feature_test_macros(7)



FEATURE_TEST_MACROS(7)     Linux Programmer's Manual    FEATURE_TEST_MACROS(7)

NAME
       feature_test_macros - feature test macros

DESCRIPTION
       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.  The requirement that the
       macro must be defined before including any header file  exists  because
       header files may freely include one another.  Thus, for example, in the
       following lines, defining the _GNU_SOURCE macro may have no effect  be-
       cause  the header <abc.h> itself includes <xyz.h> (POSIX explicitly al-
       lows this):

           #include <abc.h>
           #define _GNU_SOURCE
           #include <xys.h>

       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.   Note:
       applications  do not need to directly include <features.h>; indeed, do-
       ing so is actively discouraged.  See NOTES.

   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
           feature_test_macros(7)):

               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 be-
       fore 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)):

           #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 paragraphs below explain how feature test  macros  are  handled  in
       Linux glibc 2.x, x > 0.

       First, though a summary of a few details for the impatient:

       *  The  macros  that  you most likely need to use in modern source code
          are  _POSIX_C_SOURCE  (for  definitions  from  various  versions  of
          POSIX.1),  _XOPEN_SOURCE  (for  definitions from various versions of
          SUS), _GNU_SOURCE (for GNU and/or Linux specific  stuff),  and  _DE-
          FAULT_SOURCE  (to get definitions that would normally be provided by
          default).

       *  Certain macros are defined with default values.  Thus, although  one
          or more macros may be indicated as being required in the SYNOPSIS of
          a man page, it may not be necessary to define them explicitly.  Full
          details of the defaults are given later in this man page.

       *  Defining  _XOPEN_SOURCE  with a value of 600 or greater produces the
          same effects as defining _POSIX_C_SOURCE with a value of 200112L  or
          greater.  Where one sees

              _POSIX_C_SOURCE >= 200112L

          in  the  feature  test  macro  requirements in the SYNOPSIS of a man
          page, it is implicit that the following has the same effect:

              _XOPEN_SOURCE >= 600

       *  Defining _XOPEN_SOURCE with a value of 700 or greater  produces  the
          same  effects as defining _POSIX_C_SOURCE with a value of 200809L or
          greater.  Where one sees

              _POSIX_C_SOURCE >= 200809L

          in the feature test macro requirements in  the  SYNOPSIS  of  a  man
          page, it is implicit that the following has the same effect:

              _XOPEN_SOURCE >= 700

       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 additionally
                 exposes definitions corresponding to  the  POSIX.1-2001  base
                 specification (excluding the XSI extension).  This value also
                 causes C95 (since glibc 2.12) and C99 (since glibc 2.10) fea-
                 tures to be exposed (in other words, the equivalent of defin-
                 ing _ISOC99_SOURCE).

              o  (Since glibc 2.10) The value 200809L or greater  additionally
                 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.

              Since  this  macro is obsolete, its usage is generally not docu-
              mented when discussing feature test macro  requirements  in  the
              man pages.

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

              o  (Since  glibc 2.10) The value 700 or greater additionally ex-
                 poses definitions for  SUSv4  (i.e.,  the  POSIX.1-2008  base
                 specification plus the XSI extension).

              If  __STRICT_ANSI__  is not defined, or _XOPEN_SOURCE is defined
              with  a  value  greater  than  or  equal  to  500  and   neither
              _POSIX_SOURCE  nor  _POSIX_C_SOURCE  is explicitly defined, then
              the following macros are implicitly defined:

              o  _POSIX_SOURCE is defined with the value 1.

              o  _POSIX_C_SOURCE  is  defined,  according  to  the  value   of
                 _XOPEN_SOURCE:

                 _XOPEN_SOURCE < 500
                        _POSIX_C_SOURCE is defined with the value 2.

                 500 <= _XOPEN_SOURCE < 600
                        _POSIX_C_SOURCE is defined with the value 199506L.

                 600 <= _XOPEN_SOURCE < 700
                        _POSIX_C_SOURCE is defined with the value 200112L.

                 700 <= _XOPEN_SOURCE (since glibc 2.10)
                        _POSIX_C_SOURCE is defined with the value 200809L.

              In  addition,  defining  _XOPEN_SOURCE  with  a  value of 500 or
              greater produces the same effects as defining  _XOPEN_SOURCE_EX-
              TENDED.

       _XOPEN_SOURCE_EXTENDED
              If this macro is defined, and _XOPEN_SOURCE is defined, then ex-
              pose definitions corresponding to the XPG4v2 (SUSv1) UNIX exten-
              sions  (UNIX 95).  Defining _XOPEN_SOURCE with a value of 500 or
              more also produces the same effect as defining _XOPEN_SOURCE_EX-
              TENDED.  Use of _XOPEN_SOURCE_EXTENDED in new source code should
              be avoided.

              Since defining _XOPEN_SOURCE with a value of 500 or more has the
              same  effect as defining _XOPEN_SOURCE_EXTENDED, the latter (ob-
              solete) feature test macro is generally  not  described  in  the
              SYNOPSIS in man pages.

       _ISOC99_SOURCE (since glibc 2.1.3)
              Exposes declarations consistent with the ISO C99 standard.

              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 this macro is obsolete, glibc
              continues to recognize it for backward compatibility.

              Defining _ISOC99_SOURCE also exposes ISO C  (1990)  Amendment  1
              ("C95") definitions.  (The primary change in C95 was support for
              international character sets.)

              Invoking the C compiler with the option  -std=c99  produces  the
              same effects as defining this macro.

       _ISOC11_SOURCE (since glibc 2.16)
              Exposes  declarations  consistent  with  the  ISO  C11 standard.
              Defining this macro also enables  C99  and  C95  features  (like
              _ISOC99_SOURCE).

              Invoking  the  C  compiler with the option -std=c11 produces the
              same effects as defining this macro.

       _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 ob-
              jects (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  macro;  instead
              _FILE_OFFSET_BITS=64 should be employed.

       _LARGEFILE_SOURCE
              This  macro  was  historically  used to expose certain functions
              (specifically fseeko(3) and ftello(3)) that address  limitations
              of  earlier  APIs  (fseek(3) and ftell(3)) that use long int for
              file offsets.  This macro is implicitly defined if _XOPEN_SOURCE
              is  defined with a value greater than or equal to 500.  New pro-
              grams should not employ this macro;  defining  _XOPEN_SOURCE  as
              just  described  or defining _FILE_OFFSET_BITS with the value 64
              is the preferred mechanism to achieve the same result.

       _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 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  Giga-
              bytes, and on those systems this macro has no effect.

       _BSD_SOURCE (deprecated since glibc 2.20)
              Defining this macro with any value causes header files to expose
              BSD-derived definitions.

              In glibc versions up to and including 2.18, 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_EX-
              TENDED, or _GNU_SOURCE is defined, in which case BSD definitions
              are  disfavored.  Since glibc 2.19, _BSD_SOURCE no longer causes
              BSD definitions to be preferred in case of conflicts.

              Since glibc 2.20, this macro is deprecated.  It now has the same
              effect as defining _DEFAULT_SOURCE, but generates a compile-time
              warning (unless _DEFAULT_SOURCE  is  also  defined).   Use  _DE-
              FAULT_SOURCE  instead.   To allow code that requires _BSD_SOURCE
              in glibc 2.19 and earlier and _DEFAULT_SOURCE in glibc 2.20  and
              later  to  compile without warnings, define both _BSD_SOURCE and
              _DEFAULT_SOURCE.

       _SVID_SOURCE (deprecated since glibc 2.20)
              Defining this macro with any value causes header files to expose
              System V-derived definitions.  (SVID == System V Interface Defi-
              nition; see standards(7).)

              Since glibc 2.20, this macro is deprecated in the  same  fashion
              as _BSD_SOURCE.

       _DEFAULT_SOURCE (since glibc 2.19)
              This  macro  can be defined to ensure that the "default" defini-
              tions are provided even when the  defaults  would  otherwise  be
              disabled,  as  happens when individual macros are explicitly de-
              fined, or the compiler is invoked in one of its "standard" modes
              (e.g.,  cc -std=c99).  Defining _DEFAULT_SOURCE without defining
              other individual macros or invoking the compiler in one  of  its
              "standard" modes has no effect.

              The   "default"   definitions   comprise   those   required   by
              POSIX.1-2008 and ISO C99, as well as various definitions  origi-
              nally derived from BSD and System V.  On glibc 2.19 and earlier,
              these  defaults  were  approximately  equivalent  to  explicitly
              defining the following:

                  cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809

       _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 de-
              fined if _POSIX_C_SOURCE is defined with a value greater than or
              equal to 200809L.

       _GNU_SOURCE
              Defining  this  macro  (with  any value) implicitly defines _AT-
              FILE_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  be-
              fore 2.10; 500 in glibc versions before 2.2).  In addition, var-
              ious GNU-specific extensions are also exposed.

              Since glibc 2.19, defining _GNU_SOURCE also has  the  effect  of
              implicitly  defining  _DEFAULT_SOURCE.  In glibc versions before
              2.20, defining _GNU_SOURCE also had  the  effect  of  implicitly
              defining _BSD_SOURCE and _SVID_SOURCE.

       _REENTRANT
              Historically,  on various C libraries it was necessary to define
              this macro in all multithreaded code.   (Some  C  libraries  may
              still  require this.)  In glibc, this macro also exposed defini-
              tions of certain reentrant functions.

              However, glibc has been thread-safe by default for  many  years;
              since glibc 2.3, the only effect of defining _REENTRANT has been
              to enable one or two of the same declarations that are also  en-
              abled  by  defining  _POSIX_C_SOURCE  with a value of 199606L or
              greater.

              _REENTRANT is now obsolete.  In glibc 2.25 and  later,  defining
              _REENTRANT  is  equivalent  to defining _POSIX_C_SOURCE with the
              value 199606L.  If a higher POSIX conformance level is  selected
              by   any   other   means   (such   as   _POSIX_C_SOURCE  itself,
              _XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE),  then  defining
              _REENTRANT has no effect.

              This  macro  is  automatically  defined  if  one  compiles  with
              cc -pthread.

       _THREAD_SAFE
              Synonym for the (deprecated) _REENTRANT, provided  for  compati-
              bility 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 (for example, mem-
              cpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3),  strcat(3),
              strncat(3),  sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3),
              gets(3), and wide character variants thereof).  For  some  func-
              tions,  argument consistency is checked; for example, a check is
              made that open(2) has been supplied with a  mode  argument  when
              the  specified  flags include O_CREAT.  Not all problems are de-
              tected, just some common cases.

              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 (via  macros
              logic implemented in header files), 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 (in glibc 2.19
       and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE
       (since glibc 2.19), _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,
       _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE (in
       glibc 2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is
       explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_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  a
             value greater than or equal to 600 and less than 700.

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

          o  Older versions of glibc do not know about the values 200112L  and
             200809L  for  _POSIX_C_SOURCE, and the setting of this macro will
             depend on the glibc version.

          o  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), but is not
       present in SUSv2 and later.  _FILE_OFFSET_BITS is not specified by  any
       standard, but is employed on some other implementations.

       _BSD_SOURCE,     _SVID_SOURCE,     _DEFAULT_SOURCE,     _ATFILE_SOURCE,
       _GNU_SOURCE, _FORTIFY_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 un-
       derscores (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.

EXAMPLES
       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 _ISOC11_SOURCE
           printf("_ISOC11_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 _DEFAULT_SOURCE
           printf("_DEFAULT_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 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-04-11            FEATURE_TEST_MACROS(7)

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