Semaphore (programming)

In computer science, a semaphore is a synchronization device used to control access to a common resource by multiple threads or processes in a concurrent system, such as a multitasking operating system. It allows simultaneous access to a critical section by some predetermined number of processes.

A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to adjust that record safely (i.e., to avoid race conditions) as units are acquired or become free, and, if necessary, wait until a unit of the resource becomes available. Though semaphores are useful for preventing race conditions, they do not guarantee their absence.

Semaphores that allow an arbitrary resource count are called counting semaphores, while semaphores that are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores and are used to implement locks.

The semaphore concept was invented by Dutchcomputer scientistEdsger Dijkstra in 1962 or 1963, when Dijkstra and his team were developing an operating system for the Electrologica X8. That system eventually became known as the THE multiprogramming system.