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;
};

Can't you provide us with compilable code for the structure definitions? Or omit the `struct mystruct2`... — Jonathan Leffler, Aug 24 '11 at 18:21
Note that the keyword `typedef` is not needed in this example. It is a vacuous use of it since there is no name given to become an alias for `struct mystruct1`. — Jonathan Leffler, Jan 10 '20 at 13:57

Jonathan Leffler · Accepted Answer · 2011-08-24T18:34:15.790

14

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.

edited Aug 24 '11 at 18:34

answered Aug 24 '11 at 18:25

Jonathan Leffler

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It would be great to show an example however short it may be of it being used (declare the struct and use `offsetof`) – Evan Carroll Jun 17 '18 at 03:01
There are a number of questions where I do show examples, including [Finding offset of a structure element in C](https://stackoverflow.com/a/18749784/15168). – Jonathan Leffler Nov 27 '19 at 18:12

pmg · Answer 2 · 2011-08-24T18:34:47.273

2

Your struct mystruct1 has 1 member named u_line. You can see the offset of that member

offsetof(struct mystruct1, u_line); // should be 0

or of members down the line if you specify each "level of parenthood"

offsetof(struct mystruct1, u_line.line);
offsetof(struct mystruct1, u_line.line.buf);
offsetof(struct mystruct1, u_line.logo);

edited Aug 24 '11 at 18:34

answered Aug 24 '11 at 18:23

pmg

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score 2 · Answer 3 · edited Aug 02 '13 at 19:58

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.

edited Aug 02 '13 at 19:58

Douglas Santos

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answered Aug 24 '11 at 19:19

ACRL

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score -1 · Answer 4 · edited Jan 09 '15 at 21:56

-1

Firstly, AFAIK, offsetof is intended to be used with immediate members of the struct only (am I right on this?).

Secondly, knowing the internal details of popular offsetof implementations I can suggest trying

offsetof(struct mystruct1, u_line.line)

This should work. Whether it is standard-compliant is an open question for me.

P.S. Judging by @Jonathan Leffler's answer, this should actually work.

edited Jan 09 '15 at 21:56

Jonathan Leffler

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answered Aug 24 '11 at 18:26

AnT stands with Russia

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"am I right on this?" I don't think so (after testing gcc). gcc accepts "deep" members on `offsetof` expressions. – pmg Aug 24 '11 at 18:32
1

@pmg: Well, questions like this can't really be answered by "testing". – AnT stands with Russia Aug 24 '11 at 18:37
Right @Andrey. The quote from the Standard posted in Jonathan's answer allows for using `offsetof` with "deep" members. – pmg Aug 24 '11 at 18:49

How do you use offsetof() on a struct?

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