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POSIX (/ˈpɒzɪks/ poz-iks), an acronym for "Portable Operating System Interface", 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 compatible with variants of Unix and other operating systems.


Originally, the name "POSIX" referred to 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.


The POSIX specifications for Unix-like operating system environments originally consisted of a single document for the core programming interface, but eventually grew to 17 separate documents. 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.

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.1, Core Services (incorporates Standard ANSI C) (IEEE Std 1003.1-1988)
    • Process Creation and Control
    • Signals
    • Floating Point Exceptions
    • Segmentation / Memory Violations
    • Illegal Instructions
    • Bus Errors
    • Timers
    • File and Directory Operations
    • Pipes
    • C Library (Standard C)
    • I/O Port Interface and Control
    • Process Triggers


  • POSIX.1b, Real-time extensions (IEEE Std 1003.1b-1993)
    • Priority Scheduling
    • Real-Time Signals
    • Clocks and Timers
    • Semaphores
    • Message Passing
    • Shared Memory
    • Asynch and Synch I/O
    • Memory Locking Interface


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


  • POSIX.2, Shell and Utilities (IEEE Std 1003.2-1992)
    • Command Interpreter
    • Utility Programs

    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

    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. Its contents are available on the web.


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

    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 environmental variable POSIXLY_CORRECT was introduced to force the standards-compliant behaviour. The variable POSIX_ME_HARDER was also discussed and was implemented in a few packages 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.

    Fully POSIX-compliant

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

    • A/UX
    • AIX
    • BSD/OS
    • DSPnano
    • HP-UX
    • IRIX
    • LynxOS
    • Mac OS X
    • MINIX
    • MPE/iX
    • QNX
    • RTEMS (POSIX 1003.13-2003 Profile 52)
    • Solaris
    • Tru64
    • Unison RTOS
    • UnixWare
    • velOSity
    • VxWorks

    Mostly POSIX-compliant

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

    • BeOS / Haiku
    • FreeBSD
    • GNU/Linux (most distributions — see Linux Standard Base)
    • Contiki
    • NetBSD
    • Nucleus RTOS
    • OpenBSD
    • OpenSolaris
    • PikeOS RTOS for embedded systems with optional PSE51 and PSE52 partitions; see partition (mainframe)
    • RTEMS – POSIX API support designed to IEEE Std. 1003.13-2003 PSE52
    • Sanos
    • SkyOS
    • Syllable
    • VSTa

    POSIX for Windows

    • Cygwin provides a largely POSIX-compliant development and run-time environment for Microsoft Windows.
    • Microsoft POSIX subsystem, an optional Windows subsystem. Partial POSIX-1 — no threads, no sockets.
    • Microsoft Windows Services for UNIX 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, UNIX Services for Windows must be installed to provide POSIX compatibility. The UNIX Subsystem is built in to the Enterprise and Ultimate editions of Windows Vista and 7, and cannot be added separately to the other editions.
    • UWIN from AT&T Research implements a POSIX layer on top of the Win32 APIs.
    • MKS Toolkit

    POSIX for OS/2

    Mostly POSIX compliant environments for OS/2:

    • emx+gcc – largely POSIX compliant

    POSIX for DOS

    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.

    • eCos – POSIX is part of standard distribution, and used by many applications. 'external links' section below has more information.
    • MorphOS (through the built-in ixemul library)
    • OpenVMS (through optional POSIX package)
    • OpenVOS is an optional POSIX-compliant layer atop the Stratus VOS kernel
    • Plan 9 from Bell Labs APE - ANSI/POSIX Environment
    • Symbian OS with PIPS (PIPS Is POSIX on Symbian)
    • Windows NT kernel when using Microsoft SFU 3.5 or SUA
      • Windows 2000 Server or Professional with Service Pack 3 or later. To be POSIX compliant, one must activate optional features of Windows NT and Windows 2000 Server.
      • Windows XP Professional with Service Pack 1 or later
      • Windows Server 2003
      • Windows Vista
      • Windows 7
    • z/OS



    Published - December 2011

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