The Portable Operating System Interface (POSIX) is a family of standards 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.
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, and thus adopted it.
Unix was selected as the basis for a standard system interface partly because it was "manufacturer-neutral". However, several major versions of Unix existed—so there was a need to develop a common denominator system. 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.). The standardized user command line and scripting interface were based on the UNIX System V 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). POSIX also defines a standard threading library API which is supported by most modern operating systems. In 2008, most parts of POSIX were 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 the VSX POSIX Conformance Test Suite.
Parts before 1997
Before 1997, POSIX comprised several standards:
- POSIX.1: Core Services (incorporates Standard ANSI C) (IEEE Std 1003.1-1988)
- POSIX.1b: Real-time extensions (IEEE Std 1003.1b-1993, later appearing as librt—the Realtime Extensions library))
- 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.
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 minor updates or errata referred to as Technical Corrigenda (TCs). Its contents are available on the web.
POSIX.1-2008 (with two TCs)
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.
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008) - IEEE Standard for Information Technology--Portable Operating System Interface (POSIX(R)) Base Specifications, Issue 7 is available from either The Open Group or IEEE and is, as of 22.7.2018, the current standard.
512- vs 1024-byte blocks
POSIX mandates 512-byte default block sizes for the df and du utilities, reflecting the typical 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. This variable is now also used for a number of other behaviour quirks, where "POSIX and common sense disagree".
POSIX-oriented operating systems
The following, while not officially certified as POSIX compatible, comply in large part:
- Android (Available through Android NDK)
- BeOS (and subsequently Haiku)
- Darwin (core of OS X/macOS and iOS)
- Linux (most distributions — see Linux Standard Base)
- MINIX (now MINIX3)S
- Nucleus RTOS
- 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
- VMware ESXi
- VxWorks ( )
POSIX for Microsoft Windows
- Cygwin provides a largely POSIX-compliant development and run-time environment for Microsoft Windows.
- MinGW, 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) and dropped in 2013 (2012 R2, 8.1). It enables full POSIX compliance for certain Microsoft Windows products.
- Windows Subsystem for Linux is a compatibility layer for running Linux binary executables natively on Windows 10 using an Ubuntu image, acting as an upgrade and replacement for Windows Services for UNIX. It was released in beta in April 2016.
- 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.
- Windows C Runtime Library and Windows Sockets API implement commonly used POSIX API functions for file, time, environment, and socket access, although the support remains largely incomplete and not fully interoperable with POSIX-compliant implementations.
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:
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)
- Plan 9 from Bell Labs APE - ANSI/POSIX Environment
- RIOT (through optional POSIX module)
- 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 Server 2008 and Ultimate and Enterprise versions of Windows Vista
- Windows Server 2008 R2 and Ultimate and Enterprise versions of Windows 7
- albeit deprecated, still available for Windows Server 2012 and Enterprise version of Windows 8
- UNIX System Services that runs on z/OS (certified as compliant)
- POSIX signal
- POSIX Threads
- POSIX sockets are basically Berkeley sockets
- TRON project – alternative OS standards to POSIX
- Common User Access – User interface standard
- Interix – a full-featured POSIX and Unix environment subsystem for Microsoft's Windows NT-based operating systems
- C POSIX library
- Real-time operating system
- Portable character set, set of 103 characters which should be supported in any POSIX-compliant character set locale
- "POSIX.1 FAQ". The Open Group. 5 October 2011.
- "POSIX 1003.1 FAQ Version 1.12". 2 February 2006. Retrieved 16 July 2006.
- "POSIX". Standards. IEEE.
- "The origin of the name POSIX". 2011. Retrieved 28 September 2013.
- PASC Status (including POSIX) (Report). IEEE Computer Society. 2003-12-04. Retrieved 2015-03-01.
- "Shell Command Language - The Open Group Base Specifications Issue 7, 2013 Edition".
- "POSIX". The Open Group.
"librt(3LIB)". docs.oracle.com. man pages section 3: Library Interfaces and Headers. Oracle Corporation. 1998-08-04. Retrieved 2016-02-18.
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.
- "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.
- "IEEE Std 1003.1" (2004 ed.). Unix.org. Retrieved 26 July 2009
- "IEEE Std 1003.1" (2004 ed.). The Open Group.
- "Base Specifications, Issue 7, 2016 Edition". The Open Group. Retrieved 18 December 2014.
- "The Austin Common Standards Revision Group". The Open Group. Retrieved 1 March 2016.
- "Announce" (Google Groups replica). GNU.
- "POSIX Certification". IEEE.
- "POSIX Certified by IEEE and The Open Group - Program Guide".
- "IBM". The Open Group. Retrieved 26 January 2014.
- "Hewlett-Packard". The Open Group. Retrieved 26 January 2014.
- "Silicon Graphics, Inc". The Open Group. Retrieved 26 January 2014.
- "Huawei Technology Co., Ltd". The Open Group. Retrieved 26 May 2017.
- "The Open Brand - Register of Certified Products". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
- "Apple Inc". Register of Open Branded Products. The Open Group. Retrieved 20 May 2015.
- "Oracle Corporation". The Open Group. Retrieved 26 January 2014.
- "UnixWare ® 7.1.3 and later". The Open Group. 16 May 2003.
- "QNX Achieves New POSIX Certification". QNX. 8 April 2008. Retrieved 16 January 2016.
- "Inspur Co., Ltd". The Open Group. Retrieved 26 May 2017.
- "POSIX Certification Register". get.posixcertified.ieee.org. Retrieved 2018-03-09.
- Schweik. "POSIX utilities". FreeBSD.
- Enterprise, I. D. G. (1994-11-07). Computerworld. IDG Enterprise.
- Solter, Nicholas A.; Jelinek, Jerry; Miner, David (2011-03-21). OpenSolaris Bible. John Wiley & Sons. ISBN 9781118080313.
- Features Removed or Deprecated in Windows Server 2012
- 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
- "MSDN Library: Deprecated CRT Functions". Microsoft. Retrieved 8 Oct 2015.
- "MSDN Library: Porting Socket Applications to Winsock". Microsoft. Retrieved 8 Oct 2015.
- "Winsock Programmer's FAQ Articles: BSD Sockets Compatibility". Warren Young. 31 Aug 2015. Retrieved 8 Oct 2015.
- "APE — ANSI/POSIX Environment". Plan 9. Bell Labs.
- "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft.
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