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POSIX or "Portable Operating System Interface for uniX" is the collective name of a family of related standards specified by the IEEE to define the application programming interface (API) for software compatible with variants of the Unix operating system. They are formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. The standards emerged from a project, begun circa 1985. The term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name; before that the standards effort was called IEEE-IX. POSIX has been backronymed to Portable Operating System Interface, with the X signifying the Unix heritage of the API.
POSIX specifies the user and software interfaces to the OS in some 15 different documents. The standard user command line and scripting interface was based off the Korn shell. Other user-level programs, services and utilities include awk, echo, ed, and hundreds of others. Required program-level services include 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.
Since the IEEE charges very high rates for POSIX documentation and does not allow on-line publication of the standards, there has been a tendency toward 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.
Although used mainly for Unix systems, the POSIX standard can apply to any operating system.
POSIX has had various "upgrades":
- POSIX.1, Core Services (incorporates Standard ANSI C)
- Process Creation and Control
- Floating Point Exceptions
- Segmentation Violations
- Illegal Instructions
- Bus Errors
- File and Directory Operations
- C Library (Standard C)
- I/O Port Interface and Control
- POSIX.1b, Real-time extensions
- Priority Scheduling
- Real-Time Signals
- Clocks and Timers
- Message Passing
- Shared Memory
- Asynch and Synch I/O
- Memory Locking
- POSIX.1c, Threads extensions
- Thread Creation, Control, and Cleanup
- Thread Scheduling
- Thread Synchronization
- Signal Handling
List of POSIX-compliant operating systems
- BSD/OS 
- Mac OS X
- Windows NT (except optional POSIX features) 
- note: in order to be POSIX compliant, you must activate optional features of Windows NT.