struct A {
     int bar;
};

struct B {
     struct A a;
     int foo;
};

struct C {
    void *priv;
};

void f(struct C *c) {
    struct A *a = c->priv;
}

void g(void) {
    struct B b;
    struct C c;
    c.priv = &b;
    f(&c);
}