C/C++ Structure offset

Question

I'm looking for a piece of code that can tell me the offset of a field within a structure without allocating an instance of the structure.

IE: given

struct mstct {
    int myfield;
    int myfield2;
};

I could write:

mstct thing;
printf("offset %lu
", (unsigned long)(&thing.myfield2 - &thing));

And get offset 4 for the output. How can I do it without that mstct thing declaration/allocating one?

I know that & does not always point at the first byte of the first field of the structure, I can account for that later.

Mike_B_12711 · Answer

How about the standard offsetof() macro (in stddef.h)?

Edit: for people who might not have the offsetof() macro available for some reason, you can get the effect using something like:

#define OFFSETOF(type, field)    ((unsigned long) &(((type *) 0)->field))

Dan_20789 · Answer

Right, use the offsetof macro, which (at least with GNU CC) is available to both C and C++ code:

offsetof(struct mstct, myfield2)

user_338 · Answer

printf("offset: %d ", &((mstct*)0)->myfield2);