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From Wikipedia, the free encyclopedia

POSIX (pronounced /ˈpɒzɪks/ POZ-iks) or "Portable Operating System Interface [for Unix]"[1] is the 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 "POSIX" stood for IEEE Std 1003.1-1988, released, as the name suggests, in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945.

The standards, formerly known as IEEE-IX, emerged from a project that began circa 1985. Richard Stallman suggested the name POSIX in response to an IEEE request for a memorable name.[2]


The POSIX specifications for user and software interfaces to an operating system originally consisted of 17 separate documents.[3] 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, 10 out of these 17 parts are combined into a single standard, IEEE Std 1003.1-2008, also known as POSIX:2008.

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

  • POSIX: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.[4]

The development of the POSIX standard takes place in the Austin Group, a joint working group linking the Open Group and the ISO organization. The Austin Group has produced all POSIX revisions since 2000&2003


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 become a POSIX standard when formally approved by the ISO.


POSIX:2001 or IEEE Std 1003.1-2001 equates to the Single UNIX Specification version 3[5]

This standard consisted of:

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


POSIX:2004 or IEEE Std 1003.1-2004 involved a minor update of POSIX:2001. It incorporated two technical corrigenda.[6] Its contents are available on the web [1].


As of 2009 POSIX:2008 or IEEE Std 1003.1-2008 represents the current version.[7][8] A free online copy is available.[9]

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 what would become GNU/Linux, they objected to this on the grounds that most people think in terms of 1024 byte (or 1 KiB) blocks. The environmental variable POSIXLY_CORRECT was introduced to force the standards-compliant behaviour.[10] The variable POSIX_ME_HARDER was also discussed[11] and was implemented in a few packages[12] before being obsoleted by POSIXLY_CORRECT.

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.[13]

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, while not officially certified as POSIX compatible, conform in large part:

POSIX for Windows

  • Cygwin – enables partial POSIX compliance for certain Microsoft Windows products.
  • Microsoft POSIX subsystem, an optional Windows subsystem.
  • Microsoft Windows Services for UNIX 3.5 – enables full POSIX compliance for certain Microsoft Windows products. Windows NT-based operating systems up to Windows 2000 had a POSIX layer built into the operating system, and UNIX Services for Windows provided a UNIX-like operating environment. For Windows XP, Windows Services for UNIX must be installed to provide POSIX compatibility. The UNIX Subsystem is built in to the Enterprise and Ultimate editions of Windows Vista, and cannot be added separately to the other editions.
  • UWIN from the AT&T Research implements a POSIX layer on top of the Win32 APIs.

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

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.

Industrial resources

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

See also


  1. ^ "POSIX". Standards. IEEE. 
  2. ^ "POSIX 1003.1 FAQ Version 1.12". 2006-02-02. Retrieved 2006-07-16. 
  3. ^ "Introduction". GNU/Linux C Programming. 
  4. ^ "POSIX". NIST. 
  5. ^ The Open Group (January 30 2002). "The Open Group announces completion of the joint revision to POSIX and the Single UNIX Specification". Press release. Retrieved 2009-07-26. 
  6. ^ "IEEE Std 1003.1, 2004 Edition". Retrieved 2009-07-26. 
  7. ^ "Base Specifications, Issue 7". The Open Group. Retrieved 2009-07-27. 
  8. ^ "The Austin Common Standards Revision Group". The Open Group. Retrieved 2009-07-27. 
  9. ^
  10. ^
  11. ^
  12. ^ Search for packages implementing POSIX_ME_HARDER
  13. ^ "POSIX Certification". IEEE. 
  14. ^ Mac OS X Leopard. OS Foundation.
  15. ^ Schweik. "POSIX utilities". FreeBSD. 
  16. ^ "APE — ANSI/POSIX Environement". Plan 9. Bell Labs. 
  17. ^ "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft. 

External links


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary




  1. (computing): Portable Operating System Interface, a set of IEEE standards designed to provide application portability between UNIX variants.

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