feature_test_macros(7) - phpMan

Command: man perldoc info search(apropos)  


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 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 non-
       standard  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 re-
       quirements (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 ex-
       pose 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  _DE-
          FAULT_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 definitions for POSIX.1b
                 (real-time extensions).

              o  The value 199506L or  greater  additionally  exposes  definitions  for  POSIX.1c
                 (threads).

              o  (Since  glibc  2.3.3)  The value 200112L or greater additionally exposes defini-
                 tions 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  exten-
                 sion).

       _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:

              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 exten-
                 sion) 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 gener-
              ally 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 em-
              ploy 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  refer-
              ences  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  defi-
              nitions.

              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  indi-
              vidual  macros  or  invoking the compiler in one of its "standard" modes has no ef-
              fect.

              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 defin-
              ing 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,  _LARGE-
              FILE64_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 ex-
              posed.

              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 decla-
              rations 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 equiv-
              alent to defining _POSIX_C_SOURCE with the value 199606L.  If a higher  POSIX  con-
              formance  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 func-
              tions (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  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  per-
              formed.   With _FORTIFY_SOURCE set to 2, some more checking is added, but some con-
              forming 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,   _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 ear-
       lier) 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, <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 de-
       pending on the glibc version and what feature test macros are explicitly set.  The follow-
       ing 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 _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

COLOPHON
       This  page  is  part of release 5.10 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                                       2020-11-01                     FEATURE_TEST_MACROS(7)

Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2024-12-05 21:45 @18.97.14.87 CrawledBy CCBot/2.0 (https://commoncrawl.org/faq/)
Valid XHTML 1.0!Valid CSS!