[sem-uh-fawr, -fohr] 
noun, verb, -phored, -phor⋅ing.| 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, esp. a system by which a special flag is held in each hand and various positions of the arms indicate specific letters, numbers, etc. |
| 4. | to signal by semaphore or by some system of flags. |
[sem-uh-fawr-ik, -for-] , sem⋅a⋅phor⋅i⋅cal, adjective | sem·a·phore (sěm'ə-fôr', -fōr') n.
To send (a message) or to signal by semaphore. [Greek sēma, sign + -phore.] sem'a·phor'ic adj., sem'a·phor'i·cal·ly adv. |
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)
əˌfɔr
