Microsoft POSIX subsystem is one of 3 subsystems of several operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s from the Windows NT

Windows NT

Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

 family (together with OS/2

OS/2

OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

 and Windows subsystems).

Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 implements only the first version of the POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

 standards

Standardization

Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....

, namely POSIX.1. The official code of POSIX.1 is ISO/IEC 9945-1:1990 or IEEE standard 1003-1:1990. The subsystem was included because of 1980s US federal government

Federal government of the United States

The federal government of the United States is the national government of the constitutional republic of fifty states that is the United States of America. The federal government comprises three distinct branches of government: a legislative, an executive and a judiciary. These branches and...

's requirements listed in Federal Information Processing Standard

Federal Information Processing Standard

A Federal Information Processing Standard is a publicly announced standardization developed by the United States federal government for use in computer systems by all non-military government agencies and by government contractors, when properly invoked and tailored on a contract...

 (FIPS) 151-2. Versions Windows NT 3.5

Windows NT 3.5

Windows NT 3.5 is the second release of the Microsoft Windows NT operating system. It was released on 21 September 1994.One of the primary goals during Windows NT 3.5's development was to increase the speed of the operating system; as a result, the project was given the codename "Daytona" in...

, Windows NT 3.51

Windows NT 3.51

Windows NT 3.51 is the third release of Microsoft's Windows NT line of operating systems. It was released on 30 May 1995, nine months after Windows NT 3.5. The release provided two notable feature improvements; firstly NT 3.51 was the first of a short-lived outing of Microsoft Windows on the...

 and Windows NT 4 were certified as compliant with the FIPS 151-2.

The runtime environment of the subsystem is provided by two files: psxss.exe and psxdll.dll. A POSIX application uses psxdll.dll to communicate with the subsystem while communicating with posix.exe to provide display capabilities on the Windows desktop.

Because only the first version of POSIX (POSIX.1) is implemented, a POSIX application cannot create a thread

Thread (computer science)

In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

 or window

Window (computing)

In computing, a window is a visual area containing some kind of user interface. It usually has a rectangular shape that can overlap with the area of other windows...

, nor can it use RPC

Remote procedure call

In computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...

 or socket

Internet socket

In computer networking, an Internet socket or network socket is an endpoint of a bidirectional inter-process communication flow across an Internet Protocol-based computer network, such as the Internet....

. Instead of implementing the later versions of POSIX, Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 offers Windows Services for UNIX.

Starting with Windows XP

Windows XP

Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

, the POSIX subsystem is not included as part of standard Windows distributions and has been replaced by Interix

Interix

Interix is the name of an optional, full-featured POSIX and Unix environment subsystem for Microsoft's Windows NT-based operating systems.Interix is a component of the Services for Unix release 3.0 and 3.5...

.