linux-kernel - container

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Message-ID: <20130227191322.GA27264@arwen.pp.htv.fi>
Date:	Wed, 27 Feb 2013 21:13:22 +0200
From:	Felipe Balbi <balbi@...com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	Luciano Coelho <coelho@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: container_of() doubt

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 ?

thanks

-- 
balbi

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