Revision as of 12:15, 12 July 2016

In this tutorial, we will be discussing a Semaphore and its types ie. Binary, Mutex, Counting.
Later we will see each semaphore in detail with its pros and cons. Also, we will be looking into priority inversion and priority inheritance.

What is a Semaphore?

Semaphore is a technique for synchronizing two/more task competing for the same resources. When a task wants to use a resource, it requests for the semaphore and will be allocated if the semaphore is available. If the semaphore is not available then the requesting task will go to blocked state till the semaphore becomes free.

Consider a situation where there are two persons who want to share a bike. At one time only one person can use the bike. The one who has the bike key will get the chance to use it. And when this person gives the key to the 2nd person, then the 2nd person can use the bike.

Semaphore is just like this Key and the bike is the shared resource. Whenever a task wants access to the shared resource, it must acquire the semaphore first. The task should release the semaphore after it is done with the shared resource. Until this time all other tasks have to wait if they need access to shared resource as semaphore is not available. Even if the task trying to acquire the semaphore is of higher priority than the task acquiring the semaphore, it will be in the wait state until the semaphore is released by the lower priority task.

Types of Semaphores

There are 3-types of semaphores namely Binary, Counting and Mutex semaphore.

• Binary Semaphore:

• Counnting Semaphore:

• Mutex Semaphore: