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Lexikon

semaphore

The classic method for restricting access to shared resources (e.g. storage) in a ref="module.php?name=Lexikon&file=search&eid=1&query=multi-processing">multi-processing environment. They were invented by ref="module.php?name=Lexikon&file=search&eid=1&query=Dijkstra">Dijkstra and first used in ref="module.php?name=Lexikon&file=search&eid=1&query=T.H.E">T.H.E ref="module.php?name=Lexikon&file=search&eid=1&query=operating system">operating system. A semaphore is a ref="module.php?name=Lexikon&file=search&eid=1&query=protected variable">protected variable (or {abstract data type}) which can only be accessed using the following operations: P(s) Semaphore s { while (s == 0) /* wait until s>0 */ s = s-1 } V(s) Semaphore s { s = s+1 } Init(s, v) Semaphore s Int v { s = v } P and V stand for Dutch "Proberen", to test, and "Verhogen", to increment. The value of a semaphore is the number of units of the resource which are free (if there is only one resource a "binary semaphore" with values 0 or 1 is used). The P operation ref="module.php?name=Lexikon&file=search&eid=1&query=busy-wait">busy-waits (or maybe ref="module.php?name=Lexikon&file=search&eid=1&query=sleep">sleeps) until a resource is available whereupon it immediately claims one. V is the inverse, it simply makes a resource available again after the process has finished using it. Init is only used to initialise the semaphore before any requests are made. The P and V operations must be ref="module.php?name=Lexikon&file=search&eid=1&query=indivisible">indivisible, i.e. no other process can access the semaphore during the their execution. To avoid ref="module.php?name=Lexikon&file=search&eid=1&query=busy-wait">busy-waiting, a semaphore may have an associated ref="module.php?name=Lexikon&file=search&eid=1&query=queue">queue of processes (usually a ref="module.php?name=Lexikon&file=search&eid=1&query=FIFO">FIFO). If a process does a P on a semaphore which is zero the process is added to the semaphore' s queue. When another process increments the semaphore by doing a V and there are tasks on the queue, one is taken off and resumed. (1995-02-01)

In addition suitable contents:
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ref="module.php?name=Lexikon&op=content&tid=4514">FIFO ] [ ref="module.php?name=Lexikon&op=content&tid=4520">file ] [ ref="module.php?name=Lexikon&op=content&tid=4700">fo ] [ ref="module.php?name=Lexikon&op=content&tid=4727">for ] [ ref="module.php?name=Lexikon&op=content&tid=4828">fr ] [ ref="module.php?name=Lexikon&op=content&tid=4859">free ] [ ref="module.php?name=Lexikon&op=content&tid=4989">ga ] [ ref="module.php?name=Lexikon&op=content&tid=5057">ge ] [ ref="module.php?name=Lexikon&op=content&tid=5070">gen ] [ ref="module.php?name=Lexikon&op=content&tid=5291">gr ] [ ref="module.php?name=Lexikon&op=content&tid=5434">h ] [ ref="module.php?name=Lexikon&op=content&tid=5686">hog ] [ ref="module.php?name=Lexikon&op=content&tid=5768">hr ] [ ref="module.php?name=Lexikon&op=content&tid=5931">id ] [ ref="module.php?name=Lexikon&op=content&tid=6013">il ] [ ref="module.php?name=Lexikon&op=content&tid=6064">in ] [ ref="module.php?name=Lexikon&op=content&tid=6068">inc ] [ ref="module.php?name=Lexikon&op=content&tid=6145">initialise ] [ ref="module.php?name=Lexikon&op=content&tid=6407">inverse ] [ ref="module.php?name=Lexikon&op=content&tid=6413">io ] [ ref="module.php?name=Lexikon&op=content&tid=6449">ir ] [ ref="module.php?name=Lexikon&op=content&tid=6468">iron ] [ ref="module.php?name=Lexikon&op=content&tid=6482">is ] [ ref="module.php?name=Lexikon&op=content&tid=6558">it ] [ ref="module.php?name=Lexikon&op=content&tid=6789">ke ] [ ref="module.php?name=Lexikon&op=content&tid=6792">ken ] [ ref="module.php?name=Lexikon&op=content&tid=6918">la ] [ ref="module.php?name=Lexikon&op=content&tid=7091">Lex ] [ ref="module.php?name=Lexikon&op=content&tid=7107">li ] [ ref="module.php?name=Lexikon&op=content&tid=7410">lt ] [ ref="module.php?name=Lexikon&op=content&tid=7415">lu ] [ ref="module.php?name=Lexikon&op=content&tid=7441">ly ] [ ref="module.php?name=Lexikon&op=content&tid=7463">ma ] [ ref="module.php?name=Lexikon&op=content&tid=7607">map ] [ ref="module.php?name=Lexikon&op=content&tid=7817">method ] [ ref="module.php?name=Lexikon&op=content&tid=8019">mm ] [ ref="module.php?name=Lexikon&op=content&tid=8032">mo ] [ ref="module.php?name=Lexikon&op=content&tid=8040">mod ] [ ref="module.php?name=Lexikon&op=content&tid=8079">module ] [ ref="module.php?name=Lexikon&op=content&tid=8167">mp ] [ ref="module.php?name=Lexikon&op=content&tid=8228">ms ] [ ref="module.php?name=Lexikon&op=content&tid=8258">mu ] [ ref="module.php?name=Lexikon&op=content&tid=8386">na ] [ ref="module.php?name=Lexikon&op=content&tid=8460">nc ] [ ref="module.php?name=Lexikon&op=content&tid=8472">ne ] [ ref="module.php?name=Lexikon&op=content&tid=8627">ng ] [ ref="module.php?name=Lexikon&op=content&tid=8630">ni ] [ ref="module.php?name=Lexikon&op=content&tid=8660">nl ] [ ref="module.php?name=Lexikon&op=content&tid=8675">no ] [ ref="module.php?name=Lexikon&op=content&tid=8760">ns ] [ ref="module.php?name=Lexikon&op=content&tid=8787">nu ] [ ref="module.php?name=Lexikon&op=content&tid=8820">O ] [ ref="module.php?name=Lexikon&op=content&tid=9014">op ] [ ref="module.php?name=Lexikon&op=content&tid=9061">operating system ] [ ref="module.php?name=Lexikon&op=content&tid=9457">pe ] [ ref="module.php?name=Lexikon&op=content&tid=9550">ph ] [ ref="module.php?name=Lexikon&op=content&tid=9651">pl ] [ ref="module.php?name=Lexikon&op=content&tid=9738">ply ] [ ref="module.php?name=Lexikon&op=content&tid=9908">pr ] [ ref="module.php?name=Lexikon&op=content&tid=9989">Probe ] [ ref="module.php?name=Lexikon&op=content&tid=9995">process ] [ ref="module.php?name=Lexikon&op=content&tid=10000">processing ] [ ref="module.php?name=Lexikon&op=content&tid=10023">program ] [ ref="module.php?name=Lexikon&op=content&tid=10042">programming ] [ ref="module.php?name=Lexikon&op=content&tid=10253">query ] [ ref="module.php?name=Lexikon&op=content&tid=10256">ques ] [ ref="module.php?name=Lexikon&op=content&tid=10259">queue ] [ ref="module.php?name=Lexikon&op=content&tid=10364">rc ] [ ref="module.php?name=Lexikon&op=content&tid=10385">re ] [ ref="module.php?name=Lexikon&op=content&tid=10767">ro ] [ ref="module.php?name=Lexikon&op=content&tid=10918">S ] [ ref="module.php?name=Lexikon&op=content&tid=11150">se ] [ ref="module.php?name=Lexikon&op=content&tid=11314">sh ] [ ref="module.php?name=Lexikon&op=content&tid=11324">shar ] [ ref="module.php?name=Lexikon&op=content&tid=11376">si ] [ ref="module.php?name=Lexikon&op=content&tid=11510">sk ] [ ref="module.php?name=Lexikon&op=content&tid=11525">sl ] [ ref="module.php?name=Lexikon&op=content&tid=11537">sleep ] [ ref="module.php?name=Lexikon&op=content&tid=11651">so ] [ ref="module.php?name=Lexikon&op=content&tid=11745">source ] [ ref="module.php?name=Lexikon&op=content&tid=11934">st ] [ ref="module.php?name=Lexikon&op=content&tid=12065">storage ] [ ref="module.php?name=Lexikon&op=content&tid=12090">strict ] [ ref="module.php?name=Lexikon&op=content&tid=12133">su ] [ ref="module.php?name=Lexikon&op=content&tid=12165">sum ] [ ref="module.php?name=Lexikon&op=content&tid=12246">sy ] [ ref="module.php?name=Lexikon&op=content&tid=12312">system ] [ ref="module.php?name=Lexikon&op=content&tid=12359">T ] [ ref="module.php?name=Lexikon&op=content&tid=12360"> ] [ ref="module.php?name=Lexikon&op=content&tid=12440">tc ] [ ref="module.php?name=Lexikon&op=content&tid=12557">test ] [ ref="module.php?name=Lexikon&op=content&tid=12588">th ] [ ref="module.php?name=Lexikon&op=content&tid=12591">T.H.E ] [ ref="module.php?name=Lexikon&op=content&tid=12721">to ] [ ref="module.php?name=Lexikon&op=content&tid=12787">tr ] [ ref="module.php?name=Lexikon&op=content&tid=12970">type ] [ ref="module.php?name=Lexikon&op=content&tid=12986">ua ] [ ref="module.php?name=Lexikon&op=content&tid=13030">um ] [ ref="module.php?name=Lexikon&op=content&tid=13146">up ] [ ref="module.php?name=Lexikon&op=content&tid=13175">us ] [ ref="module.php?name=Lexikon&op=content&tid=13229">V ] [ ref="module.php?name=Lexikon&op=content&tid=13252">va ] [ ref="module.php?name=Lexikon&op=content&tid=13260">value ] [ ref="module.php?name=Lexikon&op=content&tid=13274">var ] [ ref="module.php?name=Lexikon&op=content&tid=13275">variable ] [ ref="module.php?name=Lexikon&op=content&tid=13310">ve ] [ ref="module.php?name=Lexikon&op=content&tid=13366">vi ] [ ref="module.php?name=Lexikon&op=content&tid=13694">while ] [ ref="module.php?name=Lexikon&op=content&tid=13725">win ] [ ref="module.php?name=Lexikon&op=content&tid=14102">zero ]

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