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Re: foreign objects and the garbage collector
From: |
adriano |
Subject: |
Re: foreign objects and the garbage collector |
Date: |
Tue, 21 Sep 2021 16:51:30 +0200 |
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