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Currently running asynchronous timers can be canceled. When you attempt to cancel an asynchronous timer a race condition begins between your cancellation request and expiration of the timer. It is therefore possible that the timer will expire and that your cancellation attempt will fail even though the timer had not yet expired at the instant the cancellation attempt was made.
With current versions of the Mac OS, if a primary interrupt handler queues a secondary handler that is to cancel a timer by calling CancelTimer, and if the secondary handler queues another secondary handler, the operating system guarantees that the timer will either execute or be canceled before the other secondary handler runs.