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NAME

       feature_test_macros - feature test macros

SYNOPSIS

       #include <features.h>

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.

       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.

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

       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) and also causes C95 (since glibc 2.12) and C99 (since glibc 2.10)
                 features to be exposed.

              ·  (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.

       _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.

       _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).  This macro is  also
              implicitly defined if _XOPEN_SOURCE is defined with a value of 500 or more.

       _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.)

       _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).

       _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  as  well  as
              various  definitions  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
              Defining  this  macro  exposes  definitions  of  certain  reentrant functions.  For
              multithreaded programs, use cc -pthread instead.

       _THREAD_SAFE
              Synonym for _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).

       _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

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