Re: smob mark functions in 2.0

[Ludovic Courtès]

Hello!

Andy Wingo <address@hidden> skribis:

> 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.                                              */

Arrgh!

> So, your mark function might see freed objects.  This is terrible, but
> it is the way that it is.  The key is that, if you touch a Scheme object
> in your mark function, to first do a check on that object, to see that
> it is valid.  You can check the TC bits of the first word, or otherwise
> check that other words are non-NULL.

What about making that check in libguile before invoking the user’s mark
function?

Thanks,
Ludo’.