Message-ID: <407F1769.9090700@eyal.emu.id.au>
Date: Fri, 16 Apr 2004 09:14:49 +1000
From: Eyal Lebedinsky <eyal@eyal.emu.id.au>
Organization: Eyal at Home
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To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>,
       linux-kernel list <linux-kernel@vger.kernel.org>
Subject: Re: Linux 2.4.26-rc2
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Marcelo Tosatti wrote:
> The void pointer case in here its being done math on without any problem. What is the
> problem with void pointer math 

There is a problem regarding the C standard. The semantics of 'void *' are well defined
and only allow for limited use. Basically, you can cast any pointer to and from 'void *',
but nothing else.

Pointer math says that 'p+n' means "add to 'p' the value 'n*s' where 's' is the size
on the element that 'p' points to". A 'void *' does not have a defined element size
until it is cast. So, ANSI specifically does not allow any arithmetics on 'void *'.

Some compilers are forgiving and will invent an element size of '1' and allow the
math. We should not rely on such improper usage.

>> Maybe a cast is
>>called for in bh_kmap(), like:
>>	return (char *)kmap(bh->b_page) + bh_offset(bh);
> 
> 
> Hum, that would fix the warning but I dont see much reasoning behind it. 

It will not simply 'fix' the problem, this is one case where a cast is not
a bad thing (i.e. a cheat) but the correct thing to do.

--
Eyal Lebedinsky (eyal@eyal.emu.id.au) <http://samba.org/eyal/>
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