Comtrol eCos - Manual (Page 76)

Semaphores
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 function returns success if the semaphore wait op-
eration 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 re-
turning success or failure.
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.
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