linux-kernel - Re: container-of Implementation

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Message-ID: <20723.63603.114080.51605@pilspetsen.it.uu.se>
Date:	Mon, 14 Jan 2013 13:22:11 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Schrober <franzschrober@...oo.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: container-of Implementation

Schrober writes:
 > Hi,
 > 
 > I wondered why the container_of implementation is so complicated.
 > 
 > #define container_of(ptr, type, member) ({			\
 > 	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
 > 	(type *)( (char *)__mptr - offsetof(type,member) );})
 > 
 > isn't the __mptr not unnecessary? Why not following version?
 > 
 > #define container_of(ptr, type, member) \
 >     ((type *)((char *)(ptr) - offsetof(type, member)))

Compile-time type checking.  The first version requires ptr to be
assignment-compatible with the type of the struct member, the second
version accepts random junk for ptr.
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