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semaphore

semaphore (sĕm´əfôr´), device for the visible transmission of messages. The marine semaphore, used by day between ships or between a ship and the shore, consists essentially of a post at the top of which are two pivoted arms. The arms are connected by light gearing to two operating levers. Each letter of the alphabet and each numeral is indicated by a different placing of the arms. The system can also be used by the signalman through motions of his own arms, with or without small flags as indicators. In the railroad semaphore a single projecting arm pivoted at one end and attached to a vertical post is devised to take three positions. Horizontal indicates stop, and vertical, all clear; the inclined position indicates that the locomotive may go ahead under control expecting to be stopped. See signaling.

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semaphore

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

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

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

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semaphore

semaphoretherefore, wherefore •Roquefort • semaphore • ctenophore •pinafore

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