Address of members of a struct via NULL pointer

Question

Why the following expression is not a (null pointer) runtime error?

typedef struct{
           int a,b,c;
          } st;

st obj={10,12,15};
st *ptr1=&obj;
st *ptr2=NULL;

printf("%d",*(int *)((char*)ptr1+(int)&ptr2->b));

Answer 1

Because you are performing pointer arithmetic on a NULL pointer which invokes undefined behavior - and that's not required to crash.

Answer 2

actually, in GNU C, &ptr2->b while st *ptr2=NULL; produce the data member 'b' 's byte offset in the struct, it is 4 here