Not to be confused with Unix, Unix-like, or Linux.

The Portable Operating System Interface (POSIX)[1] is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems.[2][3]


Originally, the name "POSIX" referred to IEEE Std 1003.1-1988, 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 circa 1985. Richard Stallman suggested the name POSIX to the IEEE instead of former IEEE-IX. The committee found it more easily pronounceable and memorable, and thus adopted it.[2][4]


Unix was selected as the basis for a standard system interface partly because it was "manufacturer-neutral." However, several major versions of Unix existed—so there was a need to develop a common denominator system. The POSIX specifications for Unix-like operating systems originally consisted of a single document for the core programming interface, but eventually grew to 19 separate documents (POSIX.1, POSIX.2, etc.).[5] The standardized user command line and scripting interface were based on the UNIX System V shell.[6] 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). POSIX also defines a standard threading library API which is supported by most modern operating systems. Nowadays, most parts of POSIX are combined into a single standard (IEEE Std 1003.1-2008, also known as POSIX.1-2008).

As of 2014, POSIX documentation is divided in two parts:

The development of the POSIX standard takes place in the Austin Group (a joint working group linking the IEEE, The Open Group and the ISO/IEC JTC 1 organizations).


Parts before 1997

Before 1997, POSIX comprised several standards:

Versions after 1997

After 1997, the Austin Group developed the POSIX revisions. The specifications are known under the name Single UNIX Specification, before they become a POSIX standard when formally approved by the ISO.


POSIX.1-2001 (or IEEE Std 1003.1-2001) equates to the Single UNIX Specification version 3.[9]

This standard consisted of:

POSIX.1-2004 (with two TCs)

IEEE Std 1003.1-2004 involved a minor update of POSIX.1-2001. It incorporated two minor updates or errata referred to as Technical Corrigenda.[10] Its contents are available on the web.[11]

POSIX.1-2008 (with one TC)

As of 2016, Base Specifications, Issue 7 (or IEEE Std 1003.1, 2013 edition) represents the current version.[12][13] A free online copy is available.

This standard consists of:


512- vs 1024-byte blocks

POSIX mandates 512-byte block sizes for the df and du utilities, reflecting the default size of blocks on disks. When Richard Stallman and the GNU team were implementing POSIX for the GNU operating system, they objected to this on the grounds that most people think in terms of 1024 byte (or 1 KiB) blocks. The environment variable POSIXLY_CORRECT was introduced to allow the user to force the standards-compliant behaviour.[14] The variable name POSIX_ME_HARDER was also discussed.[15] The variable POSIXLY_CORRECT is now also used for a number of other behaviour quirks, where "POSIX and common sense disagree".

POSIX-oriented operating systems

Depending upon the degree of compliance with the standards, one can classify operating systems as fully or partly POSIX compatible. Certified products can be found at the IEEE's website.[16]


Some versions of the following operating systems have been certified to conform to one or more of the various POSIX standards. This means that they passed the automated conformance tests.[17]

Mostly POSIX-compliant

The following, while not officially certified as POSIX compatible, comply in large part:

POSIX for Windows

POSIX for OS/2

Mostly POSIX compliant environments for OS/2:


Partially POSIX compliant environments for DOS include:

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.

See also


  1. "POSIX.1 FAQ". The Open Group. 5 October 2011.
  2. 1 2 "POSIX 1003.1 FAQ Version 1.12". 2 February 2006. Retrieved 16 July 2006.
  3. "POSIX". Standards. IEEE.
  4. "The origin of the name POSIX.". 2011. Retrieved 28 September 2013.
  5. PASC Status (including POSIX) (Report). IEEE Computer Society. 2003-12-04. Retrieved 2015-03-01.
  6. "Shell Command Language - The Open Group Base Specifications Issue 7, 2013 Edition".
  7. "POSIX". The Open Group.
  8. "librt(3LIB)". docs.oracle.com. man pages section 3: Library Interfaces and Headers. Oracle Corporation. 1998-08-04. Retrieved 2016-02-18. librt, libposix4- POSIX.1b Realtime Extensions library [...] librt is the preferred name for this library. The name libposix4 is maintained for backward compatibility and should be avoided. Functions in this library provide most of the interfaces specified by the POSIX.1b Realtime Extension.
  9. "The Open Group announces completion of the joint revision to POSIX and the Single UNIX Specification" (Press release). The Open Group. 30 January 2002. Retrieved 26 July 2009.
  10. "IEEE Std 1003.1" (2004 ed.). Unix.org. Retrieved 26 July 2009
  11. "IEEE Std 1003.1" (2004 ed.). The Open Group.
  12. "Base Specifications, Issue 7, 2013 Edition". The Open Group. Retrieved 18 December 2014.
  13. "The Austin Common Standards Revision Group". The Open Group. Retrieved 1 March 2016.
  14. "Announce" (Google Groups replica). GNU.
  15. "Freedom, Innovation, and Convenience: The RMS Interview". Linuxdevcenter. 22 December 2004.
  16. "POSIX Certification". IEEE.
  17. "POSIX Certified by IEEE and The Open Group - Program Guide".
  18. "IBM". The Open Group. Retrieved 26 January 2014.
  19. 1 2 "Hewlett-Packard". The Open Group. Retrieved 26 January 2014.
  20. "Silicon Graphics, Inc.". The Open Group. Retrieved 26 January 2014.
  21. "The Open Brand - Register of Certified Products". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
  22. "Apple Inc". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
  23. "Oracle Corporation". The Open Group. Retrieved 26 January 2014.
  24. "UnixWare ® 7.1.3 and later". The Open Group. 16 May 2003.
  25. "QNX Achieves New POSIX Certification". QNX. 8 April 2008. Retrieved 16 January 2016.
  26. "Register of Open Branded Products". www.opengroup.org. Retrieved 2016-01-05.
  27. "POSIX Certification - Product details". get.posixcertified.ieee.org. Retrieved 2016-03-13.
  28. Schweik. "POSIX utilities". FreeBSD.
  29. Enterprise, I. D. G. (1994-11-07). Computerworld. IDG Enterprise.
  30. Solter, Nicholas A.; Jelinek, Jerry; Miner, David (2011-03-21). OpenSolaris Bible. John Wiley & Sons. ISBN 9781118080313.
  31. Features Removed or Deprecated in Windows Server 2012
  32. Windows NT Services for UNIX Add-On Pack for NT 4; see also the November '98 press release for MKS toolkit 6.1, also archived elsewhere
  33. "MSDN Library: Deprecated CRT Functions". Microsoft. Retrieved 8 Oct 2015.
  34. "MSDN Library: Porting Socket Applications to Winsock". Microsoft. Retrieved 8 Oct 2015.
  35. "Winsock Programmer's FAQ Articles: BSD Sockets Compatibility". Warren Young. 31 Aug 2015. Retrieved 8 Oct 2015.
  36. "APE — ANSI/POSIX Environment". Plan 9. Bell Labs.
  37. "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft.
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