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-wait
ing, 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.