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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 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  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
               expose definitions corresponding to the XPG4v2 (SUSv1) UNIX ex-
               tensions (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 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 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  ex-
               pose 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  ex-
               pose System V-derived definitions.  (SVID == System V Interface
               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 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 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 ex-
               pose 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 _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  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), vs-
               nprintf(3), gets(3), and wide character variants thereof).  For
               some functions, argument consistency is checked; for example, a
               check is made that open(2) has been supplied with a mode  argu-
               ment  when  the specified flags include O_CREAT.  Not all prob-
               lems 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 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.

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 5.05 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                             2019-03-06            FEATURE_TEST_MACROS(7)

NAME | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON