[erlang-questions] NIF resources, environments and GC

[Daniel Goertzen]

This is exactly the weak reference/weak pointer pattern.  I don't see any
quick shortcuts for you here.

If you are using c++, your resources can represent shared_ptr and your
stored static variable can be a weak_ptr.  Rust has similar types.
Unfortunately you would also need to write a NIF for comparing the
underlying referred object.




On Tue, Dec 19, 2017 at 9:03 AM Loïc Hoguin <essen@REDACTED> wrote:

> Hello,
>
> I've got the following situation.
>
> I have a function SDL_CreateWindow that returns an SDL_Window* from
> which I create a resource with a destructor which is called when the
> resource variable gets GC'd. So far so good.
>
> I also have various other functions which may return the same
> SDL_Window* as the one that was created earlier. In this case I want to
> return the same resource so that it is possible to match or compare the
> resources on the latest Erlang versions so that users can confirm that
> yes, it's the same window. This is useful to know for example to know
> which window currently has focus.
>
> I therefore need to keep the ERL_NIF_TERM around along with SDL_Window*
> since recreating the term would lead to different references and
> comparisons failing. I've therefore proceded to use enif_make_copy to
> copy it to a static variable and I do enif_make_copy again when I need
> to return it.
>
> It works.
>
> Almost.
>
> The only issue with my approach is that the destructor for the
> SDL_Window* resource no longer gets called. Apparently enif_make_copy is
> the culprit. As soon as I copy the resource term to my private env, the
> destructor stops being called.
>
> Is there any way I can keep this resource term while still benefitting
> from the destructor being called when the variable is discarded?
> Basically I would like the private copy to not be reference counted. Any
> way to do this?
>
> Thanks,
>
> --
> Loïc Hoguin
> https://ninenines.eu
> _______________________________________________
> erlang-questions mailing list
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> http://erlang.org/mailman/listinfo/erlang-questions
>
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