Issue 0017.06: Are register aggregates permitted?

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: X3J11/90-056
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Cross-references: 0116
Converted from: dr.htm, dr_017.html

register on aggregates

void f(void)
 {
 register union{int i;} v;
 &v      /* Constraint error */
 &(v.i);  /*  Constraint error or undefined? */
 }

In subclause 6.3.3.2 on page 43, lines 37-38 in a constraint clause, it says “... and is not declared with the register storage-class specifier.” But in the above, the field i is not declared with the register storage-class specifier.

Footnote 58, on page 58, states that “... the address of any part of an object declared with storage-class specifier register may not be computed ...” Although the reference to this footnote is in a constraints clause I think that it is still classed as undefined behavior.

Various people have tried to find clauses in the standard that tie the storage class of an aggregate to its members. I would not use the standard to show this point. Rather I would use simple logic to show that if an object has a given storage class then any of its constituent parts must have the same storage class. Also the use of storage classes on members is syntactically illegal.

The question is not whether such a construction is legal but the status of its illegality. Is it a constraint error or undefined behavior?

It might be argued that although register does not appear on the field i, its presence is still felt. I would point out that the standard does go to some pains to state that in the case of const union{...} the const does apply to the fields. The fact that there is no such wording for register implies that register does not follow the const rule.

Comment from WG14 on 1997-09-23:

Correction

Add to subclause 6.5.1, page 58 (Semantics):

If an aggregate or union object is declared with a storage-class specifier other than typedef, the properties resulting from the storage-class specifier, except with respect to linkage, also apply to the members of the object, and so on recursively for any aggregate or union member objects.