![]() Unfortunately, those methods don’t lead us to anything useful in the long term, so I’ll describe one method that we’ll incrementally improve to offer a full range of functionality. Or handling the case where semaphore holders can crash without releasing semaphores, we could build semaphores fairly conveniently in a few different ways. We must decide who got the lock, how to handle processes that crashed with the lock, and how to handle timeouts. When building a counting semaphore, we run into many of the same concerns we had with other types of locking. Value to which the semaphore can count is 10, and the initial valueĪssigned to the count will be 0.6.3.1 Building a basic counting semaphore CreateĪ counting semaphore using xSemaphoreCreateCountingStatic(). SemaphoreHandle_t xSemaphore = NULL Counting semaphore cannot be used before they have been created. StaticSemaphore_t xSemaphoreBuffer void vATask( void * pvParameters ) If pxSemaphoreBuffer was NULL then NULL is returned. Returns If the counting semaphore was successfully created then a handle to the created counting semaphore is returned. Must point to a variable of type StaticSemaphore_t, which will then be used to hold the semaphore's data structure, removing the need for the memory to be allocated dynamically. The count value assigned to the semaphore when it is created. When the semaphore reaches this value it can no longer be 'given'. The maximum count value that can be reached. In this case it is desirable for the initial count value to be equal to the maximum count value, indicating that all resources are free. ![]() When a task finishes with the resource it 'gives' the semaphore back - incrementing the semaphore count value. When the count value reaches zero there are no free resources. To obtain control of a resource a task must first obtain a semaphore - decrementing the semaphore count value. In this usage scenario the count value indicates the number of resources available. In this case it is desirable for the initial count value to be zero. The count value is therefore the difference between the number of events that have occurred and the number that have been processed. In this usage scenario an event handler will 'give' a semaphore each time an event occurs (incrementing the semaphore count value), and a handler task will 'take' a semaphore each time it processes an event (decrementing the semaphore count value). xSemaphoreCreateCountingStatic() therefore allows a counting semaphore to be created without using any dynamic memory allocation.Ĭounting semaphores are typically used for two things: If a counting semaphore is created using xSemaphoreCreateCountingStatic() then the application writer must provide the memory. If a counting semaphore is created using xSemaphoreCreateCounting() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateCounting() function. Internally, within the FreeRTOS implementation, counting semaphores use a block of memory, in which the counting semaphore structure is stored. In many usage scenarios it is faster and more memory efficient to use a direct to task notification in place of a counting semaphore! h SemaphoreHandle_t xSemaphoreCreateCountingStatic( UBaseType_t uxMaxCount, UBaseType_t uxInitialCount, StaticSemaphore_t *pxSemaphoreBuffer )Ĭreates a new counting semaphore instance, and returns a handle by which the new counting semaphore can be referenced.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |