The best example I know of off the top of my head is wait_event() in Linux.

So long as the thread is guaranteed not to exit while blocked, you know its stack, and therefore the object allocated on it, must continue to exist. So, as long as there is no way to wake the thread except by kicking that object, the memory backing it is guaranteed to continue to exist until that object is kicked. You do have to somehow serialize the global data structure lookup (e.g. lock/dequeue/unlock/kick), if multiple threads can find and kick the object concurrently that's unsafe (the thread might exit between the first and subsequent kicks).

Generally that's true, even in pthread userspace: while there are some obvious artificial counterexamples one can construct, real world code very rarely does things like that.

Ok, I see, thanks for the example. Is this technique used to avoid the potential runtime performance cost because one would otherwise need to keep that object elsewhere/heap and not on a stack? Or is the problem definition something else?