Editing Monitor (synchronization) (section)

{{Short description|Object or module in concurrent programming}}
{{howto|date=January 2014}}

In [[concurrent computing|concurrent programming]], a '''monitor''' is a synchronization construct that prevents threads from concurrently accessing a shared object's state and allows them to wait for the state to change.  They provide a mechanism for threads to temporarily give up exclusive access in order to wait for some condition to be met, before regaining exclusive access and resuming their task.  A monitor consists of a [[lock (computer science)|mutex (lock)]] and at least one '''condition variable'''.  A '''condition variable''' is explicitly 'signalled' when the object's state is modified, temporarily passing the mutex to another thread 'waiting' on the '''condition variable'''.

Another definition of '''monitor''' is a '''thread-safe''' [[class (computer science)|class]], [[object (computer science)|object]], or [[module (programming)|module]] that wraps around a [[mutual exclusion|mutex]] in order to safely allow access to a method or variable by more than one [[thread (computer science)|thread]].  The defining characteristic of a monitor is that its methods are executed with [[mutual exclusion]]: At each point in time, at most one thread may be executing any of its [[method (computer science)|methods]]. By using one or more condition variables it can also provide the ability for threads to wait on a certain condition (thus using the above definition of a "monitor").  For the rest of this article, this sense of "monitor" will be referred to as a "thread-safe object/class/module".

Monitors were invented by [[Per Brinch Hansen]]<ref name=":0">{{Cite book|title = Operating System Principles|last = Brinch Hansen|first = Per|publisher = Prentice Hall|year = 1973|isbn = 978-0-13-637843-3|chapter = 7.2 Class Concept|chapter-url = http://brinch-hansen.net/papers/1973b.pdf|url-access = registration|url = https://archive.org/details/operatingsystemp0000brin}}</ref> and [[C. A. R. Hoare]],<ref name=":1">{{cite journal
|last1=Hoare |first1=C. A. R. |author1-link=Tony Hoare
|title=Monitors: an operating system structuring concept
|journal=[[Communications of the ACM|Comm. ACM]]
|volume=17
|issue=10
|date=October 1974
|pages=549–557 
|doi=10.1145/355620.361161
|citeseerx=10.1.1.24.6394 |s2cid=1005769 }}</ref> and were first implemented in [[Per Brinch Hansen|Brinch Hansen's]] [[Concurrent Pascal]] language.<ref>{{cite journal
|last1=Hansen |first1=P. B. |author1-link=Per Brinch Hansen
|title=The programming language Concurrent Pascal
|journal=[[IEEE Transactions on Software Engineering|IEEE Trans. Softw. Eng.]]
|volume=SE-1
|issue=2
|date=June 1975
|pages=199–207
|doi=10.1109/TSE.1975.6312840
|s2cid=2000388 |url=https://authors.library.caltech.edu/34677/1/06312840.pdf }}</ref>