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

POSIX (/ˈpɒzɪks/ poz-iks), an acronym for Portable Operating System Interface,[1] is a family ofstandards 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 namePOSIX to the IEEE instead of former IEEE-IX. The committee found it more easily pronounceable and memorable, and thus adopted it.[2][4][better source needed]


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 Korn shell[citation needed]. 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. Nowadays, most of POSIX parts 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:

  • POSIX.1, 2013 Edition: POSIX Base Definitions, System Interfaces, and Commands and Utilities (which include POSIX.1, extensions for POSIX.1, Real-time Services, Threads Interface, Real-time Extensions, Security Interface, Network File Access and Network Process-to-Process Communications, User Portability Extensions, Corrections and Extensions, Protection and Control Utilities and Batch System Utilities. This is POSIX 1003.1-2008 with Technical Corrigendum 1.)
  • POSIX Conformance Testing: A test suite for POSIX accompanies the standard: VSX-PCTS or theVSX POSIX Conformance Test Suite.[6]

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[edit]

Before 1997, POSIX comprised several standards:




  • POSIX.1c, Threads extensions (IEEE Std 1003.1c-1995)
    • Thread Creation, Control, and Cleanup
    • Thread Scheduling
    • Thread Synchronization
    • Signal Handling


Versions after 1997[edit]

After 1997, the Austin Group developed the POSIX revisions. The specifications are known under the name Single UNIX Specification, before they became 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[7]

This standard consisted of:

  • the Base Definitions, Issue 6,
  • the System Interfaces and Headers, Issue 6,
  • the Commands and Utilities, Issue 6.

POSIX.1-2004 (with two TCs)[edit]

IEEE Std 1003.1-2004 involved a minor update of POSIX.1-2001. It incorporated two technicalcorrigenda.[8] Its contents are available on the web.[9]

POSIX.1-2008 (with one TC)[edit]

As of 2014 POSIX.1-2008 with TC1 or IEEE Std 1003.1, 2013 edition represents the current version.[10][11] A free online copy is available.[12]

This standard consists of:

  • the Base Definitions, Issue 7,
  • the System Interfaces and Headers, Issue 7,
  • the Commands and Utilities, Issue 7,
  • the Rationale volume.


512- vs 1024-byte blocks[edit]

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.[13] The variable name POSIX_ME_HARDER was also discussed.[14] The variable POSIXLY_CORRECT is now also used for a number of other behaviour quirks, where “POSIX and common sense disagree”.[citation needed]

POSIX-oriented operating systems[edit]

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.[15]

Fully POSIX-compliant[edit]

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

Mostly POSIX-compliant[edit]

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

POSIX for Windows[edit]

  • Cygwin provides a largely POSIX-compliant development and run-time environment for Microsoft Windows.
    • MinGW, formerly a fork of Cygwin, provides a less POSIX-compliant development environment and supports compatible C-programmed applications via Msvcrt, Microsoft’s old Visual C runtime library.
  • Microsoft POSIX subsystem, an optional Windows subsystem included in Windows NT-based operating systems up to Windows 2000. POSIX-1 as it stood in 1990 revision, without threads or sockets.
  • Interix, originally OpenNT by Softway Systems, Inc., is an upgrade and replacement for Microsoft POSIX subsystem that was purchased by Microsoft in 1999. It was initially marketed as a stand-alone add-on product and then later included it as a component in Windows Services for UNIX (SFU) and finally incorporated it as a component in Windows Server 2003 R2 and later Windows OS releases under the name “Subsystem for UNIX-based Applications” (SUA); later made deprecated in 2012 (Windows 8)[24] and dropped in 2013 (2012 R2, 8.1). It enables full POSIX compliance for certain Microsoft Windows products[citation needed].
  • UWIN from AT&T Research implements a POSIX layer on top of the Win32 APIs.
  • MKS Toolkit, originally created for MS-DOS, is a software package produced and maintained by MKS Inc. that provides a Unix-like environment for scripting, connectivity and porting Unix and Linuxsoftware to both 32- and 64-bit Microsoft Windows systems. A subset of it was included in the first release of Windows Services for UNIX (SFU) in 1998.[25]

POSIX for OS/2[edit]

Mostly POSIX compliant environments for OS/2:

  • emx+gcc – largely POSIX compliant

POSIX for DOS[edit]

Partially POSIX compliant environments for DOS include:

  • emx+gcc – largely POSIX compliant
  • DJGPP – partially POSIX compliant
  • DR-DOS multitasking core via EMM386 /MULTI – a Pthreads frontend API extension is available

Compliant via compatibility feature[edit]

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[edit]


  1. Jump up^ “POSIX.1 FAQ”. The Open Group. October 5, 2011.
  2. ^ Jump up to:a b “POSIX 1003.1 FAQ Version 1.12”. 2 February 2006. Retrieved 16 July 2006.
  3. Jump up^ “POSIX”. Standards. IEEE.
  4. Jump up^ “The origin of the name POSIX.”. 2011. Retrieved 28 September 2013.
  5. Jump up^ PASC Status (including POSIX) (Report). IEEE Computer Society. 2003-12-04. Retrieved 2015-03-01.
  6. Jump up^ “POSIX”. The Open Group.
  7. Jump up^ “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.
  8. Jump up^ “IEEE Std 1003.1” (2004 ed.). Unix.org. Retrieved 26 July 2009
  9. Jump up^ “IEEE Std 1003.1” (2004 ed.). The Open Group.
  10. Jump up^ “Base Specifications, Issue 7, 2013 Edition”. The Open Group. Retrieved 18 December 2014.
  11. Jump up^ “The Austin Common Standards Revision Group”. The Open Group. Retrieved 27 July 2009.
  12. Jump up^ “Posix.1-2013”. The Open Group..
  13. Jump up^ “Announce” (Google Groups replica). GNU.
  14. Jump up^ “Freedom, Innovation, and Convenience: The RMS Interview”. Linuxdevcenter. 22 December 2004.
  15. Jump up^ “POSIX Certification”. IEEE.
  16. Jump up^ “IBM”. The Open Group. Retrieved 26 January 2014.
  17. ^ Jump up to:a b c “Hewlett-Packard”. The Open Group. Retrieved 26 January 2014.
  18. Jump up^ “Silicon Graphics, Inc.”. The Open Group. Retrieved 26 January 2014.
  19. Jump up^ “Register of Certified Products”. Register of Open Branded Products. The Open Group. Retrieved5 November 2014.
  20. ^ Jump up to:a b “POSIX Certified”. IEEE |chapter= ignored (help).
  21. Jump up^ “Oracle Corporation”. The Open Group. Retrieved 26 January 2014.
  22. Jump up^ “UnixWare ® 7.1.3 and later”. The Open Group. 16 May 2003.
  23. Jump up^ Schweik. “POSIX utilities”. FreeBSD.
  24. Jump up^ Features Removed or Deprecated in Windows Server 2012
  25. Jump up^ 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
  26. Jump up^ “APE — ANSI/POSIX Environment”. Plan 9. Bell Labs.
  27. Jump up^ “POSIX Compatibility”. MS Windows NT Workstation Resource Kit. Microsoft.

SOURCE: http://en.wikipedia.org/wiki/POSIX

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