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.

       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,
       doing 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
       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)):

           #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
          _DEFAULT_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  defini-
                  tions for POSIX.1b (real-time extensions).

               o  The  value  199506L  or greater additionally exposes defini-
                  tions for POSIX.1c (threads).

               o  (Since glibc 2.3.3) The value 200112L or  greater  addition-
                  ally  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) features to be  exposed  (in  other  words,  the
                  equivalent of defining _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
                  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).

               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_EXTENDED.

       _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).  Defining _XOPEN_SOURCE with a  value  of
               500   or  more  also  produces  the  same  effect  as  defining
               _XOPEN_SOURCE_EXTENDED.  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
               (obsolete) 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
               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 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  programs  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  per-
               mits  correctly written programs to use large files with only a
               recompilation 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_EXTENDED, 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  com-
               pile-time  warning  (unless  _DEFAULT_SOURCE  is also defined).
               Use _DEFAULT_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  Inter-
               face Definition; 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
               defined,  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  ear-
               lier,  these  defaults were approximately equivalent to explic-
               itly 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  defined  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
               _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 ver-
               sions before 2.10; 500 in glibc versions before 2.2).  In addi-
               tion, various 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 enabled  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
               various string and memory manipulation functions (for  example,
               memcpy(3),  memset(3),  stpcpy(3),  strcpy(3), strncpy(3), str-
               cat(3),  strncat(3),  sprintf(3),   snprintf(3),   vsprintf(3),
               vsnprintf(3),  gets(3),  and  wide character variants thereof).
               For some functions, argument consistency is checked; for  exam-
               ple, 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 detected, 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 _FOR-
               TIFY_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
               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 (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
       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 _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 4.13 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                             2017-09-15            FEATURE_TEST_MACROS(7)

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