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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  exposes  definitions
                 corresponding  to  the  POSIX.1-2001  base  specification  (excluding  the   XSI
                 extension).

              ·  (Since   glibc   2.10)   The   value  200809L  or  greater  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).

       _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.

       _ISOC95_SOURCE
              Exposes ISO C (1990) Amendment 1 definitions (also known as C95).   This  macro  is
              recognized   since  glibc  2.12.   The  primary  change  in  C95  was  support  for
              international character sets.  The C95 changes were included in the subsequent  C99
              standard (in other words, _ISOC99_SOURCE implies _ISOC95_SOURCE).

       _ISOC99_SOURCE
              Exposes  C99  extensions  to  ISO  C  (1990).  This macro is recognized since glibc
              2.1.3;  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 the latter macro is  obsolete,  glibc  continues  to  recognize  it  for
              backward compatibility.

       _ISOC11_SOURCE
              Exposes  declarations  consistent  with  the  ISO  C11  standard.   This  macro  is
              recognized since glibc 2.16.

       _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 interface; instead _FILE_OFFSET_BITS=64 should be employed.

       _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
              Defining  this  macro  with  any  value  causes  header files to expose BSD-derived
              definitions.  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.

       _SVID_SOURCE
              Defining  this  macro with any value causes header files to expose System V-derived
              definitions.  (SVID == System V Interface Definition; see standards(7).)

       _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)  is  equivalent  to defining _BSD_SOURCE,
              _SVID_SOURCE,      _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.  Where standards conflict, BSD
              definitions are disfavored.

       _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.   Not all buffer overflows are detected, just some common cases.  In the
              current implementation, checks  are  added  for  calls  to  memcpy(3),  mempcpy(3),
              memmove(3),  memset(3),  stpcpy(3),  strcpy(3),  strncpy(3), strcat(3), strncat(3),
              sprintf(3),   snprintf(3),   vsprintf(3),   vsnprintf(3),    and    gets(3).     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,  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,    _SVID_SOURCE,    _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,   or   _SVID_SOURCE  is  explicitly  defined,  then
       _BSD_SOURCE, and _SVID_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,  _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 _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 _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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