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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 libc 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. *Note:* 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 GNU libc. 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.