linux-kernel - Re: Why is the kfree() argument const?

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Message-ID: <20080118145343.GR25527@unthought.net>
Date:	Fri, 18 Jan 2008 15:53:44 +0100
From:	Jakob Oestergaard <jakob@...hought.net>
To:	Björn Steinbrink <B.Steinbrink@....de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Schwartz <davids@...master.com>,
	Johannes Weiner <hannes@...urebad.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	clameter@....com, penberg@...helsinki.fi
Subject: Re: Why is the kfree() argument const?

On Fri, Jan 18, 2008 at 02:31:16PM +0100, Björn Steinbrink wrote:
...
> 
> Do you see anything that casts the const away? No? Me neither. Still,
> the memory that p points to was changed, because there was another
> pointer and that was not const.

*another* being key here.

> 
> > *That* is the purpose of const pointers.
> 
> The only thing that const can tell you is that you should not modify the
> value _yourself_, using that pointer _directly_.

Which is pretty damn useful.

Think about it.  Don't you ever use const?  Is it ever only in the way?

...
{snip long explanation about how one can avoid the benefits of const, without
using casts}
...
> If you want to restrict the set of pointers that can be invalidated by
> an other pointer, you'll have to use something else because const does
> not talk about invalidating aliasing pointers.

Precisely, so why are we discussing this?

I claim that const is useful. You claim that it can't solve all the worlds
problems. I agree with that, but I maintain it is still useful.

But, in order for it to be useful, it requires that people do not circumvent it
in the wrong places (such as kfree).

-- 

 / jakob

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