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From: piet@cs.ruu.nl (Piet van Oostrum)
Newsgroups: comp.lang.c
Subject: Re: Ambiguous C?
Message-ID: <1301@ruuinf.cs.ruu.nl>
Date: 28 Apr 89 09:17:09 GMT
References: <111@ssp1.idca.tds.philips.nl>
Sender: piet@ruuinf.cs.ruu.nl
Reply-To: piet@cs.ruu.nl (Piet van Oostrum)
Organization: Dept of Computer Science, University of Utrecht, Holland
Lines: 28
In-reply-to: roelof@idca.tds.PHILIPS.nl (R. Vuurboom)

In article <111@ssp1.idca.tds.philips.nl>, roelof@idca (R. Vuurboom) writes:

 `#define	SCUCMD		0 
 `#define	SCU_BDID	0x2000		/* bit 14 */
 `
 `error()
 `{
 `	if ( (*(int *)(SCUCMD)) & SCU_BDID )
 `		;
 `}

In article <10136@smoke.BRL.MIL>, gwyn@smoke (Doug Gwyn) writes:

 `state of the bit is.  To force a longword access, it suffices to copy
 `the addressed object into an external longword, because the compiler
 `cannot know what other use might be made of its contents (by other
 `independently-compiled modules) and must therefore pick up the whole

Another simple way is to put the bit to be extracted in a variable, so the
compiler doesn't know to optimize it:

 long	SCU_BDID =	0x2000		/* bit 14 */

This has the advantage that it also works for bit sets.
-- 
Piet van Oostrum, Dept of Computer Science, University of Utrecht
Padualaan 14, P.O. Box 80.089, 3508 TB Utrecht, The Netherlands
Telephone: +31-30-531806. piet@cs.ruu.nl (mcvax!hp4nl!ruuinf!piet)