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POSIX ( ), an acronym for "Portable Operating System Interface",[1] is a family of standards specified by the IEEE 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]


  • Name 1
  • Overview 2
  • Versions 3
    • Parts before 1997 3.1
      • POSIX.1 3.1.1
      • POSIX.1b 3.1.2
      • POSIX.1c 3.1.3
      • POSIX.2 3.1.4
    • Versions after 1997 3.2
      • POSIX.1-2001 3.2.1
      • POSIX.1-2004 (with two TCs) 3.2.2
      • POSIX.1-2008 3.2.3
  • Controversies 4
    • 512- vs 1024-byte blocks 4.1
  • POSIX-oriented operating systems 5
    • Fully POSIX-compliant 5.1
    • Mostly POSIX-compliant 5.2
      • POSIX for Windows 5.2.1
      • POSIX for OS/2 5.2.2
      • POSIX for DOS 5.2.3
      • Compliant via compatibility feature 5.2.4
  • See also 6
  • References 7
  • External links 8


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, so the committee adopted it.[2][4]


The POSIX specifications for Unix-like operating system environments originally consisted of a single document for the core programming interface, but eventually grew to 19 separate documents (for example, POSIX.1, POSIX.2 etc.) [1]. 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. Nowadays, most of POSIX parts are combined into a single standard, IEEE Std 1003.1-2008, also known as POSIX.1-2008.

As of 2009, POSIX documentation is divided in two parts:

  • POSIX.1-2008: 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)
  • POSIX Conformance Testing: A test suite for POSIX accompanies the standard: PCTS or the POSIX Conformance Test Suite.[5]

The development of the POSIX standard takes place in the Austin Group, a joint working group linking the Open Group and the ISO organization.


Parts before 1997

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

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[6]

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)

IEEE Std 1003.1-2004 involved a minor update of POSIX.1-2001. It incorporated two technical corrigenda.[7] Its contents are available on the web.[8]


As of 2009 POSIX.1-2008 or IEEE Std 1003.1-2008 represents the current version.[9][10] A free online copy is available.[11]

This standard consists of:

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


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.[12] The variable name POSIX_ME_HARDER was also discussed.[13] 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.[14]

Fully POSIX-compliant

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

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

POSIX for Windows

  • 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)[23] and dropped in 2013 (2012 R2, 8.1). It enables full POSIX compliance for certain Microsoft Windows products.
  • 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 Linux software 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.[24]

POSIX for OS/2

Mostly POSIX compliant environments for OS/2:

  • emx+gcc – largely POSIX compliant


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

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. October 5, 2011. 
  2. ^ a b "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. ^ "POSIX". NIST. 
  6. ^ "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. 
  7. ^ "IEEE Std 1003.1" (2004 ed.). Retrieved 26 July 2009 
  8. ^ "IEEE Std 1003.1" (2004 ed.). The Open Group .
  9. ^ "Base Specifications, Issue 7". The Open Group. Retrieved 27 July 2009. 
  10. ^ "The Austin Common Standards Revision Group". The Open Group. Retrieved 27 July 2009. 
  11. ^ "POSIX.1-2008". The Open Group .
  12. ^ "Announce" (Google Groups replica). GNU .
  13. ^ "Freedom, Innovation, and Convenience: The RMS Interview". Linuxdevcenter. 22 December 2004 .
  14. ^ "POSIX Certification". IEEE. 
  15. ^ "IBM". The Open Group. Retrieved 26 January 2014. 
  16. ^ a b c "Hewlett-Packard". The Open Group. Retrieved 26 January 2014. 
  17. ^ "Silicon Graphics, Inc.". The Open Group. Retrieved 26 January 2014. 
  18. ^ "Register of Certified Products". Register of Open Branded Products. The Open Group. Retrieved 5 November 2014. 
  19. ^ a b "POSIX Certified". IEEE  .
  20. ^ "Oracle Corporation". The Open Group. Retrieved 26 January 2014. 
  21. ^ "UnixWare ® 7.1.3 and later". The Open Group. 16 May 2003. 
  22. ^ Schweik. "POSIX utilities". FreeBSD. 
  23. ^ Features Removed or Deprecated in Windows Server 2012
  24. ^ 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
  25. ^ "OpenVOS: Stratus Virtual Operating System". Stratus Technologies. Retrieved 18 October 2010. 
  26. ^ "APE — ANSI/POSIX Environment". Plan 9. Bell Labs. 
  27. ^ "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft. 

External links

  • "The Austin Group". The Open Group .
  • "The Portable Application Standards Committee" 
  • "POSIX Certification Authority". IEEE .
  • "The UNIX System" (home page). The Open Group 
  • "Base Specifications". POSIX.1-2008 (7). The Open Group 
  • "What could have been IEEE 1003.1e/2c". Tuxomania 
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