Submitter: UK C Panel
Submission Date: 2001-09-07
Source: Clive D.W. Feather <clive@demon.net>
Version: 1.3
Date: 2006-03-29
Subject: are struct fred and
union fred the same type ?
Consider the code:
union fred { int a; }
int main (void)
{
struct fred *ptr; /* Line X */
// ...
I can see nothing that forbids this code. In particular, 6.7.2.3#8 reads:
[#8] If a type specifier of the form
struct-or-union identifierorenum identifier
occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is visible, then it specifies the same type as that other declaration, and does not redeclare the tag.
At line X a declaration of fred as a tag is visible, so this line specifies the same type as that other declaration, even though this uses struct and that uses union !
It has been further pointed out to me that nothing in the Standard actually says that union x is a union type as opposed to a structure type, and vice versa.
Suggested Technical Corrigendum
Append to 6.7.2.1#6:
The keywords struct and union indicate that the type being specified is, respectively, a structure type or a union type.
Add a new paragraph following 6.7.2.3#1:
[#1a] Where two declarations that use the same tag declare the same type, they shall both use the same choice of struct, union, or enum.
Committee Discussion
The Committee is inclined to accept the suggested TC, but the issue is still being debated.
Technical Corrigendum
Append to 6.7.2.1#6:
The keywords struct and union indicate that the type being specified is, respectively, a structure type or a union type.
Add a new paragraph following 6.7.2.3#1:
[#1a] Where two declarations that use the same tag declare the same type, they shall both use the same choice of struct, union, or enum.