Issue 0040.07: Can sizeof be applied to earlier parameter names in a prototype, or to earlier fields in a struct?

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
Converted from: dr.htm, dr_040.html

sizeof various identifiers (subclause 7.1.6)

a)

void f(int c, char a[sizeof(c)]);

b)

int i;
 struct {
 int i;
 char a[sizeof(i)];
 };

Now the argument to sizeof must be an expression or a type.

In (a) is c an expression? I think not because:

expression -> object -> has storage in execution environment

and c does not have storage allocated to it. So (a) violates a semantic “shall” and is undefined behavior.

Now in (b) the field i is obviously not an expression. But is it visible? Like the outer i, it has file scope. However, it is in a different namespace. There are no rules for namespace resolution in the sizeof subclause.

So is (b) legal or undefined behavior?

Comment from WG14 on 1997-09-23:

Response

a. Example:

void f(int c, char a[sizeof(c)]);

The reference to c is an expression because the previously declared identifier designates a function parameter (cf. subclause 6.5.4.3), which is an object (subclause 3.15), thus meeting the requirement in subclause 6.3.1.

b. Another example:

int i;
 struct {
 int i;
 char a[sizeof(i)];
 };

In C, this is okay. Subclause 6.1.2.3, Name spaces of identifiers, requires that i in the sizeof expression refers to the external i, not the member.