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 <xys.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
       Linux glibc 2.x, x > 0.

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

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

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

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

              _POSIX_C_SOURCE >= 200112L

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

              _XOPEN_SOURCE >= 600

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

              _POSIX_C_SOURCE >= 200809L

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

              _XOPEN_SOURCE >= 700

       Linux glibc understands the following feature test macros:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

               ·  _POSIX_SOURCE is defined with the value 1.

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

                  _XOPEN_SOURCE < 500
                         _POSIX_C_SOURCE is defined with the value 2.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       _BSD_SOURCE (deprecated since glibc 2.20)
               Defining this macro with any value causes header files to
               expose BSD-derived definitions.

               In glibc versions up to and including 2.18, defining this macro
               also causes BSD definitions to be preferred in some situations
               where standards conflict, unless one or more of _SVID_SOURCE,
               _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE,
               _XOPEN_SOURCE_EXTENDED, or _GNU_SOURCE is defined, in which
               case BSD definitions are disfavored.  Since glibc 2.19,
               _BSD_SOURCE no longer causes BSD definitions to be preferred in
               case of conflicts.

               Since glibc 2.20, this macro is deprecated.  It now has the
               same effect as defining _DEFAULT_SOURCE, but generates a
               compile-time warning (unless _DEFAULT_SOURCE is also defined).
               Use _DEFAULT_SOURCE instead.  To allow code that requires
               _BSD_SOURCE in glibc 2.19 and earlier and _DEFAULT_SOURCE in
               glibc 2.20 and later to compile without warnings, define both
               _BSD_SOURCE and _DEFAULT_SOURCE.

       _SVID_SOURCE (deprecated since glibc 2.20)
               Defining this macro with any value causes header files to
               expose System V-derived definitions.  (SVID == System V
               Interface Definition; see standards(7).)

               Since glibc 2.20, this macro is deprecated in the same fashion
               as _BSD_SOURCE.

       _DEFAULT_SOURCE (since glibc 2.19)
               This macro can be defined to ensure that the "default"
               definitions are provided even when the defaults would otherwise
               be disabled, as happens when individual macros are explicitly
               defined, or the compiler is invoked in one of its "standard"
               modes (e.g., cc -std=c99).  Defining _DEFAULT_SOURCE without
               defining other individual macros or invoking the compiler in
               one of its "standard" modes has no effect.

               The "default" definitions comprise those required by
               POSIX.1-2008 and ISO C99, as well as various definitions
               originally derived from BSD and System V.  On glibc 2.19 and
               earlier, these defaults were approximately equivalent to
               explicitly defining the following:

                   cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809

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

       _GNU_SOURCE
               Defining this macro (with any value) implicitly defines
               _ATFILE_SOURCE, _LARGEFILE64_SOURCE, _ISOC99_SOURCE,
               _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE, _POSIX_C_SOURCE with the
               value 200809L (200112L in glibc versions before 2.10; 199506L
               in glibc versions before 2.5; 199309L in glibc versions before
               2.1) and _XOPEN_SOURCE with the value 700 (600 in glibc
               versions before 2.10; 500 in glibc versions before 2.2).  In
               addition, various GNU-specific extensions are also exposed.

               Since glibc 2.19, defining _GNU_SOURCE also has the effect of
               implicitly defining _DEFAULT_SOURCE.  In glibc versions before
               2.20, defining _GNU_SOURCE also had the effect of implicitly
               defining _BSD_SOURCE and _SVID_SOURCE.

       _REENTRANT
               Historically, on various C libraries it was necessary to define
               this macro in all multithreaded code.  (Some C libraries may
               still require this.)  In glibc, this macro also exposed
               definitions of certain reentrant functions.

               However, glibc has been thread-safe by default for many years;
               since glibc 2.3, the only effect of defining _REENTRANT has
               been to enable one or two of the same declarations that are
               also enabled by defining _POSIX_C_SOURCE with a value of
               199606L or greater.

               _REENTRANT is now obsolete.  In glibc 2.25 and later, defining
               _REENTRANT is equivalent to defining _POSIX_C_SOURCE with the
               value 199606L.  If a higher POSIX conformance level is selected
               by any other means (such as _POSIX_C_SOURCE itself,
               _XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE), then defining
               _REENTRANT has no effect.

               This macro is automatically defined if one compiles with
               cc -pthread.

       _THREAD_SAFE
               Synonym for the (deprecated) _REENTRANT, provided for
               compatibility with some other implementations.

       _FORTIFY_SOURCE (since glibc 2.3.4)
               Defining this macro causes some lightweight checks to be
               performed to detect some buffer overflow errors when employing
               various string and memory manipulation functions (for example,
               memcpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3),
               strcat(3), strncat(3), sprintf(3), snprintf(3), vsprintf(3),
               vsnprintf(3), gets(3), and wide character variants thereof).
               For some functions, argument consistency is checked; for
               example, a check is made that open(2) has been supplied with a
               mode argument when the specified flags include O_CREAT.  Not
               all problems are detected, just some common cases.

               If _FORTIFY_SOURCE is set to 1, with compiler optimization
               level 1 (gcc -O1) and above, checks that shouldn't change the
               behavior of conforming programs are performed.  With
               _FORTIFY_SOURCE set to 2, some more checking is added, but some
               conforming programs might fail.

               Some of the checks can be performed at compile time (via macros
               logic implemented in header files), and result in compiler
               warnings; other checks take place at run time, and result in a
               run-time error if the check fails.

               Use of this macro requires compiler support, available with
               gcc(1) since version 4.0.

   Default definitions, implicit definitions, and combining definitions
       If no feature test macros are explicitly defined, then the following
       feature test macros are defined by default: _BSD_SOURCE (in glibc 2.19
       and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE
       (since glibc 2.19), _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L
       in glibc versions before 2.10; 199506L in glibc versions before 2.4;
       199309L in glibc versions before 2.1).

       If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE,
       _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE (in
       glibc 2.19 and earlier), or _SVID_SOURCE (in glibc 2.19 and earlier) is
       explicitly defined, then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_SOURCE
       are not defined by default.

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

       *  _POSIX_SOURCE is defined with the value 1; and

       *  _POSIX_C_SOURCE is defined with one of the following values:

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

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

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

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

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

          ·  If _XOPEN_SOURCE is undefined, then the setting of
             _POSIX_C_SOURCE depends on the glibc version: 199506L, in glibc
             versions before 2.4; 200112L, in glibc 2.4 to 2.9; and 200809L,
             since glibc 2.10.

       Multiple macros can be defined; the results are additive.

CONFORMING TO
       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.

       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is not
       present in SUSv2 and later.  _FILE_OFFSET_BITS is not specified by any
       standard, but is employed on some other implementations.

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

NOTES
       <features.h> is a Linux/glibc-specific header file.  Other systems have
       an analogous file, but typically with a different name.  This header
       file is automatically included by other header files as required: it is
       not necessary to explicitly include it in order to employ feature test
       macros.

       According to which of the above feature test macros are defined,
       <features.h> internally defines various other macros that are checked
       by other glibc header files.  These macros have names prefixed by two
       underscores (e.g., __USE_MISC).  Programs should never define these
       macros directly: instead, the appropriate feature test macro(s) from
       the list above should be employed.

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

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

   Program source

       /* ftm.c */

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       libc(7), standards(7)

       The section "Feature Test Macros" under info libc.

       /usr/include/features.h

COLOPHON
       This page is part of release 5.01 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                             2019-03-06            FEATURE_TEST_MACROS(7)