Chapter 2. API Reference
(continued from previous page)
}
}
}
Return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime expired without the semaphore
becoming available.
Parameters
• xMutex: A handle to the mutex being obtained. This is the handle returned by xSemaphoreCre-
ateRecursiveMutex();
• xBlockTime: The time in ticks to wait for the semaphore to become available. The macro
portTICK_PERIOD_MS can be used to convert this to a real time. A block time of zero can be used
to poll the semaphore. If the task already owns the semaphore then xSemaphoreTakeRecursive()
will return immediately no matter what the value of xBlockTime.
xSemaphoreGive(xSemaphore)
Macro to release a semaphore. The semaphore must have previously been created with a call to
xSemaphoreCreateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting(). and obtained us-
ing sSemaphoreTake().
This macro must not be used from an ISR. See xSemaphoreGiveFromISR () for an alternative which can be
used from an ISR.
This macro must also not be used on semaphores created using xSemaphoreCreateRecursiveMutex().
Example usage:
SemaphoreHandle_t xSemaphore = NULL;
void vATask( void * pvParameters )
{
// Create the semaphore to guard a shared resource.
vSemaphoreCreateBinary( xSemaphore );
if( xSemaphore != NULL )
{
if( xSemaphoreGive( xSemaphore ) != pdTRUE )
{
// We would expect this call to fail because we cannot give
// a semaphore without first "taking" it!
}
// Obtain the semaphore - don't block if the semaphore is not
// immediately available.
if( xSemaphoreTake( xSemaphore, ( TickType_t ) 0 ) )
{
// We now have the semaphore and can access the shared resource.
// ...
// We have finished accessing the shared resource so can free the
// semaphore.
if( xSemaphoreGive( xSemaphore ) != pdTRUE )
{
// We would not expect this call to fail because we must have
// obtained the semaphore to get here.
}
}
}
}
Return pdTRUE if the semaphore was released. pdFALSE if an error occurred. Semaphores are imple-
mented using queues. An error can occur if there is no space on the queue to post a message - indicating
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