Home : Software Terms : Semaphore Definition

Semaphore

In computer science, a semaphore is a variable that controls access to one or more resources. It is a tool developers use to ensure functions only access valid data and run at the right time. A semaphore can prevent a deadlock or race condition by disallowing access to a resource when it is not available.

There are two main types of semaphores in computer programming: binary and counting.

Binary Semaphores

A binary semaphore is a boolean variable that can have only two possible values (0 or 1). It is often used as a lock to restrict access to a function or resource. For example, semaphore A controls function getData(). If A = 0, getData() will not excute. If A = 1, getData() will run. A binary semaphore is also considered a flag or a switch that is either on or off.

Counting Semaphores

A counting semaphore can be any non-negative integer, also known as a whole number. Its value may increase or decrease based on the results of one or more functions. For example, a counting semaphore may keep track of the number of available resources in a resource pool. If the semaphore decrements more than it increments, it will eventually reach 0. A zero value indicates there are no more resources left.

NOTE: A "semaphore" in the real world is a flag or other object used to signal from a distance. For example, a person may hold up a pole from a dock to signal to a ferry captain that he or she would like to come aboard.

Updated: February 1, 2020

Cite this definition:

https://techterms.com/definition/semaphore

TechTerms - The Tech Terms Computer Dictionary

This page contains a technical definition of Semaphore. It explains in computing terminology what Semaphore means and is one of many software terms in the TechTerms dictionary.

All definitions on the TechTerms website are written to be technically accurate but also easy to understand. If you find this Semaphore definition to be helpful, you can reference it using the citation links above. If you think a term should be updated or added to the TechTerms dictionary, please email TechTerms!