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