linux-kernel - Re: [PATCH 6/9] drivers/net/wireless/iwlwifi/iwl-4965.c: Correct use of ! and &

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Message-ID: <20080305123641.GA2365@elte.hu>
Date:	Wed, 5 Mar 2008 13:36:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Bart Van Assche <bart.vanassche@...il.com>
Cc:	Julia Lawall <julia@...u.dk>, Christopher Li <sparse@...isli.org>,
	yi.zhu@...el.com, linux-wireless@...r.kernel.org,
	ipw3945-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Harvey Harrison <harvey.harrison@...il.com>,
	Alexander Viro <viro@....linux.org.uk>,
	linux-sparse@...r.kernel.org, Josh Triplett <josh@...edesktop.org>
Subject: Re: [PATCH 6/9] drivers/net/wireless/iwlwifi/iwl-4965.c: Correct
	use of ! and &


* Bart Van Assche <bart.vanassche@...il.com> wrote:

> If someone writes (!x & !y) instead of (!x && !y) because both x and y 
> have to be evaluated, this means that both x and y have side effects.
> Please keep in mind that the C language does not specify whether x or 
> y has to be evaluated first, so if x and y have to be evaluated in 
> that order, an expression like (!x & !y) can be the cause of very 
> subtle bugs. I prefer readability above brevity.

such expressions _must_ be written as:

  ret1 = x();
  ret2 = y();

  if (ret1 && ret2)
	...

any side-effects are totally un-obvious when they are in expressions and 
someone doing cleanups later on could easily change the '&' to '&&' and 
introduce a bug.

	Ingo
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