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 standard does not require the NULL pointer to evaluate to the bit pattern 0 but can evaluate to a platform specific value.

Doing the subtraction guarantees that when converted to an integer value, NULL is 0.

Answer 2

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.