feature_test_macros(7) — Linux manual page

NAME | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

FEATURE_TEST_MACROS(7)  Linux Programmer's Manual FEATURE_TEST_MACROS(7)

NAME         top

       feature_test_macros - feature test macros

DESCRIPTION         top

       Feature test macros allow the programmer to control the
       definitions that are exposed by system header files when a
       program is compiled.

       NOTE: In order to be effective, a feature test macro must be
       defined before including any header files.  This can be done
       either in the compilation command (cc -DMACRO=value) or by
       defining the macro within the source code before including any
       headers.  The requirement that the macro must be defined before
       including any header file exists because header files may freely
       include one another.  Thus, for example, in the following lines,
       defining the _GNU_SOURCE macro may have no effect because the
       header <abc.h> itself includes <xyz.h> (POSIX explicitly allows
       this):

           #include <abc.h>
           #define _GNU_SOURCE
           #include <xyz.h>

       Some feature test macros are useful for creating portable
       applications, by preventing nonstandard definitions from being
       exposed.  Other macros can be used to expose nonstandard
       definitions that are not exposed by default.

       The precise effects of each of the feature test macros described
       below can be ascertained by inspecting the <features.h> header
       file.  Note: applications do not need to directly include
       <features.h>; indeed, doing so is actively discouraged.  See
       NOTES.

   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):

              #include <unistd.h>

              int acct(const char *filename);

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

              acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE <
              500)

       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
           #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 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:

              •  The value 1 exposes definitions conforming to
                 POSIX.1-1990 and ISO C (1990).

              •  The value 2 or greater additionally exposes definitions
                 for POSIX.2-1992.

              •  The value 199309L or greater additionally exposes
                 definitions for POSIX.1b (real-time extensions).

              •  The value 199506L or greater additionally exposes
                 definitions for POSIX.1c (threads).

              •  (Since glibc 2.3.3) The value 200112L or greater
                 additionally exposes definitions corresponding to the
                 POSIX.1-2001 base specification (excluding the XSI
                 extension).  This value also causes C95 (since glibc
                 2.12) and C99 (since glibc 2.10) features to be exposed
                 (in other words, the equivalent of defining
                 _ISOC99_SOURCE).

              •  (Since glibc 2.10) The value 200809L or greater
                 additionally exposes definitions corresponding to the
                 POSIX.1-2008 base specification (excluding the XSI
                 extension).

       _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:

              •  Defining with any value exposes definitions conforming
                 to POSIX.1, POSIX.2, and XPG4.

              •  The value 500 or greater additionally exposes
                 definitions for SUSv2 (UNIX 98).

              •  (Since glibc 2.2) The value 600 or greater additionally
                 exposes definitions for SUSv3 (UNIX 03; i.e., the
                 POSIX.1-2001 base specification plus the XSI extension)
                 and C99 definitions.

              •  (Since glibc 2.10) The value 700 or greater
                 additionally exposes definitions for SUSv4 (i.e., the
                 POSIX.1-2008 base specification plus the XSI
                 extension).

              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:

              •  _POSIX_SOURCE is defined with the value 1.

              •  _POSIX_C_SOURCE is defined, according to the value of
                 _XOPEN_SOURCE:

                 _XOPEN_SOURCE < 500
                        _POSIX_C_SOURCE is defined with the value 2.

                 500 <= _XOPEN_SOURCE < 600
                        _POSIX_C_SOURCE is defined with the value
                        199506L.

                 600 <= _XOPEN_SOURCE < 700
                        _POSIX_C_SOURCE is defined with the value
                        200112L.

                 700 <= _XOPEN_SOURCE (since glibc 2.10)
                        _POSIX_C_SOURCE is defined with the value
                        200809L.

              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.  (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 (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 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.

              Since glibc 2.19, defining _GNU_SOURCE also has the effect
              of implicitly defining _DEFAULT_SOURCE.  In glibc versions
              before 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.

              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
       (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 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, _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:

          •  2, if _XOPEN_SOURCE is defined with a value less than 500;

          •  199506L, if _XOPEN_SOURCE is defined with a value greater
             than or equal to 500 and less than 600; or

          •  (since glibc 2.4) 200112L, if _XOPEN_SOURCE is defined with
             a value greater than or equal to 600 and less than 700.

          •  (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is defined
             with a value greater than or equal to 700.

          •  Older versions of glibc do not know about the values
             200112L and 200809L for _POSIX_C_SOURCE, and the setting of
             this macro will depend on the glibc version.

          •  If _XOPEN_SOURCE is undefined, then the setting of
             _POSIX_C_SOURCE depends on the glibc version: 199506L, in
             glibc versions before 2.4; 200112L, in glibc 2.4 to 2.9;
             and 200809L, since glibc 2.10.

       Multiple macros can be defined; the results are additive.

CONFORMING TO         top

       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and
       _XOPEN_SOURCE.

       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but
       is not present in SUSv2 and later.  _FILE_OFFSET_BITS is not
       specified by any standard, but is employed on some other
       implementations.

       _BSD_SOURCE, _SVID_SOURCE, _DEFAULT_SOURCE, _ATFILE_SOURCE,
       _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are
       specific to Linux (glibc).

NOTES         top

       <features.h> is a Linux/glibc-specific header file.  Other
       systems have an analogous file, but typically with a different
       name.  This header file is automatically included by other header
       files as required: it is not necessary to explicitly include it
       in order to employ feature test macros.

       According to which of the above feature test macros are defined,
       <features.h> internally defines various other macros that are
       checked by other glibc header files.  These macros have names
       prefixed by two underscores (e.g., __USE_MISC).  Programs should
       never define these macros directly: instead, the appropriate
       feature test macro(s) from the list above should be employed.

EXAMPLES         top

       The program below can be used to explore how the various feature
       test macros are set depending on the glibc version and what
       feature test macros are explicitly set.  The following shell
       session, on a system with glibc 2.10, shows some examples of what
       we would see:

           $ cc ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 200809L
           _BSD_SOURCE defined
           _SVID_SOURCE defined
           _ATFILE_SOURCE defined
           $ cc -D_XOPEN_SOURCE=500 ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 199506L
           _XOPEN_SOURCE defined: 500
           $ cc -D_GNU_SOURCE ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 200809L
           _ISOC99_SOURCE defined
           _XOPEN_SOURCE defined: 700
           _XOPEN_SOURCE_EXTENDED defined
           _LARGEFILE64_SOURCE defined
           _BSD_SOURCE defined
           _SVID_SOURCE defined
           _ATFILE_SOURCE defined
           _GNU_SOURCE defined

   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 _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);
       }

SEE ALSO         top

       libc(7), standards(7), system_data_types(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22         FEATURE_TEST_MACROS(7)

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