POSIX (pronounced /ˈpɒzɪks/) 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.

[edit] Name

Originally, the name "POSIX" stood for IEEE Std 1002.1-1989, released, as the name suggests, in 1989. The family of POSIX standards is formally designated as IEEE 1002 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]

[edit] Overview

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^[update] 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.

[edit] Versions

[edit] Parts before 1997

Before 1997, POSIX comprised several standards:

[edit] POSIX.1

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

[edit] POSIX.1b

• 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

[edit] POSIX.1c

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

[edit] POSIX.2

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

[edit] 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.

[edit] POSIX:2001

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.

[edit] POSIX:2004

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].

[edit] POSIX:2008

As of 2009^[update] POSIX:2008 or IEEE Std 1003.1-2008 represents the current version.^[7][8] The web has a copy of the specification at http://www.opengroup.org/onlinepubs/9699919799/.

This standard consists of:

• the Base Definitions, Issue 7,
• the System Interfaces and Headers, Issue 7,
• the Commands and Utilities, Issue 7.

[edit] Controversies

[edit] 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 1k) blocks. The environmental variable POSIXLY_CORRECT was introduced to force the standards-compliant behaviour.^[9] The variable POSIX_ME_HARDER was also discussed^[10] and was implemented in a few packages^[11] before being obsoleted by POSIXLY_CORRECT.

[edit] 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.^[12]

[edit] Fully POSIX-compliant

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (June 2009)

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

• A/UX
• AIX
• BSD/OS
• Mac OS X^[13]
• HP-UX
• INTEGRITY
• IRIX
• LynxOS
• MINIX
• MPE/iX
• QNX (IEEE Std. 1003.13-2003 PSE52; see http://get.posixcertified.ieee.org/cert_prodlist.tpl)
• RTEMS (POSIX 1003.1-2003 Profile 52)
• Solaris
• UnixWare
• velOSity
• VxWorks (IEEE Std. 1003.13-2003 PSE52; see http://get.posixcertified.ieee.org/cert_prodlist.tpl)

[edit] Mostly POSIX-compliant

This section requires expansion.

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

• Nucleus RTOS
• RTEMS - POSIX API support designed to IEEE Std. 1003.13-2003 PSE52
• FreeBSD^[14]
• GNU/Linux (most distributions — see LSB)
• NetBSD
• BeOS / Haiku
• OpenBSD
• OpenSolaris
• Sanos
• SkyOS
• Syllable
• VSTa

[edit] 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.

[edit] POSIX for OS/2

Mostly POSIX compliant environments for OS/2:

• emx+gcc - largely POSIX compliant

[edit] POSIX for DOS

Partially POSIX compliant environments for DOS include:

• emx+gcc - largely POSIX compliant
• DJGPP - partially POSIX compliant

[edit] Compliant via compatibility feature

The following lack official certification 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.
• Plan 9 from Bell Labs APE - ANSI/POSIX Environment^[15]
• Symbian OS with PIPS (PIPS Is POSIX on Symbian)
• OpenVMS (through optional POSIX package)
• 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.^[16]
  • Windows XP Professional with Service Pack 1 or later
  • Windows Server 2003
  • Windows Vista
• z/OS

[edit] Industrial resources

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

[edit] See also

• POSIX signal
• POSIX Threads
• 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
• BIOS
• Real-time operating system

[edit] References

[edit] External links

• Austin Group
• The Portable Application Standards Committee
• IEEE POSIX Certification Authority
• The Open Group – The UNIX System Home Page
• POSIX:2008, The Open Group Base Specifications Issue 7
• What could have been IEEE 1003.1e/2c