Issue 0010: Is the typedef to an incomplete type valid?

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

Authors: Michael S. Ball, WG14
Date: 1992-12-10
Reference document: X3J11/90-044
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_010.html

Consider:

typedef int table[];        /* line 1 */

 table one = {1};        /* line 2 */

 table two = {1, 2};     /* line 3 */

First, is the typedef to an incomplete type legal? I can't find a prohibition in the standard. But an incomplete type is completed by a later definition, such as line 2, so what is the status of line 3?

The type, of which table is only a synonym, can't be completed by line 2 if it is to be used in line 3. And what is sizeof(table)? What old C compilers seem to do is treat the typedef as some sort of textual equivalent, which is clearly wrong.

Comment from WG14 on 1997-09-23:

Response

A typedef of an incomplete type is permitted.

Regarding objects one and two, refer to the standard subclause 6.1.2.5, page 24, lines 8-9: “An array of unknown size is an incomplete type. It is completed, for an identifier of that type, by specifying the size in a later declaration ...” [emphasis added]. The types of objects one and two are completed but the type table itself is never completed. Hence, sizeof(table) is not permitted.

An example very similar to that submitted is shown in example 6, subclause 6.5.7 on page 74, lines 16-23.