POSIX

Before 1997, POSIX comprised several standards:

• POSIX.1: Core Services (incorporates Standard ANSI C) (IEEE Std 1003.1-1988)
  • Process Creation and Control
  • Signals
    • Floating Point Exceptions
    • Segmentation / Memory Violations
    • Illegal Instructions
    • Bus Errors
    • Timers
  • File and Directory Operations
  • Pipes
  • C Library (Standard C)
  • I/O Port Interface and Control
  • Process Triggers
• POSIX.1b: Real-time extensions (IEEE Std 1003.1b-1993, later appearing as librt—the Realtime Extensions library)[8])
  • Priority Scheduling
  • Real-Time Signals
  • Clocks and Timers
  • Semaphores
  • Message Passing
  • Shared Memory
  • Asynchronous and Synchronous I/O
  • Memory Locking Interface
• 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

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.[9]

This standard consisted of:

• the Base Definitions, Issue 6,
• the System Interfaces and Headers, Issue 6,
• the Commands and Utilities, Issue 6.

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).[10] Its contents are available on the web.[11]

Base Specifications, Issue 7 (or IEEE Std 1003.1-2008, 2016 Edition) is similar to the current 2017 version (as of 22 July 2018).[12][13]

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 July 2018, the current standard. It is technically identical to POSIX.1-2008 with Technical Corrigenda 1 and 2 applied. A free online copy may still be available.[12]

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 POSIX_ME_HARDER was introduced to allow the user to force the standards-compliant behaviour.[14] The variable name was later changed to POSIXLY_CORRECT.[15] This variable is now also used for a number of other behaviour quirks.

Some versions of the following operating systems have been certified to conform to one or more of the various POSIX standards. This means that they passed the automated conformance tests.[17]

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

• 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)[32] and dropped in 2013 (2012 R2, 8.1). It enables full POSIX compliance for certain Microsoft Windows products.
• Windows Subsystem for Linux, also known as WSL, is a compatibility layer for running Linux binary executables natively on Windows 10 using a Linux image such as Ubuntu, Debian, or OpenSUSE among others, acting as an upgrade and replacement for Windows Services for UNIX. It was released in beta in April 2016. The first distribution available was Ubuntu.
• 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.[33]
• Windows C Runtime Library and Windows Sockets API implement commonly used POSIX API functions for file, time, environment, and socket access,[34] although the support remains largely incomplete and not fully interoperable with POSIX-compliant implementations.[35][36]

Mostly POSIX compliant environments for OS/2:

• emx+gcc – largely POSIX compliant

Partially POSIX compliant environments for DOS include:

• emx+gcc – largely POSIX compliant
• DJGPP – partially POSIX compliant
• DR-DOS multitasking core via EMM386 /MULTI – a POSIX threads frontend API extension is available

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 non-compliant.

• eCos – POSIX is part of the 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[37]
• 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.[38]
  • 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)