View source for Semaphore (programming)

In [[computer science]], particularly in [[Operating system|operating systems]], a '''semaphore''' is a [[Variable (computer science)|variable]] or [[abstract data type]] that is used for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment.

== Description ==

A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record as units are required or become free, and, if necessary, wait until a unit of the resource becomes available. 

Semaphores are a useful tool in the prevention of [[Race condition|race conditions]]; however, their use is by no means a guarantee that a program is free from these problems.

Semaphores which allow an arbitrary resource count are called [[counting semaphores]], while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called [[binary semaphores]].

== Invention ==

The semaphore concept was invented by Dutch computer scientist [[Edsger Dijkstra]] in 1962 or 1963.

== See also ==

* [[Abstract data type]]
* [[Computer science]]
* [[Operating system]]
* [[Race condition]]
* [[Variable (computer science)]]

== External links ==

* [https://en.wikipedia.org/wiki/Semaphore_(programming) Semaphore (programming)] @ Wikipedia


[[Category:Computer programming]]
[[Category:Computer science]]