Will this trick work in C?

Question

I want to add a field to a structure in C. So for example I have the following structure.

struct A
{
 some_type x;
 some_type y;
}

I declare a

Answer 1

Why not just call the method on the member?

some_function( &b->a );

Your code works now, but what if somebody decides to change the members of B? Or add a new member before a?