Why subtract null pointer in offsetof()?

Question

Linux\'s stddef.h defines offsetof() as:

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

whereas the Wik

Answer 1

The first version converts a pointer into an integer with a cast, which is not portable.

The second version is more portable across a wider variety of compilers, because it relies on pointer arithmetic by the compiler to get an integer result instead of a typecast.

BTW, I was the editor that added the original code to the Wiki entry, which was the Linux form. Later editors changed it to the more portable version.