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

I think you need to be a little bit careful in understanding what alloca actually is. Unlike malloc which goes to the heap, searches through buckets and linked lists of various buffers, alloca simply takes your stack register (ESP on x86) and moves it to create a "hole" on your thread's stack where you can store whatever you want. That's why it's uber-fast, just one (or few) assembly instruction.

So as others pointed out, it's not the "virtual memory" that you need to worry about but the size reserved for the stack. Although others limit themselves to "few hundred bytes", as long as you know your application and careful about it, we've allocated up to 256kb without any problems (default stack size, at least for visual studio, is 1mb and you can always increase it if you need to).

Also you really can't use alloca as a general purpose allocator (i.e. wrapping it inside another function) because whatever memory alloca allocates for you, that memory will be gone when the stack frame for current function is popped (i.e. when function exits).

I've also seen some people say that alloca is not completely cross-platform compatible, but if you are writing a specific application for a specific platform and you have the option of using alloca, sometimes it's the best option you have, as long as you understand the implications of increasing stack usage.