Issue 0040.06: Can one use offsetof(struct t1, mbr) before struct t1 is completely defined?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/91-062
Submitted against: C90
Status: Closed
Cross-references: 0097
Converted from: dr.htm, dr_040.html

The definition of the offsetof macro in subclause 7.1.6 does not cover all its possible occurrences:

a. There are no restrictions on the structure being a completed type.

struct t1 {
 char c;
 short s;
 int i[offsetof(struct t1, s)];
 }

When discussing the use of incomplete types, recourse usually has to be made to the rules relating to where an object of unknown size may appear.

Would the Committee agree that there are not any rules prohibiting the above construction?

b. In this structure we are asked to find the offset of a field that has not yet been encountered:

struct t2 {
 char c;
 union {
 int i[offsetof(struct t2, s)];
 short s;
 } u;
 };

Would the Committee agree that there do not appear to be any rules that make this construct illegal?

c. The following structure has infinitely many “solutions:”

struct t3 {
 char a[offsetof(struct t3, i)];
 int i;
 }

since char has size 1, any size of array will be the same as the offsetof the field i.

d. The following structure has no “solutions:”

struct t4 {
 int a[offsetof(struct t3, i)];
 int i;
 }

int is always larger than 1.

Comment from WG14 on 1997-09-23:

Response

a. Example:

struct t1 {
 char c;
 short s;
 int i[offsetof(struct t1, s)];
 };

This is not a valid use of the offsetof macro. The hypothetical static type t; declaration required for offsetof (cf. subclause 7.1.6) could not have validly appeared prior to the invocation of offsetof because the type struct t1 is incomplete (cf. subclause 6.7.2); therefore the offsetof invocation is not strictly conforming.

b. The answer is the same as (a) above. In addition, the members mentioned in these invocations are not in scope.

c. The answer is the same as (a) above. In addition, the members mentioned in these invocations are not in scope.

d. The answer is the same as (a) above. In addition, the members mentioned in these invocations are not in scope.