Semaphores

A semaphore is a single variable that can be incremented or decremented between zero and some specified maximum value. The value of the semaphore can communicate state information. A mail box flag is an example of a semaphore. You raise the flag to indicate that a letter is waiting in the mailbox. When the mailman picks up the letter, he lowers the flag again. You can use semaphores to keep track of how many occurrences of a particular thing are available for use.

Binary semaphores, which have a maximum value of one, are especially efficient mechanisms for indicating to some other task that something is ready. When a task or application has finished preparing data at some previously agreed to location, it raises the value of a binary semaphore that the target task waits on. The target task lowers the value of the semaphore, performs any necessary processing, and raises the value of a different binary semaphore to indicate that it is through with the data.

Semaphores are quicker and less memory intensive than other notification mechanisms, but due to their size they can convey only limited information.