Is apparent NULL pointer dereference in C actually pointer arithmetic?

Question

I\'ve got this piece of code. It appears to dereference a null pointer here, but then bitwise-ANDs the result with unsigned int. I really don\'t understand the

Answer 1

This gives you the offset in bytes of the b field inside the hi struct

((struct hi *)0) is a pointer to a hi struct, starting at address 0.

(((struct hi *)0)->b) is the b field of the above struct

& (((struct hi *)0)->b) is the address of the above field. Because the hi struct is located at address 0, this is the offset of b within the struct.

(unsigned int) & (((struct hi *)0)->b) is a conversion of that from the address type to unsigned int, so that it can be used as a number.

You're not actually dereferencing a NULL pointer. You're just doing pointer arithmetic.

---

Accessing (((struct hi *)0)->b) will give you a segmentation fault because you're trying to access a forbidden memory location.

Using & (((struct hi *)0)->b) does not give you segmentation fault because you're only taking the address of that forbidden memory location, but you're not trying to access said location.