[plt-scheme] Example of working with (lib "foreign.ss") with custodians

Probably a stupid question, but what is a custodian?

Cheers,
Alexandre


On Sep 29, 2005, at 5:06 AM, Danny Yoo wrote:

> Hi everyone,
>
> I thought this snippet of code might come in handy; I wrote some  
> code to
> see if I understood how to attach objects of a foreign origin to
> custodians.  I exercise the code by just opening a file with fopen,  
> then
> using custodian-shutdown-all, and watch with /usr/sbin/lsof to make  
> sure
> it really is closing things down.
>
> Some feedback would be useful; it would be great if something like  
> this
> were a part of foreign.ss, since it servers an analogous role to
> register-finalizer.
>
> Hope this helps!
>
>
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> ;; Example code of incorporating custodian-handled objects with
> ;; (lib "foreign.ss").
>
> (require (lib "foreign.ss"))
> (require #%foreign)
> (unsafe!)
>
> (define self-lib (ffi-lib #f))
>
> (define fopen
>   (get-ffi-obj "fopen" self-lib
>                (_fun _string _string -> _pointer)))
>
> (define fclose
>   (get-ffi-obj "fclose" self-lib
>                (_fun _pointer -> _int)))
>
>
> (define custodian-add-managed
>   (get-ffi-obj "scheme_add_managed" self-lib
>                (_fun _pointer _scheme _fpointer _pointer _int ->
> _pointer)))
>
>
> (define custodian-check-available
>   (get-ffi-obj "scheme_custodian_check_available" self-lib
>                (_fun _pointer _string _string -> _void)))
>
>
> (define (make-custodian-callback f)
>   (ffi-callback f (list _scheme _pointer) _pointer))
>
>
> (define (my-input-fopen name)
>   (let ((file (fopen "/etc/passwd" "r"))
>         (callback (make-custodian-callback
>                    (lambda (f _)
>                      (fclose f)
>                      #f))))
>     ;; Attach to the current custodian
>     (custodian-check-available #f "my-input-fopen" "file-stream")
>     (custodian-add-managed #f file callback #f 0)
>     file))
>
>
> (printf "open and shut and open and shut and ")
> (printf "give a little clap clap clap...~%")
> (let loop ()
>   (let ((custodian (make-custodian)))
>     (parameterize ((current-custodian custodian))
>       (my-input-fopen "/etc/passwd")
>       (custodian-shutdown-all (current-custodian))))
>   (loop))
>
>
>
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>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>

-- 
_,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:
Alexandre Bergel  http://www.iam.unibe.ch/~bergel
^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.