From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-42904-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 17081 invoked by alias); 15 Dec 2001 23:32:28 -0000
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
Received: (qmail 17060 invoked from network); 15 Dec 2001 23:32:28 -0000
Received: from unknown (HELO www.cgsoftware.com) (208.155.65.221)
  by sources.redhat.com with SMTP; 15 Dec 2001 23:32:28 -0000
Received: from localhost (localhost [127.0.0.1])
	by www.cgsoftware.com (8.9.3/8.9.3) with ESMTP id SAA10595;
	Sat, 15 Dec 2001 18:32:23 -0500
Date: Sat, 15 Dec 2001 16:09:00 -0000
From: Daniel Berlin <dan@cgsoftware.com>
To: Sam Lantinga <slouken@devolution.com>
cc: <gcc@gcc.gnu.org>
Subject: Re: C++ bug in structure packing and inheritance
In-Reply-To: <E16FNqC-0007Qg-00@twomix.devolution.com>
Message-ID: <Pine.LNX.4.33.0112151807180.7153-100000@www.cgsoftware.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-SW-Source: 2001-12/txt/msg00840.txt.bz2



On Sat, 15 Dec 2001, Sam Lantinga wrote:

> There seems to be a bug in structure packing in that if you have
> two classes or structures that both inherit from a class with no
> data members, then they will be the wrong size.

> A simple example follows:
>
> #include <stddef.h>
> #include <stdio.h>
>
> struct a {
> };
>
> struct b : public a {
> 	float x;
> 	float y;
> };
>
> struct c : public a {
> 	b data;
> 	float z;
> };
>
> main()
> {
> 	printf("size of a = %d (should be 0)\n", sizeof(a));
> 	printf("size of b = %d (should be 8)\n", sizeof(b));
> 	printf("size of c = %d (should be 12)\n", sizeof(c));
> 	printf("offset of c.z = %d (should be 8)\n", offsetof(c,z));
> }
>
> Sample output with the current CVS of gcc is:
> size of a = 1 (should be 0)
> size of b = 8 (should be 8)
> size of c = 16 (should be 12)
> offset of c.z = 12 (should be 8)
I have no idea whether it's correct, but i can tell you *why* it happens.
-fclass-hierarchy-dump shows:

Class a
   size=1 align=1
a (0x3015d500) 0 empty

Class b
   size=8 align=4
b (0x3015d7c0) 0
  a (0x3015d800) 0 empty

Class c
   size=16 align=4
c (0x3015db00) 0
  a (0x3015db40) 0 empty


If you force it to pack them (__attribute__ packed or whatever), you'll
get:
size of a = 1 (should be 0)
size of b = 8 (should be 8)
size of c = 13 (should be 12)
offset of c.z = 9 (should be 8)

And the class-hierarchy-dump will show:
Class a
   size=1 align=1
a (0x3015d500) 0 empty

Class b
   size=8 align=1
b (0x3015d800) 0
  a (0x3015d840) 0 empty

Class c
   size=13 align=1
c (0x3015db40) 0
  a (0x3015db80) 0 empty


I dunno whether the empty base class should have size=0 align=0, but if
it did, this should cause the others to do what you expect.

>
> If this is correct, I would appreciate it if somebody could explain it.
>
> Thanks,
> 	-Sam Lantinga, Software Engineer, Blizzard Entertainment
>