In computer science, particularly in operating systems, a semaphore is a variable or abstract data type that is used for controlling access, by multiple processes, to a common resource in a parallel programming or a multi userenvironment.
Semantics and implementation
Counting semaphores are equipped with two operations, historically denoted as V (also known as signal()) and P (or wait())(see below). Operation V increments the semaphore S, and operation P decrements it. The semantics of these operations are shown below. Square brackets are used to indicate atomic operations, i.e., operations which appear indivisible from the perspective of other processes.
The value of the semaphore S is the number of units of the resource that are currently available. The P operation wastes time or sleeps until a resource protected by the semaphore becomes available, at which time the resource is immediately claimed. The V operation is the inverse: it makes a resource available again after the process has finished using it. One important property of semaphore S is that its value cannot be changed except by using the V signal() and P wait() operations.
A simple way to understand wait() and signal() operations is:
wait(): Decrements the value of semaphore variable by 1. If the value becomes negative, the process executing wait() is blocked, i.e., added to the semaphore’s queue.signal(): Increments the value of semaphore variable by 1. After the increment, if the pre-increment value was negative (meaning there are processes waiting for a resource), it transfers a blocked process from the semaphore’s waiting queue to the ready queue.
Many operating systems provide efficient semaphore primitives that unblock a waiting process when the semaphore is incremented. This means that processes do not waste time checking the semaphore value unnecessarily.
