Semaphore (programming)

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.

based on FOLDOC