linux-kernel - Re: [PATCH v5 6/9] bug.h: Prevent double evaulation of in BUILD_BUG

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Message-ID: <50ABD5B9.6020701@att.net>
Date:	Tue, 20 Nov 2012 13:10:49 -0600
From:	Daniel Santos <danielfsantos@....net>
To:	Borislav Petkov <bp@...en8.de>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 6/9] bug.h: Prevent double evaulation of in BUILD_BUG_ON

On 11/17/2012 08:38 AM, Borislav Petkov wrote:
> On Thu, Nov 15, 2012 at 01:11:15PM -0600, Daniel Santos wrote:
>> Ah yes. I did notice that at one point, but I think it slipped
>> my mind. Also, the kernel has introduced me to the usage of the
>> !! construct, of which I'm well versed in its affects in various
>> situations and how gcc's optimizer ends up treating its usage, so
>> probably another reason I didn't change it immediately. But it's
>> basically shorthand for the expression (condition ? 1 : 0), correct?

Oh, this is embarrassing, The sentence above should have read:

... the kernel has introduced me to the usage of the !! construct, of which I'm *not* well versed in ...

hah! oh well


> I don't think so: "!!" is simply a double negation which turns the
> whatever wild construct you have into either 0 or 1, depending on what
> it evaluates to.
>
> But I don't know what the standard says so you'll have to check :-)

Here we go, from section §6.5.3.3:

The result of the logical negation operator ! is 0 if the value of its
operand compares unequal to 0, 1 if the value of its operand compares
equal to 0. The result has type int. The expression !E is equivalent to
(0==E).

I simply wasn't aware that it promised a "true" value to be one
exactly.  Hurray for expanding horizons! :)

Daniel
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