|
Semaphore - Dictionary Definition and Overview |
|
Semaphore : (noun)
1: an apparatus for visual signaling with lights or
mechanically moving arms
(verb)
1: send signals by or as if by semaphore
2: convey by semaphore, of information
Based on WordNet 2.0
|
|
Semaphore : \Sem"a*phore\, n. [Gr. sh^ma a sign _ fe`rein to bear:
cf. F. s['e]maphore.]
A signal telegraph; an apparatus for giving signals by the
disposition of lanterns, flags, oscillating arms, etc.
Based on Webster's Revised Unabridged Dictionary
|
|
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 (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)
Based on Webster's Revised Unabridged Dictionary (1913)
|
|
|
|
|
