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cpointer-arithmeticoffsetof

Why subtract null pointer in offsetof()?

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

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

whereas the Wikipedia article on offsetof() (http://en.wikipedia.org/wiki/Offsetof) defines it as:

#define offsetof(st, m) \
    ((size_t) ( (char *)&((st *)(0))->m - (char *)0 ))

Why subtract (char *)0 in the Wikipedia version? Is there any case where that would actually make a difference?

Solution

  • 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.