A semaphore is a protected variable (or abstract data type) and constitutes the classic method for restricting access to shared resources (e.g. storage) in a multi-processing environment. They were invented by Edsger Dijkstra and first used in the T.H.E operating system.
Semaphores can only be accessed using the following operations:
P(Semaphore s)
{
while (s == 0) ; /* wait until s>0 */
s = s-1;
}
V(Semaphore s)
{
s = s+1;
}
Init(Semaphore s, Integer 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, which means that each of the operations may not be executed multiple times concurrently. A process wishing to execute an operation that is already being executed by another process must wait for it to complete first.The V operation is sometimes known as "up", and the P operation as "down".
To avoid busy-waiting, a semaphore may have an associated queue of processes (usually a FIFO). If a process performs a P operation on a semaphore which has the value zero, the process is added to the semaphore's queue. When another process increments the semaphore by performing a V operation, and there are processes on the queue, one of them is removed from the queue and resumes execution.
