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 acct(2) manual page):

Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):

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

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

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

cc -D_BSD_SOURCE
cc -D_XOPEN_SOURCE           # 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
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>

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

This format is employed when the feature test macros ensure that the proper function declarations are visible, and the macros are not defined by default.

Feature test macros understood by glibc¶

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

First, though, a summary of a few details for the impatient:

•

The macros that you most likely need to use in modern source code are _POSIX_C_SOURCE (for definitions from various versions of POSIX.1), _XOPEN_SOURCE (for definitions from various versions of SUS), _GNU_SOURCE (for GNU and/or Linux specific stuff), and _DEFAULT_SOURCE (to get definitions that would normally be provided by default).

•

Certain macros are defined with default values. Thus, although one or more macros may be indicated as being required in the SYNOPSIS of a man page, it may not be necessary to define them explicitly. Full details of the defaults are given later in this man page.

•

Defining _XOPEN_SOURCE with a value of 600 or greater produces the same effects as defining _POSIX_C_SOURCE with a value of 200112L or greater. Where one sees

_POSIX_C_SOURCE >= 200112L
    

in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the following has the same effect:

_XOPEN_SOURCE >= 600
    

•

Defining _XOPEN_SOURCE with a value of 700 or greater produces the same effects as defining _POSIX_C_SOURCE with a value of 200809L or greater. Where one sees

_POSIX_C_SOURCE >= 200809L
    

in the feature test macro requirements in the SYNOPSIS of a man page, it is implicit that the following has the same effect:

_XOPEN_SOURCE >= 700
    

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.

Since this macro is obsolete, its usage is generally not documented when discussing feature test macro requirements in the man pages.

_XOPEN_SOURCE

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

If __STRICT_ANSI__ is not defined, or _XOPEN_SOURCE is defined with a value greater than or equal to 500 and neither _POSIX_SOURCE nor _POSIX_C_SOURCE is explicitly defined, then the following macros are implicitly defined:

In addition, defining _XOPEN_SOURCE with a value of 500 or greater produces the same effects as defining _XOPEN_SOURCE_EXTENDED.

_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). Defining _XOPEN_SOURCE with a value of 500 or more also produces the same effect as defining _XOPEN_SOURCE_EXTENDED. Use of _XOPEN_SOURCE_EXTENDED in new source code should be avoided.

Since defining _XOPEN_SOURCE with a value of 500 or more has the same effect as defining _XOPEN_SOURCE_EXTENDED, the latter (obsolete) feature test macro is generally not described in the SYNOPSIS in man pages.

_ISOC99_SOURCE (since glibc 2.1.3)

Exposes declarations consistent with the ISO C99 standard.

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 this macro is obsolete, glibc continues to recognize it for backward compatibility.

Defining _ISOC99_SOURCE also exposes ISO C (1990) Amendment 1 ("C95") definitions. (The primary change in C95 was support for international character sets.)

Invoking the C compiler with the option -std=c99 produces the same effects as defining this macro.

_ISOC11_SOURCE (since glibc 2.16)

Exposes declarations consistent with the ISO C11 standard. Defining this macro also enables C99 and C95 features (like _ISOC99_SOURCE).

Invoking the C compiler with the option -std=c11 produces the same effects as defining this macro.

_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 macro; instead _FILE_OFFSET_BITS=64 should be employed.

_LARGEFILE_SOURCE

This macro was historically used to expose certain functions (specifically fseeko(3) and ftello(3)) that address limitations of earlier APIs (fseek(3) and ftell(3)) that use long for file offsets. This macro is implicitly defined if _XOPEN_SOURCE is defined with a value greater than or equal to 500. New programs should not employ this macro; defining _XOPEN_SOURCE as just described or defining _FILE_OFFSET_BITS with the value 64 is the preferred mechanism to achieve the same result.

_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 filesystem operations into references to their 64-bit counterparts. This is useful for performing I/O on large files (> 2 Gigabytes) on 32-bit systems. It is also useful when calling functions like copy_file_range(2) that were added more recently and that come only in 64-bit flavors. (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.

_TIME_BITS

Defining this macro with the value 64 changes the width of time_t(3type) to 64-bit which allows handling of timestamps beyond 2038. It is closely related to _FILE_OFFSET_BITS and depending on implementation, may require it set. This macro is available as of glibc 2.34.

_BSD_SOURCE (deprecated since glibc 2.20)

Defining this macro with any value causes header files to expose BSD-derived definitions.

In glibc versions up to and including 2.18, 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. Since glibc 2.19, _BSD_SOURCE no longer causes BSD definitions to be preferred in case of conflicts.

Since glibc 2.20, this macro is deprecated. It now has the same effect as defining _DEFAULT_SOURCE, but generates a compile-time warning (unless _DEFAULT_SOURCE is also defined). Use _DEFAULT_SOURCE instead. To allow code that requires _BSD_SOURCE in glibc 2.19 and earlier and _DEFAULT_SOURCE in glibc 2.20 and later to compile without warnings, define both _BSD_SOURCE and _DEFAULT_SOURCE.

_SVID_SOURCE (deprecated since glibc 2.20)

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

Since glibc 2.20, this macro is deprecated in the same fashion as _BSD_SOURCE.

_DEFAULT_SOURCE (since glibc 2.19)

This macro can be defined to ensure that the "default" definitions are provided even when the defaults would otherwise be disabled, as happens when individual macros are explicitly defined, or the compiler is invoked in one of its "standard" modes (e.g., cc -std=c99). Defining _DEFAULT_SOURCE without defining other individual macros or invoking the compiler in one of its "standard" modes has no effect.

The "default" definitions comprise those required by POSIX.1-2008 and ISO C99, as well as various definitions originally derived from BSD and System V. On glibc 2.19 and earlier, these defaults were approximately equivalent to explicitly defining the following:

cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809
    

_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). Since glibc 2.10, this macro is also implicitly defined if _POSIX_C_SOURCE is defined with a value greater than or equal to 200809L.

_GNU_SOURCE

Defining this macro (with any value) implicitly defines _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the value 200809L (200112L before glibc 2.10; 199506L before glibc 2.5; 199309L before glibc 2.1) and _XOPEN_SOURCE with the value 700 (600 before glibc 2.10; 500 before glibc 2.2). In addition, various GNU-specific extensions are also exposed.

Since glibc 2.19, defining _GNU_SOURCE also has the effect of implicitly defining _DEFAULT_SOURCE. Before glibc 2.20, defining _GNU_SOURCE also had the effect of implicitly defining _BSD_SOURCE and _SVID_SOURCE.

_REENTRANT

Historically, on various C libraries it was necessary to define this macro in all multithreaded code. (Some C libraries may still require this.) In glibc, this macro also exposed definitions of certain reentrant functions.

However, glibc has been thread-safe by default for many years; since glibc 2.3, the only effect of defining _REENTRANT has been to enable one or two of the same declarations that are also enabled by defining _POSIX_C_SOURCE with a value of 199606L or greater.

_REENTRANT is now obsolete. In glibc 2.25 and later, defining _REENTRANT is equivalent to defining _POSIX_C_SOURCE with the value 199606L. If a higher POSIX conformance level is selected by any other means (such as _POSIX_C_SOURCE itself, _XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE), then defining _REENTRANT has no effect.

This macro is automatically defined if one compiles with cc -pthread.

_THREAD_SAFE

Synonym for the (deprecated) _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 (for example, memcpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3), gets(3), and wide character variants thereof). For some functions, argument consistency is checked; for example, a check is made that open(2) has been supplied with a mode argument when the specified flags include O_CREAT. Not all problems are detected, just some common cases.

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 (via macros logic implemented in header files), and result in compiler warnings; other checks take place at run time, and result in a run-time error if the check fails.

With _FORTIFY_SOURCE set to 3, additional checking is added to intercept some function calls used with an argument of variable size where the compiler can deduce an upper bound for its value. For example, a program where malloc(3)'s size argument is variable can now be fortified.

Use of this macro requires compiler support, available since gcc 4.0 and clang 2.6. Use of _FORTIFY_SOURCE set to 3 requires gcc 12.0 or later, or clang 9.0 or later, in conjunction with glibc 2.33 or later.

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 (in glibc 2.19 and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE (since glibc 2.19), _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L before glibc 2.10; 199506L before glibc 2.4; 199309L before glibc 2.1).

If any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc 2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc 2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_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

•

_POSIX_SOURCE is defined with the value 1; and

•

_POSIX_C_SOURCE is defined with one of the following values:

Multiple macros can be defined; the results are additive.

Program source¶

/* ftm.c */
#include <stdint.h>
#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: %jdL\n",


            (intmax_t) _POSIX_C_SOURCE);
#endif
#ifdef _ISOC99_SOURCE


    printf("_ISOC99_SOURCE defined\n");
#endif
#ifdef _ISOC11_SOURCE


    printf("_ISOC11_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 _TIME_BITS


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


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


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


    printf("_DEFAULT_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);
}