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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 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:

	      ·	 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) and also causes
		 C95 (since glibc 2.12) and C99 (since glibc 2.10) features to
		 be exposed.

	      ·	 (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:

	      ·	 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 not _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.

       _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	 (feek(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.	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), str‐
	      cpy(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 (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:

	  ·  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,	  _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 3.65 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				  2014-03-20		FEATURE_TEST_MACROS(7)
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