GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


man:feature_test_macros

FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)

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 com-
     pilation 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 man-
     ual 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   fea-
         ture_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 compila-
     tion 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:
             o  The value 1 exposes definitions conforming  to  POSIX.1-1990
                and ISO C (1990).
             o  The  value 2 or greater additionally exposes definitions for
                POSIX.2-1992.
             o  The value 199309L or greater  additionally  exposes  defini-
                tions for POSIX.1b (real-time extensions).
             o  The  value  199506L  or greater additionally exposes defini-
                tions for POSIX.1c (threads).
             o  (Since glibc 2.3.3) The value 200112L or  greater  addition-
                ally  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).
             o  (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  docu-
             mented  when  discussing feature test macro requirements in the
             man pages.
     _XOPEN_SOURCE
             Defining this macro causes header files to  expose  definitions
             as follows:
             o  Defining  with  any  value exposes definitions conforming to
                POSIX.1, POSIX.2, and XPG4.
             o  The value 500 or greater  additionally  exposes  definitions
                for SUSv2 (UNIX 98).
             o  (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.
             o  (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:
             o  _POSIX_SOURCE is defined with the value 1.
             o  _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 The alternative API consists of a set
             of new objects (i.e., functions and types) whose names are suf-
             fixed  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 per-
             mits correctly written programs to use large files with only  a
             recompilation being required.)
             64-bit systems naturally permit file sizes greater than 2 Giga-
             bytes, 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 com-
             pile-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 Inter-
             face 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" defini-
             tions 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 origi-
             nally  derived  from  BSD and System V.  On glibc 2.19 and ear-
             lier, these defaults were approximately equivalent  to  explic-
             itly 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  ver-
             sions before 2.10; 500 in glibc versions before 2.2).  In addi-
             tion, 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 defini-
             tions 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  compati-
             bility with some other implementations.
     _FORTIFY_SOURCE (since glibc 2.3.4)
             Defining  this  macro causes some lightweight checks to be per-
             formed 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),  str-
             cat(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 exam-
             ple, 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  _FOR-
             TIFY_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:
        o  2, if _XOPEN_SOURCE is defined with a value less than 500;
        o  199506L, if _XOPEN_SOURCE is defined with a value greater than or
           equal to 500 and less than 600; or
        o  (since glibc 2.4) 200112L, if _XOPEN_SOURCE  is  defined  with  a
           value greater than or equal to 600 and less than 700.
        o  (Since  glibc  2.10)  200809L, if _XOPEN_SOURCE is defined with a
           value greater than or equal to 700.
        o  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.
        o  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, <fea-
     tures.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 _LARGE-
         FILE64_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

     This  page  is  part of release 4.16 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/.

Linux 2017-09-15 FEATURE_TEST_MACROS(7)

/data/webs/external/dokuwiki/data/pages/man/feature_test_macros.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

Was this page helpful?-13+1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki