linux-kernel - Re: [PATCH 1/3] arch/x86: Use offsetof

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Date:	Wed, 26 Dec 2007 20:47:18 +0100
From:	Pavel Machek <pavel@....cz>
To:	Julia Lawall <julia@...u.dk>
Cc:	tglx@...utronix.de, viro@....linux.org.uk,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 1/3] arch/x86: Use offsetof

Hi!

> From: Julia Lawall <julia@...u.dk>
> 
> In the patch cc154ac64aa8d3396b187f64cef01ce67f433324, Al Viro observed
> that the proper way to compute the distance between two structure fields is
> to use offsetof() or a cast to a pointer to character.  The same change can
> be applied to a few more files.
> 
> The change was made using the following semantic patch
> (http://www.emn.fr/x-info/coccinelle/)
> 
> // <smpl>
> @r@
> type T;
> T s;
> type T1, T2;
> identifier fld1, fld2;
> typedef uint8_t;
> typedef u8;
> @@
> 
> (
>   (char *)&s.fld1 - (char *)&s.fld2
> |
>   (uint8_t *)&s.fld1 - (uint8_t *)&s.fld2
> |
>   (u8 *)&s.fld1 - (u8 *)&s.fld2
> |
> - (T1)&s.fld1 - (T2)&s.fld2
> + offsetof(T,fld1) - offsetof(T,fld2)
> )
> // </smpl>
> 
> Signed-off-by: Julia Lawall <julia@...u.dk>
> ---
> 
> diff -u -p a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
> --- a/arch/x86/kernel/vm86_32.c	2007-10-22 11:25:00.000000000 +0200
> +++ b/arch/x86/kernel/vm86_32.c	2007-12-26 16:27:15.000000000 +0100
> @@ -215,7 +215,9 @@ asmlinkage int sys_vm86old(struct pt_reg
>  	ret = -EFAULT;
>  	if (tmp)
>  		goto out;
> -	memset(&info.vm86plus, 0, (int)&info.regs32 - (int)&info.vm86plus);
> +	memset(&info.vm86plus, 0,
> +	       offsetof(struct kernel_vm86_struct, regs32) -
> +	       offsetof(struct kernel_vm86_struct, vm86plus));

I do not think this makes it more readable... (int) -> (char *) would
make it portable and readable, AFAICT.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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