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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.

       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

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       https://www.kernel.org/doc/man-pages/.