Portable Operating System Interface (IEEE 1003)
Year started1988; 34 years ago (1988)
Latest versionIEEE Std 1003.1-2017
2017; 5 years ago (2017)
OrganizationAustin Group (IEEE Computer Society, The Open Group, ISO/IEC JTC 1)
Related standardsISO/IEC 9945
DomainApplication programming interfaces

The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems.[1] POSIX defines both the system- and user-level application programming interfaces (API), along with command line shells and utility interfaces, for software compatibility (portability) with variants of Unix and other operating systems.[2][3] POSIX is also a trademark of the IEEE.[2] POSIX is intended to be used by both application and system developers.[4]


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 ISO/IEC standard number is ISO/IEC 9945.

The standards emerged from a project that began around 1985. Richard Stallman suggested the name POSIX (pronounced as pahz-icks, not as poh-six) to the IEEE instead of former IEEE-IX. The committee found it more easily pronounceable and memorable, and thus adopted it.[2][5]


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.).[6] The standardized user command line and scripting interface were based on the UNIX System V shell.[7] 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. In 2008, most parts of POSIX were combined into a single standard (IEEE Std 1003.1-2008, also known as POSIX.1-2008).

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

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


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 (with two TCs)

POSIX.1-2001 (or IEEE Std 1003.1-2001) equates to the Single UNIX Specification version 3.[10]

This standard consisted of:

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 (TCs).[11] Its contents are available on the web.[12]

POSIX.1-2008 (with two TCs)

Base Specifications, Issue 7 (or IEEE Std 1003.1-2008, 2016 Edition) is similar to the current 2017 version (as of 22 July 2018).[13][14]

This standard consists of:


IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008) - IEEE Standard for Information Technology—Portable Operating System Interface (POSIX(R)) Base Specifications, Issue 7 is available from either The Open Group or IEEE and is, as of 22 July 2018, the current standard. It is technically identical to POSIX.1-2008 with Technical Corrigenda 1 and 2 applied. A free online copy may still be available.[13]


512- vs 1024-byte blocks

POSIX mandates 512-byte default block sizes for the df and du utilities, reflecting the typical 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 POSIX_ME_HARDER was introduced to allow the user to force the standards-compliant behaviour.[15] The variable name was later changed to POSIXLY_CORRECT.[16] This variable is now also used for a number of other behaviour quirks.

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.[17]


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.[18]

Mostly POSIX-compliant

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

POSIX for Microsoft Windows

POSIX for OS/2

Mostly POSIX compliant environments for OS/2:


Partially POSIX compliant environments for DOS include:

Compliant via compatibility layer

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 non-compliant.

See also


  1. ^ "POSIX.1 FAQ". The Open Group. 5 October 2011.
  2. ^ a b c "POSIX 1003.1 FAQ Version 1.12". 2 February 2006. Retrieved 16 July 2006.
  3. ^ "P1003.1 - Standard for Information Technology--Portable Operating System Interface (POSIX(TM)) Base Specifications, Issue 8". IEEE Standards Association.
  4. ^ "Introduction". pubs.opengroup.org. Retrieved 22 July 2021.
  5. ^ "The origin of the name POSIX". 2011. Retrieved 28 September 2013.
  6. ^ PASC Status (including POSIX) (Report). IEEE Computer Society. 4 December 2003. Retrieved 1 March 2015.
  7. ^ "Shell Command Language - The Open Group Base Specifications Issue 7, 2013 Edition". Retrieved 28 April 2020.
  8. ^ "POSIX". The Open Group.
  9. ^ "librt(3LIB)". docs.oracle.com. man pages section 3: Library Interfaces and Headers. Oracle Corporation. 4 August 1998. Retrieved 18 February 2016. 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.
  10. ^ "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.
  11. ^ "IEEE Std 1003.1" (2004 ed.). Unix.org. Retrieved 26 July 2009.
  12. ^ "IEEE Std 1003.1" (2004 ed.). The Open Group. Cite journal requires |journal= (help).
  13. ^ a b "Base Specifications, Issue 7, 2016 Edition". The Open Group. Retrieved 18 December 2014.
  14. ^ "The Austin Common Standards Revision Group". The Open Group. Retrieved 1 March 2016.
  15. ^ Stallman, Richard (28 August 1991). "Democracy Triumphs in Disk Units". Newsgroupgnu.announce. Usenet: 9108281809.AA03552@mole.gnu.ai.mit.edu – via Google Groups.
  16. ^ "GNU Coding Standards". GNU.
  17. ^ "POSIX Certification". IEEE.
  18. ^ "POSIX Certified by IEEE and The Open Group - Program Guide".
  19. ^ a b "IBM". The Open Group. Retrieved 26 January 2014.
  20. ^ "Huawei Technology Co., Ltd". The Open Group. Retrieved 26 May 2017.
  21. ^ a b "Hewlett-Packard". The Open Group. Retrieved 26 January 2014.
  22. ^ "Inspur Co., Ltd". The Open Group. Retrieved 26 May 2017.
  23. ^ a b "POSIX Certification Register". get.posixcertified.ieee.org. Retrieved 24 December 2021.
  24. ^ "Silicon Graphics, Inc". The Open Group. Retrieved 26 January 2014.
  25. ^ "The Open Brand - Register of Certified Products". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
  26. ^ "Apple Inc". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
  27. ^ "SCO OpenServer Release 5". The Open Group. 3 May 1995. Retrieved 24 December 2021.
  28. ^ "QNX Achieves New POSIX Certification". QNX. 8 April 2008. Retrieved 16 January 2016.
  29. ^ "Oracle Corporation". The Open Group. Retrieved 26 January 2014.
  30. ^ "UnixWare ® 7.1.3 and later". The Open Group. 16 May 2003. Retrieved 24 December 2021.
  31. ^ Schweikhardt, Jens. "POSIX utilities". FreeBSD.
  32. ^ Mark Halper (7 November 1994). "HP 3000 sales catch market by surprise". Computerworld. Vol. 28 no. 4. IDG Enterprise.
  33. ^ Solter, Nicholas A.; Jelinek, Jerry; Miner, David (21 March 2011). OpenSolaris Bible. John Wiley & Sons. ISBN 9781118080313.
  34. ^ "OpenVOS POSIX.1: Conformance Guide". Status Technologies. Retrieved 26 November 2021.
  35. ^ "ULTRIX POSIX Conformance Document" (PDF). Digital. June 1990. Retrieved 13 December 2021.
  36. ^ "Features Removed or Deprecated in Windows Server 2012".
  37. ^ 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
  38. ^ "MSDN Library: Deprecated CRT Functions". Microsoft. Retrieved 8 October 2015.
  39. ^ "MSDN Library: Porting Socket Applications to Winsock". Microsoft. Retrieved 8 October 2015.
  40. ^ "Winsock Programmer's FAQ Articles: BSD Sockets Compatibility". Warren Young. 31 August 2015. Retrieved 8 October 2015.
  41. ^ "Programming IBM PASE for i" (PDF). ibm.com. IBM. 2013. Retrieved 25 November 2021.
  42. ^ "APE — ANSI/POSIX Environment". Plan 9. Bell Labs.
  43. ^ "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft.
  44. ^ "Realtime Products Technical Summary, Fifth Edition" (PDF). Digital Equipment Corporation. December 1992. Retrieved 8 December 2021.