View source for POSIX

{{Short description|Family of IEEE standards for compatibility between operating systems}}
{{Distinguish|text=[[Unix]], [[Unix-like]], or [[Linux]]}}
{{Use dmy dates|date=October 2023}}
{{Infobox technology standard
| title             = Portable Operating System Interface (IEEE 1003)
| status            = Published
| year_started      = {{Start date and age|1988}}
| version           = IEEE Std 1003.1-2024
| version_date      = {{Start date and age|2024}}
| organization      = [[Austin Group]] ([[IEEE Computer Society]], [[The Open Group]], [[ISO/IEC JTC 1/SC 22]]/WG 15)
| related_standards = [[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] 9945
| abbreviation      = POSIX
| domain            = [[Application programming interface]]s
| website           = {{URL|https://posix.opengroup.org}}
}}
The '''Portable Operating System Interface''' ('''POSIX'''; {{IPAc-en|IPA|ˈ|p|ɒ|z|.|ɪ|k|s}}<ref name="FAQ">{{cite web|date=13 June 2020|title=POSIX.1 FAQ|url=https://www.opengroup.org/austin/papers/posix_faq.html|publisher=The Open Group|access-date=20 February 2023}}</ref>) is a family of [[standardization|standard]]s specified by the [[IEEE Computer Society]] for maintaining compatibility between [[operating system]]s.<ref name="FAQ"/> In order to define a level of compatibility, POSIX specifies many aspects of functionality that can be classified as [[application programming interface]] (API), [[command-line shell]], and [[command (computing)|shell commands]]. Originally derived from commonly-found Unix APIs, shells, and commands (partly because Unix was considered manufacturer-neutral), today many systems conform to the standard {{endash}} including [[branded Unix]] systems, [[Unix-like]] systems, and many systems that were historically unrelated to Unix.<ref name="FAQ"/><ref name="IET">{{cite web|title=IEEE 1003.1-2024|url =https://standards.ieee.org/ieee/1003.1/7700/|website=IEEE Standards Association}}</ref>

The standardized user command line and [[command-line interface#Command-line interpreter |scripting interface]] were based on the [[UNIX System V]] [[Bourne shell]].<ref>{{cite web|url=https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html|title=Shell Command Language - The Open Group Base Specifications Issue 7, 2013 Edition|access-date=28 April 2020}}</ref> Many user-level programs, services, and utilities (including [[AWK|awk]], [[echo (command)|echo]], [[ed (text editor)|ed]]) were also standardized, based on UNIX System V versions of them, along with required program-level services (including basic [[input/output|I/O]]: [[computer file|file]], [[computer terminal|terminal]], and [[computer network|network]]). POSIX also defines a standard [[thread (computing)|threading]] library API which is supported by most modern operating systems.

The POSIX standard is developed by the [[Austin Group]] (a joint [[working group]] among the IEEE, [[The Open Group]], and the [[ISO/IEC JTC 1/SC 22]]/WG 15).

POSIX is intended to be used by both application and system developers.<ref>{{Cite web|title=Introduction|url=https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap01.html|access-date=22 July 2021|work=The Open Group Base Specifications Issue 7, 2018 edition}}</ref>{{Explain| as-is this is confusing and awkward; expand to make this more meaningful|date=December 2025}}

==Name==
The standards emerged from a project that began in 1984 building on work from related activity in the ''/usr/group'' association.<ref>{{Cite web|title=JimIsaak - POSIX Impact|url=https://sites.google.com/site/jimisaak/posix-impact|access-date=15 September 2022 |website=sites.google.com|language=en-US}}</ref> [[Richard Stallman]] suggested the name ''POSIX'' to the IEEE<ref>{{cite web |url=https://www.opengroup.org/austin/papers/backgrounder.html |title=A Backgrounder on IEEE Std 1003.1 |at=Footnotes}}</ref><ref>{{Cite web |title=The origin of the name POSIX. |url=https://stallman.org/articles/posix.html |access-date=2024-09-26 |website=stallman.org}}</ref> instead of the former ''IEEE-IX''.<ref>{{cite journal |url=https://www.usenix.org/legacy/bodinfo/login0207.html |title=An Update On Standards |first=Nicholas |last=Stoughton |journal=[[;login:]]}}</ref> The committee found it more easily pronounceable and memorable, and thus adopted it.{{cn|date=June 2024}}

Originally, 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 [[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] 9945.

POSIX is a [[trademark]] of the IEEE.<ref name="FAQ"/>

==Versions==
POSIX originally consisted of a single document for core services but over time additional documents were published to extend and revise the specification. Before 1997, POSIX comprised multiple documents that were published over the course of several years. After 1997, the [[Austin Group]] produces specifications titled [[Single UNIX Specification]] (SUS). Over time, the group publishes versions of this specification and later POSIX is amended per some or all of a SUS version. A SUS version consists of a collection of volumes {{endash}} each for a grouping of required behavior {{endash}} plus other information (outside of a volume).<ref>{{Cite report|date=4 December 2003|title=PASC Status (including POSIX)|url=https://collaboration.opengroup.org/external/pasc.org/standing/sd11.html|publisher=[[IEEE Computer Society]]|access-date=26 September 2024}}</ref> Each volume is assigned an issue number that is the same for each volume of a version, but is not the same value as the version. For example, SUS version 3 (SUSv3) includes volumes labeled issue 6.

{{As of | 2014 }}, POSIX documentation is divided into 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 the '''VSX POSIX Conformance Test Suite'''.<ref name = "VSX-PCTS">{{cite web | url = https://www.opengroup.org/testing/testsuites/vsxpcts2003.htm | title = Test Suites VSX-PCTS2003 | publisher = The Open Group}}</ref>

===Before 1997===

====POSIX.1====
''Core Services'' (IEEE Std 1003.1-1988) incorporates standard [[ANSI C]] and includes:
:; [[Process (computing)|Process Creation and Control]]
:; [[Signal (IPC)|Signals]]
::; [[SIGFPE |Floating Point Exceptions]]
::; [[SIGSEGV |Segmentation / Memory Violations]]
::; [[SIGILL |Illegal Instructions]]
::; [[SIGBUS |Bus Errors]]
::; [[SIGALRM |Timers]]
:; File and Directory Operations
:; [[Pipeline (Unix)|Pipes]]
:; [[C standard library |C Library (Standard C)]]
:; The [[POSIX terminal interface]]

====POSIX.1b====
''Real-time extensions'' (IEEE Std 1003.1b-1993, later appearing as librt—the Realtime Extensions library) includes:<ref>

{{cite web|url=https://docs.oracle.com/cd/E19455-01/806-0632/6j9vm89ic/index.html|title=librt(3LIB)|date=4 August 1998|website=docs.oracle.com|series=man pages section 3: Library Interfaces and Headers|publisher=Oracle Corporation|access-date=18 February 2016|quote=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.}}</ref>
:; Priority [[Scheduling (computing)|Scheduling]]
:; [[SIGRTMIN and SIGRTMAX|Real-Time Signals]]
:; Clocks and Timers
:; [[Semaphore (programming)|Semaphores]]
:; [[Message Passing]]
:; [[Shared memory|Shared Memory]]
:; [[Asynchronous I/O|Asynchronous]] and Synchronous I/O
:; Memory Locking Interface

====POSIX.1c====
[[POSIX Threads |''Threads extensions'']] (IEEE Std 1003.1c-1995) includes:

:; Thread Creation, Control, and Cleanup
:; Thread Scheduling
:; Thread Synchronization
:; Signal Handling

====POSIX.2====
''Shell and Utilities'' (IEEE Std 1003.2-1992) includes:

:; [[Command-line interpreter|Command Interpreter]]
:; Utility Programs

===POSIX.1-2001===
''POSIX.1-2001'' (IEEE Std 1003.1-2001) consists of most of SUSv3 which consists of volumes (issue 6): ''Base Definitions'', ''System Interfaces and Headers'', and ''Commands and Utilities''. The POSIX specification specifically excludes the SUSv3 requirements for a curses API (often called ''X/Open Curses'', even though there is no distinct grouping of this in SUSv3).<ref>{{cite web|url=https://unix.org/version3/overview.html|title=The Single UNIX Specification Version 3 - Overview|website=unix.org}}</ref>

IEEE Std 1003.1-2004 modifies POSIX.1-2001 via two minor updates or [[Erratum |errata]] referred to as technical corrigenda documents.<ref>{{cite web|url=http://www.unix.org/version3/ieee_std.html|title=IEEE Std 1003.1|edition = 2004|publisher=Unix.org|access-date=26 July 2009}}</ref><ref>{{cite web|url=https://pubs.opengroup.org/onlinepubs/009695399/|title=Base Specifications, Issue 6|edition=2004|access-date=20 July 2024|publisher=The Open Group}}.</ref>

===POSIX.1-2008===
Similar to its predecessor, POSIX.1-2008 (''IEEE Std 1003.1-2008'', 2016 Edition) consists of most of the normative material of SUSv4 (issue 7 of volumes ''Base Definitions'', ''System Interfaces and Headers'', ''Commands and Utilities'').<ref name=AustinGroup>{{cite web|url=https://www.opengroup.org/austin/|title=The Austin Common Standards Revision Group|access-date=20 July 2024|publisher=The Open Group}}</ref> SUSv4 also includes rationale information that largely applies to POSIX although not included per se.

===POSIX.1-2017===
POSIX.1-2017 (''IEEE Std 1003.1-2017'') revises the previous version (POSIX.1-2008) via two technical corrigenda.<ref>{{cite web|url=https://pubs.opengroup.org/onlinepubs/9699919799/|title=Base Specifications, Issue 7, 2018 Edition|edition=2018|access-date=20 July 2024|publisher=The Open Group}}</ref>

===POSIX.1-2024===
POSIX.1-2024 (''IEEE Std 1003.1-2024'') was published on 14 June 2024.<ref name=AustinGroup/><ref>{{cite web|url=https://pubs.opengroup.org/onlinepubs/9799919799/|title=Base Specifications, Issue 8|edition=2024|publisher=The Open Group}}.</ref>

As of POSIX 2024, the standard is aligned with the [[C17 (C standard revision)|C17]] language standard.

==Controversies==
===512- vs 1024-byte blocks===
{{Primary sources|section|date=April 2025}}
POSIX mandates 512-byte default [[Block (computer memory)|block]] sizes for the [[Df (Unix)|df]] and [[Du (Unix)|du]] utilities, reflecting the typical size of blocks on disks. When [[Richard M. Stallman|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 [[Kibibyte|KiB]]) blocks. The environment variable {{tt|POSIX_ME_HARDER}} was introduced to allow the user to force the standards-compliant behaviour.<ref>{{Cite newsgroup|last=Stallman|first=Richard|title=Democracy Triumphs in Disk Units|date=28 August 1991|newsgroup=gnu.announce|message-id=9108281809.AA03552@mole.gnu.ai.mit.edu|via=Google Groups |url=http://groups.google.com/group/gnu.announce/msg/6c6e20b57ddb1a82?pli=1}}</ref> The variable name was later changed to {{tt|POSIXLY_CORRECT}}.<ref>{{cite web|publisher=GNU|url=https://www.gnu.org/prep/standards/standards.html#Non_002dGNU-Standards|title=GNU Coding Standards}}</ref> As of 2025, this variable is also used for a number of other behaviour quirks.<ref>{{cite web|author=<!-- not stated -->|date=2025-05-17|title=environ(7) — Linux manual page|url=https://man7.org/linux/man-pages/man7/environ.7.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=April 7, 2025|title=bash(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/bash.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=June 17, 2025|title=file(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/file.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-07-09|title=find(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/find.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-03-21|title=grep(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/grep.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-08-09|title=more(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/more.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-05-01|title=patch(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/patch.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-04-23|title=ps(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/ps.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref><ref>{{cite web|author=<!-- not stated -->|date=2025-08-09|title=renice(1) — Linux manual page|url=https://man7.org/linux/man-pages/man1/renice.1.html|website=man7.org|publisher=Michael Kerrisk|access-date=2025-10-10}}</ref>

==Conformance==
An operating system can be classified depending upon the degree of conformance with a POSIX standard.

===Certified===
Current 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<ref name="achieve_status">{{cite web|url=https://posix.opengroup.org/certification_guide.html#Howto|title=POSIX Certified by IEEE and The Open Group - Program Guide|at=Section 2, How to Achieve Certification|access-date=26 September 2024}}</ref> and their certification has not expired and the operating system has not been discontinued.<ref name="renew_certification">{{cite web |url=https://posix.opengroup.org/certification_guide.html#Renewals|title=POSIX Certified by IEEE and The Open Group - Program Guide|at=Section 4, Renewals and Certified Product Updates|access-date=26 September 2024}}</ref><ref>{{cite web|url=https://www.opengroup.org/openbrand/register/company.htm|title=Certified Products by Company|author=<!--Not stated-->|date=n.d.|publisher=The Open Group |access-date=22 January 2022}}</ref>
<!-- Please don't add additional operating systems without also citing a source for the information. -->
{{div col|colwidth=15em}}
*[[IBM AIX|AIX]]<ref name="ibm">{{cite web |url=https://www.opengroup.org/openbrand/register/ibm.htm |title=IBM |publisher=The Open Group |access-date=23 January 2022}}</ref>
*[[HP-UX]]<ref name="hp">{{cite web |url=https://www.opengroup.org/openbrand/register/hp.htm |title=Hewlett-Packard  |publisher=The Open Group |access-date=26 January 2014}}</ref>
*[[Integrity (operating system)|INTEGRITY]]<ref name="posix-register">{{Cite web|url=https://posix.opengroup.org/register.html|title=POSIX Certification Register|website=[[The Open Group]]|access-date=22 August 2024}}</ref>
*[[macOS]] (from [[Mac OS X Leopard]]<ref>{{cite web|title=macOS version 10.5 Leopard on Intel-based Mac computers |url=https://www.opengroup.org/openbrand/register/brand3555.htm|work=Register of Open Branded Products|publisher=The Open Group |access-date=20 May 2015}}</ref> to [[macOS Tahoe]]<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/brand3720.htm|title=macOS version 26.0 Tahoe on Intel-based Mac computers|publisher=The Open Group|access-date=5 October 2025}}</ref><ref>{{cite web|url=https://www.opengroup.org/openbrand/register/brand3725.htm|title=macOS version 26.0 Tahoe on Apple Silicon-based Mac computers|publisher=The Open Group|access-date=5 October 2025}}</ref>)
*[[OpenServer]]<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/brand2574.htm|title=SCO OpenServer Release 5  |date=3 May 1995|publisher=The Open Group|access-date=24 December 2021}}</ref>
*[[UnixWare]]<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/brand2713.htm |title=UnixWare ® 7.1.3 and later  |date=16 May 2003|publisher=The Open Group|access-date=24 December 2021}}</ref>
*[[VxWorks]]<ref name="posix-register"/>
*[[z/OS]]<ref name="ibm"/>
{{div col end}}

===Formerly certified===
Some versions of the following operating systems had been certified to conform to one or more of the various POSIX standards. This means that they passed the automated conformance tests. The certification has expired and some of the operating systems have been discontinued.<ref name="renew_certification" />
<!-- Please don't add additional operating systems without also citing a source for the information. -->
{{div col|colwidth=15em}}
*[[EulerOS]] (exp. 2022)<ref>{{cite web |url=https://www.opengroup.org/openbrand/register/huawei.htm|title=Huawei Technology Co., Ltd|publisher=The Open Group|access-date=26 May 2017}}</ref>
*[[Inspur K-UX]] (exp. 2019)<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/inspur.htm|title=Inspur Co., Ltd |publisher=The Open Group|access-date=26 May 2017}}</ref>
*[[IRIX]] (defunct 2006)<ref>{{cite web |url=https://www.opengroup.org/openbrand/register/sgi.htm|title=Silicon Graphics, Inc. |publisher=The Open Group|access-date=26 January 2014}}</ref>
*[[OS/390]] (defunct 2004)<ref>{{cite web|archive-url=https://web.archive.org/web/20030802103246/http://www.opengroup.org/openbrand/register/|url=http://www.opengroup.org/openbrand/register/|title=The Open Brand - Register of Certified Products|date=23 July 2003|archive-date=2 August 2003|access-date=23 January 2022|website=Open Group}}</ref>
*[[QNX|QNX Neutrino]]<ref>{{cite press release|url=http://www.qnx.com/news/pr_2862_1.html|title=QNX Achieves New POSIX Certification|date=8 April 2008|access-date=16 January 2016 | publisher=QNX}}</ref>
*[[Solaris (operating system)|Solaris]] (exp. 2019)<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/sun.htm|title=Oracle Corporation|publisher=The Open Group|access-date=26 January 2014|archive-url=https://web.archive.org/web/20170712134003/https://www.opengroup.org/openbrand/register/sun.htm|archive-date=12 July 2017}}</ref>
*[[Tru64 UNIX|Tru64]] (defunct 2010)<ref>{{cite web|url=https://www.opengroup.org/openbrand/register/hp.htm|title=Hewlett-Packard  |publisher=The Open Group|access-date=26 January 2014|archive-url=https://web.archive.org/web/20140709114759/https://www.opengroup.org/openbrand/register/hp.htm|archive-date=9 July 2014}}</ref>
*[[LiteOS]] (defunct 2020)<ref>{{cite web |title=Huawei LiteOS / LiteOS |url=https://gitee.com/LiteOS/LiteOS |website=Gitee |publisher=Huawei |access-date=23 May 2024}}</ref>
{{div col end}}

===Partially conformant===
The following are not certified as POSIX conforming yet are considered partially conforming which is sometimes called ''compliant'':{{cn|date=December 2025}}
{{div col|colwidth=18em}}
*[[Android (operating system)|Android]] (Available through Android NDK){{citation needed|date=December 2014}}
*[[Darwin (operating system)|Darwin]] (core of [[macOS]] and [[iOS]])
*[[DragonFly BSD]]
*[[FreeBSD]]<ref name="FreeBSD">{{cite web|url=http://people.freebsd.org/~schweikh/posix-utilities.html|title=FreeBSD POSIX 2001 Utility Compliance|first=Jens|last=Schweikhardt|publisher=FreeBSD}}</ref>
*[[Haiku (operating system)|Haiku]]
*[[illumos]]
*[[Linux]] (most distributions)
*[[LynxOS]]
*[[Minix]] (since 2005 [[Minix 3]])
*[[HP Multi-Programming Executive|MPE/iX]]<ref>{{Cite magazine|magazine=Computerworld|url=https://books.google.com/books?id=-6GcSQAVaHgC&q=posix&pg=PA24|title = HP 3000 sales catch market by surprise|author=Mark Halper|volume = 28|issue=4|publisher=IDG Enterprise|date= 7 November 1994}}</ref>
*[[NetBSD]]
*[[Nucleus RTOS]]
*[[NuttX]]
*[[OpenBSD]]
*[[OpenSolaris]]<ref>{{Cite book|title=OpenSolaris Bible|url=https://books.google.com/books?id=y8qaxiZNvqAC|publisher=John Wiley & Sons|date=21 March 2011|isbn=9781118080313|language=en|first1=Nicholas A.|last1=Solter|first2=Jerry|last2=Jelinek|first3=David|last3=Miner}}</ref>
*[[PikeOS]] RTOS for embedded systems with optional PSE51 and PSE52 partitions; see [[Logical partition|partition (mainframe)]]
*[[PX5 RTOS]]<ref>{{cite web|url=https://www.embedded.com/rtos-with-standard-posix-pthreads-api-a-complement-to-embedded-linux/|title=RTOS with standard POSIX pthreads API: a complement to embedded Linux|website=Embedded.com|author-first1=Bill|author-last1=Lamie|author-first2=Rafael|author-last2=Taubinger|date=15 June 2023|access-date=13 September 2023}}</ref>
*[[Redox (operating system)|Redox]]
*[[RTEMS]] – POSIX API support designed to IEEE Std. 1003.13-2003 PSE52
*[[SerenityOS]]
*[[Stratus OpenVOS]]<ref>{{cite web|url=https://stratadoc.stratus.com/vos/19.3.0/r217m-03/wwhelp/wwhimpl/js/html/wwhelp.htm?context=r217m-03&file=ch1r217m-03.html|title=OpenVOS POSIX.1: Conformance Guide|publisher=Status Technologies|access-date=26 November 2021}}</ref>
*[[SkyOS]] (discontinued)
*[[Syllable Desktop|Syllable]] (discontinued)
*[[ULTRIX]]<ref>{{cite manual|url=http://bitsavers.org/pdf/dec/vax/ultrix-32/4.0_Jun90/AA-LY25C-TE_POSIX_Conformance_Document_Jun1990.pdf|title=ULTRIX POSIX Conformance Document|date=June 1990|publisher=Digital|access-date=13 December 2021}}</ref>
*[[VSTa]]
*[[VMware ESXi]]
*[[Xenix]]
*[[Zephyr_(operating_system)|Zephyr]]<ref>{{cite web|url=https://docs.zephyrproject.org/latest/services/portability/posix.html|title=POSIX Support|publisher=Zephyr documentation|access-date=7 March 2023}}</ref>

{{div col end}}

===Partially conformant via compatibility layer {{Anchor |Compliant via compatibility feature}}===
<!-- linked from redirect [[Ixemul.library]] -->
The following operating systems are not certified as POSIX conformant, but they conform in large part to the standard by implementing POSIX support via a compatibility feature (usually translation libraries, or a layer atop the kernel).

*[[AmigaOS]] (through the ixemul library or [[vbcc]]_PosixLib<ref>{{Cite web|url=http://aminet.net/package/dev/c/vbcc_PosixLib|title=Aminet - dev/C/Vbcc_PosixLib.lha}}</ref>)
*[[eCos]]&nbsp;– POSIX is part of the standard distribution, and used by many applications. 'external links' section below has more information.
*[[IBM i]] (through the [[IBM i#PASE|PASE]] compatibility layer)<ref>{{cite web|title=Programming IBM PASE for i|url=https://www.ibm.com/docs/de/ssw_ibm_i_73/rzalf/rzalfpdf.pdf|website=ibm.com|publisher=IBM|date=2013|access-date=25 November 2021}}</ref>
*[[MorphOS]] (through the built-in ixemul library)
*[[OpenVMS]] (through optional POSIX package)<ref>{{cite web|url=https://peren.com/fips151-2_peren_vpl.htm|title=FIPS 151-2 Conformance Validated Products List|publisher=Perennial Test Lab|access-date=14 August 2022}}</ref>
*[[Plan 9 from Bell Labs]] APE - ANSI/POSIX Environment<ref name="APE">{{cite web|url=http://plan9.bell-labs.com/sys/doc/ape.html |title=APE — ANSI/POSIX Environment|publisher=Bell Labs|work=Plan 9 documents|archive-url=https://web.archive.org/web/20170601064807/http://plan9.bell-labs.com/sys/doc/ape.html|archive-date=1 June 2017}}</ref>
*[[RIOT (operating system)|RIOT]] (through optional POSIX module)
*[[Symbian|Symbian OS]] with [[PIPS]] (PIPS Is POSIX on Symbian)
*[[VAXELN]] (partial support of 1003.1 and 1003.4 through the VAXELN POSIX runtime library)<ref>{{cite manual|url=http://www.bitsavers.org/pdf/dec/vax/realtime/EK-RPTSS-TM-004_Real-Time_Products_Technical_Summary_Dec92.pdf|title=Realtime Products Technical Summary, Fifth Edition|publisher=[[Digital Equipment Corporation]]|date=December 1992|access-date=8 December 2021}}</ref>
*[[Architecture of Windows NT |Windows NT kernel]] when using [[Windows Services for UNIX]] 3.5 or [[Subsystem for UNIX-based Applications]]. To be POSIX compliant, one must activate optional features of Windows NT and Windows 2000 Server.<ref name="MS">{{cite web|url=http://www.microsoft.com/technet/archive/ntwrkstn/reskit/poscomp.mspx|title=Chapter 29 - POSIX Compatibility|work=MS Windows NT Workstation Resource Kit|publisher=Microsoft|archive-url=https://web.archive.org/web/20080115185233/http://www.microsoft.com/technet/archive/ntwrkstn/reskit/poscomp.mspx|archive-date=15 January 2008}}</ref>

===Conformance via subsystem===
Some technologies allow an operating system to enjoy a level of conformance to POSIX even though the operating system itself has little or no conformance.

====For Windows====
Although [[Windows]] does not conform to POSIX, the following technologies provide a level of compliance.
; [[Cygwin]]: Provides a largely POSIX-compliant development and run-time environment for [[Microsoft Windows]].
; [[MinGW]]: A [[fork (software development)|fork]] of Cygwin, provides a less POSIX-compliant development environment and supports compatible [[C (programming language)|C]]-programmed applications via [[Msvcrt]], Microsoft's old Visual C [[runtime library]].
; libunistd: A largely POSIX-compliant development library originally created to build the Linux-based C/[[C++]] source code of [[CinePaint]] as is in [[Microsoft Visual Studio]]. A lightweight implementation that has POSIX-compatible header files that map POSIX APIs to call their Windows API counterparts.<ref>{{cite web|url = https://github.com/robinrowe/libunistd|title=libunistd| last=Rowe|first=Robin|date=8 September 2022|website=Github|publisher=|access-date=18 February 2023|quote=If you want to build single codebase C++ code to run on Windows, Linux and MacOS, you need this for Windows}}</ref>
; [[Microsoft POSIX subsystem]]: An optional Windows subsystem included in Windows NT-based operating systems up to Windows 2000. It supported POSIX.1 as it stood in the 1990 revision, without [[pthreads|threads]] or [[Berkeley socket|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 as a component in [[Windows Services for UNIX]] (SFU) and finally incorporated 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)<ref>{{cite web|url=https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/hh831568(v=ws.11)|title=Features Removed or Deprecated in Windows Server 2012|date=31 August 2016|website=[[Microsoft Docs]]}}</ref> and dropped in 2013 (2012 R2, 8.1). It enables full POSIX compliance for certain [[Microsoft Windows]] products.{{Citation needed|date=March 2011}}
; [[Windows Subsystem for Linux]] (WSL): A compatibility layer for running Linux binary executables natively on Windows 10 and 11 using a Linux image such as Ubuntu, Debian, or OpenSUSE among others, acting as an upgrade and replacement for Windows Services for UNIX. It was released in beta in April 2016. The first distribution available was Ubuntu.
; [[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.<ref>{{cite web|url=http://www.microsoft.com/ntserver/nts/exec/overview/sfu.asp|title=Windows NT Services for UNIX Add-On Pack|website=[[Microsoft]]|archive-url=https://web.archive.org/web/19990508011234/http://www.microsoft.com/ntserver/nts/exec/overview/sfu.asp|archive-date=8 May 1999}}</ref><ref>{{cite press release|url=http://mks.com/press/981109a.htm|title=MKS Solves Enterprise Interoperability Challenges|archive-url=https://web.archive.org/web/19990428003638/http://mks.com/press/981109a.htm|archive-date=28 April 1999}}</ref>
; [[Microsoft Windows library files#Runtime libraries|Windows C Runtime Library]] and [[Winsock|Windows Sockets API]]: Implement commonly used POSIX API functions for file, time, environment, and socket access,<ref>{{cite web |url=https://docs.microsoft.com/en-us/previous-versions/ms235384(v=vs.100) |title=Deprecated CRT Functions |date=15 October 2009 |access-date=23 July 2022 |website=[[Microsoft Docs]]}}</ref> although the support remains largely incomplete and not fully interoperable with POSIX-compliant implementations.<ref>{{cite web |url=https://docs.microsoft.com/en-us/windows/win32/winsock/porting-socket-applications-to-winsock?redirectedfrom=MSDN |title=Porting Socket Applications to Winsock |date=7 January 2021 |access-date=23 July 2022 |website=[[Microsoft Docs]]}}</ref><ref>{{cite web|url=https://tangentsoft.net/wskfaq/articles/bsd-compatibility.html|title=Winsock Programmer's FAQ Articles: BSD Sockets Compatibility|date=31 August 2015 |access-date=8 October 2015 |publisher=Warren Young}}</ref>{{discuss|BSD sockets as "POSIX"}}

====For OS/2====
POSIX environments for [[OS/2]]:
; [[EMX (programming environment) |emx+gcc]]: Largely POSIX compliant.

====For DOS====
POSIX environments for [[DOS]] include:
; [[EMX (programming environment) |emx+gcc]]: Largely POSIX compliant
; [[DJGPP]]: Partially POSIX compliant
; [[DR-DOS]]: Multitasking core via {{mono|[[EMM386]] /MULTI}} – a POSIX threads frontend API extension is available

==See also==
* {{Annotated link|C POSIX library}}
* {{Annotated link|IBM Common User Access}}
* {{Annotated link|Interix}}
* {{Annotated link|List of POSIX commands}}
* {{Annotated link|POSIX signal}}
* {{Annotated link|POSIX threads}}
* {{Annotated link|Portable character set}} {{endash}} set of characters which should be supported in any POSIX-compliant character set locale
* {{Annotated link|Real-time operating system}}
* {{Annotated link|Single UNIX specification}}
* {{Annotated link|TRON project}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|POSIX}}
* {{Cite web|url=https://pubs.opengroup.org/onlinepubs/9799919799/|publisher=The Open Group/IEEE|title=The Open Group Base Specifications Issue 8, 2024 edition IEEE Std 1003.1™-2024}}
* {{Cite web|url=https://posix.opengroup.org|publisher=The Open Group/IEEE|title=POSIX Certification home}}

{{Open Group standards}}
{{IEEE standards}}
{{ISO standards}}
{{List of International Electrotechnical Commission standards}}
{{Inter-process communication}}
{{Authority control}}

[[Category:POSIX| ]]
[[Category:Application programming interfaces]]
[[Category:Open Group standards]]
[[Category:IEC standards]]
[[Category:IEEE standards]]
[[Category:ISO standards]]