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

NAME         top

       feature_test_macros - feature test macros

SYNOPSIS         top

       #include <features.h>

DESCRIPTION         top

       Feature test macros allow the programmer to control the definitions that
       are exposed by system header files when a program is compiled.

       NOTE: In order to be effective, a feature test macro must be defined before
       including any header files.  This can be done either in the compilation
       command (cc -DMACRO=value) or by defining the macro within the source code
       before including any headers.

       Some feature test macros are useful for creating portable applications, by
       preventing nonstandard definitions from being exposed.  Other macros can be
       used to expose nonstandard definitions that are not exposed by default.
       The precise effects of each of the feature test macros described below can
       be ascertained by inspecting the <features.h> header file.

Specification of feature test macro requirements in manual pages

       When a function requires that a feature test macro is defined, the manual
       page SYNOPSIS typically includes a note of the following form (this example
       from the acct(2) manual page):

               #include <unistd.h>

               int acct(const char *filename);

           Feature Test Macro Requirements for glibc (see
           feature_test_macros(7)):

               acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)

       The || means that in order to obtain the declaration of acct(2) from
       <unistd.h>, either of the following macro definitions must be made before
       including any header files:

              #define _BSD_SOURCE
              #define _XOPEN_SOURCE        /* or any value < 500 */

       Alternatively, equivalent definitions can be included in the compilation
       command:

              cc -D_BSD_SOURCE
              cc -D_XOPEN_SOURCE           # Or any value < 500

       Note that, as described below, some feature test macros are defined by
       default, so that it may not always be necessary to explicitly specify the
       feature test macro(s) shown in the SYNOPSIS.

       In a few cases, manual pages use a shorthand for expressing the feature
       test macro requirements (this example from readahead(2)):

              #define _GNU_SOURCE
              #include <fcntl.h>

              ssize_t readahead(int fd, off64_t *offset, size_t count);

       This format is employed in cases where only a single feature test macro can
       be used to expose the function declaration, and that macro is not defined
       by default.

Feature test macros understood by glibc

       The following paragraphs explain how feature test macros are handled in
       Linux glibc 2.x, x > 0.

       Linux glibc understands the following feature test macros:

       __STRICT_ANSI__
              ISO Standard C.  This macro is implicitly defined by gcc(1) when
              invoked with, for example, the -std=c99 or -ansi flag.

       _POSIX_C_SOURCE
              Defining this macro causes header files to expose definitions as
              follows:

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

              o  (Since glibc 2.10) The value 200809L or greater exposes
                 definitions corresponding to the POSIX.1-2008 base specification
                 (excluding the XSI extension).

       _POSIX_SOURCE
              Defining this obsolete macro with any value is equivalent to
              defining _POSIX_C_SOURCE with the value 1.

       _XOPEN_SOURCE
              Defining this macro causes header files to expose definitions as
              follows:

              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).

       _XOPEN_SOURCE_EXTENDED
              If this macro is defined, and _XOPEN_SOURCE is defined, then expose
              definitions corresponding to the XPG4v2 (SUSv1) UNIX extensions
              (UNIX 95).  This macro is also implicitly defined if _XOPEN_SOURCE
              is defined with a value of 500 or more.

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

       _ISOC99_SOURCE
              Exposes C99 extensions to ISO C (1990).  This macro is recognized
              since glibc 2.1.3; earlier glibc 2.1.x versions recognized an
              equivalent macro named _ISOC9X_SOURCE (because the C99 standard had
              not then been finalized).  Although the use of the latter macro is
              obsolete, glibc continues to recognize it for backward
              compatibility.

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

       _LARGEFILE64_SOURCE
              Expose definitions for the alternative API specified by the LFS
              (Large File Summit) as a "transitional extension" to the Single UNIX
              Specification.  (See http://opengroup.org/platform/lfs.html.)  The
              alternative API consists of a set of new objects (i.e., functions
              and types) whose names are suffixed with "64" (e.g., off64_t versus
              off_t, lseek64() versus lseek(), etc.).  New programs should not
              employ this interface; instead _FILE_OFFSET_BITS=64 should be
              employed.

       _FILE_OFFSET_BITS
              Defining this macro with the value 64 automatically converts
              references to 32-bit functions and data types related to file I/O
              and file system operations into references to their 64-bit
              counterparts.  This is useful for performing I/O on large files (> 2
              Gigabytes) on 32-bit systems.  (Defining this macro permits
              correctly written programs to use large files with only a
              recompilation being required.)  64-bit systems naturally permit file
              sizes greater than 2 Gigabytes, and on those systems this macro has
              no effect.

       _BSD_SOURCE
              Defining this macro with any value causes header files to expose
              BSD-derived definitions.  Defining this macro also causes BSD
              definitions to be preferred in some situations where standards
              conflict, unless one or more of _SVID_SOURCE, _POSIX_SOURCE,
              _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, or
              _GNU_SOURCE is defined, in which case BSD definitions are
              disfavored.

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

       _ATFILE_SOURCE (since glibc 2.4)
              Defining this macro with any value causes header files to expose
              declarations of a range of functions with the suffix "at"; see
              openat(2).  Since glibc 2.10, this macro is also implicitly defined
              if _POSIX_C_SOURCE is defined with a value greater than or equal to
              200809L.

       _GNU_SOURCE
              Defining this macro (with any value) is equivalent to defining
              _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _LARGEFILE64_SOURCE,
              _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE,
              _POSIX_C_SOURCE with the value 200809L (200112L in glibc versions
              before 2.10; 199506L in glibc versions before 2.5; 199309L in glibc
              versions before 2.1) and _XOPEN_SOURCE with the value 700 (600 in
              glibc versions before 2.10; 500 in glibc versions before 2.2).  In
              addition, various GNU-specific extensions are also exposed.  Where
              standards conflict, BSD definitions are disfavored.

       _REENTRANT
              Defining this macro exposes definitions of certain reentrant
              functions.  For multithreaded programs, use cc -pthread instead.

       _THREAD_SAFE
              Synonym for _REENTRANT, provided for compatibility with some other
              implementations.

       _FORTIFY_SOURCE (since glibc 2.3.4)
              Defining this macro causes some lightweight checks to be performed
              to detect some buffer overflow errors when employing various string
              and memory manipulation functions.  Not all buffer overflows are
              detected, just some common cases.  In the current implementation
              checks are added for calls to memcpy(3), mempcpy(3), memmove(3),
              memset(3), stpcpy(3), strcpy(3), strncpy(3), strcat(3), strncat(3),
              sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3), and gets(3).  If
              _FORTIFY_SOURCE is set to 1, with compiler optimization level 1
              (gcc -O1) and above, checks that shouldn't change the behavior of
              conforming programs are performed.  With _FORTIFY_SOURCE set to 2
              some more checking is added, but some conforming programs might
              fail.  Some of the checks can be performed at compile time, and
              result in compiler warnings; other checks take place at run time,
              and result in a run-time error if the check fails.  Use of this
              macro requires compiler support, available with gcc(1) since version
              4.0.

Default definitions, implicit definitions, and combining definitions

       If no feature test macros are explicitly defined, then the following
       feature test macros are defined by default: _BSD_SOURCE, _SVID_SOURCE,
       _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L in glibc versions
       before 2.10; 199506L in glibc versions before 2.4; 199309L in glibc
       versions before 2.1).

       If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE, _POSIX_C_SOURCE,
       _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE, or _SVID_SOURCE is
       explicitly defined, then _BSD_SOURCE, and _SVID_SOURCE are not defined by
       default.

       If _POSIX_SOURCE and _POSIX_C_SOURCE are not explicitly defined, and either
       __STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with a value of
       500 or more, then

          *  _POSIX_SOURCE is defined with the value 1; and

          *  _POSIX_C_SOURCE is defined with one of the following values:

                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         top

       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1).

       _FILE_OFFSET_BITS is not specified by any standard, but is employed on some
       other implementations.

       _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _GNU_SOURCE, _FORTIFY_SOURCE,
       _REENTRANT, and _THREAD_SAFE are specific to Linux (glibc).

NOTES         top

       <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         top

       The program below can be used to explore how the various feature test
       macros are set depending on the glibc version and what feature test macros
       are explicitly set.  The following shell session, on a system with glibc
       2.10, shows some examples of what we would see:

           $ cc ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 200809L
           _BSD_SOURCE defined
           _SVID_SOURCE defined
           _ATFILE_SOURCE defined
           $ cc -D_XOPEN_SOURCE=500 ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 199506L
           _XOPEN_SOURCE defined: 500
           $ cc -D_GNU_SOURCE ftm.c
           $ ./a.out
           _POSIX_SOURCE defined
           _POSIX_C_SOURCE defined: 200809L
           _ISOC99_SOURCE defined
           _XOPEN_SOURCE defined: 700
           _XOPEN_SOURCE_EXTENDED defined
           _LARGEFILE64_SOURCE defined
           _BSD_SOURCE defined
           _SVID_SOURCE defined
           _ATFILE_SOURCE defined
           _GNU_SOURCE defined

Program source


       /* ftm.c */

       #include <stdio.h>
       #include <unistd.h>
       #include <stdlib.h>

       int
       main(int argc, char *argv[])
       {
       #ifdef _POSIX_SOURCE
           printf("_POSIX_SOURCE defined\n");
       #endif

       #ifdef _POSIX_C_SOURCE
           printf("_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
       #endif

       #ifdef _ISOC99_SOURCE
           printf("_ISOC99_SOURCE defined\n");
       #endif

       #ifdef _XOPEN_SOURCE
           printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
       #endif

       #ifdef _XOPEN_SOURCE_EXTENDED
           printf("_XOPEN_SOURCE_EXTENDED defined\n");
       #endif

       #ifdef _LARGEFILE64_SOURCE
           printf("_LARGEFILE64_SOURCE defined\n");
       #endif

       #ifdef _FILE_OFFSET_BITS
           printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
       #endif

       #ifdef _BSD_SOURCE
           printf("_BSD_SOURCE defined\n");
       #endif

       #ifdef _SVID_SOURCE
           printf("_SVID_SOURCE defined\n");
       #endif

       #ifdef _ATFILE_SOURCE
           printf("_ATFILE_SOURCE defined\n");
       #endif

       #ifdef _GNU_SOURCE
           printf("_GNU_SOURCE defined\n");
       #endif

       #ifdef _REENTRANT
           printf("_REENTRANT defined\n");
       #endif

       #ifdef _THREAD_SAFE
           printf("_THREAD_SAFE defined\n");
       #endif

       #ifdef _FORTIFY_SOURCE
           printf("_FORTIFY_SOURCE defined\n");
       #endif

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       libc(7), standards(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON         top

       This page is part of release 3.41 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                               2012-01-18              FEATURE_TEST_MACROS(7)

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