Provided by: manpages_3.35-0.1ubuntu1_all bug

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
       compilation 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
       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 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 exposes
                 definitions   corresponding   to   the   POSIX.1-2001    base
                 specification (excluding the XSI extension).

              o  (Since  glibc  2.10)  The  value  200809L  or greater 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).

       _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.

       _ISOC95_SOURCE
              Exposes  ISO  C  (1990)  Amendment  1 definitions (also known as
              C95).  This macro is recognized since glibc 2.12.   The  primary
              change in C95 was support for international character sets.  The
              C95 changes were included in the  subsequent  C99  standard  (in
              other words, _ISOC99_SOURCE implies _ISOC95_SOURCE).

       _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 backward 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).   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) 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=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.

       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,      _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,
       <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.

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 _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.

       /usr/include/features.h

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.