#define GetAggregateByMember(Aggregate,MemberName,MemberLink) \
    ((Aggregate*)(((size_t)(MemberLink))-((size_t)&((Aggregate*)(size_t)0)->MemberName)))

struct B{
    int side;
    B* peer;
};

struct A{
    B b[2];
    A(){
        b[0].side=0;
        b[1].side=1;
    }
};

B* pb;
//...

A* a1 = GetAggregateByMember(A,b[pb->side],pb);