Microsoft POSIX subsystem is one of 3 subsystems of several operating systems from the Windows NT family (together with OS/2 and Windows subsystems).
Microsoft Windows implements only the first version of the POSIX standards, namely POSIX.1. The official code of POSIX.1 is ISO/IEC 9945-1:1990 or IEEE standard 1003.1-1990. The subsystem was included because of 1980s US federal government's requirements listed in Federal Information Processing Standard (FIPS) 151-2. Versions Windows NT 3.5, Windows NT 3.51 and Windows NT 4 were certified as compliant with the FIPS 151-2.
The runtime environment of the subsystem is provided by two files: psxss.exe and psxdll.dll. A POSIX application uses psxdll.dll to communicate with the subsystem while communicating with posix.exe to provide display capabilities on the Windows desktop.
Because only the first version of POSIX (POSIX.1) is implemented, a POSIX application cannot create a thread or window, nor can it use RPC or sockets. Instead of implementing the later versions of POSIX, Microsoft offers Windows Services for UNIX.
Starting with Windows XP, the POSIX subsystem is not included as part of standard Windows distributions and has been replaced by Interix.
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