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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 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.

       _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 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‐
	      ward compatibility.

       _ISOC11_SOURCE
	      Exposes declarations consistent with the ISO C11 standard.  This
	      macro is recognized since glibc 2.16.

       _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 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 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 3.54 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				  2012-08-05		FEATURE_TEST_MACROS(7)
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