>>>>> On Tue, 29 Apr 2014 07:20:08 +0200, Willem Rein Oudshoorn said:
> Cancel-Lock: sha1:p0/p2S+buMdADzjMLfHl6LN5m6k=
>
> Martin Simmons <
martin@lispworks.com> writes:
>
> >>>>>> On Mon, 28 Apr 2014 07:26:07 +0200, Willem Rein Oudshoorn said:
> >> Cancel-Lock: sha1:zqpk08tO/PaCrz3PLTlI+ocAF9E=
> >>
> >> Joeish W <
joeish80829@yahoo.com> writes:
> >>
> >> > How do I work with the metaobjects(is that what they are called)
> >> > output by the code you gave me...If you can show me how to mem-aref
> >> > one I would really appreciate it
> >>
> >> Sorry for the short answer, but at the moment I am extremely busy.
> >> If you followed the example
> >>
> >> (mem-aref (c-pointer YOUR-POINT) ...)
> >>
> >> should work.
> >
> > It might be interesting (or scary depending on your point of view) to consider
> > what happens if the variable YOUR-POINT is the last reference to the CLOS
> > object here...the finalizer might free the foreign object before mem-aref is
> > entered if the compiler no longer keeps a pointer to the variable.
> >
> No this is not safe. In general you should never do
> `(c-pointer ...)` outside the low level parts of the bindings, and use
> it very carefully.
>
> Personally I would try to never use the `c-pointer` method outside
> the `translate-to-foreign` code. And I naively expected that this
> would be safe. But as you point out:
>
> > Or more specifically, can this ever be safe?
> >
> > (mem-aref (c-pointer (point0)) ...)
>
> This is never safe.
>
> I think the easiest fix is to change the
>
> (defmethod translate-to-foreign ((lisp-value cv-matrix) (c-type cv-mat))
> (c-pointer lisp-value))
>
> code to [UNTESTED CODE]:
>
>
> (defmethod translate-to-foreign ((lisp-value cv-matrix) (c-type cv-mat))
> (values (c-pointer lisp-value) lisp-value)
>
>
> This should keep the lisp-value around until we are done using the
> `(c-pointer ...)` value. Provided of course you are not mucking around
> with the `c-pointer` method yourself.
>
>
> In general, I would advocate to hide all the nasty c-pointer business in
> the translate methods and never deal with it outside that limited scope.
>
>
> Thank you for bringing this to my attention. I do think we need a paper
> or section in the manual on how to deal with combing GC in Lisp and
> manual memory management on the C side.
>
> When my time frees up (hopefully in a month or two) I might take a stab
> at a first draft.
IMHO, it needs to come with a huge security warning, otherwise there will be
endless use-after-free bugs when users lose the wrapper object after storing
the pointer somewhere.