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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 <xyz.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  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,   _ISOC11_SOURCE   (since   glibc   2.18),
       _POSIX_SOURCE,  _POSIX_C_SOURCE,  _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED (in glibc 2.11 and
       earlier), _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.

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 <stdint.h>
       #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: %jdL\n",
                   (intmax_t) _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), system_data_types(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON

       This  page  is  part of release 5.10 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/.