linux-kernel - Re: container

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Message-ID: <20130227191455.GB27264@arwen.pp.htv.fi>
Date:	Wed, 27 Feb 2013 21:14:55 +0200
From:	Felipe Balbi <balbi@...com>
To:	Felipe Balbi <balbi@...com>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	Luciano Coelho <coelho@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: container_of() doubt

Hi again,

On Wed, Feb 27, 2013 at 09:13:22PM +0200, Felipe Balbi wrote:
> Hi Greg,
> 
> hehe, you must've answered this over and over again, but I don't see the
> need for __mptr in the container_of() macro. I tried googling it but
> couldn't find much.
> 
> I get the macro, but not the need __mptr. Wouldn't this work the same
> way ?
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 80d3687..bbce857 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -717,8 +717,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
>   *
>   */
>  #define container_of(ptr, type, member) ({			\
> -	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
> -	(type *)( (char *)__mptr - offsetof(type,member) );})
> +	(type *)( (char *)(ptr) - offsetof(type,member) );})
>  
>  /* Trap pasters of __FUNCTION__ at compile-time */
>  #define __FUNCTION__ (__func__)
> 
> why do we need this type * NULL ?

nevermind, type safety ;-)

-- 
balbi

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