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(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 signalingsignaling,
transmission of information by visible, audible, or other detectable means. Since prehistoric times humans have sought and employed ever more effective means of communicating over distance. Signal fires on mountain tops announced awaited events.
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a fixed signaling device used on railroads equipped with semiautomatic block signaling, staff systems, and centralized switches and signals. One, two, or three arms are mounted on a semaphore tower together with signal actuators to change the position of the arms. A particular position of the arms signals the locomotive engineer that he is cleared to proceed, has orders to stop, or must change speed. At night or when the visibility is poor as a result of fog, snow, or rain, semaphores are supplemented with signal lights.

The order to stop is given when the upper arm of the semaphore is horizontal and the red stop light is on. The order to reduce speed is given when the lower arms are positioned along the axis of the tower and a yellow light is on. When the upper arm is at an angle of 135° to the axis of the tower and the light is green, the track is clear and the train is permitted to move at the normal speed.


(computer science)
A memory cell that is shared by two parallel processes which rely on each other for their continued operation, and that provides an elementary form of communication between them by indicating when significant events have taken place.


(programming, operating system)
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.


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

(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. See IPC.