[plt-scheme] scheme_add_managed with FFI

Many thanks. With some experiment, it seems the following works:

#lang scheme

(require scheme/foreign)
(unsafe!)

(define add-managed
  (get-ffi-obj "scheme_add_managed" #f
               (_fun (_pointer = #f) _scheme (_fun #:keep (box null) 
_scheme _pointer -> _void)
                     _pointer _bool -> _pointer)))

(define (make-p)
  (let ((p (malloc _pointer 'raw)))
    (add-managed p (lambda (p _) (free p)) #f #f)
    p))

(define p1 (make-p))
(define p2 (make-p))

(ptr-ref p1 _int)
(ptr-ref p2 _int)

(collect-garbage)

But I didn't store the (lambda (p _) (free p)) or the box anywhere. Is 
it really needed?

Chongkai


Matthew Flatt wrote:
> Thanks for the concrete example in PR 10538.
>
> The key feature of that example is that a single Scheme function is
> registered as a callback twice. The default handling of callbacks is
> that only the most recently generated foreign callback is retained with
> the Scheme function used as a callback. So, the second call to
> scheme_add_managed with the same function allows the foreign callback
> generated by the first call to be GCed.
>
> The simplest solution is to provides a boxed empty list for `#:keep' in
> the callback type:
>
>   (_fun #:keep (box null) _scheme _pointer -> _void)
>
>
>