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.
Last updated: 1995-02-01
Nearby terms:
semantic network ♦ semantics ♦ semaphore ♦ semi ♦ Semi-Automatic Ground Environment
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