> There is no such thing as private data, its all public by default

Not in C, put the strict declaration in the h after and the definition in an implementation source file.

Callers of the implementation functions can pass the strict around without seeing any of the fields. Only functions in the implementation can access the fields which results in all acce to the fields going through the implementation only.

Cleaner and safer than letting the caller see everything while only accessing some things.