semaphore

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sem·a·phore / ˈseməˌfôr/ • n. 1. a system of sending messages by holding the arms or two flags or poles in certain positions according to an alphabetic code. ∎  a signal sent by semaphore. 2. an apparatus for signaling in this way, consisting of an upright with movable parts. • v. [tr.] send (a message) by semaphore or by signals resembling semaphore: Josh stands facing the rear and semaphoring the driver's intentions. DERIVATIVES: sem·a·phor·ic / ˌseməˈfôrik/ adj. sem·a·phor·i·cal·ly / ˌseməˈfôrik(ə)lē/ adv.

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semaphore A special-purpose data type introduced by Edsger Dijkstra (1965). Apart from creation, initialization, and annihilation, there are only two operations on a semaphore: wait (P operation or down operation) and signal (V operation or up operation). The letters P and V derive from the Dutch words used in the original description.

A semaphore has an integer value that cannot become negative. The signal operation increases the value by one, and in general indicates that a resource has become free. The wait operation decreases the value by one when that can be done without the value going negative, and in general indicates that a free resource is about to start being used. This therefore provides a means of controlling access to critical resources by cooperating sequential processes.

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semaphore signalling apparatus. XIX. — F. sémaphore, irreg. f. Gr. séma sign, signal + -phoros -PHORE.
So semaphoric XIX.

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semaphoretherefore, wherefore •Roquefort • semaphore • ctenophore •pinafore

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