Question

I have a recursive function which can be written like:

void func(TypeName *dataStructure, LL_Node **accumulator) {
    func(datastructure->left, accumulator);
    func(datastructure->right, accumulator);

    {
        char buffer[1000];
        // do some stuff
    }

    return;
}        

I know that in reality the buffer is being allocated at the beginning of the function and putting the statement in a nested scope block doesn’t actually use a new stack frame. But I don’t want the compiler to allocate an exponential number of 1000-byte buffers at once, when they can be allocated and thrown away one at a time as each level returns.

Should I use outside global variables? A call to a helper function to force the buffer to be allocated after the recursive call? What I’m really fishing for here is advice on the cleanest, most C-idiomatic way of forcing this behavior.

Edit: One add-on question. If the exact same accumulator will be passed to every call of func, is it unheard of to leave the accumulator pointer in a global variable rather than pushing it onto the stack with every call?

Answer 1

Since it’s only used by one call at a time, you can just preallocate it and pass it into all the calls via an operand:

void func(TypeName *dataStructure, LL_Node **accumulator, char *buffer) {
    func(datastructure->left, accumulator, buffer);
    func(datastructure->right, accumulator, buffer);

    {
        // do some stuff
    }

    return;
}