SemaphoreHandle_t xSemaphoreCreateRecursiveMutexStatic( StaticSemaphore_t *pxMutexBuffer )Creates a recursive mutex, and returns a handle by which the mutex can be referenced. Recursive mutexes cannot be used in interrupt service routines. configUSE_RECURSIVE_MUTEXES and configSUPPORT_STATIC_ALLOCATION must both be set to 1 in FreeRTOSConfig.h for xSemaphoreCreateRecursiveMutexStatic() to be available.
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 xSemaphoreTakeRecursive() and 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.
pxMutexBuffer Must point to a variable of type StaticSemaphore_t, which will be used to hold the mutex type semaphore’s state.
If the recursive mutex was created successfully then a handle to the created mutex is returned. If the recursive mutex was not created because pxMutexBuffer was NULL then NULL is returned.
SemaphoreHandle_t xSemaphore = NULL;
void vATask( void * pvParameters )
/* Create a recursivemutex semaphore without using any dynamic
memory allocation. The mutex's data structures will be saved into
the xMutexBuffer variable. */
xSemaphore = xSemaphoreCreateRecursiveMutexStatic( &xMutexBuffer );
/* The pxMutexBuffer was not NULL, so it is expected that the
handle will not be NULL. */
configASSERT( xSemaphore );
/* Rest of the task code goes here. */