Difference between revisions of "RTOS Basics : Semaphore"

Revision as of 13:01, 14 April 2015

Amruta (talk) 15:30, 8 April 2015 (IST)

What is a Semaphore ?

Consider a situation where there are two person who wants 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 2nd person, then only 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. Till 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 wait state until semaphore is released by the lower priority task.

Use of Semaphore

1. Managing Shared Resource

1. Task Synchronization

Apart from managing shared resource, task synchronization can also be performed with the help of a semaphore. In this case semaphore will be like a flag not key.

1. Unilateral Rendezvous

This is one way synchronization which uses a semaphore as a flag to signal another task.

1. Bilateral Rendezvous

This is two way synchronization performed using two semaphores. A bilateral rendezvous is similar to a unilateral rendezvous, except both tasks must synchronize with one another before proceeding.

Types of semaphore

1. Binary Semaphore

Binary semaphore is used when there is only one shared resource.

1. Counting Semaphore

To handle more then one shared resource of same type, counting semaphore is used.

1. Mutual Exclusion Semaphore or Mutex

To avoid extended priority inversion, mutexes can be used. You can check Mutex Working here.

Operations on Semaphore

Basically, there are 3 operations related to the semaphore:

1. Create
2. Acquire
3. Release

API Details

 vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore )

xSemaphore : It is a handle to the created semaphore. 
It should be of type xSemaphoreHandle

 xSemaphoreTake( xSemaphoreHandle xSemaphore,
						portTickType xBlockTime)

xSemaphore : It is a handle to the semaphore being obtained. 
xBlockTime : The time in ticks to wait for the semaphore to become available. The
macro portTICK_RATE_MS can be used to convert this to a real
time. A block time of zero can be used to poll the semaphore.