The GNU C Library

This is Edition 0.12, last updated 2007-10-27, of The GNU C Library Reference Manual, for Version 2.8 of the GNU C Library.

1 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.

1.1 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 (see 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.

1.2 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.

See 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.

1.2.1 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. See 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. See 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.

1.2.2 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 (see File System Interface), device-specific terminal control functions (see Low-Level Terminal Interface), and process control functions (see 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 (see Pattern Matching).

1.2.3 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.

1.2.4 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 (see 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.

1.2.5 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.

1.3 Using the Library

This section describes some of the practical issues involved in using the GNU C library.

1.3.1 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,

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,

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 (see Feature Test Macros).

For more information about the use of header files and ‘#include’ directives, see 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.

1.3.2 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:

For example, suppose the header file stdlib.h declares a function named abs with

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.

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.

1.3.3 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:

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.

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.

1.3.4 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. See GNU CC Command Options, 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. NB: 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.

1.4 Roadmap to the Manual

If you already know the name of the facility you are interested in, you can look it up in 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.

2 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.

2.1 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 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. See 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.

2.2 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 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 (see 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 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. See 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 (see Hard Links) but also when you rename a file with rename (see 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; see 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 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 (see 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; see 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; see 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; see 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. See Socket Options.

— Macro: int EPROTONOSUPPORT

The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See 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. See Sockets.

— Macro: int EADDRINUSE

The requested socket address is already in use. See 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. See 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. See 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; see Limits for Files) or host name too long (in gethostname or sethostname; see 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. See 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 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; see 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. See 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

— Macro: int ECANCELED

Operation canceled; an asynchronous operation was canceled before it completed. See Asynchronous I/O. When you call aio_cancel, the normal result is for the operations affected to complete with this error; see Cancel AIO Operations.

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

— Macro: int ENOKEY

— Macro: int EKEYEXPIRED

— Macro: int EKEYREVOKED

— Macro: int EKEYREJECTED

— Macro: int EOWNERDEAD

— Macro: int ENOTRECOVERABLE

— Macro: int ERFKILL

2.3 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 (see 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 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. See 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.

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 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 eliminates 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 and 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:

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.

3 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.

3.1 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 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. See 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 (see 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. See 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. See 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. See Program Termination. A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are:

3.2 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.

3.2.1 Memory Allocation in C Programs

The C language supports two kinds of memory allocation through the variables in C programs:

A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C library functions.

3.2.1.1 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:

3.2.2 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).

3.2.2.1 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; see 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 (see Copying and Concatenation):

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:

3.2.2.2 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:

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:

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 (see 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 (see Changing Block Size).

3.2.2.3 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:

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.

3.2.2.4 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:

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.

3.2.2.5 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.

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.

3.2.2.6 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.

3.2.2.7 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. memalign is declared in malloc.h and posix_memalign is 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);
          }
Query Memory Parameters for more information about the memory subsystem.

3.2.2.8 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. Note that requests smaller than this threshold might still be allocated via mmap.
M_MMAP_MAX
The maximum number of chunks to allocate with mmap. Setting this to zero disables all use of mmap.
M_PERTURB
If non-zero, memory blocks are filled with values depending on some low order bits of this parameter when they are allocated (except when allocated by calloc) and freed. This can be used to debug the use of uninitialized or freed heap memory.

3.2.2.9 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 (see 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.

3.2.2.10 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.

— 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 alignment, size_t size, 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.

3.2.2.11 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 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.

3.2.2.12 Summary of malloc-Related Functions

3.2.3 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.

3.2.3.1 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 (see 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.

3.2.3.2 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:

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).

3.2.3.3 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:

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.

3.2.3.4 Interpreting the traces

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.

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:

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:

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.

3.2.4 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.

3.2.4.1 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.

3.2.4.2 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:

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 (see Unconstrained Allocation). This is done with the following pair of macro definitions:

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. See 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:

— 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 (see Program Termination) or longjmp (see Non-Local Exits) and doesn't return.
          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;

3.2.4.3 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:

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:

Contrast this with the previous example of savestring using malloc (see Basic Allocation).

3.2.4.4 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:

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 (see Preparing for Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.

3.2.4.5 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:

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:

This is the same situation that exists in ISO C for the standard library functions. See 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.

3.2.4.6 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 (see 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:

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.

3.2.4.7 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.

3.2.4.8 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)

3.2.4.9 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 aligned so that the object can hold any type of data.

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 a value that allows aligned objects to hold any type of data: for example, if its value is 3, any type of data can be stored at locations whose 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.

3.2.4.10 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 (see Creating Obstacks). Most often they are defined as macros like this:

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:

3.2.4.11 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.

3.2.5 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.

— 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)

3.2.5.1 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:

As you can see, it is simpler with alloca. But alloca has other, more important advantages, and some disadvantages.

3.2.5.2 Advantages of alloca

3.2.5.3 Disadvantages of alloca

3.2.5.4 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:

But alloca is not always equivalent to a variable-sized array, for several reasons:

NB: 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.

3.3 Resizing the Data Segment

You will not normally use the functions in this section, because the functions described in 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 (see 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: void *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.

3.4 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.

3.4.1 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:

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.

3.4.2 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). See 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. See 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.

3.4.3 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)

munlock 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. See 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.

4 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 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 (see Extended Char Intro).

4.1 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:

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. See Locales, for more information on locales.

— 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 was originally a GNU extension, but was added in ISO C99.

— 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.

4.2 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).

— 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. See SVID.

— Function: int _toupper (int c)

This is identical to toupper, and is provided for compatibility with the SVID.

4.3 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 was originally a GNU extension, but was added in ISO C99. It is declared in wchar.h.

4.4 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.

Now, with the wctype and iswctype you can avoid the if cascades, but rewriting the code as follows is wrong:

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:

See 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.

4.5 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.

5 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.

5.1 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 (see 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 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 (see 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.

5.2 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.

5.3 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.

— 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.

5.4 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 (see Formatted Output Functions) and scanf (see 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 *restrict 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 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 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 <string.h>
          #include <stdio.h>
          
          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 last non-null wide character 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 (see 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 <paths.h>
          #include <string.h>
          #include <stdio.h>
          
          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 (see 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 see 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 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:

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.

— 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.

5.5 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. See 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)’.

— 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.

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 Ä and ä 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 Ä and ä 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 characters are compared. In other words, if the two strings are the same in their first size 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.)

— 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.

5.6 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 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 (see 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.)

— 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 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 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.

The interesting part of this code for the wide character version would look like this:

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.

5.7 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).
This function is a GNU extension.

— 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:

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:

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)

wcswcs is an deprecated 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,
          strcasestr ("hello, world", "L")
              ⇒ "llo, world"
          strcasestr ("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.

5.7.1 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.

5.8 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.

— 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 (see 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. See 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. See Nonreentrancy, for a discussion of where and why reentrancy is important.

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.

— 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 <string.h>
          
          int
          main (int argc, char *argv[])
          {
            char *prog = basename (argv[0]);
          
            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\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 <libgen.h>
          
          int
          main (int argc, char *argv[])
          {
            char *prog;
            char *path = strdupa (argv[0]);
          
            prog = basename (path);
          
            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\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.

5.9 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.

— 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.

5.10 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, See Cryptographic Functions.

— 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 See Cryptographic Functions.
Portability Note: This function is unique to the GNU C library.

5.11 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 7 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, *p;
          
            /* Encode the length. */
            /* Using `htonl' is necessary so that the data can be
               decoded even on machines with different byte order.
               `l64a' can return a string shorter than 6 bytes, so 
               we pad it with encoding of 0 ('.') at the end by 
               hand. */
          
            p = stpcpy (cp, l64a (htonl (len)));
            cp = mempcpy (p, "......", 6 - (p - cp));
          
            while (len > 3)
              {
                unsigned long int n = *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                len -= 4;
                p = stpcpy (cp, l64a (htonl (n)));
                cp = mempcpy (p, "......", 6 - (p - cp));
              }
            if (len > 0)
              {
                unsigned long int n = *in++;
                if (--len > 0)
                  {
                    n = (n << 8) | *in++;
                    if (--len > 0)
                      n = (n << 8) | *in;
                  }
                cp = stpcpy (cp, l64a (htonl (n)));
              }
            *cp = '\0';
            return out;
          }
It is strange that the library does not provide the complete functionality needed but so be it.

— 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.

5.12 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).

5.12.1 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). See 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.

— 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; see 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 (see 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: void 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.

5.12.2 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.

— 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.

6 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.

6.1 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.

— 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 therefore 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 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.

6.2 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.

6.3 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:

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 (see Generic Charset Conversion).

6.3.1 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 set is always 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:

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.

6.3.2 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:

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.

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 (see 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.

6.3.3 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 is always 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):

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, using btowc is required.

— 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):

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 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.

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:

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

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.

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.

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.

6.3.4 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 (see 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):

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.

6.3.5 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.

6.4 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.

6.4.1 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. See 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. See 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. See Shift State.
The function mblen is declared in stdlib.h.

6.4.2 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 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.

6.4.3 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:

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.

6.5 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:

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.

6.5.1 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.

— 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 (see 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.

6.5.2 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.

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.

6.5.3 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:

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

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

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.

6.5.4 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.

6.5.4.1 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:

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:

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.

6.5.4.2 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 endianness, the internal representation consistently is named INTERNAL even on big-endian systems where the representations are identical.

6.5.4.3 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 (see 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.

6.5.4.4 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:

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.

7 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.

7.1 What Effects a Locale Has

Each locale specifies conventions for several purposes, including the following:

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.

7.2 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.

7.3 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.

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 Using gettextized software.

7.4 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:

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.

— 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 (see 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 (see 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.

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.

7.5 Standard Locales

The only locale names you can count on finding on all operating systems are these three standard ones:

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.

7.6 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 (see 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.

7.6.1 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.

7.6.1.1 Generic Numeric Formatting Parameters

7.6.1.2 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.

7.6.1.3 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.

7.6.2 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 (see 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:

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:

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.

7.7 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 see 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:

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.

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.

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.

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.

7.8 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.

Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read.

8 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

The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest.

8.1 X/Open Message Catalog Handling

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.

8.1.1 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 (see 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 (see 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.

8.1.2 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:

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:

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.

8.1.3 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.

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.

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.

8.1.4 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.

8.1.4.1 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 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.

8.1.4.2 Using symbolic names

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).

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:

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.)

8.1.4.3 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:

Now we want to internationalize the message and therefore replace the message with whatever the user wants.

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.

Now we can start building the program (assume the message catalog source file is named hello.msg and the program source file hello.c):

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.

8.2 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 (see GNU gettext utilities). 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.

8.2.1 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:

8.2.1.1 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 (see 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.

8.2.1.2 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:

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:

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.

8.2.1.3 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:

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:

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 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 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:

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

8.2.1.4 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.

8.2.1.5 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 an example consider the following fictional situation. A GUI program has a menu bar with the following entries:

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

Now all the strings are different and if now instead of gettext the following little wrapper function is used, everything works just fine:

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:

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.

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.

8.2.1.6 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, LC_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:

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:

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:

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:

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

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 is one more or less standardized form, originally from the X/Open specification:

Less specific locale names will be stripped of in the order of the following list:

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:

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.

8.2.2 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 automate 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.

9 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.

9.1 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 (see 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:

The header file stdlib.h defines a name for the data type of comparison functions. This type is a GNU extension.

9.2 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 if 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.

9.3 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 (see 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.

9.4 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.

9.5 The hsearch function.

The functions mentioned so far in this chapter are for 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. The following functions are declared in the header file search.h.

— 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 method used 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 inefficiently 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. Entries are never replaced.
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.
If an entry with matching key is found the action parameter is irrelevant. The found entry is returned. If no matching entry is found and the action parameter has the value FIND the function returns a NULL pointer. If no entry is found and the action parameter has the value ENTER a new entry is added to the hashing table which is initialized with the parameter item. A pointer to the newly added entry is returned.

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. It must be initialized with zero before calling this function.
The return value is non-zero if the operation was successful. If the return value is zero, something went wrong, which probably means the programs ran 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.

9.6 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. The prototypes for these functions can be found in the header file search.h.

— 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. See 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:

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.

10 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.

10.1 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.

10.2 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.

10.2.1 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 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. 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.

10.2.2 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 |.

10.2.3 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.

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.

10.3 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.

10.3.1 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. (See Matching POSIX Regexps, for how to use the compiled regular expression for matching.)

— 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 *restrict compiled, const char *restrict 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. See 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 Regexp Cleanup.

10.3.2 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.

10.3.3 Matching a Compiled POSIX Regular Expression

Once you have compiled a regular expression, as described in 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 (const regex_t *restrict compiled, const char *restrict string, size_t nmatch, regmatch_t matchptr[restrict], 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 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. See 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.

10.3.4 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.

10.3.5 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.

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.

10.3.6 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, const regex_t *restrict compiled, char *restrict 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;
          }

10.4 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.

10.4.1 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:

For the details of these transformations, and how to write the constructs that use them, see The BASH Manual (to appear).

10.4.2 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 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.

10.4.3 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 |.

10.4.4 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.

10.4.5 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.

10.4.6 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.

When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways.

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.

11 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:

11.1 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.

11.1.1 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 (see 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 (see 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 (see Formatted Input) and formatted output functions (see 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.

11.1.2 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 (see File Positioning) or the lseek function on a file descriptor (see 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.

11.2 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.

11.2.1 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 File System Interface.

11.2.2 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 (see 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 /.

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.

11.2.3 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.

11.2.4 Portability of File Names

The rules for the syntax of file names discussed in 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:

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.

12 Input/Output on Streams

This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in I/O Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.

12.1 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 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.

12.2 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.

— 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 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:

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. See Opening Streams.

The three streams stdin, stdout, and stderr are not unoriented at program start (see Streams and I18N).

12.3 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.

— 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. When using such a stream, you must call fflush (see Stream Buffering) or a file positioning function such as fseek (see File Positioning) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly.
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 (see 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. See 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. See 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. See 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; see General Limits. In BSD and GNU, it is controlled by the RLIMIT_NOFILE resource limit; see 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.

12.4 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 (see 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 (see Normal Termination), all open streams are automatically closed properly. If your program terminates in any other manner, such as by calling the abort function (see Aborting a Program) or from a fatal signal (see 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 Stream Buffering.

12.5 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 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):

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:

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:

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 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:

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).

12.6 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:

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.

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.

12.7 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 (see Block Input/Output).

12.8 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.

— 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 (see 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.

12.9 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.

— 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. See 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.

12.10 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.

12.10.1 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:

If instead of reading ‘b’ you unread the letter ‘o’, you get a situation like this:

12.10.2 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 supports this even on files opened in binary mode, but other systems might not.
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; see 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.

12.11 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.

— 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.

12.12 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.

12.12.1 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,

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.

12.12.2 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 (see Character Set Handling) are permitted in a template string.

The conversion specifications in a printf template string have the general form:

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:

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. See Declaring Attributes of Functions, for more information.

12.12.3 Table of 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.

12.12.4 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’.

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:

to print numbers using the different options for the ‘%d’ conversion gives results like:

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:

12.12.5 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.ddde[+|-]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. 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 [-]0xh.hhhp[+|-]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, see 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 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:

Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:

12.12.6 Other Output Conversions

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:

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:

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. See Error Messages. Thus:

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.)

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.

The ‘%%’ conversion prints a literal ‘%’ character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.

12.12.7 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. See 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. */
                size = nchars + 1;
                buffer = (char *) xrealloc (buffer, size);
          
                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.

12.12.8 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; see 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 a successful call to 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. See Obstacks.
The characters are written onto the end of the current object. To get at them, you must finish the object with obstack_finish (see Growing Objects).

12.12.9 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:

See Variadic Macros, 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 (see 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.)

— 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 (see Error Messages, for a description of program_invocation_short_name).

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:

This tells the compiler that eprintf uses a format string like printf (as opposed to scanf; see Formatted Input); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. See Declaring Attributes of Functions, for more information.

12.12.10 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.

Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or.

12.12.11 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).

12.13 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 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 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. See Parsing a Template String, for information about this.

Portability Note: The ability to extend the syntax of printf template strings is a GNU extension. ISO standard C has nothing similar.

12.13.1 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. See 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. See 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.

12.13.2 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.

12.13.3 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 *.

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. See 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.

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.

12.13.4 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.

12.13.5 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:
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.

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

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.

12.14 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.

12.14.1 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:

The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.

If you are trying to read input that doesn't match a single, fixed pattern, you may be better off using a tool such as Flex to generate a lexical scanner, or Bison to generate a parser, rather than using scanf. For more information about these tools, see Top, and Top.

12.14.2 Input Conversion Syntax

A scanf template string is a string that contains ordinary multibyte characters interspersed with conversion specifications that start with ‘%’.

Any whitespace character (as defined by the isspace function; see Classification of Characters) in the template causes any number of whitespace characters in the input stream to be read and discarded. The whitespace characters that are matched need not be exactly the same whitespace characters that appear in the template string. For example, write ‘ , ’ in the template to recognize a comma with optional whitespace before and after.

Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.

The conversion specifications in a scanf template string have the general form:

In more detail, an input conversion specification consists of an initial ‘%’ character followed in sequence by:

With the ‘-Wformat’ option, the GNU C compiler checks calls to scanf 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 scanf-style format string. See Declaring Attributes of Functions, for more information.

12.14.3 Table of Input Conversions

If the syntax of a conversion specification is invalid, the behavior is undefined. If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined. On the other hand, extra arguments are simply ignored.

12.14.4 Numeric Input Conversions

The ‘%d’ conversion matches an optionally signed integer in decimal radix. The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 10 for the base argument.

The ‘%i’ conversion matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant. The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 0 for the base argument. (You can print integers in this syntax with printf by using the ‘#’ flag character with the ‘%x’, ‘%o’, or ‘%d’ conversion. See Integer Conversions.)

For example, any of the strings ‘10’, ‘0xa’, or ‘012’ could be read in as integers under the ‘%i’ conversion. Each of these specifies a number with decimal value 10.

The ‘%o’, ‘%u’, and ‘%x’ conversions match unsigned integers in octal, decimal, and hexadecimal radices, respectively. The syntax that is recognized is the same as that for the strtoul function (see Parsing of Integers) with the appropriate value (8, 10, or 16) for the base argument.

The ‘%X’ conversion is identical to the ‘%x’ conversion. They both permit either uppercase or lowercase letters to be used as digits.

The default type of the corresponding argument for the %d and %i conversions is int *, and unsigned int * for the other integer conversions. You can use the following type modifiers to specify other sizes of integer:

All of the ‘%e’, ‘%f’, ‘%g’, ‘%E’, and ‘%G’ input conversions are interchangeable. They all match an optionally signed floating point number, in the same syntax as for the strtod function (see Parsing of Floats).

For the floating-point input conversions, the default argument type is float *. (This is different from the corresponding output conversions, where the default type is double; remember that float arguments to printf are converted to double by the default argument promotions, but float * arguments are not promoted to double *.) You can specify other sizes of float using these type modifiers:

For all the above number parsing formats there is an additional optional flag ‘'’. When this flag is given the scanf function expects the number represented in the input string to be formatted according to the grouping rules of the currently selected locale (see General Numeric).

If the "C" or "POSIX" locale is selected there is no difference. But for a locale which specifies values for the appropriate fields in the locale the input must have the correct form in the input. Otherwise the longest prefix with a correct form is processed.

12.14.5 String Input Conversions

This section describes the scanf input conversions for reading string and character values: ‘%s’, ‘%S’, ‘%[’, ‘%c’, and ‘%C’.

The ‘%c’ conversion is the simplest: it matches a fixed number of characters, always. The maximum field width says how many characters to read; if you don't specify the maximum, the default is 1. This conversion doesn't append a null character to the end of the text it reads. It also does not skip over initial whitespace characters. It reads precisely the next n characters, and fails if it cannot get that many. Since there is always a maximum field width with ‘%c’ (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.

If the format is ‘%lc’ or ‘%C’ the function stores wide characters which are converted using the conversion determined at the time the stream was opened from the external byte stream. The number of bytes read from the medium is limited by MB_CUR_LEN * n but at most n wide character get stored in the output string.

The ‘%s’ conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something. It stores a null character at the end of the text that it reads.

with the conversion ‘%10c’ produces " hello, wo", but reading the same input with the conversion ‘%10s’ produces "hello,".

Warning: If you do not specify a field width for ‘%s’, then the number of characters read is limited only by where the next whitespace character appears. This almost certainly means that invalid input can make your program crash—which is a bug.

The ‘%ls’ and ‘%S’ format are handled just like ‘%s’ except that the external byte sequence is converted using the conversion associated with the stream to wide characters with their own encoding. A width or precision specified with the format do not directly determine how many bytes are read from the stream since they measure wide characters. But an upper limit can be computed by multiplying the value of the width or precision by MB_CUR_MAX.

To read in characters that belong to an arbitrary set of your choice, use the ‘%[’ conversion. You specify the set between the ‘[’ character and a following ‘]’ character, using the same syntax used in regular expressions. As special cases:

As for ‘%c’ and ‘%s’ the ‘%[’ format is also modified to produce wide characters if the ‘l’ modifier is present. All what is said about ‘%ls’ above is true for ‘%l[’.

One more reminder: the ‘%s’ and ‘%[’ conversions are dangerous if you don't specify a maximum width or use the ‘a’ flag, because input too long would overflow whatever buffer you have provided for it. No matter how long your buffer is, a user could supply input that is longer. A well-written program reports invalid input with a comprehensible error message, not with a crash.

12.14.6 Dynamically Allocating String Conversions

A GNU extension to formatted input lets you safely read a string with no maximum size. Using this feature, you don't supply a buffer; instead, scanf allocates a buffer big enough to hold the data and gives you its address. To use this feature, write ‘a’ as a flag character, as in ‘%as’ or ‘%a[0-9a-z]’.

The pointer argument you supply for where to store the input should have type char **. The scanf function allocates a buffer and stores its address in the word that the argument points to. You should free the buffer with free when you no longer need it.

Here is an example of using the ‘a’ flag with the ‘%[...]’ conversion specification to read a “variable assignment” of the form ‘variable = value’.

12.14.7 Other 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 (see 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 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.

12.14.8 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.

12.14.9 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. See 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 (see 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 (see 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, see Declaring Attributes of Functions.

12.15 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 Low-Level I/O.

12.16 Recovering from errors

— Function: void clearerr (FILE *stream)

This function clears the end-of-file and error indicators for the stream stream.
The file positioning functions (see 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 (see 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.

12.17 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 Opening Streams. Without this option, fopen opens the file as a text stream.

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.

12.18 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. See 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. See Binary Streams.

The following symbolic constants are defined for use as the whence argument to fseek. They are also used with the lseek function (see I/O Primitives) and to specify offsets for file locks (see 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.

12.19 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:

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.

— 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.

12.20 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 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. See Low-Level I/O.

12.20.1 Buffering Concepts

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. See 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 Flushing Buffers.

12.20.2 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:

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.

12.20.3 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 (see 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. See 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 (see 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.

12.21 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 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 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.

12.21.1 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; see 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 (see 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:

— Function: FILE * open_memstream (char **ptr, size_t *sizeloc)

This function opens a stream for writing to a buffer. The buffer is allocated dynamically and grown as necessary, using malloc. After you've closed the stream, this buffer is your responsibility to clean up using free or realloc. See Unconstrained Allocation.
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 (see File Positioning). Moving the file position past the end of the data already written fills the intervening space with zeroes.

12.21.2 Obstack Streams

— 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 (see 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 (see 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 (see 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.)

12.21.3 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.

12.21.3.1 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.

— 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 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.

12.21.3.2 Custom Stream Hook Functions

Here are more details on how you should define the four hook functions that a custom stream needs.

This is very similar to the read function; see 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.

This is very similar to the write function; see 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.

For this function, the position and whence arguments are interpreted as for fgetpos; see Portable Positioning.

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:

— 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.

12.22 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.

12.22.1 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:

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:

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:

A last component of classification can signal the results of this message. Exactly one of the following values can be used:

— 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 (see 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:

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.

in the environment of the process before the first call to fmtmsg, where description has a value of the form

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).

12.22.2 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.

12.22.3 How to use fmtmsg and addseverity

Here is a simple example program to illustrate the use of the both functions described in this section.

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:

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 (see 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:

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:

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.

13 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 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:

13.1 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). See 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 (see 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. See 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; see 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 (see 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. See 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. See 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 (see 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.

13.2 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 (see 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. See 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. See Job Control, for more information about job control, and Signal Handling, for information about signals.
EINVAL
In some systems, when reading from a character or block device, position and size offsets must be aligned to a particular block size. This error indicates that the offsets were not properly aligned.

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; see 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 (see 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. See 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 Signal Handling.
EINVAL
In some systems, when writing to a character or block device, position and size offsets must be aligned to a particular block size. This error indicates that the offsets were not properly aligned.

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. See 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.

13.3 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. See 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; see 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,

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,

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.

13.4 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 (see 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.

— 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 Standard Streams.

13.5 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.

13.5.1 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 (or have just opened the stream), 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. See 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.

13.5.2 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:

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 Linked Channels.

13.5.3 Cleaning Streams

— 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.

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. However, a just-opened input stream might not be clean, as its input buffer might not be empty.

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. See Terminal Modes.

13.6 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.

13.7 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

— 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 (see 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. See 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. See 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.

13.8 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 (see Listening), we say that “input” is available when there are pending connections that could be accepted (see 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.
The filedes parameter must not have side effects since it is evaluated more than once.

— Macro: void FD_CLR (int filedes, fd_set *set)

This macro removes filedes from the file descriptor set set.
The filedes parameter must not have side effects since it is evaluated more than once.

— Macro: int FD_ISSET (int filedes, const 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.
The filedes parameter must not have side effects since it is evaluated more than once.

— 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 a read call will not block. This usually includes the read offset being at the end of the file or there is an error to report. A server socket is considered ready for reading if there is a pending connection which can be accepted with accept; see Accepting Connections. A client socket is ready for writing when its connection is fully established; see 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. (See 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 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 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. See Interrupted Primitives.
EINVAL
The timeout argument is invalid; one of the components is negative or too large.

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.

There is another example showing the use of select to multiplex input from multiple sockets in Server Example.

13.9 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.

13.10 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.

13.10.1 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 (see 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 (see 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. (see 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 (see 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 (see 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 (see 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.

13.10.2 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.

13.10.3 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 (see 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_fsync64 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.

13.10.4 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.

13.10.5 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.

13.11 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 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). See Duplicating Descriptors.
F_GETFD
Get flags associated with the file descriptor. See Descriptor Flags.
F_SETFD
Set flags associated with the file descriptor. See Descriptor Flags.
F_GETFL
Get flags associated with the open file. See File Status Flags.
F_SETFL
Set flags associated with the open file. See File Status Flags.
F_GETLK
Get a file lock. See File Locks.
F_SETLK
Set or clear a file lock. See File Locks.
F_SETLKW
Like F_SETLK, but wait for completion. See File Locks.
F_GETOWN
Get process or process group ID to receive SIGIO signals. See Interrupt Input.
F_SETOWN
Set process or process group ID to receive SIGIO signals. See 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.

13.12 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 (see File Status Flags), but each has its own set of file descriptor flags (see 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; see 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 (see 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.

There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in Launching Jobs.

13.13 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 (see Executing a File).

— 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 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:

13.14 File Status Flags

File status flags are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in 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; see Opening and Closing Files.

File status flags fall into three categories, which are described in the following sections.

13.14.1 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_RDONLY; only defined on GNU.

— Macro: int O_WRITE

Open the file for writing. 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.)

13.14.2 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. See Opening and Closing Files, for how to call open.

— 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; see 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. See 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.

— 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. See 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. See 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. See File Locks. This is atomic like O_SHLOCK.

13.14.3 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; see 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. See 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. See 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.

13.14.4 Getting and Setting 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:

13.15 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 (see 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 (see 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. See 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.

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.

13.16 Interrupt-Driven Input

If you set the O_ASYNC status flag on a file descriptor (see 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 Out-of-Band Data. (SIGURG is sent in any situation where select would report the socket as having an “exceptional condition”. See 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. See Job Control.

— 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 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.

13.17 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:

Although some such objects such as sockets and terminals ² have special functions of their own, it would not be practical to create functions for all these cases.

Instead these minor operations, known as IOCTLs, 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 Out-of-Band Data.

14 File System Interface

This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions (see I/O on Streams; see 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.

14.1 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 (see 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 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 (see 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:

See 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 (see 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 basically 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 (see 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.

14.2 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. Alternatively fdopendir can be used which can have advantages if the program needs to have more control over the way the directory is opened for reading. This allows, for instance, to pass the O_NOATIME flag to open.

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 I/O on Streams.

14.2.1 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. See 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. See 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. See File Attributes.

14.2.2 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 (see 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.)
ENOMEM
Not enough memory available.

The DIR type is typically implemented using a file descriptor, and the opendir function in terms of the open function. See Low-Level I/O. Directory streams and the underlying file descriptors are closed on exec (see Executing a File).

The directory which is opened for reading by opendir is identified by the name. In some situations this is not sufficient. Or the way opendir implicitly creates a file descriptor for the directory is not the way a program might want it. In these cases an alternative interface can be used.

— Function: DIR * fdopendir (int fd)

The fdopendir function works just like opendir but instead of taking a file name and opening a file descriptor for the directory the caller is required to provide a file descriptor. This file descriptor is then used in subsequent uses of the returned directory stream object.
The caller must make sure the file descriptor is associated with a directory and it allows reading.
If the fdopendir call returns successfully the file descriptor is now under the control of the system. It can be used in the same way the descriptor implicitly created by opendir can be used but the program must not close the descriptor.
In case the function is unsuccessful it returns a null pointer and the file descriptor remains to be usable by the program. The following errno error conditions are defined for this function:

EBADF
The file descriptor is not valid.
ENOTDIR
The file descriptor is not associated with a directory.
EINVAL
The descriptor does not allow reading the directory content.
ENOMEM
Not enough memory available.

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.

14.2.3 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. See 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.
Normally readdir_r returns zero and sets *result to entry. If there are no more entries in the directory or an error is detected, readdir_r sets *result to a null pointer and returns a nonzero error code, also stored in errno, as described for readdir.
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.

14.2.4 Simple Program to List a Directory

Here's a simple program that prints the names of the files in the current working directory:

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 Scanning Directory Content, and Array Sort Function.

14.2.5 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: long int 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, long int 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.

14.2.6 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 (see 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 (see 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.

14.2.7 Simple Program to List a Directory, Mark II

Here is a revised version of the directory lister found above (see 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.

Note the simple selector function in this example. Since we want to see all directory entries we always return 1.

14.3 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.
FTW_ACTIONRETVAL
If this option is specified then return values from callbacks are handled differently. If the callback returns FTW_CONTINUE, walking continues normally. FTW_STOP means walking stops and FTW_STOP is returned to the caller. If FTW_SKIP_SUBTREE is returned by the callback with FTW_D argument, the subtree is skipped and walking continues with next sibling of the directory. If FTW_SKIP_SIBLINGS is returned by the callback, all siblings of the current entry are skipped and walking continues in its parent. No other return values should be returned from the callbacks if this option is set. This option is a GNU extension.

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.

14.4 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 (see 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 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.

14.5 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.

— 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 (see 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;
            char *buffer = NULL;
          
            while (1)
              {
                buffer = (char *) xrealloc (buffer, size);
                int nchars = readlink (filename, buffer, size);
                if (nchars < 0)
                  {
                    free (buffer);
                    return NULL;
                  }
                if (nchars < size)
                  return buffer;
                size *= 2;
              }
          }
A value of -1 is returned in case of error. In addition to the usual file name errors (see 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 symbolic links 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.
If any of the path components 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)

A call to realpath where the resolved parameter is NULL behaves exactly like canonicalize_file_name. The function allocates a buffer for the file name and returns a pointer to it. If resolved is not NULL it points to a buffer into which the result is copied. It is the callers responsibility to allocate a buffer which is large enough. On systems which define PATH_MAX this means the buffer must be large enough for a pathname of this size. For systems without limitations on the pathname length the requirement cannot be met and programs should not call realpath with anything but NULL for the second parameter.
One other difference is that the buffer resolved (if nonzero) 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.

14.6 Deleting Files

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 (see 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 (see 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.

14.7 Renaming Files

— 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 (see 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.

14.8 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. See 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 (see 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.

14.9 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.

14.9.1 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 Reading Attributes.

— 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 (see Testing File Type) and the file permission bits (see 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. See File Owner.
gid_t st_gid
The group ID of the file. See 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. See File Times.
unsigned long int st_atime_usec
This is the fractional part of the last access time for the file. See File Times.
time_t st_mtime
This is the time of the last modification to the contents of the file. See 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. See File Times.
time_t st_ctime
This is the time of the last modification to the attributes of the file. See 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. See 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 (see Testing File Type) and the file permission bits (see 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. See File Owner.
gid_t st_gid
The group ID of the file. See 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. See File Times.
unsigned long int st_atime_usec
This is the fractional part of the last access time for the file. See File Times.
time_t st_mtime
This is the time of the last modification to the contents of the file. See 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. See File Times.
time_t st_ctime
This is the time of the last modification to the attributes of the file. See 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. See 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 long int.
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.

14.9.2 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 (see 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. See 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. See 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.

14.9.3 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 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. See Pipes and FIFOs.

— Macro: int S_ISLNK (mode_t m)

This macro returns non-zero if the file is a symbolic link. See Symbolic Links.

— Macro: int S_ISSOCK (mode_t m)

This macro returns non-zero if the file is a socket. See 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,

— Macro: int S_IFMT

This is a bit mask used to extract the file type code from a mode value.

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.

14.9.4 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. See 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 (see 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.

— 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 (see 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.
See 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.

14.9.5 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. See 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:

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.

14.9.6 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 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.

14.9.7 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.

— 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 (see 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. See 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.

14.9.8 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.

— 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 (see 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 (see 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.

14.9.9 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 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 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 (see 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 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]. Similar to utime, if tvp is a null pointer then the access and modification times of the file are set to the current time. This function comes from BSD.
The return values and error conditions are the same as for the utime function.

— Function: int lutimes (const char *filename, struct timeval tvp[2])

This function is like utimes, except that it does not follow symbolic links. If filename is the name of a symbolic link, lutimes sets the file access and modification times of the symbolic link special file itself (as seen by lstat; see Symbolic Links) while utimes sets the file access and modification times of the file the symbolic link refers to. This function comes from FreeBSD, and is not available on all platforms (if not available, it will fail with ENOSYS).
The return values and error conditions are the same as for the utime function.

— Function: int futimes (int fd, struct timeval tvp[2])

This function is like utimes, except that it takes an open file descriptor as an argument instead of a file name. See Low-Level I/O. This function comes from FreeBSD, and is not available on all platforms (if not available, it will fail with ENOSYS).
Like utimes, futimes returns 0 on success and -1 on failure. The following errno error conditions are defined for futimes:

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.
EBADF
The filedes argument is not a valid file descriptor.
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.

14.9.10 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 (see 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:

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.

14.10 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.

— 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). See 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 (see 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.

14.11 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.

— Function: FILE * tmpfile (void)

This function creates a temporary binary file for update mode, as if by calling fopen with mode "wb+". The file is deleted automatically when it is closed or when the program terminates. (On some other ISO C systems the file may fail to be deleted if the program terminates abnormally).
This function is reentrant.
When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact tmpfile64, i.e., the LFS interface transparently replaces the old interface.

— Function: FILE * tmpfile64 (void)

This function is similar to tmpfile, but the stream it returns a pointer to was opened using tmpfile64. Therefore this stream can be used for files larger then 2^31 bytes on 32-bit machines.
Please note that the return type is still FILE *. There is no special FILE type for the LFS interface.
If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name tmpfile and so transparently replaces the old interface.

— Function: char * tmpnam (char *result)

This function constructs and returns a valid file name that does not refer to any existing file. If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls and therefore makes this function non-reentrant. Otherwise, the result argument should be a pointer to an array of at least L_tmpnam characters, and the result is written into that array.
It is possible for tmpnam to fail if you call it too many times without removing previously-created files. This is because the limited length of the temporary file names gives room for only a finite number of different names. If tmpnam fails it returns a null pointer.
Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using tmpnam, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using tmpfile or mkstemp is a safe way to avoid this problem.

— Function: char * tmpnam_r (char *result)

This function is nearly identical to the tmpnam function, except that if result is a null pointer it returns a null pointer.
This guarantees reentrancy because the non-reentrant situation of tmpnam cannot happen here.
Warning: This function has the same security problems as tmpnam.

— Macro: int L_tmpnam

The value of this macro is an integer constant expression that represents the minimum size of a string large enough to hold a file name generated by the tmpnam function.

— Macro: int TMP_MAX

The macro TMP_MAX is a lower bound for how many temporary names you can create with tmpnam. You can rely on being able to call tmpnam at least this many times before it might fail saying you have made too many temporary file names.
With the GNU library, you can create a very large number of temporary file names. If you actually created the files, you would probably run out of disk space before you ran out of names. Some other systems have a fixed, small limit on the number of temporary files. The limit is never less than 25.

— Function: char * tempnam (const char *dir, const char *prefix)

This function generates a unique temporary file name. If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name. The return value is a string newly allocated with malloc, so you should release its storage with free when it is no longer needed.
Because the string is dynamically allocated this function is reentrant.
The directory prefix for the temporary file name is determined by testing each of the following in sequence. The directory must exist and be writable.

The environment variable TMPDIR, if it is defined. For security reasons this only happens if the program is not SUID or SGID enabled.
The dir argument, if it is not a null pointer.
The value of the P_tmpdir macro.
The directory /tmp.

This function is defined for SVID compatibility.
Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using tempnam, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using tmpfile or mkstemp is a safe way to avoid this problem.

— SVID Macro: char * P_tmpdir

This macro is the name of the default directory for temporary files.

Older Unix systems did not have the functions just described. Instead they used mktemp and mkstemp. Both of these functions work by modifying a file name template string you pass. The last six characters of this string must be ‘XXXXXX’. These six ‘X’s are replaced with six characters which make the whole string a unique file name. Usually the template string is something like ‘/tmp/prefixXXXXXX’, and each program uses a unique prefix.

NB: Because mktemp and mkstemp modify the template string, you must not pass string constants to them. String constants are normally in read-only storage, so your program would crash when mktemp or mkstemp tried to modify the string. These functions are declared in the header file stdlib.h.

— Function: char * mktemp (char *template)

The mktemp function generates a unique file name by modifying template as described above. If successful, it returns template as modified. If mktemp cannot find a unique file name, it makes template an empty string and returns that. If template does not end with ‘XXXXXX’, mktemp returns a null pointer.
Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using mktemp, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using mkstemp is a safe way to avoid this problem.

— Function: int mkstemp (char *template)

The mkstemp function generates a unique file name just as mktemp does, but it also opens the file for you with open (see Opening and Closing Files). If successful, it modifies template in place and returns a file descriptor for that file open for reading and writing. If mkstemp cannot create a uniquely-named file, it returns -1. If template does not end with ‘XXXXXX’, mkstemp returns -1 and does not modify template.
The file is opened using mode 0600. If the file is meant to be used by other users this mode must be changed explicitly.

Unlike mktemp, mkstemp is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file. This is because it works by calling open with the O_EXCL flag, which says you want to create a new file and get an error if the file already exists.

— Function: char * mkdtemp (char *template)

The mkdtemp function creates a directory with a unique name. If it succeeds, it overwrites template with the name of the directory, and returns template. As with mktemp and mkstemp, template should be a string ending with ‘XXXXXX’.
If mkdtemp cannot create an uniquely named directory, it returns NULL and sets errno appropriately. If template does not end with ‘XXXXXX’, mkdtemp returns NULL and does not modify template. errno will be set to EINVAL in this case.
The directory is created using mode 0700.

The directory created by mkdtemp cannot clash with temporary files or directories created by other users. This is because directory creation always works like open with O_EXCL. See Creating Directories.

15 Pipes and FIFOs

A pipe is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process. The data is handled in a first-in, first-out (FIFO) order. The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe.

A FIFO special file is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file. Processes open the FIFO by name in order to communicate through it.

A pipe or FIFO has to be open at both ends simultaneously. If you read from a pipe or FIFO file that doesn't have any processes writing to it (perhaps because they have all closed the file, or exited), the read returns end-of-file. Writing to a pipe or FIFO that doesn't have a reading process is treated as an error condition; it generates a SIGPIPE signal, and fails with error code EPIPE if the signal is handled or blocked.

Neither pipes nor FIFO special files allow file positioning. Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end.

15.1 Creating a Pipe

The primitive for creating a pipe is the pipe function. This creates both the reading and writing ends of the pipe. It is not very useful for a single process to use a pipe to talk to itself. In typical use, a process creates a pipe just before it forks one or more child processes (see Creating a Process). The pipe is then used for communication either between the parent or child processes, or between two sibling processes.

— Function: int pipe (int filedes[2])

The pipe function creates a pipe and puts the file descriptors for the reading and writing ends of the pipe (respectively) into filedes[0] and filedes[1].
An easy way to remember that the input end comes first is that file descriptor 0 is standard input, and file descriptor 1 is standard output.
If successful, pipe returns a value of 0. On failure, -1 is returned. The following errno error conditions are defined for this function:

EMFILE
The process has too many files open.
ENFILE
There are too many open files in the entire system. See Error Codes, for more information about ENFILE. This error never occurs in the GNU system.

Here is an example of a simple program that creates a pipe. This program uses the fork function (see Creating a Process) to create a child process. The parent process writes data to the pipe, which is read by the child process.

15.2 Pipe to a Subprocess

A common use of pipes is to send data to or receive data from a program being run as a subprocess. One way of doing this is by using a combination of pipe (to create the pipe), fork (to create the subprocess), dup2 (to force the subprocess to use the pipe as its standard input or output channel), and exec (to execute the new program). Or, you can use popen and pclose.

The advantage of using popen and pclose is that the interface is much simpler and easier to use. But it doesn't offer as much flexibility as using the low-level functions directly.

— Function: FILE * popen (const char *command, const char *mode)

The popen function is closely related to the system function; see Running a Command. It executes the shell command command as a subprocess. However, instead of waiting for the command to complete, it creates a pipe to the subprocess and returns a stream that corresponds to that pipe.
If you specify a mode argument of "r", you can read from the stream to retrieve data from the standard output channel of the subprocess. The subprocess inherits its standard input channel from the parent process.
Similarly, if you specify a mode argument of "w", you can write to the stream to send data to the standard input channel of the subprocess. The subprocess inherits its standard output channel from the parent process.
In the event of an error popen returns a null pointer. This might happen if the pipe or stream cannot be created, if the subprocess cannot be forked, or if the program cannot be executed.

— Function: int pclose (FILE *stream)

The pclose function is used to close a stream created by popen. It waits for the child process to terminate and returns its status value, as for the system function.

Here is an example showing how to use popen and pclose to filter output through another program, in this case the paging program more.

15.3 FIFO Special Files

A FIFO special file is similar to a pipe, except that it is created in a different way. Instead of being an anonymous communications channel, a FIFO special file is entered into the file system by calling mkfifo.

Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

— Function: int mkfifo (const char *filename, mode_t mode)

The mkfifo function makes a FIFO special file with name filename. The mode argument is used to set the file's permissions; see Setting Permissions.
The normal, successful return value from mkfifo is 0. In the case of an error, -1 is returned. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST
The named file already exists.
ENOSPC
The directory or file system cannot be extended.
EROFS
The directory that would contain the file resides on a read-only file system.

15.4 Atomicity of Pipe I/O

Reading or writing pipe data is atomic if the size of data written is not greater than PIPE_BUF. This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete. Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin it finishes immediately.

Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed. Also, once PIPE_BUF characters have been written, further writes will block until some characters are read.

16 Sockets

This chapter describes the GNU facilities for interprocess communication using sockets.

A socket is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. Unlike pipes sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; telnet, rlogin, ftp, talk and the other familiar network programs use sockets.

Not all operating systems support sockets. In the GNU library, the header file sys/socket.h exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail.

Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far.

16.1 Socket Concepts

When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The communication style of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these:

You must also choose a namespace for naming the socket. A socket name (“address”) is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called “domains”, but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with ‘PF_’. A corresponding symbolic name starting with ‘AF_’ designates the address format for that namespace.

Finally you must choose the protocol to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a protocol family because of this, which is why the namespace names start with ‘PF_’.

The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system and you need not know about them. What you do need to know about protocols is this:

Throughout the following description at various places variables/parameters to denote sizes are required. And here the trouble starts. In the first implementations the type of these variables was simply int. On most machines at that time an int was 32 bits wide, which created a de facto standard requiring 32-bit variables. This is important since references to variables of this type are passed to the kernel.

Then the POSIX people came and unified the interface with the words "all size values are of type size_t". On 64-bit machines size_t is 64 bits wide, so pointers to variables were no longer possible.

The Unix98 specification provides a solution by introducing a type socklen_t. This type is used in all of the cases that POSIX changed to use size_t. The only requirement of this type is that it be an unsigned type of at least 32 bits. Therefore, implementations which require that references to 32-bit variables be passed can be as happy as implementations which use 64-bit values.

16.2 Communication Styles

The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in sys/socket.h.

— Macro: int SOCK_STREAM

The SOCK_STREAM style is like a pipe (see Pipes and FIFOs). It operates over a connection with a particular remote socket and transmits data reliably as a stream of bytes.
Use of this style is covered in detail in Connections.

— Macro: int SOCK_DGRAM

The SOCK_DGRAM style is used for sending individually-addressed packets unreliably. It is the diametrical opposite of SOCK_STREAM.
Each time you write data to a socket of this kind, that data becomes one packet. Since SOCK_DGRAM sockets do not have connections, you must specify the recipient address with each packet.
The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.
The typical use for SOCK_DGRAM is in situations where it is acceptable to simply re-send a packet if no response is seen in a reasonable amount of time.
See Datagrams, for detailed information about how to use datagram sockets.

— Macro: int SOCK_RAW

This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style.

16.3 Socket Addresses

The name of a socket is normally called an address. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term “name” and sometimes using “address”. You can regard these terms as synonymous where sockets are concerned.

A socket newly created with the socket function has no address. Other processes can find it for communication only if you give it an address. We call this binding the address to the socket, and the way to do it is with the bind function.

You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.

Occasionally a client needs to specify an address because the server discriminates based on address; for example, the rsh and rlogin protocols look at the client's socket address and only bypass password checking if it is less than IPPORT_RESERVED (see Ports).

The details of socket addresses vary depending on what namespace you are using. See Local Namespace, or Internet Namespace, for specific information.

Regardless of the namespace, you use the same functions bind and getsockname to set and examine a socket's address. These functions use a phony data type, struct sockaddr *, to accept the address. In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to struct sockaddr * when you pass it to bind.

16.3.1 Address Formats

The functions bind and getsockname use the generic data type struct sockaddr * to represent a pointer to a socket address. You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace.

Thus, the usual practice is to construct an address of the proper namespace-specific type, then cast a pointer to struct sockaddr * when you call bind or getsockname.

The one piece of information that you can get from the

struct
sockaddr

data type is the address format designator. This tells you which data type to use to understand the address fully.

— Data Type: struct sockaddr

The struct sockaddr type itself has the following members:

short int sa_family
This is the code for the address format of this address. It identifies the format of the data which follows.
char sa_data[14]
This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of sa_data is essentially arbitrary.

Each address format has a symbolic name which starts with ‘AF_’. Each of them corresponds to a ‘PF_’ symbol which designates the corresponding namespace. Here is a list of address format names:

sys/socket.h defines symbols starting with ‘AF_’ for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them.

16.3.2 Setting the Address of a Socket

Use the bind function to assign an address to a socket. The prototype for bind is in the header file sys/socket.h. For examples of use, see Local Socket Example, or see Inet Example.

— Function: int bind (int socket, struct sockaddr *addr, socklen_t length)

The bind function assigns an address to the socket socket. The addr and length arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format of that namespace.
The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on this machine.
EADDRINUSE
Some other socket is already using the specified address.
EINVAL
The socket socket already has an address.
EACCES
You do not have permission to access the requested address. (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through IPPORT_RESERVED minus one; see Ports.)

Additional conditions may be possible depending on the particular namespace of the socket.

16.3.3 Reading the Address of a Socket

Use the function getsockname to examine the address of an Internet socket. The prototype for this function is in the header file sys/socket.h.

— Function: int getsockname (int socket, struct sockaddr *addr, socklen_t *length-ptr)

The getsockname function returns information about the address of the socket socket in the locations specified by the addr and length-ptr arguments. Note that the length-ptr is a pointer; you should initialize it to be the allocation size of addr, and on return it contains the actual size of the address data.
The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to struct sockaddr * to pass it to getsockname.
The return value is 0 on success and -1 on error. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOBUFS
There are not enough internal buffers available for the operation.

You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.

16.4 Interface Naming

Each network interface has a name. This usually consists of a few letters that relate to the type of interface, which may be followed by a number if there is more than one interface of that type. Examples might be lo (the loopback interface) and eth0 (the first Ethernet interface).

Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its index, which is an arbitrarily-assigned small positive integer.

The following functions, constants and data types are declared in the header file net/if.h.

— Constant: size_t IFNAMSIZ

This constant defines the maximum buffer size needed to hold an interface name, including its terminating zero byte.

— Function: unsigned int if_nametoindex (const char *ifname)

This function yields the interface index corresponding to a particular name. If no interface exists with the name given, it returns 0.

— Function: char * if_indextoname (unsigned int ifindex, char *ifname)

This function maps an interface index to its corresponding name. The returned name is placed in the buffer pointed to by ifname, which must be at least IFNAMSIZ bytes in length. If the index was invalid, the function's return value is a null pointer, otherwise it is ifname.

— Data Type: struct if_nameindex

This data type is used to hold the information about a single interface. It has the following members:

unsigned int if_index;
This is the interface index.
char *if_name
This is the null-terminated index name.

— Function: struct if_nameindex * if_nameindex (void)

This function returns an array of if_nameindex structures, one for every interface that is present. The end of the list is indicated by a structure with an interface of 0 and a null name pointer. If an error occurs, this function returns a null pointer.
The returned structure must be freed with if_freenameindex after use.

— Function: void if_freenameindex (struct if_nameindex *ptr)

This function frees the structure returned by an earlier call to if_nameindex.

16.5 The Local Namespace

This section describes the details of the local namespace, whose symbolic name (required when you create a socket) is PF_LOCAL. The local namespace is also known as “Unix domain sockets”. Another name is file namespace since socket addresses are normally implemented as file names.

16.5.1 Local Namespace Concepts

In the local namespace socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. It's common to put these files in the /tmp directory.

One peculiarity of the local namespace is that the name is only used when opening the connection; once open the address is not meaningful and may not exist.

Another peculiarity is that you cannot connect to such a socket from another machine–not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one.

After you close a socket in the local namespace, you should delete the file name from the file system. Use unlink or remove to do this; see Deleting Files.

The local namespace supports just one protocol for any communication style; it is protocol number 0.

16.5.2 Details of Local Namespace

To create a socket in the local namespace, use the constant PF_LOCAL as the namespace argument to socket or socketpair. This constant is defined in sys/socket.h.

— Macro: int PF_LOCAL

This designates the local namespace, in which socket addresses are local names, and its associated family of protocols. PF_Local is the macro used by Posix.1g.

— Macro: int PF_UNIX

This is a synonym for PF_LOCAL, for compatibility's sake.

— Macro: int PF_FILE

This is a synonym for PF_LOCAL, for compatibility's sake.

The structure for specifying socket names in the local namespace is defined in the header file sys/un.h:

— Data Type: struct sockaddr_un

This structure is used to specify local namespace socket addresses. It has the following members:

short int sun_family
This identifies the address family or format of the socket address. You should store the value AF_LOCAL to designate the local namespace. See Socket Addresses.
char sun_path[108]
This is the file name to use.
Incomplete: Why is 108 a magic number? RMS suggests making this a zero-length array and tweaking the following example to use alloca to allocate an appropriate amount of storage based on the length of the filename.

You should compute the length parameter for a socket address in the local namespace as the sum of the size of the sun_family component and the string length (not the allocation size!) of the file name string. This can be done using the macro SUN_LEN:

— Macro: int SUN_LEN (struct sockaddr_un * ptr)

The macro computes the length of socket address in the local namespace.

16.5.3 Example of Local-Namespace Sockets

Here is an example showing how to create and name a socket in the local namespace.

16.6 The Internet Namespace

This section describes the details of the protocols and socket naming conventions used in the Internet namespace.

Originally the Internet namespace used only IP version 4 (IPv4). With the growing number of hosts on the Internet, a new protocol with a larger address space was necessary: IP version 6 (IPv6). IPv6 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other features, and will eventually replace IPv4.

To create a socket in the IPv4 Internet namespace, use the symbolic name PF_INET of this namespace as the namespace argument to socket or socketpair. For IPv6 addresses you need the macro PF_INET6. These macros are defined in sys/socket.h.

— Macro: int PF_INET

This designates the IPv4 Internet namespace and associated family of protocols.

— Macro: int PF_INET6

This designates the IPv6 Internet namespace and associated family of protocols.

You must ensure that the address and port number are represented in a canonical format called network byte order. See Byte Order, for information about this.

16.6.1 Internet Socket Address Formats

In the Internet namespace, for both IPv4 (AF_INET) and IPv6 (AF_INET6), a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.

The data types for representing socket addresses in the Internet namespace are defined in the header file netinet/in.h.

— Data Type: struct sockaddr_in

This is the data type used to represent socket addresses in the Internet namespace. It has the following members:

sa_family_t sin_family
This identifies the address family or format of the socket address. You should store the value AF_INET in this member. See Socket Addresses.
struct in_addr sin_addr
This is the Internet address of the host machine. See Host Addresses, and Host Names, for how to get a value to store here.
unsigned short int sin_port
This is the port number. See Ports.

When you call bind or getsockname, you should specify sizeof (struct sockaddr_in) as the length parameter if you are using an IPv4 Internet namespace socket address.

— Data Type: struct sockaddr_in6

This is the data type used to represent socket addresses in the IPv6 namespace. It has the following members:

sa_family_t sin6_family
This identifies the address family or format of the socket address. You should store the value of AF_INET6 in this member. See Socket Addresses.
struct in6_addr sin6_addr
This is the IPv6 address of the host machine. See Host Addresses, and Host Names, for how to get a value to store here.
uint32_t sin6_flowinfo
This is a currently unimplemented field.
uint16_t sin6_port
This is the port number. See Ports.

16.6.2 Host Addresses

Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in ‘128.52.46.32’, and IPv6 numeric host addresses as sequences of up to eight numbers separated by colons, as in ‘5f03:1200:836f:c100::1’.

Each computer also has one or more host names, which are strings of words separated by periods, as in ‘mescaline.gnu.org’.

Programs that let the user specify a host typically accept both numeric addresses and host names. To open a connection a program needs a numeric address, and so must convert a host name to the numeric address it stands for.

16.6.2.1 Internet Host Addresses

An IPv4 Internet host address is a number containing four bytes of data. Historically these are divided into two parts, a network number and a local network address number within that network. In the mid-1990s classless addresses were introduced which changed this behavior. Since some functions implicitly expect the old definitions, we first describe the class-based network and will then describe classless addresses. IPv6 uses only classless addresses and therefore the following paragraphs don't apply.

The class-based IPv4 network number consists of the first one, two or three bytes; the rest of the bytes are the local address.

IPv4 network numbers are registered with the Network Information Center (NIC), and are divided into three classes—A, B and C. The local network address numbers of individual machines are registered with the administrator of the particular network.

Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specify a network. The remaining bytes of the Internet address specify the address within that network.

The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network. These uses are obsolete now but for compatibility reasons you shouldn't use network 0 and host number 0.

The Class A network 127 is reserved for loopback; you can always use the Internet address ‘127.0.0.1’ to refer to the host machine.

Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.

There are four forms of the standard numbers-and-dots notation for Internet addresses:

Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading ‘0x’ or ‘0X’ implies hexadecimal radix; a leading ‘0’ implies octal; and otherwise decimal radix is assumed.

Classless Addresses

IPv4 addresses (and IPv6 addresses also) are now considered classless; the distinction between classes A, B and C can be ignored. Instead an IPv4 host address consists of a 32-bit address and a 32-bit mask. The mask contains set bits for the network part and cleared bits for the host part. The network part is contiguous from the left, with the remaining bits representing the host. As a consequence, the netmask can simply be specified as the number of set bits. Classes A, B and C are just special cases of this general rule. For example, class A addresses have a netmask of ‘255.0.0.0’ or a prefix length of 8.

Classless IPv4 network addresses are written in numbers-and-dots notation with the prefix length appended and a slash as separator. For example the class A network 10 is written as ‘10.0.0.0/8’.

IPv6 Addresses

IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host address is usually written as eight 16-bit hexadecimal numbers that are separated by colons. Two colons are used to abbreviate strings of consecutive zeros. For example, the IPv6 loopback address ‘0:0:0:0:0:0:0:1’ can just be written as ‘::1’.

16.6.2.2 Host Address Data Type

IPv4 Internet host addresses are represented in some contexts as integers (type uint32_t). In other contexts, the integer is packaged inside a structure of type struct in_addr. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.

You will find older code that uses unsigned long int for IPv4 Internet host addresses instead of uint32_t or

struct
in_addr

. Historically unsigned long int was a 32-bit number but with 64-bit machines this has changed. Using unsigned long int might break the code if it is used on machines where this type doesn't have 32 bits. uint32_t is specified by Unix98 and guaranteed to have 32 bits.

IPv6 Internet host addresses have 128 bits and are packaged inside a structure of type struct in6_addr.

The following basic definitions for Internet addresses are declared in the header file netinet/in.h:

— Data Type: struct in_addr

This data type is used in certain contexts to contain an IPv4 Internet host address. It has just one field, named s_addr, which records the host address number as an uint32_t.

— Macro: uint32_t INADDR_LOOPBACK

You can use this constant to stand for “the address of this machine,” instead of finding its actual address. It is the IPv4 Internet address ‘127.0.0.1’, which is usually called ‘localhost’. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements INADDR_LOOPBACK specially, avoiding any network traffic for the case of one machine talking to itself.

— Macro: uint32_t INADDR_ANY

You can use this constant to stand for “any incoming address” when binding to an address. See Setting Address. This is the usual address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.

— Macro: uint32_t INADDR_BROADCAST

This constant is the address you use to send a broadcast message.

— Macro: uint32_t INADDR_NONE

This constant is returned by some functions to indicate an error.

— Data Type: struct in6_addr

This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways.

— Constant: struct in6_addr in6addr_loopback

This constant is the IPv6 address ‘::1’, the loopback address. See above for a description of what this means. The macro IN6ADDR_LOOPBACK_INIT is provided to allow you to initialize your own variables to this value.

— Constant: struct in6_addr in6addr_any

This constant is the IPv6 address ‘::’, the unspecified address. See above for a description of what this means. The macro IN6ADDR_ANY_INIT is provided to allow you to initialize your own variables to this value.

16.6.2.3 Host Address Functions

These additional functions for manipulating Internet addresses are declared in the header file arpa/inet.h. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. See Byte Order, for an explanation of network and host byte order.

— Function: int inet_aton (const char *name, struct in_addr *addr)

This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data and stores it in the struct in_addr that addr points to. inet_aton returns nonzero if the address is valid, zero if not.

— Function: uint32_t inet_addr (const char *name)

This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data. If the input is not valid, inet_addr returns INADDR_NONE. This is an obsolete interface to inet_aton, described immediately above. It is obsolete because INADDR_NONE is a valid address (255.255.255.255), and inet_aton provides a cleaner way to indicate error return.

— Function: uint32_t inet_network (const char *name)

This function extracts the network number from the address name, given in the standard numbers-and-dots notation. The returned address is in host order. If the input is not valid, inet_network returns -1.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: char * inet_ntoa (struct in_addr addr)

This function converts the IPv4 Internet host address addr to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.
In multi-threaded programs each thread has an own statically-allocated buffer. But still subsequent calls of inet_ntoa in the same thread will overwrite the result of the last call.
Instead of inet_ntoa the newer function inet_ntop which is described below should be used since it handles both IPv4 and IPv6 addresses.

— Function: struct in_addr inet_makeaddr (uint32_t net, uint32_t local)

This function makes an IPv4 Internet host address by combining the network number net with the local-address-within-network number local.

— Function: uint32_t inet_lnaof (struct in_addr addr)

This function returns the local-address-within-network part of the Internet host address addr.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: uint32_t inet_netof (struct in_addr addr)

This function returns the network number part of the Internet host address addr.
The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: int inet_pton (int af, const char *cp, void *buf)

This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. af should be either AF_INET or AF_INET6, as appropriate for the type of address being converted. cp is a pointer to the input string, and buf is a pointer to a buffer for the result. It is the caller's responsibility to make sure the buffer is large enough.

— Function: const char * inet_ntop (int af, const void *cp, char *buf, size_t len)

This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. af should be either AF_INET or AF_INET6, as appropriate. cp is a pointer to the address to be converted. buf should be a pointer to a buffer to hold the result, and len is the length of this buffer. The return value from the function will be this buffer address.

16.6.2.4 Host Names

Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address ‘158.121.106.19’ is also known as ‘alpha.gnu.org’; and other machines in the ‘gnu.org’ domain can refer to it simply as ‘alpha’.

Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file /etc/hosts or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in netdb.h. They are BSD features, defined unconditionally if you include netdb.h.

— Data Type: struct hostent

This data type is used to represent an entry in the hosts database. It has the following members:

char *h_name
This is the “official” name of the host.
char **h_aliases
These are alternative names for the host, represented as a null-terminated vector of strings.
int h_addrtype
This is the host address type; in practice, its value is always either AF_INET or AF_INET6, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than AF_INET or AF_INET6. See Socket Addresses.
int h_length
This is the length, in bytes, of each address.
char **h_addr_list
This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer.
char *h_addr
This is a synonym for h_addr_list[0]; in other words, it is the first host address.

As far as the host database is concerned, each address is just a block of memory h_length bytes long. But in other contexts there is an implicit assumption that you can convert IPv4 addresses to a struct in_addr or an uint32_t. Host addresses in a struct hostent structure are always given in network byte order; see Byte Order.

You can use gethostbyname, gethostbyname2 or gethostbyaddr to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use getaddrinfo and getnameinfo to obtain this information.

— Function: struct hostent * gethostbyname (const char *name)

The gethostbyname function returns information about the host named name. If the lookup fails, it returns a null pointer.

— Function: struct hostent * gethostbyname2 (const char *name, int af)

The gethostbyname2 function is like gethostbyname, but allows the caller to specify the desired address family (e.g. AF_INET or AF_INET6) of the result.

— Function: struct hostent * gethostbyaddr (const char *addr, size_t length, int format)

The gethostbyaddr function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of AF_INET; for an IPv6 Internet address, use AF_INET6.
If the lookup fails, gethostbyaddr returns a null pointer.

If the name lookup by gethostbyname or gethostbyaddr fails, you can find out the reason by looking at the value of the variable h_errno. (It would be cleaner design for these functions to set errno, but use of h_errno is compatible with other systems.)

The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C library a new set of functions which can be used in this context.

— Function: int gethostbyname_r (const char *restrict name, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyname_r function returns information about the host named name. The caller must pass a pointer to an object of type struct hostent in the result_buf parameter. In addition the function may need extra buffer space and the caller must pass an pointer and the size of the buffer in the buf and buflen parameters.
A pointer to the buffer, in which the result is stored, is available in *result after the function call successfully returned. If an error occurs or if no entry is found, the pointer *result is a null pointer. Success is signalled by a zero return value. If the function failed the return value is an error number. In addition to the errors defined for gethostbyname it can also be ERANGE. In this case the call should be repeated with a larger buffer. Additional error information is not stored in the global variable h_errno but instead in the object pointed to by h_errnop.
Here's a small example:
          struct hostent *
          gethostname (char *host)
          {
            struct hostent hostbuf, *hp;
            size_t hstbuflen;
            char *tmphstbuf;
            int res;
            int herr;
          
            hstbuflen = 1024;
            /* Allocate buffer, remember to free it to avoid memory leakage.  */
            tmphstbuf = malloc (hstbuflen);
          
            while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
                                           &hp, &herr)) == ERANGE)
              {
                /* Enlarge the buffer.  */
                hstbuflen *= 2;
                tmphstbuf = realloc (tmphstbuf, hstbuflen);
              }
            /*  Check for errors.  */
            if (res || hp == NULL)
              return NULL;
            return hp;
          }

— Function: int gethostbyname2_r (const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyname2_r function is like gethostbyname_r, but allows the caller to specify the desired address family (e.g. AF_INET or AF_INET6) for the result.

— Function: int gethostbyaddr_r (const char *addr, size_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyaddr_r function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of AF_INET; for an IPv6 Internet address, use AF_INET6.
Similar to the gethostbyname_r function, the caller must provide buffers for the result and memory used internally. In case of success the function returns zero. Otherwise the value is an error number where ERANGE has the special meaning that the caller-provided buffer is too small.

You can also scan the entire hosts database one entry at a time using sethostent, gethostent and endhostent. Be careful when using these functions because they are not reentrant.

— Function: void sethostent (int stayopen)

This function opens the hosts database to begin scanning it. You can then call gethostent to read the entries.
If the stayopen argument is nonzero, this sets a flag so that subsequent calls to gethostbyname or gethostbyaddr will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct hostent * gethostent (void)

This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries.

— Function: void endhostent (void)

This function closes the hosts database.

16.6.3 Internet Ports

A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.

Port numbers less than IPPORT_RESERVED are reserved for standard servers, such as finger and telnet. There is a database that keeps track of these, and you can use the getservbyname function to map a service name onto a port number; see Services Database.

If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it. Use a number greater than IPPORT_USERRESERVED; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you.

When you use a socket without specifying its address, the system generates a port number for it. This number is between IPPORT_RESERVED and IPPORT_USERRESERVED.

On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number). You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it. Normally, the system won't let you do it; bind normally insists on distinct port numbers. To reuse a port number, you must set the socket option SO_REUSEADDR. See Socket-Level Options.

— Macro: int IPPORT_RESERVED

Port numbers less than IPPORT_RESERVED are reserved for superuser use.

— Macro: int IPPORT_USERRESERVED

Port numbers greater than or equal to IPPORT_USERRESERVED are reserved for explicit use; they will never be allocated automatically.

16.6.4 The Services Database

The database that keeps track of “well-known” services is usually either the file /etc/services or an equivalent from a name server. You can use these utilities, declared in netdb.h, to access the services database.

— Data Type: struct servent

This data type holds information about entries from the services database. It has the following members:

char *s_name
This is the “official” name of the service.
char **s_aliases
These are alternate names for the service, represented as an array of strings. A null pointer terminates the array.
int s_port
This is the port number for the service. Port numbers are given in network byte order; see Byte Order.
char *s_proto
This is the name of the protocol to use with this service. See Protocols Database.

To get information about a particular service, use the getservbyname or getservbyport functions. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

— Function: struct servent * getservbyname (const char *name, const char *proto)

The getservbyname function returns information about the service named name using protocol proto. If it can't find such a service, it returns a null pointer.
This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see Listening).

— Function: struct servent * getservbyport (int port, const char *proto)

The getservbyport function returns information about the service at port port using protocol proto. If it can't find such a service, it returns a null pointer.

You can also scan the services database using setservent, getservent and endservent. Be careful when using these functions because they are not reentrant.

— Function: void setservent (int stayopen)

This function opens the services database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getservbyname or getservbyport will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct servent * getservent (void)

This function returns the next entry in the services database. If there are no more entries, it returns a null pointer.

— Function: void endservent (void)

This function closes the services database.

16.6.5 Byte Order Conversion

Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called “big-endian” order), and others put it last (“little-endian” order).

So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as network byte order.

When establishing an Internet socket connection, you must make sure that the data in the sin_port and sin_addr members of the sockaddr_in structure are represented in network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines.

If you use getservbyname and gethostbyname or inet_addr to get the port number and host address, the values are already in network byte order, and you can copy them directly into the sockaddr_in structure.

Otherwise, you have to convert the values explicitly. Use htons and ntohs to convert values for the sin_port member. Use htonl and ntohl to convert IPv4 addresses for the sin_addr member. (Remember, struct in_addr is equivalent to uint32_t.) These functions are declared in netinet/in.h.

— Function: uint16_t htons (uint16_t hostshort)

This function converts the uint16_t integer hostshort from host byte order to network byte order.

— Function: uint16_t ntohs (uint16_t netshort)

This function converts the uint16_t integer netshort from network byte order to host byte order.

— Function: uint32_t htonl (uint32_t hostlong)

This function converts the uint32_t integer hostlong from host byte order to network byte order.
This is used for IPv4 Internet addresses.

— Function: uint32_t ntohl (uint32_t netlong)

This function converts the uint32_t integer netlong from network byte order to host byte order.
This is used for IPv4 Internet addresses.

16.6.6 Protocols Database

The communications protocol used with a socket controls low-level details of how data are exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly.

The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP (“transmission control protocol”). For datagram communication, the default is UDP (“user datagram protocol”). For reliable datagram communication, the default is RDP (“reliable datagram protocol”). You should nearly always use the default.

Internet protocols are generally specified by a name instead of a number. The network protocols that a host knows about are stored in a database. This is usually either derived from the file /etc/protocols, or it may be an equivalent provided by a name server. You look up the protocol number associated with a named protocol in the database using the getprotobyname function.

Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in netdb.h.

— Data Type: struct protoent

This data type is used to represent entries in the network protocols database. It has the following members:

char *p_name
This is the official name of the protocol.
char **p_aliases
These are alternate names for the protocol, specified as an array of strings. The last element of the array is a null pointer.
int p_proto
This is the protocol number (in host byte order); use this member as the protocol argument to socket.

You can use getprotobyname and getprotobynumber to search the protocols database for a specific protocol. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

— Function: struct protoent * getprotobyname (const char *name)

The getprotobyname function returns information about the network protocol named name. If there is no such protocol, it returns a null pointer.

— Function: struct protoent * getprotobynumber (int protocol)

The getprotobynumber function returns information about the network protocol with number protocol. If there is no such protocol, it returns a null pointer.

You can also scan the whole protocols database one protocol at a time by using setprotoent, getprotoent and endprotoent. Be careful when using these functions because they are not reentrant.

— Function: void setprotoent (int stayopen)

This function opens the protocols database to begin scanning it.
If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getprotobyname or getprotobynumber will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct protoent * getprotoent (void)

This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries.

— Function: void endprotoent (void)

This function closes the protocols database.

16.6.7 Internet Socket Example

Here is an example showing how to create and name a socket in the Internet namespace. The newly created socket exists on the machine that the program is running on. Rather than finding and using the machine's Internet address, this example specifies INADDR_ANY as the host address; the system replaces that with the machine's actual address.

Here is another example, showing how you can fill in a sockaddr_in structure, given a host name string and a port number:

16.7 Other Namespaces

Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. PF_NS refers to the Xerox Network Software protocols. PF_ISO stands for Open Systems Interconnect. PF_CCITT refers to protocols from CCITT. socket.h defines these symbols and others naming protocols not actually implemented.

PF_IMPLINK is used for communicating between hosts and Internet Message Processors. For information on this and PF_ROUTE, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).

16.8 Opening and Closing Sockets

This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles.

16.8.1 Creating a Socket

The primitive for creating a socket is the socket function, declared in sys/socket.h.

— Function: int socket (int namespace, int style, int protocol)

This function creates a socket and specifies communication style style, which should be one of the socket styles listed in Communication Styles. The namespace argument specifies the namespace; it must be PF_LOCAL (see Local Namespace) or PF_INET (see Internet Namespace). protocol designates the specific protocol (see Socket Concepts); zero is usually right for protocol.
The return value from socket is the file descriptor for the new socket, or -1 in case of error. The following errno error conditions are defined for this function:

EPROTONOSUPPORT
The protocol or style is not supported by the namespace specified.
EMFILE
The process already has too many file descriptors open.
ENFILE
The system already has too many file descriptors open.
EACCES
The process does not have the privilege to create a socket of the specified style or protocol.
ENOBUFS
The system ran out of internal buffer space.

The file descriptor returned by the socket function supports both read and write operations. However, like pipes, sockets do not support file positioning operations.

16.8.2 Closing a Socket

When you have finished using a socket, you can simply close its file descriptor with close; see Opening and Closing Files. If there is still data waiting to be transmitted over the connection, normally close tries to complete this transmission. You can control this behavior using the SO_LINGER socket option to specify a timeout period; see Socket Options.

You can also shut down only reception or transmission on a connection by calling shutdown, which is declared in sys/socket.h.

— Function: int shutdown (int socket, int how)

The shutdown function shuts down the connection of socket socket. The argument how specifies what action to perform:

0
Stop receiving data for this socket. If further data arrives, reject it.
1
Stop trying to transmit data from this socket. Discard any data waiting to be sent. Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost.
2
Stop both reception and transmission.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
socket is not a valid file descriptor.
ENOTSOCK
socket is not a socket.
ENOTCONN
socket is not connected.

16.8.3 Socket Pairs

A socket pair consists of a pair of connected (but unnamed) sockets. It is very similar to a pipe and is used in much the same way. Socket pairs are created with the socketpair function, declared in sys/socket.h. A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (see Pipes and FIFOs).

— Function: int socketpair (int namespace, int style, int protocol, int filedes[2])

This function creates a socket pair, returning the file descriptors in filedes[0] and filedes[1]. The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end.
The namespace, style and protocol arguments are interpreted as for the socket function. style should be one of the communication styles listed in Communication Styles. The namespace argument specifies the namespace, which must be AF_LOCAL (see Local Namespace); protocol specifies the communications protocol, but zero is the only meaningful value.
If style specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.
The socketpair function returns 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EMFILE
The process has too many file descriptors open.
EAFNOSUPPORT
The specified namespace is not supported.
EPROTONOSUPPORT
The specified protocol is not supported.
EOPNOTSUPP
The specified protocol does not support the creation of socket pairs.

16.9 Using Sockets with Connections

The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the client) acts to request a connection, while the other side (the server) makes a socket and waits for the connection request.

16.9.1 Making a Connection

In making a connection, the client makes a connection while the server waits for and accepts the connection. Here we discuss what the client program must do with the connect function, which is declared in sys/socket.h.

— Function: int connect (int socket, struct sockaddr *addr, socklen_t length)

The connect function initiates a connection from the socket with file descriptor socket to the socket whose address is specified by the addr and length arguments. (This socket is typically on another machine, and it must be already set up as a server.) See Socket Addresses, for information about how these arguments are interpreted.
Normally, connect waits until the server responds to the request before it returns. You can set nonblocking mode on the socket socket to make connect return immediately without waiting for the response. See File Status Flags, for information about nonblocking mode.
The normal return value from connect is 0. If an error occurs, connect returns -1. The following errno error conditions are defined for this function:

EBADF
The socket socket is not a valid file descriptor.
ENOTSOCK
File descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on the remote machine.
EAFNOSUPPORT
The namespace of the addr is not supported by this socket.
EISCONN
The socket socket is already connected.
ETIMEDOUT
The attempt to establish the connection timed out.
ECONNREFUSED
The server has actively refused to establish the connection.
ENETUNREACH
The network of the given addr isn't reachable from this host.
EADDRINUSE
The socket address of the given addr is already in use.
EINPROGRESS
The socket socket is non-blocking and the connection could not be established immediately. You can determine when the connection is completely established with select; see Waiting for I/O. Another connect call on the same socket, before the connection is completely established, will fail with EALREADY.
EALREADY
The socket socket is non-blocking and already has a pending connection in progress (see EINPROGRESS above).

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

16.9.2 Listening for Connections

Now let us consider what the server process must do to accept connections on a socket. First it must use the listen function to enable connection requests on the socket, and then accept each incoming connection with a call to accept (see Accepting Connections). Once connection requests are enabled on a server socket, the select function reports when the socket has a connection ready to be accepted (see Waiting for I/O).

The listen function is not allowed for sockets using connectionless communication styles.

You can write a network server that does not even start running until a connection to it is requested. See Inetd Servers.

In the Internet namespace, there are no special protection mechanisms for controlling access to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.

In the local namespace, the ordinary file protection bits control who has access to connect to the socket.

— Function: int listen (int socket, int n)

The listen function enables the socket socket to accept connections, thus making it a server socket.
The argument n specifies the length of the queue for pending connections. When the queue fills, new clients attempting to connect fail with ECONNREFUSED until the server calls accept to accept a connection from the queue.
The listen function returns 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
The argument socket is not a valid file descriptor.
ENOTSOCK
The argument socket is not a socket.
EOPNOTSUPP
The socket socket does not support this operation.

16.9.3 Accepting Connections

When a server receives a connection request, it can complete the connection by accepting the request. Use the function accept to do this.

A socket that has been established as a server can accept connection requests from multiple clients. The server's original socket does not become part of the connection; instead, accept makes a new socket which participates in the connection. accept returns the descriptor for this socket. The server's original socket remains available for listening for further connection requests.

The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with an ECONNREFUSED error. You can specify the maximum length of this queue as an argument to the listen function, although the system may also impose its own internal limit on the length of this queue.

— Function: int accept (int socket, struct sockaddr *addr, socklen_t *length_ptr)

This function is used to accept a connection request on the server socket socket.
The accept function waits if there are no connections pending, unless the socket socket has nonblocking mode set. (You can use select to wait for a pending connection, with a nonblocking socket.) See File Status Flags, for information about nonblocking mode.
The addr and length-ptr arguments are used to return information about the name of the client socket that initiated the connection. See Socket Addresses, for information about the format of the information.
Accepting a connection does not make socket part of the connection. Instead, it creates a new socket which becomes connected. The normal return value of accept is the file descriptor for the new socket.
After accept, the original socket socket remains open and unconnected, and continues listening until you close it. You can accept further connections with socket by calling accept again.
If an error occurs, accept returns -1. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket argument is not a socket.
EOPNOTSUPP
The descriptor socket does not support this operation.
EWOULDBLOCK
socket has nonblocking mode set, and there are no pending connections immediately available.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

The accept function is not allowed for sockets using connectionless communication styles.

16.9.4 Who is Connected to Me?

— Function: int getpeername (int socket, struct sockaddr *addr, socklen_t *length-ptr)

The getpeername function returns the address of the socket that socket is connected to; it stores the address in the memory space specified by addr and length-ptr. It stores the length of the address in *length-ptr.
See Socket Addresses, for information about the format of the address. In some operating systems, getpeername works only for sockets in the Internet domain.
The return value is 0 on success and -1 on error. The following errno error conditions are defined for this function:

EBADF
The argument socket is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOTCONN
The socket socket is not connected.
ENOBUFS
There are not enough internal buffers available.

16.9.5 Transferring Data

Once a socket has been connected to a peer, you can use the ordinary read and write operations (see I/O Primitives) to transfer data. A socket is a two-way communications channel, so read and write operations can be performed at either end.

There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the recv and send functions instead of the more generic read and write functions. The recv and send functions take an additional argument which you can use to specify various flags to control special I/O modes. For example, you can specify the MSG_OOB flag to read or write out-of-band data, the MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag to control inclusion of routing information on output.

16.9.5.1 Sending Data

The send function is declared in the header file sys/socket.h. If your flags argument is zero, you can just as well use write instead of send; see I/O Primitives. If the socket was connected but the connection has broken, you get a SIGPIPE signal for any use of send or write (see Miscellaneous Signals).

— Function: int send (int socket, void *buffer, size_t size, int flags)

The send function is like write, but with the additional flags flags. The possible values of flags are described in Socket Data Options.
This function returns the number of bytes transmitted, or -1 on failure. If the socket is nonblocking, then send (like write) can return after sending just part of the data. See File Status Flags, for information about nonblocking mode.
Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.
The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
EINTR
The operation was interrupted by a signal before any data was sent. See Interrupted Primitives.
ENOTSOCK
The descriptor socket is not a socket.
EMSGSIZE
The socket type requires that the message be sent atomically, but the message is too large for this to be possible.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the write operation would block. (Normally send blocks until the operation can be completed.)
ENOBUFS
There is not enough internal buffer space available.
ENOTCONN
You never connected this socket.
EPIPE
This socket was connected but the connection is now broken. In this case, send generates a SIGPIPE signal first; if that signal is ignored or blocked, or if its handler returns, then send fails with EPIPE.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

16.9.5.2 Receiving Data

The recv function is declared in the header file sys/socket.h. If your flags argument is zero, you can just as well use read instead of recv; see I/O Primitives.

— Function: int recv (int socket, void *buffer, size_t size, int flags)

The recv function is like read, but with the additional flags flags. The possible values of flags are described in Socket Data Options.
If nonblocking mode is set for socket, and no data are available to be read, recv fails immediately rather than waiting. See File Status Flags, for information about nonblocking mode.
This function returns the number of bytes received, or -1 on failure. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the read operation would block. (Normally, recv blocks until there is input available to be read.)
EINTR
The operation was interrupted by a signal before any data was read. See Interrupted Primitives.
ENOTCONN
You never connected this socket.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

16.9.5.3 Socket Data Options

The flags argument to send and recv is a bit mask. You can bitwise-OR the values of the following macros together to obtain a value for this argument. All are defined in the header file sys/socket.h.

— Macro: int MSG_OOB

Send or receive out-of-band data. See Out-of-Band Data.

— Macro: int MSG_PEEK

Look at the data but don't remove it from the input queue. This is only meaningful with input functions such as recv, not with send.

— Macro: int MSG_DONTROUTE

Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here.

16.9.6 Byte Stream Socket Example

Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.

This program uses init_sockaddr to set up the socket address; see Inet Example.

16.9.7 Byte Stream Connection Server Example

The server end is much more complicated. Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling read or recv. Instead, the right thing to do is to use select (see Waiting for I/O) to wait for input on all of the open sockets. This also allows the server to deal with additional connection requests.

This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).

16.9.8 Out-of-Band Data

Streams with connections permit out-of-band data that is delivered with higher priority than ordinary data. Typically the reason for sending out-of-band data is to send notice of an exceptional condition. To send out-of-band data use send, specifying the flag MSG_OOB (see Sending Data).

Out-of-band data are received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use recv with the MSG_OOB flag (see Receiving Data). Ordinary read operations do not read out-of-band data; they read only ordinary data.

When a socket finds that out-of-band data are on their way, it sends a SIGURG signal to the owner process or process group of the socket. You can specify the owner using the F_SETOWN command to the fcntl function; see Interrupt Input. You must also establish a handler for this signal, as described in Signal Handling, in order to take appropriate action such as reading the out-of-band data.

Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the select function; it can wait for an exceptional condition on the socket. See Waiting for I/O, for more information about select.

Notification of out-of-band data (whether with SIGURG or with select) indicates that out-of-band data are on the way; the data may not actually arrive until later. If you try to read the out-of-band data before it arrives, recv fails with an EWOULDBLOCK error.

Sending out-of-band data automatically places a “mark” in the stream of ordinary data, showing where in the sequence the out-of-band data “would have been”. This is useful when the meaning of out-of-band data is “cancel everything sent so far”. Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark:

The integer variable atmark is set to a nonzero value if the socket's read pointer has reached the “mark”.

If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data. If you try to read out-of-band data and get an EWOULDBLOCK error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room. Here is an example:

16.10 Datagram Socket Operations

This section describes how to use communication styles that don't use connections (styles SOCK_DGRAM and SOCK_RDM). Using these styles, you group data into packets and each packet is an independent communication. You specify the destination for each packet individually.

Datagram packets are like letters: you send each one independently with its own destination address, and they may arrive in the wrong order or not at all.

The listen and accept functions are not allowed for sockets using connectionless communication styles.

16.10.1 Sending Datagrams

The normal way of sending data on a datagram socket is by using the sendto function, declared in sys/socket.h.

You can call connect on a datagram socket, but this only specifies a default destination for further data transmission on the socket. When a socket has a default destination you can use send (see Sending Data) or even write (see I/O Primitives) to send a packet there. You can cancel the default destination by calling connect using an address format of AF_UNSPEC in the addr argument. See Connecting, for more information about the connect function.

— Function: int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, socklen_t length)

The sendto function transmits the data in the buffer through the socket socket to the destination address specified by the addr and length arguments. The size argument specifies the number of bytes to be transmitted.
The flags are interpreted the same way as for send; see Socket Data Options.
The return value and error conditions are also the same as for send, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no-one at the specified address to receive it, and the operating system on your machine usually does not know this.
It is also possible for one call to sendto to report an error owing to a problem related to a previous call.
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

16.10.2 Receiving Datagrams

The recvfrom function reads a packet from a datagram socket and also tells you where it was sent from. This function is declared in sys/socket.h.

— Function: int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr)

The recvfrom function reads one packet from the socket socket into the buffer buffer. The size argument specifies the maximum number of bytes to be read.
If the packet is longer than size bytes, then you get the first size bytes of the packet and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect.
The addr and length-ptr arguments are used to return the address where the packet came from. See Socket Addresses. For a socket in the local domain the address information won't be meaningful, since you can't read the address of such a socket (see Local Namespace). You can specify a null pointer as the addr argument if you are not interested in this information.
The flags are interpreted the same way as for recv (see Socket Data Options). The return value and error conditions are also the same as for recv.
This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

You can use plain recv (see Receiving Data) instead of recvfrom if you don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike). Even read can be used if you don't want to specify flags (see I/O Primitives).

16.10.3 Datagram Socket Example

Here is a set of example programs that send messages over a datagram stream in the local namespace. Both the client and server programs use the make_named_socket function that was presented in Local Socket Example, to create and name their sockets.

First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously this isn't a particularly useful program, but it does show the general ideas involved.

16.10.4 Example of Reading Datagrams

It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client.

Keep in mind that datagram socket communications are unreliable. In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back. It's up to the user running the program to kill and restart it if desired. A more automatic solution could be to use select (see Waiting for I/O) to establish a timeout period for the reply, and in case of timeout either re-send the message or shut down the socket and exit.

16.11 The inetd Daemon

We've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.

Another way to provide a service on an Internet port is to let the daemon program inetd do the listening. inetd is a program that runs all the time and waits (using select) for messages on a specified set of ports. When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program. You specify the ports and their programs in the file /etc/inetd.conf.

16.11.1 inetd Servers

Writing a server program to be run by inetd is very simple. Each time someone requests a connection to the appropriate port, a new server process starts. The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process. Thus the server program can begin reading and writing data right away. Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by inetd.

You can also use inetd for servers that use connectionless communication styles. For these servers, inetd does not try to accept a connection since no connection is possible. It just starts the server program, which can read the incoming datagram packet from descriptor 0. The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit. You must specify which of these two techniques the server uses when you configure inetd.

16.11.2 Configuring inetd

The file /etc/inetd.conf tells inetd which ports to listen to and what server programs to run for them. Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace. Lines that start with ‘#’ are comments.

The service field says which service this program provides. It should be the name of a service defined in /etc/services. inetd uses service to decide which port to listen on for this entry.

The fields style and protocol specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with ‘SOCK_’ deleted—for example, ‘stream’ or ‘dgram’. protocol should be one of the protocols listed in /etc/protocols. The typical protocol names are ‘tcp’ for byte stream connections and ‘udp’ for unreliable datagrams.

The wait field should be either ‘wait’ or ‘nowait’. Use ‘wait’ if style is a connectionless style and the server, once started, handles multiple requests as they come in. Use ‘nowait’ if inetd should start a new process for each message or request that comes in. If style uses connections, then wait must be ‘nowait’.

user is the user name that the server should run as. inetd runs as root, so it can set the user ID of its children arbitrarily. It's best to avoid using ‘root’ for user if you can; but some servers, such as Telnet and FTP, read a username and password themselves. These servers need to be root initially so they can log in as commanded by the data coming over the network.

program together with arguments specifies the command to run to start the server. program should be an absolute file name specifying the executable file to run. arguments consists of any number of whitespace-separated words, which become the command-line arguments of program. The first word in arguments is argument zero, which should by convention be the program name itself (sans directories).

If you edit /etc/inetd.conf, you can tell inetd to reread the file and obey its new contents by sending the inetd process the SIGHUP signal. You'll have to use ps to determine the process ID of the inetd process as it is not fixed.

16.12 Socket Options

This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.

When you are manipulating a socket option, you must specify which level the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.

16.12.1 Socket Option Functions

Here are the functions for examining and modifying socket options. They are declared in sys/socket.h.

— Function: int getsockopt (int socket, int level, int optname, void *optval, socklen_t *optlen-ptr)

The getsockopt function gets information about the value of option optname at level level for socket socket.
The option value is stored in a buffer that optval points to. Before the call, you should supply in *optlen-ptr the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer.
Most options interpret the optval buffer as a single int value.
The actual return value of getsockopt is 0 on success and -1 on failure. The following errno error conditions are defined:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOPROTOOPT
The optname doesn't make sense for the given level.

— Function: int setsockopt (int socket, int level, int optname, void *optval, socklen_t optlen)

This function is used to set the socket option optname at level level for socket socket. The value of the option is passed in the buffer optval of size optlen.

16.12.2 Socket-Level Options

— Constant: int SOL_SOCKET

Use this constant as the level argument to getsockopt or setsockopt to manipulate the socket-level options described in this section.

Here is a table of socket-level option names; all are defined in the header file sys/socket.h.

16.13 Networks Database

Many systems come with a database that records a list of networks known to the system developer. This is usually kept either in the file /etc/networks or in an equivalent from a name server. This data base is useful for routing programs such as route, but it is not useful for programs that simply communicate over the network. We provide functions to access this database, which are declared in netdb.h.

— Data Type: struct netent

This data type is used to represent information about entries in the networks database. It has the following members:

char *n_name
This is the “official” name of the network.
char **n_aliases
These are alternative names for the network, represented as a vector of strings. A null pointer terminates the array.
int n_addrtype
This is the type of the network number; this is always equal to AF_INET for Internet networks.
unsigned long int n_net
This is the network number. Network numbers are returned in host byte order; see Byte Order.

Use the getnetbyname or getnetbyaddr functions to search the networks database for information about a specific network. The information is returned in a statically-allocated structure; you must copy the information if you need to save it.

— Function: struct netent * getnetbyname (const char *name)

The getnetbyname function returns information about the network named name. It returns a null pointer if there is no such network.

— Function: struct netent * getnetbyaddr (unsigned long int net, int type)

The getnetbyaddr function returns information about the network of type type with number net. You should specify a value of AF_INET for the type argument for Internet networks.
getnetbyaddr returns a null pointer if there is no such network.

You can also scan the networks database using setnetent, getnetent and endnetent. Be careful when using these functions because they are not reentrant.

— Function: void setnetent (int stayopen)

This function opens and rewinds the networks database.
If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getnetbyname or getnetbyaddr will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct netent * getnetent (void)

This function returns the next entry in the networks database. It returns a null pointer if there are no more entries.

— Function: void endnetent (void)

This function closes the networks database.

17 Low-Level Terminal Interface

This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions.

Most of the functions in this chapter operate on file descriptors. See Low-Level I/O, for more information about what a file descriptor is and how to open a file descriptor for a terminal device.

17.1 Identifying Terminals

The functions described in this chapter only work on files that correspond to terminal devices. You can find out whether a file descriptor is associated with a terminal by using the isatty function.

Prototypes for the functions in this section are declared in the header file unistd.h.

— Function: int isatty (int filedes)

This function returns 1 if filedes is a file descriptor associated with an open terminal device, and 0 otherwise.

If a file descriptor is associated with a terminal, you can get its associated file name using the ttyname function. See also the ctermid function, described in Identifying the Terminal.

— Function: char * ttyname (int filedes)

If the file descriptor filedes is associated with a terminal device, the ttyname function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file. The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined.

— Function: int ttyname_r (int filedes, char *buf, size_t len)

The ttyname_r function is similar to the ttyname function except that it places its result into the user-specified buffer starting at buf with length len.
The normal return value from ttyname_r is 0. Otherwise an error number is returned to indicate the error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
ERANGE
The buffer length len is too small to store the string to be returned.

17.2 I/O Queues

Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see I/O on Streams).

The terminal input queue is also sometimes referred to as its typeahead buffer. It holds the characters that have been received from the terminal but not yet read by any process.

The size of the input queue is described by the MAX_INPUT and _POSIX_MAX_INPUT parameters; see Limits for Files. You are guaranteed a queue size of at least MAX_INPUT, but the queue might be larger, and might even dynamically change size. If input flow control is enabled by setting the IXOFF input mode bit (see Input Modes), the terminal driver transmits STOP and START characters to the terminal when necessary to prevent the queue from overflowing. Otherwise, input may be lost if it comes in too fast from the terminal. In canonical mode, all input stays in the queue until a newline character is received, so the terminal input queue can fill up when you type a very long line. See Canonical or Not.

The terminal output queue is like the input queue, but for output; it contains characters that have been written by processes, but not yet transmitted to the terminal. If output flow control is enabled by setting the IXON input mode bit (see Input Modes), the terminal driver obeys START and STOP characters sent by the terminal to stop and restart transmission of output.

Clearing the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.

17.3 Two Styles of Input: Canonical or Not

In canonical input processing mode, terminal input is processed in lines terminated by newline ('\n'), EOF, or EOL characters. No input can be read until an entire line has been typed by the user, and the read function (see I/O Primitives) returns at most a single line of input, no matter how many bytes are requested.

In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. See Editing Characters.

The constants _POSIX_MAX_CANON and MAX_CANON parameterize the maximum number of bytes which may appear in a single line of canonical input. See Limits for Files. You are guaranteed a maximum line length of at least MAX_CANON bytes, but the maximum might be larger, and might even dynamically change size.

In noncanonical input processing mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. See Noncanonical Input.

Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.

The choice of canonical or noncanonical input is controlled by the ICANON flag in the c_lflag member of struct termios. See Local Modes.

17.4 Terminal Modes

This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file termios.h.

Don't confuse terminal attributes with file attributes. A device special file which is associated with a terminal has file attributes as described in File Attributes. These are unrelated to the attributes of the terminal device itself, which are discussed in this section.

17.4.1 Terminal Mode Data Types

The entire collection of attributes of a terminal is stored in a structure of type struct termios. This structure is used with the functions tcgetattr and tcsetattr to read and set the attributes.

— Data Type: struct termios

Structure that records all the I/O attributes of a terminal. The structure includes at least the following members:

tcflag_t c_iflag
A bit mask specifying flags for input modes; see Input Modes.
tcflag_t c_oflag
A bit mask specifying flags for output modes; see Output Modes.
tcflag_t c_cflag
A bit mask specifying flags for control modes; see Control Modes.
tcflag_t c_lflag
A bit mask specifying flags for local modes; see Local Modes.
cc_t c_cc[NCCS]
An array specifying which characters are associated with various control functions; see Special Characters.

The struct termios structure also contains members which encode input and output transmission speeds, but the representation is not specified. See Line Speed, for how to examine and store the speed values.

The following sections describe the details of the members of the struct termios structure.

— Data Type: tcflag_t

This is an unsigned integer type used to represent the various bit masks for terminal flags.

— Data Type: cc_t

This is an unsigned integer type used to represent characters associated with various terminal control functions.

— Macro: int NCCS

The value of this macro is the number of elements in the c_cc array.

17.4.2 Terminal Mode Functions

— Function: int tcgetattr (int filedes, struct termios *termios-p)

This function is used to examine the attributes of the terminal device with file descriptor filedes. The attributes are returned in the structure that termios-p points to.
If successful, tcgetattr returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.

— Function: int tcsetattr (int filedes, int when, const struct termios *termios-p)

This function sets the attributes of the terminal device with file descriptor filedes. The new attributes are taken from the structure that termios-p points to.
The when argument specifies how to deal with input and output already queued. It can be one of the following values:

TCSANOW
Make the change immediately.
TCSADRAIN
Make the change after waiting until all queued output has been written. You should usually use this option when changing parameters that affect output.
TCSAFLUSH
This is like TCSADRAIN, but also discards any queued input.
TCSASOFT
This is a flag bit that you can add to any of the above alternatives. Its meaning is to inhibit alteration of the state of the terminal hardware. It is a BSD extension; it is only supported on BSD systems and the GNU system.
Using TCSASOFT is exactly the same as setting the CIGNORE bit in the c_cflag member of the structure termios-p points to. See Control Modes, for a description of CIGNORE.

If this function is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal, in the same way as if the process were trying to write to the terminal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent. See Job Control.
If successful, tcsetattr returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
EINVAL
Either the value of the when argument is not valid, or there is something wrong with the data in the termios-p argument.

Although tcgetattr and tcsetattr specify the terminal device with a file descriptor, the attributes are those of the terminal device itself and not of the file descriptor. This means that the effects of changing terminal attributes are persistent; if another process opens the terminal file later on, it will see the changed attributes even though it doesn't have anything to do with the open file descriptor you originally specified in changing the attributes.

Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes.

17.4.3 Setting Terminal Modes Properly

When you set terminal modes, you should call tcgetattr first to get the current modes of the particular terminal device, modify only those modes that you are really interested in, and store the result with tcsetattr.

It's a bad idea to simply initialize a struct termios structure to a chosen set of attributes and pass it directly to tcsetattr. Your program may be run years from now, on systems that support members not documented in this manual. The way to avoid setting these members to unreasonable values is to avoid changing them.

What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another.

When a member contains a collection of independent flags, as the c_iflag, c_oflag and c_cflag members do, even setting the entire member is a bad idea, because particular operating systems have their own flags. Instead, you should start with the current value of the member and alter only the flags whose values matter in your program, leaving any other flags unchanged.

Here is an example of how to set one flag (ISTRIP) in the struct termios structure while properly preserving all the other data in the structure:

17.4.4 Input Modes

This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and <RET> and <LFD> characters.

All of these flags are bits in the c_iflag member of the struct termios structure. The member is an integer, and you change flags using the operators &, | and ^. Don't try to specify the entire value for c_iflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t INPCK

If this bit is set, input parity checking is enabled. If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application.
Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see Control Modes. For example, you could clear the INPCK input mode flag and set the PARENB control mode flag to ignore parity errors on input, but still generate parity on output.
If this bit is set, what happens when a parity error is detected depends on whether the IGNPAR or PARMRK bits are set. If neither of these bits are set, a byte with a parity error is passed to the application as a '\0' character.

— Macro: tcflag_t IGNPAR

If this bit is set, any byte with a framing or parity error is ignored. This is only useful if INPCK is also set.

— Macro: tcflag_t PARMRK

If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when INPCK is set and IGNPAR is not set.
The way erroneous bytes are marked is with two preceding bytes, 377 and 0. Thus, the program actually reads three bytes for one erroneous byte received from the terminal.
If a valid byte has the value 0377, and ISTRIP (see below) is not set, the program might confuse it with the prefix that marks a parity error. So a valid byte 0377 is passed to the program as two bytes, 0377 0377, in this case.

— Macro: tcflag_t ISTRIP

If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read.

— Macro: tcflag_t IGNBRK

If this bit is set, break conditions are ignored.
A break condition is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.

— Macro: tcflag_t BRKINT

If this bit is set and IGNBRK is not set, a break condition clears the terminal input and output queues and raises a SIGINT signal for the foreground process group associated with the terminal.
If neither BRKINT nor IGNBRK are set, a break condition is passed to the application as a single '\0' character if PARMRK is not set, or otherwise as a three-character sequence '\377', '\0', '\0'.

— Macro: tcflag_t IGNCR

If this bit is set, carriage return characters ('\r') are discarded on input. Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the <RET> key.

— Macro: tcflag_t ICRNL

If this bit is set and IGNCR is not set, carriage return characters ('\r') received as input are passed to the application as newline characters ('\n').

— Macro: tcflag_t INLCR

If this bit is set, newline characters ('\n') received as input are passed to the application as carriage return characters ('\r').

— Macro: tcflag_t IXOFF

If this bit is set, start/stop control on input is enabled. In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it. The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission. See Start/Stop Characters.

— Macro: tcflag_t IXON

If this bit is set, start/stop control on output is enabled. In other words, if the computer receives a STOP character, it suspends output until a START character is received. In this case, the STOP and START characters are never passed to the application program. If this bit is not set, then START and STOP can be read as ordinary characters. See Start/Stop Characters.

— Macro: tcflag_t IXANY

If this bit is set, any input character restarts output when output has been suspended with the STOP character. Otherwise, only the START character restarts output.
This is a BSD extension; it exists only on BSD systems and the GNU system.

— Macro: tcflag_t IMAXBEL

If this bit is set, then filling up the terminal input buffer sends a BEL character (code 007) to the terminal to ring the bell.
This is a BSD extension.

17.4.5 Output Modes

This section describes the terminal flags and fields that control how output characters are translated and padded for display. All of these are contained in the c_oflag member of the struct termios structure.

The c_oflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_oflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t OPOST

If this bit is set, output data is processed in some unspecified way so that it is displayed appropriately on the terminal device. This typically includes mapping newline characters ('\n') onto carriage return and linefeed pairs.
If this bit isn't set, the characters are transmitted as-is.

The following three bits are BSD features, and they exist only BSD systems and the GNU system. They are effective only if OPOST is set.

— Macro: tcflag_t ONLCR

If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed.

— Macro: tcflag_t OXTABS

If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns.

— Macro: tcflag_t ONOEOT

If this bit is set, discard C-d characters (code 004) on output. These characters cause many dial-up terminals to disconnect.

17.4.6 Control Modes

This section describes the terminal flags and fields that control parameters usually associated with asynchronous serial data transmission. These flags may not make sense for other kinds of terminal ports (such as a network connection pseudo-terminal). All of these are contained in the c_cflag member of the

struct
termios

structure.

The c_cflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_cflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t CLOCAL

If this bit is set, it indicates that the terminal is connected “locally” and that the modem status lines (such as carrier detect) should be ignored. On many systems if this bit is not set and you call open without the O_NONBLOCK flag set, open blocks until a modem connection is established.
If this bit is not set and a modem disconnect is detected, a SIGHUP signal is sent to the controlling process group for the terminal (if it has one). Normally, this causes the process to exit; see Signal Handling. Reading from the terminal after a disconnect causes an end-of-file condition, and writing causes an EIO error to be returned. The terminal device must be closed and reopened to clear the condition.

— Macro: tcflag_t HUPCL

If this bit is set, a modem disconnect is generated when all processes that have the terminal device open have either closed the file or exited.

— Macro: tcflag_t CREAD

If this bit is set, input can be read from the terminal. Otherwise, input is discarded when it arrives.

— Macro: tcflag_t CSTOPB

If this bit is set, two stop bits are used. Otherwise, only one stop bit is used.

— Macro: tcflag_t PARENB

If this bit is set, generation and detection of a parity bit are enabled. See Input Modes, for information on how input parity errors are handled.
If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.

— Macro: tcflag_t PARODD

This bit is only useful if PARENB is set. If PARODD is set, odd parity is used, otherwise even parity is used.

The control mode flags also includes a field for the number of bits per character. You can use the CSIZE macro as a mask to extract the value, like this: settings.c_cflag & CSIZE.

— Macro: tcflag_t CSIZE

This is a mask for the number of bits per character.

— Macro: tcflag_t CS5

This specifies five bits per byte.

— Macro: tcflag_t CS6

This specifies six bits per byte.

— Macro: tcflag_t CS7

This specifies seven bits per byte.

— Macro: tcflag_t CS8

This specifies eight bits per byte.

The following four bits are BSD extensions; this exist only on BSD systems and the GNU system.

— Macro: tcflag_t CCTS_OFLOW

If this bit is set, enable flow control of output based on the CTS wire (RS232 protocol).

— Macro: tcflag_t CRTS_IFLOW

If this bit is set, enable flow control of input based on the RTS wire (RS232 protocol).

— Macro: tcflag_t MDMBUF

If this bit is set, enable carrier-based flow control of output.

— Macro: tcflag_t CIGNORE

If this bit is set, it says to ignore the control modes and line speed values entirely. This is only meaningful in a call to tcsetattr.
The c_cflag member and the line speed values returned by cfgetispeed and cfgetospeed will be unaffected by the call. CIGNORE is useful if you want to set all the software modes in the other members, but leave the hardware details in c_cflag unchanged. (This is how the TCSASOFT flag to tcsettattr works.)
This bit is never set in the structure filled in by tcgetattr.

17.4.7 Local Modes

This section describes the flags for the c_lflag member of the struct termios structure. These flags generally control higher-level aspects of input processing than the input modes flags described in Input Modes, such as echoing, signals, and the choice of canonical or noncanonical input.

The c_lflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_lflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t ICANON

This bit, if set, enables canonical input processing mode. Otherwise, input is processed in noncanonical mode. See Canonical or Not.

— Macro: tcflag_t ECHO

If this bit is set, echoing of input characters back to the terminal is enabled.

— Macro: tcflag_t ECHOE

If this bit is set, echoing indicates erasure of input with the ERASE character by erasing the last character in the current line from the screen. Otherwise, the character erased is re-echoed to show what has happened (suitable for a printing terminal).
This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the ERASE character and erasure of input, without which ECHOE is simply irrelevant.

— Macro: tcflag_t ECHOPRT

This bit is like ECHOE, enables display of the ERASE character in a way that is geared to a hardcopy terminal. When you type the ERASE character, a ‘\’ character is printed followed by the first character erased. Typing the ERASE character again just prints the next character erased. Then, the next time you type a normal character, a ‘/’ character is printed before the character echoes.
This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ECHOK

This bit enables special display of the KILL character by moving to a new line after echoing the KILL character normally. The behavior of ECHOKE (below) is nicer to look at.
If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen.
This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the KILL character and erasure of input, without which ECHOK is simply irrelevant.

— Macro: tcflag_t ECHOKE

This bit is similar to ECHOK. It enables special display of the KILL character by erasing on the screen the entire line that has been killed. This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ECHONL

If this bit is set and the ICANON bit is also set, then the newline ('\n') character is echoed even if the ECHO bit is not set.

— Macro: tcflag_t ECHOCTL

If this bit is set and the ECHO bit is also set, echo control characters with ‘^’ followed by the corresponding text character. Thus, control-A echoes as ‘^A’. This is usually the preferred mode for interactive input, because echoing a control character back to the terminal could have some undesired effect on the terminal.
This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ISIG

This bit controls whether the INTR, QUIT, and SUSP characters are recognized. The functions associated with these characters are performed if and only if this bit is set. Being in canonical or noncanonical input mode has no affect on the interpretation of these characters.
You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program. See Signal Characters.

— Macro: tcflag_t IEXTEN

POSIX.1 gives IEXTEN implementation-defined meaning, so you cannot rely on this interpretation on all systems.
On BSD systems and the GNU system, it enables the LNEXT and DISCARD characters. See Other Special.

— Macro: tcflag_t NOFLSH

Normally, the INTR, QUIT, and SUSP characters cause input and output queues for the terminal to be cleared. If this bit is set, the queues are not cleared.

— Macro: tcflag_t TOSTOP

If this bit is set and the system supports job control, then SIGTTOU signals are generated by background processes that attempt to write to the terminal. See Access to the Terminal.

The following bits are BSD extensions; they exist only in BSD systems and the GNU system.

— Macro: tcflag_t ALTWERASE

This bit determines how far the WERASE character should erase. The WERASE character erases back to the beginning of a word; the question is, where do words begin?
If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those.
See Editing Characters, for more information about the WERASE character.

— Macro: tcflag_t FLUSHO

This is the bit that toggles when the user types the DISCARD character. While this bit is set, all output is discarded. See Other Special.

— Macro: tcflag_t NOKERNINFO

Setting this bit disables handling of the STATUS character. See Other Special.

— Macro: tcflag_t PENDIN

If this bit is set, it indicates that there is a line of input that needs to be reprinted. Typing the REPRINT character sets this bit; the bit remains set until reprinting is finished. See Editing Characters.

17.4.8 Line Speed

The terminal line speed tells the computer how fast to read and write data on the terminal.

If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line—if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals.

If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding.

There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions.

The speed values are stored in the struct termios structure, but don't try to access them in the struct termios structure directly. Instead, you should use the following functions to read and store them:

— Function: speed_t cfgetospeed (const struct termios *termios-p)

This function returns the output line speed stored in the structure *termios-p.

— Function: speed_t cfgetispeed (const struct termios *termios-p)

This function returns the input line speed stored in the structure *termios-p.

— Function: int cfsetospeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as the output speed. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetospeed returns -1.

— Function: int cfsetispeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as the input speed. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetospeed returns -1.

— Function: int cfsetspeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as both the input and output speeds. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetspeed returns -1. This function is an extension in 4.4 BSD.

— Data Type: speed_t

The speed_t type is an unsigned integer data type used to represent line speeds.

The functions cfsetospeed and cfsetispeed report errors only for speed values that the system simply cannot handle. If you specify a speed value that is basically acceptable, then those functions will succeed. But they do not check that a particular hardware device can actually support the specified speeds—in fact, they don't know which device you plan to set the speed for. If you use tcsetattr to set the speed of a particular device to a value that it cannot handle, tcsetattr returns -1.

Portability note: In the GNU library, the functions above accept speeds measured in bits per second as input, and return speed values measured in bits per second. Other libraries require speeds to be indicated by special codes. For POSIX.1 portability, you must use one of the following symbols to represent the speed; their precise numeric values are system-dependent, but each name has a fixed meaning: B110 stands for 110 bps, B300 for 300 bps, and so on. There is no portable way to represent any speed but these, but these are the only speeds that typical serial lines can support.

BSD defines two additional speed symbols as aliases: EXTA is an alias for B19200 and EXTB is an alias for B38400. These aliases are obsolete.

17.4.9 Special Characters

In canonical input, the terminal driver recognizes a number of special characters which perform various control functions. These include the ERASE character (usually <DEL>) for editing input, and other editing characters. The INTR character (normally C-c) for sending a SIGINT signal, and other signal-raising characters, may be available in either canonical or noncanonical input mode. All these characters are described in this section.

The particular characters used are specified in the c_cc member of the struct termios structure. This member is an array; each element specifies the character for a particular role. Each element has a symbolic constant that stands for the index of that element—for example, VINTR is the index of the element that specifies the INTR character, so storing '=' in termios.c_cc[VINTR] specifies ‘=’ as the INTR character.

On some systems, you can disable a particular special character function by specifying the value _POSIX_VDISABLE for that role. This value is unequal to any possible character code. See Options for Files, for more information about how to tell whether the operating system you are using supports _POSIX_VDISABLE.

17.4.9.1 Characters for Input Editing

These special characters are active only in canonical input mode. See Canonical or Not.

— Macro: int VEOF

This is the subscript for the EOF character in the special control character array. termios.c_cc[VEOF] holds the character itself.
The EOF character is recognized only in canonical input mode. It acts as a line terminator in the same way as a newline character, but if the EOF character is typed at the beginning of a line it causes read to return a byte count of zero, indicating end-of-file. The EOF character itself is discarded.
Usually, the EOF character is C-d.

— Macro: int VEOL

This is the subscript for the EOL character in the special control character array. termios.c_cc[VEOL] holds the character itself.
The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line.
You don't need to use the EOL character to make <RET> end a line. Just set the ICRNL flag. In fact, this is the default state of affairs.

— Macro: int VEOL2

This is the subscript for the EOL2 character in the special control character array. termios.c_cc[VEOL2] holds the character itself.
The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, by setting EOL to one of them and EOL2 to the other.
The EOL2 character is a BSD extension; it exists only on BSD systems and the GNU system.

— Macro: int VERASE

This is the subscript for the ERASE character in the special control character array. termios.c_cc[VERASE] holds the character itself.
The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded.
Usually, the ERASE character is <DEL>.

— Macro: int VWERASE

This is the subscript for the WERASE character in the special control character array. termios.c_cc[VWERASE] holds the character itself.
The WERASE character is recognized only in canonical mode. It erases an entire word of prior input, and any whitespace after it; whitespace characters before the word are not erased.
The definition of a “word” depends on the setting of the ALTWERASE mode; see Local Modes.
If the ALTWERASE mode is not set, a word is defined as a sequence of any characters except space or tab.
If the ALTWERASE mode is set, a word is defined as a sequence of characters containing only letters, numbers, and underscores, optionally followed by one character that is not a letter, number, or underscore.
The WERASE character is usually C-w.
This is a BSD extension.

— Macro: int VKILL

This is the subscript for the KILL character in the special control character array. termios.c_cc[VKILL] holds the character itself.
The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too.
The KILL character is usually C-u.

— Macro: int VREPRINT

This is the subscript for the REPRINT character in the special control character array. termios.c_cc[VREPRINT] holds the character itself.
The REPRINT character is recognized only in canonical mode. It reprints the current input line. If some asynchronous output has come while you are typing, this lets you see the line you are typing clearly again.
The REPRINT character is usually C-r.
This is a BSD extension.

17.4.9.2 Characters that Cause Signals

These special characters may be active in either canonical or noncanonical input mode, but only when the ISIG flag is set (see Local Modes).

— Macro: int VINTR

This is the subscript for the INTR character in the special control character array. termios.c_cc[VINTR] holds the character itself.
The INTR (interrupt) character raises a SIGINT signal for all processes in the foreground job associated with the terminal. The INTR character itself is then discarded. See Signal Handling, for more information about signals.
Typically, the INTR character is C-c.

— Macro: int VQUIT

This is the subscript for the QUIT character in the special control character array. termios.c_cc[VQUIT] holds the character itself.
The QUIT character raises a SIGQUIT signal for all processes in the foreground job associated with the terminal. The QUIT character itself is then discarded. See Signal Handling, for more information about signals.
Typically, the QUIT character is C-\.

— Macro: int VSUSP

This is the subscript for the SUSP character in the special control character array. termios.c_cc[VSUSP] holds the character itself.
The SUSP (suspend) character is recognized only if the implementation supports job control (see Job Control). It causes a SIGTSTP signal to be sent to all processes in the foreground job associated with the terminal. The SUSP character itself is then discarded. See Signal Handling, for more information about signals.
Typically, the SUSP character is C-z.

Few applications disable the normal interpretation of the SUSP character. If your program does this, it should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a SIGTSTP signal to the process group of the process, not just to the process itself. See Signaling Another Process.

— Macro: int VDSUSP

This is the subscript for the DSUSP character in the special control character array. termios.c_cc[VDSUSP] holds the character itself.
The DSUSP (suspend) character is recognized only if the implementation supports job control (see Job Control). It sends a SIGTSTP signal, like the SUSP character, but not right away—only when the program tries to read it as input. Not all systems with job control support DSUSP; only BSD-compatible systems (including the GNU system).
See Signal Handling, for more information about signals.
Typically, the DSUSP character is C-y.

17.4.9.3 Special Characters for Flow Control

These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags IXON and IXOFF (see Input Modes).

— Macro: int VSTART

This is the subscript for the START character in the special control character array. termios.c_cc[VSTART] holds the character itself.
The START character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a START character resumes suspended output; the START character itself is discarded. If IXANY is set, receiving any character at all resumes suspended output; the resuming character is not discarded unless it is the START character. IXOFF is set, the system may also transmit START characters to the terminal.
The usual value for the START character is C-q. You may not be able to change this value—the hardware may insist on using C-q regardless of what you specify.

— Macro: int VSTOP

This is the subscript for the STOP character in the special control character array. termios.c_cc[VSTOP] holds the character itself.
The STOP character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded. If IXOFF is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing.
The usual value for the STOP character is C-s. You may not be able to change this value—the hardware may insist on using C-s regardless of what you specify.

17.4.9.4 Other Special Characters

— Macro: int VLNEXT

This is the subscript for the LNEXT character in the special control character array. termios.c_cc[VLNEXT] holds the character itself.
The LNEXT character is recognized only when IEXTEN is set, but in both canonical and noncanonical mode. It disables any special significance of the next character the user types. Even if the character would normally perform some editing function or generate a signal, it is read as a plain character. This is the analogue of the C-q command in Emacs. “LNEXT” stands for “literal next.”
The LNEXT character is usually C-v.

— Macro: int VDISCARD

This is the subscript for the DISCARD character in the special control character array. termios.c_cc[VDISCARD] holds the character itself.
The DISCARD character is recognized only when IEXTEN is set, but in both canonical and noncanonical mode. Its effect is to toggle the discard-output flag. When this flag is set, all program output is discarded. Setting the flag also discards all output currently in the output buffer. Typing any other character resets the flag.

— Macro: int VSTATUS

This is the subscript for the STATUS character in the special control character array. termios.c_cc[VSTATUS] holds the character itself.
The STATUS character's effect is to print out a status message about how the current process is running.
The STATUS character is recognized only in canonical mode, and only if NOKERNINFO is not set.

17.4.10 Noncanonical Input

In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate.

Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting—to return immediately with whatever input is available, or with no input.

The MIN and TIME are stored in elements of the c_cc array, which is a member of the struct termios structure. Each element of this array has a particular role, and each element has a symbolic constant that stands for the index of that element. VMIN and VMAX are the names for the indices in the array of the MIN and TIME slots.

— Macro: int VMIN

This is the subscript for the MIN slot in the c_cc array. Thus, termios.c_cc[VMIN] is the value itself.
The MIN slot is only meaningful in noncanonical input mode; it specifies the minimum number of bytes that must be available in the input queue in order for read to return.

— Macro: int VTIME

This is the subscript for the TIME slot in the c_cc array. Thus, termios.c_cc[VTIME] is the value itself.
The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds.

The MIN and TIME values interact to determine the criterion for when read should return; their precise meanings depend on which of them are nonzero. There are four possible cases:

What happens if MIN is 50 and you ask to read just 10 bytes? Normally, read waits until there are 50 bytes in the buffer (or, more generally, the wait condition described above is satisfied), and then reads 10 of them, leaving the other 40 buffered in the operating system for a subsequent call to read.

Portability note: On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. The GNU library allocates separate slots for these uses.

— Function: void cfmakeraw (struct termios *termios-p)

This function provides an easy way to set up *termios-p for what has traditionally been called “raw mode” in BSD. This uses noncanonical input, and turns off most processing to give an unmodified channel to the terminal.
It does exactly this:
            termios-p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
                                          |INLCR|IGNCR|ICRNL|IXON);
            termios-p->c_oflag &= ~OPOST;
            termios-p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
            termios-p->c_cflag &= ~(CSIZE|PARENB);
            termios-p->c_cflag |= CS8;

17.5 BSD Terminal Modes

The usual way to get and set terminal modes is with the functions described in Terminal Modes. However, on some systems you can use the BSD-derived functions in this section to do some of the same thing. On many systems, these functions do not exist. Even with the GNU C library, the functions simply fail with errno = ENOSYS with many kernels, including Linux.

— Data Type: struct sgttyb

This structure is an input or output parameter list for gtty and stty.

char sg_ispeed
Line speed for input
char sg_ospeed
Line speed for output
char sg_erase
Erase character
char sg_kill
Kill character
int sg_flags
Various flags

— Function: int gtty (int filedes, struct sgttyb *attributes)

This function gets the attributes of a terminal.
gtty sets *attributes to describe the terminal attributes of the terminal which is open with file descriptor filedes.

— Function: int stty (int filedes, struct sgttyb * attributes)

This function sets the attributes of a terminal.
stty sets the terminal attributes of the terminal which is open with file descriptor filedes to those described by *filedes.

17.6 Line Control Functions

These functions perform miscellaneous control actions on terminal devices. As regards terminal access, they are treated like doing output: if any of these functions is used by a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent. See Job Control.

— Function: int tcsendbreak (int filedes, int duration)

This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor filedes. The duration of the break is controlled by the duration argument. If zero, the duration is between 0.25 and 0.5 seconds. The meaning of a nonzero value depends on the operating system.
This function does nothing if the terminal is not an asynchronous serial data port.
The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.

— Function: int tcdrain (int filedes)

The tcdrain function waits until all queued output to the terminal filedes has been transmitted.
This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time tcdrain is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to tcdrain should be protected using cancellation handlers.
The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
EINTR
The operation was interrupted by delivery of a signal. See Interrupted Primitives.

— Function: int tcflush (int filedes, int queue)

The tcflush function is used to clear the input and/or output queues associated with the terminal file filedes. The queue argument specifies which queue(s) to clear, and can be one of the following values:
TCIFLUSH
Clear any input data received, but not yet read.

TCOFLUSH
Clear any output data written, but not yet transmitted.

TCIOFLUSH
Clear both queued input and output.

The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
EINVAL
A bad value was supplied as the queue argument.

It is unfortunate that this function is named tcflush, because the term “flush” is normally used for quite another operation—waiting until all output is transmitted—and using it for discarding input or output would be confusing. Unfortunately, the name tcflush comes from POSIX and we cannot change it.

— Function: int tcflow (int filedes, int action)

The tcflow function is used to perform operations relating to XON/XOFF flow control on the terminal file specified by filedes.
The action argument specifies what operation to perform, and can be one of the following values:
TCOOFF
Suspend transmission of output.

TCOON
Restart transmission of output.

TCIOFF
Transmit a STOP character.

TCION
Transmit a START character.

For more information about the STOP and START characters, see Special Characters.
The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:
EBADF
The filedes is not a valid file descriptor.

ENOTTY
The filedes is not associated with a terminal device.

EINVAL
A bad value was supplied as the action argument.

17.7 Noncanonical Mode Example

Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo.

This program is careful to restore the original terminal modes before exiting or terminating with a signal. It uses the atexit function (see Cleanups on Exit) to make sure this is done by exit.

The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see Job Control. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so.

17.8 Pseudo-Terminals

A pseudo-terminal is a special interprocess communication channel that acts like a terminal. One end of the channel is called the master side or master pseudo-terminal device, the other side is called the slave side. Data written to the master side is received by the slave side as if it was the result of a user typing at an ordinary terminal, and data written to the slave side is sent to the master side as if it was written on an ordinary terminal.

Pseudo terminals are the way programs like xterm and emacs implement their terminal emulation functionality.

17.8.1 Allocating Pseudo-Terminals

This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file stdlib.h.

— Function: int getpt (void)

The getpt function returns a new file descriptor for the next available master pseudo-terminal. The normal return value from getpt is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. The following errno conditions are defined for this function:

ENOENT
There are no free master pseudo-terminals available.

This function is a GNU extension.

— Function: int grantpt (int filedes)

The grantpt function changes the ownership and access permission of the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. The owner is set from the real user ID of the calling process (see Process Persona), and the group is set to a special group (typically tty) or from the real group ID of the calling process. The access permission is set such that the file is both readable and writable by the owner and only writable by the group.
On some systems this function is implemented by invoking a special setuid root program (see How Change Persona). As a consequence, installing a signal handler for the SIGCHLD signal (see Job Control Signals) may interfere with a call to grantpt.
The normal return value from grantpt is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal device.
EACCES
The slave pseudo-terminal device corresponding to the master associated with filedes could not be accessed.

— Function: int unlockpt (int filedes)

The unlockpt function unlocks the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. On many systems, the slave can only be opened after unlocking, so portable applications should always call unlockpt before trying to open the slave.
The normal return value from unlockpt is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal device.

— Function: char * ptsname (int filedes)

If the file descriptor filedes is associated with a master pseudo-terminal device, the ptsname function returns a pointer to a statically-allocated, null-terminated string containing the file name of the associated slave pseudo-terminal file. This string might be overwritten by subsequent calls to ptsname.

— Function: int ptsname_r (int filedes, char *buf, size_t len)

The ptsname_r function is similar to the ptsname function except that it places its result into the user-specified buffer starting at buf with length len.
This function is a GNU extension.

Portability Note: On System V derived systems, the file returned by the ptsname and ptsname_r functions may be STREAMS-based, and therefore require additional processing after opening before it actually behaves as a pseudo terminal.

17.8.2 Opening a Pseudo-Terminal Pair

These functions, derived from BSD, are available in the separate libutil library, and declared in pty.h.

— Function: int openpty (int *amaster, int *aslave, char *name, const struct termios *termp, const struct winsize *winp)

This function allocates and opens a pseudo-terminal pair, returning the file descriptor for the master in *amaster, and the file descriptor for the slave in *aslave. If the argument name is not a null pointer, the file name of the slave pseudo-terminal device is stored in *name. If termp is not a null pointer, the terminal attributes of the slave are set to the ones specified in the structure that termp points to (see Terminal Modes). Likewise, if the winp is not a null pointer, the screen size of the slave is set to the values specified in the structure that winp points to.
The normal return value from openpty is 0; a value of -1 is returned in case of failure. The following errno conditions are defined for this function:

ENOENT
There are no free pseudo-terminal pairs available.

Warning: Using the openpty function with name not set to NULL is very dangerous because it provides no protection against overflowing the string name. You should use the ttyname function on the file descriptor returned in *slave to find out the file name of the slave pseudo-terminal device instead.

— Function: int forkpty (int *amaster, char *name, const struct termios *termp, const struct winsize *winp)

This function is similar to the openpty function, but in addition, forks a new process (see Creating a Process) and makes the newly opened slave pseudo-terminal device the controlling terminal (see Controlling Terminal) for the child process.
If the operation is successful, there are then both parent and child processes and both see forkpty return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process.
If the allocation of a pseudo-terminal pair or the process creation failed, forkpty returns a value of -1 in the parent process.
Warning: The forkpty function has the same problems with respect to the name argument as openpty.

18 Syslog

This chapter describes facilities for issuing and logging messages of system administration interest. This chapter has nothing to do with programs issuing messages to their own users or keeping private logs (One would typically do that with the facilities described in I/O on Streams).

Most systems have a facility called “Syslog” that allows programs to submit messages of interest to system administrators and can be configured to pass these messages on in various ways, such as printing on the console, mailing to a particular person, or recording in a log file for future reference.

18.1 Overview of Syslog

System administrators have to deal with lots of different kinds of messages from a plethora of subsystems within each system, and usually lots of systems as well. For example, an FTP server might report every connection it gets. The kernel might report hardware failures on a disk drive. A DNS server might report usage statistics at regular intervals.

Some of these messages need to be brought to a system administrator's attention immediately. And it may not be just any system administrator – there may be a particular system administrator who deals with a particular kind of message. Other messages just need to be recorded for future reference if there is a problem. Still others may need to have information extracted from them by an automated process that generates monthly reports.

To deal with these messages, most Unix systems have a facility called "Syslog." It is generally based on a daemon called “Syslogd” Syslogd listens for messages on a Unix domain socket named /dev/log. Based on classification information in the messages and its configuration file (usually /etc/syslog.conf), Syslogd routes them in various ways. Some of the popular routings are:

Syslogd can also handle messages from other systems. It listens on the syslog UDP port as well as the local socket for messages.

Syslog can handle messages from the kernel itself. But the kernel doesn't write to /dev/log; rather, another daemon (sometimes called “Klogd”) extracts messages from the kernel and passes them on to Syslog as any other process would (and it properly identifies them as messages from the kernel).

Syslog can even handle messages that the kernel issued before Syslogd or Klogd was running. A Linux kernel, for example, stores startup messages in a kernel message ring and they are normally still there when Klogd later starts up. Assuming Syslogd is running by the time Klogd starts, Klogd then passes everything in the message ring to it.

In order to classify messages for disposition, Syslog requires any process that submits a message to it to provide two pieces of classification information with it:

A “facility/priority” is a number that indicates both the facility and the priority.

Warning: This terminology is not universal. Some people use “level” to refer to the priority and “priority” to refer to the combination of facility and priority. A Linux kernel has a concept of a message “level,” which corresponds both to a Syslog priority and to a Syslog facility/priority (It can be both because the facility code for the kernel is zero, and that makes priority and facility/priority the same value).

The GNU C library provides functions to submit messages to Syslog. They do it by writing to the /dev/log socket. See Submitting Syslog Messages.

The GNU C library functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the syslog UDP port on that system. See Sockets.

18.2 Submitting Syslog Messages

These functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the syslog UDP port on that system. See Sockets.

18.2.1 openlog

— Function: void openlog (const char *ident, int option, int facility)

openlog opens or reopens a connection to Syslog in preparation for submitting messages.
ident is an arbitrary identification string which future syslog invocations will prefix to each message. This is intended to identify the source of the message, and people conventionally set it to the name of the program that will submit the messages.
If ident is NULL, or if openlog is not called, the default identification string used in Syslog messages will be the program name, taken from argv[0].
Please note that the string pointer ident will be retained internally by the Syslog routines. You must not free the memory that ident points to. It is also dangerous to pass a reference to an automatic variable since leaving the scope would mean ending the lifetime of the variable. If you want to change the ident string, you must call openlog again; overwriting the string pointed to by ident is not thread-safe.
You can cause the Syslog routines to drop the reference to ident and go back to the default string (the program name taken from argv[0]), by calling closelog: See closelog.
In particular, if you are writing code for a shared library that might get loaded and then unloaded (e.g. a PAM module), and you use openlog, you must call closelog before any point where your library might get unloaded, as in this example:
          #include <syslog.h>
          
          void
          shared_library_function (void)
          {
            openlog ("mylibrary", option, priority);
          
            syslog (LOG_INFO, "shared library has been invoked");
          
            closelog ();
          }
Without the call to closelog, future invocations of syslog by the program using the shared library may crash, if the library gets unloaded and the memory containing the string "mylibrary" becomes unmapped. This is a limitation of the BSD syslog interface.
openlog may or may not open the /dev/log socket, depending on option. If it does, it tries to open it and connect it as a stream socket. If that doesn't work, it tries to open it and connect it as a datagram socket. The socket has the “Close on Exec” attribute, so the kernel will close it if the process performs an exec.
You don't have to use openlog. If you call syslog without having called openlog, syslog just opens the connection implicitly and uses defaults for the information in ident and options.
options is a bit string, with the bits as defined by the following single bit masks:

LOG_PERROR
If on, openlog sets up the connection so that any syslog on this connection writes its message to the calling process' Standard Error stream in addition to submitting it to Syslog. If off, syslog does not write the message to Standard Error.
LOG_CONS
If on, openlog sets up the connection so that a syslog on this connection that fails to submit a message to Syslog writes the message instead to system console. If off, syslog does not write to the system console (but of course Syslog may write messages it receives to the console).
LOG_PID
When on, openlog sets up the connection so that a syslog on this connection inserts the calling process' Process ID (PID) into the message. When off, openlog does not insert the PID.
LOG_NDELAY
When on, openlog opens and connects the /dev/log socket. When off, a future syslog call must open and connect the socket.
Portability note: In early systems, the sense of this bit was exactly the opposite.
LOG_ODELAY
This bit does nothing. It exists for backward compatibility.

If any other bit in options is on, the result is undefined.
facility is the default facility code for this connection. A syslog on this connection that specifies default facility causes this facility to be associated with the message. See syslog for possible values. A value of zero means the default default, which is LOG_USER.
If a Syslog connection is already open when you call openlog, openlog “reopens” the connection. Reopening is like opening except that if you specify zero for the default facility code, the default facility code simply remains unchanged and if you specify LOG_NDELAY and the socket is already open and connected, openlog just leaves it that way.

18.2.2 syslog, vsyslog

— Function: void syslog (int facility_priority, char *format, ...)

syslog submits a message to the Syslog facility. It does this by writing to the Unix domain socket /dev/log.
syslog submits the message with the facility and priority indicated by facility_priority. The macro LOG_MAKEPRI generates a facility/priority from a facility and a priority, as in the following example:
          LOG_MAKEPRI(LOG_USER, LOG_WARNING)
The possible values for the facility code are (macros):

LOG_USER
A miscellaneous user process
LOG_MAIL
Mail
LOG_DAEMON
A miscellaneous system daemon
LOG_AUTH
Security (authorization)
LOG_SYSLOG
Syslog
LOG_LPR
Central printer
LOG_NEWS
Network news (e.g. Usenet)
LOG_UUCP
UUCP
LOG_CRON
Cron and At
LOG_AUTHPRIV
Private security (authorization)
LOG_FTP
Ftp server
LOG_LOCAL0
Locally defined
LOG_LOCAL1
Locally defined
LOG_LOCAL2
Locally defined
LOG_LOCAL3
Locally defined
LOG_LOCAL4
Locally defined
LOG_LOCAL5
Locally defined
LOG_LOCAL6
Locally defined
LOG_LOCAL7
Locally defined

Results are undefined if the facility code is anything else.
NB: syslog recognizes one other facility code: that of the kernel. But you can't specify that facility code with these functions. If you try, it looks the same to syslog as if you are requesting the default facility. But you wouldn't want to anyway, because any program that uses the GNU C library is not the kernel.
You can use just a priority code as facility_priority. In that case, syslog assumes the default facility established when the Syslog connection was opened. See Syslog Example.
The possible values for the priority code are (macros):

LOG_EMERG
The message says the system is unusable.
LOG_ALERT
Action on the message must be taken immediately.
LOG_CRIT
The message states a critical condition.
LOG_ERR
The message describes an error.
LOG_WARNING
The message is a warning.
LOG_NOTICE
The message describes a normal but important event.
LOG_INFO
The message is purely informational.
LOG_DEBUG
The message is only for debugging purposes.

Results are undefined if the priority code is anything else.
If the process does not presently have a Syslog connection open (i.e., it did not call openlog), syslog implicitly opens the connection the same as openlog would, with the following defaults for information that would otherwise be included in an openlog call: The default identification string is the program name. The default default facility is LOG_USER. The default for all the connection options in options is as if those bits were off. syslog leaves the Syslog connection open.
If the dev/log socket is not open and connected, syslog opens and connects it, the same as openlog with the LOG_NDELAY option would.
syslog leaves /dev/log open and connected unless its attempt to send the message failed, in which case syslog closes it (with the hope that a future implicit open will restore the Syslog connection to a usable state).
Example:
          
          #include <syslog.h>
          syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR),
                  "Unable to make network connection to %s.  Error=%m", host);
          

— Function: void vsyslog (int facility_priority, char *format, va_list arglist)

This is functionally identical to syslog, with the BSD style variable length argument.

18.2.3 closelog

— Function: void closelog (void)

closelog closes the current Syslog connection, if there is one. This includes closing the dev/log socket, if it is open. closelog also sets the identification string for Syslog messages back to the default, if openlog was called with a non-NULL argument to ident. The default identification string is the program name taken from argv[0].
If you are writing shared library code that uses openlog to generate custom syslog output, you should use closelog to drop the GNU C library's internal reference to the ident pointer when you are done. Please read the section on openlog for more information: See openlog.
closelog does not flush any buffers. You do not have to call closelog before re-opening a Syslog connection with initlog. Syslog connections are automatically closed on exec or exit.

18.2.4 setlogmask

— Function: int setlogmask (int mask)

setlogmask sets a mask (the “logmask”) that determines which future syslog calls shall be ignored. If a program has not called setlogmask, syslog doesn't ignore any calls. You can use setlogmask to specify that messages of particular priorities shall be ignored in the future.
A setlogmask call overrides any previous setlogmask call.
Note that the logmask exists entirely independently of opening and closing of Syslog connections.
Setting the logmask has a similar effect to, but is not the same as, configuring Syslog. The Syslog configuration may cause Syslog to discard certain messages it receives, but the logmask causes certain messages never to get submitted to Syslog in the first place.
mask is a bit string with one bit corresponding to each of the possible message priorities. If the bit is on, syslog handles messages of that priority normally. If it is off, syslog discards messages of that priority. Use the message priority macros described in syslog; vsyslog and the LOG_MASK to construct an appropriate mask value, as in this example:
          LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR)
or
          ~(LOG_MASK(LOG_INFO))
There is also a LOG_UPTO macro, which generates a mask with the bits on for a certain priority and all priorities above it:
          LOG_UPTO(LOG_ERROR)
The unfortunate naming of the macro is due to the fact that internally, higher numbers are used for lower message priorities.

18.2.5 Syslog Example

This example sets the logmask so that debug and informational messages get discarded without ever reaching Syslog. So the second syslog in the example does nothing.

19 Mathematics

This chapter contains information about functions for performing mathematical computations, such as trigonometric functions. Most of these functions have prototypes declared in the header file math.h. The complex-valued functions are defined in complex.h. All mathematical functions which take a floating-point argument have three variants, one each for double, float, and long double arguments. The double versions are mostly defined in ISO C89. The float and long double versions are from the numeric extensions to C included in ISO C99.

Which of the three versions of a function should be used depends on the situation. For most calculations, the float functions are the fastest. On the other hand, the long double functions have the highest precision. double is somewhere in between. It is usually wise to pick the narrowest type that can accommodate your data. Not all machines have a distinct long double type; it may be the same as double.

19.1 Predefined Mathematical Constants

The header math.h defines several useful mathematical constants. All values are defined as preprocessor macros starting with M_. The values provided are:

These constants come from the Unix98 standard and were also available in 4.4BSD; therefore they are only defined if _BSD_SOURCE or _XOPEN_SOURCE=500, or a more general feature select macro, is defined. The default set of features includes these constants. See Feature Test Macros.

All values are of type double. As an extension, the GNU C library also defines these constants with type long double. The long double macros have a lowercase ‘l’ appended to their names: M_El, M_PIl, and so forth. These are only available if _GNU_SOURCE is defined.

Note: Some programs use a constant named PI which has the same value as M_PI. This constant is not standard; it may have appeared in some old AT&T headers, and is mentioned in Stroustrup's book on C++. It infringes on the user's name space, so the GNU C library does not define it. Fixing programs written to expect it is simple: replace PI with M_PI throughout, or put ‘-DPI=M_PI’ on the compiler command line.

19.2 Trigonometric Functions

These are the familiar sin, cos, and tan functions. The arguments to all of these functions are in units of radians; recall that pi radians equals 180 degrees.

The math library normally defines M_PI to a double approximation of pi. If strict ISO and/or POSIX compliance are requested this constant is not defined, but you can easily define it yourself:

— Function: double sin (double x)

— Function: float sinf (float x)

— Function: long double sinl (long double x)

These functions return the sine of x, where x is given in radians. The return value is in the range -1 to 1.

— Function: double cos (double x)

— Function: float cosf (float x)

— Function: long double cosl (long double x)

These functions return the cosine of x, where x is given in radians. The return value is in the range -1 to 1.

— Function: double tan (double x)

— Function: float tanf (float x)

— Function: long double tanl (long double x)

These functions return the tangent of x, where x is given in radians.
Mathematically, the tangent function has singularities at odd multiples of pi/2. If the argument x is too close to one of these singularities, tan will signal overflow.

In many applications where sin and cos are used, the sine and cosine of the same angle are needed at the same time. It is more efficient to compute them simultaneously, so the library provides a function to do that.

— Function: void sincos (double x, double *sinx, double *cosx)

— Function: void sincosf (float x, float *sinx, float *cosx)

— Function: void sincosl (long double x, long double *sinx, long double *cosx)

These functions return the sine of x in *sinx and the cosine of x in *cos, where x is given in radians. Both values, *sinx and *cosx, are in the range of -1 to 1.
This function is a GNU extension. Portable programs should be prepared to cope with its absence.

ISO C99 defines variants of the trig functions which work on complex numbers. The GNU C library provides these functions, but they are only useful if your compiler supports the new complex types defined by the standard. (As of this writing GCC supports complex numbers, but there are bugs in the implementation.)

— Function: complex double csin (complex double z)

— Function: complex float csinf (complex float z)

— Function: complex long double csinl (complex long double z)

These functions return the complex sine of z. The mathematical definition of the complex sine is
sin (z) = 1/(2*i) * (exp (z*i) - exp (-z*i)).

— Function: complex double ccos (complex double z)

— Function: complex float ccosf (complex float z)

— Function: complex long double ccosl (complex long double z)

These functions return the complex cosine of z. The mathematical definition of the complex cosine is
cos (z) = 1/2 * (exp (z*i) + exp (-z*i))

— Function: complex double ctan (complex double z)

— Function: complex float ctanf (complex float z)

— Function: complex long double ctanl (complex long double z)

These functions return the complex tangent of z. The mathematical definition of the complex tangent is
tan (z) = -i * (exp (z*i) - exp (-z*i)) / (exp (z*i) + exp (-z*i))
The complex tangent has poles at pi/2 + 2n, where n is an integer. ctan may signal overflow if z is too close to a pole.

19.3 Inverse Trigonometric Functions

These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively.

— Function: double asin (double x)

— Function: float asinf (float x)

— Function: long double asinl (long double x)

These functions compute the arc sine of x—that is, the value whose sine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).
The arc sine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, asin signals a domain error.

— Function: double acos (double x)

— Function: float acosf (float x)

— Function: long double acosl (long double x)

These functions compute the arc cosine of x—that is, the value whose cosine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between 0 and pi (inclusive).
The arc cosine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, acos signals a domain error.

— Function: double atan (double x)

— Function: float atanf (float x)

— Function: long double atanl (long double x)

These functions compute the arc tangent of x—that is, the value whose tangent is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

— Function: double atan2 (double y, double x)

— Function: float atan2f (float y, float x)

— Function: long double atan2l (long double y, long double x)

This function computes the arc tangent of y/x, but the signs of both arguments are used to determine the quadrant of the result, and x is permitted to be zero. The return value is given in radians and is in the range -pi to pi, inclusive.
If x and y are coordinates of a point in the plane, atan2 returns the signed angle between the line from the origin to that point and the x-axis. Thus, atan2 is useful for converting Cartesian coordinates to polar coordinates. (To compute the radial coordinate, use hypot; see Exponents and Logarithms.)
If both x and y are zero, atan2 returns zero.

— Function: complex double casin (complex double z)

— Function: complex float casinf (complex float z)

— Function: complex long double casinl (complex long double z)

These functions compute the complex arc sine of z—that is, the value whose sine is z. The value returned is in radians.
Unlike the real-valued functions, casin is defined for all values of z.

— Function: complex double cacos (complex double z)

— Function: complex float cacosf (complex float z)

— Function: complex long double cacosl (complex long double z)

These functions compute the complex arc cosine of z—that is, the value whose cosine is z. The value returned is in radians.
Unlike the real-valued functions, cacos is defined for all values of z.

— Function: complex double catan (complex double z)

— Function: complex float catanf (complex float z)

— Function: complex long double catanl (complex long double z)

These functions compute the complex arc tangent of z—that is, the value whose tangent is z. The value is in units of radians.

19.4 Exponentiation and Logarithms

— Function: double exp (double x)

— Function: float expf (float x)

— Function: long double expl (long double x)

These functions compute e (the base of natural logarithms) raised to the power x.
If the magnitude of the result is too large to be representable, exp signals overflow.

— Function: double exp2 (double x)

— Function: float exp2f (float x)

— Function: long double exp2l (long double x)

These functions compute 2 raised to the power x. Mathematically, exp2 (x) is the same as exp (x * log (2)).

— Function: double exp10 (double x)

— Function: float exp10f (float x)

— Function: long double exp10l (long double x)

— Function: double pow10 (double x)

— Function: float pow10f (float x)

— Function: long double pow10l (long double x)

These functions compute 10 raised to the power x. Mathematically, exp10 (x) is the same as exp (x * log (10)).
These functions are GNU extensions. The name exp10 is preferred, since it is analogous to exp and exp2.

— Function: double log (double x)

— Function: float logf (float x)

— Function: long double logl (long double x)

These functions compute the natural logarithm of x. exp (log (x)) equals x, exactly in mathematics and approximately in C.
If x is negative, log signals a domain error. If x is zero, it returns negative infinity; if x is too close to zero, it may signal overflow.

— Function: double log10 (double x)

— Function: float log10f (float x)

— Function: long double log10l (long double x)

These functions return the base-10 logarithm of x. log10 (x) equals log (x) / log (10).

— Function: double log2 (double x)

— Function: float log2f (float x)

— Function: long double log2l (long double x)

These functions return the base-2 logarithm of x. log2 (x) equals log (x) / log (2).

— Function: double logb (double x)

— Function: float logbf (float x)

— Function: long double logbl (long double x)

These functions extract the exponent of x and return it as a floating-point value. If FLT_RADIX is two, logb is equal to floor (log2 (x)), except it's probably faster.
If x is de-normalized, logb returns the exponent x would have if it were normalized. If x is infinity (positive or negative), logb returns ∞. If x is zero, logb returns ∞. It does not signal.

— Function: int ilogb (double x)

— Function: int ilogbf (float x)

— Function: int ilogbl (long double x)

These functions are equivalent to the corresponding logb functions except that they return signed integer values.

Since integers cannot represent infinity and NaN, ilogb instead returns an integer that can't be the exponent of a normal floating-point number. math.h defines constants so you can check for this.

— Macro: int FP_ILOGB0

ilogb returns this value if its argument is 0. The numeric value is either INT_MIN or -INT_MAX.
This macro is defined in ISO C99.

— Macro: int FP_ILOGBNAN

ilogb returns this value if its argument is NaN. The numeric value is either INT_MIN or INT_MAX.
This macro is defined in ISO C99.

These values are system specific. They might even be the same. The proper way to test the result of ilogb is as follows:

— Function: double pow (double base, double power)

— Function: float powf (float base, float power)

— Function: long double powl (long double base, long double power)

These are general exponentiation functions, returning base raised to power.
Mathematically, pow would return a complex number when base is negative and power is not an integral value. pow can't do that, so instead it signals a domain error. pow may also underflow or overflow the destination type.

— Function: double sqrt (double x)

— Function: float sqrtf (float x)

— Function: long double sqrtl (long double x)

These functions return the nonnegative square root of x.
If x is negative, sqrt signals a domain error. Mathematically, it should return a complex number.

— Function: double cbrt (double x)

— Function: float cbrtf (float x)

— Function: long double cbrtl (long double x)

These functions return the cube root of x. They cannot fail; every representable real value has a representable real cube root.

— Function: double hypot (double x, double y)

— Function: float hypotf (float x, float y)

— Function: long double hypotl (long double x, long double y)

These functions return sqrt (x*x+y*y). This is the length of the hypotenuse of a right triangle with sides of length x and y, or the distance of the point (x, y) from the origin. Using this function instead of the direct formula is wise, since the error is much smaller. See also the function cabs in Absolute Value.

— Function: double expm1 (double x)

— Function: float expm1f (float x)

— Function: long double expm1l (long double x)

These functions return a value equivalent to exp (x) - 1. They are computed in a way that is accurate even if x is near zero—a case where exp (x) - 1 would be inaccurate owing to subtraction of two numbers that are nearly equal.

— Function: double log1p (double x)

— Function: float log1pf (float x)

— Function: long double log1pl (long double x)

These functions returns a value equivalent to log (1 + x). They are computed in a way that is accurate even if x is near zero.

ISO C99 defines complex variants of some of the exponentiation and logarithm functions.

— Function: complex double cexp (complex double z)

— Function: complex float cexpf (complex float z)

— Function: complex long double cexpl (complex long double z)

These functions return e (the base of natural logarithms) raised to the power of z. Mathematically, this corresponds to the value
exp (z) = exp (creal (z)) * (cos (cimag (z)) + I * sin (cimag (z)))

— Function: complex double clog (complex double z)

— Function: complex float clogf (complex float z)

— Function: complex long double clogl (complex long double z)

These functions return the natural logarithm of z. Mathematically, this corresponds to the value
log (z) = log (cabs (z)) + I * carg (z)
clog has a pole at 0, and will signal overflow if z equals or is very close to 0. It is well-defined for all other values of z.

— Function: complex double clog10 (complex double z)

— Function: complex float clog10f (complex float z)

— Function: complex long double clog10l (complex long double z)

These functions return the base 10 logarithm of the complex value z. Mathematically, this corresponds to the value
log (z) = log10 (cabs (z)) + I * carg (z)
These functions are GNU extensions.

— Function: complex double csqrt (complex double z)

— Function: complex float csqrtf (complex float z)

— Function: complex long double csqrtl (complex long double z)

These functions return the complex square root of the argument z. Unlike the real-valued functions, they are defined for all values of z.

— Function: complex double cpow (complex double base, complex double power)

— Function: complex float cpowf (complex float base, complex float power)

— Function: complex long double cpowl (complex long double base, complex long double power)

These functions return base raised to the power of power. This is equivalent to cexp (y * clog (x))

19.5 Hyperbolic Functions

The functions in this section are related to the exponential functions; see Exponents and Logarithms.

— Function: double sinh (double x)

— Function: float sinhf (float x)

— Function: long double sinhl (long double x)

These functions return the hyperbolic sine of x, defined mathematically as (exp (x) - exp (-x)) / 2. They may signal overflow if x is too large.

— Function: double cosh (double x)

— Function: float coshf (float x)

— Function: long double coshl (long double x)

These function return the hyperbolic cosine of x, defined mathematically as (exp (x) + exp (-x)) / 2. They may signal overflow if x is too large.

— Function: double tanh (double x)

— Function: float tanhf (float x)

— Function: long double tanhl (long double x)

These functions return the hyperbolic tangent of x, defined mathematically as sinh (x) / cosh (x). They may signal overflow if x is too large.

There are counterparts for the hyperbolic functions which take complex arguments.

— Function: complex double csinh (complex double z)

— Function: complex float csinhf (complex float z)

— Function: complex long double csinhl (complex long double z)

These functions return the complex hyperbolic sine of z, defined mathematically as (exp (z) - exp (-z)) / 2.

— Function: complex double ccosh (complex double z)

— Function: complex float ccoshf (complex float z)

— Function: complex long double ccoshl (complex long double z)

These functions return the complex hyperbolic cosine of z, defined mathematically as (exp (z) + exp (-z)) / 2.

— Function: complex double ctanh (complex double z)

— Function: complex float ctanhf (complex float z)

— Function: complex long double ctanhl (complex long double z)

These functions return the complex hyperbolic tangent of z, defined mathematically as csinh (z) / ccosh (z).

— Function: double asinh (double x)

— Function: float asinhf (float x)

— Function: long double asinhl (long double x)

These functions return the inverse hyperbolic sine of x—the value whose hyperbolic sine is x.

— Function: double acosh (double x)

— Function: float acoshf (float x)

— Function: long double acoshl (long double x)

These functions return the inverse hyperbolic cosine of x—the value whose hyperbolic cosine is x. If x is less than 1, acosh signals a domain error.

— Function: double atanh (double x)

— Function: float atanhf (float x)

— Function: long double atanhl (long double x)

These functions return the inverse hyperbolic tangent of x—the value whose hyperbolic tangent is x. If the absolute value of x is greater than 1, atanh signals a domain error; if it is equal to 1, atanh returns infinity.

— Function: complex double casinh (complex double z)

— Function: complex float casinhf (complex float z)

— Function: complex long double casinhl (complex long double z)

These functions return the inverse complex hyperbolic sine of z—the value whose complex hyperbolic sine is z.

— Function: complex double cacosh (complex double z)

— Function: complex float cacoshf (complex float z)

— Function: complex long double cacoshl (complex long double z)

These functions return the inverse complex hyperbolic cosine of z—the value whose complex hyperbolic cosine is z. Unlike the real-valued functions, there are no restrictions on the value of z.

— Function: complex double catanh (complex double z)

— Function: complex float catanhf (complex float z)

— Function: complex long double catanhl (complex long double z)

These functions return the inverse complex hyperbolic tangent of z—the value whose complex hyperbolic tangent is z. Unlike the real-valued functions, there are no restrictions on the value of z.

19.6 Special Functions

These are some more exotic mathematical functions which are sometimes useful. Currently they only have real-valued versions.

— Function: double erf (double x)

— Function: float erff (float x)
— Function: long double erfl (long double x)
erf returns the error function of x. The error function is defined as
          erf (x) = 2/sqrt(pi) * integral from 0 to x of exp(-t^2) dt

— Function: double erfc (double x)

— Function: float erfcf (float x)

— Function: long double erfcl (long double x)

erfc returns 1.0 - erf(x), but computed in a fashion that avoids round-off error when x is large.

— Function: double lgamma (double x)

— Function: float lgammaf (float x)
— Function: long double lgammal (long double x)
lgamma returns the natural logarithm of the absolute value of the gamma function of x. The gamma function is defined as
          gamma (x) = integral from 0 to &infin; of t^(x-1) e^-t dt
The sign of the gamma function is stored in the global variable signgam, which is declared in math.h. It is 1 if the intermediate result was positive or zero, or -1 if it was negative.
To compute the real gamma function you can use the tgamma function or you can compute the values as follows:
          lgam = lgamma(x);
          gam  = signgam*exp(lgam);
The gamma function has singularities at the non-positive integers. lgamma will raise the zero divide exception if evaluated at a singularity.

— Function: double lgamma_r (double x, int *signp)

— Function: float lgammaf_r (float x, int *signp)

— Function: long double lgammal_r (long double x, int *signp)

lgamma_r is just like lgamma, but it stores the sign of the intermediate result in the variable pointed to by signp instead of in the signgam global. This means it is reentrant.

— Function: double gamma (double x)

— Function: float gammaf (float x)

— Function: long double gammal (long double x)

These functions exist for compatibility reasons. They are equivalent to lgamma etc. It is better to use lgamma since for one the name reflects better the actual computation, moreover lgamma is standardized in ISO C99 while gamma is not.

— Function: double tgamma (double x)

— Function: float tgammaf (float x)
— Function: long double tgammal (long double x)
tgamma applies the gamma function to x. The gamma function is defined as
          gamma (x) = integral from 0 to &infin; of t^(x-1) e^-t dt
This function was introduced in ISO C99.

— Function: double j0 (double x)

— Function: float j0f (float x)

— Function: long double j0l (long double x)

j0 returns the Bessel function of the first kind of order 0 of x. It may signal underflow if x is too large.

— Function: double j1 (double x)

— Function: float j1f (float x)

— Function: long double j1l (long double x)

j1 returns the Bessel function of the first kind of order 1 of x. It may signal underflow if x is too large.

— Function: double jn (int n, double x)

— Function: float jnf (int n, float x)

— Function: long double jnl (int n, long double x)

jn returns the Bessel function of the first kind of order n of x. It may signal underflow if x is too large.

— Function: double y0 (double x)

— Function: float y0f (float x)

— Function: long double y0l (long double x)

y0 returns the Bessel function of the second kind of order 0 of x. It may signal underflow if x is too large. If x is negative, y0 signals a domain error; if it is zero, y0 signals overflow and returns -∞.

— Function: double y1 (double x)

— Function: float y1f (float x)

— Function: long double y1l (long double x)

y1 returns the Bessel function of the second kind of order 1 of x. It may signal underflow if x is too large. If x is negative, y1 signals a domain error; if it is zero, y1 signals overflow and returns -∞.

— Function: double yn (int n, double x)

— Function: float ynf (int n, float x)

— Function: long double ynl (int n, long double x)

yn returns the Bessel function of the second kind of order n of x. It may signal underflow if x is too large. If x is negative, yn signals a domain error; if it is zero, yn signals overflow and returns -∞.

19.7 Known Maximum Errors in Math Functions

This section lists the known errors of the functions in the math library. Errors are measured in “units of the last place”. This is a measure for the relative error. For a number z with the representation d.d...d·2^e (we assume IEEE floating-point numbers with base 2) the ULP is represented by

where p is the number of bits in the mantissa of the floating-point number representation. Ideally the error for all functions is always less than 0.5ulps. Using rounding bits this is also possible and normally implemented for the basic operations. To achieve the same for the complex math functions requires a lot more work and this has not yet been done.

Therefore many of the functions in the math library have errors. The table lists the maximum error for each function which is exposed by one of the existing tests in the test suite. The table tries to cover as much as possible and list the actual maximum error (or at least a ballpark figure) but this is often not achieved due to the large search space.

The table lists the ULP values for different architectures. Different architectures have different results since their hardware support for floating-point operations varies and also the existing hardware support is different.

Function	Generic	ix86	IA64	PowerPC	S/390
acosf	-	-	-	-	-
acos	-	-	-	-	-
acosl	-	622	-	1	-
acoshf	-	-	-	-	-
acosh	-	-	-	-	-
acoshl	-	-	-	1	-
asinf	-	-	-	-	-
asin	-	-	-	-	-
asinl	-	1	-	2	-
asinhf	-	-	-	-	-
asinh	-	-	-	-	-
asinhl	-	-	-	1	-
atanf	-	-	-	-	-
atan	-	-	-	-	-
atanl	-	-	-	-	-
atanhf	-	-	-	1	1
atanh	-	-	-	-	-
atanhl	-	1	-	-	-
atan2f	-	-	-	1	1
atan2	-	-	-	-	-
atan2l	-	-	-	1	1
cabsf	-	-	-	-	-
cabs	-	-	-	-	-
cabsl	-	-	-	1	-
cacosf	-	0 + i 1	0 + i 1	-	-
cacos	-	-	-	-	-
cacosl	-	0 + i 2	0 + i 2	1 + i 1	0 + i 1
cacoshf	-	9 + i 4	7 + i 0	7 + i 3	7 + i 3
cacosh	-	1 + i 1	1 + i 1	1 + i 1	1 + i 1
cacoshl	-	6 + i 1	7 + i 1	1 + i 0	0 + i 1
cargf	-	-	-	-	-
carg	-	-	-	-	-
cargl	-	-	-	-	-
casinf	-	1 + i 1	1 + i 1	1 + i 0	1 + i 0
casin	-	1 + i 0	1 + i 0	1 + i 0	1 + i 0
casinl	-	2 + i 2	2 + i 2	1 + i 1	0 + i 1
casinhf	-	1 + i 6	1 + i 6	1 + i 6	1 + i 6
casinh	-	5 + i 3	5 + i 3	5 + i 3	5 + i 3
casinhl	-	5 + i 5	5 + i 5	4 + i 1	4 + i 2
catanf	-	0 + i 1	0 + i 1	4 + i 1	4 + i 1
catan	-	0 + i 1	0 + i 1	0 + i 1	0 + i 1
catanl	-	-	-	1 + i 1	0 + i 1
catanhf	-	1 + i 0	-	0 + i 6	0 + i 6
catanh	-	2 + i 0	4 + i 0	4 + i 0	4 + i 0
catanhl	-	1 + i 0	1 + i 0	-	1 + i 1
cbrtf	-	-	-	-	-
cbrt	-	-	-	1	1
cbrtl	-	1	-	1	1
ccosf	-	0 + i 1	0 + i 1	1 + i 1	1 + i 1
ccos	-	1 + i 0	1 + i 0	1 + i 0	1 + i 0
ccosl	-	1 + i 1	1 + i 1	1 + i 1	1 + i 1
ccoshf	-	1 + i 1	1 + i 1	1 + i 1	1 + i 1
ccosh	-	1 + i 1	1 + i 1	1 + i 0	1 + i 0
ccoshl	-	0 + i 1	0 + i 1	1 + i 2	1 + i 1
ceilf	-	-	-	-	-
ceil	-	-	-	-	-
ceill	-	-	-	-	-
cexpf	-	-	1 + i 1	1 + i 1	1 + i 1
cexp	-	-	-	-	-
cexpl	-	1 + i 1	0 + i 1	2 + i 1	1 + i 1
cimagf	-	-	-	-	-
cimag	-	-	-	-	-
cimagl	-	-	-	-	-
clogf	-	1 + i 0	1 + i 0	1 + i 3	1 + i 3
clog	-	-	-	-	-
clogl	-	1 + i 0	1 + i 0	2 + i 1	1 + i 0
clog10f	-	1 + i 1	1 + i 1	1 + i 5	1 + i 5
clog10	-	1 + i 1	1 + i 1	0 + i 1	0 + i 1
clog10l	-	1 + i 1	1 + i 1	3 + i 1	1 + i 1
conjf	-	-	-	-	-
conj	-	-	-	-	-
conjl	-	-	-	-	-
copysignf	-	-	-	-	-
copysign	-	-	-	-	-
copysignl	-	-	-	-	-
cosf	-	1	1	1	1
cos	-	2	2	2	2
cosl	-	1	1	1	1
coshf	-	-	-	-	-
cosh	-	-	-	-	-
coshl	-	-	-	1	-
cpowf	-	4 + i 3	5 + i 3	5 + i 2	4 + i 2
cpow	-	1 + i 2	2 + i 2	2 + i 2	2 + i 2
cpowl	-	763 + i 2	6 + i 4	2 + i 2	10 + i 1
cprojf	-	-	-	-	-
cproj	-	-	-	-	-
cprojl	-	-	-	0 + i 1	-
crealf	-	-	-	-	-
creal	-	-	-	-	-
creall	-	-	-	-	-
csinf	-	1 + i 1	1 + i 1	-	-
csin	-	-	-	-	-
csinl	-	1 + i 0	1 + i 0	1 + i 0	1 + i 1
csinhf	-	1 + i 1	1 + i 1	1 + i 1	1 + i 1
csinh	-	1 + i 1	1 + i 1	0 + i 1	0 + i 1
csinhl	-	1 + i 2	1 + i 2	1 + i 1	1 + i 0
csqrtf	-	-	1 + i 0	1 + i 0	1 + i 0
csqrt	-	-	-	-	-
csqrtl	-	-	-	1 + i 1	1 + i 1
ctanf	-	0 + i 1	0 + i 1	-	-
ctan	-	1 + i 1	1 + i 1	1 + i 1	1 + i 1
ctanl	-	439 + i 3	2 + i 1	1 + i 1	1 + i 2
ctanhf	-	1 + i 1	0 + i 1	2 + i 1	2 + i 1
ctanh	-	1 + i 1	1 + i 1	1 + i 0	1 + i 0
ctanhl	-	5 + i 25	1 + i 24	1 + i 1	1 + i 1
erff	-	-	-	-	-
erf	-	1	1	1	1
erfl	-	-	-	1	-
erfcf	-	1	1	1	1
erfc	-	1	1	1	1
erfcl	-	1	1	1	1
expf	-	-	-	-	-
exp	-	-	-	-	-
expl	-	-	-	1	-
exp10f	-	-	2	2	2
exp10	-	-	6	6	6
exp10l	-	8	3	8	1
exp2f	-	-	-	-	-
exp2	-	-	-	-	-
exp2l	-	-	-	2	2
expm1f	-	-	-	1	1
expm1	-	-	-	1	1
expm1l	-	-	1	-	1
fabsf	-	-	-	-	-
fabs	-	-	-	-	-
fabsl	-	-	-	-	-
fdimf	-	-	-	-	-
fdim	-	-	-	-	-
fdiml	-	-	-	-	-
floorf	-	-	-	-	-
floor	-	-	-	-	-
floorl	-	-	-	-	-
fmaf	-	-	-	-	-
fma	-	-	-	-	-
fmal	-	-	-	-	-
fmaxf	-	-	-	-	-
fmax	-	-	-	-	-
fmaxl	-	-	-	-	-
fminf	-	-	-	-	-
fmin	-	-	-	-	-
fminl	-	-	-	-	-
fmodf	-	-	-	-	-
fmod	-	-	-	-	-
fmodl	-	-	-	-	-
frexpf	-	-	-	-	-
frexp	-	-	-	-	-
frexpl	-	-	-	-	-
gammaf	-	-	-	-	-
gamma	-	1	-	-	-
gammal	-	1	1	1	1
hypotf	-	1	1	1	1
hypot	-	-	-	-	-
hypotl	-	-	-	1	-
ilogbf	-	-	-	-	-
ilogb	-	-	-	-	-
ilogbl	-	-	-	-	-
j0f	-	2	2	2	2
j0	-	3	3	3	3
j0l	-	1	2	1	2
j1f	-	1	2	2	2
j1	-	1	1	1	1
j1l	-	1	1	1	4
jnf	-	2	4	4	4
jn	-	5	3	3	4
jnl	-	2	2	4	4
lgammaf	-	2	2	2	2
lgamma	-	1	1	1	1
lgammal	-	1	1	3	1
lrintf	-	-	-	-	-
lrint	-	-	-	-	-
lrintl	-	-	-	-	-
llrintf	-	-	-	-	-
llrint	-	-	-	-	-
llrintl	-	-	-	-	-
logf	-	1	1	-	-
log	-	-	-	-	-
logl	-	-	-	1	-
log10f	-	1	1	2	2
log10	-	-	-	1	1
log10l	-	1	1	1	1
log1pf	-	-	-	1	1
log1p	-	-	-	-	-
log1pl	-	-	-	1	1
log2f	-	-	-	-	-
log2	-	-	-	-	-
log2l	-	-	-	1	1
logbf	-	-	-	-	-
logb	-	-	-	-	-
logbl	-	-	-	-	-
lroundf	-	-	-	-	-
lround	-	-	-	-	-
lroundl	-	-	-	-	-
llroundf	-	-	-	-	-
llround	-	-	-	-	-
llroundl	-	-	-	-	-
modff	-	-	-	-	-
modf	-	-	-	-	-
modfl	-	-	-	-	-
nearbyintf	-	-	-	-	-
nearbyint	-	-	-	-	-
nearbyintl	-	-	-	-	-
nextafterf	-	-	-	-	-
nextafter	-	-	-	-	-
nextafterl	-	-	-	-	-
nexttowardf	-	-	-	-	-
nexttoward	-	-	-	-	-
nexttowardl	-	-	-	-	-
powf	-	-	-	-	-
pow	-	-	-	-	-
powl	-	-	-	1	-
remainderf	-	-	-	-	-
remainder	-	-	-	-	-
remainderl	-	-	-	-	-
remquof	-	-	-	-	-
remquo	-	-	-	-	-
remquol	-	-	-	-	-
rintf	-	-	-	-	-
rint	-	-	-	-	-
rintl	-	-	-	-	-
roundf	-	-	-	-	-
round	-	-	-	-	-
roundl	-	-	-	-	-
scalbf	-	-	-	-	-
scalb	-	-	-	-	-
scalbl	-	-	-	-	-
scalbnf	-	-	-	-	-
scalbn	-	-	-	-	-
scalbnl	-	-	-	-	-
scalblnf	-	-	-	-	-
scalbln	-	-	-	-	-
scalblnl	-	-	-	-	-
sinf	-	-	-	-	-
sin	-	-	-	-	-
sinl	-	-	-	1	-
sincosf	-	1	1	1	1
sincos	-	1	1	1	1
sincosl	-	1	1	1	1
sinhf	-	-	-	-	-
sinh	-	1	-	-	-
sinhl	-	-	-	1	-
sqrtf	-	-	-	-	-
sqrt	-	-	-	-	-
sqrtl	-	-	-	-	1
tanf	-	-	-	-	-
tan	-	1	1	1	1
tanl	-	-	-	1	-
tanhf	-	-	-	-	-
tanh	-	-	-	-	-
tanhl	-	-	-	1	1
tgammaf	-	1	1	1	1
tgamma	-	2	1	1	1
tgammal	-	1	1	1	1
truncf	-	-	-	-	-
trunc	-	-	-	-	-
truncl	-	-	-	-	-
y0f	-	1	1	1	1
y0	-	2	2	2	2
y0l	-	1	1	1	3
y1f	-	2	2	2	2
y1	-	2	3	3	3
y1l	-	1	1	2	1
ynf	-	3	2	2	2
yn	-	2	3	3	3
ynl	-	4	2	2	5

Function	SH4	Sparc 32-bit	Sparc 64-bit	x86_64/fpu
acosf	-	-	-	-
acos	-	-	-	-
acosl	-	-	-	1
acoshf	-	-	-	-
acosh	-	-	-	-
acoshl	-	-	-	-
asinf	2	-	-	-
asin	1	-	-	-
asinl	-	-	-	1
asinhf	-	-	-	-
asinh	-	-	-	-
asinhl	-	-	-	-
atanf	-	-	-	-
atan	-	-	-	-
atanl	-	-	-	-
atanhf	-	1	1	1
atanh	1	-	-	-
atanhl	-	-	-	1
atan2f	4	6	6	1
atan2	-	-	-	-
atan2l	-	1	1	-
cabsf	1	-	-	-
cabs	1	-	-	-
cabsl	-	-	-	-
cacosf	1 + i 1	-	-	0 + i 1
cacos	1 + i 0	-	-	-
cacosl	-	0 + i 1	0 + i 1	0 + i 2
cacoshf	7 + i 3	7 + i 3	7 + i 3	7 + i 3
cacosh	1 + i 1	1 + i 1	1 + i 1	1 + i 1
cacoshl	-	5 + i 1	5 + i 1	6 + i 1
cargf	-	-	-	-
carg	-	-	-	-
cargl	-	-	-	-
casinf	2 + i 1	1 + i 0	1 + i 0	1 + i 1
casin	3 + i 0	1 + i 0	1 + i 0	1 + i 0
casinl	-	0 + i 1	0 + i 1	2 + i 2
casinhf	1 + i 6	1 + i 6	1 + i 6	1 + i 6
casinh	5 + i 3	5 + i 3	5 + i 3	5 + i 3
casinhl	-	4 + i 2	4 + i 2	5 + i 5
catanf	4 + i 1	4 + i 1	4 + i 1	4 + i 1
catan	0 + i 1	0 + i 1	0 + i 1	0 + i 1
catanl	-	0 + i 1	0 + i 1	-
catanhf	1 + i 6	0 + i 6	0 + i 6	0 + i 6
catanh	4 + i 1	4 + i 0	4 + i 0	4 + i 0
catanhl	-	1 + i 1	1 + i 1	1 + i 0
cbrtf	-	-	-	-
cbrt	1	1	1	1
cbrtl	-	1	1	1
ccosf	0 + i 1	1 + i 1	1 + i 1	1 + i 1
ccos	1 + i 1	1 + i 0	1 + i 0	1 + i 0
ccosl	-	1 + i 1	1 + i 1	1 + i 1
ccoshf	1 + i 1	1 + i 1	1 + i 1	1 + i 1
ccosh	1 + i 1	1 + i 0	1 + i 0	1 + i 1
ccoshl	-	1 + i 1	1 + i 1	0 + i 1
ceilf	-	-	-	-
ceil	-	-	-	-
ceill	-	-	-	-
cexpf	1 + i 1	1 + i 1	1 + i 1	1 + i 1
cexp	1 + i 0	-	-	-
cexpl	-	1 + i 1	1 + i 1	0 + i 1
cimagf	-	-	-	-
cimag	-	-	-	-
cimagl	-	-	-	-
clogf	0 + i 3	1 + i 3	1 + i 3	1 + i 3
clog	0 + i 1	-	-	-
clogl	-	1 + i 0	1 + i 0	1 + i 0
clog10f	1 + i 5	1 + i 5	1 + i 5	1 + i 5
clog10	1 + i 1	0 + i 1	0 + i 1	1 + i 1
clog10l	-	1 + i 1	1 + i 1	1 + i 1
conjf	-	-	-	-
conj	-	-	-	-
conjl	-	-	-	-
copysignf	-	-	-	-
copysign	-	-	-	-
copysignl	-	-	-	-
cosf	1	1	1	1
cos	2	2	2	2
cosl	-	1	1	1
coshf	-	-	-	-
cosh	-	-	-	-
coshl	-	-	-	-
cpowf	4 + i 2	4 + i 2	4 + i 2	5 + i 2
cpow	1 + i 1.1031	2 + i 2	2 + i 2	2 + i 2
cpowl	-	10 + i 1	10 + i 1	5 + i 2
cprojf	-	-	-	-
cproj	-	-	-	-
cprojl	-	-	-	-
crealf	-	-	-	-
creal	-	-	-	-
creall	-	-	-	-
csinf	0 + i 1	-	-	0 + i 1
csin	-	-	-	0 + i 1
csinl	-	1 + i 1	1 + i 1	1 + i 0
csinhf	1 + i 1	1 + i 1	1 + i 1	1 + i 1
csinh	0 + i 1	0 + i 1	0 + i 1	1 + i 1
csinhl	-	1 + i 0	1 + i 0	1 + i 2
csqrtf	1 + i 1	1 + i 0	1 + i 0	1 + i 0
csqrt	1 + i 0	-	-	-
csqrtl	-	1 + i 1	1 + i 1	-
ctanf	1 + i 1	-	-	0 + i 1
ctan	1 + i 1	1 + i 1	1 + i 1	1 + i 1
ctanl	-	1 + i 2	1 + i 2	439 + i 3
ctanhf	2 + i 1	2 + i 1	2 + i 1	2 + i 1
ctanh	2 + i 2	1 + i 0	1 + i 0	1 + i 1
ctanhl	-	1 + i 1	1 + i 1	5 + i 25
erff	-	-	-	-
erf	-	1	1	1
erfl	-	-	-	-
erfcf	12	-	-	-
erfc	24	1	1	1
erfcl	-	1	1	1
expf	-	-	-	-
exp	-	-	-	-
expl	-	-	-	-
exp10f	2	2	2	2
exp10	6	6	6	6
exp10l	-	1	1	8
exp2f	-	-	-	-
exp2	-	-	-	-
exp2l	-	2	2	-
expm1f	1	1	1	1
expm1	-	1	1	1
expm1l	-	1	1	-
fabsf	-	-	-	-
fabs	-	-	-	-
fabsl	-	-	-	-
fdimf	-	-	-	-
fdim	-	-	-	-
fdiml	-	-	-	-
floorf	-	-	-	-
floor	-	-	-	-
floorl	-	-	-	-
fmaf	-	-	-	-
fma	-	-	-	-
fmal	-	-	-	-
fmaxf	-	-	-	-
fmax	-	-	-	-
fmaxl	-	-	-	-
fminf	-	-	-	-
fmin	-	-	-	-
fminl	-	-	-	-
fmodf	1	-	-	-
fmod	2	-	-	-
fmodl	-	-	-	-
frexpf	-	-	-	-
frexp	-	-	-	-
frexpl	-	-	-	-
gammaf	-	-	-	-
gamma	-	-	-	-
gammal	-	1	1	1
hypotf	1	1	1	1
hypot	1	-	-	-
hypotl	-	-	-	-
ilogbf	-	-	-	-
ilogb	-	-	-	-
ilogbl	-	-	-	-
j0f	2	2	2	2
j0	2	2	2	2
j0l	-	2	2	1
j1f	2	2	2	2
j1	1	1	1	1
j1l	-	4	4	1
jnf	4	4	4	4
jn	6	4	4	4
jnl	-	4	4	2
lgammaf	2	2	2	2
lgamma	1	1	1	1
lgammal	-	1	1	1
lrintf	-	-	-	-
lrint	-	-	-	-
lrintl	-	-	-	-
llrintf	-	-	-	-
llrint	-	-	-	-
llrintl	-	-	-	-
logf	1	-	-	-
log	1	-	-	-
logl	-	-	-	-
log10f	1	2	2	2
log10	1	1	1	1
log10l	-	1	1	1
log1pf	1	1	1	1
log1p	1	-	-	-
log1pl	-	1	1	-
log2f	1	-	-	-
log2	1	-	-	-
log2l	-	1	1	-
logbf	-	-	-	-
logb	-	-	-	-
logbl	-	-	-	-
lroundf	-	-	-	-
lround	-	-	-	-
lroundl	-	-	-	-
llroundf	-	-	-	-
llround	-	-	-	-
llroundl	-	-	-	-
modff	-	-	-	-
modf	-	-	-	-
modfl	-	-	-	-
nearbyintf	-	-	-	-
nearbyint	-	-	-	-
nearbyintl	-	-	-	-
nextafterf	-	-	-	-
nextafter	-	-	-	-
nextafterl	-	-	-	-
nexttowardf	-	-	-	-
nexttoward	-	-	-	-
nexttowardl	-	-	-	-
powf	-	-	-	-
pow	-	-	-	-
powl	-	-	-	-
remainderf	-	-	-	-
remainder	-	-	-	-
remainderl	-	-	-	-
remquof	-	-	-	-
remquo	-	-	-	-
remquol	-	-	-	-
rintf	-	-	-	-
rint	-	-	-	-
rintl	-	-	-	-
roundf	-	-	-	-
round	-	-	-	-
roundl	-	-	-	-
scalbf	-	-	-	-
scalb	-	-	-	-
scalbl	-	-	-	-
scalbnf	-	-	-	-
scalbn	-	-	-	-
scalbnl	-	-	-	-
scalblnf	-	-	-	-
scalbln	-	-	-	-
scalblnl	-	-	-	-
sinf	-	-	-	-
sin	-	-	-	-
sinl	-	-	-	-
sincosf	1	1	1	1
sincos	1	1	1	1
sincosl	-	1	1	1
sinhf	1	-	-	-
sinh	1	-	-	-
sinhl	-	-	-	-
sqrtf	-	-	-	-
sqrt	-	-	-	-
sqrtl	-	1	1	-
tanf	-	-	-	-
tan	0.5	1	1	1
tanl	-	-	-	-
tanhf	1	-	-	-
tanh	1	-	-	-
tanhl	-	1	1	-
tgammaf	1	1	1	1
tgamma	1	1	1	1
tgammal	-	1	1	1
truncf	-	-	-	-
trunc	-	-	-	-
truncl	-	-	-	-
y0f	1	1	1	1
y0	2	2	2	2
y0l	-	3	3	1
y1f	2	2	2	2
y1	3	3	3	3
y1l	-	1	1	1
ynf	2	2	2	2
yn	3	3	3	3
ynl	-	5	5	4

19.8 Pseudo-Random Numbers

This section describes the GNU facilities for generating a series of pseudo-random numbers. The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes. The random number generator works by remembering a seed value which it uses to compute the next random number and also to compute a new seed.

Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next. This is because the initial seed is always the same. This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably. If you want a different pseudo-random series each time your program runs, you must specify a different seed each time. For ordinary purposes, basing the seed on the current time works well.

You can obtain repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator. There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.

The GNU library supports the standard ISO C random number functions plus two other sets derived from BSD and SVID. The BSD and ISO C functions provide identical, somewhat limited functionality. If only a small number of random bits are required, we recommend you use the ISO C interface, rand and srand. The SVID functions provide a more flexible interface, which allows better random number generator algorithms, provides more random bits (up to 48) per call, and can provide random floating-point numbers. These functions are required by the XPG standard and therefore will be present in all modern Unix systems.

19.8.1 ISO C Random Number Functions

This section describes the random number functions that are part of the ISO C standard.

To use these facilities, you should include the header file stdlib.h in your program.

— Macro: int RAND_MAX

The value of this macro is an integer constant representing the largest value the rand function can return. In the GNU library, it is 2147483647, which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as 32767.

— Function: int rand (void)

The rand function returns the next pseudo-random number in the series. The value ranges from 0 to RAND_MAX.

— Function: void srand (unsigned int seed)

This function establishes seed as the seed for a new series of pseudo-random numbers. If you call rand before a seed has been established with srand, it uses the value 1 as a default seed.
To produce a different pseudo-random series each time your program is run, do srand (time (0)).

POSIX.1 extended the C standard functions to support reproducible random numbers in multi-threaded programs. However, the extension is badly designed and unsuitable for serious work.

— Function: int rand_r (unsigned int *seed)

This function returns a random number in the range 0 to RAND_MAX just as rand does. However, all its state is stored in the seed argument. This means the RNG's state can only have as many bits as the type unsigned int has. This is far too few to provide a good RNG.
If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random number generator. The POSIX.1 interface should only be used when the GNU extensions are not available.

19.8.2 BSD Random Number Functions

This section describes a set of random number generation functions that are derived from BSD. There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only.

— Function: long int random (void)

This function returns the next pseudo-random number in the sequence. The value returned ranges from 0 to RAND_MAX.
NB: Temporarily this function was defined to return a int32_t value to indicate that the return value always contains 32 bits even if long int is wider. The standard demands it differently. Users must always be aware of the 32-bit limitation, though.

— Function: void srandom (unsigned int seed)

The srandom function sets the state of the random number generator based on the integer seed. If you supply a seed value of 1, this will cause random to reproduce the default set of random numbers.
To produce a different set of pseudo-random numbers each time your program runs, do srandom (time (0)).

— Function: void * initstate (unsigned int seed, void *state, size_t size)

The initstate function is used to initialize the random number generator state. The argument state is an array of size bytes, used to hold the state information. It is initialized based on seed. The size must be between 8 and 256 bytes, and should be a power of two. The bigger the state array, the better.
The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.

— Function: void * setstate (void *state)

The setstate function restores the random number state information state. The argument must have been the result of a previous call to initstate or setstate.
The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.
If the function fails the return value is NULL.

The four functions described so far in this section all work on a state which is shared by all threads. The state is not directly accessible to the user and can only be modified by these functions. This makes it hard to deal with situations where each thread should have its own pseudo-random number generator.

The GNU C library contains four additional functions which contain the state as an explicit parameter and therefore make it possible to handle thread-local PRNGs. Beside this there is no difference. In fact, the four functions already discussed are implemented internally using the following interfaces.

— Data Type: struct random_data

Objects of type struct random_data contain the information necessary to represent the state of the PRNG. Although a complete definition of the type is present the type should be treated as opaque.

The functions modifying the state follow exactly the already described functions.

— Function: int random_r (struct random_data *restrict buf, int32_t *restrict result)

The random_r function behaves exactly like the random function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

— Function: int srandom_r (unsigned int seed, struct random_data *buf)

The srandom_r function behaves exactly like the srandom function except that it uses and modifies the state in the object pointed to by the second parameter instead of the global state.

— Function: int initstate_r (unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf)

The initstate_r function behaves exactly like the initstate function except that it uses and modifies the state in the object pointed to by the fourth parameter instead of the global state.

— Function: int setstate_r (char *restrict statebuf, struct random_data *restrict buf)

The setstate_r function behaves exactly like the setstate function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

19.8.3 SVID Random Number Function

The C library on SVID systems contains yet another kind of random number generator functions. They use a state of 48 bits of data. The user can choose among a collection of functions which return the random bits in different forms.

Generally there are two kinds of function. The first uses a state of the random number generator which is shared among several functions and by all threads of the process. The second requires the user to handle the state.

All functions have in common that they use the same congruential formula with the same constants. The formula is

where X is the state of the generator at the beginning and Y the state at the end. a and c are constants determining the way the generator works. By default they are

but they can also be changed by the user. m is of course 2^48 since the state consists of a 48-bit array.

— Function: double drand48 (void)

This function returns a double value in the range of 0.0 to 1.0 (exclusive). The random bits are determined by the global state of the random number generator in the C library.
Since the double type according to IEEE 754 has a 52-bit mantissa this means 4 bits are not initialized by the random number generator. These are (of course) chosen to be the least significant bits and they are initialized to 0.

— Function: double erand48 (unsigned short int xsubi[3])

This function returns a double value in the range of 0.0 to 1.0 (exclusive), similarly to drand48. The argument is an array describing the state of the random number generator.
This function can be called subsequently since it updates the array to guarantee random numbers. The array should have been initialized before initial use to obtain reproducible results.

— Function: long int lrand48 (void)

The lrand48 function returns an integer value in the range of 0 to 2^31 (exclusive). Even if the size of the long int type can take more than 32 bits, no higher numbers are returned. The random bits are determined by the global state of the random number generator in the C library.

— Function: long int nrand48 (unsigned short int xsubi[3])

This function is similar to the lrand48 function in that it returns a number in the range of 0 to 2^31 (exclusive) but the state of the random number generator used to produce the random bits is determined by the array provided as the parameter to the function.
The numbers in the array are updated afterwards so that subsequent calls to this function yield different results (as is expected of a random number generator). The array should have been initialized before the first call to obtain reproducible results.

— Function: long int mrand48 (void)

The mrand48 function is similar to lrand48. The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive).

— Function: long int jrand48 (unsigned short int xsubi[3])

The jrand48 function is similar to nrand48. The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive). For the xsubi parameter the same requirements are necessary.

The internal state of the random number generator can be initialized in several ways. The methods differ in the completeness of the information provided.

— Function: void srand48 (long int seedval)

The srand48 function sets the most significant 32 bits of the internal state of the random number generator to the least significant 32 bits of the seedval parameter. The lower 16 bits are initialized to the value 0x330E. Even if the long int type contains more than 32 bits only the lower 32 bits are used.
Owing to this limitation, initialization of the state of this function is not very useful. But it makes it easy to use a construct like srand48 (time (0)).
A side-effect of this function is that the values a and c from the internal state, which are used in the congruential formula, are reset to the default values given above. This is of importance once the user has called the lcong48 function (see below).

— Function: unsigned short int * seed48 (unsigned short int seed16v[3])

The seed48 function initializes all 48 bits of the state of the internal random number generator from the contents of the parameter seed16v. Here the lower 16 bits of the first element of see16v initialize the least significant 16 bits of the internal state, the lower 16 bits of seed16v[1] initialize the mid-order 16 bits of the state and the 16 lower bits of seed16v[2] initialize the most significant 16 bits of the state.
Unlike srand48 this function lets the user initialize all 48 bits of the state.
The value returned by seed48 is a pointer to an array containing the values of the internal state before the change. This might be useful to restart the random number generator at a certain state. Otherwise the value can simply be ignored.
As for srand48, the values a and c from the congruential formula are reset to the default values.

There is one more function to initialize the random number generator which enables you to specify even more information by allowing you to change the parameters in the congruential formula.

— Function: void lcong48 (unsigned short int param[7])

The lcong48 function allows the user to change the complete state of the random number generator. Unlike srand48 and seed48, this function also changes the constants in the congruential formula.
From the seven elements in the array param the least significant 16 bits of the entries param[0] to param[2] determine the initial state, the least significant 16 bits of param[3] to param[5] determine the 48 bit constant a and param[6] determines the 16-bit value c.

All the above functions have in common that they use the global parameters for the congruential formula. In multi-threaded programs it might sometimes be useful to have different parameters in different threads. For this reason all the above functions have a counterpart which works on a description of the random number generator in the user-supplied buffer instead of the global state.

Please note that it is no problem if several threads use the global state if all threads use the functions which take a pointer to an array containing the state. The random numbers are computed following the same loop but if the state in the array is different all threads will obtain an individual random number generator.

The user-supplied buffer must be of type struct drand48_data. This type should be regarded as opaque and not manipulated directly.

— Function: int drand48_r (struct drand48_data *buffer, double *result)

This function is equivalent to the drand48 function with the difference that it does not modify the global random number generator parameters but instead the parameters in the buffer supplied through the pointer buffer. The random number is returned in the variable pointed to by result.
The return value of the function indicates whether the call succeeded. If the value is less than 0 an error occurred and errno is set to indicate the problem.
This function is a GNU extension and should not be used in portable programs.

— Function: int erand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, double *result)

The erand48_r function works like erand48, but in addition it takes an argument buffer which describes the random number generator. The state of the random number generator is taken from the xsubi array, the parameters for the congruential formula from the global random number generator data. The random number is returned in the variable pointed to by result.
The return value is non-negative if the call succeeded.
This function is a GNU extension and should not be used in portable programs.

— Function: int lrand48_r (struct drand48_data *buffer, double *result)

This function is similar to lrand48, but in addition it takes a pointer to a buffer describing the state of the random number generator just like drand48.
If the return value of the function is non-negative the variable pointed to by result contains the result. Otherwise an error occurred.
This function is a GNU extension and should not be used in portable programs.

— Function: int nrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)

The nrand48_r function works like nrand48 in that it produces a random number in the range 0 to 2^31. But instead of using the global parameters for the congruential formula it uses the information from the buffer pointed to by buffer. The state is described by the values in xsubi.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.

— Function: int mrand48_r (struct drand48_data *buffer, double *result)

This function is similar to mrand48 but like the other reentrant functions it uses the random number generator described by the value in the buffer pointed to by buffer.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.

— Function: int jrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)

The jrand48_r function is similar to jrand48. Like the other reentrant functions of this function family it uses the congruential formula parameters from the buffer pointed to by buffer.
If the return value is non-negative the variable pointed to by result contains the result.
This function is a GNU extension and should not be used in portable programs.

Before any of the above functions are used the buffer of type struct drand48_data should be initialized. The easiest way to do this is to fill the whole buffer with null bytes, e.g. by

Using any of the reentrant functions of this family now will automatically initialize the random number generator to the default values for the state and the parameters of the congruential formula.

The other possibility is to use any of the functions which explicitly initialize the buffer. Though it might be obvious how to initialize the buffer from looking at the parameter to the function, it is highly recommended to use these functions since the result might not always be what you expect.

— Function: int srand48_r (long int seedval, struct drand48_data *buffer)

The description of the random number generator represented by the information in buffer is initialized similarly to what the function srand48 does. The state is initialized from the parameter seedval and the parameters for the congruential formula are initialized to their default values.
If the return value is non-negative the function call succeeded.
This function is a GNU extension and should not be used in portable programs.

— Function: int seed48_r (unsigned short int seed16v[3], struct drand48_data *buffer)

This function is similar to srand48_r but like seed48 it initializes all 48 bits of the state from the parameter seed16v.
If the return value is non-negative the function call succeeded. It does not return a pointer to the previous state of the random number generator like the seed48 function does. If the user wants to preserve the state for a later re-run s/he can copy the whole buffer pointed to by buffer.
This function is a GNU extension and should not be used in portable programs.

— Function: int lcong48_r (unsigned short int param[7], struct drand48_data *buffer)

This function initializes all aspects of the random number generator described in buffer with the data in param. Here it is especially true that the function does more than just copying the contents of param and buffer. More work is required and therefore it is important to use this function rather than initializing the random number generator directly.
If the return value is non-negative the function call succeeded.
This function is a GNU extension and should not be used in portable programs.

19.9 Is Fast Code or Small Code preferred?

If an application uses many floating point functions it is often the case that the cost of the function calls themselves is not negligible. Modern processors can often execute the operations themselves very fast, but the function call disrupts the instruction pipeline.

For this reason the GNU C Library provides optimizations for many of the frequently-used math functions. When GNU CC is used and the user activates the optimizer, several new inline functions and macros are defined. These new functions and macros have the same names as the library functions and so are used instead of the latter. In the case of inline functions the compiler will decide whether it is reasonable to use them, and this decision is usually correct.

This means that no calls to the library functions may be necessary, and can increase the speed of generated code significantly. The drawback is that code size will increase, and the increase is not always negligible.

There are two kind of inline functions: Those that give the same result as the library functions and others that might not set errno and might have a reduced precision and/or argument range in comparison with the library functions. The latter inline functions are only available if the flag -ffast-math is given to GNU CC.

In cases where the inline functions and macros are not wanted the symbol __NO_MATH_INLINES should be defined before any system header is included. This will ensure that only library functions are used. Of course, it can be determined for each file in the project whether giving this option is preferable or not.

Not all hardware implements the entire IEEE 754 standard, and even if it does there may be a substantial performance penalty for using some of its features. For example, enabling traps on some processors forces the FPU to run un-pipelined, which can more than double calculation time.

20 Arithmetic Functions

This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts or retrieving the imaginary part of a complex value. These functions are declared in the header files math.h and complex.h.

20.1 Integers

The C language defines several integer data types: integer, short integer, long integer, and character, all in both signed and unsigned varieties. The GNU C compiler extends the language to contain long long integers as well. The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines. The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on.

To address this problem, the GNU C library contains C type definitions you can use to declare integers that meet your exact needs. Because the GNU C library header files are customized to a specific machine, your program source code doesn't have to be.

These typedefs are in stdint.h. If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness:

If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist.

If you don't need a specific storage size, but want the smallest data structure with at least N bits, use one of these:

If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these:

If you want an integer with the widest range possible on the platform on which it is being used, use one of the following. If you use these, you should write code that takes into account the variable size and range of the integer.

The GNU C library also provides macros that tell you the maximum and minimum possible values for each integer data type. The macro names follow these examples: INT32_MAX, UINT8_MAX, INT_FAST32_MIN, INT_LEAST64_MIN, UINTMAX_MAX, INTMAX_MAX, INTMAX_MIN. Note that there are no macros for unsigned integer minima. These are always zero. There are similar macros for use with C's built in integer types which should come with your C compiler. These are described in Data Type Measurements.

Don't forget you can use the C sizeof function with any of these data types to get the number of bytes of storage each uses.

20.2 Integer Division

This section describes functions for performing integer division. These functions are redundant when GNU CC is used, because in GNU C the ‘/’ operator always rounds towards zero. But in other C implementations, ‘/’ may round differently with negative arguments. div and ldiv are useful because they specify how to round the quotient: towards zero. The remainder has the same sign as the numerator.

These functions are specified to return a result r such that the value r.quot*denominator + r.rem equals numerator.

To use these facilities, you should include the header file stdlib.h in your program.

— Data Type: div_t

This is a structure type used to hold the result returned by the div function. It has the following members:

int quot
The quotient from the division.
int rem
The remainder from the division.

— Function: div_t div (int numerator, int denominator)

This function div computes the quotient and remainder from the division of numerator by denominator, returning the result in a structure of type div_t.
If the result cannot be represented (as in a division by zero), the behavior is undefined.
Here is an example, albeit not a very useful one.
          div_t result;
          result = div (20, -6);
Now result.quot is -3 and result.rem is 2.

— Data Type: ldiv_t

This is a structure type used to hold the result returned by the ldiv function. It has the following members:

long int quot
The quotient from the division.
long int rem
The remainder from the division.

(This is identical to div_t except that the components are of type long int rather than int.)

— Function: ldiv_t ldiv (long int numerator, long int denominator)

The ldiv function is similar to div, except that the arguments are of type long int and the result is returned as a structure of type ldiv_t.

— Data Type: lldiv_t

This is a structure type used to hold the result returned by the lldiv function. It has the following members:

long long int quot
The quotient from the division.
long long int rem
The remainder from the division.

(This is identical to div_t except that the components are of type long long int rather than int.)

— Function: lldiv_t lldiv (long long int numerator, long long int denominator)

The lldiv function is like the div function, but the arguments are of type long long int and the result is returned as a structure of type lldiv_t.
The lldiv function was added in ISO C99.

— Data Type: imaxdiv_t

This is a structure type used to hold the result returned by the imaxdiv function. It has the following members:

intmax_t quot
The quotient from the division.
intmax_t rem
The remainder from the division.

(This is identical to div_t except that the components are of type intmax_t rather than int.)
See Integers for a description of the intmax_t type.

— Function: imaxdiv_t imaxdiv (intmax_t numerator, intmax_t denominator)

The imaxdiv function is like the div function, but the arguments are of type intmax_t and the result is returned as a structure of type imaxdiv_t.
See Integers for a description of the intmax_t type.
The imaxdiv function was added in ISO C99.

20.3 Floating Point Numbers

Most computer hardware has support for two different kinds of numbers: integers (...-3, -2, -1, 0, 1, 2, 3...) and floating-point numbers. Floating-point numbers have three parts: the mantissa, the exponent, and the sign bit. The real number represented by a floating-point value is given by (s ? -1 : 1) · 2^e · M where s is the sign bit, e the exponent, and M the mantissa. See Floating Point Concepts, for details. (It is possible to have a different base for the exponent, but all modern hardware uses 2.)

Floating-point numbers can represent a finite subset of the real numbers. While this subset is large enough for most purposes, it is important to remember that the only reals that can be represented exactly are rational numbers that have a terminating binary expansion shorter than the width of the mantissa. Even simple fractions such as 1/5 can only be approximated by floating point.

Mathematical operations and functions frequently need to produce values that are not representable. Often these values can be approximated closely enough for practical purposes, but sometimes they can't. Historically there was no way to tell when the results of a calculation were inaccurate. Modern computers implement the IEEE 754 standard for numerical computations, which defines a framework for indicating to the program when the results of calculation are not trustworthy. This framework consists of a set of exceptions that indicate why a result could not be represented, and the special values infinity and not a number (NaN).

20.4 Floating-Point Number Classification Functions

ISO C99 defines macros that let you determine what sort of floating-point number a variable holds.

— Macro: int fpclassify (float-type x)

This is a generic macro which works on all floating-point types and which returns a value of type int. The possible values are:

FP_NAN
The floating-point number x is “Not a Number” (see Infinity and NaN)
FP_INFINITE
The value of x is either plus or minus infinity (see Infinity and NaN)
FP_ZERO
The value of x is zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if x is negative zero.
FP_SUBNORMAL
Numbers whose absolute value is too small to be represented in the normal format are represented in an alternate, denormalized format (see Floating Point Concepts). This format is less precise but can represent values closer to zero. fpclassify returns this value for values of x in this alternate format.
FP_NORMAL
This value is returned for all other values of x. It indicates that there is nothing special about the number.

fpclassify is most useful if more than one property of a number must be tested. There are more specific macros which only test one property at a time. Generally these macros execute faster than fpclassify, since there is special hardware support for them. You should therefore use the specific macros whenever possible.

— Macro: int isfinite (float-type x)

This macro returns a nonzero value if x is finite: not plus or minus infinity, and not NaN. It is equivalent to
          (fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)
isfinite is implemented as a macro which accepts any floating-point type.

— Macro: int isnormal (float-type x)

This macro returns a nonzero value if x is finite and normalized. It is equivalent to
          (fpclassify (x) == FP_NORMAL)

— Macro: int isnan (float-type x)

This macro returns a nonzero value if x is NaN. It is equivalent to
          (fpclassify (x) == FP_NAN)

Another set of floating-point classification functions was provided by BSD. The GNU C library also supports these functions; however, we recommend that you use the ISO C99 macros in new code. Those are standard and will be available more widely. Also, since they are macros, you do not have to worry about the type of their argument.

— Function: int isinf (double x)

— Function: int isinff (float x)

— Function: int isinfl (long double x)

This function returns -1 if x represents negative infinity, 1 if x represents positive infinity, and 0 otherwise.

— Function: int isnan (double x)

— Function: int isnanf (float x)
— Function: int isnanl (long double x)
This function returns a nonzero value if x is a “not a number” value, and zero otherwise.
NB: The isnan macro defined by ISO C99 overrides the BSD function. This is normally not a problem, because the two routines behave identically. However, if you really need to get the BSD function for some reason, you can write
          (isnan) (x)

— Function: int finite (double x)

— Function: int finitef (float x)

— Function: int finitel (long double x)

This function returns a nonzero value if x is finite or a “not a number” value, and zero otherwise.

20.5 Errors in Floating-Point Calculations

20.5.1 FP Exceptions

The IEEE 754 standard defines five exceptions that can occur during a calculation. Each corresponds to a particular sort of error, such as overflow.

When exceptions occur (when exceptions are raised, in the language of the standard), one of two things can happen. By default the exception is simply noted in the floating-point status word, and the program continues as if nothing had happened. The operation produces a default value, which depends on the exception (see the table below). Your program can check the status word to find out which exceptions happened.

Alternatively, you can enable traps for exceptions. In that case, when an exception is raised, your program will receive the SIGFPE signal. The default action for this signal is to terminate the program. See Signal Handling, for how you can change the effect of the signal.

In the System V math library, the user-defined function matherr is called when certain exceptions occur inside math library functions. However, the Unix98 standard deprecates this interface. We support it for historical compatibility, but recommend that you do not use it in new programs.

20.5.2 Infinity and NaN

IEEE 754 floating point numbers can represent positive or negative infinity, and NaN (not a number). These three values arise from calculations whose result is undefined or cannot be represented accurately. You can also deliberately set a floating-point variable to any of them, which is sometimes useful. Some examples of calculations that produce infinity or NaN:

When a calculation produces any of these values, an exception also occurs; see FP Exceptions.

The basic operations and math functions all accept infinity and NaN and produce sensible output. Infinities propagate through calculations as one would expect: for example, 2 + ∞ = ∞, 4/∞ = 0, atan (∞) = π/2. NaN, on the other hand, infects any calculation that involves it. Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN.

In comparison operations, positive infinity is larger than all values except itself and NaN, and negative infinity is smaller than all values except itself and NaN. NaN is unordered: it is not equal to, greater than, or less than anything, including itself.

x ==
x

is false if the value of x is NaN. You can use this to test whether a value is NaN or not, but the recommended way to test for NaN is with the isnan function (see Floating Point Classes). In addition, <, >, <=, and >= will raise an exception when applied to NaNs.

math.h defines macros that allow you to explicitly set a variable to infinity or NaN.

— Macro: float INFINITY

An expression representing positive infinity. It is equal to the value produced by mathematical operations like 1.0 / 0.0. -INFINITY represents negative infinity.
You can test whether a floating-point value is infinite by comparing it to this macro. However, this is not recommended; you should use the isfinite macro instead. See Floating Point Classes.
This macro was introduced in the ISO C99 standard.

— Macro: float NAN

An expression representing a value which is “not a number”. This macro is a GNU extension, available only on machines that support the “not a number” value—that is to say, on all machines that support IEEE floating point.
You can use ‘#ifdef NAN’ to test whether the machine supports NaN. (Of course, you must arrange for GNU extensions to be visible, such as by defining _GNU_SOURCE, and then you must include math.h.)

IEEE 754 also allows for another unusual value: negative zero. This value is produced when you divide a positive number by negative infinity, or when a negative result is smaller than the limits of representation. Negative zero behaves identically to zero in all calculations, unless you explicitly test the sign bit with signbit or copysign.

20.5.3 Examining the FPU status word

ISO C99 defines functions to query and manipulate the floating-point status word. You can use these functions to check for untrapped exceptions when it's convenient, rather than worrying about them in the middle of a calculation.

These constants represent the various IEEE 754 exceptions. Not all FPUs report all the different exceptions. Each constant is defined if and only if the FPU you are compiling for supports that exception, so you can test for FPU support with ‘#ifdef’. They are defined in fenv.h.

The macro FE_ALL_EXCEPT is the bitwise OR of all exception macros which are supported by the FP implementation.

These functions allow you to clear exception flags, test for exceptions, and save and restore the set of exceptions flagged.

— Function: int feclearexcept (int excepts)

This function clears all of the supported exception flags indicated by excepts.
The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feraiseexcept (int excepts)

This function raises the supported exceptions indicated by excepts. If more than one exception bit in excepts is set the order in which the exceptions are raised is undefined except that overflow (FE_OVERFLOW) or underflow (FE_UNDERFLOW) are raised before inexact (FE_INEXACT). Whether for overflow or underflow the inexact exception is also raised is also implementation dependent.
The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int fetestexcept (int excepts)

Test whether the exception flags indicated by the parameter except are currently set. If any of them are, a nonzero value is returned which specifies which exceptions are set. Otherwise the result is zero.

To understand these functions, imagine that the status word is an integer variable named status. feclearexcept is then equivalent to ‘status &= ~excepts’ and fetestexcept is equivalent to ‘(status & excepts)’. The actual implementation may be very different, of course.

Exception flags are only cleared when the program explicitly requests it, by calling feclearexcept. If you want to check for exceptions from a set of calculations, you should clear all the flags first. Here is a simple example of the way to use fetestexcept:

You cannot explicitly set bits in the status word. You can, however, save the entire status word and restore it later. This is done with the following functions:

— Function: int fegetexceptflag (fexcept_t *flagp, int excepts)

This function stores in the variable pointed to by flagp an implementation-defined value representing the current setting of the exception flags indicated by excepts.
The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int fesetexceptflag (const fexcept_t *flagp, int excepts)

This function restores the flags for the exceptions indicated by excepts to the values stored in the variable pointed to by flagp.
The function returns zero in case the operation was successful, a non-zero value otherwise.

Note that the value stored in fexcept_t bears no resemblance to the bit mask returned by fetestexcept. The type may not even be an integer. Do not attempt to modify an fexcept_t variable.

20.5.4 Error Reporting by Mathematical Functions

Many of the math functions are defined only over a subset of the real or complex numbers. Even if they are mathematically defined, their result may be larger or smaller than the range representable by their return type. These are known as domain errors, overflows, and underflows, respectively. Math functions do several things when one of these errors occurs. In this manual we will refer to the complete response as signalling a domain error, overflow, or underflow.

When a math function suffers a domain error, it raises the invalid exception and returns NaN. It also sets errno to EDOM; this is for compatibility with old systems that do not support IEEE 754 exception handling. Likewise, when overflow occurs, math functions raise the overflow exception and return ∞ or -∞ as appropriate. They also set errno to ERANGE. When underflow occurs, the underflow exception is raised, and zero (appropriately signed) is returned. errno may be set to ERANGE, but this is not guaranteed.

Some of the math functions are defined mathematically to result in a complex value over parts of their domains. The most familiar example of this is taking the square root of a negative number. The complex math functions, such as csqrt, will return the appropriate complex value in this case. The real-valued functions, such as sqrt, will signal a domain error.

Some older hardware does not support infinities. On that hardware, overflows instead return a particular very large number (usually the largest representable number). math.h defines macros you can use to test for overflow on both old and new hardware.

— Macro: double HUGE_VAL

— Macro: float HUGE_VALF

— Macro: long double HUGE_VALL

An expression representing a particular very large number. On machines that use IEEE 754 floating point format, HUGE_VAL is infinity. On other machines, it's typically the largest positive number that can be represented.
Mathematical functions return the appropriately typed version of HUGE_VAL or −HUGE_VAL when the result is too large to be represented.

20.6 Rounding Modes

Floating-point calculations are carried out internally with extra precision, and then rounded to fit into the destination type. This ensures that results are as precise as the input data. IEEE 754 defines four possible rounding modes:

fenv.h defines constants which you can use to refer to the various rounding modes. Each one will be defined if and only if the FPU supports the corresponding rounding mode.

Underflow is an unusual case. Normally, IEEE 754 floating point numbers are always normalized (see Floating Point Concepts). Numbers smaller than 2^r (where r is the minimum exponent, FLT_MIN_RADIX-1 for float) cannot be represented as normalized numbers. Rounding all such numbers to zero or 2^r would cause some algorithms to fail at 0. Therefore, they are left in denormalized form. That produces loss of precision, since some bits of the mantissa are stolen to indicate the decimal point.

If a result is too small to be represented as a denormalized number, it is rounded to zero. However, the sign of the result is preserved; if the calculation was negative, the result is negative zero. Negative zero can also result from some operations on infinity, such as 4/-∞. Negative zero behaves identically to zero except when the copysign or signbit functions are used to check the sign bit directly.

At any time one of the above four rounding modes is selected. You can find out which one with this function:

— Function: int fegetround (void)

Returns the currently selected rounding mode, represented by one of the values of the defined rounding mode macros.

— Function: int fesetround (int round)

Changes the currently selected rounding mode to round. If round does not correspond to one of the supported rounding modes nothing is changed. fesetround returns zero if it changed the rounding mode, a nonzero value if the mode is not supported.

You should avoid changing the rounding mode if possible. It can be an expensive operation; also, some hardware requires you to compile your program differently for it to work. The resulting code may run slower. See your compiler documentation for details.

20.7 Floating-Point Control Functions

IEEE 754 floating-point implementations allow the programmer to decide whether traps will occur for each of the exceptions, by setting bits in the control word. In C, traps result in the program receiving the SIGFPE signal; see Signal Handling.

NB: IEEE 754 says that trap handlers are given details of the exceptional situation, and can set the result value. C signals do not provide any mechanism to pass this information back and forth. Trapping exceptions in C is therefore not very useful.

It is sometimes necessary to save the state of the floating-point unit while you perform some calculation. The library provides functions which save and restore the exception flags, the set of exceptions that generate traps, and the rounding mode. This information is known as the floating-point environment.

The functions to save and restore the floating-point environment all use a variable of type fenv_t to store information. This type is defined in fenv.h. Its size and contents are implementation-defined. You should not attempt to manipulate a variable of this type directly.

— Function: int fegetenv (fenv_t *envp)

Store the floating-point environment in the variable pointed to by envp.
The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feholdexcept (fenv_t *envp)

Store the current floating-point environment in the object pointed to by envp. Then clear all exception flags, and set the FPU to trap no exceptions. Not all FPUs support trapping no exceptions; if feholdexcept cannot set this mode, it returns nonzero value. If it succeeds, it returns zero.

The functions which restore the floating-point environment can take these kinds of arguments:

— Function: int fesetenv (const fenv_t *envp)

Set the floating-point environment to that described by envp.
The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feupdateenv (const fenv_t *envp)

Like fesetenv, this function sets the floating-point environment to that described by envp. However, if any exceptions were flagged in the status word before feupdateenv was called, they remain flagged after the call. In other words, after feupdateenv is called, the status word is the bitwise OR of the previous status word and the one saved in envp.
The function returns zero in case the operation was successful, a non-zero value otherwise.

To control for individual exceptions if raising them causes a trap to occur, you can use the following two functions.

— Function: int feenableexcept (int excepts)

This functions enables traps for each of the exceptions as indicated by the parameter except. The individual excepetions are described in Status bit operations. Only the specified exceptions are enabled, the status of the other exceptions is not changed.
The function returns the previous enabled exceptions in case the operation was successful, -1 otherwise.

— Function: int fedisableexcept (int excepts)

This functions disables traps for each of the exceptions as indicated by the parameter except. The individual excepetions are described in Status bit operations. Only the specified exceptions are disabled, the status of the other exceptions is not changed.
The function returns the previous enabled exceptions in case the operation was successful, -1 otherwise.

— Function: int fegetexcept (int excepts)

The function returns a bitmask of all currently enabled exceptions. It returns -1 in case of failure.

20.8 Arithmetic Functions

The C library provides functions to do basic operations on floating-point numbers. These include absolute value, maximum and minimum, normalization, bit twiddling, rounding, and a few others.

20.8.1 Absolute Value

These functions are provided for obtaining the absolute value (or magnitude) of a number. The absolute value of a real number x is x if x is positive, −x if x is negative. For a complex number z, whose real part is x and whose imaginary part is y, the absolute value is sqrt (x*x + y*y).

Prototypes for abs, labs and llabs are in stdlib.h; imaxabs is declared in inttypes.h; fabs, fabsf and fabsl are declared in math.h. cabs, cabsf and cabsl are declared in complex.h.

— Function: int abs (int number)

— Function: long int labs (long int number)

— Function: long long int llabs (long long int number)

— Function: intmax_t imaxabs (intmax_t number)

These functions return the absolute value of number.
Most computers use a two's complement integer representation, in which the absolute value of INT_MIN (the smallest possible int) cannot be represented; thus, abs (INT_MIN) is not defined.
llabs and imaxdiv are new to ISO C99.
See Integers for a description of the intmax_t type.

— Function: double fabs (double number)

— Function: float fabsf (float number)

— Function: long double fabsl (long double number)

This function returns the absolute value of the floating-point number number.

— Function: double cabs (complex double z)

— Function: float cabsf (complex float z)
— Function: long double cabsl (complex long double z)
These functions return the absolute value of the complex number z (see Complex Numbers). The absolute value of a complex number is:
          sqrt (creal (z) * creal (z) + cimag (z) * cimag (z))
This function should always be used instead of the direct formula because it takes special care to avoid losing precision. It may also take advantage of hardware support for this operation. See hypot in Exponents and Logarithms.

20.8.2 Normalization Functions

The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see Floating Point Concepts. These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.

— Function: double frexp (double value, int *exponent)

— Function: float frexpf (float value, int *exponent)

— Function: long double frexpl (long double value, int *exponent)

These functions are used to split the number value into a normalized fraction and an exponent.
If the argument value is not zero, the return value is value times a power of two, and is always in the range 1/2 (inclusive) to 1 (exclusive). The corresponding exponent is stored in *exponent; the return value multiplied by 2 raised to this exponent equals the original number value.
For example, frexp (12.8, &exponent) returns 0.8 and stores 4 in exponent.
If value is zero, then the return value is zero and zero is stored in *exponent.

— Function: double ldexp (double value, int exponent)

— Function: float ldexpf (float value, int exponent)

— Function: long double ldexpl (long double value, int exponent)

These functions return the result of multiplying the floating-point number value by 2 raised to the power exponent. (It can be used to reassemble floating-point numbers that were taken apart by frexp.)
For example, ldexp (0.8, 4) returns 12.8.

The following functions, which come from BSD, provide facilities equivalent to those of ldexp and frexp. See also the ISO C function logb which originally also appeared in BSD.

— Function: double scalb (double value, int exponent)

— Function: float scalbf (float value, int exponent)

— Function: long double scalbl (long double value, int exponent)

The scalb function is the BSD name for ldexp.

— Function: long long int scalbn (double x, int n)

— Function: long long int scalbnf (float x, int n)

— Function: long long int scalbnl (long double x, int n)

scalbn is identical to scalb, except that the exponent n is an int instead of a floating-point number.

— Function: long long int scalbln (double x, long int n)

— Function: long long int scalblnf (float x, long int n)

— Function: long long int scalblnl (long double x, long int n)

scalbln is identical to scalb, except that the exponent n is a long int instead of a floating-point number.

— Function: long long int significand (double x)

— Function: long long int significandf (float x)

— Function: long long int significandl (long double x)

significand returns the mantissa of x scaled to the range [1, 2). It is equivalent to scalb (x, (double) -ilogb (x)).
This function exists mainly for use in certain standardized tests of IEEE 754 conformance.

20.8.3 Rounding Functions

The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating point numbers to integer values. They are all declared in math.h.

You can also convert floating-point numbers to integers simply by casting them to int. This discards the fractional part, effectively rounding towards zero. However, this only works if the result can actually be represented as an int—for very large numbers, this is impossible. The functions listed here return the result as a double instead to get around this problem.

— Function: double ceil (double x)

— Function: float ceilf (float x)

— Function: long double ceill (long double x)

These functions round x upwards to the nearest integer, returning that value as a double. Thus, ceil (1.5) is 2.0.

— Function: double floor (double x)

— Function: float floorf (float x)

— Function: long double floorl (long double x)

These functions round x downwards to the nearest integer, returning that value as a double. Thus, floor (1.5) is 1.0 and floor (-1.5) is -2.0.

— Function: double trunc (double x)

— Function: float truncf (float x)

— Function: long double truncl (long double x)

The trunc functions round x towards zero to the nearest integer (returned in floating-point format). Thus, trunc (1.5) is 1.0 and trunc (-1.5) is -1.0.

— Function: double rint (double x)

— Function: float rintf (float x)

— Function: long double rintl (long double x)

These functions round x to an integer value according to the current rounding mode. See Floating Point Parameters, for information about the various rounding modes. The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicitly select another.
If x was not initially an integer, these functions raise the inexact exception.

— Function: double nearbyint (double x)

— Function: float nearbyintf (float x)

— Function: long double nearbyintl (long double x)

These functions return the same value as the rint functions, but do not raise the inexact exception if x is not an integer.

— Function: double round (double x)

— Function: float roundf (float x)

— Function: long double roundl (long double x)

These functions are similar to rint, but they round halfway cases away from zero instead of to the nearest integer (or other current rounding mode).

— Function: long int lrint (double x)

— Function: long int lrintf (float x)

— Function: long int lrintl (long double x)

These functions are just like rint, but they return a long int instead of a floating-point number.

— Function: long long int llrint (double x)

— Function: long long int llrintf (float x)

— Function: long long int llrintl (long double x)

These functions are just like rint, but they return a long long int instead of a floating-point number.

— Function: long int lround (double x)

— Function: long int lroundf (float x)

— Function: long int lroundl (long double x)

These functions are just like round, but they return a long int instead of a floating-point number.

— Function: long long int llround (double x)

— Function: long long int llroundf (float x)

— Function: long long int llroundl (long double x)

These functions are just like round, but they return a long long int instead of a floating-point number.

— Function: double modf (double value, double *integer-part)

— Function: float modff (float value, float *integer-part)

— Function: long double modfl (long double value, long double *integer-part)

These functions break the argument value into an integer part and a fractional part (between -1 and 1, exclusive). Their sum equals value. Each of the parts has the same sign as value, and the integer part is always rounded toward zero.
modf stores the integer part in *integer-part, and returns the fractional part. For example, modf (2.5, &intpart) returns 0.5 and stores 2.0 into intpart.

20.8.4 Remainder Functions

The functions in this section compute the remainder on division of two floating-point numbers. Each is a little different; pick the one that suits your problem.

— Function: double fmod (double numerator, double denominator)

— Function: float fmodf (float numerator, float denominator)

— Function: long double fmodl (long double numerator, long double denominator)

These functions compute the remainder from the division of numerator by denominator. Specifically, the return value is numerator - n * denominator, where n is the quotient of numerator divided by denominator, rounded towards zero to an integer. Thus, fmod (6.5, 2.3) returns 1.9, which is 6.5 minus 4.6.
The result has the same sign as the numerator and has magnitude less than the magnitude of the denominator.
If denominator is zero, fmod signals a domain error.

— Function: double drem (double numerator, double denominator)

— Function: float dremf (float numerator, float denominator)

— Function: long double dreml (long double numerator, long double denominator)

These functions are like fmod except that they round the internal quotient n to the nearest integer instead of towards zero to an integer. For example, drem (6.5, 2.3) returns -0.4, which is 6.5 minus 6.9.
The absolute value of the result is less than or equal to half the absolute value of the denominator. The difference between fmod (numerator, denominator) and drem (numerator, denominator) is always either denominator, minus denominator, or zero.
If denominator is zero, drem signals a domain error.

— Function: double remainder (double numerator, double denominator)

— Function: float remainderf (float numerator, float denominator)

— Function: long double remainderl (long double numerator, long double denominator)

This function is another name for drem.

20.8.5 Setting and modifying single bits of FP values

There are some operations that are too complicated or expensive to perform by hand on floating-point numbers. ISO C99 defines functions to do these operations, which mostly involve changing single bits.

— Function: double copysign (double x, double y)

— Function: float copysignf (float x, float y)

— Function: long double copysignl (long double x, long double y)

These functions return x but with the sign of y. They work even if x or y are NaN or zero. Both of these can carry a sign (although not all implementations support it) and this is one of the few operations that can tell the difference.
copysign never raises an exception.
This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

— Function: int signbit (float-type x)

signbit is a generic macro which can work on all floating-point types. It returns a nonzero value if the value of x has its sign bit set.
This is not the same as x < 0.0, because IEEE 754 floating point allows zero to be signed. The comparison -0.0 < 0.0 is false, but signbit (-0.0) will return a nonzero value.

— Function: double nextafter (double x, double y)

— Function: float nextafterf (float x, float y)

— Function: long double nextafterl (long double x, long double y)

The nextafter function returns the next representable neighbor of x in the direction towards y. The size of the step between x and the result depends on the type of the result. If x = y the function simply returns y. If either value is NaN, NaN is returned. Otherwise a value corresponding to the value of the least significant bit in the mantissa is added or subtracted, depending on the direction. nextafter will signal overflow or underflow if the result goes outside of the range of normalized numbers.
This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

— Function: double nexttoward (double x, long double y)

— Function: float nexttowardf (float x, long double y)

— Function: long double nexttowardl (long double x, long double y)

These functions are identical to the corresponding versions of nextafter except that their second argument is a long double.

— Function: double nan (const char *tagp)

— Function: float nanf (const char *tagp)

— Function: long double nanl (const char *tagp)

The nan function returns a representation of NaN, provided that NaN is supported by the target platform. nan ("n-char-sequence") is equivalent to strtod ("NAN(n-char-sequence)").
The argument tagp is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and tagp selects one. On other systems it may do nothing.

20.8.6 Floating-Point Comparison Functions

The standard C comparison operators provoke exceptions when one or other of the operands is NaN. For example,

will raise an exception if a is NaN. (This does not happen with == and !=; those merely return false and true, respectively, when NaN is examined.) Frequently this exception is undesirable. ISO C99 therefore defines comparison functions that do not raise exceptions when NaN is examined. All of the functions are implemented as macros which allow their arguments to be of any floating-point type. The macros are guaranteed to evaluate their arguments only once.

— Macro: int isgreater (real-floating x, real-floating y)

This macro determines whether the argument x is greater than y. It is equivalent to (x) > (y), but no exception is raised if x or y are NaN.

— Macro: int isgreaterequal (real-floating x, real-floating y)

This macro determines whether the argument x is greater than or equal to y. It is equivalent to (x) >= (y), but no exception is raised if x or y are NaN.

— Macro: int isless (real-floating x, real-floating y)

This macro determines whether the argument x is less than y. It is equivalent to (x) < (y), but no exception is raised if x or y are NaN.

— Macro: int islessequal (real-floating x, real-floating y)

This macro determines whether the argument x is less than or equal to y. It is equivalent to (x) <= (y), but no exception is raised if x or y are NaN.

— Macro: int islessgreater (real-floating x, real-floating y)

This macro determines whether the argument x is less or greater than y. It is equivalent to (x) < (y) || (x) > (y) (although it only evaluates x and y once), but no exception is raised if x or y are NaN.
This macro is not equivalent to x != y, because that expression is true if x or y are NaN.

— Macro: int isunordered (real-floating x, real-floating y)

This macro determines whether its arguments are unordered. In other words, it is true if x or y are NaN, and false otherwise.

Not all machines provide hardware support for these operations. On machines that don't, the macros can be very slow. Therefore, you should not use these functions when NaN is not a concern.

NB: There are no macros isequal or isunequal. They are unnecessary, because the == and != operators do not throw an exception if one or both of the operands are NaN.

20.8.7 Miscellaneous FP arithmetic functions

The functions in this section perform miscellaneous but common operations that are awkward to express with C operators. On some processors these functions can use special machine instructions to perform these operations faster than the equivalent C code.

— Function: double fmin (double x, double y)

— Function: float fminf (float x, float y)
— Function: long double fminl (long double x, long double y)
The fmin function returns the lesser of the two values x and y. It is similar to the expression
          ((x) < (y) ? (x) : (y))
except that x and y are only evaluated once.
If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.

— Function: double fmax (double x, double y)

— Function: float fmaxf (float x, float y)

— Function: long double fmaxl (long double x, long double y)

The fmax function returns the greater of the two values x and y.
If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.

— Function: double fdim (double x, double y)

— Function: float fdimf (float x, float y)

— Function: long double fdiml (long double x, long double y)

The fdim function returns the positive difference between x and y. The positive difference is x - y if x is greater than y, and 0 otherwise.
If x, y, or both are NaN, NaN is returned.

— Function: double fma (double x, double y, double z)

— Function: float fmaf (float x, float y, float z)

— Function: long double fmal (long double x, long double y, long double z)

The fma function performs floating-point multiply-add. This is the operation (x · y) + z, but the intermediate result is not rounded to the destination type. This can sometimes improve the precision of a calculation.
This function was introduced because some processors have a special instruction to perform multiply-add. The C compiler cannot use it directly, because the expression ‘x*y + z’ is defined to round the intermediate result. fma lets you choose when you want to round only once.
On processors which do not implement multiply-add in hardware, fma can be very slow since it must avoid intermediate rounding. math.h defines the symbols FP_FAST_FMA, FP_FAST_FMAF, and FP_FAST_FMAL when the corresponding version of fma is no slower than the expression ‘x*y + z’. In the GNU C library, this always means the operation is implemented in hardware.

20.9 Complex Numbers

ISO C99 introduces support for complex numbers in C. This is done with a new type qualifier, complex. It is a keyword if and only if complex.h has been included. There are three complex types, corresponding to the three real types: float complex, double complex, and long double complex.

To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, complex.h defines two macros that can be used to create complex numbers.

— Macro: const float complex _Complex_I

This macro is a representation of the complex number “0+1i”. Multiplying a real floating-point value by _Complex_I gives a complex number whose value is purely imaginary. You can use this to construct complex constants:
          3.0 + 4.0i = 3.0 + 4.0 * _Complex_I
Note that _Complex_I * _Complex_I has the value -1, but the type of that value is complex.

_Complex_I is a bit of a mouthful. complex.h also defines a shorter name for the same constant.

— Macro: const float complex I

This macro has exactly the same value as _Complex_I. Most of the time it is preferable. However, it causes problems if you want to use the identifier I for something else. You can safely write
          #include <complex.h>
          #undef I
if you need I for your own purposes. (In that case we recommend you also define some other short name for _Complex_I, such as J.)

20.10 Projections, Conjugates, and Decomposing of Complex Numbers

ISO C99 also defines functions that perform basic operations on complex numbers, such as decomposition and conjugation. The prototypes for all these functions are in complex.h. All functions are available in three variants, one for each of the three complex types.

— Function: double creal (complex double z)

— Function: float crealf (complex float z)

— Function: long double creall (complex long double z)

These functions return the real part of the complex number z.

— Function: double cimag (complex double z)

— Function: float cimagf (complex float z)

— Function: long double cimagl (complex long double z)

These functions return the imaginary part of the complex number z.

— Function: complex double conj (complex double z)

— Function: complex float conjf (complex float z)

— Function: complex long double conjl (complex long double z)

These functions return the conjugate value of the complex number z. The conjugate of a complex number has the same real part and a negated imaginary part. In other words, ‘conj(a + bi) = a + -bi’.

— Function: double carg (complex double z)

— Function: float cargf (complex float z)

— Function: long double cargl (complex long double z)

These functions return the argument of the complex number z. The argument of a complex number is the angle in the complex plane between the positive real axis and a line passing through zero and the number. This angle is measured in the usual fashion and ranges from 0 to 2π.
carg has a branch cut along the positive real axis.

— Function: complex double cproj (complex double z)

— Function: complex float cprojf (complex float z)
— Function: complex long double cprojl (complex long double z)
These functions return the projection of the complex value z onto the Riemann sphere. Values with a infinite imaginary part are projected to positive infinity on the real axis, even if the real part is NaN. If the real part is infinite, the result is equivalent to
          INFINITY + I * copysign (0.0, cimag (z))

20.11 Parsing of Numbers

This section describes functions for “reading” integer and floating-point numbers from a string. It may be more convenient in some cases to use sscanf or one of the related functions; see Formatted Input. But often you can make a program more robust by finding the tokens in the string by hand, then converting the numbers one by one.

20.11.1 Parsing of Integers

The ‘str’ functions are declared in stdlib.h and those beginning with ‘wcs’ are declared in wchar.h. One might wonder about the use of restrict in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well.

— Function: long int strtol (const char *restrict string, char **restrict tailptr, int base)

The strtol (“string-to-long”) function converts the initial part of string to a signed integer, which is returned as a value of type long int.
This function attempts to decompose string as follows:

A (possibly empty) sequence of whitespace characters. Which characters are whitespace is determined by the isspace function (see Classification of Characters). These are discarded.
An optional plus or minus sign (‘+’ or ‘-’).
A nonempty sequence of digits in the radix specified by base.
If base is zero, decimal radix is assumed unless the series of digits begins with ‘0’ (specifying octal radix), or ‘0x’ or ‘0X’ (specifying hexadecimal radix); in other words, the same syntax used for integer constants in C.
Otherwise base must have a value between 2 and 36. If base is 16, the digits may optionally be preceded by ‘0x’ or ‘0X’. If base has no legal value the value returned is 0l and the global variable errno is set to EINVAL.
Any remaining characters in the string. If tailptr is not a null pointer, strtol stores a pointer to this tail in *tailptr.

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for an integer in the specified base, no conversion is performed. In this case, strtol returns a value of zero and the value stored in *tailptr is the value of string.
In a locale other than the standard "C" locale, this function may recognize additional implementation-dependent syntax.
If the string has valid syntax for an integer but the value is not representable because of overflow, strtol returns either LONG_MAX or LONG_MIN (see Range of Type), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.
You should not check for errors by examining the return value of strtol, because the string might be a valid representation of 0l, LONG_MAX, or LONG_MIN. Instead, check whether tailptr points to what you expect after the number (e.g. '\0' if the string should end after the number). You also need to clear errno before the call and check it afterward, in case there was overflow.
There is an example at the end of this section.

— Function: long int wcstol (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstol function is equivalent to the strtol function in nearly all aspects but handles wide character strings.
The wcstol function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long int strtoul (const char *retrict string, char **restrict tailptr, int base)

The strtoul (“string-to-unsigned-long”) function is like strtol except it converts to an unsigned long int value. The syntax is the same as described above for strtol. The value returned on overflow is ULONG_MAX (see Range of Type).
If string depicts a negative number, strtoul acts the same as strtol but casts the result to an unsigned integer. That means for example that strtoul on "-1" returns ULONG_MAX and an input more negative than LONG_MIN returns (ULONG_MAX + 1) / 2.
strtoul sets errno to EINVAL if base is out of range, or ERANGE on overflow.

— Function: unsigned long int wcstoul (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoul function is equivalent to the strtoul function in nearly all aspects but handles wide character strings.
The wcstoul function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoll (const char *restrict string, char **restrict tailptr, int base)

The strtoll function is like strtol except that it returns a long long int value, and accepts numbers with a correspondingly larger range.
If the string has valid syntax for an integer but the value is not representable because of overflow, strtoll returns either LONG_LONG_MAX or LONG_LONG_MIN (see Range of Type), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.
The strtoll function was introduced in ISO C99.

— Function: long long int wcstoll (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoll function is equivalent to the strtoll function in nearly all aspects but handles wide character strings.
The wcstoll function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoq (const char *restrict string, char **restrict tailptr, int base)

strtoq (“string-to-quad-word”) is the BSD name for strtoll.

— Function: long long int wcstoq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoq function is equivalent to the strtoq function in nearly all aspects but handles wide character strings.
The wcstoq function is a GNU extension.

— Function: unsigned long long int strtoull (const char *restrict string, char **restrict tailptr, int base)

The strtoull function is related to strtoll the same way strtoul is related to strtol.
The strtoull function was introduced in ISO C99.

— Function: unsigned long long int wcstoull (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoull function is equivalent to the strtoull function in nearly all aspects but handles wide character strings.
The wcstoull function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long long int strtouq (const char *restrict string, char **restrict tailptr, int base)

strtouq is the BSD name for strtoull.

— Function: unsigned long long int wcstouq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstouq function is equivalent to the strtouq function in nearly all aspects but handles wide character strings.
The wcstouq function is a GNU extension.

— Function: intmax_t strtoimax (const char *restrict string, char **restrict tailptr, int base)

The strtoimax function is like strtol except that it returns a intmax_t value, and accepts numbers of a corresponding range.
If the string has valid syntax for an integer but the value is not representable because of overflow, strtoimax returns either INTMAX_MAX or INTMAX_MIN (see Integers), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.
See Integers for a description of the intmax_t type. The strtoimax function was introduced in ISO C99.

— Function: intmax_t wcstoimax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoimax function is equivalent to the strtoimax function in nearly all aspects but handles wide character strings.
The wcstoimax function was introduced in ISO C99.

— Function: uintmax_t strtoumax (const char *restrict string, char **restrict tailptr, int base)

The strtoumax function is related to strtoimax the same way that strtoul is related to strtol.
See Integers for a description of the intmax_t type. The strtoumax function was introduced in ISO C99.

— Function: uintmax_t wcstoumax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoumax function is equivalent to the strtoumax function in nearly all aspects but handles wide character strings.
The wcstoumax function was introduced in ISO C99.

— Function: long int atol (const char *string)

This function is similar to the strtol function with a base argument of 10, except that it need not detect overflow errors. The atol function is provided mostly for compatibility with existing code; using strtol is more robust.

— Function: int atoi (const char *string)

This function is like atol, except that it returns an int. The atoi function is also considered obsolete; use strtol instead.

— Function: long long int atoll (const char *string)

This function is similar to atol, except it returns a long long int.
The atoll function was introduced in ISO C99. It too is obsolete (despite having just been added); use strtoll instead.

All the functions mentioned in this section so far do not handle alternative representations of characters as described in the locale data. Some locales specify thousands separator and the way they have to be used which can help to make large numbers more readable. To read such numbers one has to use the scanf functions with the ‘'’ flag.

Here is a function which parses a string as a sequence of integers and returns the sum of them:

20.11.2 Parsing of Floats

— Function: double strtod (const char *restrict string, char **restrict tailptr)

The strtod (“string-to-double”) function converts the initial part of string to a floating-point number, which is returned as a value of type double.
This function attempts to decompose string as follows:

A (possibly empty) sequence of whitespace characters. Which characters are whitespace is determined by the isspace function (see Classification of Characters). These are discarded.
An optional plus or minus sign (‘+’ or ‘-’).
A floating point number in decimal or hexadecimal format. The decimal format is:

A nonempty sequence of digits optionally containing a decimal-point character—normally ‘.’, but it depends on the locale (see General Numeric).
An optional exponent part, consisting of a character ‘e’ or ‘E’, an optional sign, and a sequence of digits.

The hexadecimal format is as follows:

A 0x or 0X followed by a nonempty sequence of hexadecimal digits optionally containing a decimal-point character—normally ‘.’, but it depends on the locale (see General Numeric).
An optional binary-exponent part, consisting of a character ‘p’ or ‘P’, an optional sign, and a sequence of digits.

Any remaining characters in the string. If tailptr is not a null pointer, a pointer to this tail of the string is stored in *tailptr.

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for a floating-point number, no conversion is performed. In this case, strtod returns a value of zero and the value returned in *tailptr is the value of string.
In a locale other than the standard "C" or "POSIX" locales, this function may recognize additional locale-dependent syntax.
If the string has valid syntax for a floating-point number but the value is outside the range of a double, strtod will signal overflow or underflow as described in Math Error Reporting.
strtod recognizes four special input strings. The strings "inf" and "infinity" are converted to ∞, or to the largest representable value if the floating-point format doesn't support infinities. You can prepend a "+" or "-" to specify the sign. Case is ignored when scanning these strings.
The strings "nan" and "nan(chars...)" are converted to NaN. Again, case is ignored. If chars... are provided, they are used in some unspecified fashion to select a particular representation of NaN (there can be several).
Since zero is a valid result as well as the value returned on error, you should check for errors in the same way as for strtol, by examining errno and tailptr.

— Function: float strtof (const char *string, char **tailptr)

— Function: long double strtold (const char *string, char **tailptr)

These functions are analogous to strtod, but return float and long double values respectively. They report errors in the same way as strtod. strtof can be substantially faster than strtod, but has less precision; conversely, strtold can be much slower but has more precision (on systems where long double is a separate type).
These functions have been GNU extensions and are new to ISO C99.

— Function: double wcstod (const wchar_t *restrict string, wchar_t **restrict tailptr)

— Function: float wcstof (const wchar_t *string, wchar_t **tailptr)

— Function: long double wcstold (const wchar_t *string, wchar_t **tailptr)

The wcstod, wcstof, and wcstol functions are equivalent in nearly all aspect to the strtod, strtof, and strtold functions but it handles wide character string.
The wcstod function was introduced in Amendment 1 of ISO C90. The wcstof and wcstold functions were introduced in ISO C99.

— Function: double atof (const char *string)

This function is similar to the strtod function, except that it need not detect overflow and underflow errors. The atof function is provided mostly for compatibility with existing code; using strtod is more robust.

The GNU C library also provides ‘_l’ versions of these functions, which take an additional argument, the locale to use in conversion. See Parsing of Integers.

20.12 Old-fashioned System V number-to-string functions

The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics. The GNU C library also provides these functions and some natural extensions.

These functions are only available in glibc and on systems descended from AT&T Unix. Therefore, unless these functions do precisely what you need, it is better to use sprintf, which is standard.

— Function: char * ecvt (double value, int ndigit, int *decpt, int *neg)

The function ecvt converts the floating-point number value to a string with at most ndigit decimal digits. The returned string contains no decimal point or sign. The first digit of the string is non-zero (unless value is actually zero) and the last digit is rounded to nearest. *decpt is set to the index in the string of the first digit after the decimal point. *neg is set to a nonzero value if value is negative, zero otherwise.
If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call to ecvt.
If value is zero, it is implementation defined whether *decpt is 0 or 1.
For example: ecvt (12.3, 5, &d, &n) returns "12300" and sets d to 2 and n to 0.

— Function: char * fcvt (double value, int ndigit, int *decpt, int *neg)

The function fcvt is like ecvt, but ndigit specifies the number of digits after the decimal point. If ndigit is less than zero, value is rounded to the ndigit+1'th place to the left of the decimal point. For example, if ndigit is -1, value will be rounded to the nearest 10. If ndigit is negative and larger than the number of digits to the left of the decimal point in value, value will be rounded to one significant digit.
If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call to fcvt.

— Function: char * gcvt (double value, int ndigit, char *buf)

gcvt is functionally equivalent to ‘sprintf(buf, "%*g", ndigit, value’. It is provided only for compatibility's sake. It returns buf.
If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.

As extensions, the GNU C library provides versions of these three functions that take long double arguments.

— Function: char * qecvt (long double value, int ndigit, int *decpt, int *neg)

This function is equivalent to ecvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

— Function: char * qfcvt (long double value, int ndigit, int *decpt, int *neg)

This function is equivalent to fcvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

— Function: char * qgcvt (long double value, int ndigit, char *buf)

This function is equivalent to gcvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

The ecvt and fcvt functions, and their long double equivalents, all return a string located in a static buffer which is overwritten by the next call to the function. The GNU C library provides another set of extended functions which write the converted string into a user-supplied buffer. These have the conventional _r suffix.

— Function: int ecvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The ecvt_r function is the same as ecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.
This function is a GNU extension.

— Function: int fcvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The fcvt_r function is the same as fcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.
This function is a GNU extension.

— Function: int qecvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The qecvt_r function is the same as qecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.
This function is a GNU extension.

— Function: int qfcvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The qfcvt_r function is the same as qfcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.
This function is a GNU extension.

21 Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations.

21.1 Time Basics

Discussing time in a technical manual can be difficult because the word “time” in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word “time” for is to talk about the abstract concept.

A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called “absolute time”. We don't speak of a “date”, because that is inherent in a calendar time. An interval is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980. An elapsed time is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word “interval” to refer to the elapsed time of some interval. An amount of time is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.

A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events. CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process. Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above.

21.2 Elapsed Time

One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times. This function is declared in time.h.

— Function: double difftime (time_t time1, time_t time0)

The difftime function returns the number of seconds of elapsed time between calendar time time1 and calendar time time0, as a value of type double. The difference ignores leap seconds unless leap second support is enabled.
In the GNU system, you can simply subtract time_t values. But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

The GNU C library provides two data types specifically for representing an elapsed time. They are used by various GNU C library functions, and you can use them for your own purposes too. They're exactly the same except that one has a resolution in microseconds, and the other, newer one, is in nanoseconds.

— Data Type: struct timeval

The struct timeval structure represents an elapsed time. It is declared in sys/time.h and has the following members:

long int tv_sec
This represents the number of whole seconds of elapsed time.
long int tv_usec
This is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds. It is always less than one million.

— Data Type: struct timespec

The struct timespec structure represents an elapsed time. It is declared in time.h and has the following members:

long int tv_sec
This represents the number of whole seconds of elapsed time.
long int tv_nsec
This is the rest of the elapsed time (a fraction of a second), represented as the number of nanoseconds. It is always less than one billion.

It is often necessary to subtract two values of type struct timeval or struct timespec. Here is the best way to do this. It works even on some peculiar operating systems where the tv_sec member has an unsigned type.

Common functions that use struct timeval are gettimeofday and settimeofday.

There are no GNU C library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time, and alarm and sleeping functions have a lot to do with them.

21.3 Processor And CPU Time

If you're trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses. For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section.

CPU time (see Time Basics) is represented by the data type clock_t, which is a number of clock ticks. It gives the total amount of time a process has actively used a CPU since some arbitrary event. On the GNU system, that event is the creation of the process. While arbitrary in general, the event is always the same event for any particular process, so you can always measure how much time on the CPU a particular computation takes by examining the process' CPU time before and after the computation. In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the macro CLOCKS_PER_SEC can be either integer or floating-point types. Casting CPU time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Note that the clock can wrap around. On a 32bit system with CLOCKS_PER_SEC set to one million this function will return the same value approximately every 72 minutes.

For additional functions to examine a process' use of processor time, and to control it, see Resource Usage And Limitation.

21.3.1 CPU Time Inquiry

To get a process' CPU time, you can use the clock function. This facility is declared in the header file time.h. In typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values, and then divide by CLOCKS_PER_SEC (the number of clock ticks per second) to get processor time, like this:

Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. See Processor Time.

Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.

— Macro: int CLOCKS_PER_SEC

The value of this macro is the number of clock ticks per second measured by the clock function. POSIX requires that this value be one million independent of the actual resolution.

— Macro: int CLK_TCK

This is an obsolete name for CLOCKS_PER_SEC.

— Data Type: clock_t

This is the type of the value returned by the clock function. Values of type clock_t are numbers of clock ticks.

— Function: clock_t clock (void)

This function returns the calling process' current CPU time. If the CPU time is not available or cannot be represented, clock returns the value (clock_t)(-1).

21.3.2 Processor Time Inquiry

The times function returns information about a process' consumption of processor time in a struct tms object, in addition to the process' CPU time. See Time Basics. You should include the header file sys/times.h to use this facility.

— Data Type: struct tms

The tms structure is used to return information about process times. It contains at least the following members:

clock_t tms_utime
This is the total processor time the calling process has used in executing the instructions of its program.
clock_t tms_stime
This is the processor time the system has used on behalf of the calling process.
clock_t tms_cutime
This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid; see Process Completion. In other words, it represents the total processor time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by wait or waitpid.
clock_t tms_cstime
This is similar to tms_cutime, but represents the total processor time system has used on behalf of all the terminated child processes of the calling process.

All of the times are given in numbers of clock ticks. Unlike CPU time, these are the actual amounts of time; not relative to any event. See Creating a Process.

— Function: clock_t times (struct tms *buffer)

The times function stores the processor time information for the calling process in buffer.
The return value is the calling process' CPU time (the same value you get from clock(). times returns (clock_t)(-1) to indicate failure.

Portability Note: The clock function described in CPU Time is specified by the ISO C standard. The times function is a feature of POSIX.1. In the GNU system, the CPU time is defined to be equivalent to the sum of the tms_utime and tms_stime fields returned by times.

21.4 Calendar Time

This section describes facilities for keeping track of calendar time. See Time Basics.

21.4.1 Simple Calendar Time

This section describes the time_t data type for representing calendar time as simple time, and the functions which operate on simple time objects. These facilities are declared in the header file time.h.

— Data Type: time_t

This is the data type used to represent simple time. Sometimes, it also represents an elapsed time. When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This calendar time is sometimes referred to as the epoch.) POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set TZ to certain values (see TZ Variable).
Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is.
In the GNU C library, time_t is equivalent to long int. In other systems, time_t might be either an integer or floating-point type.

The function difftime tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting. See Elapsed Time.

— Function: time_t time (time_t *result)

The time function returns the current calendar time as a value of type time_t. If the argument result is not a null pointer, the calendar time value is also stored in *result. If the current calendar time is not available, the value (time_t)(-1) is returned.

— Function: int stime (time_t *newtime)

stime sets the system clock, i.e., it tells the system that the current calendar time is newtime, where newtime is interpreted as described in the above definition of time_t.
settimeofday is a newer function which sets the system clock to better than one second precision. settimeofday is generally a better choice than stime. See High-Resolution Calendar.
Only the superuser can set the system clock.
If the function succeeds, the return value is zero. Otherwise, it is -1 and errno is set accordingly:

EPERM
The process is not superuser.

21.4.2 High-Resolution Calendar

The time_t data type used to represent simple times has a resolution of only one second. Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in sys/time.h.

— Data Type: struct timezone

The struct timezone structure is used to hold minimal information about the local time zone. It has the following members:

int tz_minuteswest
This is the number of minutes west of UTC.
int tz_dsttime
If nonzero, Daylight Saving Time applies during some part of the year.

The struct timezone type is obsolete and should never be used. Instead, use the facilities described in Time Zone Functions.

— Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)

The gettimeofday function returns the current calendar time as the elapsed time since the epoch in the struct timeval structure indicated by tp. (see Elapsed Time for a description of struct timeval). Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.
The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

ENOSYS
The operating system does not support getting time zone information, and tzp is not a null pointer. The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in Time Zone Functions.

— Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)

The settimeofday function sets the current calendar time in the system clock according to the arguments. As for gettimeofday, the calendar time is represented as the elapsed time since the epoch. As for gettimeofday, time zone information is ignored if tzp is a null pointer.
You must be a privileged user in order to use settimeofday.
Some kernels automatically set the system clock from some source such as a hardware clock when they start up. Others, including Linux, place the system clock in an “invalid” state (in which attempts to read the clock fail). A call of stime removes the system clock from an invalid state, and system startup scripts typically run a program that calls stime.
settimeofday causes a sudden jump forwards or backwards, which can cause a variety of problems in a system. Use adjtime (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock.
With a Linux kernel, adjtimex does the same thing and can also make permanent changes to the speed of the system clock so it doesn't need to be corrected as often.
The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
This process cannot set the clock because it is not privileged.
ENOSYS
The operating system does not support setting time zone information, and tzp is not a null pointer.

— Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)

This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock.
The delta argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while.
If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.
This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.
With a Linux kernel, you can use the adjtimex function to permanently change the clock speed.
The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM
You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday, and adjtime functions are derived from BSD.

— Function: int adjtimex (struct timex *timex)

adjtimex is functionally identical to ntp_adjtime. See High Accuracy Clock.
This function is present only with a Linux kernel.

21.4.3 Broken-down Time

Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information.

A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.

— Data Type: struct tm

This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order.

int tm_sec
This is the number of full seconds since the top of the minute (normally in the range 0 through 59, but the actual upper limit is 60, to allow for leap seconds if leap second support is available).
int tm_min
This is the number of full minutes since the top of the hour (in the range 0 through 59).
int tm_hour
This is the number of full hours past midnight (in the range 0 through 23).
int tm_mday
This is the ordinal day of the month (in the range 1 through 31). Watch out for this one! As the only ordinal number in the structure, it is inconsistent with the rest of the structure.
int tm_mon
This is the number of full calendar months since the beginning of the year (in the range 0 through 11). Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
This is the number of full calendar years since 1900.
int tm_wday
This is the number of full days since Sunday (in the range 0 through 6).
int tm_yday
This is the number of full days since the beginning of the year (in the range 0 through 365).
int tm_isdst
This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.
long int tm_gmtoff
This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is -5*60*60. The tm_gmtoff field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.
const char *tm_zone
This field is the name for the time zone that was used to compute this broken-down time value. Like tm_gmtoff, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

— Function: struct tm * localtime (const time_t *time)

The localtime function converts the simple time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.
The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to ctime, gmtime, or localtime. (But no other library function overwrites the contents of this object.)
The return value is the null pointer if time cannot be represented as a broken-down time; typically this is because the year cannot fit into an int.
Calling localtime has one other effect: it sets the variable tzname with information about the current time zone. See Time Zone Functions.

Using the localtime function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a variant of this function.

— Function: struct tm * localtime_r (const time_t *time, struct tm *resultp)

The localtime_r function works just like the localtime function. It takes a pointer to a variable containing a simple time and converts it to the broken-down time format.
But the result is not placed in a static buffer. Instead it is placed in the object of type struct tm to which the parameter resultp points.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

— Function: struct tm * gmtime (const time_t *time)

This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)) rather than relative to a local time zone.

As for the localtime function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for gmtime.

— Function: struct tm * gmtime_r (const time_t *time, struct tm *resultp)

This function is similar to localtime_r, except that it converts just like gmtime the given time as Coordinated Universal Time.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

— Function: time_t mktime (struct tm *brokentime)

The mktime function is used to convert a broken-down time structure to a simple time representation. It also “normalizes” the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.
The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure. It uses the values of the other components to determine the calendar time; it's permissible for these components to have unnormalized values outside their normal ranges. The last thing that mktime does is adjust the components of the brokentime structure (including the tm_wday and tm_yday).
If the specified broken-down time cannot be represented as a simple time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.
Calling mktime also sets the variable tzname with information about the current time zone. See Time Zone Functions.

— Function: time_t timelocal (struct tm *brokentime)

timelocal is functionally identical to mktime, but more mnemonically named. Note that it is the inverse of the localtime function.
Portability note: mktime is essentially universally available. timelocal is rather rare.

— Function: time_t timegm (struct tm *brokentime)

timegm is functionally identical to mktime except it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting.
Note that timegm is the inverse of gmtime.
Portability note: mktime is essentially universally available. timegm is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the TZ environment variable to UTC, call mktime, then set TZ back.

21.4.4 High Accuracy Clock

The ntp_gettime and ntp_adjtime functions provide an interface to monitor and manipulate the system clock to maintain high accuracy time. For example, you can fine tune the speed of the clock or synchronize it with another time source.

A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks.

— Data Type: struct ntptimeval

This structure is used for information about the system clock. It contains the following members:

struct timeval time
This is the current calendar time, expressed as the elapsed time since the epoch. The struct timeval data type is described in Elapsed Time.
long int maxerror
This is the maximum error, measured in microseconds. Unless updated via ntp_adjtime periodically, this value will reach some platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can be set by ntp_adjtime to indicate the estimated offset of the system clock from the true calendar time.

— Function: int ntp_gettime (struct ntptimeval *tptr)

The ntp_gettime function sets the structure pointed to by tptr to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a ntp_adjtime call is necessary.
The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR
The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.

— Data Type: struct timex

This structure is used to control and monitor the system clock. It contains the following members:

unsigned int modes
This variable controls whether and which values are set. Several symbolic constants have to be combined with binary or to specify the effective mode. These constants start with MOD_.
long int offset
This value indicates the current offset of the system clock from the true calendar time. The value is given in microseconds. If bit MOD_OFFSET is set in modes, the offset (and possibly other dependent values) can be set. The offset's absolute value must not exceed MAXPHASE.
long int frequency
This value indicates the difference in frequency between the true calendar time and the system clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is 1 << SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but the absolute value must not exceed MAXFREQ.
long int maxerror
This is the maximum error, measured in microseconds. A new value can be set using bit MOD_MAXERROR. Unless updated via ntp_adjtime periodically, this value will increase steadily and reach some platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can be set using bit MOD_ESTERROR.
int status
This variable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with STA_. Some of these flags can be updated using the MOD_STATUS bit.
long int constant
This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit MOD_TIMECONST.
long int precision
This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds.
long int tolerance
This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the maxerror every second.
struct timeval time
The current calendar time.
long int tick
The elapsed time between clock ticks in microseconds. A clock tick is a periodic timer interrupt on which the system clock is based.
long int ppsfreq
This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal.
long int jitter
This value expresses a median filtered average of the PPS signal's dispersion in microseconds.
int shift
This value is a binary exponent for the duration of the PPS calibration interval, ranging from PPS_SHIFT to PPS_SHIFTMAX.
long int stabil
This value represents the median filtered dispersion of the PPS frequency in scaled PPM.
long int jitcnt
This counter represents the number of pulses where the jitter exceeded the allowed maximum MAXTIME.
long int calcnt
This counter reflects the number of successful calibration intervals.
long int errcnt
This counter represents the number of calibration errors (caused by large offsets or jitter).
long int stbcnt
This counter denotes the number of calibrations where the stability exceeded the threshold.

— Function: int ntp_adjtime (struct timex *tptr)

The ntp_adjtime function sets the structure specified by tptr to current values.
In addition, ntp_adjtime updates some settings to match what you pass to it in *tptr. Use the modes element of *tptr to select what settings to update. You can set offset, freq, maxerror, esterror, status, constant, and tick.
modes = zero means set nothing.
Only the superuser can update settings.
The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR
The high accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed.
EPERM
The process specified a settings update, but is not superuser.

For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.
Portability note: Early versions of the GNU C library did not have this function but did have the synonymous adjtimex.

21.4.5 Formatting Calendar Time

The functions described in this section format calendar time values as strings. These functions are declared in the header file time.h.

— Function: char * asctime (const struct tm *brokentime)

The asctime function converts the broken-down time value that brokentime points to into a string in a standard format:
          "Tue May 21 13:46:22 1991\n"
The abbreviations for the days of week are: ‘Sun’, ‘Mon’, ‘Tue’, ‘Wed’, ‘Thu’, ‘Fri’, and ‘Sat’.
The abbreviations for the months are: ‘Jan’, ‘Feb’, ‘Mar’, ‘Apr’, ‘May’, ‘Jun’, ‘Jul’, ‘Aug’, ‘Sep’, ‘Oct’, ‘Nov’, and ‘Dec’.
The return value points to a statically allocated string, which might be overwritten by subsequent calls to asctime or ctime. (But no other library function overwrites the contents of this string.)

— Function: char * asctime_r (const struct tm *brokentime, char *buffer)

This function is similar to asctime but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter buffer. This buffer should have room for at least 26 bytes, including the terminating null.
If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

— Function: char * ctime (const time_t *time)

The ctime function is similar to asctime, except that you specify the calendar time argument as a time_t simple time value rather than in broken-down local time format. It is equivalent to
          asctime (localtime (time))
ctime sets the variable tzname, because localtime does so. See Time Zone Functions.

— Function: char * ctime_r (const time_t *time, char *buffer)

This function is similar to ctime, but places the result in the string pointed to by buffer. It is equivalent to (written using gcc extensions, see Statement Exprs):
          ({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })
If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

— Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)

This function is similar to the sprintf function (see Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see Locales).
Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a ‘%’ character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:

_
The number is padded with spaces.
-
The number is not padded at all.
0
The number is padded with zeros even if the format specifies padding with spaces.
^
The output uses uppercase characters, but only if this is possible (see Case Conversion).

The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.
Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.
An optional modifier can follow the optional flag and width specification. The modifiers, which were first standardized by POSIX.2-1992 and by ISO C99, are:

E
Use the locale's alternate representation for date and time. This modifier applies to the %c, %C, %x, %X, %y and %Y format specifiers. In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns.
O
Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers.

If the format supports the modifier but no alternate representation is available, it is ignored.
The conversion specifier ends with a format specifier taken from the following list. The whole ‘%’ sequence is replaced in the output string as follows:

%a
The abbreviated weekday name according to the current locale.
%A
The full weekday name according to the current locale.
%b
The abbreviated month name according to the current locale.
%B
The full month name according to the current locale.
Using %B together with %d produces grammatically incorrect results for some locales.
%c
The preferred calendar time representation for the current locale.
%C
The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%d
The day of the month as a decimal number (range 01 through 31).
%D
The date using the format %m/%d/%y.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%e
The day of the month like with %d, but padded with blank (range 1 through 31).
This format was first standardized by POSIX.2-1992 and by ISO C99.
%F
The date using the format %Y-%m-%d. This is the form specified in the ISO 8601 standard and is the preferred form for all uses.
This format was first standardized by ISO C99 and by POSIX.1-2001.
%g
The year corresponding to the ISO week number, but without the century (range 00 through 99). This has the same format and value as %y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.
This format was first standardized by ISO C99 and by POSIX.1-2001.
%G
The year corresponding to the ISO week number. This has the same format and value as %Y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.
This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.
%h
The abbreviated month name according to the current locale. The action is the same as for %b.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%H
The hour as a decimal number, using a 24-hour clock (range 00 through 23).
%I
The hour as a decimal number, using a 12-hour clock (range 01 through 12).
%j
The day of the year as a decimal number (range 001 through 366).
%k
The hour as a decimal number, using a 24-hour clock like %H, but padded with blank (range 0 through 23).
This format is a GNU extension.
%l
The hour as a decimal number, using a 12-hour clock like %I, but padded with blank (range 1 through 12).
This format is a GNU extension.
%m
The month as a decimal number (range 01 through 12).
%M
The minute as a decimal number (range 00 through 59).
%n
A single ‘\n’ (newline) character.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%p
Either ‘AM’ or ‘PM’, according to the given time value; or the corresponding strings for the current locale. Noon is treated as ‘PM’ and midnight as ‘AM’. In most locales ‘AM’/‘PM’ format is not supported, in such cases "%p" yields an empty string.
%P
Either ‘am’ or ‘pm’, according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as ‘pm’ and midnight as ‘am’. In most locales ‘AM’/‘PM’ format is not supported, in such cases "%P" yields an empty string.
This format is a GNU extension.
%r
The complete calendar time using the AM/PM format of the current locale.
This format was first standardized by POSIX.2-1992 and by ISO C99. In the POSIX locale, this format is equivalent to %I:%M:%S %p.
%R
The hour and minute in decimal numbers using the format %H:%M.
This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.
%s
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available.
This format is a GNU extension.
%S
The seconds as a decimal number (range 00 through 60).
%t
A single ‘\t’ (tabulator) character.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%T
The time of day using decimal numbers using the format %H:%M:%S.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%u
The day of the week as a decimal number (range 1 through 7), Monday being 1.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%U
The week number of the current year as a decimal number (range 00 through 53), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week 00.
%V
The ISO 8601:1988 week number as a decimal number (range 01 through 53). ISO weeks start with Monday and end with Sunday. Week 01 of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week 01 of a year can contain days from the previous year. The week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year.
This format was first standardized by POSIX.2-1992 and by ISO C99.
%w
The day of the week as a decimal number (range 0 through 6), Sunday being 0.
%W
The week number of the current year as a decimal number (range 00 through 53), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week 00.
%x
The preferred date representation for the current locale.
%X
The preferred time of day representation for the current locale.
%y
The year without a century as a decimal number (range 00 through 99). This is equivalent to the year modulo 100.
%Y
The year as a decimal number, using the Gregorian calendar. Years before the year 1 are numbered 0, -1, and so on.
%z
RFC 822/ISO 8601:1988 style numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable.
This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.
In the POSIX locale, a full RFC 822 timestamp is generated by the format ‘"%a, %d %b %Y %H:%M:%S %z"’ (or the equivalent ‘"%a, %d %b %Y %T %z"’).
%Z
The time zone abbreviation (empty if the time zone can't be determined).
%%
A literal ‘%’ character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character. If the formatted time requires more than size characters, strftime returns zero and the contents of the array s are undefined. Otherwise the return value indicates the number of characters placed in the array s, not including the terminating null character.
Warning: This convention for the return value which is prescribed in ISO C can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales "%p" will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used:
          buf[0] = '\1';
          len = strftime (buf, bufsize, format, tp);
          if (len == 0 && buf[0] != '\0')
            {
              /* Something went wrong in the strftime call.  */
              ...
            }
If s is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.
According to POSIX.1 every call to strftime implies a call to tzset. So the contents of the environment variable TZ is examined before any output is produced.
For an example of strftime, see Time Functions Example.

— Function: size_t wcsftime (wchar_t *s, size_t size, const wchar_t *template, const struct tm *brokentime)

The wcsftime function is equivalent to the strftime function with the difference that it operates on wide character strings. The buffer where the result is stored, pointed to by s, must be an array of wide characters. The parameter size which specifies the size of the output buffer gives the number of wide character, not the number of bytes.
Also the format string template is a wide character string. Since all characters needed to specify the format string are in the basic character set it is portably possible to write format strings in the C source code using the L"..." notation. The parameter brokentime has the same meaning as in the strftime call.
The wcsftime function supports the same flags, modifiers, and format specifiers as the strftime function.
The return value of wcsftime is the number of wide characters stored in s. When more characters would have to be written than can be placed in the buffer s the return value is zero, with the same problems indicated in the strftime documentation.

21.4.6 Convert textual time and date information back

The ISO C standard does not specify any functions which can convert the output of the strftime function back into a binary format. This led to a variety of more-or-less successful implementations with different interfaces over the years. Then the Unix standard was extended by the addition of two functions: strptime and getdate. Both have strange interfaces but at least they are widely available.

21.4.6.1 Interpret string according to given format

The first function is rather low-level. It is nevertheless frequently used in software since it is better known. Its interface and implementation are heavily influenced by the getdate function, which is defined and implemented in terms of calls to strptime.

— Function: char * strptime (const char *s, const char *fmt, struct tm *tp)

The strptime function parses the input string s according to the format string fmt and stores its results in the structure tp.
The input string could be generated by a strftime call or obtained any other way. It does not need to be in a human-recognizable format; e.g. a date passed as "02:1999:9" is acceptable, even though it is ambiguous without context. As long as the format string fmt matches the input string the function will succeed.
The user has to make sure, though, that the input can be parsed in a unambiguous way. The string "1999112" can be parsed using the format "%Y%m%d" as 1999-1-12, 1999-11-2, or even 19991-1-2. It is necessary to add appropriate separators to reliably get results.
The format string consists of the same components as the format string of the strftime function. The only difference is that the flags _, -, 0, and ^ are not allowed. Several of the distinct formats of strftime do the same work in strptime since differences like case of the input do not matter. For reasons of symmetry all formats are supported, though.
The modifiers E and O are also allowed everywhere the strftime function allows them.
The formats are:

%a
%A
The weekday name according to the current locale, in abbreviated form or the full name.
%b
%B
%h
The month name according to the current locale, in abbreviated form or the full name.
%c
The date and time representation for the current locale.
%Ec
Like %c but the locale's alternative date and time format is used.
%C
The century of the year.
It makes sense to use this format only if the format string also contains the %y format.
%EC
The locale's representation of the period.
Unlike %C it sometimes makes sense to use this format since some cultures represent years relative to the beginning of eras instead of using the Gregorian years.
%d
%e
The day of the month as a decimal number (range 1 through 31). Leading zeroes are permitted but not required.
%Od
%Oe
Same as %d but using the locale's alternative numeric symbols.
Leading zeroes are permitted but not required.
%D
Equivalent to %m/%d/%y.
%F
Equivalent to %Y-%m-%d, which is the ISO 8601 date format.
This is a GNU extension following an ISO C99 extension to strftime.
%g
The year corresponding to the ISO week number, but without the century (range 00 through 99).
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
This format is a GNU extension following a GNU extension of strftime.
%G
The year corresponding to the ISO week number.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
This format is a GNU extension following a GNU extension of strftime.
%H
%k
The hour as a decimal number, using a 24-hour clock (range 00 through 23).
%k is a GNU extension following a GNU extension of strftime.
%OH
Same as %H but using the locale's alternative numeric symbols.
%I
%l
The hour as a decimal number, using a 12-hour clock (range 01 through 12).
%l is a GNU extension following a GNU extension of strftime.
%OI
Same as %I but using the locale's alternative numeric symbols.
%j
The day of the year as a decimal number (range 1 through 366).
Leading zeroes are permitted but not required.
%m
The month as a decimal number (range 1 through 12).
Leading zeroes are permitted but not required.
%Om
Same as %m but using the locale's alternative numeric symbols.
%M
The minute as a decimal number (range 0 through 59).
Leading zeroes are permitted but not required.
%OM
Same as %M but using the locale's alternative numeric symbols.
%n
%t
Matches any white space.
%p
%P
The locale-dependent equivalent to ‘AM’ or ‘PM’.
This format is not useful unless %I or %l is also used. Another complication is that the locale might not define these values at all and therefore the conversion fails.
%P is a GNU extension following a GNU extension to strftime.
%r
The complete time using the AM/PM format of the current locale.
A complication is that the locale might not define this format at all and therefore the conversion fails.
%R
The hour and minute in decimal numbers using the format %H:%M.
%R is a GNU extension following a GNU extension to strftime.
%s
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available.
%s is a GNU extension following a GNU extension to strftime.
%S
The seconds as a decimal number (range 0 through 60).
Leading zeroes are permitted but not required.
NB: The Unix specification says the upper bound on this value is 61, a result of a decision to allow double leap seconds. You will not see the value 61 because no minute has more than one leap second, but the myth persists.
%OS
Same as %S but using the locale's alternative numeric symbols.
%T
Equivalent to the use of %H:%M:%S in this place.
%u
The day of the week as a decimal number (range 1 through 7), Monday being 1.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
%U
The week number of the current year as a decimal number (range 0 through 53).
Leading zeroes are permitted but not required.
%OU
Same as %U but using the locale's alternative numeric symbols.
%V
The ISO 8601:1988 week number as a decimal number (range 1 through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
%w
The day of the week as a decimal number (range 0 through 6), Sunday being 0.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
%Ow
Same as %w but using the locale's alternative numeric symbols.
%W
The week number of the current year as a decimal number (range 0 through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
%OW
Same as %W but using the locale's alternative numeric symbols.
%x
The date using the locale's date format.
%Ex
Like %x but the locale's alternative data representation is used.
%X
The time using the locale's time format.
%EX
Like %X but the locale's alternative time representation is used.
%y
The year without a century as a decimal number (range 0 through 99).
Leading zeroes are permitted but not required.
Note that it is questionable to use this format without the %C format. The strptime function does regard input values in the range 68 to 99 as the years 1969 to 1999 and the values 0 to 68 as the years 2000 to 2068. But maybe this heuristic fails for some input data.
Therefore it is best to avoid %y completely and use %Y instead.
%Ey
The offset from %EC in the locale's alternative representation.
%Oy
The offset of the year (from %C) using the locale's alternative numeric symbols.
%Y
The year as a decimal number, using the Gregorian calendar.
%EY
The full alternative year representation.
%z
The offset from GMT in ISO 8601/RFC822 format.
%Z
The timezone name.
Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.
%%
A literal ‘%’ character.

All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more whitespace characters in the format string.
Portability Note: The XPG standard advises applications to use at least one whitespace character (as specified by isspace) or other non-alphanumeric characters between any two conversion specifications. The GNU C Library does not have this limitation but other libraries might have trouble parsing formats like "%d%m%Y%H%M%S".
The strptime function processes the input string from right to left. Each of the three possible input elements (white space, literal, or format) are handled one after the other. If the input cannot be matched to the format string the function stops. The remainder of the format and input strings are not processed.
The function returns a pointer to the first character it was unable to process. If the input string contains more characters than required by the format string the return value points right after the last consumed input character. If the whole input string is consumed the return value points to the NULL byte at the end of the string. If an error occurs, i.e., strptime fails to match all of the format string, the function returns NULL.

The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information. Most importantly, it does not specify what happens to those elements of tm which are not directly initialized by the different formats. The implementations on different Unix systems vary here.

The GNU libc implementation does not touch those fields which are not directly initialized. Exceptions are the tm_wday and tm_yday elements, which are recomputed if any of the year, month, or date elements changed. This has two implications:

The following example shows a function which parses a string which is contains the date information in either US style or ISO 8601 form:

`"alnum"`	`"alpha"`	`"cntrl"`	`"digit"`
`"graph"`	`"lower"`	`"print"`	`"punct"`
`"space"`	`"upper"`	`"xdigit"`

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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`

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16	`E`	`F`	`G`	`H`	`I`	`J`	`K`	`L`
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56	`s`	`t`	`u`	`v`	`w`	`x`	`y`	`z`

The GNU C Library

Short Contents

Table of Contents

Main Menu

1 Introduction

1.1 Getting Started

1.2 Standards and Portability

1.2.1 ISO C

1.2.2 POSIX (The Portable Operating System Interface)

1.2.3 Berkeley Unix

1.2.4 SVID (The System V Interface Description)

1.2.5 XPG (The X/Open Portability Guide)

1.3 Using the Library

1.3.1 Header Files

1.3.2 Macro Definitions of Functions

1.3.3 Reserved Names

1.3.4 Feature Test Macros

1.4 Roadmap to the Manual

2 Error Reporting

2.1 Checking for Errors

2.2 Error Codes

2.3 Error Messages

3 Virtual Memory Allocation And Paging

3.1 Process Memory Concepts

3.2 Allocating Storage For Program Data

3.2.1 Memory Allocation in C Programs

3.2.1.1 Dynamic Memory Allocation

3.2.2 Unconstrained Allocation

3.2.2.1 Basic Memory Allocation

3.2.2.2 Examples of malloc

3.2.2.3 Freeing Memory Allocated with malloc

3.2.2.4 Changing the Size of a Block

3.2.2.5 Allocating Cleared Space

3.2.2.6 Efficiency Considerations for malloc

3.2.2.7 Allocating Aligned Memory Blocks

3.2.2.8 Malloc Tunable Parameters

3.2.2.9 Heap Consistency Checking

3.2.2.10 Memory Allocation Hooks

3.2.2.11 Statistics for Memory Allocation with malloc

3.2.2.12 Summary of malloc-Related Functions

3.2.3 Allocation Debugging

3.2.3.1 How to install the tracing functionality

3.2.3.2 Example program excerpts

3.2.3.3 Some more or less clever ideas

3.2.3.4 Interpreting the traces

3.2.4 Obstacks

3.2.4.1 Creating Obstacks

3.2.4.2 Preparing for Using Obstacks

3.2.4.3 Allocation in an Obstack

3.2.4.4 Freeing Objects in an Obstack

3.2.4.5 Obstack Functions and Macros

3.2.4.6 Growing Objects

3.2.4.7 Extra Fast Growing Objects

3.2.4.8 Status of an Obstack

3.2.4.9 Alignment of Data in Obstacks

3.2.4.10 Obstack Chunks

3.2.4.11 Summary of Obstack Functions

3.2.5 Automatic Storage with Variable Size

3.2.5.1 alloca Example

3.2.5.2 Advantages of alloca

3.2.5.3 Disadvantages of alloca

3.2.5.4 GNU C Variable-Size Arrays

3.3 Resizing the Data Segment

3.4 Locking Pages

3.4.1 Why Lock Pages

3.4.2 Locked Memory Details

3.4.3 Functions To Lock And Unlock Pages

4 Character Handling

4.1 Classification of Characters

4.2 Case Conversion

4.3 Character class determination for wide characters

4.4 Notes on using the wide character classes

4.5 Mapping of wide characters.

5 String and Array Utilities

5.1 Representation of Strings

5.2 String and Array Conventions

5.3 String Length

5.4 Copying and Concatenation

5.5 String/Array Comparison

5.6 Collation Functions

3.2.2.2 Examples of `malloc`

3.2.2.3 Freeing Memory Allocated with `malloc`

3.2.2.6 Efficiency Considerations for `malloc`

3.2.2.11 Statistics for Memory Allocation with `malloc`

3.2.2.12 Summary of `malloc`-Related Functions

3.2.5.1 `alloca` Example

3.2.5.2 Advantages of `alloca`

3.2.5.3 Disadvantages of `alloca`

6.5.2 A complete `iconv` example

6.5.3 Some Details about other `iconv` Implementations

6.5.4 The `iconv` Implementation in the GNU C library

6.5.4.1 Format of `gconv-modules` files

6.5.4.2 Finding the conversion path in `iconv`

6.5.4.3 `iconv` module data structures

6.5.4.4 `iconv` module interfaces

7.6.1 `localeconv`: It is portable but ...

8.1.1 The `catgets` function family

8.1.4 How to use the `catgets` interface

8.2.1 The `gettext` family of functions

8.2.1.4 How to specify the output character set `gettext` uses

8.2.1.5 How to use `gettext` in GUI programs

8.2.1.6 User influence on `gettext`

8.2.2 Programs to handle message catalogs for `gettext`

9.5 The `hsearch` function.

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