container_of

[Simon Brand]

Good evening,

i read the article about the container_of function:
http://www.kroah.com/log/linux/container_of.html

 I understand what it does and how it works.

I thought, it could be simplified by replacing it through this:

#define container_of(ptr, type, member) ({ \
                (type *)( (char *)ptr - offsetof(type,member) );})

Original:
#define container_of(ptr, type, member) ({ \
                const typeof( ((type *)0)->member ) *__mptr = (ptr); 
                (type *)( (char *)__mptr - offsetof(type,member) );})


ptr has the type of a pointer to the member, which should be the same
as __mptr? The value should although be the same.

First I tried it in a self written script, then replaced it in
include/linux/kernel.h and compiled it as usermode linux -> working
well.

Then I compiled it and run it in a VM, but it is not working.

Can you please explain to me, why the original version is always working
and "mine" is not? 

Thank you for your time!

Regards,
Simon