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.
       This can be 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.

       In order to be effective, a feature test macro must be defined before including any header files.  This can either be done in the  compila-
       tion command (cc -DMACRO=value) or by defining the macro within the source code before including any headers.

   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 spec-
       ify 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 (exclud-
		 ing 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 spec-
		 ification 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.

       _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 backwards 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  Giga-
	      bytes)  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 defi-
	      nitions 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 Defini-
	      tion; 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 ope-
	      nat(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 implemen-
	      tation 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 opti-
	      mization level 1 (gcc -O1) and above, checks that shouldn't change the behavior of conforming programs are  performed.   With  _FOR-
	      TIFY_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
");
       #endif

       #ifdef _POSIX_C_SOURCE
	   printf("_POSIX_C_SOURCE defined: %ldL
", (long) _POSIX_C_SOURCE);
       #endif

       #ifdef _ISOC99_SOURCE
	   printf("_ISOC99_SOURCE defined
");
       #endif

       #ifdef _XOPEN_SOURCE
	   printf("_XOPEN_SOURCE defined: %d
", _XOPEN_SOURCE);
       #endif

       #ifdef _XOPEN_SOURCE_EXTENDED
	   printf("_XOPEN_SOURCE_EXTENDED defined
");
       #endif

       #ifdef _LARGEFILE64_SOURCE
	   printf("_LARGEFILE64_SOURCE defined
");
       #endif

       #ifdef _FILE_OFFSET_BITS
	   printf("_FILE_OFFSET_BITS defined: %d
", _FILE_OFFSET_BITS);
       #endif

       #ifdef _BSD_SOURCE
	   printf("_BSD_SOURCE defined
");
       #endif

       #ifdef _SVID_SOURCE
	   printf("_SVID_SOURCE defined
");
       #endif

       #ifdef _ATFILE_SOURCE
	   printf("_ATFILE_SOURCE defined
");
       #endif

       #ifdef _GNU_SOURCE
	   printf("_GNU_SOURCE defined
");
       #endif

       #ifdef _REENTRANT
	   printf("_REENTRANT defined
");
       #endif

       #ifdef _THREAD_SAFE
	   printf("_THREAD_SAFE defined
");
       #endif

       #ifdef _FORTIFY_SOURCE
	   printf("_FORTIFY_SOURCE defined
");
       #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 3.25 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								    2009-12-13						    FEATURE_TEST_MACROS(7)