They call it an "alertable state", and that's: > A thread enters an alertable state when it calls the SleepEx, SignalObjectAndWait, MsgWaitForMultipleObjectsEx

They call it an "alertable state", and that's: > A thread enters an alertable state when it calls the SleepEx, SignalObjectAndWait, MsgWaitForMultipleObjectsEx, WaitForMultipleObjectsEx, or WaitForSingleObjectEx function. So this is a lot less like Unix signals. It only really works if the thread you're doing the async procedure call to is one that's likely to use those. So APCs are safe enough -- a lot safer than Unix signal handlers.