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POSIX An IEEE trial-use standard (P1003) that defines the behavior of a set of supervisor calls, basing these closely on those found in UNIX. However, POSIX is not itself an operating system so much as a formal description of one form of operating system of which UNIX is a specific instance. The intention is that a program written in such a way as to use only those functions defined by the POSIX specifications will be readily portable between different operating systems, provided that these are all conformant to the POSIX definitions.