(libc.info.gz) Feature Test Macros

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 1.3.4 Feature Test Macros
 -------------------------
 
 The exact set of features available when you compile a source file is
 controlled by which "feature test macros" you define.
 
    If you compile your programs using 'gcc -ansi', you get only the ISO C
 library features, unless you explicitly request additional features by
 defining one or more of the feature macros.   GNU CC Command
 Options (gcc.info)Invoking GCC, for more information about GCC options.
 
    You should define these macros by using '#define' preprocessor
 directives at the top of your source code files.  These directives
 _must_ come before any '#include' of a system header file.  It is best
 to make them the very first thing in the file, preceded only by
 comments.  You could also use the '-D' option to GCC, but it's better if
 you make the source files indicate their own meaning in a self-contained
 way.
 
    This system exists to allow the library to conform to multiple
 standards.  Although the different standards are often described as
 supersets of each other, they are usually incompatible because larger
 standards require functions with names that smaller ones reserve to the
 user program.  This is not mere pedantry -- it has been a problem in
 practice.  For instance, some non-GNU programs define functions named
 'getline' that have nothing to do with this library's 'getline'.  They
 would not be compilable if all features were enabled indiscriminately.
 
    This should not be used to verify that a program conforms to a
 limited standard.  It is insufficient for this purpose, as it will not
 protect you from including header files outside the standard, or relying
 on semantics undefined within the standard.
 
  -- Macro: _POSIX_SOURCE
      If you define this macro, then the functionality from the POSIX.1
      standard (IEEE Standard 1003.1) is available, as well as all of the
      ISO C facilities.
 
      The state of '_POSIX_SOURCE' is irrelevant if you define the macro
      '_POSIX_C_SOURCE' to a positive integer.
 
  -- Macro: _POSIX_C_SOURCE
      Define this macro to a positive integer to control which POSIX
      functionality is made available.  The greater the value of this
      macro, the more functionality is made available.
 
      If you define this macro to a value greater than or equal to '1',
      then the functionality from the 1990 edition of the POSIX.1
      standard (IEEE Standard 1003.1-1990) is made available.
 
      If you define this macro to a value greater than or equal to '2',
      then the functionality from the 1992 edition of the POSIX.2
      standard (IEEE Standard 1003.2-1992) is made available.
 
      If you define this macro to a value greater than or equal to
      '199309L', then the functionality from the 1993 edition of the
      POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.
 
      Greater values for '_POSIX_C_SOURCE' will enable future extensions.
      The POSIX standards process will define these values as necessary,
      and the GNU C Library should support them some time after they
      become standardized.  The 1996 edition of POSIX.1 (ISO/IEC 9945-1:
      1996) states that if you define '_POSIX_C_SOURCE' to a value
      greater than or equal to '199506L', then the functionality from the
      1996 edition is made available.
 
  -- Macro: _BSD_SOURCE
      If you define this macro, functionality derived from 4.3 BSD Unix
      is included as well as the ISO C, POSIX.1, and POSIX.2 material.
 
      Some of the features derived from 4.3 BSD Unix conflict with the
      corresponding features specified by the POSIX.1 standard.  If this
      macro is defined, the 4.3 BSD definitions take precedence over the
      POSIX definitions.
 
      Due to the nature of some of the conflicts between 4.3 BSD and
      POSIX.1, you need to use a special "BSD compatibility library" when
      linking programs compiled for BSD compatibility.  This is because
      some functions must be defined in two different ways, one of them
      in the normal C library, and one of them in the compatibility
      library.  If your program defines '_BSD_SOURCE', you must give the
      option '-lbsd-compat' to the compiler or linker when linking the
      program, to tell it to find functions in this special compatibility
      library before looking for them in the normal C library.
 
  -- Macro: _SVID_SOURCE
      If you define this macro, functionality derived from SVID is
      included as well as the ISO C, POSIX.1, POSIX.2, and X/Open
      material.
 
  -- Macro: _XOPEN_SOURCE
  -- Macro: _XOPEN_SOURCE_EXTENDED
      If you define this macro, functionality described in the X/Open
      Portability Guide is included.  This is a superset of the POSIX.1
      and POSIX.2 functionality and in fact '_POSIX_SOURCE' and
      '_POSIX_C_SOURCE' are automatically defined.
 
      As the unification of all Unices, functionality only available in
      BSD and SVID is also included.
 
      If the macro '_XOPEN_SOURCE_EXTENDED' is also defined, even more
      functionality is available.  The extra functions will make all
      functions available which are necessary for the X/Open Unix brand.
 
      If the macro '_XOPEN_SOURCE' has the value 500 this includes all
      functionality described so far plus some new definitions from the
      Single Unix Specification, version 2.
 
  -- Macro: _LARGEFILE_SOURCE
      If this macro is defined some extra functions are available which
      rectify a few shortcomings in all previous standards.
      Specifically, the functions 'fseeko' and 'ftello' are available.
      Without these functions the difference between the ISO C interface
      ('fseek', 'ftell') and the low-level POSIX interface ('lseek')
      would lead to problems.
 
      This macro was introduced as part of the Large File Support
      extension (LFS).
 
  -- Macro: _LARGEFILE64_SOURCE
      If you define this macro an additional set of functions is made
      available which enables 32 bit systems to use files of sizes beyond
      the usual limit of 2GB. This interface is not available if the
      system does not support files that large.  On systems where the
      natural file size limit is greater than 2GB (i.e., on 64 bit
      systems) the new functions are identical to the replaced functions.
 
      The new functionality is made available by a new set of types and
      functions which replace the existing ones.  The names of these new
      objects contain '64' to indicate the intention, e.g., 'off_t' vs.
      'off64_t' and 'fseeko' vs.  'fseeko64'.
 
      This macro was introduced as part of the Large File Support
      extension (LFS). It is a transition interface for the period when 64 bit
      offsets are not generally used (see '_FILE_OFFSET_BITS').
 
  -- Macro: _FILE_OFFSET_BITS
      This macro determines which file system interface shall be used,
      one replacing the other.  Whereas '_LARGEFILE64_SOURCE' makes the 64 bit
      interface available as an additional interface, '_FILE_OFFSET_BITS'
      allows the 64 bit interface to replace the old interface.
 
      If '_FILE_OFFSET_BITS' is undefined, or if it is defined to the
      value '32', nothing changes.  The 32 bit interface is used and
      types like 'off_t' have a size of 32 bits on 32 bit systems.
 
      If the macro is defined to the value '64', the large file interface
      replaces the old interface.  I.e., the functions are not made
      available under different names (as they are with
      '_LARGEFILE64_SOURCE').  Instead the old function names now
      reference the new functions, e.g., a call to 'fseeko' now indeed
      calls 'fseeko64'.
 
      This macro should only be selected if the system provides
      mechanisms for handling large files.  On 64 bit systems this macro
      has no effect since the '*64' functions are identical to the normal
      functions.
 
      This macro was introduced as part of the Large File Support
      extension (LFS).
 
  -- Macro: _ISOC99_SOURCE
      Until the revised ISO C standard is widely adopted the new features
      are not automatically enabled.  The GNU C Library nevertheless has
      a complete implementation of the new standard and to enable the new
      features the macro '_ISOC99_SOURCE' should be defined.
 
  -- Macro: _GNU_SOURCE
      If you define this macro, everything is included: ISO C89, ISO C99,
      POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions.  In
      the cases where POSIX.1 conflicts with BSD, the POSIX definitions
      take precedence.
 
      If you want to get the full effect of '_GNU_SOURCE' but make the
      BSD definitions take precedence over the POSIX definitions, use
      this sequence of definitions:
 
           #define _GNU_SOURCE
           #define _BSD_SOURCE
           #define _SVID_SOURCE
 
      Note that if you do this, you must link your program with the BSD
      compatibility library by passing the '-lbsd-compat' option to the
      compiler or linker.  *NB:* If you forget to do this, you may get
      very strange errors at run time.
 
  -- Macro: _REENTRANT
  -- Macro: _THREAD_SAFE
      If you define one of these macros, reentrant versions of several
      functions get declared.  Some of the functions are specified in
      POSIX.1c but many others are only available on a few other systems
      or are unique to the GNU C Library.  The problem is the delay in
      the standardization of the thread safe C library interface.
 
      Unlike on some other systems, no special version of the C library
      must be used for linking.  There is only one version but while
      compiling this it must have been specified to compile as thread
      safe.
 
    We recommend you use '_GNU_SOURCE' in new programs.  If you don't
 specify the '-ansi' option to GCC and don't define any of these macros
 explicitly, the effect is the same as defining '_POSIX_C_SOURCE' to 2
 and '_POSIX_SOURCE', '_SVID_SOURCE', and '_BSD_SOURCE' to 1.
 
    When you define a feature test macro to request a larger class of
 features, it is harmless to define in addition a feature test macro for
 a subset of those features.  For example, if you define
 '_POSIX_C_SOURCE', then defining '_POSIX_SOURCE' as well has no effect.
 Likewise, if you define '_GNU_SOURCE', then defining either
 '_POSIX_SOURCE' or '_POSIX_C_SOURCE' or '_SVID_SOURCE' as well has no
 effect.
 
    Note, however, that the features of '_BSD_SOURCE' are not a subset of
 any of the other feature test macros supported.  This is because it
 defines BSD features that take precedence over the POSIX features that
 are requested by the other macros.  For this reason, defining
 '_BSD_SOURCE' in addition to the other feature test macros does have an
 effect: it causes the BSD features to take priority over the conflicting
 POSIX features.
 
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