linux-kernel - Re: somebody dropped a (warning) bomb

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Message-ID: <Pine.LNX.4.64.0702081528070.8424@woody.linux-foundation.org>
Date:	Thu, 8 Feb 2007 15:37:38 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Jeff Garzik <jeff@...zik.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: somebody dropped a (warning) bomb

On Thu, 8 Feb 2007, David Rientjes wrote:
> 
> And a compiler that makes a_variable.flag unsigned would be brain-dead 
> because "int" is always signed.

No, making bitfields unsigned is actually usually a good idea. It allows 
you to often generate better code, and it actually tends to be what 
programmers _expect_. A lot of people seem to be surprised to hear that a 
one-bit bitfield actually often encodes -1/0, and not 0/1.

So unsigned bitfields are not only traditional K&R, they are also usually 
_faster_ (which is probably why they are traditional K&R - along with 
allowing "char" to be unsigned by default). Don't knock them.  It's much 
better to just remember that bitfields simply don't _have_ any standard 
sign unless you specify it explicitly, than saying "it should be signed 
because 'int' is signed".

I will actually argue that having signed bit-fields is almost always a 
bug, and that as a result you should _never_ use "int" at all. Especially 
as you might as well just write it as

	signed a:1;

if you really want a signed bitfield.

So I would reall yrecommend that you never use "int a:<bits>" AT ALL, 
because there really is never any good reason to do so. Do it as

	unsigned a:3;
	signed b:2;

but never 

	int c:4;

because the latter really isn't sensible.

"sparse" will actually complain about single-bit signed bitfields, and it 
found a number of cases where people used that "int x:1" kind of syntax. 

Just don't do it.

			Linus
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