🚀 go-pugleaf

I wrote:

>Well, actually I'd probably use a macro that I've declared for my own
>use...

>#define CONTAINER_P(p,s,m) (s*)((char *)(p)-offsetof(s,m))

>...which, for pointer p which is assumed to be a pointer to member m
>of aggregate s, gives a pointer to s.  There's a slight bit more type
>checking than the simple cast (I'd love it if I could also be assured
                                --------------------------------------
>that p is really the same type as &s.m).  If the member is first,
 --------------------------------------
>typically the subtraction is optimized away since the offset is a
>constant zero.

I've tested the following and it does just what I want:

#define CONTAINER_PTR(p,s,m)  \
  ((sizeof(&((s *)0)->m == (p))), \
   (s *) ((char *)(p) - offsetof(s,m)))

--
Richard Krehbiel, Kastle Systems, Arlington VA USA
rich@kastle.com (work) or richk@cais.com (personal)