I read an article by a guy talking about stupid C tricks. One of them was to 'mangle' raw pointers into an index before passing them. And then de-magle them to get back a raw pointer. Advantages are you can pass meta data with the 'pointer'. Which also allows you to invalidate a pointer. The pointer can't be dereferenced. The enclosed variables/data isn't accessible and cannot be modified by the target.

For callbacks the overhead likely isn't significant.