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

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Date:	Wed, 5 Mar 2008 13:30:54 +0100
From:	"Bart Van Assche" <bart.vanassche@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
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 &

On Wed, Mar 5, 2008 at 1:20 PM, Ingo Molnar <mingo@...e.hu> wrote:
>
>  * Julia Lawall <julia@...u.dk> wrote:
>
>  > There are some legitimate uses of !x & y which are actually of the
>  > form !x & !y, where x and y are function calls.  That is a not
>  > particularly elegant way of getting both x and y to be evaluated and
>  > then combining the results using "and".  If such code is considered
>  > acceptable, then perhaps the sparse patch should be more complicated.
>
>  i tend to be of the opinion that the details in C source code should be
>  visually obvious and should be heavily simplified down from what is
>  'possible' language-wise - with most deviations and complications that
>  depart from convention considered an error. I'd consider "!fn1() &
>  !fn2()" a borderline coding style violation in any case - and it costs
>  nothing to change it to "!fn1() && !fn2()".

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.

Bart Van Assche.
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