with a number or an alphanumeric identifier (with the underscore character included). The following characters (starting after the first whitespace character) will form the string which gets associated with the currently selected set and the message number represented by the number and identifier respectively. If the start of the line is a number the message number is obvious. It is an error if the same message number already appeared for this set. If the leading token was an identifier the message number gets automatically assigned. The value is the current maximum messages number for this set plus one. It is an error if the identifier was already used for a message in this set. It is OK to reuse the identifier for a message in another thread. How to use the symbolic identifiers will be explained below (*note Common Usage::). There is one limitation with the identifier: it must not be `Set'. The reason will be explained below. The text of the messages can contain escape characters. The usual bunch of characters known from the ISO C language are recognized (`\n', `\t', `\v', `\b', `\r', `\f', `\\', and `\NNN', where NNN is the octal coding of a character code). *Important:* The handling of identifiers instead of numbers for the set and messages is a GNU extension. Systems strictly following the X/Open specification do not have this feature. An example for a message catalog file is this: $ This is a leading comment. $quote " $set SetOne 1 Message with ID 1. two " Message with ID \"two\", which gets the value 2 assigned" $set SetTwo $ Since the last set got the number 1 assigned this set has number 2. 4000 "The numbers can be arbitrary, they need not start at one." This small example shows various aspects: * Lines 1 and 9 are comments since they start with `$' followed by a whitespace. * The quoting character is set to `"'. Otherwise the quotes in the message definition would have to be left away and in this case the message with the identifier `two' would loose its leading whitespace. * Mixing numbered messages with message having symbolic names is no problem and the numbering happens automatically. While this file format is pretty easy it is not the best possible for use in a running program. The `catopen' function would have to parser the file and handle syntactic errors gracefully. This is not so easy and the whole process is pretty slow. Therefore the `catgets' functions expect the data in another more compact and ready-to-use file format. There is a special program `gencat' which is explained in detail in the next section. Files in this other format are not human readable. To be easy to use by programs it is a binary file. But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library). Details about the binary file format are not important to know since these files are always created by the `gencat' program. The sources of the GNU C Library also provide the sources for the `gencat' program and so the interested reader can look through these source files to learn about the file format. Generate Message Catalogs files ------------------------------- The `gencat' program is specified in the X/Open standard and the GNU implementation follows this specification and so processes all correctly formed input files. Additionally some extension are implemented which help to work in a more reasonable way with the `catgets' functions. The `gencat' program can be invoked in two ways: `gencat [OPTION]... [OUTPUT-FILE [INPUT-FILE]...]` This is the interface defined in the X/Open standard. If no INPUT-FILE parameter is given input will be read from standard input. Multiple input files will be read as if they are concatenated. If OUTPUT-FILE is also missing, the output will be written to standard output. To provide the interface one is used to from other programs a second interface is provided. `gencat [OPTION]... -o OUTPUT-FILE [INPUT-FILE]...` The option `-o' is used to specify the output file and all file arguments are used as input files. Beside this one can use `-' or `/dev/stdin' for INPUT-FILE to denote the standard input. Corresponding one can use `-' and `/dev/stdout' for OUTPUT-FILE to denote standard output. Using `-' as a file name is allowed in X/Open while using the device names is a GNU extension. The `gencat' program works by concatenating all input files and then *merge* the resulting collection of message sets with a possibly existing output file. This is done by removing all messages with set/message number tuples matching any of the generated messages from the output file and then adding all the new messages. To regenerate a catalog file while ignoring the old contents therefore requires to remove the output file if it exists. If the output is written to standard output no merging takes place. The following table shows the options understood by the `gencat' program. The X/Open standard does not specify any option for the program so all of these are GNU extensions. `-V' `--version' Print the version information and exit. `-h' `--help' Print a usage message listing all available options, then exit successfully. `--new' Do never merge the new messages from the input files with the old content of the output files. The old content of the output file is discarded. `-H' `--header=name' This option is used to emit the symbolic names given to sets and messages in the input files for use in the program. Details about how to use this are given in the next section. The NAME parameter to this option specifies the name of the output file. It will contain a number of C preprocessor `#define's to associate a name with a number. Please note that the generated file only contains the symbols from the input files. If the output is merged with the previous content of the output file the possibly existing symbols from the file(s) which generated the old output files are not in the generated header file. How to use the `catgets' interface ---------------------------------- The `catgets' functions can be used in two different ways. By following slavishly the X/Open specs and not relying on the extension and by using the GNU extensions. We will take a look at the former method first to understand the benefits of extensions. Not using symbolic names ........................ Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like catgets (catdesc, set, msg, "string") CATGETS is retrieved from a call to `catopen' which is normally done once at the program start. The `"string"' is the string we want to translate. The problems start with the set and message numbers. In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts). The allocation process can be relaxed a bit by different set numbers for different parts of the program. So the number of developers who have to coordinate the allocation can be reduced. But still lists must be keep track of the allocation and errors can easily happen. These errors cannot be discovered by the compiler or the `catgets' functions. Only the user of the program might see wrong messages printed. In the worst cases the messages are so irritating that they cannot be recognized as wrong. Think about the translations for `"true"' and `"false"' being exchanged. This could result in a disaster. Using symbolic names .................... The problems mentioned in the last section derive from the fact that: 1. the numbers are allocated once and due to the possibly frequent use of them it is difficult to change a number later. 2. the numbers do not allow to guess anything about the string and therefore collisions can easily happen. By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself. This is necessary since the symbolic names must be mapped to numbers before the program sources can be compiled. In the last section it was described how to generate a header containing the mapping of the names. E.g., for the example message file given in the last section we could call the `gencat' program as follow (assume `ex.msg' contains the sources). gencat -H ex.h -o ex.cat ex.msg This generates a header file with the following content: #define SetTwoSet 0x2 /* ex.msg:8 */ #define SetOneSet 0x1 /* ex.msg:4 */ #define SetOnetwo 0x2 /* ex.msg:6 */ As can be seen the various symbols given in the source file are mangled to generate unique identifiers and these identifiers get numbers assigned. Reading the source file and knowing about the rules will allow to predict the content of the header file (it is deterministic) but this is not necessary. The `gencat' program can take care for everything. All the programmer has to do is to put the generated header file in the dependency list of the source files of her/his project and to add a rules to regenerate the header of any of the input files change. One word about the symbol mangling. Every symbol consists of two parts: the name of the message set plus the name of the message or the special string `Set'. So `SetOnetwo' means this macro can be used to access the translation with identifier `two' in the message set `SetOne'. The other names denote the names of the message sets. The special string `Set' is used in the place of the message identifier. If in the code the second string of the set `SetOne' is used the C code should look like this: catgets (catdesc, SetOneSet, SetOnetwo, " Message with ID \"two\", which gets the value 2 assigned") Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.) How does to this allow to develop ................................. To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual: #include int main (void) { printf ("Hello, world!\n"); return 0; } Now we want to internationalize the message and therefore replace the message with whatever the user wants. #include #include #include "msgnrs.h" int main (void) { nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE); printf (catgets (catdesc, SetMainSet, SetMainHello, "Hello, world!\n")); catclose (catdesc); return 0; } We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation. What remains unspecified here are the constants `SetMainSet' and `SetMainHello'. These are the symbolic names describing the message. To get the actual definitions which match the information in the catalog file we have to create the message catalog source file and process it using the `gencat' program. $ Messages for the famous greeting program. $quote " $set Main Hello "Hallo, Welt!\n" Now we can start building the program (assume the message catalog source file is named `hello.msg' and the program source file `hello.c'): % gencat -H msgnrs.h -o hello.cat hello.msg % cat msgnrs.h #define MainSet 0x1 /* hello.msg:4 */ #define MainHello 0x1 /* hello.msg:5 */ % gcc -o hello hello.c -I. % cp hello.cat /usr/share/locale/de/LC_MESSAGES % echo $LC_ALL de % ./hello Hallo, Welt! % The call of the `gencat' program creates the missing header file `msgnrs.h' as well as the message catalog binary. The former is used in the compilation of `hello.c' while the later is placed in a directory in which the `catopen' function will try to locate it. Please check the `LC_ALL' environment variable and the default path for `catopen' presented in the description above. The Uniforum approach to Message Translation ============================================ Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group. There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU project and the GNU `gettext' package provides support for this outside the GNU C Library. The code of the `libintl' from GNU `gettext' is the same as the code in the GNU C Library. So the documentation in the GNU `gettext' manual is also valid for the functionality here. The following text will describe the library functions in detail. But the numerous helper programs are not described in this manual. Instead people should read the GNU `gettext' manual (*note GNU gettext utilities: (gettext)Top.). We will only give a short overview. Though the `catgets' functions are available by default on more systems the `gettext' interface is at least as portable as the former. The GNU `gettext' package can be used wherever the functions are not available. The `gettext' family of functions --------------------------------- The paradigms underlying the `gettext' approach to message translations is different from that of the `catgets' functions the basic functionally is equivalent. There are functions of the following categories: What has to be done to translate a message? ........................................... The `gettext' functions have a very simple interface. The most basic function just takes the string which shall be translated as the argument and it returns the translation. This is fundamentally different from the `catgets' approach where an extra key is necessary and the original string is only used for the error case. If the string which has to be translated is the only argument this of course means the string itself is the key. I.e., the translation will be selected based on the original string. The message catalogs must therefore contain the original strings plus one translation for any such string. The task of the `gettext' function is it to compare the argument string with the available strings in the catalog and return the appropriate translation. Of course this process is optimized so that this process is not more expensive than an access using an atomic key like in `catgets'. The `gettext' approach has some advantages but also some disadvantages. Please see the GNU `gettext' manual for a detailed discussion of the pros and cons. All the definitions and declarations for `gettext' can be found in the `libintl.h' header file. On systems where these functions are not part of the C library they can be found in a separate library named `libintl.a' (or accordingly different for shared libraries). - Function: char * gettext (const char *MSGID) The `gettext' function searches the currently selected message catalogs for a string which is equal to MSGID. If there is such a string available it is returned. Otherwise the argument string MSGID is returned. Please note that all though the return value is `char *' the returned string must not be changed. This broken type results from the history of the function and does not reflect the way the function should be used. Please note that above we wrote "message catalogs" (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (*note Locating gettext catalog::). The `gettext' function does not modify the value of the global ERRNO variable. This is necessary to make it possible to write something like printf (gettext ("Operation failed: %m\n")); Here the ERRNO value is used in the `printf' function while processing the `%m' format element and if the `gettext' function would change this value (it is called before `printf' is called) we would get a wrong message. So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation. The remaining two functions to access the message catalog add some functionality to select a message catalog which is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time. The C library itself is an example: internally it uses the `gettext' functions but since it must not depend on a currently selected default message catalog it must specify all ambiguous information. - Function: char * dgettext (const char *DOMAINNAME, const char *MSGID) The `dgettext' functions acts just like the `gettext' function. It only takes an additional first argument DOMAINNAME which guides the selection of the message catalogs which are searched for the translation. If the DOMAINNAME parameter is the null pointer the `dgettext' function is exactly equivalent to `gettext' since the default value for the domain name is used. As for `gettext' the return value type is `char *' which is an anachronism. The returned string must never be modified. - Function: char * dcgettext (const char *DOMAINNAME, const char *MSGID, int CATEGORY) The `dcgettext' adds another argument to those which `dgettext' takes. This argument CATEGORY specifies the last piece of information needed to localize the message catalog. I.e., the domain name and the locale category exactly specify which message catalog has to be used (relative to a given directory, see below). The `dgettext' function can be expressed in terms of `dcgettext' by using dcgettext (domain, string, LC_MESSAGES) instead of dgettext (domain, string) This also shows which values are expected for the third parameter. One has to use the available selectors for the categories available in `locale.h'. Normally the available values are `LC_CTYPE', `LC_COLLATE', `LC_MESSAGES', `LC_MONETARY', `LC_NUMERIC', and `LC_TIME'. Please note that `LC_ALL' must not be used and even though the names might suggest this, there is no relation to the environments variables of this name. The `dcgettext' function is only implemented for compatibility with other systems which have `gettext' functions. There is not really any situation where it is necessary (or useful) to use a different value but `LC_MESSAGES' in for the CATEGORY parameter. We are dealing with messages here and any other choice can only be irritating. As for `gettext' the return value type is `char *' which is an anachronism. The returned string must never be modified. When using the three functions above in a program it is a frequent case that the MSGID argument is a constant string. So it is worth to optimize this case. Thinking shortly about this one will realize that as long as no new message catalog is loaded the translation of a message will not change. This optimization is actually implemented by the `gettext', `dgettext' and `dcgettext' functions. How to determine which catalog to be used ......................................... The functions to retrieve the translations for a given message have a remarkable simple interface. But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this. The code is complicated the use is easy. Basically we have two different tasks to perform which can also be performed by the `catgets' functions: 1. Locate the set of message catalogs. There are a number of files for different languages and which all belong to the package. Usually they are all stored in the filesystem below a certain directory. There can be arbitrary many packages installed and they can follow different guidelines for the placement of their files. 2. Relative to the location specified by the package the actual translation files must be searched, based on the wishes of the user. I.e., for each language the user selects the program should be able to locate the appropriate file. This is the functionality required by the specifications for `gettext' and this is also what the `catgets' functions are able to do. But there are some problems unresolved: * The language to be used can be specified in several different ways. There is no generally accepted standard for this and the user always expects the program understand what s/he means. E.g., to select the German translation one could write `de', `german', or `deutsch' and the program should always react the same. * Sometimes the specification of the user is too detailed. If s/he, e.g., specifies `de_DE.ISO-8859-1' which means German, spoken in Germany, coded using the ISO 8859-1 character set there is the possibility that a message catalog matching this exactly is not available. But there could be a catalog matching `de' and if the character set used on the machine is always ISO 8859-1 there is no reason why this later message catalog should not be used. (We call this "message inheritance".) * If a catalog for a wanted language is not available it is not always the second best choice to fall back on the language of the developer and simply not translate any message. Instead a user might be better able to read the messages in another language and so the user of the program should be able to define an precedence order of languages. We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user. We will start with the functions the programmer can use since the user configuration will be based on this. As the functions described in the last sections already mention separate sets of messages can be selected by a "domain name". This is a simple string which should be unique for each program part with uses a separate domain. It is possible to use in one program arbitrary many domains at the same time. E.g., the GNU C Library itself uses a domain named `libc' while the program using the C Library could use a domain named `foo'. The important point is that at any time exactly one domain is active. This is controlled with the following function. - Function: char * textdomain (const char *DOMAINNAME) The `textdomain' function sets the default domain, which is used in all future `gettext' calls, to DOMAINNAME. Please note that `dgettext' and `dcgettext' calls are not influenced if the DOMAINNAME parameter of these functions is not the null pointer. Before the first call to `textdomain' the default domain is `messages'. This is the name specified in the specification of the `gettext' API. This name is as good as any other name. No program should ever really use a domain with this name since this can only lead to problems. The function returns the value which is from now on taken as the default domain. If the system went out of memory the returned value is `NULL' and the global variable ERRNO is set to `ENOMEM'. Despite the return value type being `char *' the return string must not be changed. It is allocated internally by the `textdomain' function. If the DOMAINNAME parameter is the null pointer no new default domain is set. Instead the currently selected default domain is returned. If the DOMAINNAME parameter is the empty string the default domain is reset to its initial value, the domain with the name `messages'. This possibility is questionable to use since the domain `messages' really never should be used. - Function: char * bindtextdomain (const char *DOMAINNAME, const char *DIRNAME) The `bindtextdomain' function can be used to specify the directory which contains the message catalogs for domain DOMAINNAME for the different languages. To be correct, this is the directory where the hierarchy of directories is expected. Details are explained below. For the programmer it is important to note that the translations which come with the program have be placed in a directory hierarchy starting at, say, `/foo/bar'. Then the program should make a `bindtextdomain' call to bind the domain for the current program to this directory. So it is made sure the catalogs are found. A correctly running program does not depend on the user setting an environment variable. The `bindtextdomain' function can be used several times and if the DOMAINNAME argument is different the previously bound domains will not be overwritten. If the program which wish to use `bindtextdomain' at some point of time use the `chdir' function to change the current working directory it is important that the DIRNAME strings ought to be an absolute pathname. Otherwise the addressed directory might vary with the time. If the DIRNAME parameter is the null pointer `bindtextdomain' returns the currently selected directory for the domain with the name DOMAINNAME. The `bindtextdomain' function returns a pointer to a string containing the name of the selected directory name. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of `bindtextdomain' the return value is `NULL' and the global variable ERRNO is set accordingly. Additional functions for more complicated situations .................................................... The functions of the `gettext' family described so far (and all the `catgets' functions as well) have one problem in the real world which have been neglected completely in all existing approaches. What is meant here is the handling of plural forms. Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following: printf ("%d file%s deleted", n, n == 1 ? "" : "s"); After the first complaints from people internationalizing the code people either completely avoided formulations like this or used strings like `"file(s)"'. Both look unnatural and should be avoided. First tries to solve the problem correctly looked like this: if (n == 1) printf ("%d file deleted", n); else printf ("%d files deleted", n); But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. There are two things we can differ between (and even inside language families); * The form how plural forms are build differs. This is a problem with language which have many irregularities. German, for instance, is a drastic case. Though English and German are part of the same language family (Germanic), the almost regular forming of plural noun forms (appending an `s') is hardly found in German. * The number of plural forms differ. This is somewhat surprising for those who only have experiences with Romanic and Germanic languages since here the number is the same (there are two). But other language families have only one form or many forms. More information on this in an extra section. The consequence of this is that application writers should not try to solve the problem in their code. This would be localization since it is only usable for certain, hardcoded language environments. Instead the extended `gettext' interface should be used. These extra functions are taking instead of the one key string two strings and an numerical argument. The idea behind this is that using the numerical argument and the first string as a key, the implementation can select using rules specified by the translator the right plural form. The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal `gettext' behavior). In this case the rules for Germanic language is used and it is assumed that the first string argument is the singular form, the second the plural form. This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language. This is a limitation but since the GNU C library (as well as the GNU `gettext' package) are written as part of the GNU package and the coding standards for the GNU project require program being written in English, this solution nevertheless fulfills its purpose. - Function: char * ngettext (const char *MSGID1, const char *MSGID2, unsigned long int N) The `ngettext' function is similar to the `gettext' function as it finds the message catalogs in the same way. But it takes two extra arguments. The MSGID1 parameter must contain the singular form of the string to be converted. It is also used as the key for the search in the catalog. The MSGID2 parameter is the plural form. The parameter N is used to determine the plural form. If no message catalog is found MSGID1 is returned if `n == 1', otherwise `msgid2'. An example for the us of this function is: printf (ngettext ("%d file removed", "%d files removed", n), n); Please note that the numeric value N has to be passed to the `printf' function as well. It is not sufficient to pass it only to `ngettext'. - Function: char * dngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N) The `dngettext' is similar to the `dgettext' function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way `ngettext' handles them. - Function: char * dcngettext (const char *DOMAIN, const char *MSGID1, const char *MSGID2, unsigned long int N, int CATEGORY) The `dcngettext' is similar to the `dcgettext' function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way `ngettext' handles them. The problem of plural forms ........................... A description of the problem can be found at the beginning of the last section. Now there is the question how to solve it. Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language. Therefore the solution implemented is to allow the translator to specify the rules of how to select the plural form. Since the formula varies with every language this is the only viable solution except for hardcoding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages). The details are explained in the GNU `gettext' manual. Here only a a bit of information is provided. The information about the plural form selection has to be stored in the header entry (the one with the empty (`msgid' string). It looks like this: Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1; The `nplurals' value must be a decimal number which specifies how many different plural forms exist for this language. The string following `plural' is an expression which is using the C language syntax. Exceptions are that no negative number are allowed, numbers must be decimal, and the only variable allowed is `n'. This expression will be evaluated whenever one of the functions `ngettext', `dngettext', or `dcngettext' is called. The numeric value passed to these functions is then substituted for all uses of the variable `n' in the expression. The resulting value then must be greater or equal to zero and smaller than the value given as the value of `nplurals'. The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).(1) Only one form: Some languages only require one single form. There is no distinction between the singular and plural form. An appropriate header entry would look like this: Plural-Forms: nplurals=1; plural=0; Languages with this property include: Finno-Ugric family Hungarian Asian family Japanese Turkic/Altaic family Turkish Two forms, singular used for one only This is the form used in most existing programs since it is what English is using. A header entry would look like this: Plural-Forms: nplurals=2; plural=n != 1; (Note: this uses the feature of C expressions that boolean expressions have to value zero or one.) Languages with this property include: Germanic family Danish, Dutch, English, German, Norwegian, Swedish Finno-Ugric family Estonian, Finnish Latin/Greek family Greek Semitic family Hebrew Romance family Italian, Spanish Artificial Esperanto Two forms, singular used for zero and one Exceptional case in the language family. The header entry would be: Plural-Forms: nplurals=2; plural=n>1; Languages with this property include: Romanic family French Three forms, special cases for one and two The header entry would be: Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2; Languages with this property include: Celtic Gaeilge Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4] The header entry would look like this: Plural-Forms: nplurals=3; \ plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1; Languages with this property include: Slavic family Czech, Russian Three forms, special cases for 1 and 2, 3, 4 The header entry would look like this: Plural-Forms: nplurals=3; \ plural=(n==1) ? 1 : (n>=2 && n<=4) ? 2 : 0; Languages with this property include: Slavic family Slovak Three forms, special case for one and some numbers ending in 2, 3, or 4 The header entry would look like this: Plural-Forms: nplurals=3; \ plural=n==1 ? 0 : \ n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2; (Continuation in the next line is possible.) Languages with this property include: Slavic family Polish Four forms, special case for one and all numbers ending in 2, 3, or 4 The header entry would look like this: Plural-Forms: nplurals=4; \ plural=n==1 ? 0 : n%10==2 ? 1 : n%10==3 || n%10==4 ? 2 : 3; Languages with this property include: Slavic family Slovenian ---------- Footnotes ---------- (1) Additions are welcome. Send appropriate information to . How to specify the output character set `gettext' uses ...................................................... `gettext' not only looks up a translation in a message catalog. It also converts the translation on the fly to the desired output character set. This is useful if the user is working in a different character set than the translator who created the message catalog, because it avoids distributing variants of message catalogs which differ only in the character set. The output character set is, by default, the value of `nl_langinfo (CODESET)', which depends on the `LC_CTYPE' part of the current locale. But programs which store strings in a locale independent way (e.g. UTF-8) can request that `gettext' and related functions return the translations in that encoding, by use of the `bind_textdomain_codeset' function. Note that the MSGID argument to `gettext' is not subject to character set conversion. Also, when `gettext' does not find a translation for MSGID, it returns MSGID unchanged - independently of the current output character set. It is therefore recommended that all MSGIDs be US-ASCII strings. - Function: char * bind_textdomain_codeset (const char *DOMAINNAME, const char *CODESET) The `bind_textdomain_codeset' function can be used to specify the output character set for message catalogs for domain DOMAINNAME. The CODESET argument must be a valid codeset name which can be used for the `iconv_open' function, or a null pointer. If the CODESET parameter is the null pointer, `bind_textdomain_codeset' returns the currently selected codeset for the domain with the name DOMAINNAME. It returns `NULL' if no codeset has yet been selected. The `bind_textdomain_codeset' function can be used several times. If used multiple times with the same DOMAINNAME argument, the later call overrides the settings made by the earlier one. The `bind_textdomain_codeset' function returns a pointer to a string containing the name of the selected codeset. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of `bind_textdomain_codeset', the return value is `NULL' and the global variable ERRNO is set accordingly. How to use `gettext' in GUI programs .................................... One place where the `gettext' functions, if used normally, have big problems is within programs with graphical user interfaces (GUIs). The problem is that many of the strings which have to be translated are very short. They have to appear in pull-down menus which restricts the length. But strings which are not containing entire sentences or at least large fragments of a sentence may appear in more than one situation in the program but might have different translations. This is especially true for the one-word strings which are frequently used in GUI programs. As a consequence many people say that the `gettext' approach is wrong and instead `catgets' should be used which indeed does not have this problem. But there is a very simple and powerful method to handle these kind of problems with the `gettext' functions. As as example consider the following fictional situation. A GUI program has a menu bar with the following entries: +------------+------------+--------------------------------------+ | File | Printer | | +------------+------------+--------------------------------------+ | Open | | Select | | New | | Open | +----------+ | Connect | +----------+ To have the strings `File', `Printer', `Open', `New', `Select', and `Connect' translated there has to be at some point in the code a call to a function of the `gettext' family. But in two places the string passed into the function would be `Open'. The translations might not be the same and therefore we are in the dilemma described above. One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions. To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as Menu|File Menu|Printer Menu|File|Open Menu|File|New Menu|Printer|Select Menu|Printer|Open Menu|Printer|Connect Now all the strings are different and if now instead of `gettext' the following little wrapper function is used, everything works just fine: char * sgettext (const char *msgid) { char *msgval = gettext (msgid); if (msgval == msgid) msgval = strrchr (msgid, '|') + 1; return msgval; } What this little function does is to recognize the case when no translation is available. This can be done very efficiently by a pointer comparison since the return value is the input value. If there is no translation we know that the input string is in the format we used for the Menu entries and therefore contains a `|' character. We simply search for the last occurrence of this character and return a pointer to the character following it. That's it! If one now consistently uses the enlengthened string form and replaces the `gettext' calls with calls to `sgettext' (this is normally limited to very few places in the GUI implementation) then it is possible to produce a program which can be internationalized. With advanced compilers (such as GNU C) one can write the `sgettext' functions as an inline function or as a macro like this: #define sgettext(msgid) \ ({ const char *__msgid = (msgid); \ char *__msgstr = gettext (__msgid); \ if (__msgval == __msgid) \ __msgval = strrchr (__msgid, '|') + 1; \ __msgval; }) The other `gettext' functions (`dgettext', `dcgettext' and the `ngettext' equivalents) can and should have corresponding functions as well which look almost identical, except for the parameters and the call to the underlying function. Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question. * They are easy to write and therefore can be provided by the project they are used in. This is not an answer by itself and must be seen together with the second part which is: * There is no way the C library can contain a version which can work everywhere. The problem is the selection of the character to separate the prefix from the actual string in the enlenghtened string. The examples above used `|' which is a quite good choice because it resembles a notation frequently used in this context and it also is a character not often used in message strings. But what if the character is used in message strings. Or if the chose character is not available in the character set on the machine one compiles (e.g., `|' is not required to exist for ISO C; this is why the `iso646.h' file exists in ISO C programming environments). There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this. User influence on `gettext' ........................... The last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them. The POSIX locale model uses the environment variables `LC_COLLATE', `LC_CTYPE', `LC_MESSAGES', `LC_MONETARY', `NUMERIC', and `LC_TIME' to select the locale which is to be used. This way the user can influence lots of functions. As we mentioned above the `gettext' functions also take advantage of this. To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows: DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo The default value for DIR_NAME is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is `/usr' or `/'. For the former the complete DIR_NAME is: /usr/share/locale We can use `/usr/share' since the `.mo' files containing the message catalogs are system independent, so all systems can use the same files. If the program executed the `bindtextdomain' function for the message domain that is currently handled, the `dir_name' component is exactly the value which was given to the function as the second parameter. I.e., `bindtextdomain' allows overwriting the only system dependent and fixed value to make it possible to address files anywhere in the filesystem. The CATEGORY is the name of the locale category which was selected in the program code. For `gettext' and `dgettext' this is always `LC_MESSAGES', for `dcgettext' this is selected by the value of the third parameter. As said above it should be avoided to ever use a category other than `LC_MESSAGES'. The LOCALE component is computed based on the category used. Just like for the `setlocale' function here comes the user selection into the play. Some environment variables are examined in a fixed order and the first environment variable set determines the return value of the lookup process. In detail, for the category `LC_xxx' the following variables in this order are examined: `LANGUAGE' `LC_ALL' `LC_xxx' `LANG' This looks very familiar. With the exception of the `LANGUAGE' environment variable this is exactly the lookup order the `setlocale' function uses. But why introducing the `LANGUAGE' variable? The reason is that the syntax of the values these variables can have is different to what is expected by the `setlocale' function. If we would set `LC_ALL' to a value following the extended syntax that would mean the `setlocale' function will never be able to use the value of this variable as well. An additional variable removes this problem plus we can select the language independently of the locale setting which sometimes is useful. While for the `LC_xxx' variables the value should consist of exactly one specification of a locale the `LANGUAGE' variable's value can consist of a colon separated list of locale names. The attentive reader will realize that this is the way we manage to implement one of our additional demands above: we want to be able to specify an ordered list of language. Back to the constructed filename we have only one component missing. The DOMAIN_NAME part is the name which was either registered using the `textdomain' function or which was given to `dgettext' or `dcgettext' as the first parameter. Now it becomes obvious that a good choice for the domain name in the program code is a string which is closely related to the program/package name. E.g., for the GNU C Library the domain name is `libc'. A limit piece of example code should show how the programmer is supposed to work: { setlocale (LC_ALL, ""); textdomain ("test-package"); bindtextdomain ("test-package", "/usr/local/share/locale"); puts (gettext ("Hello, world!")); } At the program start the default domain is `messages', and the default locale is "C". The `setlocale' call sets the locale according to the user's environment variables; remember that correct functioning of `gettext' relies on the correct setting of the `LC_MESSAGES' locale (for looking up the message catalog) and of the `LC_CTYPE' locale (for the character set conversion). The `textdomain' call changes the default domain to `test-package'. The `bindtextdomain' call specifies that the message catalogs for the domain `test-package' can be found below the directory `/usr/local/share/locale'. If now the user set in her/his environment the variable `LANGUAGE' to `de' the `gettext' function will try to use the translations from the file /usr/local/share/locale/de/LC_MESSAGES/test-package.mo From the above descriptions it should be clear which component of this filename is determined by which source. In the above example we assumed that the `LANGUAGE' environment variable to `de'. This might be an appropriate selection but what happens if the user wants to use `LC_ALL' because of the wider usability and here the required value is `de_DE.ISO-8859-1'? We already mentioned above that a situation like this is not infrequent. E.g., a person might prefer reading a dialect and if this is not available fall back on the standard language. The `gettext' functions know about situations like this and can handle them gracefully. The functions recognize the format of the value of the environment variable. It can split the value is different pieces and by leaving out the only or the other part it can construct new values. This happens of course in a predictable way. To understand this one must know the format of the environment variable value. There are two more or less standardized forms: _X/Open Format_ `language[_territory[.codeset]][@modifier]' _CEN Format (European Community Standard)_ `language[_territory][+audience][+special][,[sponsor][_revision]]' The functions will automatically recognize which format is used. Less specific locale names will be stripped of in the order of the following list: 1. `revision' 2. `sponsor' 3. `special' 4. `codeset' 5. `normalized codeset' 6. `territory' 7. `audience'/`modifier' From the last entry one can see that the meaning of the `modifier' field in the X/Open format and the `audience' format have the same meaning. Beside one can see that the `language' field for obvious reasons never will be dropped. The only new thing is the `normalized codeset' entry. This is another goodie which is introduced to help reducing the chaos which derives from the inability of the people to standardize the names of character sets. Instead of ISO-8859-1 one can often see 8859-1, 88591, iso8859-1, or iso_8859-1. The `normalized codeset' value is generated from the user-provided character set name by applying the following rules: 1. Remove all characters beside numbers and letters. 2. Fold letters to lowercase. 3. If the same only contains digits prepend the string `"iso"'. So all of the above name will be normalized to `iso88591'. This allows the program user much more freely choosing the locale name. Even this extended functionality still does not help to solve the problem that completely different names can be used to denote the same locale (e.g., `de' and `german'). To be of help in this situation the locale implementation and also the `gettext' functions know about aliases. The file `/usr/share/locale/locale.alias' (replace `/usr' with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling. In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful. Programs to handle message catalogs for `gettext' ------------------------------------------------- The GNU C Library does not contain the source code for the programs to handle message catalogs for the `gettext' functions. As part of the GNU project the GNU gettext package contains everything the developer needs. The functionality provided by the tools in this package by far exceeds the abilities of the `gencat' program described above for the `catgets' functions. There is a program `msgfmt' which is the equivalent program to the `gencat' program. It generates from the human-readable and -editable form of the message catalog a binary file which can be used by the `gettext' functions. But there are several more programs available. The `xgettext' program can be used to automatically extract the translatable messages from a source file. I.e., the programmer need not take care for the translations and the list of messages which have to be translated. S/He will simply wrap the translatable string in calls to `gettext' et.al and the rest will be done by `xgettext'. This program has a lot of option which help to customize the output or do help to understand the input better. Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. here it should only be noted that using all the tools in GNU gettext it is possible to _completely_ automize the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves. Searching and Sorting ********************* This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements. Defining the Comparison Function ================================ In order to use the sorted array library functions, you have to describe how to compare the elements of the array. To do this, you supply a comparison function to compare two elements of the array. The library will call this function, passing as arguments pointers to two array elements to be compared. Your comparison function should return a value the way `strcmp' (*note String/Array Comparison::) does: negative if the first argument is "less" than the second, zero if they are "equal", and positive if the first argument is "greater". Here is an example of a comparison function which works with an array of numbers of type `double': int compare_doubles (const void *a, const void *b) { const double *da = (const double *) a; const double *db = (const double *) b; return (*da > *db) - (*da < *db); } The header file `stdlib.h' defines a name for the data type of comparison functions. This type is a GNU extension. int comparison_fn_t (const void *, const void *); Array Search Function ===================== Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in `search.h'. - Function: void * lfind (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lfind' function searches in the array with `*NMEMB' elements of SIZE bytes pointed to by BASE for an element which matches the one pointed to by KEY. The function pointed to by COMPAR is used decide whether two elements match. The return value is a pointer to the matching element in the array starting at BASE if it is found. If no matching element is available `NULL' is returned. The mean runtime of this function is `*NMEMB'/2. This function should only be used elements often get added to or deleted from the array in which case it might not be useful to sort the array before searching. - Function: void * lsearch (const void *KEY, void *BASE, size_t *NMEMB, size_t SIZE, comparison_fn_t COMPAR) The `lsearch' function is similar to the `lfind' function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found the `lsearch' function adds the object pointed to by KEY (with a size of SIZE bytes) at the end of the array and it increments the value of `*NMEMB' to reflect this addition. This means for the caller that if it is not sure that the array contains the element one is searching for the memory allocated for the array starting at BASE must have room for at least SIZE more bytes. If one is sure the element is in the array it is better to use `lfind' so having more room in the array is always necessary when calling `lsearch'. To search a sorted array for an element matching the key, use the `bsearch' function. The prototype for this function is in the header file `stdlib.h'. - Function: void * bsearch (const void *KEY, const void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The `bsearch' function searches the sorted array ARRAY for an object that is equivalent to KEY. The array contains COUNT elements, each of which is of size SIZE bytes. The COMPARE function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the ARRAY must already be sorted in ascending order according to this comparison function. The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified. This function derives its name from the fact that it is implemented using the binary search algorithm. Array Sort Function =================== To sort an array using an arbitrary comparison function, use the `qsort' function. The prototype for this function is in `stdlib.h'. - Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE, comparison_fn_t COMPARE) The QSORT function sorts the array ARRAY. The array contains COUNT elements, each of which is of size SIZE. The COMPARE function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. *Warning:* If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects. If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary. Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (*note Comparison Functions::): { double *array; int size; ... qsort (array, size, sizeof (double), compare_doubles); } The `qsort' function derives its name from the fact that it was originally implemented using the "quick sort" algorithm. The implementation of `qsort' in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array. Searching and Sorting Example ============================= Here is an example showing the use of `qsort' and `bsearch' with an array of structures. The objects in the array are sorted by comparing their `name' fields with the `strcmp' function. Then, we can look up individual objects based on their names. #include #include #include /* Define an array of critters to sort. */ struct critter { const char *name; const char *species; }; struct critter muppets[] = { {"Kermit", "frog"}, {"Piggy", "pig"}, {"Gonzo", "whatever"}, {"Fozzie", "bear"}, {"Sam", "eagle"}, {"Robin", "frog"}, {"Animal", "animal"}, {"Camilla", "chicken"}, {"Sweetums", "monster"}, {"Dr. Strangepork", "pig"}, {"Link Hogthrob", "pig"}, {"Zoot", "human"}, {"Dr. Bunsen Honeydew", "human"}, {"Beaker", "human"}, {"Swedish Chef", "human"} }; int count = sizeof (muppets) / sizeof (struct critter); /* This is the comparison function used for sorting and searching. */ int critter_cmp (const struct critter *c1, const struct critter *c2) { return strcmp (c1->name, c2->name); } /* Print information about a critter. */ void print_critter (const struct critter *c) { printf ("%s, the %s\n", c->name, c->species); } /* Do the lookup into the sorted array. */ void find_critter (const char *name) { struct critter target, *result; target.name = name; result = bsearch (&target, muppets, count, sizeof (struct critter), critter_cmp); if (result) print_critter (result); else printf ("Couldn't find %s.\n", name); } /* Main program. */ int main (void) { int i; for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); qsort (muppets, count, sizeof (struct critter), critter_cmp); for (i = 0; i < count; i++) print_critter (&muppets[i]); printf ("\n"); find_critter ("Kermit"); find_critter ("Gonzo"); find_critter ("Janice"); return 0; } The output from this program looks like: Kermit, the frog Piggy, the pig Gonzo, the whatever Fozzie, the bear Sam, the eagle Robin, the frog Animal, the animal Camilla, the chicken Sweetums, the monster Dr. Strangepork, the pig Link Hogthrob, the pig Zoot, the human Dr. Bunsen Honeydew, the human Beaker, the human Swedish Chef, the human Animal, the animal Beaker, the human Camilla, the chicken Dr. Bunsen Honeydew, the human Dr. Strangepork, the pig Fozzie, the bear Gonzo, the whatever Kermit, the frog Link Hogthrob, the pig Piggy, the pig Robin, the frog Sam, the eagle Swedish Chef, the human Sweetums, the monster Zoot, the human Kermit, the frog Gonzo, the whatever Couldn't find Janice. The `hsearch' function. ======================= The functions mentioned so far in this chapter are searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables. - Function: int hcreate (size_t NEL) The `hcreate' function creates a hashing table which can contain at least NEL elements. There is no possibility to grow this table so it is necessary to choose the value for NEL wisely. The used methods to implement this function might make it necessary to make the number of elements in the hashing table larger than the expected maximal number of elements. Hashing tables usually work inefficient if they are filled 80% or more. The constant access time guaranteed by hashing can only be achieved if few collisions exist. See Knuth's "The Art of Computer Programming, Part 3: Searching and Sorting" for more information. The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables. It is possible to use more than one hashing table in the program run if the former table is first destroyed by a call to `hdestroy'. The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory. - Function: void hdestroy (void) The `hdestroy' function can be used to free all the resources allocated in a previous call of `hcreate'. After a call to this function it is again possible to call `hcreate' and allocate a new table with possibly different size. It is important to remember that the elements contained in the hashing table at the time `hdestroy' is called are _not_ freed by this function. It is the responsibility of the program code to free those strings (if necessary at all). Freeing all the element memory is not possible without extra, separately kept information since there is no function to iterate through all available elements in the hashing table. If it is really necessary to free a table and all elements the programmer has to keep a list of all table elements and before calling `hdestroy' s/he has to free all element's data using this list. This is a very unpleasant mechanism and it also shows that this kind of hashing tables is mainly meant for tables which are created once and used until the end of the program run. Entries of the hashing table and keys for the search are defined using this type: - Data type: struct ENTRY Both elements of this structure are pointers to zero-terminated strings. This is a limiting restriction of the functionality of the `hsearch' functions. They can only be used for data sets which use the NUL character always and solely to terminate the records. It is not possible to handle general binary data. `char *key' Pointer to a zero-terminated string of characters describing the key for the search or the element in the hashing table. `char *data' Pointer to a zero-terminated string of characters describing the data. If the functions will be called only for searching an existing entry this element might stay undefined since it is not used. - Function: ENTRY * hsearch (ENTRY ITEM, ACTION ACTION) To search in a hashing table created using `hcreate' the `hsearch' function must be used. This function can perform simple search for an element (if ACTION has the `FIND') or it can alternatively insert the key element into the hashing table, possibly replacing a previous value (if ACTION is `ENTER'). The key is denoted by a pointer to an object of type `ENTRY'. For locating the corresponding position in the hashing table only the `key' element of the structure is used. The return value depends on the ACTION parameter value. If it is `FIND' the value is a pointer to the matching element in the hashing table or `NULL' if no matching element exists. If ACTION is `ENTER' the return value is only `NULL' if the programs runs out of memory while adding the new element to the table. Otherwise the return value is a pointer to the element in the hashing table which contains newly added element based on the data in KEY. As mentioned before the hashing table used by the functions described so far is global and there can be at any time at most one hashing table in the program. A solution is to use the following functions which are a GNU extension. All have in common that they operate on a hashing table which is described by the content of an object of the type `struct hsearch_data'. This type should be treated as opaque, none of its members should be changed directly. - Function: int hcreate_r (size_t NEL, struct hsearch_data *HTAB) The `hcreate_r' function initializes the object pointed to by HTAB to contain a hashing table with at least NEL elements. So this function is equivalent to the `hcreate' function except that the initialized data structure is controlled by the user. This allows having more than one hashing table at one time. The memory necessary for the `struct hsearch_data' object can be allocated dynamically. The return value is non-zero if the operation were successful. if the return value is zero something went wrong which probably means the programs runs out of memory. - Function: void hdestroy_r (struct hsearch_data *HTAB) The `hdestroy_r' function frees all resources allocated by the `hcreate_r' function for this very same object HTAB. As for `hdestroy' it is the programs responsibility to free the strings for the elements of the table. - Function: int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL, struct hsearch_data *HTAB) The `hsearch_r' function is equivalent to `hsearch'. The meaning of the first two arguments is identical. But instead of operating on a single global hashing table the function works on the table described by the object pointed to by HTAB (which is initialized by a call to `hcreate_r'). Another difference to `hcreate' is that the pointer to the found entry in the table is not the return value of the functions. It is returned by storing it in a pointer variables pointed to by the RETVAL parameter. The return value of the function is an integer value indicating success if it is non-zero and failure if it is zero. In the latter case the global variable ERRNO signals the reason for the failure. `ENOMEM' The table is filled and `hsearch_r' was called with an so far unknown key and ACTION set to `ENTER'. `ESRCH' The ACTION parameter is `FIND' and no corresponding element is found in the table. The `tsearch' function. ======================= Another common form to organize data for efficient search is to use trees. The `tsearch' function family provides a nice interface to functions to organize possibly large amounts of data by providing a mean access time proportional to the logarithm of the number of elements. The GNU C library implementation even guarantees that this bound is never exceeded even for input data which cause problems for simple binary tree implementations. The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable. In contrast to the `hsearch' functions the `tsearch' functions can be used with arbitrary data and not only zero-terminated strings. The `tsearch' functions have the advantage that no function to initialize data structures is necessary. A simple pointer of type `void *' initialized to `NULL' is a valid tree and can be extended or searched. - Function: void * tsearch (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) The `tsearch' function searches in the tree pointed to by `*ROOTP' for an element matching KEY. The function pointed to by COMPAR is used to determine whether two elements match. *Note Comparison Functions::, for a specification of the functions which can be used for the COMPAR parameter. If the tree does not contain a matching entry the KEY value will be added to the tree. `tsearch' does not make a copy of the object pointed to by KEY (how could it since the size is unknown). Instead it adds a reference to this object which means the object must be available as long as the tree data structure is used. The tree is represented by a pointer to a pointer since it is sometimes necessary to change the root node of the tree. So it must not be assumed that the variable pointed to by ROOTP has the same value after the call. This also shows that it is not safe to call the `tsearch' function more than once at the same time using the same tree. It is no problem to run it more than once at a time on different trees. The return value is a pointer to the matching element in the tree. If a new element was created the pointer points to the new data (which is in fact KEY). If an entry had to be created and the program ran out of space `NULL' is returned. - Function: void * tfind (const void *KEY, void *const *ROOTP, comparison_fn_t COMPAR) The `tfind' function is similar to the `tsearch' function. It locates an element matching the one pointed to by KEY and returns a pointer to this element. But if no matching element is available no new element is entered (note that the ROOTP parameter points to a constant pointer). Instead the function returns `NULL'. Another advantage of the `tsearch' function in contrast to the `hsearch' functions is that there is an easy way to remove elements. - Function: void * tdelete (const void *KEY, void **ROOTP, comparison_fn_t COMPAR) To remove a specific element matching KEY from the tree `tdelete' can be used. It locates the matching element using the same method as `tfind'. The corresponding element is then removed and a pointer to the parent of the deleted node is returned by the function. If there is no matching entry in the tree nothing can be deleted and the function returns `NULL'. If the root of the tree is deleted `tdelete' returns some unspecified value not equal to `NULL'. - Function: void tdestroy (void *VROOT, __free_fn_t FREEFCT) If the complete search tree has to be removed one can use `tdestroy'. It frees all resources allocated by the `tsearch' function to generate the tree pointed to by VROOT. For the data in each tree node the function FREEFCT is called. The pointer to the data is passed as the argument to the function. If no such work is necessary FREEFCT must point to a function doing nothing. It is called in any case. This function is a GNU extension and not covered by the System V or X/Open specifications. In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type: void __action_fn_t (const void *nodep, VISIT value, int level); The NODEP is the data value of the current node (once given as the KEY argument to `tsearch'). LEVEL is a numeric value which corresponds to the depth of the current node in the tree. The root node has the depth 0 and its children have a depth of 1 and so on. The `VISIT' type is an enumeration type. - Data Type: VISIT The `VISIT' value indicates the status of the current node in the tree and how the function is called. The status of a node is either `leaf' or `internal node'. For each leaf node the function is called exactly once, for each internal node it is called three times: before the first child is processed, after the first child is processed and after both children are processed. This makes it possible to handle all three methods of tree traversal (or even a combination of them). `preorder' The current node is an internal node and the function is called before the first child was processed. `postorder' The current node is an internal node and the function is called after the first child was processed. `endorder' The current node is an internal node and the function is called after the second child was processed. `leaf' The current node is a leaf. - Function: void twalk (const void *ROOT, __action_fn_t ACTION) For each node in the tree with a node pointed to by ROOT, the `twalk' function calls the function provided by the parameter ACTION. For leaf nodes the function is called exactly once with VALUE set to `leaf'. For internal nodes the function is called three times, setting the VALUE parameter or ACTION to the appropriate value. The LEVEL argument for the ACTION function is computed while descending the tree with increasing the value by one for the descend to a child, starting with the value 0 for the root node. Since the functions used for the ACTION parameter to `twalk' must not modify the tree data, it is safe to run `twalk' in more than one thread at the same time, working on the same tree. It is also safe to call `tfind' in parallel. Functions which modify the tree must not be used, otherwise the behavior is undefined. Pattern Matching **************** The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does. Wildcard Matching ================= This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in `fnmatch.h'. - Function: int fnmatch (const char *PATTERN, const char *STRING, int FLAGS) This function tests whether the string STRING matches the pattern PATTERN. It returns `0' if they do match; otherwise, it returns the nonzero value `FNM_NOMATCH'. The arguments PATTERN and STRING are both strings. The argument FLAGS is a combination of flag bits that alter the details of matching. See below for a list of the defined flags. In the GNU C Library, `fnmatch' cannot experience an "error"--it always returns an answer for whether the match succeeds. However, other implementations of `fnmatch' might sometimes report "errors". They would do so by returning nonzero values that are not equal to `FNM_NOMATCH'. These are the available flags for the FLAGS argument: `FNM_FILE_NAME' Treat the `/' character specially, for matching file names. If this flag is set, wildcard constructs in PATTERN cannot match `/' in STRING. Thus, the only way to match `/' is with an explicit `/' in PATTERN. `FNM_PATHNAME' This is an alias for `FNM_FILE_NAME'; it comes from POSIX.2. We don't recommend this name because we don't use the term "pathname" for file names. `FNM_PERIOD' Treat the `.' character specially if it appears at the beginning of STRING. If this flag is set, wildcard constructs in PATTERN cannot match `.' as the first character of STRING. If you set both `FNM_PERIOD' and `FNM_FILE_NAME', then the special treatment applies to `.' following `/' as well as to `.' at the beginning of STRING. (The shell uses the `FNM_PERIOD' and `FNM_FILE_NAME' flags together for matching file names.) `FNM_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `FNM_NOESCAPE', then `\' is an ordinary character. `FNM_LEADING_DIR' Ignore a trailing sequence of characters starting with a `/' in STRING; that is to say, test whether STRING starts with a directory name that PATTERN matches. If this flag is set, either `foo*' or `foobar' as a pattern would match the string `foobar/frobozz'. `FNM_CASEFOLD' Ignore case in comparing STRING to PATTERN. `FNM_EXTMATCH' Recognize beside the normal patterns also the extended patterns introduced in `ksh'. The patterns are written in the form explained in the following table where PATTERN-LIST is a `|' separated list of patterns. `?(PATTERN-LIST)' The pattern matches if zero or one occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `*(PATTERN-LIST)' The pattern matches if zero or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `+(PATTERN-LIST)' The pattern matches if one or more occurrences of any of the patterns in the PATTERN-LIST allow matching the input string. `@(PATTERN-LIST)' The pattern matches if exactly one occurrence of any of the patterns in the PATTERN-LIST allows matching the input string. `!(PATTERN-LIST)' The pattern matches if the input string cannot be matched with any of the patterns in the PATTERN-LIST. Globbing ======== The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called "globbing". You could do this using `fnmatch', by reading the directory entries one by one and testing each one with `fnmatch'. But that would be slow (and complex, since you would have to handle subdirectories by hand). The library provides a function `glob' to make this particular use of wildcards convenient. `glob' and the other symbols in this section are declared in `glob.h'. Calling `glob' -------------- The result of globbing is a vector of file names (strings). To return this vector, `glob' uses a special data type, `glob_t', which is a structure. You pass `glob' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: glob_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat *)'. This is a GNU extension. For use in the `glob64' function `glob.h' contains another definition for a very similar type. `glob64_t' differs from `glob_t' only in the types of the members `gl_readdir', `gl_stat', and `gl_lstat'. - Data Type: glob64_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions. `gl_pathc' The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below). `gl_pathv' The address of the vector. This field has type `char **'. `gl_offs' The offset of the first real element of the vector, from its nominal address in the `gl_pathv' field. Unlike the other fields, this is always an input to `glob', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `glob' function fills them with null pointers.) The `gl_offs' field is meaningful only if you use the `GLOB_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. `gl_closedir' The address of an alternative implementation of the `closedir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void (*) (void *)'. This is a GNU extension. `gl_readdir' The address of an alternative implementation of the `readdir64' function used to read the contents of a directory. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `struct dirent64 *(*) (void *)'. This is a GNU extension. `gl_opendir' The address of an alternative implementation of the `opendir' function. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `void *(*) (const char *)'. This is a GNU extension. `gl_stat' The address of an alternative implementation of the `stat64' function to get information about an object in the filesystem. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. `gl_lstat' The address of an alternative implementation of the `lstat64' function to get information about an object in the filesystems, not following symbolic links. It is used if the `GLOB_ALTDIRFUNC' bit is set in the flag parameter. The type of this field is `int (*) (const char *, struct stat64 *)'. This is a GNU extension. - Function: int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR) The function `glob' does globbing using the pattern PATTERN in the current directory. It puts the result in a newly allocated vector, and stores the size and address of this vector into `*VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Globbing::, for details of the flags. The result of globbing is a sequence of file names. The function `glob' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `glob' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*VECTOR-PTR'. Normally, `glob' sorts the file names alphabetically before returning them. You can turn this off with the flag `GLOB_NOSORT' if you want to get the information as fast as possible. Usually it's a good idea to let `glob' sort them--if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making. If `glob' succeeds, it returns 0. Otherwise, it returns one of these error codes: `GLOB_ABORTED' There was an error opening a directory, and you used the flag `GLOB_ERR' or your specified ERRFUNC returned a nonzero value. *Note Flags for Globbing::, for an explanation of the `GLOB_ERR' flag and ERRFUNC. `GLOB_NOMATCH' The pattern didn't match any existing files. If you use the `GLOB_NOCHECK' flag, then you never get this error code, because that flag tells `glob' to _pretend_ that the pattern matched at least one file. `GLOB_NOSPACE' It was impossible to allocate memory to hold the result. In the event of an error, `glob' stores information in `*VECTOR-PTR' about all the matches it has found so far. It is important to notice that the `glob' function will not fail if it encounters directories or files which cannot be handled without the LFS interfaces. The implementation of `glob' is supposed to use these functions internally. This at least is the assumptions made by the Unix standard. The GNU extension of allowing the user to provide own directory handling and `stat' functions complicates things a bit. If these callback functions are used and a large file or directory is encountered `glob' _can_ fail. - Function: int glob64 (const char *PATTERN, int FLAGS, int (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t *VECTOR-PTR) The `glob64' function was added as part of the Large File Summit extensions but is not part of the original LFS proposal. The reason for this is simple: it is not necessary. The necessity for a `glob64' function is added by the extensions of the GNU `glob' implementation which allows the user to provide own directory handling and `stat' functions. The `readdir' and `stat' functions do depend on the choice of `_FILE_OFFSET_BITS' since the definition of the types `struct dirent' and `struct stat' will change depending on the choice. Beside this difference the `glob64' works just like `glob' in all aspects. This function is a GNU extension. Flags for Globbing ------------------ This section describes the flags that you can specify in the FLAGS argument to `glob'. Choose the flags you want, and combine them with the C bitwise OR operator `|'. `GLOB_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `glob'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `glob'. And, if you set `GLOB_DOOFFS' in the first call to `glob', you must also set it when you append to the results. Note that the pointer stored in `gl_pathv' may no longer be valid after you call `glob' the second time, because `glob' might have relocated the vector. So always fetch `gl_pathv' from the `glob_t' structure after each `glob' call; *never* save the pointer across calls. `GLOB_DOOFFS' Leave blank slots at the beginning of the vector of words. The `gl_offs' field says how many slots to leave. The blank slots contain null pointers. `GLOB_ERR' Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand PATTERN fully. Such difficulties might include a directory in which you don't have the requisite access. Normally, `glob' tries its best to keep on going despite any errors, reading whatever directories it can. You can exercise even more control than this by specifying an error-handler function ERRFUNC when you call `glob'. If ERRFUNC is not a null pointer, then `glob' doesn't give up right away when it can't read a directory; instead, it calls ERRFUNC with two arguments, like this: (*ERRFUNC) (FILENAME, ERROR-CODE) The argument FILENAME is the name of the directory that `glob' couldn't open or couldn't read, and ERROR-CODE is the `errno' value that was reported to `glob'. If the error handler function returns nonzero, then `glob' gives up right away. Otherwise, it continues. `GLOB_MARK' If the pattern matches the name of a directory, append `/' to the directory's name when returning it. `GLOB_NOCHECK' If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched. (Normally, when the pattern doesn't match anything, `glob' returns that there were no matches.) `GLOB_NOSORT' Don't sort the file names; return them in no particular order. (In practice, the order will depend on the order of the entries in the directory.) The only reason _not_ to sort is to save time. `GLOB_NOESCAPE' Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character. If you use `GLOB_NOESCAPE', then `\' is an ordinary character. `glob' does its work by calling the function `fnmatch' repeatedly. It handles the flag `GLOB_NOESCAPE' by turning on the `FNM_NOESCAPE' flag in calls to `fnmatch'. More Flags for Globbing ----------------------- Beside the flags described in the last section, the GNU implementation of `glob' allows a few more flags which are also defined in the `glob.h' file. Some of the extensions implement functionality which is available in modern shell implementations. `GLOB_PERIOD' The `.' character (period) is treated special. It cannot be matched by wildcards. *Note Wildcard Matching::, `FNM_PERIOD'. `GLOB_MAGCHAR' The `GLOB_MAGCHAR' value is not to be given to `glob' in the FLAGS parameter. Instead, `glob' sets this bit in the GL_FLAGS element of the GLOB_T structure provided as the result if the pattern used for matching contains any wildcard character. `GLOB_ALTDIRFUNC' Instead of the using the using the normal functions for accessing the filesystem the `glob' implementation uses the user-supplied functions specified in the structure pointed to by PGLOB parameter. For more information about the functions refer to the sections about directory handling see *Note Accessing Directories::, and *Note Reading Attributes::. `GLOB_BRACE' If this flag is given the handling of braces in the pattern is changed. It is now required that braces appear correctly grouped. I.e., for each opening brace there must be a closing one. Braces can be used recursively. So it is possible to define one brace expression in another one. It is important to note that the range of each brace expression is completely contained in the outer brace expression (if there is one). The string between the matching braces is separated into single expressions by splitting at `,' (comma) characters. The commas themselves are discarded. Please note what we said above about recursive brace expressions. The commas used to separate the subexpressions must be at the same level. Commas in brace subexpressions are not matched. They are used during expansion of the brace expression of the deeper level. The example below shows this glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result) is equivalent to the sequence glob ("foo/", GLOB_BRACE, NULL, &result) glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result) glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result) if we leave aside error handling. `GLOB_NOMAGIC' If the pattern contains no wildcard constructs (it is a literal file name), return it as the sole "matching" word, even if no file exists by that name. `GLOB_TILDE' If this flag is used the character `~' (tilde) is handled special if it appears at the beginning of the pattern. Instead of being taken verbatim it is used to represent the home directory of a known user. If `~' is the only character in pattern or it is followed by a `/' (slash), the home directory of the process owner is substituted. Using `getlogin' and `getpwnam' the information is read from the system databases. As an example take user `bart' with his home directory at `/home/bart'. For him a call like glob ("~/bin/*", GLOB_TILDE, NULL, &result) would return the contents of the directory `/home/bart/bin'. Instead of referring to the own home directory it is also possible to name the home directory of other users. To do so one has to append the user name after the tilde character. So the contents of user `homer''s `bin' directory can be retrieved by glob ("~homer/bin/*", GLOB_TILDE, NULL, &result) If the user name is not valid or the home directory cannot be determined for some reason the pattern is left untouched and itself used as the result. I.e., if in the last example `home' is not available the tilde expansion yields to `"~homer/bin/*"' and `glob' is not looking for a directory named `~homer'. This functionality is equivalent to what is available in C-shells if the `nonomatch' flag is set. `GLOB_TILDE_CHECK' If this flag is used `glob' behaves like as if `GLOB_TILDE' is given. The only difference is that if the user name is not available or the home directory cannot be determined for other reasons this leads to an error. `glob' will return `GLOB_NOMATCH' instead of using the pattern itself as the name. This functionality is equivalent to what is available in C-shells if `nonomatch' flag is not set. `GLOB_ONLYDIR' If this flag is used the globbing function takes this as a *hint* that the caller is only interested in directories matching the pattern. If the information about the type of the file is easily available non-directories will be rejected but no extra work will be done to determine the information for each file. I.e., the caller must still be able to filter directories out. This functionality is only available with the GNU `glob' implementation. It is mainly used internally to increase the performance but might be useful for a user as well and therefore is documented here. Calling `glob' will in most cases allocate resources which are used to represent the result of the function call. If the same object of type `glob_t' is used in multiple call to `glob' the resources are freed or reused so that no leaks appear. But this does not include the time when all `glob' calls are done. - Function: void globfree (glob_t *PGLOB) The `globfree' function frees all resources allocated by previous calls to `glob' associated with the object pointed to by PGLOB. This function should be called whenever the currently used `glob_t' typed object isn't used anymore. - Function: void globfree64 (glob64_t *PGLOB) This function is equivalent to `globfree' but it frees records of type `glob64_t' which were allocated by `glob64'. Regular Expression Matching =========================== The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years. Both interfaces are declared in the header file `regex.h'. If you define `_POSIX_C_SOURCE', then only the POSIX.2 functions, structures, and constants are declared. POSIX Regular Expression Compilation ------------------------------------ Before you can actually match a regular expression, you must "compile" it. This is not true compilation--it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to "execute" the pattern fast. (*Note Matching POSIX Regexps::, for how to use the compiled regular expression for matching.) There is a special data type for compiled regular expressions: - Data Type: regex_t This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at: `re_nsub' This field holds the number of parenthetical subexpressions in the regular expression that was compiled. There are several other fields, but we don't describe them here, because only the functions in the library should use them. After you create a `regex_t' object, you can compile a regular expression into it by calling `regcomp'. - Function: int regcomp (regex_t *COMPILED, const char *PATTERN, int CFLAGS) The function `regcomp' "compiles" a regular expression into a data structure that you can use with `regexec' to match against a string. The compiled regular expression format is designed for efficient matching. `regcomp' stores it into `*COMPILED'. It's up to you to allocate an object of type `regex_t' and pass its address to `regcomp'. The argument CFLAGS lets you specify various options that control the syntax and semantics of regular expressions. *Note Flags for POSIX Regexps::. If you use the flag `REG_NOSUB', then `regcomp' omits from the compiled regular expression the information necessary to record how subexpressions actually match. In this case, you might as well pass `0' for the MATCHPTR and NMATCH arguments when you call `regexec'. If you don't use `REG_NOSUB', then the compiled regular expression does have the capacity to record how subexpressions match. Also, `regcomp' tells you how many subexpressions PATTERN has, by storing the number in `COMPILED->re_nsub'. You can use that value to decide how long an array to allocate to hold information about subexpression matches. `regcomp' returns `0' if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below). You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. Here are the possible nonzero values that `regcomp' can return: `REG_BADBR' There was an invalid `\{...\}' construct in the regular expression. A valid `\{...\}' construct must contain either a single number, or two numbers in increasing order separated by a comma. `REG_BADPAT' There was a syntax error in the regular expression. `REG_BADRPT' A repetition operator such as `?' or `*' appeared in a bad position (with no preceding subexpression to act on). `REG_ECOLLATE' The regular expression referred to an invalid collating element (one not defined in the current locale for string collation). *Note Locale Categories::. `REG_ECTYPE' The regular expression referred to an invalid character class name. `REG_EESCAPE' The regular expression ended with `\'. `REG_ESUBREG' There was an invalid number in the `\DIGIT' construct. `REG_EBRACK' There were unbalanced square brackets in the regular expression. `REG_EPAREN' An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced `\(' and `\)'. `REG_EBRACE' The regular expression had unbalanced `\{' and `\}'. `REG_ERANGE' One of the endpoints in a range expression was invalid. `REG_ESPACE' `regcomp' ran out of memory. Flags for POSIX Regular Expressions ----------------------------------- These are the bit flags that you can use in the CFLAGS operand when compiling a regular expression with `regcomp'. `REG_EXTENDED' Treat the pattern as an extended regular expression, rather than as a basic regular expression. `REG_ICASE' Ignore case when matching letters. `REG_NOSUB' Don't bother storing the contents of the MATCHES-PTR array. `REG_NEWLINE' Treat a newline in STRING as dividing STRING into multiple lines, so that `$' can match before the newline and `^' can match after. Also, don't permit `.' to match a newline, and don't permit `[^...]' to match a newline. Otherwise, newline acts like any other ordinary character. Matching a Compiled POSIX Regular Expression -------------------------------------------- Once you have compiled a regular expression, as described in *Note POSIX Regexp Compilation::, you can match it against strings using `regexec'. A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (`^' or `$'). - Function: int regexec (regex_t *COMPILED, char *STRING, size_t NMATCH, regmatch_t MATCHPTR [], int EFLAGS) This function tries to match the compiled regular expression `*COMPILED' against STRING. `regexec' returns `0' if the regular expression matches; otherwise, it returns a nonzero value. See the table below for what nonzero values mean. You can use `regerror' to produce an error message string describing the reason for a nonzero value; see *Note Regexp Cleanup::. The argument EFLAGS is a word of bit flags that enable various options. If you want to get information about what part of STRING actually matched the regular expression or its subexpressions, use the arguments MATCHPTR and NMATCH. Otherwise, pass `0' for NMATCH, and `NULL' for MATCHPTR. *Note Regexp Subexpressions::. You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression. The function `regexec' accepts the following flags in the EFLAGS argument: `REG_NOTBOL' Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it. `REG_NOTEOL' Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it. Here are the possible nonzero values that `regexec' can return: `REG_NOMATCH' The pattern didn't match the string. This isn't really an error. `REG_ESPACE' `regexec' ran out of memory. Match Results with Subexpressions --------------------------------- When `regexec' matches parenthetical subexpressions of PATTERN, it records which parts of STRING they match. It returns that information by storing the offsets into an array whose elements are structures of type `regmatch_t'. The first element of the array (index `0') records the part of the string that matched the entire regular expression. Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression. - Data Type: regmatch_t This is the data type of the MATCHARRAY array that you pass to `regexec'. It contains two structure fields, as follows: `rm_so' The offset in STRING of the beginning of a substring. Add this value to STRING to get the address of that part. `rm_eo' The offset in STRING of the end of the substring. - Data Type: regoff_t `regoff_t' is an alias for another signed integer type. The fields of `regmatch_t' have type `regoff_t'. The `regmatch_t' elements correspond to subexpressions positionally; the first element (index `1') records where the first subexpression matched, the second element records the second subexpression, and so on. The order of the subexpressions is the order in which they begin. When you call `regexec', you specify how long the MATCHPTR array is, with the NMATCH argument. This tells `regexec' how many elements to store. If the actual regular expression has more than NMATCH subexpressions, then you won't get offset information about the rest of them. But this doesn't alter whether the pattern matches a particular string or not. If you don't want `regexec' to return any information about where the subexpressions matched, you can either supply `0' for NMATCH, or use the flag `REG_NOSUB' when you compile the pattern with `regcomp'. Complications in Subexpression Matching --------------------------------------- Sometimes a subexpression matches a substring of no characters. This happens when `f\(o*\)' matches the string `fum'. (It really matches just the `f'.) In this case, both of the offsets identify the point in the string where the null substring was found. In this example, the offsets are both `1'. Sometimes the entire regular expression can match without using some of its subexpressions at all--for example, when `ba\(na\)*' matches the string `ba', the parenthetical subexpression is not used. When this happens, `regexec' stores `-1' in both fields of the element for that subexpression. Sometimes matching the entire regular expression can match a particular subexpression more than once--for example, when `ba\(na\)*' matches the string `bananana', the parenthetical subexpression matches three times. When this happens, `regexec' usually stores the offsets of the last part of the string that matched the subexpression. In the case of `bananana', these offsets are `6' and `8'. But the last match is not always the one that is chosen. It's more accurate to say that the last _opportunity_ to match is the one that takes precedence. What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression. For an example, consider `\(ba\(na\)*s \)*' matching the string `bananas bas '. The last time the inner expression actually matches is near the end of the first word. But it is _considered_ again in the second word, and fails to match there. `regexec' reports nonuse of the "na" subexpression. Another place where this rule applies is when the regular expression \(ba\(na\)*s \|nefer\(ti\)* \)* matches `bananas nefertiti'. The "na" subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there. Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the "na" subexpression is not used. So `regexec' reports nonuse of the "na" subexpression. POSIX Regexp Matching Cleanup ----------------------------- When you are finished using a compiled regular expression, you can free the storage it uses by calling `regfree'. - Function: void regfree (regex_t *COMPILED) Calling `regfree' frees all the storage that `*COMPILED' points to. This includes various internal fields of the `regex_t' structure that aren't documented in this manual. `regfree' does not free the object `*COMPILED' itself. You should always free the space in a `regex_t' structure with `regfree' before using the structure to compile another regular expression. When `regcomp' or `regexec' reports an error, you can use the function `regerror' to turn it into an error message string. - Function: size_t regerror (int ERRCODE, regex_t *COMPILED, char *BUFFER, size_t LENGTH) This function produces an error message string for the error code ERRCODE, and stores the string in LENGTH bytes of memory starting at BUFFER. For the COMPILED argument, supply the same compiled regular expression structure that `regcomp' or `regexec' was working with when it got the error. Alternatively, you can supply `NULL' for COMPILED; you will still get a meaningful error message, but it might not be as detailed. If the error message can't fit in LENGTH bytes (including a terminating null character), then `regerror' truncates it. The string that `regerror' stores is always null-terminated even if it has been truncated. The return value of `regerror' is the minimum length needed to store the entire error message. If this is less than LENGTH, then the error message was not truncated, and you can use it. Otherwise, you should call `regerror' again with a larger buffer. Here is a function which uses `regerror', but always dynamically allocates a buffer for the error message: char *get_regerror (int errcode, regex_t *compiled) { size_t length = regerror (errcode, compiled, NULL, 0); char *buffer = xmalloc (length); (void) regerror (errcode, compiled, buffer, length); return buffer; } Shell-Style Word Expansion ========================== "Word expansion" means the process of splitting a string into "words" and substituting for variables, commands, and wildcards just as the shell does. For example, when you write `ls -l foo.c', this string is split into three separate words--`ls', `-l' and `foo.c'. This is the most basic function of word expansion. When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called "wildcard expansion", and it is also a part of word expansion. When you use `echo $PATH' to print your path, you are taking advantage of "variable substitution", which is also part of word expansion. Ordinary programs can perform word expansion just like the shell by calling the library function `wordexp'. The Stages of Word Expansion ---------------------------- When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here: 1. "Tilde expansion": Replacement of `~foo' with the name of the home directory of `foo'. 2. Next, three different transformations are applied in the same step, from left to right: * "Variable substitution": Environment variables are substituted for references such as `$foo'. * "Command substitution": Constructs such as ``cat foo`' and the equivalent `$(cat foo)' are replaced with the output from the inner command. * "Arithmetic expansion": Constructs such as `$(($x-1))' are replaced with the result of the arithmetic computation. 3. "Field splitting": subdivision of the text into "words". 4. "Wildcard expansion": The replacement of a construct such as `*.c' with a list of `.c' file names. Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words. 5. "Quote removal": The deletion of string-quotes, now that they have done their job by inhibiting the above transformations when appropriate. For the details of these transformations, and how to write the constructs that use them, see `The BASH Manual' (to appear). Calling `wordexp' ----------------- All the functions, constants and data types for word expansion are declared in the header file `wordexp.h'. Word expansion produces a vector of words (strings). To return this vector, `wordexp' uses a special data type, `wordexp_t', which is a structure. You pass `wordexp' the address of the structure, and it fills in the structure's fields to tell you about the results. - Data Type: wordexp_t This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. `we_wordc' The number of elements in the vector. `we_wordv' The address of the vector. This field has type `char **'. `we_offs' The offset of the first real element of the vector, from its nominal address in the `we_wordv' field. Unlike the other fields, this is always an input to `wordexp', rather than an output from it. If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The `wordexp' function fills them with null pointers.) The `we_offs' field is meaningful only if you use the `WRDE_DOOFFS' flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector. - Function: int wordexp (const char *WORDS, wordexp_t *WORD-VECTOR-PTR, int FLAGS) Perform word expansion on the string WORDS, putting the result in a newly allocated vector, and store the size and address of this vector into `*WORD-VECTOR-PTR'. The argument FLAGS is a combination of bit flags; see *Note Flags for Wordexp::, for details of the flags. You shouldn't use any of the characters `|&;<>' in the string WORDS unless they are quoted; likewise for newline. If you use these characters unquoted, you will get the `WRDE_BADCHAR' error code. Don't use parentheses or braces unless they are quoted or part of a word expansion construct. If you use quotation characters `'"`', they should come in pairs that balance. The results of word expansion are a sequence of words. The function `wordexp' allocates a string for each resulting word, then allocates a vector of type `char **' to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the "word vector". To return this vector, `wordexp' stores both its address and its length (number of elements, not counting the terminating null pointer) into `*WORD-VECTOR-PTR'. If `wordexp' succeeds, it returns 0. Otherwise, it returns one of these error codes: `WRDE_BADCHAR' The input string WORDS contains an unquoted invalid character such as `|'. `WRDE_BADVAL' The input string refers to an undefined shell variable, and you used the flag `WRDE_UNDEF' to forbid such references. `WRDE_CMDSUB' The input string uses command substitution, and you used the flag `WRDE_NOCMD' to forbid command substitution. `WRDE_NOSPACE' It was impossible to allocate memory to hold the result. In this case, `wordexp' can store part of the results--as much as it could allocate room for. `WRDE_SYNTAX' There was a syntax error in the input string. For example, an unmatched quoting character is a syntax error. - Function: void wordfree (wordexp_t *WORD-VECTOR-PTR) Free the storage used for the word-strings and vector that `*WORD-VECTOR-PTR' points to. This does not free the structure `*WORD-VECTOR-PTR' itself--only the other data it points to. Flags for Word Expansion ------------------------ This section describes the flags that you can specify in the FLAGS argument to `wordexp'. Choose the flags you want, and combine them with the C operator `|'. `WRDE_APPEND' Append the words from this expansion to the vector of words produced by previous calls to `wordexp'. This way you can effectively expand several words as if they were concatenated with spaces between them. In order for appending to work, you must not modify the contents of the word vector structure between calls to `wordexp'. And, if you set `WRDE_DOOFFS' in the first call to `wordexp', you must also set it when you append to the results. `WRDE_DOOFFS' Leave blank slots at the beginning of the vector of words. The `we_offs' field says how many slots to leave. The blank slots contain null pointers. `WRDE_NOCMD' Don't do command substitution; if the input requests command substitution, report an error. `WRDE_REUSE' Reuse a word vector made by a previous call to `wordexp'. Instead of allocating a new vector of words, this call to `wordexp' will use the vector that already exists (making it larger if necessary). Note that the vector may move, so it is not safe to save an old pointer and use it again after calling `wordexp'. You must fetch `we_pathv' anew after each call. `WRDE_SHOWERR' Do show any error messages printed by commands run by command substitution. More precisely, allow these commands to inherit the standard error output stream of the current process. By default, `wordexp' gives these commands a standard error stream that discards all output. `WRDE_UNDEF' If the input refers to a shell variable that is not defined, report an error. `wordexp' Example ----------------- Here is an example of using `wordexp' to expand several strings and use the results to run a shell command. It also shows the use of `WRDE_APPEND' to concatenate the expansions and of `wordfree' to free the space allocated by `wordexp'. int expand_and_execute (const char *program, const char *options) { wordexp_t result; pid_t pid int status, i; /* Expand the string for the program to run. */ switch (wordexp (program, &result, 0)) { case 0: /* Successful. */ break; case WRDE_NOSPACE: /* If the error was `WRDE_NOSPACE', then perhaps part of the result was allocated. */ wordfree (&result); default: /* Some other error. */ return -1; } /* Expand the strings specified for the arguments. */ for (i = 0; args[i]; i++) { if (wordexp (options, &result, WRDE_APPEND)) { wordfree (&result); return -1; } } pid = fork (); if (pid == 0) { /* This is the child process. Execute the command. */ execv (result.we_wordv[0], result.we_wordv); exit (EXIT_FAILURE); } else if (pid < 0) /* The fork failed. Report failure. */ status = -1; else /* This is the parent process. Wait for the child to complete. */ if (waitpid (pid, &status, 0) != pid) status = -1; wordfree (&result); return status; } Details of Tilde Expansion -------------------------- It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~USER' to stand for USER's home directory. "Tilde expansion" is the process of converting these abbreviations to the directory names that they stand for. Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way. Plain `~' uses the value of the environment variable `HOME' as the proper home directory name. `~' followed by a user name uses `getpwname' to look up that user in the user database, and uses whatever directory is recorded there. Thus, `~' followed by your own name can give different results from plain `~', if the value of `HOME' is not really your home directory. Details of Variable Substitution -------------------------------- Part of ordinary shell syntax is the use of `$VARIABLE' to substitute the value of a shell variable into a command. This is called "variable substitution", and it is one part of doing word expansion. There are two basic ways you can write a variable reference for substitution: `${VARIABLE}' If you write braces around the variable name, then it is completely unambiguous where the variable name ends. You can concatenate additional letters onto the end of the variable value by writing them immediately after the close brace. For example, `${foo}s' expands into `tractors'. `$VARIABLE' If you do not put braces around the variable name, then the variable name consists of all the alphanumeric characters and underscores that follow the `$'. The next punctuation character ends the variable name. Thus, `$foo-bar' refers to the variable `foo' and expands into `tractor-bar'. When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways. `${VARIABLE:-DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead. `${VARIABLE:=DEFAULT}' Substitute the value of VARIABLE, but if that is empty or undefined, use DEFAULT instead and set the variable to DEFAULT. `${VARIABLE:?MESSAGE}' If VARIABLE is defined and not empty, substitute its value. Otherwise, print MESSAGE as an error message on the standard error stream, and consider word expansion a failure. `${VARIABLE:+REPLACEMENT}' Substitute REPLACEMENT, but only if VARIABLE is defined and nonempty. Otherwise, substitute nothing for this construct. `${#VARIABLE}' Substitute a numeral which expresses in base ten the number of characters in the value of VARIABLE. `${#foo}' stands for `7', because `tractor' is seven characters. These variants of variable substitution let you remove part of the variable's value before substituting it. The PREFIX and SUFFIX are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names. `${VARIABLE%%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the longest possible match. Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'. `${VARIABLE%SUFFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the end that matches the pattern SUFFIX. If there is more than one alternative for how to match against SUFFIX, this construct uses the shortest possible alternative. Thus, `${foo%%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'. `${VARIABLE##PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the longest possible match. Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'. `${VARIABLE#PREFIX}' Substitute the value of VARIABLE, but first discard from that variable any portion at the beginning that matches the pattern PREFIX. If there is more than one alternative for how to match against PREFIX, this construct uses the shortest possible alternative. Thus, `${foo%%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'. Input/Output Overview ********************* Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate! This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are: * *Note I/O on Streams::, which covers the high-level functions that operate on streams, including formatted input and output. * *Note Low-Level I/O::, which covers the basic I/O and control functions on file descriptors. * *Note File System Interface::, which covers functions for operating on directories and for manipulating file attributes such as access modes and ownership. * *Note Pipes and FIFOs::, which includes information on the basic interprocess communication facilities. * *Note Sockets::, which covers a more complicated interprocess communication facility with support for networking. * *Note Low-Level Terminal Interface::, which covers functions for changing how input and output to terminals or other serial devices are processed. Input/Output Concepts ===================== Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called "opening" the file. You can open a file for reading, writing, or both. The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors. When you have finished reading to or writing from the file, you can terminate the connection by "closing" the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it. Streams and File Descriptors ---------------------------- When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type `int', while streams are represented as `FILE *' objects. File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (*note File Status Flags::). Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike--the sole exception being the three styles of buffering that you can choose (*note Stream Buffering::). The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors. The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (`printf' and `scanf') as well as functions for character- and line-oriented input and output. Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor. In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (*note Formatted Input::) and formatted output functions (*note Formatted Output::). If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however. File Position ------------- One of the attributes of an open file is its "file position" that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file. The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally "sequential". Ordinary files permit read or write operations at any position within the file. Some other kinds of files may also permit this. Files which do permit this are sometimes referred to as "random-access" files. You can change the file position using the `fseek' function on a stream (*note File Positioning::) or the `lseek' function on a file descriptor (*note I/O Primitives::). If you try to change the file position on a file that doesn't support random access, you get the `ESPIPE' error. Streams and descriptors that are opened for "append access" are treated specially for output: output to such files is _always_ appended sequentially to the _end_ of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done. If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do. In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions. By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other. File Names ========== In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings--even files which are actually devices such as tape drives or terminals. These strings are called "file names". You specify the file name to say which file you want to open or operate on. This section describes the conventions for file names and how the operating system works with them. Directories ----------- In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories. A "directory" is a file that contains information to associate other files with names; these associations are called "links" or "directory entries". Sometimes, people speak of "files in a directory", but in reality, a directory only contains pointers to files, not the files themselves. The name of a file contained in a directory entry is called a "file name component". In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on. Some other documents, such as the POSIX standard, use the term "pathname" for what we call a file name, and either "filename" or "pathname component" for what this manual calls a file name component. We don't use this terminology because a "path" is something completely different (a list of directories to search), and we think that "pathname" used for something else will confuse users. We always use "file name" and "file name component" (or sometimes just "component", where the context is obvious) in GNU documentation. Some macros use the POSIX terminology in their names, such as `PATH_MAX'. These macros are defined by the POSIX standard, so we cannot change their names. You can find more detailed information about operations on directories in *Note File System Interface::. File Name Resolution -------------------- A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character. The process of determining what file a file name refers to is called "file name resolution". This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails. If a file name begins with a `/', the first component in the file name is located in the "root directory" of the process (usually all processes on the system have the same root directory). Such a file name is called an "absolute file name". Otherwise, the first component in the file name is located in the current working directory (*note Working Directory::). This kind of file name is called a "relative file name". The file name components `.' ("dot") and `..' ("dot-dot") have special meanings. Every directory has entries for these file name components. The file name component `.' refers to the directory itself, while the file name component `..' refers to its "parent directory" (the directory that contains the link for the directory in question). As a special case, `..' in the root directory refers to the root directory itself, since it has no parent; thus `/..' is the same as `/'. Here are some examples of file names: `/a' The file named `a', in the root directory. `/a/b' The file named `b', in the directory named `a' in the root directory. `a' The file named `a', in the current working directory. `/a/./b' This is the same as `/a/b'. `./a' The file named `a', in the current working directory. `../a' The file named `a', in the parent directory of the current working directory. A file name that names a directory may optionally end in a `/'. You can specify a file name of `/' to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of `.' or `./'. Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names--for example, files containing C source code usually have names suffixed with `.c'--but there is nothing in the file system itself that enforces this kind of convention. File Name Errors ---------------- Functions that accept file name arguments usually detect these `errno' error conditions relating to the file name syntax or trouble finding the named file. These errors are referred to throughout this manual as the "usual file name errors". `EACCES' The process does not have search permission for a directory component of the file name. `ENAMETOOLONG' This error is used when either the total length of a file name is greater than `PATH_MAX', or when an individual file name component has a length greater than `NAME_MAX'. *Note Limits for Files::. In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component. `ENOENT' This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist. *Note Symbolic Links::. `ENOTDIR' A file that is referenced as a directory component in the file name exists, but it isn't a directory. `ELOOP' Too many symbolic links were resolved while trying to look up the file name. The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops. *Note Symbolic Links::. Portability of File Names ------------------------- The rules for the syntax of file names discussed in *Note File Names::, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions. There are two reasons why it can be important for you to be aware of file name portability issues: * If your program makes assumptions about file name syntax, or contains embedded literal file name strings, it is more difficult to get it to run under other operating systems that use different syntax conventions. * Even if you are not concerned about running your program on machines that run other operating systems, it may still be possible to access files that use different naming conventions. For example, you may be able to access file systems on another computer running a different operating system over a network, or read and write disks in formats used by other operating systems. The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others. The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings. Input/Output on Streams *********************** This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in *Note I/O Overview::, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process. Streams ======= For historical reasons, the type of the C data structure that represents a stream is called `FILE' rather than "stream". Since most of the library functions deal with objects of type `FILE *', sometimes the term "file pointer" is also used to mean "stream". This leads to unfortunate confusion over terminology in many books on C. This manual, however, is careful to use the terms "file" and "stream" only in the technical sense. The `FILE' type is declared in the header file `stdio.h'. - Data Type: FILE This is the data type used to represent stream objects. A `FILE' object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information. Each stream also has error and end-of-file status indicators that can be tested with the `ferror' and `feof' functions; see *Note EOF and Errors::. `FILE' objects are allocated and managed internally by the input/output library functions. Don't try to create your own objects of type `FILE'; let the library do it. Your programs should deal only with pointers to these objects (that is, `FILE *' values) rather than the objects themselves. Standard Streams ================ When the `main' function of your program is invoked, it already has three predefined streams open and available for use. These represent the "standard" input and output channels that have been established for the process. These streams are declared in the header file `stdio.h'. - Variable: FILE * stdin The "standard input" stream, which is the normal source of input for the program. - Variable: FILE * stdout The "standard output" stream, which is used for normal output from the program. - Variable: FILE * stderr The "standard error" stream, which is used for error messages and diagnostics issued by the program. In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in *Note File System Interface::.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary. In the GNU C library, `stdin', `stdout', and `stderr' are normal variables which you can set just like any others. For example, to redirect the standard output to a file, you could do: fclose (stdout); stdout = fopen ("standard-output-file", "w"); Note however, that in other systems `stdin', `stdout', and `stderr' are macros that you cannot assign to in the normal way. But you can use `freopen' to get the effect of closing one and reopening it. *Note Opening Streams::. The three streams `stdin', `stdout', and `stderr' are not unoriented at program start (*note Streams and I18N::). Opening Streams =============== Opening a file with the `fopen' function creates a new stream and establishes a connection between the stream and a file. This may involve creating a new file. Everything described in this section is declared in the header file `stdio.h'. - Function: FILE * fopen (const char *FILENAME, const char *OPENTYPE) The `fopen' function opens a stream for I/O to the file FILENAME, and returns a pointer to the stream. The OPENTYPE argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters: `r' Open an existing file for reading only. `w' Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created. `a' Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created. `r+' Open an existing file for both reading and writing. The initial contents of the file are unchanged and the initial file position is at the beginning of the file. `w+' Open a file for both reading and writing. If the file already exists, it is truncated to zero length. Otherwise, a new file is created. `a+' Open or create file for both reading and appending. If the file exists, its initial contents are unchanged. Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file. As you can see, `+' requests a stream that can do both input and output. The ISO standard says that when using such a stream, you must call `fflush' (*note Stream Buffering::) or a file positioning function such as `fseek' (*note File Positioning::) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly. The GNU C library does not have this limitation; you can do arbitrary reading and writing operations on a stream in whatever order. Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems. The GNU C library defines one additional character for use in OPENTYPE: the character `x' insists on creating a new file--if a file FILENAME already exists, `fopen' fails rather than opening it. If you use `x' you are guaranteed that you will not clobber an existing file. This is equivalent to the `O_EXCL' option to the `open' function (*note Opening and Closing Files::). The character `b' in OPENTYPE has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. *Note Binary Streams::. If the OPENTYPE string contains the sequence `,ccs=STRING' then STRING is taken as the name of a coded character set and `fopen' will mark the stream as wide-oriented which appropriate conversion functions in place to convert from and to the character set STRING is place. Any other stream is opened initially unoriented and the orientation is decided with the first file operation. If the first operation is a wide character operation, the stream is not only marked as wide-oriented, also the conversion functions to convert to the coded character set used for the current locale are loaded. This will not change anymore from this point on even if the locale selected for the `LC_CTYPE' category is changed. Any other characters in OPENTYPE are simply ignored. They may be meaningful in other systems. If the open fails, `fopen' returns a null pointer. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `fopen64' since the LFS interface replaces transparently the old interface. You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. *Note Stream/Descriptor Precautions::. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. *Note File Locks::. - Function: FILE * fopen64 (const char *FILENAME, const char *OPENTYPE) This function is similar to `fopen' but the stream it returns a pointer for is opened using `open64'. Therefore this stream can be used even on files larger then 2^31 bytes on 32 bit machines. Please note that the return type is still `FILE *'. There is no special `FILE' type for the LFS interface. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fopen' and so transparently replaces the old interface. - Macro: int FOPEN_MAX The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously. You might be able to open more than this many streams, but that is not guaranteed. The value of this constant is at least eight, which includes the three standard streams `stdin', `stdout', and `stderr'. In POSIX.1 systems this value is determined by the `OPEN_MAX' parameter; *note General Limits::. In BSD and GNU, it is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. - Function: FILE * freopen (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is like a combination of `fclose' and `fopen'. It first closes the stream referred to by STREAM, ignoring any errors that are detected in the process. (Because errors are ignored, you should not use `freopen' on an output stream if you have actually done any output using the stream.) Then the file named by FILENAME is opened with mode OPENTYPE as for `fopen', and associated with the same stream object STREAM. If the operation fails, a null pointer is returned; otherwise, `freopen' returns STREAM. `freopen' has traditionally been used to connect a standard stream such as `stdin' with a file of your own choice. This is useful in programs in which use of a standard stream for certain purposes is hard-coded. In the GNU C library, you can simply close the standard streams and open new ones with `fopen'. But other systems lack this ability, so using `freopen' is more portable. When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is in fact `freopen64' since the LFS interface replaces transparently the old interface. - Function: FILE * freopen64 (const char *FILENAME, const char *OPENTYPE, FILE *STREAM) This function is similar to `freopen'. The only difference is that on 32 bit machine the stream returned is able to read beyond the 2^31 bytes limits imposed by the normal interface. It should be noted that the stream pointed to by STREAM need not be opened using `fopen64' or `freopen64' since its mode is not important for this function. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `freopen' and so transparently replaces the old interface. In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library. - Function: int __freadable (FILE *STREAM) The `__freadable' function determines whether the stream STREAM was opened to allow reading. In this case the return value is nonzero. For write-only streams the function returns zero. This function is declared in `stdio_ext.h'. - Function: int __fwritable (FILE *STREAM) The `__fwritable' function determines whether the stream STREAM was opened to allow writing. In this case the return value is nonzero. For read-only streams the function returns zero. This function is declared in `stdio_ext.h'. For slightly different kind of problems there are two more functions. They provide even finer-grained information. - Function: int __freading (FILE *STREAM) The `__freading' function determines whether the stream STREAM was last read from or whether it is opened read-only. In this case the return value is nonzero, otherwise it is zero. Determining whether a stream opened for reading and writing was last used for writing allows to draw conclusions about the content about the buffer, among other things. This function is declared in `stdio_ext.h'. - Function: int __fwriting (FILE *STREAM) The `__fwriting' function determines whether the stream STREAM was last written to or whether it is opened write-only. In this case the return value is nonzero, otherwise it is zero. This function is declared in `stdio_ext.h'. Closing Streams =============== When a stream is closed with `fclose', the connection between the stream and the file is canceled. After you have closed a stream, you cannot perform any additional operations on it. - Function: int fclose (FILE *STREAM) This function causes STREAM to be closed and the connection to the corresponding file to be broken. Any buffered output is written and any buffered input is discarded. The `fclose' function returns a value of `0' if the file was closed successfully, and `EOF' if an error was detected. It is important to check for errors when you call `fclose' to close an output stream, because real, everyday errors can be detected at this time. For example, when `fclose' writes the remaining buffered output, it might get an error because the disk is full. Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS. The function `fclose' is declared in `stdio.h'. To close all streams currently available the GNU C Library provides another function. - Function: int fcloseall (void) This function causes all open streams of the process to be closed and the connection to corresponding files to be broken. All buffered data is written and any buffered input is discarded. The `fcloseall' function returns a value of `0' if all the files were closed successfully, and `EOF' if an error was detected. This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (*note Standard Streams::) will also be closed. The function `fcloseall' is declared in `stdio.h'. If the `main' function to your program returns, or if you call the `exit' function (*note Normal Termination::), all open streams are automatically closed properly. If your program terminates in any other manner, such as by calling the `abort' function (*note Aborting a Program::) or from a fatal signal (*note Signal Handling::), open streams might not be closed properly. Buffered output might not be flushed and files may be incomplete. For more information on buffering of streams, see *Note Stream Buffering::. Streams and Threads =================== Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of a the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming. The POSIX standard requires that by default the stream operations are atomic. I.e., issuing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done. But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code. - Function: void flockfile (FILE *STREAM) The `flockfile' function acquires the internal locking object associated with the stream STREAM. This ensures that no other thread can explicitly through `flockfile'/`ftrylockfile' or implicit through a call of a stream function lock the stream. The thread will block until the lock is acquired. An explicit call to `funlockfile' has to be used to release the lock. - Function: int ftrylockfile (FILE *STREAM) The `ftrylockfile' function tries to acquire the internal locking object associated with the stream STREAM just like `flockfile'. But unlike `flockfile' this function does not block if the lock is not available. `ftrylockfile' returns zero if the lock was successfully acquired. Otherwise the stream is locked by another thread. - Function: void funlockfile (FILE *STREAM) The `funlockfile' function releases the internal locking object of the stream STREAM. The stream must have been locked before by a call to `flockfile' or a successful call of `ftrylockfile'. The implicit locking performed by the stream operations do not count. The `funlockfile' function does not return an error status and the behavior of a call for a stream which is not locked by the current thread is undefined. The following example shows how the functions above can be used to generate an output line atomically even in multi-threaded applications (yes, the same job could be done with one `fprintf' call but it is sometimes not possible): FILE *fp; { ... flockfile (fp); fputs ("This is test number ", fp); fprintf (fp, "%d\n", test); funlockfile (fp) } Without the explicit locking it would be possible for another thread to use the stream FP after the `fputs' call return and before `fprintf' was called with the result that the number does not follow the word `number'. From this description it might already be clear that the locking objects in streams are no simple mutexes. Since locking the same stream twice in the same thread is allowed the locking objects must be equivalent to recursive mutexes. These mutexes keep track of the owner and the number of times the lock is acquired. The same number of `funlockfile' calls by the same threads is necessary to unlock the stream completely. For instance: void foo (FILE *fp) { ftrylockfile (fp); fputs ("in foo\n", fp); /* This is very wrong!!! */ funlockfile (fp); } It is important here that the `funlockfile' function is only called if the `ftrylockfile' function succeeded in locking the stream. It is therefore always wrong to ignore the result of `ftrylockfile'. And it makes no sense since otherwise one would use `flockfile'. The result of code like that above is that either `funlockfile' tries to free a stream that hasn't been locked by the current thread or it frees the stream prematurely. The code should look like this: void foo (FILE *fp) { if (ftrylockfile (fp) == 0) { fputs ("in foo\n", fp); funlockfile (fp); } } Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations that are safe in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is not needed - because the code in question is never used in a context where two or more threads may use a stream at a time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking. There are two basic mechanisms to avoid locking. The first is to use the `_unlocked' variants of the stream operations. The POSIX standard defines quite a few of those and the GNU library adds a few more. These variants of the functions behave just like the functions with the name without the suffix except that they do not lock the stream. Using these functions is very desirable since they are potentially much faster. This is not only because the locking operation itself is avoided. More importantly, functions like `putc' and `getc' are very simple and traditionally (before the introduction of threads) were implemented as macros which are very fast if the buffer is not empty. With the addition of locking requirements these functions are no longer implemented as macros since they would would expand to too much code. But these macros are still available with the same functionality under the new names `putc_unlocked' and `getc_unlocked'. This possibly huge difference of speed also suggests the use of the `_unlocked' functions even if locking is required. The difference is that the locking then has to be performed in the program: void foo (FILE *fp, char *buf) { flockfile (fp); while (*buf != '/') putc_unlocked (*buf++, fp); funlockfile (fp); } If in this example the `putc' function would be used and the explicit locking would be missing the `putc' function would have to acquire the lock in every call, potentially many times depending on when the loop terminates. Writing it the way illustrated above allows the `putc_unlocked' macro to be used which means no locking and direct manipulation of the buffer of the stream. A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well. - Function: int __fsetlocking (FILE *STREAM, int TYPE) The `__fsetlocking' function can be used to select whether the stream operations will implicitly acquire the locking object of the stream STREAM. By default this is done but it can be disabled and reinstated using this function. There are three values defined for the TYPE parameter. `FSETLOCKING_INTERNAL' The stream `stream' will from now on use the default internal locking. Every stream operation with exception of the `_unlocked' variants will implicitly lock the stream. `FSETLOCKING_BYCALLER' After the `__fsetlocking' function returns the user is responsible for locking the stream. None of the stream operations will implicitly do this anymore until the state is set back to `FSETLOCKING_INTERNAL'. `FSETLOCKING_QUERY' `__fsetlocking' only queries the current locking state of the stream. The return value will be `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state. The return value of `__fsetlocking' is either `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the state of the stream before the call. This function and the values for the TYPE parameter are declared in `stdio_ext.h'. This function is especially useful when program code has to be used which is written without knowledge about the `_unlocked' functions (or if the programmer was too lazy to use them). Streams in Internationalized Applications ========================================= ISO C90 introduced the new type `wchar_t' to allow handling larger character sets. What was missing was a possibility to output strings of `wchar_t' directly. One had to convert them into multibyte strings using `mbstowcs' (there was no `mbsrtowcs' yet) and then use the normal stream functions. While this is doable it is very cumbersome since performing the conversions is not trivial and greatly increases program complexity and size. The Unix standard early on (I think in XPG4.2) introduced two additional format specifiers for the `printf' and `scanf' families of functions. Printing and reading of single wide characters was made possible using the `%C' specifier and wide character strings can be handled with `%S'. These modifiers behave just like `%c' and `%s' only that they expect the corresponding argument to have the wide character type and that the wide character and string are transformed into/from multibyte strings before being used. This was a beginning but it is still not good enough. Not always is it desirable to use `printf' and `scanf'. The other, smaller and faster functions cannot handle wide characters. Second, it is not possible to have a format string for `printf' and `scanf' consisting of wide characters. The result is that format strings would have to be generated if they have to contain non-basic characters. In the Amendment 1 to ISO C90 a whole new set of functions was added to solve the problem. Most of the stream functions got a counterpart which take a wide character or wide character string instead of a character or string respectively. The new functions operate on the same streams (like `stdout'). This is different from the model of the C++ runtime library where separate streams for wide and normal I/O are used. Being able to use the same stream for wide and normal operations comes with a restriction: a stream can be used either for wide operations or for normal operations. Once it is decided there is no way back. Only a call to `freopen' or `freopen64' can reset the "orientation". The orientation can be decided in three ways: * If any of the normal character functions is used (this includes the `fread' and `fwrite' functions) the stream is marked as not wide oriented. * If any of the wide character functions is used the stream is marked as wide oriented. * The `fwide' function can be used to set the orientation either way. It is important to never mix the use of wide and not wide operations on a stream. There are no diagnostics issued. The application behavior will simply be strange or the application will simply crash. The `fwide' function can help avoiding this. - Function: int fwide (FILE *STREAM, int MODE) The `fwide' function can be used to set and query the state of the orientation of the stream STREAM. If the MODE parameter has a positive value the streams get wide oriented, for negative values narrow oriented. It is not possible to overwrite previous orientations with `fwide'. I.e., if the stream STREAM was already oriented before the call nothing is done. If MODE is zero the current orientation state is queried and nothing is changed. The `fwide' function returns a negative value, zero, or a positive value if the stream is narrow, not at all, or wide oriented respectively. This function was introduced in Amendment 1 to ISO C90 and is declared in `wchar.h'. It is generally a good idea to orient a stream as early as possible. This can prevent surprise especially for the standard streams `stdin', `stdout', and `stderr'. If some library function in some situations uses one of these streams and this use orients the stream in a different way the rest of the application expects it one might end up with hard to reproduce errors. Remember that no errors are signal if the streams are used incorrectly. Leaving a stream unoriented after creation is normally only necessary for library functions which create streams which can be used in different contexts. When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this. void print_f (FILE *fp) { if (fwide (fp, 0) > 0) /* Positive return value means wide orientation. */ fputwc (L'f', fp); else fputc ('f', fp); } Note that in this case the function `print_f' decides about the orientation of the stream if it was unoriented before (will not happen if the advise above is followed). The encoding used for the `wchar_t' values is unspecified and the user must not make any assumptions about it. For I/O of `wchar_t' values this means that it is impossible to write these values directly to the stream. This is not what follows from the ISO C locale model either. What happens instead is that the bytes read from or written to the underlying media are first converted into the internal encoding chosen by the implementation for `wchar_t'. The external encoding is determined by the `LC_CTYPE' category of the current locale or by the `ccs' part of the mode specification given to `fopen', `fopen64', `freopen', or `freopen64'. How and when the conversion happens is unspecified and it happens invisible to the user. Since a stream is created in the unoriented state it has at that point no conversion associated with it. The conversion which will be used is determined by the `LC_CTYPE' category selected at the time the stream is oriented. If the locales are changed at the runtime this might produce surprising results unless one pays attention. This is just another good reason to orient the stream explicitly as soon as possible, perhaps with a call to `fwide'. Simple Output by Characters or Lines ==================================== This section describes functions for performing character- and line-oriented output. These narrow streams functions are declared in the header file `stdio.h' and the wide stream functions in `wchar.h'. - Function: int fputc (int C, FILE *STREAM) The `fputc' function converts the character C to type `unsigned char', and writes it to the stream STREAM. `EOF' is returned if a write error occurs; otherwise the character C is returned. - Function: wint_t fputwc (wchar_t WC, FILE *STREAM) The `fputwc' function writes the wide character WC to the stream STREAM. `WEOF' is returned if a write error occurs; otherwise the character WC is returned. - Function: int fputc_unlocked (int C, FILE *STREAM) The `fputc_unlocked' function is equivalent to the `fputc' function except that it does not implicitly lock the stream. - Function: wint_t fputwc_unlocked (wint_t WC, FILE *STREAM) The `fputwc_unlocked' function is equivalent to the `fputwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putc (int C, FILE *STREAM) This is just like `fputc', except that most systems implement it as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putc' is usually the best function to use for writing a single character. - Function: wint_t putwc (wchar_t WC, FILE *STREAM) This is just like `fputwc', except that it can be implement as a macro, making it faster. One consequence is that it may evaluate the STREAM argument more than once, which is an exception to the general rule for macros. `putwc' is usually the best function to use for writing a single wide character. - Function: int putc_unlocked (int C, FILE *STREAM) The `putc_unlocked' function is equivalent to the `putc' function except that it does not implicitly lock the stream. - Function: wint_t putwc_unlocked (wchar_t WC, FILE *STREAM) The `putwc_unlocked' function is equivalent to the `putwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int putchar (int C) The `putchar' function is equivalent to `putc' with `stdout' as the value of the STREAM argument. - Function: wint_t putwchar (wchar_t WC) The `putwchar' function is equivalent to `putwc' with `stdout' as the value of the STREAM argument. - Function: int putchar_unlocked (int C) The `putchar_unlocked' function is equivalent to the `putchar' function except that it does not implicitly lock the stream. - Function: wint_t putwchar_unlocked (wchar_t WC) The `putwchar_unlocked' function is equivalent to the `putwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputs (const char *S, FILE *STREAM) The function `fputs' writes the string S to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `EOF' if a write error occurs, and otherwise a non-negative value. For example: fputs ("Are ", stdout); fputs ("you ", stdout); fputs ("hungry?\n", stdout); outputs the text `Are you hungry?' followed by a newline. - Function: int fputws (const wchar_t *WS, FILE *STREAM) The function `fputws' writes the wide character string WS to the stream STREAM. The terminating null character is not written. This function does _not_ add a newline character, either. It outputs only the characters in the string. This function returns `WEOF' if a write error occurs, and otherwise a non-negative value. - Function: int fputs_unlocked (const char *S, FILE *STREAM) The `fputs_unlocked' function is equivalent to the `fputs' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int fputws_unlocked (const wchar_t *WS, FILE *STREAM) The `fputws_unlocked' function is equivalent to the `fputws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int puts (const char *S) The `puts' function writes the string S to the stream `stdout' followed by a newline. The terminating null character of the string is not written. (Note that `fputs' does _not_ write a newline as this function does.) `puts' is the most convenient function for printing simple messages. For example: puts ("This is a message."); outputs the text `This is a message.' followed by a newline. - Function: int putw (int W, FILE *STREAM) This function writes the word W (that is, an `int') to STREAM. It is provided for compatibility with SVID, but we recommend you use `fwrite' instead (*note Block Input/Output::). Character Input =============== This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file `stdio.h' and the wide character functions are declared in `wchar.h'. These functions return an `int' or `wint_t' value (for narrow and wide stream functions respectively) that is either a character of input, or the special value `EOF'/`WEOF' (usually -1). For the narrow stream functions it is important to store the result of these functions in a variable of type `int' instead of `char', even when you plan to use it only as a character. Storing `EOF' in a `char' variable truncates its value to the size of a character, so that it is no longer distinguishable from the valid character `(char) -1'. So always use an `int' for the result of `getc' and friends, and check for `EOF' after the call; once you've verified that the result is not `EOF', you can be sure that it will fit in a `char' variable without loss of information. - Function: int fgetc (FILE *STREAM) This function reads the next character as an `unsigned char' from the stream STREAM and returns its value, converted to an `int'. If an end-of-file condition or read error occurs, `EOF' is returned instead. - Function: wint_t fgetwc (FILE *STREAM) This function reads the next wide character from the stream STREAM and returns its value. If an end-of-file condition or read error occurs, `WEOF' is returned instead. - Function: int fgetc_unlocked (FILE *STREAM) The `fgetc_unlocked' function is equivalent to the `fgetc' function except that it does not implicitly lock the stream. - Function: wint_t fgetwc_unlocked (FILE *STREAM) The `fgetwc_unlocked' function is equivalent to the `fgetwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getc (FILE *STREAM) This is just like `fgetc', except that it is permissible (and typical) for it to be implemented as a macro that evaluates the STREAM argument more than once. `getc' is often highly optimized, so it is usually the best function to use to read a single character. - Function: wint_t getwc (FILE *STREAM) This is just like `fgetwc', except that it is permissible for it to be implemented as a macro that evaluates the STREAM argument more than once. `getwc' can be highly optimized, so it is usually the best function to use to read a single wide character. - Function: int getc_unlocked (FILE *STREAM) The `getc_unlocked' function is equivalent to the `getc' function except that it does not implicitly lock the stream. - Function: wint_t getwc_unlocked (FILE *STREAM) The `getwc_unlocked' function is equivalent to the `getwc' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: int getchar (void) The `getchar' function is equivalent to `getc' with `stdin' as the value of the STREAM argument. - Function: wint_t getwchar (void) The `getwchar' function is equivalent to `getwc' with `stdin' as the value of the STREAM argument. - Function: int getchar_unlocked (void) The `getchar_unlocked' function is equivalent to the `getchar' function except that it does not implicitly lock the stream. - Function: wint_t getwchar_unlocked (void) The `getwchar_unlocked' function is equivalent to the `getwchar' function except that it does not implicitly lock the stream. This function is a GNU extension. Here is an example of a function that does input using `fgetc'. It would work just as well using `getc' instead, or using `getchar ()' instead of `fgetc (stdin)'. The code would also work the same for the wide character stream functions. int y_or_n_p (const char *question) { fputs (question, stdout); while (1) { int c, answer; /* Write a space to separate answer from question. */ fputc (' ', stdout); /* Read the first character of the line. This should be the answer character, but might not be. */ c = tolower (fgetc (stdin)); answer = c; /* Discard rest of input line. */ while (c != '\n' && c != EOF) c = fgetc (stdin); /* Obey the answer if it was valid. */ if (answer == 'y') return 1; if (answer == 'n') return 0; /* Answer was invalid: ask for valid answer. */ fputs ("Please answer y or n:", stdout); } } - Function: int getw (FILE *STREAM) This function reads a word (that is, an `int') from STREAM. It's provided for compatibility with SVID. We recommend you use `fread' instead (*note Block Input/Output::). Unlike `getc', any `int' value could be a valid result. `getw' returns `EOF' when it encounters end-of-file or an error, but there is no way to distinguish this from an input word with value -1. Line-Oriented Input =================== Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream. Standard C has functions to do this, but they aren't very safe: null characters and even (for `gets') long lines can confuse them. So the GNU library provides the nonstandard `getline' function that makes it easy to read lines reliably. Another GNU extension, `getdelim', generalizes `getline'. It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character. All these functions are declared in `stdio.h'. - Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM) This function reads an entire line from STREAM, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in `*LINEPTR'. Before calling `getline', you should place in `*LINEPTR' the address of a buffer `*N' bytes long, allocated with `malloc'. If this buffer is long enough to hold the line, `getline' stores the line in this buffer. Otherwise, `getline' makes the buffer bigger using `realloc', storing the new buffer address back in `*LINEPTR' and the increased size back in `*N'. *Note Unconstrained Allocation::. If you set `*LINEPTR' to a null pointer, and `*N' to zero, before the call, then `getline' allocates the initial buffer for you by calling `malloc'. In either case, when `getline' returns, `*LINEPTR' is a `char *' which points to the text of the line. When `getline' is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator. This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable. If an error occurs or end of file is reached without any bytes read, `getline' returns `-1'. - Function: ssize_t getdelim (char **LINEPTR, size_t *N, int DELIMITER, FILE *STREAM) This function is like `getline' except that the character which tells it to stop reading is not necessarily newline. The argument DELIMITER specifies the delimiter character; `getdelim' keeps reading until it sees that character (or end of file). The text is stored in LINEPTR, including the delimiter character and a terminating null. Like `getline', `getdelim' makes LINEPTR bigger if it isn't big enough. `getline' is in fact implemented in terms of `getdelim', just like this: ssize_t getline (char **lineptr, size_t *n, FILE *stream) { return getdelim (lineptr, n, '\n', stream); } - Function: char * fgets (char *S, int COUNT, FILE *STREAM) The `fgets' function reads characters from the stream STREAM up to and including a newline character and stores them in the string S, adding a null character to mark the end of the string. You must supply COUNT characters worth of space in S, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null character at the end of the string. If the system is already at end of file when you call `fgets', then the contents of the array S are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer S. *Warning:* If the input data has a null character, you can't tell. So don't use `fgets' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. We recommend using `getline' instead of `fgets'. - Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws' function reads wide characters from the stream STREAM up to and including a newline character and stores them in the string WS, adding a null wide character to mark the end of the string. You must supply COUNT wide characters worth of space in WS, but the number of characters read is at most COUNT - 1. The extra character space is used to hold the null wide character at the end of the string. If the system is already at end of file when you call `fgetws', then the contents of the array WS are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer WS. *Warning:* If the input data has a null wide character (which are null bytes in the input stream), you can't tell. So don't use `fgetws' unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. - Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM) The `fgets_unlocked' function is equivalent to the `fgets' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE *STREAM) The `fgetws_unlocked' function is equivalent to the `fgetws' function except that it does not implicitly lock the stream. This function is a GNU extension. - Deprecated function: char * gets (char *S) The function `gets' reads characters from the stream `stdin' up to the next newline character, and stores them in the string S. The newline character is discarded (note that this differs from the behavior of `fgets', which copies the newline character into the string). If `gets' encounters a read error or end-of-file, it returns a null pointer; otherwise it returns S. *Warning:* The `gets' function is *very dangerous* because it provides no protection against overflowing the string S. The GNU library includes it for compatibility only. You should *always* use `fgets' or `getline' instead. To remind you of this, the linker (if using GNU `ld') will issue a warning whenever you use `gets'. Unreading ========= In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called "peeking ahead" at the input because your program gets a glimpse of the input it will read next. Using stream I/O, you can peek ahead at input by first reading it and then "unreading" it (also called "pushing it back" on the stream). Unreading a character makes it available to be input again from the stream, by the next call to `fgetc' or other input function on that stream. What Unreading Means -------------------- Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this: f o o b a r ^ so the next input character will be `b'. If instead of reading `b' you unread the letter `o', you get a situation like this: f o o b a r | o-- ^ so that the next input characters will be `o' and `b'. If you unread `9' instead of `o', you get this situation: f o o b a r | 9-- ^ so that the next input characters will be `9' and `b'. Using `ungetc' To Do Unreading ------------------------------ The function to unread a character is called `ungetc', because it reverses the action of `getc'. - Function: int ungetc (int C, FILE *STREAM) The `ungetc' function pushes back the character C onto the input stream STREAM. So the next input from STREAM will read C before anything else. If C is `EOF', `ungetc' does nothing and just returns `EOF'. This lets you call `ungetc' with the return value of `getc' without needing to check for an error from `getc'. The character that you push back doesn't have to be the same as the last character that was actually read from the stream. In fact, it isn't necessary to actually read any characters from the stream before unreading them with `ungetc'! But that is a strange way to write a program; usually `ungetc' is used only to unread a character that was just read from the same stream. The GNU C library only supports one character of pushback--in other words, it does not work to call `ungetc' twice without doing input in between. Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed. Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected. If a file positioning function (such as `fseek', `fseeko' or `rewind'; *note File Positioning::) is called, any pending pushed-back characters are discarded. Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file. - Function: wint_t ungetwc (wint_t WC, FILE *STREAM) The `ungetwc' function behaves just like `ungetc' just that it pushes back a wide character. Here is an example showing the use of `getc' and `ungetc' to skip over whitespace characters. When this function reaches a non-whitespace character, it unreads that character to be seen again on the next read operation on the stream. #include #include void skip_whitespace (FILE *stream) { int c; do /* No need to check for `EOF' because it is not `isspace', and `ungetc' ignores `EOF'. */ c = getc (stream); while (isspace (c)); ungetc (c, stream); } Block Input/Output ================== This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines. Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory--not just character or string objects--can be written to a binary file, and meaningfully read in again by the same program. Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers. These functions are declared in `stdio.h'. - Function: size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function reads up to COUNT objects of size SIZE into the array DATA, from the stream STREAM. It returns the number of objects actually read, which might be less than COUNT if a read error occurs or the end of the file is reached. This function returns a value of zero (and doesn't read anything) if either SIZE or COUNT is zero. If `fread' encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object. Therefore, the stream remains at the actual end of the file. - Function: size_t fread_unlocked (void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fread_unlocked' function is equivalent to the `fread' function except that it does not implicitly lock the stream. This function is a GNU extension. - Function: size_t fwrite (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) This function writes up to COUNT objects of size SIZE from the array DATA, to the stream STREAM. The return value is normally COUNT, if the call succeeds. Any other value indicates some sort of error, such as running out of space. - Function: size_t fwrite_unlocked (const void *DATA, size_t SIZE, size_t COUNT, FILE *STREAM) The `fwrite_unlocked' function is equivalent to the `fwrite' function except that it does not implicitly lock the stream. This function is a GNU extension. Formatted Output ================ The functions described in this section (`printf' and related functions) provide a convenient way to perform formatted output. You call `printf' with a "format string" or "template string" that specifies how to format the values of the remaining arguments. Unless your program is a filter that specifically performs line- or character-oriented processing, using `printf' or one of the other related functions described in this section is usually the easiest and most concise way to perform output. These functions are especially useful for printing error messages, tables of data, and the like. Formatted Output Basics ----------------------- The `printf' function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them. Ordinary characters in the template string are simply written to the output stream as-is, while "conversion specifications" introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example, int pct = 37; char filename[] = "foo.txt"; printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", filename, pct); produces output like Processing of `foo.txt' is 37% finished. Please be patient. This example shows the use of the `%d' conversion to specify that an `int' argument should be printed in decimal notation, the `%s' conversion to specify printing of a string argument, and the `%%' conversion to print a literal `%' character. There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c'). Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number. You can control formatting more precisely by writing "modifiers" between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field. The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look "prettier" in tables. Output Conversion Syntax ------------------------ This section provides details about the precise syntax of conversion specifications that can appear in a `printf' template string. Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (*note Character Set Handling::) are permitted in a template string. The conversion specifications in a `printf' template string have the general form: % [ PARAM-NO $] FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION For example, in the conversion specifier `%-10.8ld', the `-' is a flag, `10' specifies the field width, the precision is `8', the letter `l' is a type modifier, and `d' specifies the conversion style. (This particular type specifier says to print a `long int' argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.) In more detail, output conversion specifications consist of an initial `%' character followed in sequence by: * An optional specification of the parameter used for this format. Normally the parameters to the `printf' function are assigned to the formats in the order of appearance in the format string. But in some situations (such as message translation) this is not desirable and this extension allows an explicit parameter to be specified. The PARAM-NO part of the format must be an integer in the range of 1 to the maximum number of arguments present to the function call. Some implementations limit this number to a certainly upper bound. The exact limit can be retrieved by the following constant. - Macro: NL_ARGMAX The value of `ARGMAX' is the maximum value allowed for the specification of an positional parameter in a `printf' call. The actual value in effect at runtime can be retrieved by using `sysconf' using the `_SC_NL_ARGMAX' parameter *note Sysconf Definition::. Some system have a quite low limit such as 9 for System V systems. The GNU C library has no real limit. If any of the formats has a specification for the parameter position all of them in the format string shall have one. Otherwise the behavior is undefined. * Zero or more "flag characters" that modify the normal behavior of the conversion specification. * An optional decimal integer specifying the "minimum field width". If the normal conversion produces fewer characters than this, the field is padded with spaces to the specified width. This is a _minimum_ value; if the normal conversion produces more characters than this, the field is _not_ truncated. Normally, the output is right-justified within the field. You can also specify a field width of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the field width. The value must be an `int'. If the value is negative, this means to set the `-' flag (see below) and to use the absolute value as the field width. * An optional "precision" to specify the number of digits to be written for the numeric conversions. If the precision is specified, it consists of a period (`.') followed optionally by a decimal integer (which defaults to zero if omitted). You can also specify a precision of `*'. This means that the next argument in the argument list (before the actual value to be printed) is used as the precision. The value must be an `int', and is ignored if it is negative. If you specify `*' for both the field width and precision, the field width argument precedes the precision argument. Other C library versions may not recognize this syntax. * An optional "type modifier character", which is used to specify the data type of the corresponding argument if it differs from the default type. (For example, the integer conversions assume a type of `int', but you can specify `h', `l', or `L' for other integer types.) * A character that specifies the conversion to be applied. The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use. With the `-Wformat' option, the GNU C compiler checks calls to `printf' and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a `printf'-style format string. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information. Table of Output Conversions --------------------------- Here is a table summarizing what all the different conversions do: `%d', `%i' Print an integer as a signed decimal number. *Note Integer Conversions::, for details. `%d' and `%i' are synonymous for output, but are different when used with `scanf' for input (*note Table of Input Conversions::). `%o' Print an integer as an unsigned octal number. *Note Integer Conversions::, for details. `%u' Print an integer as an unsigned decimal number. *Note Integer Conversions::, for details. `%x', `%X' Print an integer as an unsigned hexadecimal number. `%x' uses lower-case letters and `%X' uses upper-case. *Note Integer Conversions::, for details. `%f' Print a floating-point number in normal (fixed-point) notation. *Note Floating-Point Conversions::, for details. `%e', `%E' Print a floating-point number in exponential notation. `%e' uses lower-case letters and `%E' uses upper-case. *Note Floating-Point Conversions::, for details. `%g', `%G' Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude. `%g' uses lower-case letters and `%G' uses upper-case. *Note Floating-Point Conversions::, for details. `%a', `%A' Print a floating-point number in a hexadecimal fractional notation which the exponent to base 2 represented in decimal digits. `%a' uses lower-case letters and `%A' uses upper-case. *Note Floating-Point Conversions::, for details. `%c' Print a single character. *Note Other Output Conversions::. `%C' This is an alias for `%lc' which is supported for compatibility with the Unix standard. `%s' Print a string. *Note Other Output Conversions::. `%S' This is an alias for `%ls' which is supported for compatibility with the Unix standard. `%p' Print the value of a pointer. *Note Other Output Conversions::. `%n' Get the number of characters printed so far. *Note Other Output Conversions::. Note that this conversion specification never produces any output. `%m' Print the string corresponding to the value of `errno'. (This is a GNU extension.) *Note Other Output Conversions::. `%%' Print a literal `%' character. *Note Other Output Conversions::. If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful. Integer Conversions ------------------- This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats. The `%d' and `%i' conversion specifications both print an `int' argument as a signed decimal number; while `%o', `%u', and `%x' print the argument as an unsigned octal, decimal, or hexadecimal number (respectively). The `%X' conversion specification is just like `%x' except that it uses the characters `ABCDEF' as digits instead of `abcdef'. The following flags are meaningful: `-' Left-justify the result in the field (instead of the normal right-justification). `+' For the signed `%d' and `%i' conversions, print a plus sign if the value is positive. ` ' For the signed `%d' and `%i' conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' For the `%o' conversion, this forces the leading digit to be `0', as if by increasing the precision. For `%x' or `%X', this prefixes a leading `0x' or `0X' (respectively) to the result. This doesn't do anything useful for the `%d', `%i', or `%u' conversions. Using this flag produces output which can be parsed by the `strtoul' function (*note Parsing of Integers::) and `scanf' with the `%i' conversion (*note Numeric Input Conversions::). `'' Separate the digits into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces. The zeros are placed after any indication of sign or base. This flag is ignored if the `-' flag is also specified, or if a precision is specified. If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced. Without a type modifier, the corresponding argument is treated as an `int' (for the signed conversions `%i' and `%d') or `unsigned int' (for the unsigned conversions `%o', `%u', `%x', and `%X'). Recall that since `printf' and friends are variadic, any `char' and `short' arguments are automatically converted to `int' by the default argument promotions. For arguments of other integer types, you can use these modifiers: `hh' Specifies that the argument is a `signed char' or `unsigned char', as appropriate. A `char' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `char' again. This modifier was introduced in ISO C99. `h' Specifies that the argument is a `short int' or `unsigned short int', as appropriate. A `short' argument is converted to an `int' or `unsigned int' by the default argument promotions anyway, but the `h' modifier says to convert it back to a `short' again. `j' Specifies that the argument is a `intmax_t' or `uintmax_t', as appropriate. This modifier was introduced in ISO C99. `l' Specifies that the argument is a `long int' or `unsigned long int', as appropriate. Two `l' characters is like the `L' modifier, below. If used with `%c' or `%s' the corresponding parameter is considered as a wide character or wide character string respectively. This use of `l' was introduced in Amendment 1 to ISO C90. `L' `ll' `q' Specifies that the argument is a `long long int'. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as `long int'.) The `q' modifier is another name for the same thing, which comes from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'. `t' Specifies that the argument is a `ptrdiff_t'. This modifier was introduced in ISO C99. `z' `Z' Specifies that the argument is a `size_t'. `z' was introduced in ISO C99. `Z' is a GNU extension predating this addition and should not be used in new code. Here is an example. Using the template string: "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n" to print numbers using the different options for the `%d' conversion gives results like: | 0|0 | +0|+0 | 0|00000| | 00|0| | 1|1 | +1|+1 | 1|00001| 1| 01|1| | -1|-1 | -1|-1 | -1|-0001| -1| -01|-1| |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000| In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified. Here are some more examples showing how unsigned integers print under various format options, using the template string: "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n" | 0| 0| 0| 0| 0| 0| 0| 00000000| | 1| 1| 1| 1| 01| 0x1| 0X1|0x00000001| |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0| Floating-Point Conversions -------------------------- This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions. The `%f' conversion prints its argument in fixed-point notation, producing output of the form [`-']DDD`.'DDD, where the number of digits following the decimal point is controlled by the precision you specify. The `%e' conversion prints its argument in exponential notation, producing output of the form [`-']D`.'DDD`e'[`+'|`-']DD. Again, the number of digits following the decimal point is controlled by the precision. The exponent always contains at least two digits. The `%E' conversion is similar but the exponent is marked with the letter `E' instead of `e'. The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. A precision of `0', is taken as 1. is Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit. The `%a' and `%A' conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines. The numbers are represented is the form [`-']`0x'H`.'HHH`p'[`+'|`-']DD. At the left of the decimal-point character exactly one digit is print. This character is only `0' if the number is denormalized. Otherwise the value is unspecified; it is implementation dependent how many bits are used. The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision. If the precision is zero it is determined to be large enough to provide an exact representation of the number (or it is large enough to distinguish two adjacent values if the `FLT_RADIX' is not a power of 2, *note Floating Point Parameters::). For the `%a' conversion lower-case characters are used to represent the hexadecimal number and the prefix and exponent sign are printed as `0x' and `p' respectively. Otherwise upper-case characters are used and `0X' and `P' are used for the representation of prefix and exponent string. The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly. If the value to be printed represents infinity or a NaN, the output is [`-']`inf' or `nan' respectively if the conversion specifier is `%a', `%e', `%f', or `%g' and it is [`-']`INF' or `NAN' respectively if the conversion is `%A', `%E', or `%G'. The following flags can be used to modify the behavior: `-' Left-justify the result in the field. Normally the result is right-justified. `+' Always include a plus or minus sign in the result. ` ' If the result doesn't start with a plus or minus sign, prefix it with a space instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them. `#' Specifies that the result should always include a decimal point, even if no digits follow it. For the `%g' and `%G' conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed. `'' Separate the digits of the integer part of the result into groups as specified by the locale specified for the `LC_NUMERIC' category; *note General Numeric::. This flag is a GNU extension. `0' Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the `-' flag is also specified. The precision specifies how many digits follow the decimal-point character for the `%f', `%e', and `%E' conversions. For these conversions, the default precision is `6'. If the precision is explicitly `0', this suppresses the decimal point character entirely. For the `%g' and `%G' conversions, the precision specifies how many significant digits to print. Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is `0' or not specified for `%g' or `%G', it is treated like a value of `1'. If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits. Without a type modifier, the floating-point conversions use an argument of type `double'. (By the default argument promotions, any `float' arguments are automatically converted to `double'.) The following type modifier is supported: `L' An uppercase `L' specifies that the argument is a `long double'. Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string: "|%13.4a|%13.4f|%13.4e|%13.4g|\n" Here is the output: | 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05| Notice how the `%g' conversion drops trailing zeros. Other Output Conversions ------------------------ This section describes miscellaneous conversions for `printf'. The `%c' conversion prints a single character. In case there is no `l' modifier the `int' argument is first converted to an `unsigned char'. Then, if used in a wide stream function, the character is converted into the corresponding wide character. The `-' flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example: printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o'); prints `hello'. If there is a `l' modifier present the argument is expected to be of type `wint_t'. If used in a multibyte function the wide character is converted into a multibyte character before being added to the output. In this case more than one output byte can be produced. The `%s' conversion prints a string. If no `l' modifier is present the corresponding argument must be of type `char *' (or `const char *'). If used in a wide stream function the string is first converted in a wide character string. A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream. The `-' flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example: printf ("%3s%-6s", "no", "where"); prints ` nowhere '. If there is a `l' modifier present the argument is expected to be of type `wchar_t' (or `const wchar_t *'). If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally. The `%m' conversion prints the string corresponding to the error code in `errno'. *Note Error Messages::. Thus: fprintf (stderr, "can't open `%s': %m\n", filename); is equivalent to: fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno)); The `%m' conversion is a GNU C library extension. The `%p' conversion prints a pointer value. The corresponding argument must be of type `void *'. In practice, you can use any type of pointer. In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.) For example: printf ("%p", "testing"); prints `0x' followed by a hexadecimal number--the address of the string constant `"testing"'. It does not print the word `testing'. You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined. The `%n' conversion is unlike any of the other output conversions. It uses an argument which must be a pointer to an `int', but instead of printing anything it stores the number of characters printed so far by this call at that location. The `h' and `l' type modifiers are permitted to specify that the argument is of type `short int *' or `long int *' instead of `int *', but no flags, field width, or precision are permitted. For example, int nchar; printf ("%d %s%n\n", 3, "bears", &nchar); prints: 3 bears and sets `nchar' to `7', because `3 bears' is seven characters. The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted. Formatted Output Functions -------------------------- This section describes how to call `printf' and related functions. Prototypes for these functions are in the header file `stdio.h'. Because these functions take a variable number of arguments, you _must_ declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include `stdio.h'. - Function: int printf (const char *TEMPLATE, ...) The `printf' function prints the optional arguments under the control of the template string TEMPLATE to the stream `stdout'. It returns the number of characters printed, or a negative value if there was an output error. - Function: int wprintf (const wchar_t *TEMPLATE, ...) The `wprintf' function prints the optional arguments under the control of the wide template string TEMPLATE to the stream `stdout'. It returns the number of wide characters printed, or a negative value if there was an output error. - Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `printf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wprintf', except that the output is written to the stream STREAM instead of `stdout'. - Function: int sprintf (char *S, const char *TEMPLATE, ...) This is like `printf', except that the output is stored in the character array S instead of written to a stream. A null character is written to mark the end of the string. The `sprintf' function returns the number of characters stored in the array S, not including the terminating null character. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to be printed under control of the `%s' conversion. *Note Copying and Concatenation::. *Warning:* The `sprintf' function can be *dangerous* because it can potentially output more characters than can fit in the allocation size of the string S. Remember that the field width given in a conversion specification is only a _minimum_ value. To avoid this problem, you can use `snprintf' or `asprintf', described below. - Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, ...) This is like `wprintf', except that the output is stored in the wide character array WS instead of written to a stream. A null wide character is written to mark the end of the string. The SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE wide characters for the string WS. The return value is the number of characters generated for the given input, excluding the trailing null. If not all output fits into the provided buffer a negative value is returned. You should try again with a bigger output string. _Note:_ this is different from how `snprintf' handles this situation. Note that the corresponding narrow stream function takes fewer parameters. `swprintf' in fact corresponds to the `snprintf' function. Since the `sprintf' function can be dangerous and should be avoided the ISO C committee refused to make the same mistake again and decided to not define an function exactly corresponding to `sprintf'. - Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE, ...) The `snprintf' function is similar to `sprintf', except that the SIZE argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least SIZE characters for the string S. The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to SIZE, not all characters from the result have been stored in S. You should try again with a bigger output string. Here is an example of doing this: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { /* Guess we need no more than 100 chars of space. */ int size = 100; char *buffer = (char *) xmalloc (size); int nchars; if (buffer == NULL) return NULL; /* Try to print in the allocated space. */ nchars = snprintf (buffer, size, "value of %s is %s", name, value); if (nchars >= size) { /* Reallocate buffer now that we know how much space is needed. */ buffer = (char *) xrealloc (buffer, nchars + 1); if (buffer != NULL) /* Try again. */ snprintf (buffer, size, "value of %s is %s", name, value); } /* The last call worked, return the string. */ return buffer; } In practice, it is often easier just to use `asprintf', below. *Attention:* In versions of the GNU C library prior to 2.1 the return value is the number of characters stored, not including the terminating null; unless there was not enough space in S to store the result in which case `-1' is returned. This was changed in order to comply with the ISO C99 standard. Dynamically Allocating Formatted Output --------------------------------------- The functions in this section do formatted output and place the results in dynamically allocated memory. - Function: int asprintf (char **PTR, const char *TEMPLATE, ...) This function is similar to `sprintf', except that it dynamically allocates a string (as with `malloc'; *note Unconstrained Allocation::) to hold the output, instead of putting the output in a buffer you allocate in advance. The PTR argument should be the address of a `char *' object, and `asprintf' stores a pointer to the newly allocated string at that location. The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated. Here is how to use `asprintf' to get the same result as the `snprintf' example, but more easily: /* Construct a message describing the value of a variable whose name is NAME and whose value is VALUE. */ char * make_message (char *name, char *value) { char *result; if (asprintf (&result, "value of %s is %s", name, value) < 0) return NULL; return result; } - Function: int obstack_printf (struct obstack *OBSTACK, const char *TEMPLATE, ...) This function is similar to `asprintf', except that it uses the obstack OBSTACK to allocate the space. *Note Obstacks::. The characters are written onto the end of the current object. To get at them, you must finish the object with `obstack_finish' (*note Growing Objects::). Variable Arguments Output Functions ----------------------------------- The functions `vprintf' and friends are provided so that you can define your own variadic `printf'-like functions that make use of the same internals as the built-in formatted output functions. The most natural way to define such functions would be to use a language construct to say, "Call `printf' and pass this template plus all of my arguments after the first five." But there is no way to do this in C, and it would be hard to provide a way, since at the C language level there is no way to tell how many arguments your function received. Since that method is impossible, we provide alternative functions, the `vprintf' series, which lets you pass a `va_list' to describe "all of my arguments after the first five." When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example: #define myprintf(a, b, c, d, e, rest...) \ printf (mytemplate , ## rest...) *Note Macros with Variable Numbers of Arguments: (gcc.info)Macro Varargs, for details. But this is limited to macros, and does not apply to real functions at all. Before calling `vprintf' or the other functions listed in this section, you _must_ call `va_start' (*note Variadic Functions::) to initialize a pointer to the variable arguments. Then you can call `va_arg' to fetch the arguments that you want to handle yourself. This advances the pointer past those arguments. Once your `va_list' pointer is pointing at the argument of your choice, you are ready to call `vprintf'. That argument and all subsequent arguments that were passed to your function are used by `vprintf' along with the template that you specified separately. In some other systems, the `va_list' pointer may become invalid after the call to `vprintf', so you must not use `va_arg' after you call `vprintf'. Instead, you should call `va_end' to retire the pointer from service. However, you can safely call `va_start' on another pointer variable and begin fetching the arguments again through that pointer. Calling `vprintf' does not destroy the argument list of your function, merely the particular pointer that you passed to it. GNU C does not have such restrictions. You can safely continue to fetch arguments from a `va_list' pointer after passing it to `vprintf', and `va_end' is a no-op. (Note, however, that subsequent `va_arg' calls will fetch the same arguments which `vprintf' previously used.) Prototypes for these functions are declared in `stdio.h'. - Function: int vprintf (const char *TEMPLATE, va_list AP) This function is similar to `printf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vwprintf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wprintf' except that, instead of taking a variable number of arguments directly, it takes an argument list pointer AP. - Function: int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsprintf (char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vswprintf (wchar_t *S, size_t SIZE, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swprintf' with the variable argument list specified directly as for `vwprintf'. - Function: int vsnprintf (char *S, size_t SIZE, const char *TEMPLATE, va_list AP) This is the equivalent of `snprintf' with the variable argument list specified directly as for `vprintf'. - Function: int vasprintf (char **PTR, const char *TEMPLATE, va_list AP) The `vasprintf' function is the equivalent of `asprintf' with the variable argument list specified directly as for `vprintf'. - Function: int obstack_vprintf (struct obstack *OBSTACK, const char *TEMPLATE, va_list AP) The `obstack_vprintf' function is the equivalent of `obstack_printf' with the variable argument list specified directly as for `vprintf'. Here's an example showing how you might use `vfprintf'. This is a function that prints error messages to the stream `stderr', along with a prefix indicating the name of the program (*note Error Messages::, for a description of `program_invocation_short_name'). #include #include void eprintf (const char *template, ...) { va_list ap; extern char *program_invocation_short_name; fprintf (stderr, "%s: ", program_invocation_short_name); va_start (ap, template); vfprintf (stderr, template, ap); va_end (ap); } You could call `eprintf' like this: eprintf ("file `%s' does not exist\n", filename); In GNU C, there is a special construct you can use to let the compiler know that a function uses a `printf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For example, take this declaration of `eprintf': void eprintf (const char *template, ...) __attribute__ ((format (printf, 1, 2))); This tells the compiler that `eprintf' uses a format string like `printf' (as opposed to `scanf'; *note Formatted Input::); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. *Note Declaring Attributes of Functions: (gcc.info)Function Attributes, for more information. Parsing a Template String ------------------------- You can use the function `parse_printf_format' to obtain information about the number and types of arguments that are expected by a given template string. This function permits interpreters that provide interfaces to `printf' to avoid passing along invalid arguments from the user's program, which could cause a crash. All the symbols described in this section are declared in the header file `printf.h'. - Function: size_t parse_printf_format (const char *TEMPLATE, size_t N, int *ARGTYPES) This function returns information about the number and types of arguments expected by the `printf' template string TEMPLATE. The information is stored in the array ARGTYPES; each element of this array describes one argument. This information is encoded using the various `PA_' macros, listed below. The argument N specifies the number of elements in the array ARGTYPES. This is the maximum number of elements that `parse_printf_format' will try to write. `parse_printf_format' returns the total number of arguments required by TEMPLATE. If this number is greater than N, then the information returned describes only the first N arguments. If you want information about additional arguments, allocate a bigger array and call `parse_printf_format' again. The argument types are encoded as a combination of a basic type and modifier flag bits. - Macro: int PA_FLAG_MASK This macro is a bitmask for the type modifier flag bits. You can write the expression `(argtypes[i] & PA_FLAG_MASK)' to extract just the flag bits for an argument, or `(argtypes[i] & ~PA_FLAG_MASK)' to extract just the basic type code. Here are symbolic constants that represent the basic types; they stand for integer values. `PA_INT' This specifies that the base type is `int'. `PA_CHAR' This specifies that the base type is `int', cast to `char'. `PA_STRING' This specifies that the base type is `char *', a null-terminated string. `PA_POINTER' This specifies that the base type is `void *', an arbitrary pointer. `PA_FLOAT' This specifies that the base type is `float'. `PA_DOUBLE' This specifies that the base type is `double'. `PA_LAST' You can define additional base types for your own programs as offsets from `PA_LAST'. For example, if you have data types `foo' and `bar' with their own specialized `printf' conversions, you could define encodings for these types as: #define PA_FOO PA_LAST #define PA_BAR (PA_LAST + 1) Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or. `PA_FLAG_PTR' If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value. For example, `PA_INT|PA_FLAG_PTR' represents the type `int *'. `PA_FLAG_SHORT' If this bit is set, it indicates that the base type is modified with `short'. (This corresponds to the `h' type modifier.) `PA_FLAG_LONG' If this bit is set, it indicates that the base type is modified with `long'. (This corresponds to the `l' type modifier.) `PA_FLAG_LONG_LONG' If this bit is set, it indicates that the base type is modified with `long long'. (This corresponds to the `L' type modifier.) `PA_FLAG_LONG_DOUBLE' This is a synonym for `PA_FLAG_LONG_LONG', used by convention with a base type of `PA_DOUBLE' to indicate a type of `long double'. For an example of using these facilities, see *Note Example of Parsing::. Example of Parsing a Template String ------------------------------------ Here is an example of decoding argument types for a format string. We assume this is part of an interpreter which contains arguments of type `NUMBER', `CHAR', `STRING' and `STRUCTURE' (and perhaps others which are not valid here). /* Test whether the NARGS specified objects in the vector ARGS are valid for the format string FORMAT: if so, return 1. If not, return 0 after printing an error message. */ int validate_args (char *format, int nargs, OBJECT *args) { int *argtypes; int nwanted; /* Get the information about the arguments. Each conversion specification must be at least two characters long, so there cannot be more specifications than half the length of the string. */ argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int)); nwanted = parse_printf_format (string, nelts, argtypes); /* Check the number of arguments. */ if (nwanted > nargs) { error ("too few arguments (at least %d required)", nwanted); return 0; } /* Check the C type wanted for each argument and see if the object given is suitable. */ for (i = 0; i < nwanted; i++) { int wanted; if (argtypes[i] & PA_FLAG_PTR) wanted = STRUCTURE; else switch (argtypes[i] & ~PA_FLAG_MASK) { case PA_INT: case PA_FLOAT: case PA_DOUBLE: wanted = NUMBER; break; case PA_CHAR: wanted = CHAR; break; case PA_STRING: wanted = STRING; break; case PA_POINTER: wanted = STRUCTURE; break; } if (TYPE (args[i]) != wanted) { error ("type mismatch for arg number %d", i); return 0; } } return 1; } Customizing `printf' ==================== The GNU C library lets you define your own custom conversion specifiers for `printf' template strings, to teach `printf' clever ways to print the important data structures of your program. The way you do this is by registering the conversion with the function `register_printf_function'; see *Note Registering New Conversions::. One of the arguments you pass to this function is a pointer to a handler function that produces the actual output; see *Note Defining the Output Handler::, for information on how to write this function. You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. *Note Parsing a Template String::, for information about this. The facilities of this section are declared in the header file `printf.h'. *Portability Note:* The ability to extend the syntax of `printf' template strings is a GNU extension. ISO standard C has nothing similar. Registering New Conversions --------------------------- The function to register a new output conversion is `register_printf_function', declared in `printf.h'. - Function: int register_printf_function (int SPEC, printf_function HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION) This function defines the conversion specifier character SPEC. Thus, if SPEC is `'Y'', it defines the conversion `%Y'. You can redefine the built-in conversions like `%s', but flag characters like `#' and type modifiers like `l' can never be used as conversions; calling `register_printf_function' for those characters has no effect. It is advisable not to use lowercase letters, since the ISO C standard warns that additional lowercase letters may be standardized in future editions of the standard. The HANDLER-FUNCTION is the function called by `printf' and friends when this conversion appears in a template string. *Note Defining the Output Handler::, for information about how to define a function to pass as this argument. If you specify a null pointer, any existing handler function for SPEC is removed. The ARGINFO-FUNCTION is the function called by `parse_printf_format' when this conversion appears in a template string. *Note Parsing a Template String::, for information about this. *Attention:* In the GNU C library versions before 2.0 the ARGINFO-FUNCTION function did not need to be installed unless the user used the `parse_printf_format' function. This has changed. Now a call to any of the `printf' functions will call this function when this format specifier appears in the format string. The return value is `0' on success, and `-1' on failure (which occurs if SPEC is out of range). You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this. Conversion Specifier Options ---------------------------- If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template. Both the HANDLER-FUNCTION and ARGINFO-FUNCTION accept an argument that points to a `struct printf_info', which contains information about the options appearing in an instance of the conversion specifier. This data type is declared in the header file `printf.h'. - Type: struct printf_info This structure is used to pass information about the options appearing in an instance of a conversion specifier in a `printf' template string to the handler and arginfo functions for that specifier. It contains the following members: `int prec' This is the precision specified. The value is `-1' if no precision was specified. If the precision was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `int width' This is the minimum field width specified. The value is `0' if no width was specified. If the field width was given as `*', the `printf_info' structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of `INT_MIN', since the actual value is not known. `wchar_t spec' This is the conversion specifier character specified. It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called. `unsigned int is_long_double' This is a boolean that is true if the `L', `ll', or `q' type modifier was specified. For integer conversions, this indicates `long long int', as opposed to `long double' for floating point conversions. `unsigned int is_char' This is a boolean that is true if the `hh' type modifier was specified. `unsigned int is_short' This is a boolean that is true if the `h' type modifier was specified. `unsigned int is_long' This is a boolean that is true if the `l' type modifier was specified. `unsigned int alt' This is a boolean that is true if the `#' flag was specified. `unsigned int space' This is a boolean that is true if the ` ' flag was specified. `unsigned int left' This is a boolean that is true if the `-' flag was specified. `unsigned int showsign' This is a boolean that is true if the `+' flag was specified. `unsigned int group' This is a boolean that is true if the `'' flag was specified. `unsigned int extra' This flag has a special meaning depending on the context. It could be used freely by the user-defined handlers but when called from the `printf' function this variable always contains the value `0'. `unsigned int wide' This flag is set if the stream is wide oriented. `wchar_t pad' This is the character to use for padding the output to the minimum field width. The value is `'0'' if the `0' flag was specified, and `' '' otherwise. Defining the Output Handler --------------------------- Now let's look at how to define the handler and arginfo functions which are passed as arguments to `register_printf_function'. *Compatibility Note:* The interface changed in GNU libc version 2.0. Previously the third argument was of type `va_list *'. You should define your handler functions with a prototype like: int FUNCTION (FILE *stream, const struct printf_info *info, const void *const *args) The STREAM argument passed to the handler function is the stream to which it should write output. The INFO argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. *Note Conversion Specifier Options::, for a description of this data structure. The ARGS is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user. Your handler function should return a value just like `printf' does: it should return the number of characters it has written, or a negative value to indicate an error. - Data Type: printf_function This is the data type that a handler function should have. If you are going to use `parse_printf_format' in your application, you must also define a function to pass as the ARGINFO-FUNCTION argument for each new conversion you install with `register_printf_function'. You have to define these functions with a prototype like: int FUNCTION (const struct printf_info *info, size_t n, int *argtypes) The return value from the function should be the number of arguments the conversion expects. The function should also fill in no more than N elements of the ARGTYPES array with information about the types of each of these arguments. This information is encoded using the various `PA_' macros. (You will notice that this is the same calling convention `parse_printf_format' itself uses.) - Data Type: printf_arginfo_function This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier. `printf' Extension Example -------------------------- Here is an example showing how to define a `printf' handler function. This program defines a data structure called a `Widget' and defines the `%W' conversion to print information about `Widget *' arguments, including the pointer value and the name stored in the data structure. The `%W' conversion supports the minimum field width and left-justification options, but ignores everything else. #include #include #include typedef struct { char *name; } Widget; int print_widget (FILE *stream, const struct printf_info *info, const void *const *args) { const Widget *w; char *buffer; int len; /* Format the output into a string. */ w = *((const Widget **) (args[0])); len = asprintf (&buffer, "", w, w->name); if (len == -1) return -1; /* Pad to the minimum field width and print to the stream. */ len = fprintf (stream, "%*s", (info->left ? -info->width : info->width), buffer); /* Clean up and return. */ free (buffer); return len; } int print_widget_arginfo (const struct printf_info *info, size_t n, int *argtypes) { /* We always take exactly one argument and this is a pointer to the structure.. */ if (n > 0) argtypes[0] = PA_POINTER; return 1; } int main (void) { /* Make a widget to print. */ Widget mywidget; mywidget.name = "mywidget"; /* Register the print function for widgets. */ register_printf_function ('W', print_widget, print_widget_arginfo); /* Now print the widget. */ printf ("|%W|\n", &mywidget); printf ("|%35W|\n", &mywidget); printf ("|%-35W|\n", &mywidget); return 0; } The output produced by this program looks like: || | | | | Predefined `printf' Handlers ---------------------------- The GNU libc also contains a concrete and useful application of the `printf' handler extension. There are two functions available which implement a special way to print floating-point numbers. - Function: int printf_size (FILE *FP, const struct printf_info *INFO, const void *const *ARGS) Print a given floating point number as for the format `%f' except that there is a postfix character indicating the divisor for the number to make this less than 1000. There are two possible divisors: powers of 1024 or powers of 1000. Which one is used depends on the format character specified while registered this handler. If the character is of lower case, 1024 is used. For upper case characters, 1000 is used. The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is: +------+--------------+--------+--------+---------------+ |low|Multiplier|From|Upper|Multiplier| +------+--------------+--------+--------+---------------+ |' '|1||' '|1| +------+--------------+--------+--------+---------------+ |k|2^10 (1024)|kilo|K|10^3 (1000)| +------+--------------+--------+--------+---------------+ |m|2^20|mega|M|10^6| +------+--------------+--------+--------+---------------+ |g|2^30|giga|G|10^9| +------+--------------+--------+--------+---------------+ |t|2^40|tera|T|10^12| +------+--------------+--------+--------+---------------+ |p|2^50|peta|P|10^15| +------+--------------+--------+--------+---------------+ |e|2^60|exa|E|10^18| +------+--------------+--------+--------+---------------+ |z|2^70|zetta|Z|10^21| +------+--------------+--------+--------+---------------+ |y|2^80|yotta|Y|10^24| +------+--------------+--------+--------+---------------+ The default precision is 3, i.e., 1024 is printed with a lower-case format character as if it were `%.3fk' and will yield `1.000k'. Due to the requirements of `register_printf_function' we must also provide the function which returns information about the arguments. - Function: int printf_size_info (const struct printf_info *INFO, size_t N, int *ARGTYPES) This function will return in ARGTYPES the information about the used parameters in the way the `vfprintf' implementation expects it. The format always takes one argument. To use these functions both functions must be registered with a call like register_printf_function ('B', printf_size, printf_size_info); Here we register the functions to print numbers as powers of 1000 since the format character `'B'' is an upper-case character. If we would additionally use `'b'' in a line like register_printf_function ('b', printf_size, printf_size_info); we could also print using a power of 1024. Please note that all that is different in these two lines is the format specifier. The `printf_size' function knows about the difference between lower and upper case format specifiers. The use of `'B'' and `'b'' is no coincidence. Rather it is the preferred way to use this functionality since it is available on some other systems which also use format specifiers. Formatted Input =============== The functions described in this section (`scanf' and related functions) provide facilities for formatted input analogous to the formatted output facilities. These functions provide a mechanism for reading arbitrary values under the control of a "format string" or "template string". Formatted Input Basics ---------------------- Calls to `scanf' are superficially similar to calls to `printf' in that arbitrary arguments are read under the control of a template string. While the syntax of the conversion specifications in the template is very similar to that for `printf', the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting. For example, most `scanf' conversions skip over any amount of "white space" (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions. Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream. Another area of difference between `scanf' and `printf' is that you must remember to supply pointers rather than immediate values as the optional arguments to `scanf'; the values that are read are stored in the objects that the pointers point to. Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to `scanf', you might want to double-check this. When a "matching failure" occurs, `scanf' returns immediately, leaving the first non-matching character as the next character to be read from the stream. The normal return value from `scanf' is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read. The `scanf' function is typically used for things like reading in the contents of tables. For example, here is a function that uses `scanf' to initialize an array of `double': void readarray (double *array, int n) { int i; for (i=0; i scanf ("%a[a-zA-Z0-9] = %a[^\n]\n", &variable, &value)) { invalid_input_error (); return 0; } ... } Other Input Conversions ----------------------- This section describes the miscellaneous input conversions. The `%p' conversion is used to read a pointer value. It recognizes the same syntax used by the `%p' output conversion for `printf' (*note Other Output Conversions::); that is, a hexadecimal number just as the `%x' conversion accepts. The corresponding argument should be of type `void **'; that is, the address of a place to store a pointer. The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in. The `%n' conversion produces the number of characters read so far by this call. The corresponding argument should be of type `int *'. This conversion works in the same way as the `%n' conversion for `printf'; see *Note Other Output Conversions::, for an example. The `%n' conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the `%n' follows the locus of a matching failure, then no value is stored for it since `scanf' returns before processing the `%n'. If you store `-1' in that argument slot before calling `scanf', the presence of `-1' after `scanf' indicates an error occurred before the `%n' was reached. Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified. Formatted Input Functions ------------------------- Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file `stdio.h'. - Function: int scanf (const char *TEMPLATE, ...) The `scanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `EOF' is returned. - Function: int wscanf (const wchar_t *TEMPLATE, ...) The `wscanf' function reads formatted input from the stream `stdin' under the control of the template string TEMPLATE. The optional arguments are pointers to the places which receive the resulting values. The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then `WEOF' is returned. - Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...) This function is just like `scanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...) This function is just like `wscanf', except that the input is read from the stream STREAM instead of `stdin'. - Function: int sscanf (const char *S, const char *TEMPLATE, ...) This is like `scanf', except that the characters are taken from the null-terminated string S instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if S is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion. - Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...) This is like `wscanf', except that the characters are taken from the null-terminated string WS instead of from a stream. Reaching the end of the string is treated as an end-of-file condition. The behavior of this function is undefined if copying takes place between objects that overlap--for example, if WS is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion. Variable Arguments Input Functions ---------------------------------- The functions `vscanf' and friends are provided so that you can define your own variadic `scanf'-like functions that make use of the same internals as the built-in formatted output functions. These functions are analogous to the `vprintf' series of output functions. *Note Variable Arguments Output::, for important information on how to use them. *Portability Note:* The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions. - Function: int vscanf (const char *TEMPLATE, va_list AP) This function is similar to `scanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP) This function is similar to `wscanf', but instead of taking a variable number of arguments directly, it takes an argument list pointer AP of type `va_list' (*note Variadic Functions::). - Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list AP) This is the equivalent of `fscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `fwscanf' with the variable argument list specified directly as for `vwscanf'. - Function: int vsscanf (const char *S, const char *TEMPLATE, va_list AP) This is the equivalent of `sscanf' with the variable argument list specified directly as for `vscanf'. - Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE, va_list AP) This is the equivalent of `swscanf' with the variable argument list specified directly as for `vwscanf'. In GNU C, there is a special construct you can use to let the compiler know that a function uses a `scanf'-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For details, *Note Declaring Attributes of Functions: (gcc.info)Function Attributes. End-Of-File and Errors ====================== Many of the functions described in this chapter return the value of the macro `EOF' to indicate unsuccessful completion of the operation. Since `EOF' is used to report both end of file and random errors, it's often better to use the `feof' function to check explicitly for end of file and `ferror' to check for errors. These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream. - Macro: int EOF This macro is an integer value that is returned by a number of narrow stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `EOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `stdio.h'. - Macro: int WEOF This macro is an integer value that is returned by a number of wide stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, `WEOF' is `-1'. In other libraries, its value may be some other negative number. This symbol is declared in `wchar.h'. - Function: int feof (FILE *STREAM) The `feof' function returns nonzero if and only if the end-of-file indicator for the stream STREAM is set. This symbol is declared in `stdio.h'. - Function: int feof_unlocked (FILE *STREAM) The `feof_unlocked' function is equivalent to the `feof' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. - Function: int ferror (FILE *STREAM) The `ferror' function returns nonzero if and only if the error indicator for the stream STREAM is set, indicating that an error has occurred on a previous operation on the stream. This symbol is declared in `stdio.h'. - Function: int ferror_unlocked (FILE *STREAM) The `ferror_unlocked' function is equivalent to the `ferror' function except that it does not implicitly lock the stream. This function is a GNU extension. This symbol is declared in `stdio.h'. In addition to setting the error indicator associated with the stream, the functions that operate on streams also set `errno' in the same way as the corresponding low-level functions that operate on file descriptors. For example, all of the functions that perform output to a stream--such as `fputc', `printf', and `fflush'--are implemented in terms of `write', and all of the `errno' error conditions defined for `write' are meaningful for these functions. For more information about the descriptor-level I/O functions, see *Note Low-Level I/O::. Recovering from errors ====================== You may explicitly clear the error and EOF flags with the `clearerr' function. - Function: void clearerr (FILE *STREAM) This function clears the end-of-file and error indicators for the stream STREAM. The file positioning functions (*note File Positioning::) also clear the end-of-file indicator for the stream. - Function: void clearerr_unlocked (FILE *STREAM) The `clearerr_unlocked' function is equivalent to the `clearerr' function except that it does not implicitly lock the stream. This function is a GNU extension. Note that it is _not_ correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data. A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying. Most errors that can happen are not recoverable -- a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic. One important exception is `EINTR' (*note Interrupted Primitives::). Many stream I/O implementations will treat it as an ordinary error, which can be quite inconvenient. You can avoid this hassle by installing all signals with the `SA_RESTART' flag. For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable. Text and Binary Streams ======================= The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems. When you open a stream, you can specify either a "text stream" or a "binary stream". You indicate that you want a binary stream by specifying the `b' modifier in the OPENTYPE argument to `fopen'; see *Note Opening Streams::. Without this option, `fopen' opens the file as a text stream. Text and binary streams differ in several ways: * The data read from a text stream is divided into "lines" which are terminated by newline (`'\n'') characters, while a binary stream is simply a long series of characters. A text stream might on some systems fail to handle lines more than 254 characters long (including the terminating newline character). * On some systems, text files can contain only printing characters, horizontal tab characters, and newlines, and so text streams may not support other characters. However, binary streams can handle any character value. * Space characters that are written immediately preceding a newline character in a text stream may disappear when the file is read in again. * More generally, there need not be a one-to-one mapping between characters that are read from or written to a text stream, and the characters in the actual file. Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write "an ordinary file of text" that can work with other text-oriented programs is through a text stream. In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have. File Positioning ================ The "file position" of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. *Note File Position::. During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as "random-access" files. You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file `stdio.h'. - Function: long int ftell (FILE *STREAM) This function returns the current file position of the stream STREAM. This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a `long int', and possibly for other reasons as well. If a failure occurs, a value of `-1' is returned. - Function: off_t ftello (FILE *STREAM) The `ftello' function is similar to `ftell', except that it returns a value of type `off_t'. Systems which support this type use it to describe all file positions, unlike the POSIX specification which uses a long int. The two are not necessarily the same size. Therefore, using ftell can lead to problems if the implementation is written on top of a POSIX compliant low-level I/O implementation, and using `ftello' is preferable whenever it is available. If this function fails it returns `(off_t) -1'. This can happen due to missing support for file positioning or internal errors. Otherwise the return value is the current file position. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `ftello64'. I.e., the LFS interface transparently replaces the old interface. - Function: off64_t ftello64 (FILE *STREAM) This function is similar to `ftello' with the only difference that the return value is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `ftello' and so transparently replaces the old interface. - Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE) The `fseek' function is used to change the file position of the stream STREAM. The value of WHENCE must be one of the constants `SEEK_SET', `SEEK_CUR', or `SEEK_END', to indicate whether the OFFSET is relative to the beginning of the file, the current file position, or the end of the file, respectively. This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure. A successful call also clears the end-of-file indicator of STREAM and discards any characters that were "pushed back" by the use of `ungetc'. `fseek' either flushes any buffered output before setting the file position or else remembers it so it will be written later in its proper place in the file. - Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE) This function is similar to `fseek' but it corrects a problem with `fseek' in a system with POSIX types. Using a value of type `long int' for the offset is not compatible with POSIX. `fseeko' uses the correct type `off_t' for the OFFSET parameter. For this reason it is a good idea to prefer `ftello' whenever it is available since its functionality is (if different at all) closer the underlying definition. The functionality and return value is the same as for `fseek'. The function is an extension defined in the Unix Single Specification version 2. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system this function is in fact `fseeko64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE) This function is similar to `fseeko' with the only difference that the OFFSET parameter is of type `off64_t'. This also requires that the stream STREAM was opened using either `fopen64', `freopen64', or `tmpfile64' since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fseeko' and so transparently replaces the old interface. *Portability Note:* In non-POSIX systems, `ftell', `ftello', `fseek' and `fseeko' might work reliably only on binary streams. *Note Binary Streams::. The following symbolic constants are defined for use as the WHENCE argument to `fseek'. They are also used with the `lseek' function (*note I/O Primitives::) and to specify offsets for file locks (*note Control Operations::). - Macro: int SEEK_SET This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the beginning of the file. - Macro: int SEEK_CUR This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the current file position. - Macro: int SEEK_END This is an integer constant which, when used as the WHENCE argument to the `fseek' or `fseeko' function, specifies that the offset provided is relative to the end of the file. - Function: void rewind (FILE *STREAM) The `rewind' function positions the stream STREAM at the beginning of the file. It is equivalent to calling `fseek' or `fseeko' on the STREAM with an OFFSET argument of `0L' and a WHENCE argument of `SEEK_SET', except that the return value is discarded and the error indicator for the stream is reset. These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'. `L_SET' An alias for `SEEK_SET'. `L_INCR' An alias for `SEEK_CUR'. `L_XTND' An alias for `SEEK_END'. Portable File-Position Functions ================================ On the GNU system, the file position is truly a character count. You can specify any character count value as an argument to `fseek' or `fseeko' and get reliable results for any random access file. However, some ISO C systems do not represent file positions in this way. On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record. As a consequence, if you want your programs to be portable to these systems, you must observe certain rules: * The value returned from `ftell' on a text stream has no predictable relationship to the number of characters you have read so far. The only thing you can rely on is that you can use it subsequently as the OFFSET argument to `fseek' or `fseeko' to move back to the same file position. * In a call to `fseek' or `fseeko' on a text stream, either the OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the OFFSET must be the result of an earlier call to `ftell' on the same stream. * The value of the file position indicator of a text stream is undefined while there are characters that have been pushed back with `ungetc' that haven't been read or discarded. *Note Unreading::. But even if you observe these rules, you may still have trouble for long files, because `ftell' and `fseek' use a `long int' value to represent the file position. This type may not have room to encode all the file positions in a large file. Using the `ftello' and `fseeko' functions might help here since the `off_t' type is expected to be able to hold all file position values but this still does not help to handle additional information which must be associated with a file position. So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions `fgetpos' and `fsetpos' instead. These functions represent the file position using the data type `fpos_t', whose internal representation varies from system to system. These symbols are declared in the header file `stdio.h'. - Data Type: fpos_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos' and `fsetpos'. In the GNU system, `fpos_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this type is in fact equivalent to `fpos64_t' since the LFS interface transparently replaces the old interface. - Data Type: fpos64_t This is the type of an object that can encode information about the file position of a stream, for use by the functions `fgetpos64' and `fsetpos64'. In the GNU system, `fpos64_t' is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation. - Function: int fgetpos (FILE *STREAM, fpos_t *POSITION) This function stores the value of the file position indicator for the stream STREAM in the `fpos_t' object pointed to by POSITION. If successful, `fgetpos' returns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fgetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION) This function is similar to `fgetpos' but the file position is returned in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fgetpos' and so transparently replaces the old interface. - Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION) This function sets the file position indicator for the stream STREAM to the position POSITION, which must have been set by a previous call to `fgetpos' on the same stream. If successful, `fsetpos' clears the end-of-file indicator on the stream, discards any characters that were "pushed back" by the use of `ungetc', and returns a value of zero. Otherwise, `fsetpos' returns a nonzero value and stores an implementation-defined positive value in `errno'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit system the function is in fact `fsetpos64'. I.e., the LFS interface transparently replaces the old interface. - Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION) This function is similar to `fsetpos' but the file position used for positioning is provided in a variable of type `fpos64_t' to which POSITION points. If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is available under the name `fsetpos' and so transparently replaces the old interface. Stream Buffering ================ Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called "buffering". If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior. This section deals only with controlling when characters are transmitted between the stream and the file or device, and _not_ with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see *Note Low-Level Terminal Interface::. You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. *Note Low-Level I/O::. Buffering Concepts ------------------ There are three different kinds of buffering strategies: * Characters written to or read from an "unbuffered" stream are transmitted individually to or from the file as soon as possible. * Characters written to a "line buffered" stream are transmitted to the file in blocks when a newline character is encountered. * Characters written to or read from a "fully buffered" stream are transmitted to or from the file in blocks of arbitrary size. Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. *Note Controlling Buffering::, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open. The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately--which is usually what you want. Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with `fflush', as described in *Note Flushing Buffers::. Flushing Buffers ---------------- "Flushing" output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically: * When you try to do output and the output buffer is full. * When the stream is closed. *Note Closing Streams::. * When the program terminates by calling `exit'. *Note Normal Termination::. * When a newline is written, if the stream is line buffered. * Whenever an input operation on _any_ stream actually reads data from its file. If you want to flush the buffered output at another time, call `fflush', which is declared in the header file `stdio.h'. - Function: int fflush (FILE *STREAM) This function causes any buffered output on STREAM to be delivered to the file. If STREAM is a null pointer, then `fflush' causes buffered output on _all_ open output streams to be flushed. This function returns `EOF' if a write error occurs, or zero otherwise. - Function: int fflush_unlocked (FILE *STREAM) The `fflush_unlocked' function is equivalent to the `fflush' function except that it does not implicitly lock the stream. The `fflush' function can be used to flush all streams currently opened. While this is useful in some situations it does often more than necessary since it might be done in situations when terminal input is required and the program wants to be sure that all output is visible on the terminal. But this means that only line buffered streams have to be flushed. Solaris introduced a function especially for this. It was always available in the GNU C library in some form but never officially exported. - Function: void _flushlbf (void) The `_flushlbf' function flushes all line buffered streams currently opened. This function is declared in the `stdio_ext.h' header. *Compatibility Note:* Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this "feature" seems to be becoming less common. You do not need to worry about this in the GNU system. In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used. - Function: void __fpurge (FILE *STREAM) The `__fpurge' function causes the buffer of the stream STREAM to be emptied. If the stream is currently in read mode all input in the buffer is lost. If the stream is in output mode the buffered output is not written to the device (or whatever other underlying storage) and the buffer the cleared. This function is declared in `stdio_ext.h'. Controlling Which Kind of Buffering ----------------------------------- After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the `setvbuf' function. The facilities listed in this section are declared in the header file `stdio.h'. - Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t SIZE) This function is used to specify that the stream STREAM should have the buffering mode MODE, which can be either `_IOFBF' (for full buffering), `_IOLBF' (for line buffering), or `_IONBF' (for unbuffered input/output). If you specify a null pointer as the BUF argument, then `setvbuf' allocates a buffer itself using `malloc'. This buffer will be freed when you close the stream. Otherwise, BUF should be a character array that can hold at least SIZE characters. You should not free the space for this array as long as the stream remains open and this array remains its buffer. You should usually either allocate it statically, or `malloc' (*note Unconstrained Allocation::) the buffer. Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array. While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering. The `setvbuf' function returns zero on success, or a nonzero value if the value of MODE is not valid or if the request could not be honored. - Macro: int _IOFBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be fully buffered. - Macro: int _IOLBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be line buffered. - Macro: int _IONBF The value of this macro is an integer constant expression that can be used as the MODE argument to the `setvbuf' function to specify that the stream should be unbuffered. - Macro: int BUFSIZ The value of this macro is an integer constant expression that is good to use for the SIZE argument to `setvbuf'. This value is guaranteed to be at least `256'. The value of `BUFSIZ' is chosen on each system so as to make stream I/O efficient. So it is a good idea to use `BUFSIZ' as the size for the buffer when you call `setvbuf'. Actually, you can get an even better value to use for the buffer size by means of the `fstat' system call: it is found in the `st_blksize' field of the file attributes. *Note Attribute Meanings::. Sometimes people also use `BUFSIZ' as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with `fgets' (*note Character Input::). There is no particular reason to use `BUFSIZ' for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size. - Function: void setbuf (FILE *STREAM, char *BUF) If BUF is a null pointer, the effect of this function is equivalent to calling `setvbuf' with a MODE argument of `_IONBF'. Otherwise, it is equivalent to calling `setvbuf' with BUF, and a MODE of `_IOFBF' and a SIZE argument of `BUFSIZ'. The `setbuf' function is provided for compatibility with old code; use `setvbuf' in all new programs. - Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE) If BUF is a null pointer, this function makes STREAM unbuffered. Otherwise, it makes STREAM fully buffered using BUF as the buffer. The SIZE argument specifies the length of BUF. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. - Function: void setlinebuf (FILE *STREAM) This function makes STREAM be line buffered, and allocates the buffer for you. This function is provided for compatibility with old BSD code. Use `setvbuf' instead. It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library. - Function: int __flbf (FILE *STREAM) The `__flbf' function will return a nonzero value in case the stream STREAM is line buffered. Otherwise the return value is zero. This function is declared in the `stdio_ext.h' header. Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris. - Function: size_t __fbufsize (FILE *STREAM) The `__fbufsize' function return the size of the buffer in the stream STREAM. This value can be used to optimize the use of the stream. This function is declared in the `stdio_ext.h' header. - Function: size_t __fpending (FILE *STREAM) The `__fpending' function returns the number of bytes currently in the output buffer. For wide-oriented stream the measuring unit is wide characters. This function should not be used on buffers in read mode or opened read-only. This function is declared in the `stdio_ext.h' header. Other Kinds of Streams ====================== The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file. One such type of stream takes input from or writes output to a string. These kinds of streams are used internally to implement the `sprintf' and `sscanf' functions. You can also create such a stream explicitly, using the functions described in *Note String Streams::. More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in *Note Custom Streams::. *Portability Note:* The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality. String Streams -------------- The `fmemopen' and `open_memstream' functions allow you to do I/O to a string or memory buffer. These facilities are declared in `stdio.h'. - Function: FILE * fmemopen (void *BUF, size_t SIZE, const char *OPENTYPE) This function opens a stream that allows the access specified by the OPENTYPE argument, that reads from or writes to the buffer specified by the argument BUF. This array must be at least SIZE bytes long. If you specify a null pointer as the BUF argument, `fmemopen' dynamically allocates an array SIZE bytes long (as with `malloc'; *note Unconstrained Allocation::). This is really only useful if you are going to write things to the buffer and then read them back in again, because you have no way of actually getting a pointer to the buffer (for this, try `open_memstream', below). The buffer is freed when the stream is closed. The argument OPENTYPE is the same as in `fopen' (*note Opening Streams::). If the OPENTYPE specifies append mode, then the initial file position is set to the first null character in the buffer. Otherwise the initial file position is at the beginning of the buffer. When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the SIZE argument to account for this. Attempts to write more than SIZE bytes to the buffer result in an error. For a stream open for reading, null characters (zero bytes) in the buffer do not count as "end of file". Read operations indicate end of file only when the file position advances past SIZE bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the SIZE argument. Here is an example of using `fmemopen' to create a stream for reading from a string: #include static char buffer[] = "foobar"; int main (void) { int ch; FILE *stream; stream = fmemopen (buffer, strlen (buffer), "r"); while ((ch = fgetc (stream)) != EOF) printf ("Got %c\n", ch); fclose (stream); return 0; } This program produces the following output: Got f Got o Got o Got b Got a Got r - Function: FILE * open_memstream (char **PTR, size_t *SIZELOC) This function opens a stream for writing to a buffer. The buffer is allocated dynamically (as with `malloc'; *note Unconstrained Allocation::) and grown as necessary. When the stream is closed with `fclose' or flushed with `fflush', the locations PTR and SIZELOC are updated to contain the pointer to the buffer and its size. The values thus stored remain valid only as long as no further output on the stream takes place. If you do more output, you must flush the stream again to store new values before you use them again. A null character is written at the end of the buffer. This null character is _not_ included in the size value stored at SIZELOC. You can move the stream's file position with `fseek' or `fseeko' (*note File Positioning::). Moving the file position past the end of the data already written fills the intervening space with zeroes. Here is an example of using `open_memstream': #include int main (void) { char *bp; size_t size; FILE *stream; stream = open_memstream (&bp, &size); fprintf (stream, "hello"); fflush (stream); printf ("buf = `%s', size = %d\n", bp, size); fprintf (stream, ", world"); fclose (stream); printf ("buf = `%s', size = %d\n", bp, size); return 0; } This program produces the following output: buf = `hello', size = 5 buf = `hello, world', size = 12 Obstack Streams --------------- You can open an output stream that puts it data in an obstack. *Note Obstacks::. - Function: FILE * open_obstack_stream (struct obstack *OBSTACK) This function opens a stream for writing data into the obstack OBSTACK. This starts an object in the obstack and makes it grow as data is written (*note Growing Objects::). Calling `fflush' on this stream updates the current size of the object to match the amount of data that has been written. After a call to `fflush', you can examine the object temporarily. You can move the file position of an obstack stream with `fseek' or `fseeko' (*note File Positioning::). Moving the file position past the end of the data written fills the intervening space with zeros. To make the object permanent, update the obstack with `fflush', and then use `obstack_finish' to finalize the object and get its address. The following write to the stream starts a new object in the obstack, and later writes add to that object until you do another `fflush' and `obstack_finish'. But how do you find out how long the object is? You can get the length in bytes by calling `obstack_object_size' (*note Status of an Obstack::), or you can null-terminate the object like this: obstack_1grow (OBSTACK, 0); Whichever one you do, you must do it _before_ calling `obstack_finish'. (You can do both if you wish.) Here is a sample function that uses `open_obstack_stream': char * make_message_string (const char *a, int b) { FILE *stream = open_obstack_stream (&message_obstack); output_task (stream); fprintf (stream, ": "); fprintf (stream, a, b); fprintf (stream, "\n"); fclose (stream); obstack_1grow (&message_obstack, 0); return obstack_finish (&message_obstack); } Programming Your Own Custom Streams ----------------------------------- This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these "custom streams". The functions and types described here are all GNU extensions. Custom Streams and Cookies .......................... Inside every custom stream is a special object called the "cookie". This is an object supplied by you which records where to fetch or store the data read or written. It is up to you to define a data type to use for the cookie. The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type `void *'. To implement a custom stream, you must specify _how_ to fetch or store the data in the specified place. You do this by defining "hook functions" to read, write, change "file position", and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know. When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type `cookie_io_functions_t'. These facilities are declared in `stdio.h'. - Data Type: cookie_io_functions_t This is a structure type that holds the functions that define the communications protocol between the stream and its cookie. It has the following members: `cookie_read_function_t *read' This is the function that reads data from the cookie. If the value is a null pointer instead of a function, then read operations on this stream always return `EOF'. `cookie_write_function_t *write' This is the function that writes data to the cookie. If the value is a null pointer instead of a function, then data written to the stream is discarded. `cookie_seek_function_t *seek' This is the function that performs the equivalent of file positioning on the cookie. If the value is a null pointer instead of a function, calls to `fseek' or `fseeko' on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an `ESPIPE' error. `cookie_close_function_t *close' This function performs any appropriate cleanup on the cookie when closing the stream. If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed. - Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE, cookie_io_functions_t IO-FUNCTIONS) This function actually creates the stream for communicating with the COOKIE using the functions in the IO-FUNCTIONS argument. The OPENTYPE argument is interpreted as for `fopen'; see *Note Opening Streams::. (But note that the "truncate on open" option is ignored.) The new stream is fully buffered. The `fopencookie' function returns the newly created stream, or a null pointer in case of an error. Custom Stream Hook Functions ............................ Here are more details on how you should define the four hook functions that a custom stream needs. You should define the function to read data from the cookie as: ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE) This is very similar to the `read' function; see *Note I/O Primitives::. Your function should transfer up to SIZE bytes into the BUFFER, and return the number of bytes read, or zero to indicate end-of-file. You can return a value of `-1' to indicate an error. You should define the function to write data to the cookie as: ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE) This is very similar to the `write' function; see *Note I/O Primitives::. Your function should transfer up to SIZE bytes from the buffer, and return the number of bytes written. You can return a value of `-1' to indicate an error. You should define the function to perform seek operations on the cookie as: int SEEKER (void *COOKIE, fpos_t *POSITION, int WHENCE) For this function, the POSITION and WHENCE arguments are interpreted as for `fgetpos'; see *Note Portable Positioning::. In the GNU library, `fpos_t' is equivalent to `off_t' or `long int', and simply represents the number of bytes from the beginning of the file. After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in POSITION. Your function should return a value of `0' on success and `-1' to indicate an error. You should define the function to do cleanup operations on the cookie appropriate for closing the stream as: int CLEANER (void *COOKIE) Your function should return `-1' to indicate an error, and `0' otherwise. - Data Type: cookie_read_function This is the data type that the read function for a custom stream should have. If you declare the function as shown above, this is the type it will have. - Data Type: cookie_write_function The data type of the write function for a custom stream. - Data Type: cookie_seek_function The data type of the seek function for a custom stream. - Data Type: cookie_close_function The data type of the close function for a custom stream. Formatted Messages ================== On systems which are based on System V messages of programs (especially the system tools) are printed in a strict form using the `fmtmsg' function. The uniformity sometimes helps the user to interpret messages and the strictness tests of the `fmtmsg' function ensure that the programmer follows some minimal requirements. Printing Formatted Messages --------------------------- Messages can be printed to standard error and/or to the console. To select the destination the programmer can use the following two values, bitwise OR combined if wanted, for the CLASSIFICATION parameter of `fmtmsg': `MM_PRINT' Display the message in standard error. `MM_CONSOLE' Display the message on the system console. The erroneous piece of the system can be signalled by exactly one of the following values which also is bitwise ORed with the CLASSIFICATION parameter to `fmtmsg': `MM_HARD' The source of the condition is some hardware. `MM_SOFT' The source of the condition is some software. `MM_FIRM' The source of the condition is some firmware. A third component of the CLASSIFICATION parameter to `fmtmsg' can describe the part of the system which detects the problem. This is done by using exactly one of the following values: `MM_APPL' The erroneous condition is detected by the application. `MM_UTIL' The erroneous condition is detected by a utility. `MM_OPSYS' The erroneous condition is detected by the operating system. A last component of CLASSIFICATION can signal the results of this message. Exactly one of the following values can be used: `MM_RECOVER' It is a recoverable error. `MM_NRECOV' It is a non-recoverable error. - Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL, int SEVERITY, const char *TEXT, const char *ACTION, const char *TAG) Display a message described by its parameters on the device(s) specified in the CLASSIFICATION parameter. The LABEL parameter identifies the source of the message. The string should consist of two colon separated parts where the first part has not more than 10 and the second part not more than 14 characters. The TEXT parameter describes the condition of the error, the ACTION parameter possible steps to recover from the error and the TAG parameter is a reference to the online documentation where more information can be found. It should contain the LABEL value and a unique identification number. Each of the parameters can be a special value which means this value is to be omitted. The symbolic names for these values are: `MM_NULLLBL' Ignore LABEL parameter. `MM_NULLSEV' Ignore SEVERITY parameter. `MM_NULLMC' Ignore CLASSIFICATION parameter. This implies that nothing is actually printed. `MM_NULLTXT' Ignore TEXT parameter. `MM_NULLACT' Ignore ACTION parameter. `MM_NULLTAG' Ignore TAG parameter. There is another way certain fields can be omitted from the output to standard error. This is described below in the description of environment variables influencing the behavior. The SEVERITY parameter can have one of the values in the following table: `MM_NOSEV' Nothing is printed, this value is the same as `MM_NULLSEV'. `MM_HALT' This value is printed as `HALT'. `MM_ERROR' This value is printed as `ERROR'. `MM_WARNING' This value is printed as `WARNING'. `MM_INFO' This value is printed as `INFO'. The numeric value of these five macros are between `0' and `4'. Using the environment variable `SEV_LEVEL' or using the `addseverity' function one can add more severity levels with their corresponding string to print. This is described below (*note Adding Severity Classes::). If no parameter is ignored the output looks like this: LABEL: SEVERITY-STRING: TEXT TO FIX: ACTION TAG The colons, new line characters and the `TO FIX' string are inserted if necessary, i.e., if the corresponding parameter is not ignored. This function is specified in the X/Open Portability Guide. It is also available on all systems derived from System V. The function returns the value `MM_OK' if no error occurred. If only the printing to standard error failed, it returns `MM_NOMSG'. If printing to the console fails, it returns `MM_NOCON'. If nothing is printed `MM_NOTOK' is returned. Among situations where all outputs fail this last value is also returned if a parameter value is incorrect. There are two environment variables which influence the behavior of `fmtmsg'. The first is `MSGVERB'. It is used to control the output actually happening on standard error (_not_ the console output). Each of the five fields can explicitly be enabled. To do this the user has to put the `MSGVERB' variable with a format like the following in the environment before calling the `fmtmsg' function the first time: MSGVERB=KEYWORD[:KEYWORD[:...]] Valid KEYWORDs are `label', `severity', `text', `action', and `tag'. If the environment variable is not given or is the empty string, a not supported keyword is given or the value is somehow else invalid, no part of the message is masked out. The second environment variable which influences the behavior of `fmtmsg' is `SEV_LEVEL'. This variable and the change in the behavior of `fmtmsg' is not specified in the X/Open Portability Guide. It is available in System V systems, though. It can be used to introduce new severity levels. By default, only the five severity levels described above are available. Any other numeric value would make `fmtmsg' print nothing. If the user puts `SEV_LEVEL' with a format like SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]] in the environment of the process before the first call to `fmtmsg', where DESCRIPTION has a value of the form SEVERITY-KEYWORD,LEVEL,PRINTSTRING The SEVERITY-KEYWORD part is not used by `fmtmsg' but it has to be present. The LEVEL part is a string representation of a number. The numeric value must be a number greater than 4. This value must be used in the SEVERITY parameter of `fmtmsg' to select this class. It is not possible to overwrite any of the predefined classes. The PRINTSTRING is the string printed when a message of this class is processed by `fmtmsg' (see above, `fmtsmg' does not print the numeric value but instead the string representation). Adding Severity Classes ----------------------- There is another possibility to introduce severity classes besides using the environment variable `SEV_LEVEL'. This simplifies the task of introducing new classes in a running program. One could use the `setenv' or `putenv' function to set the environment variable, but this is toilsome. - Function: int addseverity (int SEVERITY, const char *STRING) This function allows the introduction of new severity classes which can be addressed by the SEVERITY parameter of the `fmtmsg' function. The SEVERITY parameter of `addseverity' must match the value for the parameter with the same name of `fmtmsg', and STRING is the string printed in the actual messages instead of the numeric value. If STRING is `NULL' the severity class with the numeric value according to SEVERITY is removed. It is not possible to overwrite or remove one of the default severity classes. All calls to `addseverity' with SEVERITY set to one of the values for the default classes will fail. The return value is `MM_OK' if the task was successfully performed. If the return value is `MM_NOTOK' something went wrong. This could mean that no more memory is available or a class is not available when it has to be removed. This function is not specified in the X/Open Portability Guide although the `fmtsmg' function is. It is available on System V systems. How to use `fmtmsg' and `addseverity' ------------------------------------- Here is a simple example program to illustrate the use of the both functions described in this section. #include int main (void) { addseverity (5, "NOTE2"); fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2"); fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual", "UX:cat:001"); fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag"); return 0; } The second call to `fmtmsg' illustrates a use of this function as it usually occurs on System V systems, which heavily use this function. It seems worthwhile to give a short explanation here of how this system works on System V. The value of the LABEL field (`UX:cat') says that the error occurred in the Unix program `cat'. The explanation of the error follows and the value for the ACTION parameter is `"refer to manual"'. One could be more specific here, if necessary. The TAG field contains, as proposed above, the value of the string given for the LABEL parameter, and additionally a unique ID (`001' in this case). For a GNU environment this string could contain a reference to the corresponding node in the Info page for the program. Running this program without specifying the `MSGVERB' and `SEV_LEVEL' function produces the following output: UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 We see the different fields of the message and how the extra glue (the colons and the `TO FIX' string) are printed. But only one of the three calls to `fmtmsg' produced output. The first call does not print anything because the LABEL parameter is not in the correct form. The string must contain two fields, separated by a colon (*note Printing Formatted Messages::). The third `fmtmsg' call produced no output since the class with the numeric value `6' is not defined. Although a class with numeric value `5' is also not defined by default, the call to `addseverity' introduces it and the second call to `fmtmsg' produces the above output. When we change the environment of the program to contain `SEV_LEVEL=XXX,6,NOTE' when running it we get a different result: UX:cat: NOTE2: invalid syntax TO FIX: refer to manual UX:cat:001 label:foo: NOTE: text TO FIX: action tag Now the third call to `fmtmsg' produced some output and we see how the string `NOTE' from the environment variable appears in the message. Now we can reduce the output by specifying which fields we are interested in. If we additionally set the environment variable `MSGVERB' to the value `severity:label:action' we get the following output: UX:cat: NOTE2 TO FIX: refer to manual label:foo: NOTE TO FIX: action I.e., the output produced by the TEXT and the TAG parameters to `fmtmsg' vanished. Please also note that now there is no colon after the `NOTE' and `NOTE2' strings in the output. This is not necessary since there is no more output on this line because the text is missing. Low-Level Input/Output ********************** This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in *Note I/O on Streams::, as well as functions for performing low-level control operations for which there are no equivalents on streams. Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons: * For reading binary files in large chunks. * For reading an entire file into core before parsing it. * To perform operations other than data transfer, which can only be done with a descriptor. (You can use `fileno' to get the descriptor corresponding to a stream.) * To pass descriptors to a child process. (The child can create its own stream to use a descriptor that it inherits, but cannot inherit a stream directly.) Opening and Closing Files ========================= This section describes the primitives for opening and closing files using file descriptors. The `open' and `creat' functions are declared in the header file `fcntl.h', while `close' is declared in `unistd.h'. - Function: int open (const char *FILENAME, int FLAGS[, mode_t MODE]) The `open' function creates and returns a new file descriptor for the file named by FILENAME. Initially, the file position indicator for the file is at the beginning of the file. The argument MODE is used only when a file is created, but it doesn't hurt to supply the argument in any case. The FLAGS argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). *Note File Status Flags::, for the parameters available. The normal return value from `open' is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' The file exists but is not readable/writable as requested by the FLAGS argument, the file does not exist and the directory is unwritable so it cannot be created. `EEXIST' Both `O_CREAT' and `O_EXCL' are set, and the named file already exists. `EINTR' The `open' operation was interrupted by a signal. *Note Interrupted Primitives::. `EISDIR' The FLAGS argument specified write access, and the file is a directory. `EMFILE' The process has too many files open. The maximum number of file descriptors is controlled by the `RLIMIT_NOFILE' resource limit; *note Limits on Resources::. `ENFILE' The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.) `ENOENT' The named file does not exist, and `O_CREAT' is not specified. `ENOSPC' The directory or file system that would contain the new file cannot be extended, because there is no disk space left. `ENXIO' `O_NONBLOCK' and `O_WRONLY' are both set in the FLAGS argument, the file named by FILENAME is a FIFO (*note Pipes and FIFOs::), and no process has the file open for reading. `EROFS' The file resides on a read-only file system and any of `O_WRONLY', `O_RDWR', and `O_TRUNC' are set in the FLAGS argument, or `O_CREAT' is set and the file does not already exist. If on a 32 bit machine the sources are translated with `_FILE_OFFSET_BITS == 64' the function `open' returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 bytes in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `open' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `open' should be protected using cancellation handlers. The `open' function is the underlying primitive for the `fopen' and `freopen' functions, that create streams. - Function: int open64 (const char *FILENAME, int FLAGS[, mode_t MODE]) This function is similar to `open'. It returns a file descriptor which can be used to access the file named by FILENAME. The only difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `open'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Obsolete function: int creat (const char *FILENAME, mode_t MODE) This function is obsolete. The call: creat (FILENAME, MODE) is equivalent to: open (FILENAME, O_WRONLY | O_CREAT | O_TRUNC, MODE) If on a 32 bit machine the sources are translated with `_FILE_OFFSET_BITS == 64' the function `creat' returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced. - Obsolete function: int creat64 (const char *FILENAME, mode_t MODE) This function is similar to `creat'. It returns a file descriptor which can be used to access the file named by FILENAME. The only the difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits. To use this file descriptor one must not use the normal operations but instead the counterparts named `*64', e.g., `read64'. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `open'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Function: int close (int FILEDES) The function `close' closes the file descriptor FILEDES. Closing a file has the following consequences: * The file descriptor is deallocated. * Any record locks owned by the process on the file are unlocked. * When all file descriptors associated with a pipe or FIFO have been closed, any unread data is discarded. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `close' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `close' should be protected using cancellation handlers. The normal return value from `close' is 0; a value of -1 is returned in case of failure. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINTR' The `close' call was interrupted by a signal. *Note Interrupted Primitives::. Here is an example of how to handle `EINTR' properly: TEMP_FAILURE_RETRY (close (desc)); `ENOSPC' `EIO' `EDQUOT' When the file is accessed by NFS, these errors from `write' can sometimes not be detected until `close'. *Note I/O Primitives::, for details on their meaning. Please note that there is _no_ separate `close64' function. This is not necessary since this function does not determine nor depend on the mode of the file. The kernel which performs the `close' operation knows which mode the descriptor is used for and can handle this situation. To close a stream, call `fclose' (*note Closing Streams::) instead of trying to close its underlying file descriptor with `close'. This flushes any buffered output and updates the stream object to indicate that it is closed. Input and Output Primitives =========================== This section describes the functions for performing primitive input and output operations on file descriptors: `read', `write', and `lseek'. These functions are declared in the header file `unistd.h'. - Data Type: ssize_t This data type is used to represent the sizes of blocks that can be read or written in a single operation. It is similar to `size_t', but must be a signed type. - Function: ssize_t read (int FILEDES, void *BUFFER, size_t SIZE) The `read' function reads up to SIZE bytes from the file with descriptor FILEDES, storing the results in the BUFFER. (This is not necessarily a character string, and no terminating null character is added.) The return value is the number of bytes actually read. This might be less than SIZE; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than SIZE bytes is not an error. A value of zero indicates end-of-file (except if the value of the SIZE argument is also zero). This is not considered an error. If you keep calling `read' while at end-of-file, it will keep returning zero and doing nothing else. If `read' returns at least one character, there is no way you can tell whether end-of-file was reached. But if you did reach the end, the next read will return zero. In case of an error, `read' returns -1. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, when no input is immediately available, `read' waits for some input. But if the `O_NONBLOCK' flag is set for the file (*note File Status Flags::), `read' returns immediately without reading any data, and reports this error. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, reading a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem never happens in the GNU system. Any condition that could result in `EAGAIN' can instead result in a successful `read' which returns fewer bytes than requested. Calling `read' again immediately would result in `EAGAIN'. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for reading. `EINTR' `read' was interrupted by a signal while it was waiting for input. *Note Interrupted Primitives::. A signal will not necessary cause `read' to return `EINTR'; it may instead result in a successful `read' which returns fewer bytes than requested. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `EIO' also occurs when a background process tries to read from the controlling terminal, and the normal action of stopping the process by sending it a `SIGTTIN' signal isn't working. This might happen if the signal is being blocked or ignored, or because the process group is orphaned. *Note Job Control::, for more information about job control, and *Note Signal Handling::, for information about signals. Please note that there is no function named `read64'. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally, the `read' function can be used for all cases. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `read' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `read' should be protected using cancellation handlers. The `read' function is the underlying primitive for all of the functions that read from streams, such as `fgetc'. - Function: ssize_t pread (int FILEDES, void *BUFFER, size_t SIZE, off_t OFFSET) The `pread' function is similar to the `read' function. The first three arguments are identical, and the return values and error codes also correspond. The difference is the fourth argument and its handling. The data block is not read from the current position of the file descriptor `filedes'. Instead the data is read from the file starting at position OFFSET. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `pread' function is in fact `pread64' and the type `off_t' has 64 bits, which makes it possible to handle files up to 2^63 bytes in length. The return value of `pread' describes the number of bytes read. In the error case it returns -1 like `read' does and the error codes are also the same, with these additions: `EINVAL' The value given for OFFSET is negative and therefore illegal. `ESPIPE' The file descriptor FILEDES is associate with a pipe or a FIFO and this device does not allow positioning of the file pointer. The function is an extension defined in the Unix Single Specification version 2. - Function: ssize_t pread64 (int FILEDES, void *BUFFER, size_t SIZE, off64_t OFFSET) This function is similar to the `pread' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is actually available under the name `pread' and so transparently replaces the 32 bit interface. - Function: ssize_t write (int FILEDES, const void *BUFFER, size_t SIZE) The `write' function writes up to SIZE bytes from BUFFER to the file with descriptor FILEDES. The data in BUFFER is not necessarily a character string and a null character is output like any other character. The return value is the number of bytes actually written. This may be SIZE, but can always be smaller. Your program should always call `write' in a loop, iterating until all the data is written. Once `write' returns, the data is enqueued to be written and can be read back right away, but it is not necessarily written out to permanent storage immediately. You can use `fsync' when you need to be sure your data has been permanently stored before continuing. (It is more efficient for the system to batch up consecutive writes and do them all at once when convenient. Normally they will always be written to disk within a minute or less.) Modern systems provide another function `fdatasync' which guarantees integrity only for the file data and is therefore faster. You can use the `O_FSYNC' open mode to make `write' always store the data to disk before returning; *note Operating Modes::. In the case of an error, `write' returns -1. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, `write' blocks until the write operation is complete. But if the `O_NONBLOCK' flag is set for the file (*note Control Operations::), it returns immediately without writing any data and reports this error. An example of a situation that might cause the process to block on output is writing to a terminal device that supports flow control, where output has been suspended by receipt of a STOP character. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, writing a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem does not arise in the GNU system. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for writing. `EFBIG' The size of the file would become larger than the implementation can support. `EINTR' The `write' operation was interrupted by a signal while it was blocked waiting for completion. A signal will not necessarily cause `write' to return `EINTR'; it may instead result in a successful `write' which writes fewer bytes than requested. *Note Interrupted Primitives::. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `ENOSPC' The device containing the file is full. `EPIPE' This error is returned when you try to write to a pipe or FIFO that isn't open for reading by any process. When this happens, a `SIGPIPE' signal is also sent to the process; see *Note Signal Handling::. Unless you have arranged to prevent `EINTR' failures, you should check `errno' after each failing call to `write', and if the error was `EINTR', you should simply repeat the call. *Note Interrupted Primitives::. The easy way to do this is with the macro `TEMP_FAILURE_RETRY', as follows: nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count)); Please note that there is no function named `write64'. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally the `write' function can be used for all cases. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `write' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `write' should be protected using cancellation handlers. The `write' function is the underlying primitive for all of the functions that write to streams, such as `fputc'. - Function: ssize_t pwrite (int FILEDES, const void *BUFFER, size_t SIZE, off_t OFFSET) The `pwrite' function is similar to the `write' function. The first three arguments are identical, and the return values and error codes also correspond. The difference is the fourth argument and its handling. The data block is not written to the current position of the file descriptor `filedes'. Instead the data is written to the file starting at position OFFSET. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `pwrite' function is in fact `pwrite64' and the type `off_t' has 64 bits, which makes it possible to handle files up to 2^63 bytes in length. The return value of `pwrite' describes the number of written bytes. In the error case it returns -1 like `write' does and the error codes are also the same, with these additions: `EINVAL' The value given for OFFSET is negative and therefore illegal. `ESPIPE' The file descriptor FILEDES is associated with a pipe or a FIFO and this device does not allow positioning of the file pointer. The function is an extension defined in the Unix Single Specification version 2. - Function: ssize_t pwrite64 (int FILEDES, const void *BUFFER, size_t SIZE, off64_t OFFSET) This function is similar to the `pwrite' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine this function is actually available under the name `pwrite' and so transparently replaces the 32 bit interface. Setting the File Position of a Descriptor ========================================= Just as you can set the file position of a stream with `fseek', you can set the file position of a descriptor with `lseek'. This specifies the position in the file for the next `read' or `write' operation. *Note File Positioning::, for more information on the file position and what it means. To read the current file position value from a descriptor, use `lseek (DESC, 0, SEEK_CUR)'. - Function: off_t lseek (int FILEDES, off_t OFFSET, int WHENCE) The `lseek' function is used to change the file position of the file with descriptor FILEDES. The WHENCE argument specifies how the OFFSET should be interpreted, in the same way as for the `fseek' function, and it must be one of the symbolic constants `SEEK_SET', `SEEK_CUR', or `SEEK_END'. `SEEK_SET' Specifies that WHENCE is a count of characters from the beginning of the file. `SEEK_CUR' Specifies that WHENCE is a count of characters from the current file position. This count may be positive or negative. `SEEK_END' Specifies that WHENCE is a count of characters from the end of the file. A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end. If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position. The return value from `lseek' is normally the resulting file position, measured in bytes from the beginning of the file. You can use this feature together with `SEEK_CUR' to read the current file position. If you want to append to the file, setting the file position to the current end of file with `SEEK_END' is not sufficient. Another process may write more data after you seek but before you write, extending the file so the position you write onto clobbers their data. Instead, use the `O_APPEND' operating mode; *note Operating Modes::. You can set the file position past the current end of the file. This does not by itself make the file longer; `lseek' never changes the file. But subsequent output at that position will extend the file. Characters between the previous end of file and the new position are filled with zeros. Extending the file in this way can create a "hole": the blocks of zeros are not actually allocated on disk, so the file takes up less space than it appears to; it is then called a "sparse file". If the file position cannot be changed, or the operation is in some way invalid, `lseek' returns a value of -1. The following `errno' error conditions are defined for this function: `EBADF' The FILEDES is not a valid file descriptor. `EINVAL' The WHENCE argument value is not valid, or the resulting file offset is not valid. A file offset is invalid. `ESPIPE' The FILEDES corresponds to an object that cannot be positioned, such as a pipe, FIFO or terminal device. (POSIX.1 specifies this error only for pipes and FIFOs, but in the GNU system, you always get `ESPIPE' if the object is not seekable.) When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `lseek' function is in fact `lseek64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `lseek' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `lseek' should be protected using cancellation handlers. The `lseek' function is the underlying primitive for the `fseek', `fseeko', `ftell', `ftello' and `rewind' functions, which operate on streams instead of file descriptors. - Function: off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE) This function is similar to the `lseek' function. The difference is that the OFFSET parameter is of type `off64_t' instead of `off_t' which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor `filedes' must be opened using `open64' since otherwise the large offsets possible with `off64_t' will lead to errors with a descriptor in small file mode. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `lseek' and so transparently replaces the 32 bit interface. You can have multiple descriptors for the same file if you open the file more than once, or if you duplicate a descriptor with `dup'. Descriptors that come from separate calls to `open' have independent file positions; using `lseek' on one descriptor has no effect on the other. For example, { int d1, d2; char buf[4]; d1 = open ("foo", O_RDONLY); d2 = open ("foo", O_RDONLY); lseek (d1, 1024, SEEK_SET); read (d2, buf, 4); } will read the first four characters of the file `foo'. (The error-checking code necessary for a real program has been omitted here for brevity.) By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example, { int d1, d2, d3; char buf1[4], buf2[4]; d1 = open ("foo", O_RDONLY); d2 = dup (d1); d3 = dup (d2); lseek (d3, 1024, SEEK_SET); read (d1, buf1, 4); read (d2, buf2, 4); } will read four characters starting with the 1024'th character of `foo', and then four more characters starting with the 1028'th character. - Data Type: off_t This is an arithmetic data type used to represent file sizes. In the GNU system, this is equivalent to `fpos_t' or `long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `off64_t'. - Data Type: off64_t This type is used similar to `off_t'. The difference is that even on 32 bit machines, where the `off_t' type would have 32 bits, `off64_t' has 64 bits and so is able to address files up to 2^63 bytes in length. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `off_t'. These aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: `fcntl.h' and `sys/file.h'. `L_SET' An alias for `SEEK_SET'. `L_INCR' An alias for `SEEK_CUR'. `L_XTND' An alias for `SEEK_END'. Descriptors and Streams ======================= Given an open file descriptor, you can create a stream for it with the `fdopen' function. You can get the underlying file descriptor for an existing stream with the `fileno' function. These functions are declared in the header file `stdio.h'. - Function: FILE * fdopen (int FILEDES, const char *OPENTYPE) The `fdopen' function returns a new stream for the file descriptor FILEDES. The OPENTYPE argument is interpreted in the same way as for the `fopen' function (*note Opening Streams::), except that the `b' option is not permitted; this is because GNU makes no distinction between text and binary files. Also, `"w"' and `"w+"' do not cause truncation of the file; these have an effect only when opening a file, and in this case the file has already been opened. You must make sure that the OPENTYPE argument matches the actual mode of the open file descriptor. The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the OPENTYPE argument), a null pointer is returned instead. In some other systems, `fdopen' may fail to detect that the modes for file descriptor do not permit the access specified by `opentype'. The GNU C library always checks for this. For an example showing the use of the `fdopen' function, see *Note Creating a Pipe::. - Function: int fileno (FILE *STREAM) This function returns the file descriptor associated with the stream STREAM. If an error is detected (for example, if the STREAM is not valid) or if STREAM does not do I/O to a file, `fileno' returns -1. - Function: int fileno_unlocked (FILE *STREAM) The `fileno_unlocked' function is equivalent to the `fileno' function except that it does not implicitly lock the stream if the state is `FSETLOCKING_INTERNAL'. This function is a GNU extension. There are also symbolic constants defined in `unistd.h' for the file descriptors belonging to the standard streams `stdin', `stdout', and `stderr'; see *Note Standard Streams::. `STDIN_FILENO' This macro has value `0', which is the file descriptor for standard input. `STDOUT_FILENO' This macro has value `1', which is the file descriptor for standard output. `STDERR_FILENO' This macro has value `2', which is the file descriptor for standard error output. Dangers of Mixing Streams and Descriptors ========================================= You can have multiple file descriptors and streams (let's call both streams and descriptors "channels" for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: "linked" channels that share a single file position value, and "independent" channels that have their own file positions. It's best to use just one channel in your program for actual data transfer to any given file, except when all the access is for input. For example, if you open a pipe (something you can only do at the file descriptor level), either do all I/O with the descriptor, or construct a stream from the descriptor with `fdopen' and then do all I/O with the stream. Linked Channels --------------- Channels that come from a single opening share the same file position; we call them "linked" channels. Linked channels result when you make a stream from a descriptor using `fdopen', when you get a descriptor from a stream with `fileno', when you copy a descriptor with `dup' or `dup2', and when descriptors are inherited during `fork'. For files that don't support random access, such as terminals and pipes, _all_ channels are effectively linked. On random-access files, all append-type output streams are effectively linked to each other. If you have been using a stream for I/O, and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first "clean up" the stream that you have been using. *Note Cleaning Streams::. Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them. Independent Channels -------------------- When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called "independent channels". The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. However, if some of the channels are streams, you must take these precautions: * You should clean an output stream after use, before doing anything else that might read or write from the same part of the file. * You should clean an input stream before reading data that may have been modified using an independent channel. Otherwise, you might read obsolete data that had been in the stream's buffer. If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. You cannot reliably set their file positions to the new end of file before writing, because the file can always be extended by another process between when you set the file position and when you write the data. Instead, use an append-type descriptor or stream; they always output at the current end of the file. In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream. It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see *Note Linked Channels::. Cleaning Streams ---------------- On the GNU system, you can clean up any stream with `fclean': - Function: int fclean (FILE *STREAM) Clean up the stream STREAM so that its buffer is empty. If STREAM is doing output, force it out. If STREAM is doing input, give the data in the buffer back to the system, arranging to reread it. On other systems, you can use `fflush' to clean a stream in most cases. You can skip the `fclean' or `fflush' if you know the stream is already clean. A stream is clean whenever its buffer is empty. For example, an unbuffered stream is always clean. An input stream that is at end-of-file is clean. A line-buffered stream is clean when the last character output was a newline. There is one case in which cleaning a stream is impossible on most systems. This is when the stream is doing input from a file that is not random-access. Such streams typically read ahead, and when the file is not random access, there is no way to give back the excess data already read. When an input stream reads from a random-access file, `fflush' does clean the stream, but leaves the file pointer at an unpredictable place; you must set the file pointer before doing any further I/O. On the GNU system, using `fclean' avoids both of these problems. Closing an output-only stream also does `fflush', so this is a valid way of cleaning an output stream. On the GNU system, closing an input stream does `fclean'. You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already "output" to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure "past" output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. *Note Terminal Modes::. Fast Scatter-Gather I/O ======================= Some applications may need to read or write data to multiple buffers, which are separated in memory. Although this can be done easily enough with multiple calls to `read' and `write', it is inefficient because there is overhead associated with each kernel call. Instead, many platforms provide special high-speed primitives to perform these "scatter-gather" operations in a single kernel call. The GNU C library will provide an emulation on any system that lacks these primitives, so they are not a portability threat. They are defined in `sys/uio.h'. These functions are controlled with arrays of `iovec' structures, which describe the location and size of each buffer. - Data Type: struct iovec The `iovec' structure describes a buffer. It contains two fields: `void *iov_base' Contains the address of a buffer. `size_t iov_len' Contains the length of the buffer. - Function: ssize_t readv (int FILEDES, const struct iovec *VECTOR, int COUNT) The `readv' function reads data from FILEDES and scatters it into the buffers described in VECTOR, which is taken to be COUNT structures long. As each buffer is filled, data is sent to the next. Note that `readv' is not guaranteed to fill all the buffers. It may stop at any point, for the same reasons `read' would. The return value is a count of bytes (_not_ buffers) read, 0 indicating end-of-file, or -1 indicating an error. The possible errors are the same as in `read'. - Function: ssize_t writev (int FILEDES, const struct iovec *VECTOR, int COUNT) The `writev' function gathers data from the buffers described in VECTOR, which is taken to be COUNT structures long, and writes them to `filedes'. As each buffer is written, it moves on to the next. Like `readv', `writev' may stop midstream under the same conditions `write' would. The return value is a count of bytes written, or -1 indicating an error. The possible errors are the same as in `write'. Note that if the buffers are small (under about 1kB), high-level streams may be easier to use than these functions. However, `readv' and `writev' are more efficient when the individual buffers themselves (as opposed to the total output), are large. In that case, a high-level stream would not be able to cache the data effectively. Memory-mapped I/O ================= On modern operating systems, it is possible to "mmap" (pronounced "em-map") a file to a region of memory. When this is done, the file can be accessed just like an array in the program. This is more efficient than `read' or `write', as only the regions of the file that a program actually accesses are loaded. Accesses to not-yet-loaded parts of the mmapped region are handled in the same way as swapped out pages. Since mmapped pages can be stored back to their file when physical memory is low, it is possible to mmap files orders of magnitude larger than both the physical memory _and_ swap space. The only limit is address space. The theoretical limit is 4GB on a 32-bit machine - however, the actual limit will be smaller since some areas will be reserved for other purposes. If the LFS interface is used the file size on 32-bit systems is not limited to 2GB (offsets are signed which reduces the addressable area of 4GB by half); the full 64-bit are available. Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. To determine the size of a page the machine uses one should use size_t page_size = (size_t) sysconf (_SC_PAGESIZE); These functions are declared in `sys/mman.h'. - Function: void * mmap (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off_t OFFSET) The `mmap' function creates a new mapping, connected to bytes (OFFSET) to (OFFSET + LENGTH - 1) in the file open on FILEDES. A new reference for the file specified by FILEDES is created, which is not removed by closing the file. ADDRESS gives a preferred starting address for the mapping. `NULL' expresses no preference. Any previous mapping at that address is automatically removed. The address you give may still be changed, unless you use the `MAP_FIXED' flag. PROTECT contains flags that control what kind of access is permitted. They include `PROT_READ', `PROT_WRITE', and `PROT_EXEC', which permit reading, writing, and execution, respectively. Inappropriate access will cause a segfault (*note Program Error Signals::). Note that most hardware designs cannot support write permission without read permission, and many do not distinguish read and execute permission. Thus, you may receive wider permissions than you ask for, and mappings of write-only files may be denied even if you do not use `PROT_READ'. FLAGS contains flags that control the nature of the map. One of `MAP_SHARED' or `MAP_PRIVATE' must be specified. They include: `MAP_PRIVATE' This specifies that writes to the region should never be written back to the attached file. Instead, a copy is made for the process, and the region will be swapped normally if memory runs low. No other process will see the changes. Since private mappings effectively revert to ordinary memory when written to, you must have enough virtual memory for a copy of the entire mmapped region if you use this mode with `PROT_WRITE'. `MAP_SHARED' This specifies that writes to the region will be written back to the file. Changes made will be shared immediately with other processes mmaping the same file. Note that actual writing may take place at any time. You need to use `msync', described below, if it is important that other processes using conventional I/O get a consistent view of the file. `MAP_FIXED' This forces the system to use the exact mapping address specified in ADDRESS and fail if it can't. `MAP_ANONYMOUS' `MAP_ANON' This flag tells the system to create an anonymous mapping, not connected to a file. FILEDES and OFF are ignored, and the region is initialized with zeros. Anonymous maps are used as the basic primitive to extend the heap on some systems. They are also useful to share data between multiple tasks without creating a file. On some systems using private anonymous mmaps is more efficient than using `malloc' for large blocks. This is not an issue with the GNU C library, as the included `malloc' automatically uses `mmap' where appropriate. `mmap' returns the address of the new mapping, or -1 for an error. Possible errors include: `EINVAL' Either ADDRESS was unusable, or inconsistent FLAGS were given. `EACCES' FILEDES was not open for the type of access specified in PROTECT. `ENOMEM' Either there is not enough memory for the operation, or the process is out of address space. `ENODEV' This file is of a type that doesn't support mapping. `ENOEXEC' The file is on a filesystem that doesn't support mapping. - Function: void * mmap64 (void *ADDRESS, size_t LENGTH,int PROTECT, int FLAGS, int FILEDES, off64_t OFFSET) The `mmap64' function is equivalent to the `mmap' function but the OFFSET parameter is of type `off64_t'. On 32-bit systems this allows the file associated with the FILEDES descriptor to be larger than 2GB. FILEDES must be a descriptor returned from a call to `open64' or `fopen64' and `freopen64' where the descriptor is retrieved with `fileno'. When the sources are translated with `_FILE_OFFSET_BITS == 64' this function is actually available under the name `mmap'. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API. - Function: int munmap (void *ADDR, size_t LENGTH) `munmap' removes any memory maps from (ADDR) to (ADDR + LENGTH). LENGTH should be the length of the mapping. It is safe to unmap multiple mappings in one command, or include unmapped space in the range. It is also possible to unmap only part of an existing mapping. However, only entire pages can be removed. If LENGTH is not an even number of pages, it will be rounded up. It returns 0 for success and -1 for an error. One error is possible: `EINVAL' The memory range given was outside the user mmap range or wasn't page aligned. - Function: int msync (void *ADDRESS, size_t LENGTH, int FLAGS) When using shared mappings, the kernel can write the file at any time before the mapping is removed. To be certain data has actually been written to the file and will be accessible to non-memory-mapped I/O, it is necessary to use this function. It operates on the region ADDRESS to (ADDRESS + LENGTH). It may be used on part of a mapping or multiple mappings, however the region given should not contain any unmapped space. FLAGS can contain some options: `MS_SYNC' This flag makes sure the data is actually written _to disk_. Normally `msync' only makes sure that accesses to a file with conventional I/O reflect the recent changes. `MS_ASYNC' This tells `msync' to begin the synchronization, but not to wait for it to complete. `msync' returns 0 for success and -1 for error. Errors include: `EINVAL' An invalid region was given, or the FLAGS were invalid. `EFAULT' There is no existing mapping in at least part of the given region. - Function: void * mremap (void *ADDRESS, size_t LENGTH, size_t NEW_LENGTH, int FLAG) This function can be used to change the size of an existing memory area. ADDRESS and LENGTH must cover a region entirely mapped in the same `mmap' statement. A new mapping with the same characteristics will be returned with the length NEW_LENGTH. One option is possible, `MREMAP_MAYMOVE'. If it is given in FLAGS, the system may remove the existing mapping and create a new one of the desired length in another location. The address of the resulting mapping is returned, or -1. Possible error codes include: `EFAULT' There is no existing mapping in at least part of the original region, or the region covers two or more distinct mappings. `EINVAL' The address given is misaligned or inappropriate. `EAGAIN' The region has pages locked, and if extended it would exceed the process's resource limit for locked pages. *Note Limits on Resources::. `ENOMEM' The region is private writable, and insufficient virtual memory is available to extend it. Also, this error will occur if `MREMAP_MAYMOVE' is not given and the extension would collide with another mapped region. This function is only available on a few systems. Except for performing optional optimizations one should not rely on this function. Not all file descriptors may be mapped. Sockets, pipes, and most devices only allow sequential access and do not fit into the mapping abstraction. In addition, some regular files may not be mmapable, and older kernels may not support mapping at all. Thus, programs using `mmap' should have a fallback method to use should it fail. *Note Mmap: (standards)Mmap. - Function: int madvise (void *ADDR, size_t LENGTH, int ADVICE) This function can be used to provide the system with ADVICE about the intended usage patterns of the memory region starting at ADDR and extending LENGTH bytes. The valid BSD values for ADVICE are: `MADV_NORMAL' The region should receive no further special treatment. `MADV_RANDOM' The region will be accessed via random page references. The kernel should page-in the minimal number of pages for each page fault. `MADV_SEQUENTIAL' The region will be accessed via sequential page references. This may cause the kernel to aggressively read-ahead, expecting further sequential references after any page fault within this region. `MADV_WILLNEED' The region will be needed. The pages within this region may be pre-faulted in by the kernel. `MADV_DONTNEED' The region is no longer needed. The kernel may free these pages, causing any changes to the pages to be lost, as well as swapped out pages to be discarded. The POSIX names are slightly different, but with the same meanings: `POSIX_MADV_NORMAL' This corresponds with BSD's `MADV_NORMAL'. `POSIX_MADV_RANDOM' This corresponds with BSD's `MADV_RANDOM'. `POSIX_MADV_SEQUENTIAL' This corresponds with BSD's `MADV_SEQUENTIAL'. `POSIX_MADV_WILLNEED' This corresponds with BSD's `MADV_WILLNEED'. `POSIX_MADV_DONTNEED' This corresponds with BSD's `MADV_DONTNEED'. `msync' returns 0 for success and -1 for error. Errors include: `EINVAL' An invalid region was given, or the ADVICE was invalid. `EFAULT' There is no existing mapping in at least part of the given region. Waiting for Input or Output =========================== Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets. You cannot normally use `read' for this purpose, because this blocks the program until input is available on one particular file descriptor; input on other channels won't wake it up. You could set nonblocking mode and poll each file descriptor in turn, but this is very inefficient. A better solution is to use the `select' function. This blocks the program until input or output is ready on a specified set of file descriptors, or until a timer expires, whichever comes first. This facility is declared in the header file `sys/types.h'. In the case of a server socket (*note Listening::), we say that "input" is available when there are pending connections that could be accepted (*note Accepting Connections::). `accept' for server sockets blocks and interacts with `select' just as `read' does for normal input. The file descriptor sets for the `select' function are specified as `fd_set' objects. Here is the description of the data type and some macros for manipulating these objects. - Data Type: fd_set The `fd_set' data type represents file descriptor sets for the `select' function. It is actually a bit array. - Macro: int FD_SETSIZE The value of this macro is the maximum number of file descriptors that a `fd_set' object can hold information about. On systems with a fixed maximum number, `FD_SETSIZE' is at least that number. On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an `fd_set'; if you get a file descriptor with a value as high as `FD_SETSIZE', you cannot put that descriptor into an `fd_set'. - Macro: void FD_ZERO (fd_set *SET) This macro initializes the file descriptor set SET to be the empty set. - Macro: void FD_SET (int FILEDES, fd_set *SET) This macro adds FILEDES to the file descriptor set SET. - Macro: void FD_CLR (int FILEDES, fd_set *SET) This macro removes FILEDES from the file descriptor set SET. - Macro: int FD_ISSET (int FILEDES, fd_set *SET) This macro returns a nonzero value (true) if FILEDES is a member of the file descriptor set SET, and zero (false) otherwise. Next, here is the description of the `select' function itself. - Function: int select (int NFDS, fd_set *READ-FDS, fd_set *WRITE-FDS, fd_set *EXCEPT-FDS, struct timeval *TIMEOUT) The `select' function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired. The file descriptors specified by the READ-FDS argument are checked to see if they are ready for reading; the WRITE-FDS file descriptors are checked to see if they are ready for writing; and the EXCEPT-FDS file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition. A file descriptor is considered ready for reading if it is not at end of file. A server socket is considered ready for reading if there is a pending connection which can be accepted with `accept'; *note Accepting Connections::. A client socket is ready for writing when its connection is fully established; *note Connecting::. "Exceptional conditions" does not mean errors--errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (*Note Sockets::, for information on urgent messages.) The `select' function checks only the first NFDS file descriptors. The usual thing is to pass `FD_SETSIZE' as the value of this argument. The TIMEOUT specifies the maximum time to wait. If you pass a null pointer for this argument, it means to block indefinitely until one of the file descriptors is ready. Otherwise, you should provide the time in `struct timeval' format; see *Note High-Resolution Calendar::. Specify zero as the time (a `struct timeval' containing all zeros) if you want to find out which descriptors are ready without waiting if none are ready. The normal return value from `select' is the total number of ready file descriptors in all of the sets. Each of the argument sets is overwritten with information about the descriptors that are ready for the corresponding operation. Thus, to see if a particular descriptor DESC has input, use `FD_ISSET (DESC, READ-FDS)' after `select' returns. If `select' returns because the timeout period expires, it returns a value of zero. Any signal will cause `select' to return immediately. So if your program uses signals, you can't rely on `select' to keep waiting for the full time specified. If you want to be sure of waiting for a particular amount of time, you must check for `EINTR' and repeat the `select' with a newly calculated timeout based on the current time. See the example below. See also *Note Interrupted Primitives::. If an error occurs, `select' returns `-1' and does not modify the argument file descriptor sets. The following `errno' error conditions are defined for this function: `EBADF' One of the file descriptor sets specified an invalid file descriptor. `EINTR' The operation was interrupted by a signal. *Note Interrupted Primitives::. `EINVAL' The TIMEOUT argument is invalid; one of the components is negative or too large. *Portability Note:* The `select' function is a BSD Unix feature. Here is an example showing how you can use `select' to establish a timeout period for reading from a file descriptor. The `input_timeout' function blocks the calling process until input is available on the file descriptor, or until the timeout period expires. #include #include #include #include int input_timeout (int filedes, unsigned int seconds) { fd_set set; struct timeval timeout; /* Initialize the file descriptor set. */ FD_ZERO (&set); FD_SET (filedes, &set); /* Initialize the timeout data structure. */ timeout.tv_sec = seconds; timeout.tv_usec = 0; /* `select' returns 0 if timeout, 1 if input available, -1 if error. */ return TEMP_FAILURE_RETRY (select (FD_SETSIZE, &set, NULL, NULL, &timeout)); } int main (void) { fprintf (stderr, "select returned %d.\n", input_timeout (STDIN_FILENO, 5)); return 0; } There is another example showing the use of `select' to multiplex input from multiple sockets in *Note Server Example::. Synchronizing I/O operations ============================ In most modern operating systems, the normal I/O operations are not executed synchronously. I.e., even if a `write' system call returns, this does not mean the data is actually written to the media, e.g., the disk. In situations where synchronization points are necessary, you can use special functions which ensure that all operations finish before they return. - Function: int sync (void) A call to this function will not return as long as there is data which has not been written to the device. All dirty buffers in the kernel will be written and so an overall consistent system can be achieved (if no other process in parallel writes data). A prototype for `sync' can be found in `unistd.h'. The return value is zero to indicate no error. Programs more often want to ensure that data written to a given file is committed, rather than all data in the system. For this, `sync' is overkill. - Function: int fsync (int FILDES) The `fsync' function can be used to make sure all data associated with the open file FILDES is written to the device associated with the descriptor. The function call does not return unless all actions have finished. A prototype for `fsync' can be found in `unistd.h'. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `fsync' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to `fsync' should be protected using cancellation handlers. The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable ERRNO is set to the following values: `EBADF' The descriptor FILDES is not valid. `EINVAL' No synchronization is possible since the system does not implement this. Sometimes it is not even necessary to write all data associated with a file descriptor. E.g., in database files which do not change in size it is enough to write all the file content data to the device. Meta-information, like the modification time etc., are not that important and leaving such information uncommitted does not prevent a successful recovering of the file in case of a problem. - Function: int fdatasync (int FILDES) When a call to the `fdatasync' function returns, it is ensured that all of the file data is written to the device. For all pending I/O operations, the parts guaranteeing data integrity finished. Not all systems implement the `fdatasync' operation. On systems missing this functionality `fdatasync' is emulated by a call to `fsync' since the performed actions are a superset of those required by `fdatasync'. The prototype for `fdatasync' is in `unistd.h'. The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable ERRNO is set to the following values: `EBADF' The descriptor FILDES is not valid. `EINVAL' No synchronization is possible since the system does not implement this. Perform I/O Operations in Parallel ================================== The POSIX.1b standard defines a new set of I/O operations which can significantly reduce the time an application spends waiting at I/O. The new functions allow a program to initiate one or more I/O operations and then immediately resume normal work while the I/O operations are executed in parallel. This functionality is available if the `unistd.h' file defines the symbol `_POSIX_ASYNCHRONOUS_IO'. These functions are part of the library with realtime functions named `librt'. They are not actually part of the `libc' binary. The implementation of these functions can be done using support in the kernel (if available) or using an implementation based on threads at userlevel. In the latter case it might be necessary to link applications with the thread library `libpthread' in addition to `librt'. All AIO operations operate on files which were opened previously. There might be arbitrarily many operations running for one file. The asynchronous I/O operations are controlled using a data structure named `struct aiocb' ("AIO control block"). It is defined in `aio.h' as follows. - Data Type: struct aiocb The POSIX.1b standard mandates that the `struct aiocb' structure contains at least the members described in the following table. There might be more elements which are used by the implementation, but depending upon these elements is not portable and is highly deprecated. `int aio_fildes' This element specifies the file descriptor to be used for the operation. It must be a legal descriptor, otherwise the operation will fail. The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an `lseek' call would lead to an error. `off_t aio_offset' This element specifies the offset in the file at which the operation (input or output) is performed. Since the operations are carried out in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor. `volatile void *aio_buf' This is a pointer to the buffer with the data to be written or the place where the read data is stored. `size_t aio_nbytes' This element specifies the length of the buffer pointed to by `aio_buf'. `int aio_reqprio' If the platform has defined `_POSIX_PRIORITIZED_IO' and `_POSIX_PRIORITY_SCHEDULING', the AIO requests are processed based on the current scheduling priority. The `aio_reqprio' element can then be used to lower the priority of the AIO operation. `struct sigevent aio_sigevent' This element specifies how the calling process is notified once the operation terminates. If the `sigev_notify' element is `SIGEV_NONE', no notification is sent. If it is `SIGEV_SIGNAL', the signal determined by `sigev_signo' is sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD'. In this case, a thread is created which starts executing the function pointed to by `sigev_notify_function'. `int aio_lio_opcode' This element is only used by the `lio_listio' and `lio_listio64' functions. Since these functions allow an arbitrary number of operations to start at once, and each operation can be input or output (or nothing), the information must be stored in the control block. The possible values are: `LIO_READ' Start a read operation. Read from the file at position `aio_offset' and store the next `aio_nbytes' bytes in the buffer pointed to by `aio_buf'. `LIO_WRITE' Start a write operation. Write `aio_nbytes' bytes starting at `aio_buf' into the file starting at position `aio_offset'. `LIO_NOP' Do nothing for this control block. This value is useful sometimes when an array of `struct aiocb' values contains holes, i.e., some of the values must not be handled although the whole array is presented to the `lio_listio' function. When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine, this type is in fact `struct aiocb64', since the LFS interface transparently replaces the `struct aiocb' definition. For use with the AIO functions defined in the LFS, there is a similar type defined which replaces the types of the appropriate members with larger types but otherwise is equivalent to `struct aiocb'. Particularly, all member names are the same. - Data Type: struct aiocb64 `int aio_fildes' This element specifies the file descriptor which is used for the operation. It must be a legal descriptor since otherwise the operation fails for obvious reasons. The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an `lseek' call would lead to an error. `off64_t aio_offset' This element specifies at which offset in the file the operation (input or output) is performed. Since the operation are carried in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor. `volatile void *aio_buf' This is a pointer to the buffer with the data to be written or the place where the read data is stored. `size_t aio_nbytes' This element specifies the length of the buffer pointed to by `aio_buf'. `int aio_reqprio' If for the platform `_POSIX_PRIORITIZED_IO' and `_POSIX_PRIORITY_SCHEDULING' are defined the AIO requests are processed based on the current scheduling priority. The `aio_reqprio' element can then be used to lower the priority of the AIO operation. `struct sigevent aio_sigevent' This element specifies how the calling process is notified once the operation terminates. If the `sigev_notify', element is `SIGEV_NONE' no notification is sent. If it is `SIGEV_SIGNAL', the signal determined by `sigev_signo' is sent. Otherwise, `sigev_notify' must be `SIGEV_THREAD' in which case a thread which starts executing the function pointed to by `sigev_notify_function'. `int aio_lio_opcode' This element is only used by the `lio_listio' and `[lio_listio64' functions. Since these functions allow an arbitrary number of operations to start at once, and since each operation can be input or output (or nothing), the information must be stored in the control block. See the description of `struct aiocb' for a description of the possible values. When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a 32 bit machine, this type is available under the name `struct aiocb64', since the LFS transparently replaces the old interface. Asynchronous Read and Write Operations -------------------------------------- - Function: int aio_read (struct aiocb *AIOCBP) This function initiates an asynchronous read operation. It immediately returns after the operation was enqueued or when an error was encountered. The first `aiocbp->aio_nbytes' bytes of the file for which `aiocbp->aio_fildes' is a descriptor are written to the buffer starting at `aiocbp->aio_buf'. Reading starts at the absolute position `aiocbp->aio_offset' in the file. If prioritized I/O is supported by the platform the `aiocbp->aio_reqprio' value is used to adjust the priority before the request is actually enqueued. The calling process is notified about the termination of the read request according to the `aiocbp->aio_sigevent' value. When `aio_read' returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found, the function returns -1 and sets `errno' to one of the following values: `EAGAIN' The request was not enqueued due to (temporarily) exceeded resource limitations. `ENOSYS' The `aio_read' function is not implemented. `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously. `EINVAL' The `aiocbp->aio_offset' or `aiocbp->aio_reqpiro' value is invalid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously. If `aio_read' returns zero, the current status of the request can be queried using `aio_error' and `aio_return' functions. As long as the value returned by `aio_error' is `EINPROGRESS' the operation has not yet completed. If `aio_error' returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be obtained using a call to `aio_return'. The returned value is the same as an equivalent call to `read' would have returned. Possible error codes returned by `aio_error' are: `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. `ECANCELED' The operation was canceled before the operation was finished (*note Cancel AIO Operations::) `EINVAL' The `aiocbp->aio_offset' value is invalid. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_read64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_read64 (struct aiocb *AIOCBP) This function is similar to the `aio_read' function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, `aio_read64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the reading, as opposed to `lseek' functionality used in `aio_read'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_read' and so transparently replaces the interface for small files on 32 bit machines. To write data asynchronously to a file, there exists an equivalent pair of functions with a very similar interface. - Function: int aio_write (struct aiocb *AIOCBP) This function initiates an asynchronous write operation. The function call immediately returns after the operation was enqueued or if before this happens an error was encountered. The first `aiocbp->aio_nbytes' bytes from the buffer starting at `aiocbp->aio_buf' are written to the file for which `aiocbp->aio_fildes' is an descriptor, starting at the absolute position `aiocbp->aio_offset' in the file. If prioritized I/O is supported by the platform, the `aiocbp->aio_reqprio' value is used to adjust the priority before the request is actually enqueued. The calling process is notified about the termination of the read request according to the `aiocbp->aio_sigevent' value. When `aio_write' returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found the function returns -1 and sets `errno' to one of the following values. `EAGAIN' The request was not enqueued due to (temporarily) exceeded resource limitations. `ENOSYS' The `aio_write' function is not implemented. `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. This condition may not be recognized before enqueueing the request, and so this error might also be signaled asynchronously. `EINVAL' The `aiocbp->aio_offset' or `aiocbp->aio_reqprio' value is invalid. This condition may not be recognized before enqueueing the request and so this error might also be signaled asynchronously. In the case `aio_write' returns zero, the current status of the request can be queried using `aio_error' and `aio_return' functions. As long as the value returned by `aio_error' is `EINPROGRESS' the operation has not yet completed. If `aio_error' returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be get using a call to `aio_return'. The returned value is the same as an equivalent call to `read' would have returned. Possible error codes returned by `aio_error' are: `EBADF' The `aiocbp->aio_fildes' descriptor is not valid. `ECANCELED' The operation was canceled before the operation was finished. (*note Cancel AIO Operations::) `EINVAL' The `aiocbp->aio_offset' value is invalid. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `aio_write64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_write64 (struct aiocb *AIOCBP) This function is similar to the `aio_write' function. The only difference is that on 32 bit machines the file descriptor should be opened in the large file mode. Internally `aio_write64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the writing, as opposed to `lseek' functionality used in `aio_write'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_write' and so transparently replaces the interface for small files on 32 bit machines. Besides these functions with the more or less traditional interface, POSIX.1b also defines a function which can initiate more than one operation at a time, and which can handle freely mixed read and write operations. It is therefore similar to a combination of `readv' and `writev'. - Function: int lio_listio (int MODE, struct aiocb *const LIST[], int NENT, struct sigevent *SIG) The `lio_listio' function can be used to enqueue an arbitrary number of read and write requests at one time. The requests can all be meant for the same file, all for different files or every solution in between. `lio_listio' gets the NENT requests from the array pointed to by LIST. The operation to be performed is determined by the `aio_lio_opcode' member in each element of LIST. If this field is `LIO_READ' a read operation is enqueued, similar to a call of `aio_read' for this element of the array (except that the way the termination is signalled is different, as we will see below). If the `aio_lio_opcode' member is `LIO_WRITE' a write operation is enqueued. Otherwise the `aio_lio_opcode' must be `LIO_NOP' in which case this element of LIST is simply ignored. This "operation" is useful in situations where one has a fixed array of `struct aiocb' elements from which only a few need to be handled at a time. Another situation is where the `lio_listio' call was canceled before all requests are processed (*note Cancel AIO Operations::) and the remaining requests have to be reissued. The other members of each element of the array pointed to by `list' must have values suitable for the operation as described in the documentation for `aio_read' and `aio_write' above. The MODE argument determines how `lio_listio' behaves after having enqueued all the requests. If MODE is `LIO_WAIT' it waits until all requests terminated. Otherwise MODE must be `LIO_NOWAIT' and in this case the function returns immediately after having enqueued all the requests. In this case the caller gets a notification of the termination of all requests according to the SIG parameter. If SIG is `NULL' no notification is send. Otherwise a signal is sent or a thread is started, just as described in the description for `aio_read' or `aio_write'. If MODE is `LIO_WAIT', the return value of `lio_listio' is 0 when all requests completed successfully. Otherwise the function return -1 and `errno' is set accordingly. To find out which request or requests failed one has to use the `aio_error' function on all the elements of the array LIST. In case MODE is `LIO_NOWAIT', the function returns 0 if all requests were enqueued correctly. The current state of the requests can be found using `aio_error' and `aio_return' as described above. If `lio_listio' returns -1 in this mode, the global variable `errno' is set accordingly. If a request did not yet terminate, a call to `aio_error' returns `EINPROGRESS'. If the value is different, the request is finished and the error value (or 0) is returned and the result of the operation can be retrieved using `aio_return'. Possible values for `errno' are: `EAGAIN' The resources necessary to queue all the requests are not available at the moment. The error status for each element of LIST must be checked to determine which request failed. Another reason could be that the system wide limit of AIO requests is exceeded. This cannot be the case for the implementation on GNU systems since no arbitrary limits exist. `EINVAL' The MODE parameter is invalid or NENT is larger than `AIO_LISTIO_MAX'. `EIO' One or more of the request's I/O operations failed. The error status of each request should be checked to determine which one failed. `ENOSYS' The `lio_listio' function is not supported. If the MODE parameter is `LIO_NOWAIT' and the caller cancels a request, the error status for this request returned by `aio_error' is `ECANCELED'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `lio_listio64' since the LFS interface transparently replaces the normal implementation. - Function: int lio_listio64 (int MODE, struct aiocb *const LIST, int NENT, struct sigevent *SIG) This function is similar to the `lio_listio' function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, `lio_listio64' uses functionality equivalent to `lseek64' (*note File Position Primitive::) to position the file descriptor correctly for the reading or writing, as opposed to `lseek' functionality used in `lio_listio'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `lio_listio' and so transparently replaces the interface for small files on 32 bit machines. Getting the Status of AIO Operations ------------------------------------ As already described in the documentation of the functions in the last section, it must be possible to get information about the status of an I/O request. When the operation is performed truly asynchronously (as with `aio_read' and `aio_write' and with `lio_listio' when the mode is `LIO_NOWAIT'), one sometimes needs to know whether a specific request already terminated and if so, what the result was. The following two functions allow you to get this kind of information. - Function: int aio_error (const struct aiocb *AIOCBP) This function determines the error state of the request described by the `struct aiocb' variable pointed to by AIOCBP. If the request has not yet terminated the value returned is always `EINPROGRESS'. Once the request has terminated the value `aio_error' returns is either 0 if the request completed successfully or it returns the value which would be stored in the `errno' variable if the request would have been done using `read', `write', or `fsync'. The function can return `ENOSYS' if it is not implemented. It could also return `EINVAL' if the AIOCBP parameter does not refer to an asynchronous operation whose return status is not yet known. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_error64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_error64 (const struct aiocb64 *AIOCBP) This function is similar to `aio_error' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_error' and so transparently replaces the interface for small files on 32 bit machines. - Function: ssize_t aio_return (const struct aiocb *AIOCBP) This function can be used to retrieve the return status of the operation carried out by the request described in the variable pointed to by AIOCBP. As long as the error status of this request as returned by `aio_error' is `EINPROGRESS' the return of this function is undefined. Once the request is finished this function can be used exactly once to retrieve the return value. Following calls might lead to undefined behavior. The return value itself is the value which would have been returned by the `read', `write', or `fsync' call. The function can return `ENOSYS' if it is not implemented. It could also return `EINVAL' if the AIOCBP parameter does not refer to an asynchronous operation whose return status is not yet known. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_return64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_return64 (const struct aiocb64 *AIOCBP) This function is similar to `aio_return' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_return' and so transparently replaces the interface for small files on 32 bit machines. Getting into a Consistent State ------------------------------- When dealing with asynchronous operations it is sometimes necessary to get into a consistent state. This would mean for AIO that one wants to know whether a certain request or a group of request were processed. This could be done by waiting for the notification sent by the system after the operation terminated, but this sometimes would mean wasting resources (mainly computation time). Instead POSIX.1b defines two functions which will help with most kinds of consistency. The `aio_fsync' and `aio_fsync64' functions are only available if the symbol `_POSIX_SYNCHRONIZED_IO' is defined in `unistd.h'. - Function: int aio_fsync (int OP, struct aiocb *AIOCBP) Calling this function forces all I/O operations operating queued at the time of the function call operating on the file descriptor `aiocbp->aio_fildes' into the synchronized I/O completion state (*note Synchronizing I/O::). The `aio_fsync' function returns immediately but the notification through the method described in `aiocbp->aio_sigevent' will happen only after all requests for this file descriptor have terminated and the file is synchronized. This also means that requests for this very same file descriptor which are queued after the synchronization request are not affected. If OP is `O_DSYNC' the synchronization happens as with a call to `fdatasync'. Otherwise OP should be `O_SYNC' and the synchronization happens as with `fsync'. As long as the synchronization has not happened, a call to `aio_error' with the reference to the object pointed to by AIOCBP returns `EINPROGRESS'. Once the synchronization is done `aio_error' return 0 if the synchronization was not successful. Otherwise the value returned is the value to which the `fsync' or `fdatasync' function would have set the `errno' variable. In this case nothing can be assumed about the consistency for the data written to this file descriptor. The return value of this function is 0 if the request was successfully enqueued. Otherwise the return value is -1 and `errno' is set to one of the following values: `EAGAIN' The request could not be enqueued due to temporary lack of resources. `EBADF' The file descriptor `aiocbp->aio_fildes' is not valid or not open for writing. `EINVAL' The implementation does not support I/O synchronization or the OP parameter is other than `O_DSYNC' and `O_SYNC'. `ENOSYS' This function is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_return64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_fsync64 (int OP, struct aiocb64 *AIOCBP) This function is similar to `aio_fsync' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_fsync' and so transparently replaces the interface for small files on 32 bit machines. Another method of synchronization is to wait until one or more requests of a specific set terminated. This could be achieved by the `aio_*' functions to notify the initiating process about the termination but in some situations this is not the ideal solution. In a program which constantly updates clients somehow connected to the server it is not always the best solution to go round robin since some connections might be slow. On the other hand letting the `aio_*' function notify the caller might also be not the best solution since whenever the process works on preparing data for on client it makes no sense to be interrupted by a notification since the new client will not be handled before the current client is served. For situations like this `aio_suspend' should be used. - Function: int aio_suspend (const struct aiocb *const LIST[], int NENT, const struct timespec *TIMEOUT) When calling this function, the calling thread is suspended until at least one of the requests pointed to by the NENT elements of the array LIST has completed. If any of the requests has already completed at the time `aio_suspend' is called, the function returns immediately. Whether a request has terminated or not is determined by comparing the error status of the request with `EINPROGRESS'. If an element of LIST is `NULL', the entry is simply ignored. If no request has finished, the calling process is suspended. If TIMEOUT is `NULL', the process is not woken until a request has finished. If TIMEOUT is not `NULL', the process remains suspended at least as long as specified in TIMEOUT. In this case, `aio_suspend' returns with an error. The return value of the function is 0 if one or more requests from the LIST have terminated. Otherwise the function returns -1 and `errno' is set to one of the following values: `EAGAIN' None of the requests from the LIST completed in the time specified by TIMEOUT. `EINTR' A signal interrupted the `aio_suspend' function. This signal might also be sent by the AIO implementation while signalling the termination of one of the requests. `ENOSYS' The `aio_suspend' function is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `aio_suspend64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_suspend64 (const struct aiocb64 *const LIST[], int NENT, const struct timespec *TIMEOUT) This function is similar to `aio_suspend' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `aio_suspend' and so transparently replaces the interface for small files on 32 bit machines. Cancellation of AIO Operations ------------------------------ When one or more requests are asynchronously processed, it might be useful in some situations to cancel a selected operation, e.g., if it becomes obvious that the written data is no longer accurate and would have to be overwritten soon. As an example, assume an application, which writes data in files in a situation where new incoming data would have to be written in a file which will be updated by an enqueued request. The POSIX AIO implementation provides such a function, but this function is not capable of forcing the cancellation of the request. It is up to the implementation to decide whether it is possible to cancel the operation or not. Therefore using this function is merely a hint. - Function: int aio_cancel (int FILDES, struct aiocb *AIOCBP) The `aio_cancel' function can be used to cancel one or more outstanding requests. If the AIOCBP parameter is `NULL', the function tries to cancel all of the outstanding requests which would process the file descriptor FILDES (i.e., whose `aio_fildes' member is FILDES). If AIOCBP is not `NULL', `aio_cancel' attempts to cancel the specific request pointed to by AIOCBP. For requests which were successfully canceled, the normal notification about the termination of the request should take place. I.e., depending on the `struct sigevent' object which controls this, nothing happens, a signal is sent or a thread is started. If the request cannot be canceled, it terminates the usual way after performing the operation. After a request is successfully canceled, a call to `aio_error' with a reference to this request as the parameter will return `ECANCELED' and a call to `aio_return' will return -1. If the request wasn't canceled and is still running the error status is still `EINPROGRESS'. The return value of the function is `AIO_CANCELED' if there were requests which haven't terminated and which were successfully canceled. If there is one or more requests left which couldn't be canceled, the return value is `AIO_NOTCANCELED'. In this case `aio_error' must be used to find out which of the, perhaps multiple, requests (in AIOCBP is `NULL') weren't successfully canceled. If all requests already terminated at the time `aio_cancel' is called the return value is `AIO_ALLDONE'. If an error occurred during the execution of `aio_cancel' the function returns -1 and sets `errno' to one of the following values. `EBADF' The file descriptor FILDES is not valid. `ENOSYS' `aio_cancel' is not implemented. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is in fact `aio_cancel64' since the LFS interface transparently replaces the normal implementation. - Function: int aio_cancel64 (int FILDES, struct aiocb64 *AIOCBP) This function is similar to `aio_cancel' with the only difference that the argument is a reference to a variable of type `struct aiocb64'. When the sources are compiled with `_FILE_OFFSET_BITS == 64', this function is available under the name `aio_cancel' and so transparently replaces the interface for small files on 32 bit machines. How to optimize the AIO implementation -------------------------------------- The POSIX standard does not specify how the AIO functions are implemented. They could be system calls, but it is also possible to emulate them at userlevel. At the point of this writing, the available implementation is a userlevel implementation which uses threads for handling the enqueued requests. While this implementation requires making some decisions about limitations, hard limitations are something which is best avoided in the GNU C library. Therefore, the GNU C library provides a means for tuning the AIO implementation according to the individual use. - Data Type: struct aioinit This data type is used to pass the configuration or tunable parameters to the implementation. The program has to initialize the members of this struct and pass it to the implementation using the `aio_init' function. `int aio_threads' This member specifies the maximal number of threads which may be used at any one time. `int aio_num' This number provides an estimate on the maximal number of simultaneously enqueued requests. `int aio_locks' Unused. `int aio_usedba' Unused. `int aio_debug' Unused. `int aio_numusers' Unused. `int aio_reserved[2]' Unused. - Function: void aio_init (const struct aioinit *INIT) This function must be called before any other AIO function. Calling it is completely voluntary, as it is only meant to help the AIO implementation perform better. Before calling the `aio_init', function the members of a variable of type `struct aioinit' must be initialized. Then a reference to this variable is passed as the parameter to `aio_init' which itself may or may not pay attention to the hints. The function has no return value and no error cases are defined. It is a extension which follows a proposal from the SGI implementation in Irix 6. It is not covered by POSIX.1b or Unix98. Control Operations on Files =========================== This section describes how you can perform various other operations on file descriptors, such as inquiring about or setting flags describing the status of the file descriptor, manipulating record locks, and the like. All of these operations are performed by the function `fcntl'. The second argument to the `fcntl' function is a command that specifies which operation to perform. The function and macros that name various flags that are used with it are declared in the header file `fcntl.h'. Many of these flags are also used by the `open' function; see *Note Opening and Closing Files::. - Function: int fcntl (int FILEDES, int COMMAND, ...) The `fcntl' function performs the operation specified by COMMAND on the file descriptor FILEDES. Some commands require additional arguments to be supplied. These additional arguments and the return value and error conditions are given in the detailed descriptions of the individual commands. Briefly, here is a list of what the various commands are. `F_DUPFD' Duplicate the file descriptor (return another file descriptor pointing to the same open file). *Note Duplicating Descriptors::. `F_GETFD' Get flags associated with the file descriptor. *Note Descriptor Flags::. `F_SETFD' Set flags associated with the file descriptor. *Note Descriptor Flags::. `F_GETFL' Get flags associated with the open file. *Note File Status Flags::. `F_SETFL' Set flags associated with the open file. *Note File Status Flags::. `F_GETLK' Get a file lock. *Note File Locks::. `F_SETLK' Set or clear a file lock. *Note File Locks::. `F_SETLKW' Like `F_SETLK', but wait for completion. *Note File Locks::. `F_GETOWN' Get process or process group ID to receive `SIGIO' signals. *Note Interrupt Input::. `F_SETOWN' Set process or process group ID to receive `SIGIO' signals. *Note Interrupt Input::. This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time `fcntl' is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to `fcntl' should be protected using cancellation handlers. Duplicating Descriptors ======================= You can "duplicate" a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (*note File Status Flags::), but each has its own set of file descriptor flags (*note Descriptor Flags::). The major use of duplicating a file descriptor is to implement "redirection" of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to. You can perform this operation using the `fcntl' function with the `F_DUPFD' command, but there are also convenient functions `dup' and `dup2' for duplicating descriptors. The `fcntl' function and flags are declared in `fcntl.h', while prototypes for `dup' and `dup2' are in the header file `unistd.h'. - Function: int dup (int OLD) This function copies descriptor OLD to the first available descriptor number (the first number not currently open). It is equivalent to `fcntl (OLD, F_DUPFD, 0)'. - Function: int dup2 (int OLD, int NEW) This function copies the descriptor OLD to descriptor number NEW. If OLD is an invalid descriptor, then `dup2' does nothing; it does not close NEW. Otherwise, the new duplicate of OLD replaces any previous meaning of descriptor NEW, as if NEW were closed first. If OLD and NEW are different numbers, and OLD is a valid descriptor number, then `dup2' is equivalent to: close (NEW); fcntl (OLD, F_DUPFD, NEW) However, `dup2' does this atomically; there is no instant in the middle of calling `dup2' at which NEW is closed and not yet a duplicate of OLD. - Macro: int F_DUPFD This macro is used as the COMMAND argument to `fcntl', to copy the file descriptor given as the first argument. The form of the call in this case is: fcntl (OLD, F_DUPFD, NEXT-FILEDES) The NEXT-FILEDES argument is of type `int' and specifies that the file descriptor returned should be the next available one greater than or equal to this value. The return value from `fcntl' with this command is normally the value of the new file descriptor. A return value of -1 indicates an error. The following `errno' error conditions are defined for this command: `EBADF' The OLD argument is invalid. `EINVAL' The NEXT-FILEDES argument is invalid. `EMFILE' There are no more file descriptors available--your program is already using the maximum. In BSD and GNU, the maximum is controlled by a resource limit that can be changed; *note Limits on Resources::, for more information about the `RLIMIT_NOFILE' limit. `ENFILE' is not a possible error code for `dup2' because `dup2' does not create a new opening of a file; duplicate descriptors do not count toward the limit which `ENFILE' indicates. `EMFILE' is possible because it refers to the limit on distinct descriptor numbers in use in one process. Here is an example showing how to use `dup2' to do redirection. Typically, redirection of the standard streams (like `stdin') is done by a shell or shell-like program before calling one of the `exec' functions (*note Executing a File::) to execute a new program in a child process. When the new program is executed, it creates and initializes the standard streams to point to the corresponding file descriptors, before its `main' function is invoked. So, to redirect standard input to a file, the shell could do something like: pid = fork (); if (pid == 0) { char *filename; char *program; int file; ... file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY)); dup2 (file, STDIN_FILENO); TEMP_FAILURE_RETRY (close (file)); execv (program, NULL); } There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in *Note Launching Jobs::. File Descriptor Flags ===================== "File descriptor flags" are miscellaneous attributes of a file descriptor. These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags. Currently there is just one file descriptor flag: `FD_CLOEXEC', which causes the descriptor to be closed if you use any of the `exec...' functions (*note Executing a File::). The symbols in this section are defined in the header file `fcntl.h'. - Macro: int F_GETFD This macro is used as the COMMAND argument to `fcntl', to specify that it should return the file descriptor flags associated with the FILEDES argument. The normal return value from `fcntl' with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags (except that currently there is only one flag to use). In case of an error, `fcntl' returns -1. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETFD This macro is used as the COMMAND argument to `fcntl', to specify that it should set the file descriptor flags associated with the FILEDES argument. This requires a third `int' argument to specify the new flags, so the form of the call is: fcntl (FILEDES, F_SETFD, NEW-FLAGS) The normal return value from `fcntl' with this command is an unspecified value other than -1, which indicates an error. The flags and error conditions are the same as for the `F_GETFD' command. The following macro is defined for use as a file descriptor flag with the `fcntl' function. The value is an integer constant usable as a bit mask value. - Macro: int FD_CLOEXEC This flag specifies that the file descriptor should be closed when an `exec' function is invoked; see *Note Executing a File::. When a file descriptor is allocated (as with `open' or `dup'), this bit is initially cleared on the new file descriptor, meaning that descriptor will survive into the new program after `exec'. If you want to modify the file descriptor flags, you should get the current flags with `F_GETFD' and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag `FD_CLOEXEC' without altering any other flags: /* Set the `FD_CLOEXEC' flag of DESC if VALUE is nonzero, or clear the flag if VALUE is 0. Return 0 on success, or -1 on error with `errno' set. */ int set_cloexec_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFD, 0); /* If reading the flags failed, return error indication now. if (oldflags < 0) return oldflags; /* Set just the flag we want to set. */ if (value != 0) oldflags |= FD_CLOEXEC; else oldflags &= ~FD_CLOEXEC; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFD, oldflags); } File Status Flags ================= "File status flags" are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in *Note Descriptor Flags::, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file. The file status flags are specified with the FLAGS argument to `open'; *note Opening and Closing Files::. File status flags fall into three categories, which are described in the following sections. * *Note Access Modes::, specify what type of access is allowed to the file: reading, writing, or both. They are set by `open' and are returned by `fcntl', but cannot be changed. * *Note Open-time Flags::, control details of what `open' will do. These flags are not preserved after the `open' call. * *Note Operating Modes::, affect how operations such as `read' and `write' are done. They are set by `open', and can be fetched or changed with `fcntl'. The symbols in this section are defined in the header file `fcntl.h'. File Access Modes ----------------- The file access modes allow a file descriptor to be used for reading, writing, or both. (In the GNU system, they can also allow none of these, and allow execution of the file as a program.) The access modes are chosen when the file is opened, and never change. - Macro: int O_RDONLY Open the file for read access. - Macro: int O_WRONLY Open the file for write access. - Macro: int O_RDWR Open the file for both reading and writing. In the GNU system (and not in other systems), `O_RDONLY' and `O_WRONLY' are independent bits that can be bitwise-ORed together, and it is valid for either bit to be set or clear. This means that `O_RDWR' is the same as `O_RDONLY|O_WRONLY'. A file access mode of zero is permissible; it allows no operations that do input or output to the file, but does allow other operations such as `fchmod'. On the GNU system, since "read-only" or "write-only" is a misnomer, `fcntl.h' defines additional names for the file access modes. These names are preferred when writing GNU-specific code. But most programs will want to be portable to other POSIX.1 systems and should use the POSIX.1 names above instead. - Macro: int O_READ Open the file for reading. Same as `O_RDWR'; only defined on GNU. - Macro: int O_WRITE Open the file for reading. Same as `O_WRONLY'; only defined on GNU. - Macro: int O_EXEC Open the file for executing. Only defined on GNU. To determine the file access mode with `fcntl', you must extract the access mode bits from the retrieved file status flags. In the GNU system, you can just test the `O_READ' and `O_WRITE' bits in the flags word. But in other POSIX.1 systems, reading and writing access modes are not stored as distinct bit flags. The portable way to extract the file access mode bits is with `O_ACCMODE'. - Macro: int O_ACCMODE This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be `O_RDONLY', `O_WRONLY', or `O_RDWR'. (In the GNU system it could also be zero, and it never includes the `O_EXEC' bit.) Open-time Flags --------------- The open-time flags specify options affecting how `open' will behave. These options are not preserved once the file is open. The exception to this is `O_NONBLOCK', which is also an I/O operating mode and so it _is_ saved. *Note Opening and Closing Files::, for how to call `open'. There are two sorts of options specified by open-time flags. * "File name translation flags" affect how `open' looks up the file name to locate the file, and whether the file can be created. * "Open-time action flags" specify extra operations that `open' will perform on the file once it is open. Here are the file name translation flags. - Macro: int O_CREAT If set, the file will be created if it doesn't already exist. - Macro: int O_EXCL If both `O_CREAT' and `O_EXCL' are set, then `open' fails if the specified file already exists. This is guaranteed to never clobber an existing file. - Macro: int O_NONBLOCK This prevents `open' from blocking for a "long time" to open the file. This is only meaningful for some kinds of files, usually devices such as serial ports; when it is not meaningful, it is harmless and ignored. Often opening a port to a modem blocks until the modem reports carrier detection; if `O_NONBLOCK' is specified, `open' will return immediately without a carrier. Note that the `O_NONBLOCK' flag is overloaded as both an I/O operating mode and a file name translation flag. This means that specifying `O_NONBLOCK' in `open' also sets nonblocking I/O mode; *note Operating Modes::. To open the file without blocking but do normal I/O that blocks, you must call `open' with `O_NONBLOCK' set and then call `fcntl' to turn the bit off. - Macro: int O_NOCTTY If the named file is a terminal device, don't make it the controlling terminal for the process. *Note Job Control::, for information about what it means to be the controlling terminal. In the GNU system and 4.4 BSD, opening a file never makes it the controlling terminal and `O_NOCTTY' is zero. However, other systems may use a nonzero value for `O_NOCTTY' and set the controlling terminal when you open a file that is a terminal device; so to be portable, use `O_NOCTTY' when it is important to avoid this. The following three file name translation flags exist only in the GNU system. - Macro: int O_IGNORE_CTTY Do not recognize the named file as the controlling terminal, even if it refers to the process's existing controlling terminal device. Operations on the new file descriptor will never induce job control signals. *Note Job Control::. - Macro: int O_NOLINK If the named file is a symbolic link, open the link itself instead of the file it refers to. (`fstat' on the new file descriptor will return the information returned by `lstat' on the link's name.) - Macro: int O_NOTRANS If the named file is specially translated, do not invoke the translator. Open the bare file the translator itself sees. The open-time action flags tell `open' to do additional operations which are not really related to opening the file. The reason to do them as part of `open' instead of in separate calls is that `open' can do them atomically. - Macro: int O_TRUNC Truncate the file to zero length. This option is only useful for regular files, not special files such as directories or FIFOs. POSIX.1 requires that you open the file for writing to use `O_TRUNC'. In BSD and GNU you must have permission to write the file to truncate it, but you need not open for write access. This is the only open-time action flag specified by POSIX.1. There is no good reason for truncation to be done by `open', instead of by calling `ftruncate' afterwards. The `O_TRUNC' flag existed in Unix before `ftruncate' was invented, and is retained for backward compatibility. The remaining operating modes are BSD extensions. They exist only on some systems. On other systems, these macros are not defined. - Macro: int O_SHLOCK Acquire a shared lock on the file, as with `flock'. *Note File Locks::. If `O_CREAT' is specified, the locking is done atomically when creating the file. You are guaranteed that no other process will get the lock on the new file first. - Macro: int O_EXLOCK Acquire an exclusive lock on the file, as with `flock'. *Note File Locks::. This is atomic like `O_SHLOCK'. I/O Operating Modes ------------------- The operating modes affect how input and output operations using a file descriptor work. These flags are set by `open' and can be fetched and changed with `fcntl'. - Macro: int O_APPEND The bit that enables append mode for the file. If set, then all `write' operations write the data at the end of the file, extending it, regardless of the current file position. This is the only reliable way to append to a file. In append mode, you are guaranteed that the data you write will always go to the current end of the file, regardless of other processes writing to the file. Conversely, if you simply set the file position to the end of file and write, then another process can extend the file after you set the file position but before you write, resulting in your data appearing someplace before the real end of file. - Macro: int O_NONBLOCK The bit that enables nonblocking mode for the file. If this bit is set, `read' requests on the file can return immediately with a failure status if there is no input immediately available, instead of blocking. Likewise, `write' requests can also return immediately with a failure status if the output can't be written immediately. Note that the `O_NONBLOCK' flag is overloaded as both an I/O operating mode and a file name translation flag; *note Open-time Flags::. - Macro: int O_NDELAY This is an obsolete name for `O_NONBLOCK', provided for compatibility with BSD. It is not defined by the POSIX.1 standard. The remaining operating modes are BSD and GNU extensions. They exist only on some systems. On other systems, these macros are not defined. - Macro: int O_ASYNC The bit that enables asynchronous input mode. If set, then `SIGIO' signals will be generated when input is available. *Note Interrupt Input::. Asynchronous input mode is a BSD feature. - Macro: int O_FSYNC The bit that enables synchronous writing for the file. If set, each `write' call will make sure the data is reliably stored on disk before returning. Synchronous writing is a BSD feature. - Macro: int O_SYNC This is another name for `O_FSYNC'. They have the same value. - Macro: int O_NOATIME If this bit is set, `read' will not update the access time of the file. *Note File Times::. This is used by programs that do backups, so that backing a file up does not count as reading it. Only the owner of the file or the superuser may use this bit. This is a GNU extension. Getting and Setting File Status Flags ------------------------------------- The `fcntl' function can fetch or change file status flags. - Macro: int F_GETFL This macro is used as the COMMAND argument to `fcntl', to read the file status flags for the open file with descriptor FILEDES. The normal return value from `fcntl' with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with `O_ACCMODE' to compare them. In case of an error, `fcntl' returns -1. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETFL This macro is used as the COMMAND argument to `fcntl', to set the file status flags for the open file corresponding to the FILEDES argument. This command requires a third `int' argument to specify the new flags, so the call looks like this: fcntl (FILEDES, F_SETFL, NEW-FLAGS) You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing. The normal return value from `fcntl' with this command is an unspecified value other than -1, which indicates an error. The error conditions are the same as for the `F_GETFL' command. If you want to modify the file status flags, you should get the current flags with `F_GETFL' and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag `O_NONBLOCK' without altering any other flags: /* Set the `O_NONBLOCK' flag of DESC if VALUE is nonzero, or clear the flag if VALUE is 0. Return 0 on success, or -1 on error with `errno' set. */ int set_nonblock_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFL, 0); /* If reading the flags failed, return error indication now. */ if (oldflags == -1) return -1; /* Set just the flag we want to set. */ if (value != 0) oldflags |= O_NONBLOCK; else oldflags &= ~O_NONBLOCK; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFL, oldflags); } File Locks ========== The remaining `fcntl' commands are used to support "record locking", which permits multiple cooperating programs to prevent each other from simultaneously accessing parts of a file in error-prone ways. An "exclusive" or "write" lock gives a process exclusive access for writing to the specified part of the file. While a write lock is in place, no other process can lock that part of the file. A "shared" or "read" lock prohibits any other process from requesting a write lock on the specified part of the file. However, other processes can request read locks. The `read' and `write' functions do not actually check to see whether there are any locks in place. If you want to implement a locking protocol for a file shared by multiple processes, your application must do explicit `fcntl' calls to request and clear locks at the appropriate points. Locks are associated with processes. A process can only have one kind of lock set for each byte of a given file. When any file descriptor for that file is closed by the process, all of the locks that process holds on that file are released, even if the locks were made using other descriptors that remain open. Likewise, locks are released when a process exits, and are not inherited by child processes created using `fork' (*note Creating a Process::). When making a lock, use a `struct flock' to specify what kind of lock and where. This data type and the associated macros for the `fcntl' function are declared in the header file `fcntl.h'. - Data Type: struct flock This structure is used with the `fcntl' function to describe a file lock. It has these members: `short int l_type' Specifies the type of the lock; one of `F_RDLCK', `F_WRLCK', or `F_UNLCK'. `short int l_whence' This corresponds to the WHENCE argument to `fseek' or `lseek', and specifies what the offset is relative to. Its value can be one of `SEEK_SET', `SEEK_CUR', or `SEEK_END'. `off_t l_start' This specifies the offset of the start of the region to which the lock applies, and is given in bytes relative to the point specified by `l_whence' member. `off_t l_len' This specifies the length of the region to be locked. A value of `0' is treated specially; it means the region extends to the end of the file. `pid_t l_pid' This field is the process ID (*note Process Creation Concepts::) of the process holding the lock. It is filled in by calling `fcntl' with the `F_GETLK' command, but is ignored when making a lock. - Macro: int F_GETLK This macro is used as the COMMAND argument to `fcntl', to specify that it should get information about a lock. This command requires a third argument of type `struct flock *' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_GETLK, LOCKP) If there is a lock already in place that would block the lock described by the LOCKP argument, information about that lock overwrites `*LOCKP'. Existing locks are not reported if they are compatible with making a new lock as specified. Thus, you should specify a lock type of `F_WRLCK' if you want to find out about both read and write locks, or `F_RDLCK' if you want to find out about write locks only. There might be more than one lock affecting the region specified by the LOCKP argument, but `fcntl' only returns information about one of them. The `l_whence' member of the LOCKP structure is set to `SEEK_SET' and the `l_start' and `l_len' fields set to identify the locked region. If no lock applies, the only change to the LOCKP structure is to update the `l_type' to a value of `F_UNLCK'. The normal return value from `fcntl' with this command is an unspecified value other than -1, which is reserved to indicate an error. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. `EINVAL' Either the LOCKP argument doesn't specify valid lock information, or the file associated with FILEDES doesn't support locks. - Macro: int F_SETLK This macro is used as the COMMAND argument to `fcntl', to specify that it should set or clear a lock. This command requires a third argument of type `struct flock *' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_SETLK, LOCKP) If the process already has a lock on any part of the region, the old lock on that part is replaced with the new lock. You can remove a lock by specifying a lock type of `F_UNLCK'. If the lock cannot be set, `fcntl' returns immediately with a value of -1. This function does not block waiting for other processes to release locks. If `fcntl' succeeds, it return a value other than -1. The following `errno' error conditions are defined for this function: `EAGAIN' `EACCES' The lock cannot be set because it is blocked by an existing lock on the file. Some systems use `EAGAIN' in this case, and other systems use `EACCES'; your program should treat them alike, after `F_SETLK'. (The GNU system always uses `EAGAIN'.) `EBADF' Either: the FILEDES argument is invalid; you requested a read lock but the FILEDES is not open for read access; or, you requested a write lock but the FILEDES is not open for write access. `EINVAL' Either the LOCKP argument doesn't specify valid lock information, or the file associated with FILEDES doesn't support locks. `ENOLCK' The system has run out of file lock resources; there are already too many file locks in place. Well-designed file systems never report this error, because they have no limitation on the number of locks. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine. - Macro: int F_SETLKW This macro is used as the COMMAND argument to `fcntl', to specify that it should set or clear a lock. It is just like the `F_SETLK' command, but causes the process to block (or wait) until the request can be specified. This command requires a third argument of type `struct flock *', as for the `F_SETLK' command. The `fcntl' return values and errors are the same as for the `F_SETLK' command, but these additional `errno' error conditions are defined for this command: `EINTR' The function was interrupted by a signal while it was waiting. *Note Interrupted Primitives::. `EDEADLK' The specified region is being locked by another process. But that process is waiting to lock a region which the current process has locked, so waiting for the lock would result in deadlock. The system does not guarantee that it will detect all such conditions, but it lets you know if it notices one. The following macros are defined for use as values for the `l_type' member of the `flock' structure. The values are integer constants. `F_RDLCK' This macro is used to specify a read (or shared) lock. `F_WRLCK' This macro is used to specify a write (or exclusive) lock. `F_UNLCK' This macro is used to specify that the region is unlocked. As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file. One example of such a program might be a game that uses a file to keep track of high scores. Another example might be a program that records usage or accounting information for billing purposes. Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up. But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file. If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock. While the read lock is set, no other process can lock that part of the file for writing. Remember that file locks are only a _voluntary_ protocol for controlling access to a file. There is still potential for access to the file by programs that don't use the lock protocol. Interrupt-Driven Input ====================== If you set the `O_ASYNC' status flag on a file descriptor (*note File Status Flags::), a `SIGIO' signal is sent whenever input or output becomes possible on that file descriptor. The process or process group to receive the signal can be selected by using the `F_SETOWN' command to the `fcntl' function. If the file descriptor is a socket, this also selects the recipient of `SIGURG' signals that are delivered when out-of-band data arrives on that socket; see *Note Out-of-Band Data::. (`SIGURG' is sent in any situation where `select' would report the socket as having an "exceptional condition". *Note Waiting for I/O::.) If the file descriptor corresponds to a terminal device, then `SIGIO' signals are sent to the foreground process group of the terminal. *Note Job Control::. The symbols in this section are defined in the header file `fcntl.h'. - Macro: int F_GETOWN This macro is used as the COMMAND argument to `fcntl', to specify that it should get information about the process or process group to which `SIGIO' signals are sent. (For a terminal, this is actually the foreground process group ID, which you can get using `tcgetpgrp'; see *Note Terminal Access Functions::.) The return value is interpreted as a process ID; if negative, its absolute value is the process group ID. The following `errno' error condition is defined for this command: `EBADF' The FILEDES argument is invalid. - Macro: int F_SETOWN This macro is used as the COMMAND argument to `fcntl', to specify that it should set the process or process group to which `SIGIO' signals are sent. This command requires a third argument of type `pid_t' to be passed to `fcntl', so that the form of the call is: fcntl (FILEDES, F_SETOWN, PID) The PID argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID. The return value from `fcntl' with this command is -1 in case of error and some other value if successful. The following `errno' error conditions are defined for this command: `EBADF' The FILEDES argument is invalid. `ESRCH' There is no process or process group corresponding to PID. Generic I/O Control operations ============================== The GNU system can handle most input/output operations on many different devices and objects in terms of a few file primitives - `read', `write' and `lseek'. However, most devices also have a few peculiar operations which do not fit into this model. Such as: * Changing the character font used on a terminal. * Telling a magnetic tape system to rewind or fast forward. (Since they cannot move in byte increments, `lseek' is inapplicable). * Ejecting a disk from a drive. * Playing an audio track from a CD-ROM drive. * Maintaining routing tables for a network. Although some such objects such as sockets and terminals (1) have special functions of their own, it would not be practical to create functions for all these cases. Instead these minor operations, known as "IOCTL"s, are assigned code numbers and multiplexed through the `ioctl' function, defined in `sys/ioctl.h'. The code numbers themselves are defined in many different headers. - Function: int ioctl (int FILEDES, int COMMAND, ...) The `ioctl' function performs the generic I/O operation COMMAND on FILEDES. A third argument is usually present, either a single number or a pointer to a structure. The meaning of this argument, the returned value, and any error codes depends upon the command used. Often -1 is returned for a failure. On some systems, IOCTLs used by different devices share the same numbers. Thus, although use of an inappropriate IOCTL _usually_ only produces an error, you should not attempt to use device-specific IOCTLs on an unknown device. Most IOCTLs are OS-specific and/or only used in special system utilities, and are thus beyond the scope of this document. For an example of the use of an IOCTL, see *Note Out-of-Band Data::. ---------- Footnotes ---------- (1) Actually, the terminal-specific functions are implemented with IOCTLs on many platforms. File System Interface ********************* This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions (*note I/O on Streams::; *note Low-Level I/O::), these functions are concerned with operating on the files themselves rather than on their contents. Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times. Working Directory ================= Each process has associated with it a directory, called its "current working directory" or simply "working directory", that is used in the resolution of relative file names (*note File Name Resolution::). When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database. You can find any user's home directory using the `getpwuid' or `getpwnam' functions; see *Note User Database::. Users can change the working directory using shell commands like `cd'. The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory. Prototypes for these functions are declared in the header file `unistd.h'. - Function: char * getcwd (char *BUFFER, size_t SIZE) The `getcwd' function returns an absolute file name representing the current working directory, storing it in the character array BUFFER that you provide. The SIZE argument is how you tell the system the allocation size of BUFFER. The GNU library version of this function also permits you to specify a null pointer for the BUFFER argument. Then `getcwd' allocates a buffer automatically, as with `malloc' (*note Unconstrained Allocation::). If the SIZE is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result. The return value is BUFFER on success and a null pointer on failure. The following `errno' error conditions are defined for this function: `EINVAL' The SIZE argument is zero and BUFFER is not a null pointer. `ERANGE' The SIZE argument is less than the length of the working directory name. You need to allocate a bigger array and try again. `EACCES' Permission to read or search a component of the file name was denied. You could implement the behavior of GNU's `getcwd (NULL, 0)' using only the standard behavior of `getcwd': char * gnu_getcwd () { size_t size = 100; while (1) { char *buffer = (char *) xmalloc (size); if (getcwd (buffer, size) == buffer) return buffer; free (buffer); if (errno != ERANGE) return 0; size *= 2; } } *Note Malloc Examples::, for information about `xmalloc', which is not a library function but is a customary name used in most GNU software. - Deprecated Function: char * getwd (char *BUFFER) This is similar to `getcwd', but has no way to specify the size of the buffer. The GNU library provides `getwd' only for backwards compatibility with BSD. The BUFFER argument should be a pointer to an array at least `PATH_MAX' bytes long (*note Limits for Files::). In the GNU system there is no limit to the size of a file name, so this is not necessarily enough space to contain the directory name. That is why this function is deprecated. - Function: char * get_current_dir_name (void) This `get_current_dir_name' function is bascially equivalent to `getcwd (NULL, 0)'. The only difference is that the value of the `PWD' variable is returned if this value is correct. This is a subtle difference which is visible if the path described by the `PWD' value is using one or more symbol links in which case the value returned by `getcwd' can resolve the symbol links and therefore yield a different result. This function is a GNU extension. - Function: int chdir (const char *FILENAME) This function is used to set the process's working directory to FILENAME. The normal, successful return value from `chdir' is `0'. A value of `-1' is returned to indicate an error. The `errno' error conditions defined for this function are the usual file name syntax errors (*note File Name Errors::), plus `ENOTDIR' if the file FILENAME is not a directory. - Function: int fchdir (int FILEDES) This function is used to set the process's working directory to directory associated with the file descriptor FILEDES. The normal, successful return value from `fchdir' is `0'. A value of `-1' is returned to indicate an error. The following `errno' error conditions are defined for this function: `EACCES' Read permission is denied for the directory named by `dirname'. `EBADF' The FILEDES argument is not a valid file descriptor. `ENOTDIR' The file descriptor FILEDES is not associated with a directory. `EINTR' The function call was interrupt by a signal. `EIO' An I/O error occurred. Accessing Directories ===================== The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu. The `opendir' function opens a "directory stream" whose elements are directory entries. You use the `readdir' function on the directory stream to retrieve these entries, represented as `struct dirent' objects. The name of the file for each entry is stored in the `d_name' member of this structure. There are obvious parallels here to the stream facilities for ordinary files, described in *Note I/O on Streams::. Format of a Directory Entry --------------------------- This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file `dirent.h'. - Data Type: struct dirent This is a structure type used to return information about directory entries. It contains the following fields: `char d_name[]' This is the null-terminated file name component. This is the only field you can count on in all POSIX systems. `ino_t d_fileno' This is the file serial number. For BSD compatibility, you can also refer to this member as `d_ino'. In the GNU system and most POSIX systems, for most files this the same as the `st_ino' member that `stat' will return for the file. *Note File Attributes::. `unsigned char d_namlen' This is the length of the file name, not including the terminating null character. Its type is `unsigned char' because that is the integer type of the appropriate size `unsigned char d_type' This is the type of the file, possibly unknown. The following constants are defined for its value: `DT_UNKNOWN' The type is unknown. On some systems this is the only value returned. `DT_REG' A regular file. `DT_DIR' A directory. `DT_FIFO' A named pipe, or FIFO. *Note FIFO Special Files::. `DT_SOCK' A local-domain socket. `DT_CHR' A character device. `DT_BLK' A block device. This member is a BSD extension. The symbol `_DIRENT_HAVE_D_TYPE' is defined if this member is available. On systems where it is used, it corresponds to the file type bits in the `st_mode' member of `struct statbuf'. If the value cannot be determine the member value is DT_UNKNOWN. These two macros convert between `d_type' values and `st_mode' values: - Function: int IFTODT (mode_t MODE) This returns the `d_type' value corresponding to MODE. - Function: mode_t DTTOIF (int DTYPE) This returns the `st_mode' value corresponding to DTYPE. This structure may contain additional members in the future. Their availability is always announced in the compilation environment by a macro names `_DIRENT_HAVE_D_XXX' where XXX is replaced by the name of the new member. For instance, the member `d_reclen' available on some systems is announced through the macro `_DIRENT_HAVE_D_RECLEN'. When a file has multiple names, each name has its own directory entry. The only way you can tell that the directory entries belong to a single file is that they have the same value for the `d_fileno' field. File attributes such as size, modification times etc., are part of the file itself, not of any particular directory entry. *Note File Attributes::. Opening a Directory Stream -------------------------- This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'. - Data Type: DIR The `DIR' data type represents a directory stream. You shouldn't ever allocate objects of the `struct dirent' or `DIR' data types, since the directory access functions do that for you. Instead, you refer to these objects using the pointers returned by the following functions. - Function: DIR * opendir (const char *DIRNAME) The `opendir' function opens and returns a directory stream for reading the directory whose file name is DIRNAME. The stream has type `DIR *'. If unsuccessful, `opendir' returns a null pointer. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Read permission is denied for the directory named by `dirname'. `EMFILE' The process has too many files open. `ENFILE' The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.) The `DIR' type is typically implemented using a file descriptor, and the `opendir' function in terms of the `open' function. *Note Low-Level I/O::. Directory streams and the underlying file descriptors are closed on `exec' (*note Executing a File::). In some situations it can be desirable to get hold of the file descriptor which is created by the `opendir' call. For instance, to switch the current working directory to the directory just read the `fchdir' function could be used. Historically the `DIR' type was exposed and programs could access the fields. This does not happen in the GNU C library. Instead a separate function is provided to allow access. - Function: int dirfd (DIR *DIRSTREAM) The function `dirfd' returns the file descriptor associated with the directory stream DIRSTREAM. This descriptor can be used until the directory is closed with `closedir'. If the directory stream implementation is not using file descriptors the return value is `-1'. Reading and Closing a Directory Stream -------------------------------------- This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file `dirent.h'. - Function: struct dirent * readdir (DIR *DIRSTREAM) This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call. *Portability Note:* On some systems `readdir' may not return entries for `.' and `..', even though these are always valid file names in any directory. *Note File Name Resolution::. If there are no more entries in the directory or an error is detected, `readdir' returns a null pointer. The following `errno' error conditions are defined for this function: `EBADF' The DIRSTREAM argument is not valid. `readdir' is not thread safe. Multiple threads using `readdir' on the same DIRSTREAM may overwrite the return value. Use `readdir_r' when this is critical. - Function: int readdir_r (DIR *DIRSTREAM, struct dirent *ENTRY, struct dirent **RESULT) This function is the reentrant version of `readdir'. Like `readdir' it returns the next entry from the directory. But to prevent conflicts between simultaneously running threads the result is not stored in statically allocated memory. Instead the argument ENTRY points to a place to store the result. The return value is `0' in case the next entry was read successfully. In this case a pointer to the result is returned in *RESULT. It is not required that *RESULT is the same as ENTRY. If something goes wrong while executing `readdir_r' the function returns a value indicating the error (as described for `readdir'). If there are no more directory entries, `readdir_r''s return value is `0', and *RESULT is set to `NULL'. *Portability Note:* On some systems `readdir_r' may not return a NUL terminated string for the file name, even when there is no `d_reclen' field in `struct dirent' and the file name is the maximum allowed size. Modern systems all have the `d_reclen' field, and on old systems multi-threading is not critical. In any case there is no such problem with the `readdir' function, so that even on systems without the `d_reclen' member one could use multiple threads by using external locking. It is also important to look at the definition of the `struct dirent' type. Simply passing a pointer to an object of this type for the second parameter of `readdir_r' might not be enough. Some systems don't define the `d_name' element sufficiently long. In this case the user has to provide additional space. There must be room for at least `NAME_MAX + 1' characters in the `d_name' array. Code to call `readdir_r' could look like this: union { struct dirent d; char b[offsetof (struct dirent, d_name) + NAME_MAX + 1]; } u; if (readdir_r (dir, &u.d, &res) == 0) ... To support large filesystems on 32-bit machines there are LFS variants of the last two functions. - Function: struct dirent64 * readdir64 (DIR *DIRSTREAM) The `readdir64' function is just like the `readdir' function except that it returns a pointer to a record of type `struct dirent64'. Some of the members of this data type (notably `d_ino') might have a different size to allow large filesystems. In all other aspects this function is equivalent to `readdir'. - Function: int readdir64_r (DIR *DIRSTREAM, struct dirent64 *ENTRY, struct dirent64 **RESULT) The `readdir64_r' function is equivalent to the `readdir_r' function except that it takes parameters of base type `struct dirent64' instead of `struct dirent' in the second and third position. The same precautions mentioned in the documentation of `readdir_r' also apply here. - Function: int closedir (DIR *DIRSTREAM) This function closes the directory stream DIRSTREAM. It returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for this function: `EBADF' The DIRSTREAM argument is not valid. Simple Program to List a Directory ---------------------------------- Here's a simple program that prints the names of the files in the current working directory: #include #include #include #include int main (void) { DIR *dp; struct dirent *ep; dp = opendir ("./"); if (dp != NULL) { while (ep = readdir (dp)) puts (ep->d_name); (void) closedir (dp); } else perror ("Couldn't open the directory"); return 0; } The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see *Note Scanning Directory Content::, and *Note Array Sort Function::. Random Access in a Directory Stream ----------------------------------- This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file `dirent.h'. - Function: void rewinddir (DIR *DIRSTREAM) The `rewinddir' function is used to reinitialize the directory stream DIRSTREAM, so that if you call `readdir' it returns information about the first entry in the directory again. This function also notices if files have been added or removed to the directory since it was opened with `opendir'. (Entries for these files might or might not be returned by `readdir' if they were added or removed since you last called `opendir' or `rewinddir'.) - Function: off_t telldir (DIR *DIRSTREAM) The `telldir' function returns the file position of the directory stream DIRSTREAM. You can use this value with `seekdir' to restore the directory stream to that position. - Function: void seekdir (DIR *DIRSTREAM, off_t POS) The `seekdir' function sets the file position of the directory stream DIRSTREAM to POS. The value POS must be the result of a previous call to `telldir' on this particular stream; closing and reopening the directory can invalidate values returned by `telldir'. Scanning the Content of a Directory ----------------------------------- A higher-level interface to the directory handling functions is the `scandir' function. With its help one can select a subset of the entries in a directory, possibly sort them and get a list of names as the result. - Function: int scandir (const char *DIR, struct dirent ***NAMELIST, int (*SELECTOR) (const struct dirent *), int (*CMP) (const void *, const void *)) The `scandir' function scans the contents of the directory selected by DIR. The result in *NAMELIST is an array of pointers to structure of type `struct dirent' which describe all selected directory entries and which is allocated using `malloc'. Instead of always getting all directory entries returned, the user supplied function SELECTOR can be used to decide which entries are in the result. Only the entries for which SELECTOR returns a non-zero value are selected. Finally the entries in *NAMELIST are sorted using the user-supplied function CMP. The arguments passed to the CMP function are of type `struct dirent **', therefore one cannot directly use the `strcmp' or `strcoll' functions; instead see the functions `alphasort' and `versionsort' below. The return value of the function is the number of entries placed in *NAMELIST. If it is `-1' an error occurred (either the directory could not be opened for reading or the malloc call failed) and the global variable `errno' contains more information on the error. As described above the fourth argument to the `scandir' function must be a pointer to a sorting function. For the convenience of the programmer the GNU C library contains implementations of functions which are very helpful for this purpose. - Function: int alphasort (const void *A, const void *B) The `alphasort' function behaves like the `strcoll' function (*note String/Array Comparison::). The difference is that the arguments are not string pointers but instead they are of type `struct dirent **'. The return value of `alphasort' is less than, equal to, or greater than zero depending on the order of the two entries A and B. - Function: int versionsort (const void *A, const void *B) The `versionsort' function is like `alphasort' except that it uses the `strverscmp' function internally. If the filesystem supports large files we cannot use the `scandir' anymore since the `dirent' structure might not able to contain all the information. The LFS provides the new type `struct dirent64'. To use this we need a new function. - Function: int scandir64 (const char *DIR, struct dirent64 ***NAMELIST, int (*SELECTOR) (const struct dirent64 *), int (*CMP) (const void *, const void *)) The `scandir64' function works like the `scandir' function except that the directory entries it returns are described by elements of type `struct dirent64'. The function pointed to by SELECTOR is again used to select the desired entries, except that SELECTOR now must point to a function which takes a `struct dirent64 *' parameter. Similarly the CMP function should expect its two arguments to be of type `struct dirent64 **'. As CMP is now a function of a different type, the functions `alphasort' and `versionsort' cannot be supplied for that argument. Instead we provide the two replacement functions below. - Function: int alphasort64 (const void *A, const void *B) The `alphasort64' function behaves like the `strcoll' function (*note String/Array Comparison::). The difference is that the arguments are not string pointers but instead they are of type `struct dirent64 **'. Return value of `alphasort64' is less than, equal to, or greater than zero depending on the order of the two entries A and B. - Function: int versionsort64 (const void *A, const void *B) The `versionsort64' function is like `alphasort64', excepted that it uses the `strverscmp' function internally. It is important not to mix the use of `scandir' and the 64-bit comparison functions or vice versa. There are systems on which this works but on others it will fail miserably. Simple Program to List a Directory, Mark II ------------------------------------------- Here is a revised version of the directory lister found above (*note Simple Directory Lister::). Using the `scandir' function we can avoid the functions which work directly with the directory contents. After the call the returned entries are available for direct use. #include #include static int one (struct dirent *unused) { return 1; } int main (void) { struct dirent **eps; int n; n = scandir ("./", &eps, one, alphasort); if (n >= 0) { int cnt; for (cnt = 0; cnt < n; ++cnt) puts (eps[cnt]->d_name); } else perror ("Couldn't open the directory"); return 0; } Note the simple selector function in this example. Since we want to see all directory entries we always return `1'. Working with Directory Trees ============================ The functions described so far for handling the files in a directory have allowed you to either retrieve the information bit by bit, or to process all the files as a group (see `scandir'). Sometimes it is useful to process whole hierarchies of directories and their contained files. The X/Open specification defines two functions to do this. The simpler form is derived from an early definition in System V systems and therefore this function is available on SVID-derived systems. The prototypes and required definitions can be found in the `ftw.h' header. There are four functions in this family: `ftw', `nftw' and their 64-bit counterparts `ftw64' and `nftw64'. These functions take as one of their arguments a pointer to a callback function of the appropriate type. - Data Type: __ftw_func_t int (*) (const char *, const struct stat *, int) The type of callback functions given to the `ftw' function. The first parameter points to the file name, the second parameter to an object of type `struct stat' which is filled in for the file named in the first parameter. The last parameter is a flag giving more information about the current file. It can have the following values: `FTW_F' The item is either a normal file or a file which does not fit into one of the following categories. This could be special files, sockets etc. `FTW_D' The item is a directory. `FTW_NS' The `stat' call failed and so the information pointed to by the second paramater is invalid. `FTW_DNR' The item is a directory which cannot be read. `FTW_SL' The item is a symbolic link. Since symbolic links are normally followed seeing this value in a `ftw' callback function means the referenced file does not exist. The situation for `nftw' is different. This value is only available if the program is compiled with `_BSD_SOURCE' or `_XOPEN_EXTENDED' defined before including the first header. The original SVID systems do not have symbolic links. If the sources are compiled with `_FILE_OFFSET_BITS == 64' this type is in fact `__ftw64_func_t' since this mode changes `struct stat' to be `struct stat64'. For the LFS interface and for use in the function `ftw64', the header `ftw.h' defines another function type. - Data Type: __ftw64_func_t int (*) (const char *, const struct stat64 *, int) This type is used just like `__ftw_func_t' for the callback function, but this time is called from `ftw64'. The second parameter to the function is a pointer to a variable of type `struct stat64' which is able to represent the larger values. - Data Type: __nftw_func_t int (*) (const char *, const struct stat *, int, struct FTW *) The first three arguments are the same as for the `__ftw_func_t' type. However for the third argument some additional values are defined to allow finer differentiation: `FTW_DP' The current item is a directory and all subdirectories have already been visited and reported. This flag is returned instead of `FTW_D' if the `FTW_DEPTH' flag is passed to `nftw' (see below). `FTW_SLN' The current item is a stale symbolic link. The file it points to does not exist. The last parameter of the callback function is a pointer to a structure with some extra information as described below. If the sources are compiled with `_FILE_OFFSET_BITS == 64' this type is in fact `__nftw64_func_t' since this mode changes `struct stat' to be `struct stat64'. For the LFS interface there is also a variant of this data type available which has to be used with the `nftw64' function. - Data Type: __nftw64_func_t int (*) (const char *, const struct stat64 *, int, struct FTW *) This type is used just like `__nftw_func_t' for the callback function, but this time is called from `nftw64'. The second parameter to the function is this time a pointer to a variable of type `struct stat64' which is able to represent the larger values. - Data Type: struct FTW The information contained in this structure helps in interpreting the name parameter and gives some information about the current state of the traversal of the directory hierarchy. `int base' The value is the offset into the string passed in the first parameter to the callback function of the beginning of the file name. The rest of the string is the path of the file. This information is especially important if the `FTW_CHDIR' flag was set in calling `nftw' since then the current directory is the one the current item is found in. `int level' Whilst processing, the code tracks how many directories down it has gone to find the current file. This nesting level starts at 0 for files in the initial directory (or is zero for the initial file if a file was passed). - Function: int ftw (const char *FILENAME, __ftw_func_t FUNC, int DESCRIPTORS) The `ftw' function calls the callback function given in the parameter FUNC for every item which is found in the directory specified by FILENAME and all directories below. The function follows symbolic links if necessary but does not process an item twice. If FILENAME is not a directory then it itself is the only object returned to the callback function. The file name passed to the callback function is constructed by taking the FILENAME parameter and appending the names of all passed directories and then the local file name. So the callback function can use this parameter to access the file. `ftw' also calls `stat' for the file and passes that information on to the callback function. If this `stat' call was not successful the failure is indicated by setting the third argument of the callback function to `FTW_NS'. Otherwise it is set according to the description given in the account of `__ftw_func_t' above. The callback function is expected to return 0 to indicate that no error occurred and that processing should continue. If an error occurred in the callback function or it wants `ftw' to return immediately, the callback function can return a value other than 0. This is the only correct way to stop the function. The program must not use `setjmp' or similar techniques to continue from another place. This would leave resources allocated by the `ftw' function unfreed. The DESCRIPTORS parameter to `ftw' specifies how many file descriptors it is allowed to consume. The function runs faster the more descriptors it can use. For each level in the directory hierarchy at most one descriptor is used, but for very deep ones any limit on open file descriptors for the process or the system may be exceeded. Moreover, file descriptor limits in a multi-threaded program apply to all the threads as a group, and therefore it is a good idea to supply a reasonable limit to the number of open descriptors. The return value of the `ftw' function is 0 if all callback function calls returned 0 and all actions performed by the `ftw' succeeded. If a function call failed (other than calling `stat' on an item) the function returns -1. If a callback function returns a value other than 0 this value is returned as the return value of `ftw'. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `ftw64', i.e. the LFS interface transparently replaces the old interface. - Function: int ftw64 (const char *FILENAME, __ftw64_func_t FUNC, int DESCRIPTORS) This function is similar to `ftw' but it can work on filesystems with large files. File information is reported using a variable of type `struct stat64' which is passed by reference to the callback function. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is available under the name `ftw' and transparently replaces the old implementation. - Function: int nftw (const char *FILENAME, __nftw_func_t FUNC, int DESCRIPTORS, int FLAG) The `nftw' function works like the `ftw' functions. They call the callback function FUNC for all items found in the directory FILENAME and below. At most DESCRIPTORS file descriptors are consumed during the `nftw' call. One difference is that the callback function is of a different type. It is of type `struct FTW *' and provides the callback function with the extra information described above. A second difference is that `nftw' takes a fourth argument, which is 0 or a bitwise-OR combination of any of the following values. `FTW_PHYS' While traversing the directory symbolic links are not followed. Instead symbolic links are reported using the `FTW_SL' value for the type parameter to the callback function. If the file referenced by a symbolic link does not exist `FTW_SLN' is returned instead. `FTW_MOUNT' The callback function is only called for items which are on the same mounted filesystem as the directory given by the FILENAME parameter to `nftw'. `FTW_CHDIR' If this flag is given the current working directory is changed to the directory of the reported object before the callback function is called. When `ntfw' finally returns the current directory is restored to its original value. `FTW_DEPTH' If this option is specified then all subdirectories and files within them are processed before processing the top directory itself (depth-first processing). This also means the type flag given to the callback function is `FTW_DP' and not `FTW_D'. The return value is computed in the same way as for `ftw'. `nftw' returns 0 if no failures occurred and all callback functions returned 0. In case of internal errors, such as memory problems, the return value is -1 and ERRNO is set accordingly. If the return value of a callback invocation was non-zero then that value is returned. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is in fact `nftw64', i.e. the LFS interface transparently replaces the old interface. - Function: int nftw64 (const char *FILENAME, __nftw64_func_t FUNC, int DESCRIPTORS, int FLAG) This function is similar to `nftw' but it can work on filesystems with large files. File information is reported using a variable of type `struct stat64' which is passed by reference to the callback function. When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32-bit system this function is available under the name `nftw' and transparently replaces the old implementation. Hard Links ========== In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others. To add a name to a file, use the `link' function. (The new name is also called a "hard link" to the file.) Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file's existing name or names. One file can have names in several directories, so the organization of the file system is not a strict hierarchy or tree. In most implementations, it is not possible to have hard links to the same file in multiple file systems. `link' reports an error if you try to make a hard link to the file from another file system when this cannot be done. The prototype for the `link' function is declared in the header file `unistd.h'. - Function: int link (const char *OLDNAME, const char *NEWNAME) The `link' function makes a new link to the existing file named by OLDNAME, under the new name NEWNAME. This function returns a value of `0' if it is successful and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::) for both OLDNAME and NEWNAME, the following `errno' error conditions are defined for this function: `EACCES' You are not allowed to write to the directory in which the new link is to be written. `EEXIST' There is already a file named NEWNAME. If you want to replace this link with a new link, you must remove the old link explicitly first. `EMLINK' There are already too many links to the file named by OLDNAME. (The maximum number of links to a file is `LINK_MAX'; see *Note Limits for Files::.) `ENOENT' The file named by OLDNAME doesn't exist. You can't make a link to a file that doesn't exist. `ENOSPC' The directory or file system that would contain the new link is full and cannot be extended. `EPERM' In the GNU system and some others, you cannot make links to directories. Many systems allow only privileged users to do so. This error is used to report the problem. `EROFS' The directory containing the new link can't be modified because it's on a read-only file system. `EXDEV' The directory specified in NEWNAME is on a different file system than the existing file. `EIO' A hardware error occurred while trying to read or write the to filesystem. Symbolic Links ============== The GNU system supports "soft links" or "symbolic links". This is a kind of "file" that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file. The reason symbolic links work the way they do is that special things happen when you try to open the link. The `open' function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead. The `stat' function likewise operates on the file that the symbolic link points to, instead of on the link itself. By contrast, other operations such as deleting or renaming the file operate on the link itself. The functions `readlink' and `lstat' also refrain from following symbolic links, because their purpose is to obtain information about the link. `link', the function that makes a hard link, does too. It makes a hard link to the symbolic link, which one rarely wants. Some systems have for some functions operating on files have a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the `sys/param.h' header file. - Macro: int MAXSYMLINKS The macro `MAXSYMLINKS' specifies how many symlinks some function will follow before returning `ELOOP'. Not all functions behave the same and this value is not the same a that returned for `_SC_SYMLOOP' by `sysconf'. In fact, the `sysconf' result can indicate that there is no fixed limit although `MAXSYMLINKS' exists and has a finite value. Prototypes for most of the functions listed in this section are in `unistd.h'. - Function: int symlink (const char *OLDNAME, const char *NEWNAME) The `symlink' function makes a symbolic link to OLDNAME named NEWNAME. The normal return value from `symlink' is `0'. A return value of `-1' indicates an error. In addition to the usual file name syntax errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EEXIST' There is already an existing file named NEWNAME. `EROFS' The file NEWNAME would exist on a read-only file system. `ENOSPC' The directory or file system cannot be extended to make the new link. `EIO' A hardware error occurred while reading or writing data on the disk. - Function: int readlink (const char *FILENAME, char *BUFFER, size_t SIZE) The `readlink' function gets the value of the symbolic link FILENAME. The file name that the link points to is copied into BUFFER. This file name string is _not_ null-terminated; `readlink' normally returns the number of characters copied. The SIZE argument specifies the maximum number of characters to copy, usually the allocation size of BUFFER. If the return value equals SIZE, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call `readlink' again. Here is an example: char * readlink_malloc (const char *filename) { int size = 100; while (1) { char *buffer = (char *) xmalloc (size); int nchars = readlink (filename, buffer, size); if (nchars < 0) return NULL; if (nchars < size) return buffer; free (buffer); size *= 2; } } A value of `-1' is returned in case of error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EINVAL' The named file is not a symbolic link. `EIO' A hardware error occurred while reading or writing data on the disk. In some situations it is desirable to resolve all the to get the real name of a file where no prefix names a symbolic link which is followed and no filename in the path is `.' or `..'. This is for instance desirable if files have to be compare in which case different names can refer to the same inode. - Function: char * canonicalize_file_name (const char *NAME) The `canonicalize_file_name' function returns the absolute name of the file named by NAME which contains no `.', `..' components nor any repeated path separators (`/') or symlinks. The result is passed back as the return value of the function in a block of memory allocated with `malloc'. If the result is not used anymore the memory should be freed with a call to `free'. In any of the path components except the last one is missing the function returns a NULL pointer. This is also what is returned if the length of the path reaches or exceeds `PATH_MAX' characters. In any case `errno' is set accordingly. `ENAMETOOLONG' The resulting path is too long. This error only occurs on systems which have a limit on the file name length. `EACCES' At least one of the path components is not readable. `ENOENT' The input file name is empty. `ENOENT' At least one of the path components does not exist. `ELOOP' More than `MAXSYMLINKS' many symlinks have been followed. This function is a GNU extension and is declared in `stdlib.h'. The Unix standard includes a similar function which differs from `canonicalize_file_name' in that the user has to provide the buffer where the result is placed in. - Function: char * realpath (const char *restrict NAME, char *restrict RESOLVED) The `realpath' function behaves just like `canonicalize_file_name' but instead of allocating a buffer for the result it is placed in the buffer pointed to by RESOLVED. One other difference is that the buffer RESOLVED will contain the part of the path component which does not exist or is not readable if the function returns `NULL' and `errno' is set to `EACCES' or `ENOENT'. This function is declared in `stdlib.h'. The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long path on systems which have no limits on the file name length. Deleting Files ============== You can delete a file with `unlink' or `remove'. Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other remaining names (*note Hard Links::), it remains accessible under those names. - Function: int unlink (const char *FILENAME) The `unlink' function deletes the file name FILENAME. If this is a file's sole name, the file itself is also deleted. (Actually, if any process has the file open when this happens, deletion is postponed until all processes have closed the file.) The function `unlink' is declared in the header file `unistd.h'. This function returns `0' on successful completion, and `-1' on error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Write permission is denied for the directory from which the file is to be removed, or the directory has the sticky bit set and you do not own the file. `EBUSY' This error indicates that the file is being used by the system in such a way that it can't be unlinked. For example, you might see this error if the file name specifies the root directory or a mount point for a file system. `ENOENT' The file name to be deleted doesn't exist. `EPERM' On some systems `unlink' cannot be used to delete the name of a directory, or at least can only be used this way by a privileged user. To avoid such problems, use `rmdir' to delete directories. (In the GNU system `unlink' can never delete the name of a directory.) `EROFS' The directory containing the file name to be deleted is on a read-only file system and can't be modified. - Function: int rmdir (const char *FILENAME) The `rmdir' function deletes a directory. The directory must be empty before it can be removed; in other words, it can only contain entries for `.' and `..'. In most other respects, `rmdir' behaves like `unlink'. There are two additional `errno' error conditions defined for `rmdir': `ENOTEMPTY' `EEXIST' The directory to be deleted is not empty. These two error codes are synonymous; some systems use one, and some use the other. The GNU system always uses `ENOTEMPTY'. The prototype for this function is declared in the header file `unistd.h'. - Function: int remove (const char *FILENAME) This is the ISO C function to remove a file. It works like `unlink' for files and like `rmdir' for directories. `remove' is declared in `stdio.h'. Renaming Files ============== The `rename' function is used to change a file's name. - Function: int rename (const char *OLDNAME, const char *NEWNAME) The `rename' function renames the file OLDNAME to NEWNAME. The file formerly accessible under the name OLDNAME is afterwards accessible as NEWNAME instead. (If the file had any other names aside from OLDNAME, it continues to have those names.) The directory containing the name NEWNAME must be on the same file system as the directory containing the name OLDNAME. One special case for `rename' is when OLDNAME and NEWNAME are two names for the same file. The consistent way to handle this case is to delete OLDNAME. However, in this case POSIX requires that `rename' do nothing and report success--which is inconsistent. We don't know what your operating system will do. If OLDNAME is not a directory, then any existing file named NEWNAME is removed during the renaming operation. However, if NEWNAME is the name of a directory, `rename' fails in this case. If OLDNAME is a directory, then either NEWNAME must not exist or it must name a directory that is empty. In the latter case, the existing directory named NEWNAME is deleted first. The name NEWNAME must not specify a subdirectory of the directory `oldname' which is being renamed. One useful feature of `rename' is that the meaning of NEWNAME changes "atomically" from any previously existing file by that name to its new meaning (i.e. the file that was called OLDNAME). There is no instant at which NEWNAME is non-existent "in between" the old meaning and the new meaning. If there is a system crash during the operation, it is possible for both names to still exist; but NEWNAME will always be intact if it exists at all. If `rename' fails, it returns `-1'. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' One of the directories containing NEWNAME or OLDNAME refuses write permission; or NEWNAME and OLDNAME are directories and write permission is refused for one of them. `EBUSY' A directory named by OLDNAME or NEWNAME is being used by the system in a way that prevents the renaming from working. This includes directories that are mount points for filesystems, and directories that are the current working directories of processes. `ENOTEMPTY' `EEXIST' The directory NEWNAME isn't empty. The GNU system always returns `ENOTEMPTY' for this, but some other systems return `EEXIST'. `EINVAL' OLDNAME is a directory that contains NEWNAME. `EISDIR' NEWNAME is a directory but the OLDNAME isn't. `EMLINK' The parent directory of NEWNAME would have too many links (entries). `ENOENT' The file OLDNAME doesn't exist. `ENOSPC' The directory that would contain NEWNAME has no room for another entry, and there is no space left in the file system to expand it. `EROFS' The operation would involve writing to a directory on a read-only file system. `EXDEV' The two file names NEWNAME and OLDNAME are on different file systems. Creating Directories ==================== Directories are created with the `mkdir' function. (There is also a shell command `mkdir' which does the same thing.) - Function: int mkdir (const char *FILENAME, mode_t MODE) The `mkdir' function creates a new, empty directory with name FILENAME. The argument MODE specifies the file permissions for the new directory file. *Note Permission Bits::, for more information about this. A return value of `0' indicates successful completion, and `-1' indicates failure. In addition to the usual file name syntax errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' Write permission is denied for the parent directory in which the new directory is to be added. `EEXIST' A file named FILENAME already exists. `EMLINK' The parent directory has too many links (entries). Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine. `ENOSPC' The file system doesn't have enough room to create the new directory. `EROFS' The parent directory of the directory being created is on a read-only file system and cannot be modified. To use this function, your program should include the header file `sys/stat.h'. File Attributes =============== When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, etc. These are called the "file attributes", and are associated with the file itself and not a particular one of its names. This section contains information about how you can inquire about and modify the attributes of a file. The meaning of the File Attributes ---------------------------------- When you read the attributes of a file, they come back in a structure called `struct stat'. This section describes the names of the attributes, their data types, and what they mean. For the functions to read the attributes of a file, see *Note Reading Attributes::. The header file `sys/stat.h' declares all the symbols defined in this section. - Data Type: struct stat The `stat' structure type is used to return information about the attributes of a file. It contains at least the following members: `mode_t st_mode' Specifies the mode of the file. This includes file type information (*note Testing File Type::) and the file permission bits (*note Permission Bits::). `ino_t st_ino' The file serial number, which distinguishes this file from all other files on the same device. `dev_t st_dev' Identifies the device containing the file. The `st_ino' and `st_dev', taken together, uniquely identify the file. The `st_dev' value is not necessarily consistent across reboots or system crashes, however. `nlink_t st_nlink' The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total. `uid_t st_uid' The user ID of the file's owner. *Note File Owner::. `gid_t st_gid' The group ID of the file. *Note File Owner::. `off_t st_size' This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to. `time_t st_atime' This is the last access time for the file. *Note File Times::. `unsigned long int st_atime_usec' This is the fractional part of the last access time for the file. *Note File Times::. `time_t st_mtime' This is the time of the last modification to the contents of the file. *Note File Times::. `unsigned long int st_mtime_usec' This is the fractional part of the time of the last modification to the contents of the file. *Note File Times::. `time_t st_ctime' This is the time of the last modification to the attributes of the file. *Note File Times::. `unsigned long int st_ctime_usec' This is the fractional part of the time of the last modification to the attributes of the file. *Note File Times::. `blkcnt_t st_blocks' This is the amount of disk space that the file occupies, measured in units of 512-byte blocks. The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk. You can tell (approximately) whether a file is sparse by comparing this value with `st_size', like this: (st.st_blocks * 512 < st.st_size) This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem. `unsigned int st_blksize' The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to `st_blocks'.) The extensions for the Large File Support (LFS) require, even on 32-bit machines, types which can handle file sizes up to 2^63. Therefore a new definition of `struct stat' is necessary. - Data Type: struct stat64 The members of this type are the same and have the same names as those in `struct stat'. The only difference is that the members `st_ino', `st_size', and `st_blocks' have a different type to support larger values. `mode_t st_mode' Specifies the mode of the file. This includes file type information (*note Testing File Type::) and the file permission bits (*note Permission Bits::). `ino64_t st_ino' The file serial number, which distinguishes this file from all other files on the same device. `dev_t st_dev' Identifies the device containing the file. The `st_ino' and `st_dev', taken together, uniquely identify the file. The `st_dev' value is not necessarily consistent across reboots or system crashes, however. `nlink_t st_nlink' The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total. `uid_t st_uid' The user ID of the file's owner. *Note File Owner::. `gid_t st_gid' The group ID of the file. *Note File Owner::. `off64_t st_size' This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to. `time_t st_atime' This is the last access time for the file. *Note File Times::. `unsigned long int st_atime_usec' This is the fractional part of the last access time for the file. *Note File Times::. `time_t st_mtime' This is the time of the last modification to the contents of the file. *Note File Times::. `unsigned long int st_mtime_usec' This is the fractional part of the time of the last modification to the contents of the file. *Note File Times::. `time_t st_ctime' This is the time of the last modification to the attributes of the file. *Note File Times::. `unsigned long int st_ctime_usec' This is the fractional part of the time of the last modification to the attributes of the file. *Note File Times::. `blkcnt64_t st_blocks' This is the amount of disk space that the file occupies, measured in units of 512-byte blocks. `unsigned int st_blksize' The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to `st_blocks'.) Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them. - Data Type: mode_t This is an integer data type used to represent file modes. In the GNU system, this is equivalent to `unsigned int'. - Data Type: ino_t This is an arithmetic data type used to represent file serial numbers. (In Unix jargon, these are sometimes called "inode numbers".) In the GNU system, this type is equivalent to `unsigned long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `ino64_t'. - Data Type: ino64_t This is an arithmetic data type used to represent file serial numbers for the use in LFS. In the GNU system, this type is equivalent to `unsigned long longint'. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `ino_t'. - Data Type: dev_t This is an arithmetic data type used to represent file device numbers. In the GNU system, this is equivalent to `int'. - Data Type: nlink_t This is an arithmetic data type used to represent file link counts. In the GNU system, this is equivalent to `unsigned short int'. - Data Type: blkcnt_t This is an arithmetic data type used to represent block counts. In the GNU system, this is equivalent to `unsigned long int'. If the source is compiled with `_FILE_OFFSET_BITS == 64' this type is transparently replaced by `blkcnt64_t'. - Data Type: blkcnt64_t This is an arithmetic data type used to represent block counts for the use in LFS. In the GNU system, this is equivalent to `unsigned long long int'. When compiling with `_FILE_OFFSET_BITS == 64' this type is available under the name `blkcnt_t'. Reading the Attributes of a File -------------------------------- To examine the attributes of files, use the functions `stat', `fstat' and `lstat'. They return the attribute information in a `struct stat' object. All three functions are declared in the header file `sys/stat.h'. - Function: int stat (const char *FILENAME, struct stat *BUF) The `stat' function returns information about the attributes of the file named by FILENAME in the structure pointed to by BUF. If FILENAME is the name of a symbolic link, the attributes you get describe the file that the link points to. If the link points to a nonexistent file name, then `stat' fails reporting a nonexistent file. The return value is `0' if the operation is successful, or `-1' on failure. In addition to the usual file name errors (*note File Name Errors::, the following `errno' error conditions are defined for this function: `ENOENT' The file named by FILENAME doesn't exist. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `stat64' since the LFS interface transparently replaces the normal implementation. - Function: int stat64 (const char *FILENAME, struct stat64 *BUF) This function is similar to `stat' but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type `struct stat64' to which BUF must point. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `stat' and so transparently replaces the interface for small files on 32-bit machines. - Function: int fstat (int FILEDES, struct stat *BUF) The `fstat' function is like `stat', except that it takes an open file descriptor as an argument instead of a file name. *Note Low-Level I/O::. Like `stat', `fstat' returns `0' on success and `-1' on failure. The following `errno' error conditions are defined for `fstat': `EBADF' The FILEDES argument is not a valid file descriptor. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `fstat64' since the LFS interface transparently replaces the normal implementation. - Function: int fstat64 (int FILEDES, struct stat64 *BUF) This function is similar to `fstat' but is able to work on large files on 32-bit platforms. For large files the file descriptor FILEDES should be obtained by `open64' or `creat64'. The BUF pointer points to a variable of type `struct stat64' which is able to represent the larger values. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `fstat' and so transparently replaces the interface for small files on 32-bit machines. - Function: int lstat (const char *FILENAME, struct stat *BUF) The `lstat' function is like `stat', except that it does not follow symbolic links. If FILENAME is the name of a symbolic link, `lstat' returns information about the link itself; otherwise `lstat' works like `stat'. *Note Symbolic Links::. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is in fact `lstat64' since the LFS interface transparently replaces the normal implementation. - Function: int lstat64 (const char *FILENAME, struct stat64 *BUF) This function is similar to `lstat' but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type `struct stat64' to which BUF must point. When the sources are compiled with `_FILE_OFFSET_BITS == 64' this function is available under the name `lstat' and so transparently replaces the interface for small files on 32-bit machines. Testing the Type of a File -------------------------- The "file mode", stored in the `st_mode' field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the type code, which you can use to tell whether the file is a directory, socket, symbolic link, and so on. For details about access permissions see *Note Permission Bits::. There are two ways you can access the file type information in a file mode. Firstly, for each file type there is a "predicate macro" which examines a given file mode and returns whether it is of that type or not. Secondly, you can mask out the rest of the file mode to leave just the file type code, and compare this against constants for each of the supported file types. All of the symbols listed in this section are defined in the header file `sys/stat.h'. The following predicate macros test the type of a file, given the value M which is the `st_mode' field returned by `stat' on that file: - Macro: int S_ISDIR (mode_t M) This macro returns non-zero if the file is a directory. - Macro: int S_ISCHR (mode_t M) This macro returns non-zero if the file is a character special file (a device like a terminal). - Macro: int S_ISBLK (mode_t M) This macro returns non-zero if the file is a block special file (a device like a disk). - Macro: int S_ISREG (mode_t M) This macro returns non-zero if the file is a regular file. - Macro: int S_ISFIFO (mode_t M) This macro returns non-zero if the file is a FIFO special file, or a pipe. *Note Pipes and FIFOs::. - Macro: int S_ISLNK (mode_t M) This macro returns non-zero if the file is a symbolic link. *Note Symbolic Links::. - Macro: int S_ISSOCK (mode_t M) This macro returns non-zero if the file is a socket. *Note Sockets::. An alternate non-POSIX method of testing the file type is supported for compatibility with BSD. The mode can be bitwise AND-ed with `S_IFMT' to extract the file type code, and compared to the appropriate constant. For example, S_ISCHR (MODE) is equivalent to: ((MODE & S_IFMT) == S_IFCHR) - Macro: int S_IFMT This is a bit mask used to extract the file type code from a mode value. These are the symbolic names for the different file type codes: `S_IFDIR' This is the file type constant of a directory file. `S_IFCHR' This is the file type constant of a character-oriented device file. `S_IFBLK' This is the file type constant of a block-oriented device file. `S_IFREG' This is the file type constant of a regular file. `S_IFLNK' This is the file type constant of a symbolic link. `S_IFSOCK' This is the file type constant of a socket. `S_IFIFO' This is the file type constant of a FIFO or pipe. The POSIX.1b standard introduced a few more objects which possibly can be implemented as object in the filesystem. These are message queues, semaphores, and shared memory objects. To allow differentiating these objects from other files the POSIX standard introduces three new test macros. But unlike the other macros it does not take the value of the `st_mode' field as the parameter. Instead they expect a pointer to the whole `struct stat' structure. - Macro: int S_TYPEISMQ (struct stat *S) If the system implement POSIX message queues as distinct objects and the file is a message queue object, this macro returns a non-zero value. In all other cases the result is zero. - Macro: int S_TYPEISSEM (struct stat *S) If the system implement POSIX semaphores as distinct objects and the file is a semaphore object, this macro returns a non-zero value. In all other cases the result is zero. - Macro: int S_TYPEISSHM (struct stat *S) If the system implement POSIX shared memory objects as distinct objects and the file is an shared memory object, this macro returns a non-zero value. In all other cases the result is zero. File Owner ---------- Every file has an "owner" which is one of the registered user names defined on the system. Each file also has a "group" which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control. The file owner and group play a role in determining access because the file has one set of access permission bits for the owner, another set that applies to users who belong to the file's group, and a third set of bits that applies to everyone else. *Note Access Permission::, for the details of how access is decided based on this data. When a file is created, its owner is set to the effective user ID of the process that creates it (*note Process Persona::). The file's group ID may be set to either the effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rules, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior no matter what kind of system you run it on. You can change the owner and/or group owner of an existing file using the `chown' function. This is the primitive for the `chown' and `chgrp' shell commands. The prototype for this function is declared in `unistd.h'. - Function: int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP) The `chown' function changes the owner of the file FILENAME to OWNER, and its group owner to GROUP. Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID permission bits. (This is because those bits may not be appropriate for the new owner.) Other file permission bits are not changed. The return value is `0' on success and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EPERM' This process lacks permission to make the requested change. Only privileged users or the file's owner can change the file's group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on. *Note Options for Files::, for information about the `_POSIX_CHOWN_RESTRICTED' macro. `EROFS' The file is on a read-only file system. - Function: int fchown (int FILEDES, int OWNER, int GROUP) This is like `chown', except that it changes the owner of the open file with descriptor FILEDES. The return value from `fchown' is `0' on success and `-1' on failure. The following `errno' error codes are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument corresponds to a pipe or socket, not an ordinary file. `EPERM' This process lacks permission to make the requested change. For details see `chmod' above. `EROFS' The file resides on a read-only file system. The Mode Bits for Access Permission ----------------------------------- The "file mode", stored in the `st_mode' field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the access permission bits, which control who can read or write the file. *Note Testing File Type::, for information about the file type code. All of the symbols listed in this section are defined in the header file `sys/stat.h'. These symbolic constants are defined for the file mode bits that control access permission for the file: `S_IRUSR' `S_IREAD' Read permission bit for the owner of the file. On many systems this bit is 0400. `S_IREAD' is an obsolete synonym provided for BSD compatibility. `S_IWUSR' `S_IWRITE' Write permission bit for the owner of the file. Usually 0200. `S_IWRITE' is an obsolete synonym provided for BSD compatibility. `S_IXUSR' `S_IEXEC' Execute (for ordinary files) or search (for directories) permission bit for the owner of the file. Usually 0100. `S_IEXEC' is an obsolete synonym provided for BSD compatibility. `S_IRWXU' This is equivalent to `(S_IRUSR | S_IWUSR | S_IXUSR)'. `S_IRGRP' Read permission bit for the group owner of the file. Usually 040. `S_IWGRP' Write permission bit for the group owner of the file. Usually 020. `S_IXGRP' Execute or search permission bit for the group owner of the file. Usually 010. `S_IRWXG' This is equivalent to `(S_IRGRP | S_IWGRP | S_IXGRP)'. `S_IROTH' Read permission bit for other users. Usually 04. `S_IWOTH' Write permission bit for other users. Usually 02. `S_IXOTH' Execute or search permission bit for other users. Usually 01. `S_IRWXO' This is equivalent to `(S_IROTH | S_IWOTH | S_IXOTH)'. `S_ISUID' This is the set-user-ID on execute bit, usually 04000. *Note How Change Persona::. `S_ISGID' This is the set-group-ID on execute bit, usually 02000. *Note How Change Persona::. `S_ISVTX' This is the "sticky" bit, usually 01000. For a directory it gives permission to delete a file in that directory only if you own that file. Ordinarily, a user can either delete all the files in a directory or cannot delete any of them (based on whether the user has write permission for the directory). The same restriction applies--you must have both write permission for the directory and own the file you want to delete. The one exception is that the owner of the directory can delete any file in the directory, no matter who owns it (provided the owner has given himself write permission for the directory). This is commonly used for the `/tmp' directory, where anyone may create files but not delete files created by other users. Originally the sticky bit on an executable file modified the swapping policies of the system. Normally, when a program terminated, its pages in core were immediately freed and reused. If the sticky bit was set on the executable file, the system kept the pages in core for a while as if the program were still running. This was advantageous for a program likely to be run many times in succession. This usage is obsolete in modern systems. When a program terminates, its pages always remain in core as long as there is no shortage of memory in the system. When the program is next run, its pages will still be in core if no shortage arose since the last run. On some modern systems where the sticky bit has no useful meaning for an executable file, you cannot set the bit at all for a non-directory. If you try, `chmod' fails with `EFTYPE'; *note Setting Permissions::. Some systems (particularly SunOS) have yet another use for the sticky bit. If the sticky bit is set on a file that is _not_ executable, it means the opposite: never cache the pages of this file at all. The main use of this is for the files on an NFS server machine which are used as the swap area of diskless client machines. The idea is that the pages of the file will be cached in the client's memory, so it is a waste of the server's memory to cache them a second time. With this usage the sticky bit also implies that the filesystem may fail to record the file's modification time onto disk reliably (the idea being that no-one cares for a swap file). This bit is only available on BSD systems (and those derived from them). Therefore one has to use the `_BSD_SOURCE' feature select macro to get the definition (*note Feature Test Macros::). The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed. *Warning:* Writing explicit numbers for file permissions is bad practice. Not only is it not portable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean use the symbolic names. How Your Access to a File is Decided ------------------------------------ Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in *Note Process Persona::. If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits. Otherwise, permissions are controlled by the "other" bits. Privileged users, like `root', can access any file regardless of its permission bits. As a special case, for a file to be executable even by a privileged user, at least one of its execute bits must be set. Assigning File Permissions -------------------------- The primitive functions for creating files (for example, `open' or `mkdir') take a MODE argument, which specifies the file permissions to give the newly created file. This mode is modified by the process's "file creation mask", or "umask", before it is used. The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the MODE argument passed to the create function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant. Programs that create files typically specify a MODE argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask. To change the permission of an existing file given its name, call `chmod'. This function uses the specified permission bits and ignores the file creation mask. In normal use, the file creation mask is initialized by the user's login shell (using the `umask' shell command), and inherited by all subprocesses. Application programs normally don't need to worry about the file creation mask. It will automatically do what it is supposed to do. When your program needs to create a file and bypass the umask for its access permissions, the easiest way to do this is to use `fchmod' after opening the file, rather than changing the umask. In fact, changing the umask is usually done only by shells. They use the `umask' function. The functions in this section are declared in `sys/stat.h'. - Function: mode_t umask (mode_t MASK) The `umask' function sets the file creation mask of the current process to MASK, and returns the previous value of the file creation mask. Here is an example showing how to read the mask with `umask' without changing it permanently: mode_t read_umask (void) { mode_t mask = umask (0); umask (mask); return mask; } However, it is better to use `getumask' if you just want to read the mask value, because it is reentrant (at least if you use the GNU operating system). - Function: mode_t getumask (void) Return the current value of the file creation mask for the current process. This function is a GNU extension. - Function: int chmod (const char *FILENAME, mode_t MODE) The `chmod' function sets the access permission bits for the file named by FILENAME to MODE. If FILENAME is a symbolic link, `chmod' changes the permissions of the file pointed to by the link, not those of the link itself. This function returns `0' if successful and `-1' if not. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `ENOENT' The named file doesn't exist. `EPERM' This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them. `EROFS' The file resides on a read-only file system. `EFTYPE' MODE has the `S_ISVTX' bit (the "sticky bit") set, and the named file is not a directory. Some systems do not allow setting the sticky bit on non-directory files, and some do (and only some of those assign a useful meaning to the bit for non-directory files). You only get `EFTYPE' on systems where the sticky bit has no useful meaning for non-directory files, so it is always safe to just clear the bit in MODE and call `chmod' again. *Note Permission Bits::, for full details on the sticky bit. - Function: int fchmod (int FILEDES, int MODE) This is like `chmod', except that it changes the permissions of the currently open file given by FILEDES. The return value from `fchmod' is `0' on success and `-1' on failure. The following `errno' error codes are defined for this function: `EBADF' The FILEDES argument is not a valid file descriptor. `EINVAL' The FILEDES argument corresponds to a pipe or socket, or something else that doesn't really have access permissions. `EPERM' This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them. `EROFS' The file resides on a read-only file system. Testing Permission to Access a File ----------------------------------- In some situations it is desirable to allow programs to access files or devices even if this is not possible with the permissions granted to the user. One possible solution is to set the setuid-bit of the program file. If such a program is started the _effective_ user ID of the process is changed to that of the owner of the program file. So to allow write access to files like `/etc/passwd', which normally can be written only by the super-user, the modifying program will have to be owned by `root' and the setuid-bit must be set. But beside the files the program is intended to change the user should not be allowed to access any file to which s/he would not have access anyway. The program therefore must explicitly check whether _the user_ would have the necessary access to a file, before it reads or writes the file. To do this, use the function `access', which checks for access permission based on the process's _real_ user ID rather than the effective user ID. (The setuid feature does not alter the real user ID, so it reflects the user who actually ran the program.) There is another way you could check this access, which is easy to describe, but very hard to use. This is to examine the file mode bits and mimic the system's own access computation. This method is undesirable because many systems have additional access control features; your program cannot portably mimic them, and you would not want to try to keep track of the diverse features that different systems have. Using `access' is simple and automatically does whatever is appropriate for the system you are using. `access' is _only_ only appropriate to use in setuid programs. A non-setuid program will always use the effective ID rather than the real ID. The symbols in this section are declared in `unistd.h'. - Function: int access (const char *FILENAME, int HOW) The `access' function checks to see whether the file named by FILENAME can be accessed in the way specified by the HOW argument. The HOW argument either can be the bitwise OR of the flags `R_OK', `W_OK', `X_OK', or the existence test `F_OK'. This function uses the _real_ user and group IDs of the calling process, rather than the _effective_ IDs, to check for access permission. As a result, if you use the function from a `setuid' or `setgid' program (*note How Change Persona::), it gives information relative to the user who actually ran the program. The return value is `0' if the access is permitted, and `-1' otherwise. (In other words, treated as a predicate function, `access' returns true if the requested access is _denied_.) In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' The access specified by HOW is denied. `ENOENT' The file doesn't exist. `EROFS' Write permission was requested for a file on a read-only file system. These macros are defined in the header file `unistd.h' for use as the HOW argument to the `access' function. The values are integer constants. - Macro: int R_OK Flag meaning test for read permission. - Macro: int W_OK Flag meaning test for write permission. - Macro: int X_OK Flag meaning test for execute/search permission. - Macro: int F_OK Flag meaning test for existence of the file. File Times ---------- Each file has three time stamps associated with it: its access time, its modification time, and its attribute modification time. These correspond to the `st_atime', `st_mtime', and `st_ctime' members of the `stat' structure; see *Note File Attributes::. All of these times are represented in calendar time format, as `time_t' objects. This data type is defined in `time.h'. For more information about representation and manipulation of time values, see *Note Calendar Time::. Reading from a file updates its access time attribute, and writing updates its modification time. When a file is created, all three time stamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated. Adding a new name for a file with the `link' function updates the attribute change time field of the file being linked, and both the attribute change time and modification time fields of the directory containing the new name. These same fields are affected if a file name is deleted with `unlink', `remove' or `rmdir'. Renaming a file with `rename' affects only the attribute change time and modification time fields of the two parent directories involved, and not the times for the file being renamed. Changing the attributes of a file (for example, with `chmod') updates its attribute change time field. You can also change some of the time stamps of a file explicitly using the `utime' function--all except the attribute change time. You need to include the header file `utime.h' to use this facility. - Data Type: struct utimbuf The `utimbuf' structure is used with the `utime' function to specify new access and modification times for a file. It contains the following members: `time_t actime' This is the access time for the file. `time_t modtime' This is the modification time for the file. - Function: int utime (const char *FILENAME, const struct utimbuf *TIMES) This function is used to modify the file times associated with the file named FILENAME. If TIMES is a null pointer, then the access and modification times of the file are set to the current time. Otherwise, they are set to the values from the `actime' and `modtime' members (respectively) of the `utimbuf' structure pointed to by TIMES. The attribute modification time for the file is set to the current time in either case (since changing the time stamps is itself a modification of the file attributes). The `utime' function returns `0' if successful and `-1' on failure. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EACCES' There is a permission problem in the case where a null pointer was passed as the TIMES argument. In order to update the time stamp on the file, you must either be the owner of the file, have write permission for the file, or be a privileged user. `ENOENT' The file doesn't exist. `EPERM' If the TIMES argument is not a null pointer, you must either be the owner of the file or be a privileged user. `EROFS' The file lives on a read-only file system. Each of the three time stamps has a corresponding microsecond part, which extends its resolution. These fields are called `st_atime_usec', `st_mtime_usec', and `st_ctime_usec'; each has a value between 0 and 999,999, which indicates the time in microseconds. They correspond to the `tv_usec' field of a `timeval' structure; see *Note High-Resolution Calendar::. The `utimes' function is like `utime', but also lets you specify the fractional part of the file times. The prototype for this function is in the header file `sys/time.h'. - Function: int utimes (const char *FILENAME, struct timeval TVP[2]) This function sets the file access and modification times of the file FILENAME. The new file access time is specified by `TVP[0]', and the new modification time by `TVP[1]'. This function comes from BSD. The return values and error conditions are the same as for the `utime' function. File Size --------- Normally file sizes are maintained automatically. A file begins with a size of 0 and is automatically extended when data is written past its end. It is also possible to empty a file completely by an `open' or `fopen' call. However, sometimes it is necessary to _reduce_ the size of a file. This can be done with the `truncate' and `ftruncate' functions. They were introduced in BSD Unix. `ftruncate' was later added to POSIX.1. Some systems allow you to extend a file (creating holes) with these functions. This is useful when using memory-mapped I/O (*note Memory-mapped I/O::), where files are not automatically extended. However, it is not portable but must be implemented if `mmap' allows mapping of files (i.e., `_POSIX_MAPPED_FILES' is defined). Using these functions on anything other than a regular file gives _undefined_ results. On many systems, such a call will appear to succeed, without actually accomplishing anything. - Function: int truncate (const char *FILENAME, off_t LENGTH) The `truncate' function changes the size of FILENAME to LENGTH. If LENGTH is shorter than the previous length, data at the end will be lost. The file must be writable by the user to perform this operation. If LENGTH is longer, holes will be added to the end. However, some systems do not support this feature and will leave the file unchanged. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `truncate' function is in fact `truncate64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. The return value is 0 for success, or -1 for an error. In addition to the usual file name errors, the following errors may occur: `EACCES' The file is a directory or not writable. `EINVAL' LENGTH is negative. `EFBIG' The operation would extend the file beyond the limits of the operating system. `EIO' A hardware I/O error occurred. `EPERM' The file is "append-only" or "immutable". `EINTR' The operation was interrupted by a signal. - Function: int truncate64 (const char *NAME, off64_t LENGTH) This function is similar to the `truncate' function. The difference is that the LENGTH argument is 64 bits wide even on 32 bits machines, which allows the handling of files with sizes up to 2^63 bytes. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `truncate' and so transparently replaces the 32 bits interface. - Function: int ftruncate (int FD, off_t LENGTH) This is like `truncate', but it works on a file descriptor FD for an opened file instead of a file name to identify the object. The file must be opened for writing to successfully carry out the operation. The POSIX standard leaves it implementation defined what happens if the specified new LENGTH of the file is bigger than the original size. The `ftruncate' function might simply leave the file alone and do nothing or it can increase the size to the desired size. In this later case the extended area should be zero-filled. So using `ftruncate' is no reliable way to increase the file size but if it is possible it is probably the fastest way. The function also operates on POSIX shared memory segments if these are implemented by the system. `ftruncate' is especially useful in combination with `mmap'. Since the mapped region must have a fixed size one cannot enlarge the file by writing something beyond the last mapped page. Instead one has to enlarge the file itself and then remap the file with the new size. The example below shows how this works. When the source file is compiled with `_FILE_OFFSET_BITS == 64' the `ftruncate' function is in fact `ftruncate64' and the type `off_t' has 64 bits which makes it possible to handle files up to 2^63 bytes in length. The return value is 0 for success, or -1 for an error. The following errors may occur: `EBADF' FD does not correspond to an open file. `EACCES' FD is a directory or not open for writing. `EINVAL' LENGTH is negative. `EFBIG' The operation would extend the file beyond the limits of the operating system. `EIO' A hardware I/O error occurred. `EPERM' The file is "append-only" or "immutable". `EINTR' The operation was interrupted by a signal. - Function: int ftruncate64 (int ID, off64_t LENGTH) This function is similar to the `ftruncate' function. The difference is that the LENGTH argument is 64 bits wide even on 32 bits machines which allows the handling of files with sizes up to 2^63 bytes. When the source file is compiled with `_FILE_OFFSET_BITS == 64' on a 32 bits machine this function is actually available under the name `ftruncate' and so transparently replaces the 32 bits interface. As announced here is a little example of how to use `ftruncate' in combination with `mmap': int fd; void *start; size_t len; int add (off_t at, void *block, size_t size) { if (at + size > len) { /* Resize the file and remap. */ size_t ps = sysconf (_SC_PAGESIZE); size_t ns = (at + size + ps - 1) & ~(ps - 1); void *np; if (ftruncate (fd, ns) < 0) return -1; np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (np == MAP_FAILED) return -1; start = np; len = ns; } memcpy ((char *) start + at, block, size); return 0; } The function `add' writes a block of memory at an arbitrary position in the file. If the current size of the file is too small it is extended. Note the it is extended by a round number of pages. This is a requirement of `mmap'. The program has to keep track of the real size, and when it has finished a final `ftruncate' call should set the real size of the file. Making Special Files ==================== The `mknod' function is the primitive for making special files, such as files that correspond to devices. The GNU library includes this function for compatibility with BSD. The prototype for `mknod' is declared in `sys/stat.h'. - Function: int mknod (const char *FILENAME, int MODE, int DEV) The `mknod' function makes a special file with name FILENAME. The MODE specifies the mode of the file, and may include the various special file bits, such as `S_IFCHR' (for a character special file) or `S_IFBLK' (for a block special file). *Note Testing File Type::. The DEV argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created. The return value is `0' on success and `-1' on error. In addition to the usual file name errors (*note File Name Errors::), the following `errno' error conditions are defined for this function: `EPERM' The calling process is not privileged. Only the superuser can create special files. `ENOSPC' The directory or file system that would contain the new file is full and cannot be extended. `EROFS' The directory containing the new file can't be modified because it's on a read-only file system. `EEXIST' There is already a file named FILENAME. If you want to replace this file, you must remove the old file explicitly first. Temporary Files =============== If you need to use a temporary file in your program, you can use the `tmpfile' function to open it. Or you can use the `tmpnam' (better: `tmpnam_r') function to provide a name for a temporary file and then you can open it in the usual way with `fopen'. The `tempnam' function is like `tmpnam' but lets you choose what directory temporary files will go in, and something about what their file names will look like. Important for multi-threaded programs is that `tempnam' is reentrant, while `tmpnam' is not since it returns a pointer to a static buffer. These facilities are declared in the header file `stdio.h'. - Function: FILE * tmpfile (void) This function creates a temporary binary file for update mode, as if by calling `fopen' with mode `"wb+"'. The file is