xSemaphoreTake

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().

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. A block time of portMAX_DELAY can be used to block indefinitely (provided INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).

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.
    vSemaphoreCreateBinary( xSemaphore );
 }

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.
        }
    }
 }