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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 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 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 dec-
       laration, 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  corresponding  to  the
                 POSIX.1-2001 base specification (excluding the XSI extension).

              ?  (Since glibc 2.10) The value 200809L or greater exposes definitions corresponding 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 definitions 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 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 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/platform/lfs.html.)  The  alter-
              native  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 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 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 Definition; 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 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 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  pro-
              grams  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  warn-
              ings;  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, _REENTRANT, and _THREAD_SAFE are spe-
       cific 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 neces-
       sary 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\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 informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-04-03            FEATURE_TEST_MACROS(7)