What does (s *)0 in (size_t)&(((s *)0)->m) mean [duplicate]

Question

This is one of the series of macros in stddef.h.

#define offsetof(s,m)   (size_t)&(((s *)0)->m)

What does (s *)0 mean?

Answer 1

It is a way of writing a NULL pointer of type pointer-to-s . By taking the address of the m member of an s whose address is 0, you get the offset of m within an s.

Answer 2

It's a typecast, converting 0 to pointer to s.