Chapter 2. API Reference
becoming available.
xSemaphoreTakeRecursive(xMutex, xBlockTime)
Macro to recursively obtain, or ‘take’, a mutex type semaphore. The mutex must have previously been
created using a call to xSemaphoreCreateRecursiveMutex();
configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this macro to be available.
This macro must not be used on mutexes created using xSemaphoreCreateMutex().
A mutex used recursively can be ‘taken’repeatedly by the owner. The mutex doesn’t become available
again until the owner has called xSemaphoreGiveRecursive() for each successful‘take’request. For example,
if a task successfully ‘takes’the same mutex 5 times then the mutex will not be available to any other task
until it has also ‘given’the mutex back exactly five times.
Example usage:
SemaphoreHandle_t xMutex = NULL;
// A task that creates a mutex.
void vATask( void * pvParameters )
{
// Create the mutex to guard a shared resource.
xMutex = xSemaphoreCreateRecursiveMutex();
}
// A task that uses the mutex.
void vAnotherTask( void * pvParameters )
{
// ... Do other things.
if( xMutex != NULL )
{
// See if we can obtain the mutex. If the mutex is not available
// wait 10 ticks to see if it becomes free.
if( xSemaphoreTakeRecursive( xSemaphore, ( TickType_t ) 10 ) == pdTRUE )
{
// We were able to obtain the mutex and can now access the
// shared resource.
// ...
// For some reason due to the nature of the code further calls to
// xSemaphoreTakeRecursive() are made on the same mutex. In real
// code these would not be just sequential calls as this would make
// no sense. Instead the calls are likely to be buried inside
// a more complex call structure.
xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );
xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );
// The mutex has now been 'taken' three times, so will not be
// available to another task until it has also been given back
// three times. Again it is unlikely that real code would have
// these calls sequentially, but instead buried in a more complex
// call structure. This is just for illustrative purposes.
xSemaphoreGiveRecursive( xMutex );
xSemaphoreGiveRecursive( xMutex );
xSemaphoreGiveRecursive( xMutex );
// Now the mutex can be taken by other tasks.
}
else
{
// We could not obtain the mutex and can therefore not access
// the shared resource safely.
(continues on next page)
Espressif Systems 904
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