Hi Ricard,
Thank you for your answer. My problem was my incorrect (?) understanding
of "s->foo". I thought it being equivalent to "(*s).foo" rather than a
direct offset calculation.
Do you have any references for me? I could not find anything in the
ANSI-C standard.
Regards,
Hendrik
Ricard Wanderlof wrote:
On Wed, 4 Oct 2006, Hendrik Post wrote:
While studying the Infiniband drivers if found an interesting line of
code within the preprocessed files from kernel 2.6.14.1:
// 85 "drivers/infiniband/core/mad.c"
entry = (struct ib_mad_port_private *) ((char *)__mptr - (unsigned
int) (&((struct ib_mad_port_private *)0) ->port_list) );
This line is the result of a macro expansion from list.h. I wonder why
it is safe to dereference the NULL-pointer in the second part of the
right side:
(& ((struct ib_mad_port_private *)0) ->port_list) )
The NULL is never dereferenced. (Note that -> has higher priority than &).
I.e. you take the address of
((struct ib_mad_port_private *)0) ->port_list
which is simply the offset of port_list in the struct. The 0 is never
deferenced, because the (supposed) value at port_list is never used,
only its address.
/Ricard
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Ricard Wolf Wanderlöf ricardw(at)axis.com
Axis Communications AB, Lund, Sweden www.axis.com
Phone +46 46 272 2016 Fax +46 46 13 61 30
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