Semaphores

Description

Counting semaphores are a synchronization primitive that allow threads to wait until an event has occurred. The event may be generated by a producer thread, or by a DSR in response to a hardware interrupt. Associated with each semaphore is an integer counter that keeps track of the number of events that have not yet been processed. If this counter is zero, an attempt by a consumer thread to wait on the semaphore will block until some other thread or a DSR posts a new event to the semaphore. If the counter is greater than zero then an attempt to wait on the semaphore will consume one event, in other words decrement the counter, and return immediately. Posting to a semaphore will wake up the first thread that is currently waiting, which will then resume inside the semaphore wait operation and decrement the counter again.

Another use of semaphores is for certain forms of resource management. The counter would correspond to how many of a certain type of resource are currently available, with threads waiting on the semaphore to claim a resource and posting to release the resource again. In practice condition variables are usually much better suited for operations like this.

cyg_semaphore_init is used to initialize a semaphore. It takes two arguments, a pointer to a cyg_sem_t structure and an initial value for the counter. Note that semaphore operations, unlike some other parts of the kernel API, use pointers to data structures rather than handles. This makes it easier to embed semaphores in a larger data structure. The initial counter value can be any number, zero, positive or negative, but typically a value of zero is used to indicate that no events have occurred yet.

cyg_semaphore_wait is used by a consumer thread to wait for an event. If the current counter is greater than 0, in other words if the event has already occurred in the past, then the counter will be decremented and the call will return immediately. Otherwise the current thread will be blocked until there is a cyg_semaphore_post call.

cyg_semaphore_post is called when an event has occurs. This increments the counter and wakes up the first thread waiting on the semaphore (if any). Usually that thread will then continue running inside cyg_semaphore_wait and decrement the counter again. However other scenarioes are possible. For example the thread calling cyg_semaphore_post may be running at high priority, some other thread running at medium priority may be about to call cyg_semaphore_wait when it next gets a chance to run, and a low priority thread may be waiting on the semaphore. What will happen is that the current high priority thread continues running until it is descheduled for some reason, then the medium priority thread runs and its call to cyg_semaphore_wait succeeds immediately, and later on the low priority thread runs again, discovers a counter value of 0, and blocks until another event is posted. If there are multiple threads blocked on a semaphore then the configuration option CYGIMP_KERNEL_SCHED_SORTED_QUEUES determines which one will be woken up by a post operation.

cyg_semaphore_wait returns a boolean. Normally it will block until it has successfully decremented the counter, retrying as necessary, and return success. However the wait operation may be aborted by a call to cyg_thread_release, and cyg_semaphore_wait will then return false.

cyg_semaphore_timed_wait is a variant of cyg_semaphore_wait. It can be used to wait until either an event has occurred or a number of clock ticks have happened. The number of ticks is specified as an absolute, not relative tick count, and so in order to wait for a relative number of ticks, the return value of the cyg_current_time() function should be added to determine the absolute number of ticks. The function returns success if the semaphore wait operation succeeded, or false if the operation timed out or was aborted by cyg_thread_release. If support for the real-time clock has been removed from the current configuration then this function will not be available. cyg_semaphore_trywait is another variant which will always return immediately rather than block, again returning success or failure. If cyg_semaphore_timedwait is given a timeout in the past, it operates like cyg_semaphore_trywait.

cyg_semaphore_peek can be used to get hold of the current counter value. This function is rarely useful except for debugging purposes since the counter value may change at any time if some other thread or a DSR performs a semaphore operation.

Valid contexts

cyg_semaphore_init is normally called during initialization but may also be called from thread context. cyg_semaphore_wait and cyg_semaphore_timed_wait may only be called from thread context because these operations may block. cyg_semaphore_trywait, cyg_semaphore_post and cyg_semaphore_peek may be called from thread or DSR context.