SemaphoreHandle_t xSemaphoreCreateRecursiveMutex( void )
Creates a recursive mutex,
and returns a handle by which the mutex can be
referenced. Recursive mutexes cannot be used in interrupt service routines.
and configUSE_RECURSIVE_MUTEXES must both be set to 1 in FreeRTOSConfig.h for
xSemaphoreCreateRecursiveMutex() to be available (configSUPPORT_DYNAMIC_ALLOCATION
can also be left undefined, in which case it will defailt to 1).
Each recursive mutex require a small amount of RAM that is used to hold the
recursive mutex’s state. If a mutex is created using xSemaphoreCreateRecursiveMutex()
then the required RAM is automatically allocated from the FreeRTOS heap.
If a recursive mutex is created using xSemaphoreCreateRecursiveMutexStatic()
then the RAM is provided by the application writer, which requires an additional
parameter, but allows the RAM to be statically allocated at compile
time. See the Static Vs
Dynamic allocation page for more information.
Recursive mutexes are ‘taken’ (obtained) using the
given (released) using the
xSemaphoreGiveRecursive() API functions respectively.
xSemaphoreTake() and xSemaphoreGive() must not be used.
Non-recursive mutexes are created using xSemaphoreCreateMutex() and
xSemaphoreCreateMutexStatic(). A non-recursive mutex can only be ‘taken’ by a
task once. Any attempt by a task to take a non-recursive mutex that it already
holds will fail – and the mutex will always be given back the first time the
task ‘gives’ the mutex.
Contrary to non-recursive mutexes, a task can ‘take’ a recursive mutex multiple
times, and the recursive mutex will only be returned after the holding task has
‘given’ the mutex the same number of times it ‘took’ the mutex.
Like non-recursive mutexes, recursive mutexes implement a priority inheritance
algorithm. The priority of a task that ‘takes’ a mutex will be temporarily 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.
If the recursive mutex was created successfully then a handle to
the created mutex is returned. If the recursive mutex was not
created because the memory required to hold the mutex
could not be allocated
then NULL is returned.
void vATask( void * pvParameters )
Create a recursive mutex.
xMutex = xSemaphoreCreateRecursiveMutex();
if( xMutex != NULL )
/* The recursive mutex was created successfully and
can now be used. */
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