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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 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

       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.

       _TIME_BITS
              Defining this macro with the value 64 changes the width of time_t(3type) to  64-bit
              which  allows  handling  of  timestamps  beyond  2038.   It  is  closely related to
              _FILE_OFFSET_BITS and depending on implementation, may require it set.  This  macro
              is available as of glibc 2.34.

       _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 before glibc 2.10; 199506L before
              glibc 2.5; 199309L before glibc 2.1) and _XOPEN_SOURCE  with  the  value  700  (600
              before  glibc  2.10;  500  before  glibc  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.   Before  glibc  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.

              With _FORTIFY_SOURCE set to 3, additional  checking  is  added  to  intercept  some
              function calls used with an argument of variable size where the compiler can deduce
              an upper bound for its value.   For  example,  a  program  where  malloc(3)'s  size
              argument is variable can now be fortified.

              Use of this macro requires compiler support, available with gcc(1) since glibc 4.0.

              Use of _FORTIFY_SOURCE set to 3 requires gcc(1) version 12.0 or later.

   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 before glibc 2.10;  199506L  before  glibc  2.4;  199309L
       before glibc 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, before glibc 2.4; 200112L, since glibc 2.4 to glibc 2.9; and
             200809L, since glibc 2.10.

       Multiple macros can be defined; the results are additive.

STANDARDS

       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 _TIME_BITS
           printf("_TIME_BITS defined: %d\n", _TIME_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