POSIX (// 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 compatibility with variants of Unix and other operating systems.
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.) . 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[update], 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.
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)
- 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.1-2001 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.1-2001 (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.
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.
The following operating systems conform (i.e., are 100% compliant) to one or more of the various POSIX standards.
|This section requires expansion. (January 2007)|
The following, while not officially certified as POSIX compatible, comply in large part:
- BeOS / Haiku
- Linux (most distributions — see Linux Standard Base)
- MINIX (now MINIX3)
- 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
- VxWorks ( VxWorks is often used as a shell around non-posix Kernels i.e. TiMOS/SROS )
POSIX for Windows
- Cygwin provides a largely POSIX-compliant development and run-time environment for Microsoft Windows.
- Microsoft POSIX subsystem, an optional Windows subsystem included in Windows NT-based operating systems up to Windows 2000. Partial POSIX-1 — no threads, no 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 Vista and later Windows OS releases under the name "Subsystem for UNIX-based Applications" (SUA). 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 demonstration version of this was included in the first release of Windows Services for UNIX (SFU).
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)
- 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 (Ultimate / Enterprise)
- Windows 7 (Ultimate / Enterprise)
- POSIX signal
- POSIX Threads
- POSIX sockets are basically Berkeley sockets
- TRON project – alternative OS standard to POSIX
- Interix – a full-featured POSIX and Unix environment subsystem for Microsoft's Windows NT-based operating systems
- C POSIX library
- Real-time operating system
- "POSIX 1003.1 FAQ Version 1.12". 2 February 2006. Retrieved 16 July 2006.
- "POSIX". Standards. IEEE.
- "POSIX". NIST.
- "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". The Open Group. Retrieved 27 July 2009.
- "The Austin Common Standards Revision Group". The Open Group. Retrieved 27 July 2009.
- POSIX.1-2008, The Open Group.
- Announce (Google Groups replica), GNU.
- Freedom, Innovation, and Convenience: The RMS Interview, Linuxdevcenter, 22 December 2004.
- "POSIX Certification". IEEE.
- "New UNIX®; 03 Certification for Apple Inc.". Register of Open Branded Products. The Open Group. Retrieved 7 August 2012.
- "IEEE Std. 1003.13-2003 PSE52", POSIX Certified, IEEE.
- "UnixWare ® 7.1.3 and later". The Open Group. 16-May-2003.
- Schweik. "POSIX utilities". FreeBSD.
- "OpenVOS: Stratus Virtual Operating System". Stratus Technologies. Retrieved 18 October 2010.
- "APE — ANSI/POSIX Environment". Plan 9. Bell Labs.
- "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft.
- 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