Callbacks only deal with their arguments, but almost every callback I see in the wild has a first argument "void * cookie" which immediately gets typecast into a pointer to the structure (oops, I mean object) being manipulated.
Right, that's how you do it in C, where you don't have closures. Closures take some heavy machinery behind the scenes in a hard-compiled language. They sort of fall out of the dynamic binding mechanism in Python and Javascript.
