Hmm ... I don't know C's pointer arithmetic sufficiently well, but I wouldn't call this functional programming, so callbacks alone are not enough.

For instance, can you write a generator function which returns an iterator function?

What about closures?

So probably are First class citizen a prerequisite for FP but not the whole picture.

update

here we go: Some authors require it be possible to create new functions at runtime to call them 'first-class'. As a result, functions in C are not first-class objects; instead, they are sometimes called second-class objects, because they can still be manipulated in most of the above fashions (via function pointers).

see also First class functions