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       feature_test_macros - feature test macros


       #include <features.h>


       Feature  test  macros  allow  the programmer to control the definitions
       that are exposed by system header files when  a  program  is  compiled.
       This  can  be  useful for creating portable applications, by preventing
       non-standard definitions from being exposed.  Other macros can be  used
       to  expose  non-standard  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.

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

   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 chmod(2) manual page):

              #include <sys/stat.h>

              int chmod(const char *path, mode_t mode);
              int fchmod(int fd, mode_t mode);

          Feature    Test    Macro     Requirements     for     glibc     (see

              fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

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

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

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

              cc -D_BSD_SOURCE
              cc -D_XOPEN_SOURCE=500        # 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:

              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

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

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

              Defining this macro causes header files to expose definitions as

              ·  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.

              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.

              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 backwards compatibility.

              Expose  definitions for the alternative API specified by the LFS
              (Large File Summit) as a "transitional extension" to the  Single
              UNIX                     Specification.                     (See
       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.

              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.

              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,
              _GNU_SOURCE is  defined,  in  which  case  BSD  definitions  are

              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

              Defining  this  macro (with any value) is equivalent to defining
              _POSIX_C_SOURCE  with  the  value  200112L  (199506L  in   glibc
              versions  before 2.5), and _XOPEN_SOURCE with the value 600 (500
              in glibc  versions  before  2.2).   In  addition,  various  GNU-
              specific extensions are also exposed.  Where standards conflict,
              BSD definitions are disfavored.

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

              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=200112L (199506L in glibc versions
       before 2.4).

       If   any    of    __STRICT_ANSI__,    _ISOC99_SOURCE,    _POSIX_SOURCE,
       _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:

                ·  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

                ·  200112L  (199506L  in  glibc  versions  before   2.4),   if
                   XOPEN_SOURCE  is  undefined,  or  is  defined  with a value
                   greater than or equal to 600.

       Multiple macros can be defined; the results are additive.


       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.

       _FORTIFY_SOURCE,  _REENTRANT,  and  _THREAD_SAFE  are specific to Linux


       <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

       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.



       The section "Feature Test Macros" under info libc.



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