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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.
       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 <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 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):

              #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    fea-
          ture_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 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
              #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 def-
                 initions corresponding to the POSIX.1-2001 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  extension)  and
                 C99 definitions.

       _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  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.

       _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/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.

       _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 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).

       _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 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.

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

       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  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.

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.

SEE ALSO
       standards(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON
       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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