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xSemaphoreCreateMutex
[Semaphores]

Only available from FreeRTOS.org V4.5.0 onwards.

semphr. h

xSemaphoreHandle xSemaphoreCreateMutex( void )

Macro that creates a mutex semaphore by using the existing queue mechanism.

Mutexes created using this macro can be accessed using the xSemaphoreTake() and xSemaphoreGive() macros. The xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros should not be used.

Mutexes and binary semaphores are very similar but have some subtle differences: Mutexes include a priority inheritance mechanism, binary semaphores do not. This makes binary semaphores the better choice for implementing synchronisation (between tasks or between tasks and an interrupt), and mutexes the better choice for implementing simple mutual exclusion.

The priority of a task that 'takes' a mutex can potentially be raised if another task of higher priority attempts to obtain the same mutex. The task that owns the mutex 'inherits' the priority of the task attempting to 'take' the same mutex. This means the mutex must always be 'given' back - otherwise the higher priority task will never be able to obtain the mutex, and the lower priority task will never 'disinherit' the priority. An example of a mutex being used to implement mutual exclusion is provided on the xSemaphoreTake() documentation page.

A binary semaphore need not be given back once obtained, so task synchronisation can be implemented by one task/interrupt continuously 'giving' the semaphore while another continuously 'takes' the semaphore. This is demonstrated by the sample code on the xSemaphoreGiveFromISR() documentation page.

Both mutex and binary semaphores are assigned to variables of type xSemaphoreHandle and can be used in any API function that takes a parameter of this type.

Return:

Handle to the created semaphore. Should be of type xSemaphoreHandle.

Example usage:

 xSemaphoreHandle xSemaphore;

 void vATask( void * pvParameters )
 {
    // Mutex semaphores cannot be used before a call to 
    // xSemaphoreCreateMutex().  The created mutex is returned.
    xSemaphore = xSemaphoreCreateMutex();

    if( xSemaphore != NULL )
    {
        // The semaphore was created successfully.
        // The semaphore can now be used.  
    }
 }
 

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