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.
