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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 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
	     feature_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
	      #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:

	      ·	 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 exposes def‐
		 initions corresponding to the POSIX.1-2001 base specification
		 (excluding the XSI extension).

	      ·	 (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:

	      ·	 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).

       _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
	      Exposes  C99  extensions	to ISO C (1990).  This macro is recog‐
	      nized since glibc 2.1.3; earlier glibc 2.1.x versions recognized
	      an  equivalent macro named _ISOC9X_SOURCE (because the C99 stan‐
	      dard had not then been finalized).  Although the use of the lat‐
	      ter macro is obsolete, glibc continues to recognize it for back‐
	      wards 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/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).   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  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
	      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.  Not  all	buffer
	      overflows	 are detected, just some common cases.	In the current
	      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:

		·  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
       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,	 _FOR‐
       TIFY_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 _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 3.27 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2010-09-10		FEATURE_TEST_MACROS(7)
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