Re: foreign objects and the garbage collector

[adriano <randomlooser@riseup.net>]

Il giorno mar, 21/09/2021 alle 10.25 -0400, Olivier Dion ha scritto:
> On Tue, 21 Sep 2021, adriano <randomlooser@riseup.net> wrote:
> > Hi Olivier,
> > 
> > thank you very much for your reply
> > 
> > Il giorno dom, 19/09/2021 alle 14.11 -0400, Olivier Dion ha
> > scritto:
> > > On Sun, 19 Sep 2021, adriano <randomlooser@riseup.net> wrote:
> > > > 
> > > > 
> > 
> > 
> > 
> > 
> > > > It'd be so nice to have an example
> > > 
> > > (define (with-my-resource token proc)
> > >   (let ((resource #f))
> > >     (dynamic-wind
> > >       (lambda ()
> > >         (set! resource (open-my-resource% token)))
> > > 
> > >       (proc resource)
> > > 
> > >       (lambda ()
> > >         (when resource
> > >           (close-my-resource% resource))))))
> > > 
> > > (with-my-resource "some-internal-token" (lambda ()))
> > 
> > Oh my, thank you for this !
> > 
> > This should be included in the manual !
> > 
> > The example that's there currently is totally indequate, in my
> > opinion
> > 
> > > > 
> > > 
> > > Says you have `open_my_resource()` and `close_my_resource()` in C
> > > in
> > > library "libfoo.so" where open_my_resource takes a C string and
> > > returns
> > > an integer for the resource while close_my_resource takes the
> > > integer
> > > of
> > > the resource:
> > > 
> > > (define open-my-resource%
> > >   (eval-when (eval load compile)
> > >     (let ((this-lib (load-foreign-library "libfoo")))
> > >       (foreign-library-function this-lib "open_my_resource"
> > >                                 #:return-type int
> > >                                 #:arg-types (list '*))))
> > > 
> > > (define open-my-resource%
> > >   (eval-when (eval load compile)
> > >     (let ((this-lib (load-foreign-library "libfoo")))
> > >       (foreign-library-function this-lib "open_my_resource"
> > >                                 #:return-type int
> > >                                 #:arg-types (list int)))))
> > 
> > Uhmm... I see 2 versions of open-my-resource%
> > The only slight difference I see is in the #:arg-types
> > 
> > The first one has 
> > 
> > (list '*) 
> > 
> > and the second one has 
> > 
> > (list int)
> > 
> > 
> > Maybe you you got confused while editing ?
> 
> You're right.  I copy paste the form two times and forget to edit the
> second
> binding name.  I meant `close-my-resource%` for the second define.
> 
> > 
> > Thank you again
> 
> Just make sure to read what Maxime says about escape continuation.
> Dynamic wind are not bullet proof.
> 
> As the manual says:
> 
>         If, any time during the execution of THUNK, the dynamic
> extent of
>         the ‘dynamic-wind’ expression is escaped non-locally,
> OUT_GUARD is
>         called.  If the dynamic extent of the dynamic-wind is re-
> entered,
>         IN_GUARD is called.  Thus IN_GUARD and OUT_GUARD may be
> called any
>         number of times.
> 
> Thus, you could also do a `(set! resource #f)` after freeing it, so
> that
> the overall dynamic-wind is reentrant and you don't end up with
> double
> free of resource.
> 

After seeing your example of usage of dinamic-wind, I understand what
IN_GUARD and OUT_GUARD are supposed to be

Before that it was quite nebulous

Thanks again