mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-22 13:00:06 +00:00
96 lines
4.4 KiB
Plaintext
96 lines
4.4 KiB
Plaintext
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
|
|
ANSI 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.
|
|
|
|
- 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
|
|
ANSI C facilities.
|
|
|
|
- Macro: _POSIX_C_SOURCE
|
|
If you define this macro with a value of `1', then the
|
|
functionality from the POSIX.1 standard (IEEE Standard 1003.1) is
|
|
made available. If you define this macro with a value of `2',
|
|
then both the functionality from the POSIX.1 standard and the
|
|
functionality from the POSIX.2 standard (IEEE Standard 1003.2) are
|
|
made available. This is in addition to the ANSI C facilities.
|
|
|
|
- Macro: _BSD_SOURCE
|
|
If you define this macro, functionality derived from 4.3 BSD Unix
|
|
is included as well as the ANSI 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 ANSI C, POSIX.1, and POSIX.2 material.
|
|
|
|
- Macro: _GNU_SOURCE
|
|
If you define this macro, everything is included: ANSI C, POSIX.1,
|
|
POSIX.2, BSD, SVID, 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.
|
|
|
|
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 `_GNU_SOURCE'.
|
|
|
|
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.
|
|
|