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|Defination ||<syntaxhighlight  lang = "c"> vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore )</syntaxhighlight>
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|Defination ||<syntaxhighlight  lang = "c"> vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore)</syntaxhighlight>
 
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| Input Arguments ||<syntaxhighlight  lang = "c">xSemaphore : It is a handle to the created semaphore.  
 
| Input Arguments ||<syntaxhighlight  lang = "c">xSemaphore : It is a handle to the created semaphore.  
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|Defination ||<syntaxhighlight  lang = "c"> xSemaphoreTake( xSemaphoreHandle xSemaphore,
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|Defination ||<syntaxhighlight  lang = "c"> xSemaphoreTake( xSemaphoreHandle xSemaphore, portTickType xBlockTime)</syntaxhighlight>
portTickType xBlockTime)</syntaxhighlight>
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| Input Arguments ||<syntaxhighlight  lang = "c">xSemaphore : It is a handle to the semaphore being obtained.  
 
| Input Arguments ||<syntaxhighlight  lang = "c">xSemaphore : It is a handle to the semaphore being obtained.  
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| Return Value|| none
 
| Return Value|| none
 
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| Description || <br />
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| Description ||  
 
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| Usage ||<syntaxhighlight  lang = "c"></syntaxhighlight>
 
| Usage ||<syntaxhighlight  lang = "c"></syntaxhighlight>

Revision as of 13:07, 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

{{#Widget:LibTable}}
Defination
 vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore)
Input Arguments
xSemaphore : It is a handle to the created semaphore. 
It should be of type xSemaphoreHandle
Return Value none
Description
Usage
 


{{#Widget:LibTable}}
Defination
 xSemaphoreTake( xSemaphoreHandle xSemaphore, portTickType xBlockTime)
Input Arguments
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.
Return Value none
Description
Usage