POSIX.1‐2008 only describes the behavior of systems that claim
conformance to it. However, application developers who want to
write applications that adapt to other versions of this standard
(or to systems that do not conform to any POSIX standard) may
find it useful to code them so as to conditionally compile
different code depending on the value of _POSIX_VERSION, for
example:
#if _POSIX_VERSION >= 200112L
/* Use the newer function that copes with large files. */
off_t pos=ftello(fp);
#else
/* Either this is an old version of POSIX, or _POSIX_VERSION is
not even defined, so use the traditional function. */
long pos=ftell(fp);
#endif
Earlier versions of POSIX.1‐2008 and of the Single UNIX
Specification can be identified by the following macros:
POSIX.1‐1988 standard
_POSIX_VERSION==198808L
POSIX.1‐1990 standard
_POSIX_VERSION==199009L
ISO POSIX‐1:1996 standard
_POSIX_VERSION==199506L
Single UNIX Specification, Version 1
_XOPEN_UNIX and _XOPEN_VERSION==4
Single UNIX Specification, Version 2
_XOPEN_UNIX and _XOPEN_VERSION==500
ISO POSIX‐1:2001 and Single UNIX Specification, Version 3
_POSIX_VERSION==200112L, plus (if the XSI option is
supported) _XOPEN_UNIX and _XOPEN_VERSION==600
POSIX.1‐2008 does not make any attempt to define application
binary interaction with the underlying operating system. However,
application developers may find it useful to query _SC_VERSION at
runtime via sysconf() to determine whether the current version of
the operating system supports the necessary functionality as in
the following program fragment:
if (sysconf(_SC_VERSION) < 200809L) {
fprintf(stderr, "POSIX.1-2008 system required, terminating \n");
exit(1);
}
New applications should not use _XOPEN_SHM or _XOPEN_ENH_I18N.
