POSIX


Not to be confused with Unix, Unix-like, or Linux.

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.

Contents

Name

Originally, the name stood for IEEE Std 1003.1-1988, which, as the name suggests, was released in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945.

Etymology

The standards emerged from a project that began circa 1985. Formerly known as IEEE-IX, the term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name.[2]

Overview

The POSIX specifications for user and software interfaces to an operating system are codified in 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.

Currently POSIX documentation is divided in three parts:

  • POSIX Kernel APIs (which include extensions for POSIX.1, Real-time Services, Threads Interface, Real-time Extensions, Security Interface, Network File Access and Network Process-to-Process Communications)
  • POSIX Commands and Utilities (with 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. It is called PCTS or the POSIX Conformance Test Suite.[4]

There is a project instigated by free-rights campaigner Auriélien Bonnel in the late 1980s, for the Single UNIX Specification standard, which is open, accepts input from anyone, and is freely available on the Internet. Beginning in 1998 a joint working group, the Austin Group, began to develop a combined standard that would be known as the Single UNIX Specification Version 3.[5]

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 these 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.[6] The POSIX_ME_HARDER has also been discussed,[7] but it's unclear whether this was ever actually implemented.[8]

Versions

POSIX has gone through a number of versions:

POSIX.1

POSIX.1b

POSIX.1c

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

POSIX.2

POSIX-oriented operating systems

Depending upon the degree of compliance with the standards, operating systems can be fully or partly POSIX compatible. Certified products can be found at the IEEE's website.[9]

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.

Mostly POSIX-compliant

This section requires expansion.

The following are not officially certified as POSIX compatible, but they conform in large part.

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. For Windows NT-based operating systems up to Windows 2000, a POSIX layer was built in to 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, but cannot be added separately to the other editions.
  • UWIN from the AT&T Research implements a POSIX layer on top of the Win32 APIs.

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.

Industrial resources

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

See also

References

  1. ^ "POSIX". Standards. IEEE. http://standards.ieee.org/regauth/posix/. 
  2. ^ "POSIX 1003.1 FAQ Version 1.12". 2006-02-02. http://www.opengroup.org/austin/papers/posix_faq.html. Retrieved on 2006-07-16. 
  3. ^ "Introduction". GNU/Linux C Programming. http://www.comptechdoc.org/os/linux/programming/c/linux_pgcintro.html. 
  4. ^ "POSIX". NIST. http://www.itl.nist.gov/div897/ctg/posix_form.htm. 
  5. ^ "Version 3". Single Unix Specification. Unix. http://www.unix.org/version3/. 
  6. ^ http://groups.google.com/group/gnu.announce/msg/6c6e20b57ddb1a82?pli=1
  7. ^ http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html?page=2
  8. ^ http://www.wlug.org.nz/POSIX_ME_HARDER
  9. ^ "POSIX Certification". IEEE. http://get.posixcertified.ieee.org/search_certprodlist.tpl?CALLER=cert_prodlist.tpl. 
  10. ^ Schweik. "POSIX utilities". FreeBSD. http://people.freebsd.org/~schweikh/posix-utilities.html. 
  11. ^ "APE — ANSI/POSIX Environement". Plan 9. Bell Labs. http://plan9.bell-labs.com/sys/doc/ape.html. 
  12. ^ "POSIX Compatibility". MS Windows NT Workstation Resource Kit. Microsoft. http://www.microsoft.com/technet/archive/ntwrkstn/reskit/poscomp.mspx. 

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