Re: smob mark functions in 2.0

[Mark H Weaver]

Andy Wingo <address@hidden> writes:
> Specifically, there is a warning in gc/gc_mark.h:
>
>     /* WARNING: Such a mark procedure may be invoked on an unused object    */
>     /* residing on a free list.  Such objects are cleared, except for a     */
>     /* free list link field in the first word.  Thus mark procedures may    */
>     /* not count on the presence of a type descriptor, and must handle this */
>     /* case correctly somehow.                                              */
>
> So, your mark function might see freed objects.

How can this happen?  If you are marking an object, then presumably it
is still reachable, and therefore the objects it references are also
still reachable.  If any of those reachable objects has been freed,
isn't that already a bug of a different kind?  What am I missing?

   Thanks,
     Mark