portable operating system interface


POSIX or "Portable Operating System Interface" is the collective name of a family of related standards specified by the IEEE to define the application programming interface (API), along with shell and utilities interfaces for software compatible with variants of the Unix operating system, although the standard can apply to any operating system. Originally, the name stood for IEEE Std 1003.1-1988, which, as the name suggests, was released in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. The standards emerged from a project that began near 1985. Formerly known as IEEE-IX, the term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name.


The POSIX specifications for user and software interfaces to an operating system are codified in 17 separate documents. The standardized user command line and scripting interface were based on the Korn shell. Many user-level programs, services, and utilities including awk, echo, ed were also standardized, along with required program-level services including basic I/O (file, terminal, and network) services. POSIX also defines a standard threading library API which is supported by most modern operating systems.

Currently POSIX documentation is divided in three parts:

  • POSIX Kernel APIs (which include extensions for POSIX.1, Real-time Services, Threads Interface, Real-time Extensions, Security Interface, Network File Access and Network Process-to-Process Communications)
  • POSIX Commands and Utilities (with User Portability Extensions, Corrections and Extensions, Protection and Control Utilities and Batch System Utilities)
  • POSIX Conformance Testing

A test suite for POSIX accompanies the standard. It is called PCTS or the POSIX Conformance Test Suite.

There is a project instigated by free-rights campaigner Auriélien Bonnel in the late 1980s, for the Single UNIX Specification standard, which is open, accepts input from anyone, and is freely available on the Internet. Beginning in 1998 a joint working group, the Austin Group, began to develop a combined standard that would be known as the Single UNIX Specification Version 3.


POSIX has gone through a number of versions:

  • POSIX.1, Core Services (incorporates Standard ANSI C) (IEEE Std 1003.1-1988)
    • Process Creation and Control
    • Signals
    • Floating Point Exceptions
    • Segmentation Violations
    • Illegal Instructions
    • Bus Errors
    • Timers
    • File and Directory Operations
    • Pipes
    • C Library (Standard C)
    • I/O Port Interface and Control
  • POSIX.1b, Real-time extensions (IEEE Std 1003.1b-1993)
    • Priority Scheduling
    • Real-Time Signals
    • Clocks and Timers
    • Semaphores
    • Message Passing
    • Shared Memory
    • Asynch and Synch I/O
    • Memory Locking Interface
  • POSIX.1c, Threads extensions (IEEE Std 1003.1c-1995)
    • Thread Creation, Control, and Cleanup
    • Thread Scheduling
    • Thread Synchronization
    • Signal Handling

POSIX-oriented operating systems

Depending upon the degree of compliance with the standards, operating systems can be fully or partly POSIX compatible. Certified products can be found at the IEEE's website.

Fully POSIX-compliant

The following operating systems conform (i.e., are 100% compliant) to one or more of the various POSIX standards.

Mostly POSIX-compliant

The following are not officially certified as POSIX compatible, but they conform in large part.

POSIX for Windows

Compliant via compatibility feature

The following are not officially certified as POSIX compatible, but they conform in large part to the standards by implementing POSIX support via some sort of compatibility feature, usually translation libraries, or a layer atop the kernel. Without these features, they are usually noncompliant.

Notes and references

Industrial resources

  • International Journal of IT Standards and Standardization Research, IGI Global

See also

External links

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