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 13:35:29 +0100 (MET)
From:	Julia Lawall <julia@...u.dk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	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, 5 Mar 2008, Ingo Molnar 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()".

!fn1() && !fn2() does not have the same semantics as !fn1() & !fn2().  In
!fn1() & !fn2() both function calls are evaluated.  In !fn1() && !fn2(),
if !fn1() returns false then !fn2() is not evaluated.  I haven't studied
the particular instances of fn2(), though, to know whether it makes a
difference.

One could instead do something like:

x = fn1();
y = fn2();
if (!x && !y) ...

It would certainly be clearer, but more verbose.

julia

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