feature_test_macros


SYNOPSIS
       #include <features.h>

DESCRIPTION
       Feature  test  macros  allow  the programmer to control the definitions
       that are exposed by system header files when  a  program  is  compiled.
       This  can  be  useful for creating portable applications, by preventing
       non-standard definitions from being exposed.  Other macros can be  used
       to  expose  non-standard  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.

       In  order  to be effective, a feature test macro must be defined before
       including any header files.  This can either be done in the compilation
       command  (cc  -DMACRO=value) or by defining the macro within the source
       code before including any headers.

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

              #include <sys/stat.h>

              int chmod(const char *path, mode_t mode);
              int fchmod(int fd, mode_t mode);

          Feature   Test   Macro   Requirements   for    glibc    (see    fea-
          ture_test_macros(7)):

              fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

       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 compila-
       tion 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

       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:

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

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

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

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

              o  (Since glibc 2.3.3) The value 200112L or greater exposes def-
                 initions corresponding to the POSIX.1-2001 base specification
                 (excluding the XSI extension).

              o  (Since glibc 2.10) The value 200809L or greater exposes defi-
                 nitions  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.

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

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

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

              o  (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.

              o  (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).

       _XOPEN_SOURCE_EXTENDED

       _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/plat-
              form/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 recom-
              pilation 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 disfa-
              vored.

       _SVID_SOURCE
              Defining this macro with any value causes header files to expose
              System V-derived definitions.  (SVID == System V Interface Defi-
              nition; 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 ver-
              sions 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
              implementation  checks  are  added  for calls to memcpy(3), mem-
              pcpy(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=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,   _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

          *  _POSIX_SOURCE is defined with the value 1; and

          *  _POSIX_C_SOURCE is defined with one of the following values:

                o  2, if XOPEN_SOURCE is defined with a value less than 500;

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

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

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

                o  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.

                o  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.


NOTES
       <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,  <fea-
       tures.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.

EXAMPLE
       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 <stdio.h>
       #include <unistd.h>
       #include <stdlib.h>

           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);
       }

SEE ALSO
       libc(7), standards(7)

       The section "Feature Test Macros" under info libc.
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