From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-help-return-9856-listarch-gcc-help=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 30130 invoked by alias); 17 Aug 2002 18:58:58 -0000
Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-help@gcc.gnu.org>
Sender: gcc-help-owner@gcc.gnu.org
Received: (qmail 30120 invoked from network); 17 Aug 2002 18:58:54 -0000
Received: from unknown (HELO hofr.at) (80.120.128.82)
  by sources.redhat.com with SMTP; 17 Aug 2002 18:58:54 -0000
Received: (from root@localhost)
	by hofr.at (8.10.2/8.9.3/SuSE Linux 8.9.3-0.1) id g7HI2wN24657;
	Sat, 17 Aug 2002 20:02:58 +0200
From: Der Herr Hofrat <der.herr@mail.hofr.at>
Message-Id: <200208171802.g7HI2wN24657@hofr.at>
Subject: Re: Bitfield packing
In-Reply-To: <20020817073337.GA816@fericom.net> from "eddy@fericom.net" at "Aug
 17, 2002 09:33:37 am"
To: eddy@fericom.net
Date: Sat, 17 Aug 2002 11:58:00 -0000
CC: gcc-help@gcc.gnu.org
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
X-SW-Source: 2002-08/txt/msg00154.txt.bz2

> Hi, 
> 
> I am using gcc for a microcontroller application. I want to use
> bitfields to describe the processor registers, 
> similar to this: 
> 
> typedef union {
> 	IO_BYTE byte; 
> 	struct {
> 		IO_BYTE P30 :1; 
> 		IO_BYTE P31 :1; 
> 		IO_BYTE P32 :1; 
> 		IO_BYTE P33 :1; 
> 		IO_BYTE P34 :1; 
> 		IO_BYTE P35 :1; 
> 		IO_BYTE P36 :1; 
> 		IO_BYTE P37 :1; 
> 	} bit; 
> } PDR2;
> 
> How does gcc handle bitfields like thisone? Above I have defined 8 bits,
> are they distributed on several bytes with some optimization levels?

if you don't use any __attribute__((packed)) on the bitfields the size of
this object will be alligned machin dependant (probably 32bit on a 32bit box)
you can see that by using the __alignof__() (just like sizeof) function to
report allignement - internal ordering I guess is nothing you should ever 
relie on. See the section "storage layout" in the gcc manual for infos on
what may influence your setup (i.e. BITS_PER_UNIT).

If you want to be shure of the bit-positions I guess you must use 
bit-operators and allocate an unsigned char / u8 object.

hofrat