How do you use offsetof() on a struct?

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;
};