This constant represents the version of the POSIX.1 standard to which
the implementation conforms. For an implementation conforming to the
1995 POSIX.1 standard, the value is the integer
_POSIX_VERSION is always defined (in unistd.h) in any
Usage Note: Don't try to test whether the system supports POSIX
by including unistd.h and then checking whether
_POSIX_VERSION is defined. On a non-POSIX system, this will
probably fail because there is no unistd.h. We do not know of
any way you can reliably test at compilation time whether your
target system supports POSIX or whether unistd.h exists.
The GNU C compiler predefines the symbol
__POSIX__ if the target
system is a POSIX system. Provided you do not use any other compilers
on POSIX systems, testing
defined (__POSIX__) will reliably
detect such systems.