Re: foreign objects and the garbage collector

[Olivier Dion via General Guile related discussions <guile-user@gnu.org>]

On Sun, 19 Sep 2021, adriano <randomlooser@riseup.net> wrote:
>> If you want to avoid the problem, you should explicitely bind and
>> call the gps-close function and not rely on the garbage collector to
>> do it for you. You can use dynamic-wind to open and close
>> resources as needed.
>
> 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 ()))

>
>> That being said, make-pointer (from (system foreign-library)) is
>> probably what you are expecting. It should work with gps_close.
>
> Another exmple of what you mean, here, would be so nice :-)

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)))))
                                
Note that you probably need to do a wrapper named `open-my-resource` to
do the conversion of scm_string to C raw pointer before calling
`open-my-resource%` in this particular case.

This is just an example, but it shows you that you can call foreign C
primitives easily without any C code, and that you have to use a
dynamic context to manage the lifetime of the C resources.

-- 
Olivier Dion
Polymtl