Elias Mårtenson <lokedhs@gmail.com> schrieb am Fr., 3. Feb. 2017 um
03:56 Uhr:

> I'm currently building an Emacs module to provide access to GSSAPI. In it,
> I have to use the make_user_ptr() function in a few places. When creating a
> user_ptr, the function accepts a pointer to a function which will be called
> when the object is GC'ed.
>
> The problem is that the free function is only called with a single
> argument; the pointer itself. In my case, the underlying GSSAPI function
> that releases the object may return some diagnostics, and I would like to
> be able to call the Elisp function ‘warn’ with a message describing what
> happened.
>
> I think that the callback should accept 3 arguments instead of 1. In
> addition to the pointer itself, the ‘emacs_env’ pointer is needed. An
> arbitrary ‘void *data’ pointer would be useful too.
>

This would require two things:
- It would need to be possible to run arbitrary Lisp code during GC. I
don't think that's the case right now.
- It would require a second type of user pointer because the existing one
is already published.

The second requirement wouldn't be a big deal from a technical perspective,
but would make the interface more complex, and I don't know whether it's a
good tradeoff just to show a warning. The first requirement would likely be
a deal-breaker.