mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-25 22:40:05 +00:00
224 lines
10 KiB
Plaintext
224 lines
10 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
|
|
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.
|
|
|