🚀 go-pugleaf

Richard Krehbiel (rich@kastle.com) wrote:
: 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)))

Er... That's really quite clever. Keep the old version around
in case you hit a compiler that doens't agree.

The '==' makes sure the pointer types agree, but why the
sizeof?

I hope you put _lots_ of comments above that macro! I just
hate it when I am trying to understand someone else's program,
and I hit a macro that looks like it came straight out of
the International Obfuscated C Coding Competition.

--
Erik Corry ehcorry@inet.uni-c.dk