How do you use offsetof() on a struct?

Question

I want the offsetof() the param line in mystruct1. I've tried

offsetof(struct mystruct1, rec.structPtr1.u_line.line) 

and also

offsetof(struct mystruct1, line)  

but neither works.

union {
    struct mystruct1 structPtr1;
    struct mystruct2 structPtr2;
} rec;

typedef struct mystruct1 {
    union {
        struct {
            short len;
            char buf[2];
        } line;

        struct {
            short len;
        } logo;

    } u_line;
};

Answer 1

The offsetof() macro takes two arguments. The C99 standard says (in §7.17 <stddef.h>):

offsetof(type, member-designator)

which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given

static type t;

then the expression &(t.member-designator) evaluates to an address constant.

So, you need to write:

offsetof(struct mystruct1, u_line.line);

However, we can observe that the answer will be zero since mystruct1 contains a union as the first member (and only), and the line part of it is one element of the union, so it will be at offset 0.

Answer 2

A great article to read on this is:

Learn a new trick with the offsetof() macro

I use the offsetof macro frequently in my embedded code, together with the modified SIZEOF macro as discussed in the article.