Specification of feature test macro requirements in manual pages¶

When a function requires that a feature test macro is defined, the manual page SYNOPSIS typically includes a note of the following form (this example from the chmod(2) manual page):

The || means that in order to obtain the declaration of fchmod(2) from <sys/stat.h>, either of the following macro definitions must be made before including any header files:

#define _BSD_SOURCE
#define _XOPEN_SOURCE 500     /* or any value > 500 */

Alternatively, equivalent definitions can be included in the compilation command:

cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE=500        # Or any value > 500

Note that, as described below, some feature test macros are defined by default, so that it may not always be necessary to explicitly specify the feature test macro(s) shown in the SYNOPSIS.

In a few cases, manual pages use a shorthand for expressing the feature test macro requirements (this example from readahead(2)):

#define _GNU_SOURCE
#include <fcntl.h>

ssize_t readahead(int fd, off64_t *offset, size_t count);

This format is employed in cases where only a single feature test macro can be used to expose the function declaration, and that macro is not defined by default.

Feature test macros understood by glibc¶

The following paragraphs explain how feature test macros are handled in Linux glibc 2.x, x > 0.

Linux glibc understands the following feature test macros:

__STRICT_ANSI__

ISO Standard C. This macro is implicitly defined by gcc(1) when invoked with, for example, the -std=c99 or -ansi flag.

_POSIX_C_SOURCE

Defining this macro causes header files to expose definitions as follows:

_POSIX_SOURCE

Defining this obsolete macro with any value is equivalent to defining _POSIX_C_SOURCE with the value 1.

_XOPEN_SOURCE

Defining this macro causes header files to expose definitions as follows:

_XOPEN_SOURCE_EXTENDED

If this macro is defined, and _XOPEN_SOURCE is defined, then expose definitions corresponding to the XPG4v2 (SUSv1) UNIX extensions (UNIX 95). This macro is also implicitly defined if _XOPEN_SOURCE is defined with a value of 500 or more.

_ISOC99_SOURCE

Exposes C99 extensions to ISO C (1990). This macro is recognized since glibc 2.1.3; earlier glibc 2.1.x versions recognized an equivalent macro named _ISOC9X_SOURCE (because the C99 standard had not then been finalized). Although the use of the latter macro is obsolete, glibc continues to recognize it for backwards compatibility.

_LARGEFILE64_SOURCE

Expose definitions for the alternative API specified by the LFS (Large File Summit) as a "transitional extension" to the Single UNIX Specification. (See http://opengroup.org/platform/lfs.html.) The alternative API consists of a set of new objects (i.e., functions and types) whose names are suffixed with "64" (e.g., off64_t versus off_t, lseek64() versus lseek(), etc.). New programs should not employ this interface; instead _FILE_OFFSET_BITS=64 should be employed.

_FILE_OFFSET_BITS

Defining this macro with the value 64 automatically converts references to 32-bit functions and data types related to file I/O and file system operations into references to their 64-bit counterparts. This is useful for performing I/O on large files (> 2 Gigabytes) on 32-bit systems. (Defining this macro permits correctly written programs to use large files with only a recompilation being required.) 64-bit systems naturally permit file sizes greater than 2 Gigabytes, and on those systems this macro has no effect.

_BSD_SOURCE

Defining this macro with any value causes header files to expose BSD-derived definitions. Defining this macro also causes BSD definitions to be preferred in some situations where standards conflict, unless one or more of _SVID_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, or _GNU_SOURCE is defined, in which case BSD definitions are disfavored.

_SVID_SOURCE

Defining this macro with any value causes header files to expose System V-derived definitions. (SVID == System V Interface Definition; see standards(7).)

_ATFILE_SOURCE (since glibc 2.4)

Defining this macro with any value causes header files to expose declarations of a range of functions with the suffix "at"; see openat(2).

_GNU_SOURCE

Defining this macro (with any value) is equivalent to defining _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L (200112L in glibc versions before 2.10; 199506L in glibc versions before 2.5; 199309L in glibc versions before 2.1) and _XOPEN_SOURCE with the value 700 (600 in glibc versions before 2.10; 500 in glibc versions before 2.2). In addition, various GNU-specific extensions are also exposed. Where standards conflict, BSD definitions are disfavored.

_REENTRANT

Defining this macro exposes definitions of certain reentrant functions. For multithreaded programs, use cc -pthread instead.

_THREAD_SAFE

Synonym for _REENTRANT, provided for compatibility with some other implementations.

_FORTIFY_SOURCE (since glibc 2.3.4)

Defining this macro causes some lightweight checks to be performed to detect some buffer overflow errors when employing various string and memory manipulation functions. Not all buffer overflows are detected, just some common cases. In the current implementation checks are added for calls to memcpy(3), mempcpy(3), memmove(3), memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3), and gets(3). If _FORTIFY_SOURCE is set to 1, with compiler optimization level 1 (gcc -O1) and above, checks that shouldn't change the behavior of conforming programs are performed. With _FORTIFY_SOURCE set to 2 some more checking is added, but some conforming programs might fail. Some of the checks can be performed at compile time, and result in compiler warnings; other checks take place at run time, and result in a run-time error if the check fails. Use of this macro requires compiler support, available with gcc(1) since version 4.0.

Default definitions, implicit definitions, and combining definitions¶

If no feature test macros are explicitly defined, then the following feature test macros are defined by default: _BSD_SOURCE, _SVID_SOURCE, _POSIX_SOURCE, and _POSIX_C_SOURCE=200909L (200112L in glibc versions before 2.10; 199506L in glibc versions before 2.4; 199309L in glibc versions before 2.1).

If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE, or _SVID_SOURCE is explicitly defined, then _BSD_SOURCE, and _SVID_SOURCE are not defined by default.

If _POSIX_SOURCE and _POSIX_C_SOURCE are not explicitly defined, and either __STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with a value of 500 or more, then

Multiple macros can be defined; the results are additive.

Program source¶

/* ftm.c */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
#ifdef _POSIX_SOURCE


    printf("_POSIX_SOURCE defined\n");
#endif
#ifdef _POSIX_C_SOURCE


    printf("_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
#endif
#ifdef _ISOC99_SOURCE


    printf("_ISOC99_SOURCE defined\n");
#endif
#ifdef _XOPEN_SOURCE


    printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
#endif
#ifdef _XOPEN_SOURCE_EXTENDED


    printf("_XOPEN_SOURCE_EXTENDED defined\n");
#endif
#ifdef _LARGEFILE64_SOURCE


    printf("_LARGEFILE64_SOURCE defined\n");
#endif
#ifdef _FILE_OFFSET_BITS


    printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
#endif
#ifdef _BSD_SOURCE


    printf("_BSD_SOURCE defined\n");
#endif
#ifdef _SVID_SOURCE


    printf("_SVID_SOURCE defined\n");
#endif
#ifdef _ATFILE_SOURCE


    printf("_ATFILE_SOURCE defined\n");
#endif
#ifdef _GNU_SOURCE


    printf("_GNU_SOURCE defined\n");
#endif
#ifdef _REENTRANT


    printf("_REENTRANT defined\n");
#endif
#ifdef _THREAD_SAFE


    printf("_THREAD_SAFE defined\n");
#endif
#ifdef _FORTIFY_SOURCE


    printf("_FORTIFY_SOURCE defined\n");
#endif


    exit(EXIT_SUCCESS);
}