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FEATURE_TEST_MACROS(7)	   Linux Programmers Manual    FEATURE_TEST_MACROS(7)

       feature_test_macros - feature test macros


       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
       non-standard definitions from being exposed.  Other macros can be  used
       to  expose  non-standard  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  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 compilation
       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 man
       ual page SYNOPSIS typically includes a note of the following form (this
       example from the chmod(2) manual page):


	      int chmod(const char *path, mode_t mode);
	      int fchmod(int fd, mode_t mode);

	  Feature   Test   Macro   Requirements   for	 glibc	  (see	  fea

	      fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

       The  || means that in order to obtain the declaration of fchmod(2) from
       , 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

	      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:

	      ISO Standard C.  This macro is implicitly defined by gcc(1) when
	      invoked with, for example, the -std=c99 or -ansi flag.

	      Defining this macro causes header files to expose definitions as

		The value 1 exposes definitions  conforming  to  POSIX.1-1990
		 and ISO C (1990).

		The  value  2 or greater additionally exposes definitions for

		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).

	      Defining this obsolete macro with any  value  is	equivalent  to
	      defining _POSIX_C_SOURCE with the value 1.

	      Defining this macro causes header files to expose definitions as

		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.

	      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.

	      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.

	      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.

	      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.

	      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,
	      _GNU_SOURCE is defined, in which case BSD definitions are disfa

	      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

	      Defining this macro (with any value) is equivalent  to  defining
	      _POSIX_C_SOURCE  with  the  value 200112L (199506L in glibc ver
	      sions before 2.5), and _XOPEN_SOURCE with the value 600 (500  in
	      glibc  versions  before 2.2).  In addition, various GNU-specific
	      extensions are also exposed.  Where standards conflict, BSD def
	      initions are disfavored.

	      Defining	this  macro  exposes  definitions of certain reentrant
	      functions.  For multithreaded programs, use cc -pthread instead.

	      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 shouldnt 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=200112L (199506L in  glibc  versions
       before 2.4).

       If    any    of	  __STRICT_ANSI__,    _ISOC99_SOURCE,	_POSIX_SOURCE,
       _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

		  200112L   (199506L   in  glibc  versions  before  2.4),  if
		   XOPEN_SOURCE is undefined,  or  is  defined	with  a  value
		   greater than or equal to 600.

       Multiple macros can be defined; the results are additive.

       _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.

       TIFY_SOURCE,  _REENTRANT,  and  _THREAD_SAFE  are  specific  to	 Linux

        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

       According  to which of the above feature test macros are defined,  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.


       The section "Feature Test Macros" under info libc.


       This page is part of release 3.05 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				  2008-01-02		FEATURE_TEST_MACROS(7)

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