glibc/manual/creature.texi

@node Feature Test Macros
@subsection Feature Test Macros

@cindex feature test macros
The exact set of features available when you compile a source file
is controlled by which @dfn{feature test macros} you define.

If you compile your programs using @samp{gcc -ansi}, you get only the
@w{ISO C} library features, unless you explicitly request additional
features by defining one or more of the feature macros.
@xref{Invoking GCC,, GNU CC Command Options, gcc.info, The GNU CC Manual},
for more information about GCC options.@refill

You should define these macros by using @samp{#define} preprocessor
directives at the top of your source code files.  These directives
@emph{must} come before any @code{#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 @samp{-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 @code{getline} that have
nothing to do with this library's @code{getline}.  They would not be
compilable if all features were enabled indescriminantly. 

This should not be used to verify that a program conforms to a limited
standard.  It is insufficent for this purpose, as it will not protect you
from including header files outside the standard, or relying on semantics
undefined within the standard. 

@comment (none)
@comment POSIX.1
@defvr 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
@w{ISO C} facilities.

The state of @code{_POSIX_SOURCE} is irrelevant if you define the
macro @code{_POSIX_C_SOURCE} to a positive integer.
@end defvr

@comment (none)
@comment POSIX.2
@defvr 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 @code{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 @code{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 @code{199309L},
then the functionality from the 1993 edition of the POSIX.1b standard
(IEEE Standard 1003.1b-1993) is made available.

Greater values for @code{_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 @code{_POSIX_C_SOURCE} to a value greater than
or equal to @code{199506L}, then the functionality from the 1996
edition is made available.

The Single Unix Specification specify that setting this macro to the
value @code{199506L} selects all the values specified by the POSIX
standards plus those of the Single Unix Specification, i.e., is the
same as if @code{_XOPEN_SOURCE} is set to @code{500} (see below).
@end defvr

@comment (none)
@comment GNU
@defvr Macro _BSD_SOURCE
If you define this macro, functionality derived from 4.3 BSD Unix is
included as well as the @w{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 @dfn{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 @code{_BSD_SOURCE}, you must give the option @samp{-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.
@pindex -lbsd-compat
@pindex bsd-compat
@cindex BSD compatibility library.
@end defvr

@comment (none)
@comment GNU
@defvr Macro _SVID_SOURCE
If you define this macro, functionality derived from SVID is
included as well as the @w{ISO C}, POSIX.1, POSIX.2, and X/Open material.
@end defvr

@comment (none)
@comment X/Open
@defvr Macro _XOPEN_SOURCE
@defvrx 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 @code{_POSIX_SOURCE} and
@code{_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 @code{_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 @code{_XOPEN_SOURCE} has the value @math{500} this includes
all functionality described so far plus some new definitions from the
Single Unix Specification, @w{version 2}.
@end defvr

@comment (NONE)
@comment X/Open
@defvr Macro _LARGEFILE_SOURCE
If this macro is defined some extra functions are available which
rectify a few shortcomings in all previous standards.  More concrete
the functions @code{fseeko} and @code{ftello} are available.  Without
these functions the difference between the @w{ISO C} interface
(@code{fseek}, @code{ftell}) and the low-level POSIX interface
(@code{lseek}) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).
@end defvr

@comment (NONE)
@comment X/Open
@defvr Macro _LARGEFILE64_SOURCE
If you define this macro an additional set of function gets available
which enables to use on @w{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 @w{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 existing.  The names of these new objects
contain @code{64} to indicate the intention, e.g., @code{off_t}
vs. @code{off64_t} and @code{fseeko} vs. @code{fseeko64}.

This macro was introduced as part of the Large File Support extension
(LFS).  It is a transition interface for the time @w{64 bit} offsets are
not generally used (see @code{_FILE_OFFSET_BITS}.
@end defvr

@comment (NONE)
@comment X/Open
@defvr Macro _FILE_OFFSET_BITS
This macro lets decide which file system interface shall be used, one
replacing the other.  While @code{_LARGEFILE64_SOURCE} makes the @w{64
bit} interface available as an additional interface
@code{_FILE_OFFSET_BITS} allows to use the @w{64 bit} interface to
replace the old interface.

If @code{_FILE_OFFSET_BITS} is undefined or if it is defined to the
value @code{32} nothing changes.  The @w{32 bit} interface is used and
types like @code{off_t} have a size of @w{32 bits} on @w{32 bit}
systems.

If the macro is defined to the value @code{64} the large file interface
replaces the old interface.  I.e., the functions are not made available
under different names as @code{_LARGEFILE64_SOURCE} does.  Instead the
old function names now reference the new functions, e.g., a call to
@code{fseeko} now indeed calls @code{fseeko64}.

This macro should only be selected if the system provides mechanisms for
handling large files.  On @w{64 bit} systems this macro has no effect
since the @code{*64} functions are identical to the normal functions.

This macro was introduced as part of the Large File Support extension
(LFS).
@end defvr

@comment (none)
@comment GNU
@defvr Macro _GNU_SOURCE
If you define this macro, everything is included: @w{ISO C}, 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 @code{_GNU_SOURCE} but make the
BSD definitions take precedence over the POSIX definitions, use this
sequence of definitions:

@smallexample
#define _GNU_SOURCE
#define _BSD_SOURCE
#define _SVID_SOURCE
@end smallexample

Note that if you do this, you must link your program with the BSD
compatibility library by passing the @samp{-lbsd-compat} option to the
compiler or linker.  @strong{Note:} If you forget to do this, you may
get very strange errors at run time.
@end defvr

@comment (none)
@comment GNU
@defvr Macro _REENTRANT
@defvrx 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 that the standardization of the thread safe C library
interface still is behind.

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.
@end defvr

We recommend you use @code{_GNU_SOURCE} in new programs.  If you don't
specify the @samp{-ansi} option to GCC and don't define any of these
macros explicitly, the effect is the same as defining
@code{_POSIX_C_SOURCE} to 2 and @code{_POSIX_SOURCE},
@code{_SVID_SOURCE}, and @code{_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 @code{_POSIX_C_SOURCE}, then
defining @code{_POSIX_SOURCE} as well has no effect.  Likewise, if you
define @code{_GNU_SOURCE}, then defining either @code{_POSIX_SOURCE} or
@code{_POSIX_C_SOURCE} or @code{_SVID_SOURCE} as well has no effect.

Note, however, that the features of @code{_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
@code{_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.