treeTraverse(Tree *ptr, (void *) call_back(Tree *ptr));

myFunc(Tree *ptr, int a, int b)

void treeTraverse(Tree *ptr,
                  (void *) call_back(Tree *ptr, void *arg),
                  void *call_back_arg);
struct {
  int a;
  int b;
} args = {...};
treeTraverse(ptr, myCallback, &args);