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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 manual page SYN-
       OPSIS 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 includ-
       ing 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:

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

              ?  The value 2 or greater additionally exposes definitions for POSIX.2-1992.

              ?  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 definitions cor-
                 responding to the POSIX.1-2001  base  specification  (excluding  the  XSI
                 extension).

              ?  (Since  glibc 2.10) The value 200809L or greater exposes definitions cor-
                 responding 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:

              ?  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  defini-
                 tions  for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification plus
                 the XSI extension) and C99 definitions.

              ?  (Since glibc 2.10) The value 700 or greater additionally exposes  defini-
                 tions  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  defini-
              tions  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  recog-
              nize 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  opera-
              tions into references to their 64-bit counterparts.  This is useful for per-
              forming 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
              definitions.  Defining this macro also causes BSD  definitions  to  be  pre-
              ferred  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 defini-
              tions 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 stan-
              dards(7).)

       _ATFILE_SOURCE (since glibc 2.4)
              Defining this macro with any value causes header files  to  expose  declara-
              tions 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
              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 implemen-
              tations.

       _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 manipu-
              lation functions.  Not all buffer overflows are detected, just  some  common
              cases.   In  the  current  implementation checks are added for calls to mem-
              cpy(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  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=200909L  (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:

                ?  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

                ?  (since  glibc  2.4)  200112L,  if  XOPEN_SOURCE is defined with a value
                   greater than or equal to 600 and less than 700.

                ?  (Since glibc 2.10) 200809L, if XOPEN_SOURCE is  defined  with  a  value
                   greater than or equal to 700.

                ?  Older  versions  of  glibc  do  not  know about the values 2001212L and
                   200809L for _POSIX_C_SOURCE, and the setting of this macro will  depend
                   on the glibc version.

                ?  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, _REEN-
       TRANT, and _THREAD_SAFE are specific to Linux (glibc).

NOTES
       <features.h> is a Linux/glibc-specific header file.  Other systems have  an  analo-
       gous  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  fea-
       ture 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\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 3.22 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                             2009-04-03            FEATURE_TEST_MACROS(7)

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