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semaphore [n]

1)	an apparatus for visual signaling with lights or mechanically moving arms

Synonyms :

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apparatus

semaphore [v]

1)	send signals by or as if by semaphore

Synonyms :

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sign

2)	convey by semaphore, of information

Synonyms :

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communicate

From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)

The classic method for restricting access to shared resources (e.g. storage) in a multi-processing environment. They were invented by Dijkstra and first used in T.H.E operating system.

A semaphore is a 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 busy-waits (or maybe 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 indivisible, i.e. no other process can access the semaphore during the their execution.

To avoid busy-waiting, a semaphore may have an associated queue of processes (usually a 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)

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