xSemaphoreTake
[Semaphores]

semphr. h

 xSemaphoreTake( 
                   xSemaphoreHandle xSemaphore, 
                   portTickType xBlockTime 
               )

Macro to obtain a semaphore. The semaphore must have previously been created with a call to vSemaphoreCreateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting().

This macro must not be called from an ISR. xQueueReceiveFromISR() can be used to take a semaphore from within an interrupt if required, although this would not be a normal operation. Semaphores use queues as their underlying mechanism, so functions are to some extent interoperable.

xSemaphoreTake() is part of the fully featured intertask communications API. xSemaphoreAltTake() is the alternative API equivalent. Both versions require the same parameters and return the same values.

Parameters:

xSemaphore	A handle to the semaphore being taken - obtained when the semaphore was created.
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. If INCLUDE_vTaskSuspend is set to '1' then specifying the block time as portMAX_DELAY will cause the task to block indefinitely (without a timeout).

Returns:

pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime expired without the semaphore becoming available.

Example usage:

 xSemaphoreHandle xSemaphore = NULL;

 // A task that creates a semaphore.
 void vATask( void * pvParameters )
 {
    // Create the semaphore to guard a shared resource.  As we are using
    // the semaphore for mutual exclusion we create a mutex semaphore
    // rather than a binary semaphore.
    xSemaphore = xSemaphoreCreateMutex();
 }

 // A task that uses the semaphore.
 void vAnotherTask( void * pvParameters )
 {
    // ... Do other things.

    if( xSemaphore != NULL )
    {
        // See if we can obtain the semaphore.  If the semaphore is not available
        // wait 10 ticks to see if it becomes free.	
        if( xSemaphoreTake( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
        {
            // We were able to obtain the semaphore and can now access the
            // shared resource.

            // ...

            // We have finished accessing the shared resource.  Release the 
            // semaphore.
            xSemaphoreGive( xSemaphore );
        }
        else
        {
            // We could not obtain the semaphore and can therefore not access
            // the shared resource safely.
        }
    }
 }
 

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