On the use and abuse of alloca

Question

I am working on a soft-realtime event processing system. I would like to minimise as many calls in my code that have non-deterministic timing. I need to construct a message

Answer 1

Firstly, it's because alloca memory is very hard to control. It's untyped, dies at the earliest opportunity, which makes it not very helpful. In addition, alloca has some unfortunate side effects, and those side effects are that regular stack variables now have to be dynamically indexed instead of constants, which can affect your performance in even basic operations accessing them and consumes register/stack space to store the dynamic offsets. This means that the real cost of using alloca isn't recorded in just the time it takes for the function to return. In addition, stack memory is very limited compared to heap memory- on Windows the stack limit is 8MB by default I believe, whereas the heap can be nearly the entire user address space. More than that, ultimately, whatever data you want to return has to be on the heap, so you may as well just use that as working space.