#include <features.h>

       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

               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)):

       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

              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

              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).

              o  (Since glibc 2.10) The value 200809L or greater exposes defi-
                 nitions corresponding to the POSIX.1-2008 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

              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.

              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).

              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.

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

              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.

              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.

              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
              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.

              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 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,
       _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
                   and 200809L, since glibc 2.10.

       Multiple macros can be defined; the results are additive.

       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.

       TIFY_SOURCE,   _REENTRANT,  and  _THREAD_SAFE  are  specific  to  Linux

       <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

       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.

       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

       #include <stdlib.h>

       main(int argc, char *argv[])
       #ifdef _POSIX_SOURCE
           printf("_POSIX_SOURCE defined\n");

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

       #ifdef _ISOC99_SOURCE
           printf("_ISOC99_SOURCE defined\n");

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

           printf("_XOPEN_SOURCE_EXTENDED defined\n");

       #ifdef _LARGEFILE64_SOURCE
           printf("_LARGEFILE64_SOURCE defined\n");

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

       #ifdef _BSD_SOURCE
           printf("_BSD_SOURCE defined\n");

       #ifdef _SVID_SOURCE
           printf("_SVID_SOURCE defined\n");

       #ifdef _ATFILE_SOURCE
           printf("_ATFILE_SOURCE defined\n");

       #ifdef _GNU_SOURCE
           printf("_GNU_SOURCE defined\n");

       #ifdef _REENTRANT
           printf("_REENTRANT defined\n");

       #ifdef _THREAD_SAFE

       The section "Feature Test Macros" under info libc.


       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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