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authorJoseph Myers <joseph@codesourcery.com>2012-03-21 16:56:28 +0000
committerJoseph Myers <joseph@codesourcery.com>2012-03-21 16:56:28 +0000
commitbe22ce65d7643ffcd2dcb55b73eae4d7da6f0d71 (patch)
tree4d0e8f60788036ecdeff4caed7465ee733016485 /NOTES
parent40b601fbb774cd2be2f6cbe7155570340fc475a2 (diff)
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-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.  *Note 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.
-