Martin Simmons martin@lispworks.com writes:
On Mon, 28 Apr 2014 07:26:07 +0200, Willem Rein Oudshoorn said:
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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.
Kind regards, Wim Oudshoorn.