Re: VM code question

[Nick Piggin]

William Lee Irwin III wrote:

> On Tue, Oct 14, 2003 at 11:32:27AM +1000, Darren Williams wrote:
>
> > I have a small question wrt some VM code.
> > source file is include/linux/kernel.h
> > #define container_of(ptr, type, member) ({                      \
> >        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
> >        (type *)( (char *)__mptr - offsetof(type,member) );})
> > what is the use of the 0 (zero) in the typeof? I am thinking
> > that we are casting 0 to (type *) then referencing 'member' of
> > 'type', however why do we require the 0 ?
> > Just curious
>
> It's an address calculation method. We subtract the address of the
> start of the structure from the address of the member inside the
> structure.

AFAIKS the 0 is not part of the address calculation method though. It
is only used in the argument to the typeof operator. I think 0 is used
simply because its as good a place as any, right?


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