On Fri, Jan 27, 2012 at 10:10:21PM +0000, Alexander Nasonov wrote:
> Mindaugas Rasiukevicius wrote:
> > + pcq = kmem_zalloc(offsetof(pcq_t, pcq_items[nitems]), kmflags);
>
> I'm not sure that this is a valid use of offsetof. C99 (7.17/3) defines
> it as
>
> offsetof(type, member-designator)
>
> I don't see a definition of member-designator, but the designator is
> defined as
>
> designator:
> [ constant-expression ]
> . identifier
>
> nitems in pcq_items[nitems] is not a constant expression.
At least some versions of gcc complain when the result of offsetof()
isn't a compile time constant.
IIRC something in the standard requires the result to be a compile time
constant - so that it can be used to initialise static data [1].
I suspect uses like the above were not thought of when the standard
was written!
You can probably use:
#define offsetof_array(type, member, index) \
(offsetof(type, member[0]) + (index) * sizeof ((type *)0)->member[0])
But I can't think of a way that doesn't involve casting a pointer.
David
[1] One common compiler doesn't treat 'offsetof(s, a[10])/4' as a
compile time constant.
--
David Laight: da...@l8s.co.uk
