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
        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.