The number of tokens in the semaphore represents the number of additional threads that may enter. To allow multiple threads to run the function, initialize a semaphore to the maximum number of threads that can be allowed.
For example, this could be a function accessing DMA resources which can only support a limited number of calls.
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Downey which is available for free download.Ī multiplex limits the number of threads that can access a critical section of code. The following explains a use case for semaphores, taken from the book The Little Book Of Semaphores by Allen B. At the same time, they are perhaps the most challenging RTOS object to understand. Semaphore Use Casesĭue to their flexibility, semaphores cover a wide range of synchronizing applications. Refer to Semaphore Configuration for RTX5 configuration options. Using semaphores, access to a group of identical peripherals can be managed (for example multiple DMA channels).ĬMSIS-RTOS Semaphore States Note The functions osSemaphoreAcquire, osSemaphoreGetCount, and osSemaphoreRelease can be called from Interrupt Service Routines. Whereas a Mutex permits just one thread to access a shared resource at a time, a semaphore can be used to permit a fixed number of threads/ISRs to access a pool of shared resources. Semaphores are used to manage and protect access to shared resources. OsSemaphoreDelete ( osSemaphoreId_t semaphore_id) OsSemaphoreGetCount ( osSemaphoreId_t semaphore_id) Release a Semaphore token up to the initial maximum count. OsSemaphoreRelease ( osSemaphoreId_t semaphore_id) OsSemaphoreAcquire ( osSemaphoreId_t semaphore_id, uint32_t timeout)Īcquire a Semaphore token or timeout if no tokens are available. OsSemaphoreGetName ( osSemaphoreId_t semaphore_id) OsSemaphoreNew (uint32_t max_count, uint32_t initial_count, const osSemaphoreAttr_t *attr)Ĭreate and Initialize a Semaphore object. Access shared resources simultaneously from different threads.Īttributes structure for semaphore.
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