If you really mean that, you had better go and read the standard[1]. However, to a reasonable approximation, POSIX (1003.1) is just the C run time library and indeed the POSIX standard makes reference to the ANSI C standard [2]. Unfortunately, different computers have somewhat different C run time libraries. The point of POSIX is to define a common run time library that will be available on all computers.
PSX POSIX interface routines