linux-kernel - void* arithmnetic

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Message-ID: <20071129010531.22b3a71f@werewolf>
Date:	Thu, 29 Nov 2007 01:05:31 +0100
From:	"J.A. Magallón" <jamagallon@....com>
To:	"Linux-Kernel, " <linux-kernel@...r.kernel.org>
Subject: void* arithmnetic

Hi all...

Since begin of the ages the build of the nvidia driver says things like
this:

include/asm/compat.h:210: warning: pointer of type 'void *' used in arithmetic

There are several of this warnings. The code in question for this example
is:

static __inline__ void __user *compat_alloc_user_space(long len)
{
    struct pt_regs *regs = task_pt_regs(current);
    return (void __user *)regs->rsp - len;
}

As this is dealing with mem blocks, I suppose it's counting in bytes, so
we could do something like:

   return (void __user *)((u8*)regs->rsp - len);

so the arithmetic knows how to inc/dec for each unity...
I think the warning is correct and that void* arithmetic is undefined in C,
isn't it ?

TIA

--
J.A. Magallon <jamagallon()ono!com>     \               Software is like sex:
                                         \         It's better when it's free
Mandriva Linux release 2008.1 (Cooker) for i586
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