feature_test_macros(7) — Linux manual page


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

NAME         top

       feature_test_macros - feature test macros

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.  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 applica‐
       tions, 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

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

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

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

       Alternatively, equivalent definitions can be included in the compila‐
       tion command:

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

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

       In a few cases, manual pages use a shorthand for expressing the fea‐
       ture 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 pro‐
          vided by default).

       *  Certain macros are defined with default values.  Thus, although
          one or more macros may be indicated as being required in the SYN‐
          OPSIS 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:

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

              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

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

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

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

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

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

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

              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 < 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

              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
              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

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

              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 ver‐
              sus off_t, lseek64() versus lseek(), etc.).  New programs
              should not employ this macro; instead _FILE_OFFSET_BITS=64
              should be employed.

              This macro was historically used to expose certain functions
              (specifically fseeko(3) and ftello(3)) that address limita‐
              tions 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.

              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 sit‐
              uations where standards conflict, unless one or more of
              _XOPEN_SOURCE_EXTENDED, or _GNU_SOURCE is defined, in which
              case BSD definitions are disfavored.  Since glibc 2.19,
              _BSD_SOURCE no longer causes BSD definitions to be preferred
              in case of conflicts.

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

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

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

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

              The "default" definitions comprise those required by
              POSIX.1-2008 and ISO C99, as well as various definitions orig‐
              inally derived from BSD and System V.  On glibc 2.19 and ear‐
              lier, these defaults were approximately equivalent to explic‐
              itly defining the following:

                  cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809

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

              Defining this macro (with any value) implicitly defines
              the value 200809L (200112L in glibc versions before 2.10;
              199506L in glibc versions before 2.5; 199309L in glibc ver‐
              sions 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.

              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.

              Synonym for the (deprecated) _REENTRANT, provided for compati‐
              bility with some other implementations.

       _FORTIFY_SOURCE (since glibc 2.3.4)
              Defining this macro causes some lightweight checks to be per‐
              formed to detect some buffer overflow errors when employing
              various string and memory manipulation functions (for example,
              memcpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3),
              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 exam‐
              ple, a check is made that open(2) has been supplied with a
              mode argument when the specified flags include O_CREAT.  Not
              all problems are detected, just some common cases.

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

              Some of the checks can be performed at compile time (via
              macros logic implemented in header files), and result in com‐
              piler 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,
       (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:

          ·  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         top


       _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.

       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.

EXAMPLES         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>

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

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

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

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

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

           printf("_XOPEN_SOURCE_EXTENDED defined\n");

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

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

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

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

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

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

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

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

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

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


SEE ALSO         top

       libc(7), standards(7)

       The section "Feature Test Macros" under info libc.


COLOPHON         top

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Linux                            2020-04-11           FEATURE_TEST_MACROS(7)

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