Word:

Semaphore

Sem´a`phore
n.1.A signal telegraph; an apparatus for giving signals by the disposition of lanterns, flags, oscillating arms, etc.
Noun1.semaphore - an apparatus for visual signaling with lights or mechanically moving arms
Verb1.semaphore - send signals by or as if by semaphore
2.semaphore - convey by semaphore, of information
(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.
Roman candle, aid to navigation, alarm, amber light, balefire, beacon, beacon fire, bell, bell buoy, blinker, blue peter, buoy, caution light, flare, fog bell, fog signal, fog whistle, foghorn, glance, go light, gong buoy, green light, heliograph, high sign, international alphabet flag, international numeral pennant, kick, leer, marker beacon, nod, nudge, parachute flare, pilot flag, poke, police whistle, quarantine flag, radio beacon, red flag, red light, rocket, sailing aid, semaphore flag, semaphore telegraph, sign, signal, signal beacon, signal bell, signal fire, signal flag, signal gong, signal gun, signal lamp, signal light, signal mast, signal post, signal rocket, signal shot, signal siren, signal tower, spar buoy, stop light, the nod, the wink, touch, traffic light, traffic signal, watch fire, white flag, wigwag, wigwag flag, wink, yellow flag
Translate Semaphore to German, Translate Semaphore to French
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