[clappski]

You'd have an error about an incomplete type - see https://godbolt.org/z/G4Gsfn7MT

> a pointer, whose size is always known

Yeah, this is exactly how it works. You work with a pointer that acts like a void* in your code, and the library with the definition is allowed to reach into the fields of that pointer. Normally you'd have a C API like

    struct Op;
    Op* init_op();
    void free_op( Op* );
    void do_something_with_op( Op* );

in the header provided by the library that you compile as part of your code, and the definition/implementation in some .a or .so/.dll that you'll link against.*