In C, will working with global variables help the stack?

Question

No I'm really serious.

I've just witnessed someone move variables local to a function up to global status, with the commit message "Relieve the stack".

Is there really some rationale towards doing this sort of thing?

Answer 1

First, putting variables in global variables doesn't improve CPU usage directly. Stack initialization is generally a single add/subtract at function entry/exit, independent of stack frame size.

However, if the function requires a very large working set, it's better to put it in something other than stack; the stack's size is usually rather limited. The usual choice is the heap; however this does take time to allocate and free, and so if you're going to be calling the function frequently it can be expensive. It's also a problem on embedded systems, where they may not have a proper heap implementation.

So if heap is a problem, globals can be a solution. However, they have their own downsides - in particular, you don't want to have multiple threads messing with the global data at the same time, nor can you recurse through this function without taking a lot of care, or the recursed bits may corrupt the earlier calls to the function.

So it's a technique which, in some cases, can be helpful. I wouldn't recommend using it as a first choice however, because of threading issues.

Also, for what it's worth, you can get the same memory effects with static variables. I would recommend using these instead, as otherwise you'll end up polluting the global namespace.