[cffi-devel] Re: CFFI defcallback closures?

[Nikodemus Siivola]

"Hoehle, Joerg-Cyril" <Joerg-Cyril.Hoehle at t-systems.com> writes:

> I'm curious to know how precisely SBCL handles the details.
>
> As far as CLISP is concerned, the second example works (if converted to CLISP FFI syntax), but will leak malloc()'ed memory if it's never deallocated.

>From memory: each callback gets allocated separately, and will never
be reclaimed (currently). Each alien-lambda closure gets its own
callback.

> That is, if your application does not provide for a means to receive
> the callback address from the alien world, so that the Lisp side can
> free it, then your application leaks memory.

Right.

> This tells us how a lambda callback ought to be designed:
>  + can create one,
>  + be able to keep its handle on the Lisp side,
>  + to be able to destroy it
>    - via garbage collection
>    - and/or via stack semantics
>    - from the C side(?)
> CLISP's FFI does not directly fir the bill.  You need a little wrapper code.
> I haven't looked at how CFFI fits these requirements.

I'm not sure I agree. I think the best we can do for callbacks
(anonymous or not) is invalidate them: switch the C->Lisp tramp to
point to an error-signalling Lisp function, after which the original
callback trampoline can be freed (but then we need to lock the
trampoline in order not to yank it from beneath another thread -- or
do something really clever).

That way any C-function that still happens to have the address we gave it
will "act reasonably", instead of landing at a random location -- possibly
a new and totally different callback.

I would not like callbacks to ever be released by default, nor from
the C-side.

> Second, you say "expensive".  What do you mean?
> o The allocation of memory for the callback structure?

The allocation and contruction of the callback trampoline. There are a couple
of shortcuts that could be taken to make it significantly more efficient,
but they have not been implemented yet.

Cheers,

  -- Nikodemus              Schemer: "Buddha is small, clean, and serious."
                   Lispnik: "Buddha is big, has hairy armpits, and laughs."