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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 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 non-
       standard  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  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 compilation 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:

	      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 correspond-
		 ing to the POSIX.1-2001 base specification (excluding the XSI extension).

	      o  (Since  glibc 2.10) The value 200809L or greater exposes definitions correspond-
		 ing 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 exten-
		 sion) 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.

       _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  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://open-
	      group.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  ver-
	      sus  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 refer-
	      ences 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  defi-
	      nitions.	 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, vari-
	      ous GNU-specific extensions are also exposed.  Where standards conflict, BSD  defi-
	      nitions are disfavored.

       _REENTRANT
	      Defining this macro exposes definitions of certain reentrant functions.  For multi-
	      threaded 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 func-
	      tions.  Not all buffer overflows are detected, just some common cases.  In the cur-
	      rent  implementation  checks  are  added	for  calls to memcpy(3), mempcpy(3), mem-
	      move(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  _FOR-
	      TIFY_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 pro-
	      grams  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 ver-
		   sion.

		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 imple-
       mentations.

       _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 fol-
       lowing 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.ker-
       nel.org/doc/man-pages/.

Linux					    2010-09-10			   FEATURE_TEST_MACROS(7)
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