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NAME

       ftm - feature test macros

SYNOPSIS

       #include <features.h>

DESCRIPTION

       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.

       Linux/glibc understands the following feature test macros:

       _POSIX_C_SOURCE
              Defining this macro with the value  1  causes  header  files  to
              expose  definitions conforming to POSIX.1-1990 and ISO C (1990).
              Defining with the value 199309 or greater  additionally  exposes
              definitions  for POSIX.1b (real-time extensions).  Defining with
              the value 199506 or greater additionally exposes definitions for
              POSIX.1c  (threads).   Defining  with  the  value 200112 exposes
              definitions corresponding to the POSIX.1-2001 base specification
              (excluding the XSI extension).

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

       _XOPEN_SOURCE
              Defining this macro with any value causes header files to expose
              definitions  conforming to POSIX.1, POSIX.2, and XPG4.  Defining
              with the value 500 or greater additionally  exposes  definitions
              for  SUSv2  (UNIX  98).   Defining with the value 600 or greater
              additionally exposes definitions for SUSv3 (UNIX 03;  i.e.,  the
              POSIX.1  base  specification  plus  the  XSI extension) and C 99
              definitions.

       _XOPEN_SOURCE_EXTENDED
              If this macro is defined with the value 1, and the _XOPEN_SOURCE
              is  defined, then expose definitions corresponding to the XPG4v2
              UNIX extensions.

       _ISOC99_SOURCE
              Exposes C 99 extensions to ISO C (1990).

       _LARGEFILE64_SOURCE
              Expose definitions for the alternative API specified by the  LFS
              (Large  File  Summit)  as  a "transitional extension" the Single
              UNIX Specification.

       _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.

       _BSD_SOURCE
              Defining this macro with any value cause header files to  expose
              BSD-derived  definitions.   Defining  this macro also causes BSD
              definitions to be preferred in some situations  where  standards
              conflict.

       _SVID_SOURCE
              Defining  this macro with any value cause header files to expose
              System V-derived  definitions.   (SVID  ==  System  V  Interface
              Definition.)

       _GNU_SOURCE
              Defining  this  macro (with any value) is equivalent to defining
              _BSD_SOURCE, _SVID_SOURCE,  _LARGEFILE64_SOURCE,  _ISOC99_SOURCE
              _POSIX_C_SOURCE  with  the value 1999506, and _XOPEN_SOURCE with
              the value 600.  In addition, various GNU-specific extensions are
              also exposed.

       When  gcc(1)  is  invoked,  then  the  following  macros are defined by
       default:     _BSD_SOURCE,     _SVID_SOURCE,     _POSIX_SOURCE,      and
       POSIX_C_SOURCE=199506.   If  individual  macros are defined, then other
       macros  are  disabled  unless  they  are   also   explicitly   defined.
       (Exception:  if  POSIX_C_SOURCE  is  not  otherwise defined, then it is
       always defined with the value 199506, unless the compiler is invoked in
       one  of  is  standard modes, e.g., the -std=c99 flag.)  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).
       _FILE_OFFSET_BITS is not specified by any standard, but is employed  on
       some other implementations.  _BSD_SOURCE, _SVID_SOURCE, and _GNU_SOURCE
       are Linux (glibc) specific.

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.