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
       compilation command (cc -DMACRO=value) or by defining the macro within
       the source code before including any headers.  The requirement that the
       macro must be defined before including any header file exists because
       header files may freely include one another.  Thus, for example, in the
       following lines, defining the _GNU_SOURCE macro may have no effect
       because the header <abc.h> itself includes <xyz.h> (POSIX explicitly
       allows this):

           #include <abc.h>
           #define _GNU_SOURCE
           #include <xyz.h>

       Some feature test macros are useful for creating portable applications,
       by preventing nonstandard definitions from being exposed.  Other macros
       can be used to expose nonstandard definitions that are not exposed by
       default.

       The precise effects of each of the feature test macros described below
       can be ascertained by inspecting the <features.h> header file.  Note:
       applications do not need to directly include <features.h>; indeed, doing
       so is actively discouraged.  See NOTES.

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

              #include <unistd.h>

              int acct(const char *filename);

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

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

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

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

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

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

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

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

           #define _GNU_SOURCE
           #include <fcntl.h>

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

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

   Feature test macros understood by glibc
       The paragraphs below explain how feature test macros are handled in glibc
       2.x, x > 0.

       First, though, a summary of a few details for the impatient:

       *  The macros that you most likely need to use in modern source code are
          _POSIX_C_SOURCE (for definitions from various versions of POSIX.1),
          _XOPEN_SOURCE (for definitions from various versions of SUS),
          _GNU_SOURCE (for GNU and/or Linux specific stuff), and _DEFAULT_SOURCE
          (to get definitions that would normally be provided by default).

       *  Certain macros are defined with default values.  Thus, although one or
          more macros may be indicated as being required in the SYNOPSIS of a
          man page, it may not be necessary to define them explicitly.  Full
          details of the defaults are given later in this man page.

       *  Defining _XOPEN_SOURCE with a value of 600 or greater produces the
          same effects as defining _POSIX_C_SOURCE with a value of 200112L or
          greater.  Where one sees

              _POSIX_C_SOURCE >= 200112L

          in the feature test macro requirements in the SYNOPSIS of a man page,
          it is implicit that the following has the same effect:

              _XOPEN_SOURCE >= 600

       *  Defining _XOPEN_SOURCE with a value of 700 or greater produces the
          same effects as defining _POSIX_C_SOURCE with a value of 200809L or
          greater.  Where one sees

              _POSIX_C_SOURCE >= 200809L

          in the feature test macro requirements in the SYNOPSIS of a man page,
          it is implicit that the following has the same effect:

              _XOPEN_SOURCE >= 700

       Glibc understands the following feature test macros:

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

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

              •  The value 1 exposes definitions conforming to POSIX.1-1990 and
                 ISO C (1990).

              •  The value 2 or greater additionally exposes definitions for
                 POSIX.2-1992.

              •  The value 199309L or greater additionally exposes definitions
                 for POSIX.1b (real-time extensions).

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

              •  (Since glibc 2.3.3) The value 200112L or greater additionally
                 exposes definitions corresponding to the POSIX.1-2001 base
                 specification (excluding the XSI extension).  This value also
                 causes C95 (since glibc 2.12) and C99 (since glibc 2.10)
                 features to be exposed (in other words, the equivalent of
                 defining _ISOC99_SOURCE).

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

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

              Since this macro is obsolete, its usage is generally not
              documented when discussing feature test macro requirements in the
              man pages.

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

              •  Defining with any value exposes definitions conforming to
                 POSIX.1, POSIX.2, and XPG4.

              •  The value 500 or greater additionally exposes definitions for
                 SUSv2 (UNIX 98).

              •  (Since glibc 2.2) The value 600 or greater additionally exposes
                 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base
                 specification plus the XSI extension) and C99 definitions.

              •  (Since glibc 2.10) The value 700 or greater additionally
                 exposes definitions for SUSv4 (i.e., the POSIX.1-2008 base
                 specification plus the XSI extension).

              If __STRICT_ANSI__ is not defined, or _XOPEN_SOURCE is defined
              with a value greater than or equal to 500 and neither
              _POSIX_SOURCE nor _POSIX_C_SOURCE is explicitly defined, then the
              following macros are implicitly defined:

              •  _POSIX_SOURCE is defined with the value 1.

              •  _POSIX_C_SOURCE is defined, according to the value of
                 _XOPEN_SOURCE:

                 _XOPEN_SOURCE < 500
                        _POSIX_C_SOURCE is defined with the value 2.

                 500 <= _XOPEN_SOURCE < 600
                        _POSIX_C_SOURCE is defined with the value 199506L.

                 600 <= _XOPEN_SOURCE < 700
                        _POSIX_C_SOURCE is defined with the value 200112L.

                 700 <= _XOPEN_SOURCE (since glibc 2.10)
                        _POSIX_C_SOURCE is defined with the value 200809L.

              In addition, defining _XOPEN_SOURCE with a value of 500 or greater
              produces the same effects as defining _XOPEN_SOURCE_EXTENDED.

       _XOPEN_SOURCE_EXTENDED
              If this macro is defined, and _XOPEN_SOURCE is defined, then
              expose definitions corresponding to the XPG4v2 (SUSv1) UNIX
              extensions (UNIX 95).  Defining _XOPEN_SOURCE with a value of 500
              or more also produces the same effect as defining
              _XOPEN_SOURCE_EXTENDED.  Use of _XOPEN_SOURCE_EXTENDED in new
              source code should be avoided.

              Since defining _XOPEN_SOURCE with a value of 500 or more has the
              same effect as defining _XOPEN_SOURCE_EXTENDED, the latter
              (obsolete) feature test macro is generally not described in the
              SYNOPSIS in man pages.

       _ISOC99_SOURCE (since glibc 2.1.3)
              Exposes declarations consistent with the ISO C99 standard.

              Earlier glibc 2.1.x versions recognized an equivalent macro named
              _ISOC9X_SOURCE (because the C99 standard had not then been
              finalized).  Although the use of this macro is obsolete, glibc
              continues to recognize it for backward compatibility.

              Defining _ISOC99_SOURCE also exposes ISO C (1990) Amendment 1
              ("C95") definitions.  (The primary change in C95 was support for
              international character sets.)

              Invoking the C compiler with the option -std=c99 produces the same
              effects as defining this macro.

       _ISOC11_SOURCE (since glibc 2.16)
              Exposes declarations consistent with the ISO C11 standard.
              Defining this macro also enables C99 and C95 features (like
              _ISOC99_SOURCE).

              Invoking the C compiler with the option -std=c11 produces the same
              effects as defining this macro.

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

       _LARGEFILE_SOURCE
              This macro was historically used to expose certain functions
              (specifically fseeko(3) and ftello(3)) that address limitations of
              earlier APIs (fseek(3) and ftell(3)) that use long for file
              offsets.  This macro is implicitly defined if _XOPEN_SOURCE is
              defined with a value greater than or equal to 500.  New programs
              should not employ this macro; defining _XOPEN_SOURCE as just
              described or defining _FILE_OFFSET_BITS with the value 64 is the
              preferred mechanism to achieve the same result.

       _FILE_OFFSET_BITS
              Defining this macro with the value 64 automatically converts
              references to 32-bit functions and data types related to file I/O
              and filesystem operations into references to their 64-bit
              counterparts.  This is useful for performing I/O on large files (>
              2 Gigabytes) on 32-bit systems.  (Defining this macro permits
              correctly written programs to use large files with only a
              recompilation being required.)

              64-bit systems naturally permit file sizes greater than 2
              Gigabytes, and on those systems this macro has no effect.

       _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, _ISOC11_SOURCE (since glibc
       2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE,
       _XOPEN_SOURCE_EXTENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc
       2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is
       explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_SOURCE
       are not defined by default.

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

       *  _POSIX_SOURCE is defined with the value 1; and

       *  _POSIX_C_SOURCE is defined with one of the following values:

          •  2, if _XOPEN_SOURCE is defined with a value less than 500;

          •  199506L, if _XOPEN_SOURCE is defined with a value greater than or
             equal to 500 and less than 600; or

          •  (since glibc 2.4) 200112L, if _XOPEN_SOURCE is defined with a value
             greater than or equal to 600 and less than 700.

          •  (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is defined with a
             value greater than or equal to 700.

          •  Older versions of glibc do not know about the values 200112L and
             200809L for _POSIX_C_SOURCE, and the setting of this macro will
             depend on the glibc version.

          •  If _XOPEN_SOURCE is undefined, then the setting of _POSIX_C_SOURCE
             depends on the glibc version: 199506L, 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 depending on the glibc version and what feature test
       macros are explicitly set.  The following shell session, on a system with
       glibc 2.10, shows some examples of what we would see:

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

   Program source

       /* ftm.c */

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

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

       #ifdef _POSIX_C_SOURCE
           printf("_POSIX_C_SOURCE defined: %jdL\n",
                   (intmax_t) _POSIX_C_SOURCE);
       #endif

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

       #ifdef _ISOC11_SOURCE
           printf("_ISOC11_SOURCE defined\n");
       #endif

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

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

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

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

       #ifdef _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 5.11 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                              2021-03-22             FEATURE_TEST_MACROS(7)