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Message-ID: <19971102174709.12838@gil.physik.rwth-aachen.de>
Date: Sun, 2 Nov 1997 17:47:09 +0100
From: Christoph Kukulies <kuku@gilberto.physik.RWTH-Aachen.DE>
To: John Milford <jwm@CSUA.Berkeley.EDU>
Cc: Christoph Kukulies <kuku@gilberto.physik.RWTH-Aachen.DE>,
        freebsd-hackers@freefall.freebsd.org
Subject: Re: gcc and bitfields
References: <kuku@gilberto.physik.RWTH-Aachen.DE> <199711021618.IAA02063@soda.CSUA.Berkeley.EDU>
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In-Reply-To: <199711021618.IAA02063@soda.CSUA.Berkeley.EDU>; from John Milford on Sun, Nov 02, 1997 at 08:18:29AM -0800
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On Sun, Nov 02, 1997 at 08:18:29AM -0800, John Milford wrote:
> Christoph Kukulies <kuku@gilberto.physik.RWTH-Aachen.DE>  wrote:
> 
> >
> > During porting a piece of a DOS program to make a driver
> > for FreeBSD of it for a certain device I'm stuck at the
> > point where there is a structure:
> >
> >     struct C_OPEN {
> >          int a;
> >          int b;
> >          unsigned xdt:1;
> >          unsigned reserved:15;
> >     }
> >
> > The sizeof this structure is 10 under DOS (borland C)
> > and evaluates to 12 under cc (gcc) on FreeBSD.
> >
> > There are a lot of these definitions and it would be
> > tedious to find a workaround.
> >
> > Does anyone know if I can pack the structure respectively
> > enforce the bitfield to a short int?
> 
> By default gcc pads structures to make make the size a multiple of 4 bytes.
> Try:
> 
>    struct C_OPEN {
>         int a;
> 	int b;
> 	unsigned xdt:1;
> 	unsigned reserved:15;
>    } __attribute__ ((packed));

I tried that already to no avail. 

struct C_OPEN
   {
      int a;
      int b; 
      unsigned xtd:1;
      unsigned reserved:15; } __attribute__ ((packed));


main()
{
   printf("sizeof struct C_OPEN = %d\n", sizeof (struct C_OPEN));
}
$ ./a.out
12

Ah, I found it: making the bitfield declarations unsigned short
works.

> 
> 
> 
> 		--John

-- 
Chris Christoph P. U. Kukulies kuku@gil.physik.rwth-aachen.de