NAME
ObtainSemaphore -- gain exclusive access to a semaphore
SYNOPSIS
ObtainSemaphore(signalSemaphore)
A0
void ObtainSemaphore(struct SignalSemaphore *);Links: SignalSemaphore
FUNCTION
Signal semaphores are used to gain exclusive access to an object. ObtainSemaphore is the call used to gain this access. If another user currently has the semaphore locked the call will block until the object is available.
If the current task already has locked the semaphore and attempts to lock it again the call will still succeed. A "nesting count" is incremented each time the current owning task of the semaphore calls ObtainSemaphore(). This counter is decremented each time ReleaseSemaphore is called. When the counter returns to zero the semaphore is actually released, and the next waiting task is called.
A queue of waiting tasks is maintained on the stacks of the waiting tasks. Each will be called in turn as soon as the current task releases the semaphore.
Signal Semaphores are different than Procure()/Vacate() semaphores. The former requires less CPU time, especially if the semaphore is not currently locked. They require very little set up and user thought. The latter flavor of semaphore make no assumptions about how they are used -- they are completely general. Unfortunately they are not as efficient as signal semaphores, and require the locker to have done some setup before doing the call.
INPUT signalSemaphore -- an initialized signal semaphore structure
NOTE This function preserves all registers (see BUGS).
BUGS Until V37, this function could destroy A0.
SEE ALSO
ObtainSemaphoreShared, InitSemaphore, ReleaseSemaphore, AttemptSemaphore, ObtainSemaphoreList