FEATURE_TEST_MACROS(7)     Linux Programmer's Manual    FEATURE_TEST_MACROS(7)
NAME
       feature_test_macros - 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.

       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 com-
       pilation command (cc -DMACRO=value) or by defining the macro within the
       source code before including any headers.

       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.

   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 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  compila-
       tion 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 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:

              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 additionally
                 exposes definitions corresponding to  the  POSIX.1-2001  base
                 specification  (excluding  the XSI extension) and also causes
                 C95 (since glibc 2.12) and C99 (since glibc 2.10) features to
                 be exposed.

              o  (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.

       _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).

              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:

              o  _POSIX_SOURCE is defined with the value 1.

              o  _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.

       _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 (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.)

       _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).

       _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 int for
              file offsets.  This macro is implicitly defined if _XOPEN_SOURCE
              is  defined with a value greater than or equal to 500.  New pro-
              grams 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 recom-
              pilation being required.)

              64-bit systems naturally permit file sizes greater than 2  Giga-
              bytes, 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 con-
              flicts.

              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 Defi-
              nition; 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"  defini-
              tions  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 as well as various definitions 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
              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 per-
              formed to detect some buffer overflow errors when employing var-
              ious string and memory manipulation functions (for example, mem-
              cpy(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  func-
              tions,  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 warn-
              ings; 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,   _POSIX_SOURCE,
       _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _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:

          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.

       Multiple macros can be defined; the results are additive.

CONFORMING TO
       POSIX.1  specifies  _POSIX_C_SOURCE,  _POSIX_SOURCE, and _XOPEN_SOURCE.
       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1).

       _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
       <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>

       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 _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
       libc(7), standards(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON
       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-03-29            FEATURE_TEST_MACROS(7)