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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 compilation 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  because the header <abc.h> itself includes
       <xyz.h> (POSIX explicitly allows 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, 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  manual  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 compilation 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:

               ·  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  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) features to be exposed (in other words, the equivalent of  defining
                  _ISOC99_SOURCE).

               ·  (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  documented  when
               discussing feature test macro requirements in the man pages.

       _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).

               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:

               ·  _POSIX_SOURCE is defined with the value 1.

               ·  _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  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 (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   compile-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 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" definitions 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 originally 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  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 versions before 2.10; 500 in  glibc
               versions  before  2.2).   In  addition,  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 definitions 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 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 (for example, memcpy(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  functions,  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 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  _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 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:

          ·  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 200112L 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), 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,  <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 _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 5.02 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/.