Semaphore

noun
1.
an apparatus for conveying information by means of visual signals, as a light whose position may be changed.
2.
any of various devices for signaling by changing the position of a light, flag, etc.
3.
a system of signaling, especially a system by which a special flag is held in each hand and various positions of the arms indicate specific letters, numbers, etc.
verb (used with or without object), semaphored, semaphoring.
4.
to signal by semaphore or by some system of flags.
noun
1.
an apparatus for conveying information by means of visual signals, as with movable arms or railway signals, flags, etc
2.
a system of signalling by holding a flag in each hand and moving the arms to designated positions to denote each letter of the alphabet
verb
3.
to signal (information) by means of semaphore
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 (s == 0) ; /* wait until s>0 */ s = s-1;
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.
(1995-02-01)