semaphore (redirected from Semaphore (communication))

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

Method of visual signaling, usually with flags or lights. Before radio, a semaphore system was widely used to send messages between ships. A person would stand with arms extended, moving two flags to specific angles to indicate letters or numbers. Before the invention of the telegraph, semaphore signaling with lights on high towers was used to transmit messages between distant points; messages were read by telescope. Modern semaphores have included movable arms or rows of light simulating arms, displayed from towers and used to signal railroad trains.

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semaphore

(1) A hardware or software flag used to indicate the status of some activity.

(2) A shared space for interprocess communications (IPC) controlled by "wake up" and "sleep" commands. The source process fills a queue and goes to sleep until the destination process uses the data and tells the source process to wake up.

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(programming, operating system)

semaphore - 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 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.