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_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.
The value of this symbol says nothing about the utilities installed on the system.
Usage Note: You can use this macro to tell whether a POSIX.1
system library supports POSIX.2 as well. Any POSIX.1 system contains
`unistd.h', so include that file and then test
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