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Re: foreign objects and the garbage collector
From: |
Olivier Dion |
Subject: |
Re: foreign objects and the garbage collector |
Date: |
Tue, 21 Sep 2021 10:25:47 -0400 |
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.
--
Olivier Dion
Polymtl