linux-kernel - Re: double exclamation (!!) suckage in the kernel

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Message-ID: <20070526165009.GT4095@ftp.linux.org.uk>
Date:	Sat, 26 May 2007 17:50:09 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Bodo Eggert <7eggert@....de>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	David Miller <davem@...emloft.net>, satyam.sharma@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: double exclamation (!!) suckage in the kernel

On Sat, May 26, 2007 at 06:38:07PM +0200, Bodo Eggert wrote:
> Not exactly, if(foo) is the same as if( (int) foo), which is not
> guaranteed to result in non-null values for non-null pointers.

RTFStandard.

> ISO 9899/1999 says: "Any pointer may be converted to an integer type. [...]
>          The result need not be in the range of values of any integer type."

... and it sure as hell does *NOT* say that controlling expression of
if statement is converted to an integer type.  For pointers of for anything
else.  if (v) is the same as if (v != 0) [6.8.4.1p2], and if v is a pointer,
0 in that comparison becomes a null pointer constant [6.5.9p5].
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