An acronym for Portable Operating System Interface for UNIX, one of several standards developed by the Institute of Electrical and Electronics Engineers (IEEE) for cross-platform implementations of UNIX. POSIX is a standard operating system interface and environment that supports portability of applications at the source code level.
POSIX arose because different vendors were producing different “flavors” of UNIX; code written for one platform would not run on another platform unless it was modified appropriately.
The solution was to develop a standard in which code could be portable between all UNIX flavors, so that a POSIX call in one program would work on any other POSIX-compliant system.
Because of vendor-specific enhancements most UNIX systems are not 100 percent POSIX-compliant. The POSIX.1 standard defines the portability of C language code by specifying a standard application programming interface (API). Microsoft Windows NT and Windows 2000 are fully compliant with the POSIX.1 standard, which means that the POSIX subsystem of Windows NT or Windows 2000 can run native C language code written to the POSIX.1 standard. Windows NT does not support other POSIX standards.
| Standard | Description |
| POSIX.1 | Specifies the base system interfaces for C language programming |
| POSIX.2 | Specifies standards for shells and system utilities plus additional C language interfaces |
| POSIX.3 | Specifies methods for testing conformance to POSIX |
| POSIX.4 | Adds real-time extensions to POSIX.1 |
| POSIX.5 | Extends POSIX.1 to the Ada programming language |
| POSIX.9 | Extends POSIX.1 to the FORTRAN77 language |