Get Position Of Field In Struct

Question

A fairly simple question (I hope). Given a struct in C (the layout of which is known at compile time), is there a way (via macro, or other) that I can access the byte position of a named field in the struct?

Support for unions would be a bonus. The compiler is VC++ 2008. Assume that #pragma pack(1) is used.

Cheers

Answer 1

You're looking for offsetof. It should be in stddef.h, but in case you don't have that, a sample implementation (from wikipedia):

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

For a union, the offset of every field is 0.