Your approach may work for this case but I wouldn’t call it best practice. You can’t (in general) copy a ___MOVABLE closure to a ___STILL closure with ##still-copy because a closure may contain a circular reference that will not be copied properly by the shallow ##still-copy. For example :

(define (foo x) (letrec ((bar (lambda (y) (list x y bar)))) bar)) (define clo (foo 42)) (eq? clo (##closure-ref clo 2))

#t

This shows that the closure’s structure contains a cycle in its environment.

The best practice is to store the reference to the closure in a still object, such as a still box. It is also a good idea to increment this still object's reference count so that the GC knows that “the C world” has a pointer to that still object (in case the only way to reach the object is from C). The reference count should be decremented when C no longer has a reference to the object. This refcount management can be avoided if you can guarantee that the C world will only ever use the object while the Scheme world refers to the object.

Marc

On Apr 27, 2021, at 4:09 PM, Phillip Suero philsuero@gmail.com wrote:

Dear Marc,

I ran this code:

(c-declare "___SCMOBJ storage;")

(define store! (c-lambda (scheme-object) void "storage = ___arg1;"))

(define get (c-lambda () scheme-object "___return(storage);"))

(define (gen-c) (let ((abc #f)) (##still-copy (lambda (v) (define o v) (set! abc v) o))))

(define gen-i (eval '(lambda () (let ((abc #f)) (##still-copy (lambda (v) (define o v) (set! abc v) o))))))

To test the hypothesis that a ____STILL closure can be successfully passed as a ___SCMOBJ from Scheme to C, stay in C for a while and then be moved back to Scheme anytime.

A normal closure will not work, since it changes memory address. However, thanks to ##still-copy , I can get a closure that has a fixed memory address all up to collection, and such a closure's ___SCMOBJ can then be kept in C for as long as desired.

Also, this ___STILL closure will still be collected, so there is no memory leak problem.

The code above outputs:

...

(load "test")

"/path/to/test.o1"

...

(define x (gen-c)) (print x " ") (store! x) (##gc) (print (get) "\n") #<procedure #2 x> > > > #<procedure #2 x>

(define x (gen-i)) (print x " ") (store! x) (##gc) (print (get) "\n")

...

#<procedure #3 x> > > > #<procedure #3 x>

The only operational requirement for the transfer of a ___SCMOBJ from C to Scheme is that the object is still alive. In the example code above, that is guaranteed by that a strong reference to the closure is kept in |x| up to that (get) has made the transfer from C to Scheme, which then produces a new strong reference to the closure, which is sufficient in itself.

Example to illustrate clean transfer of the strong reference:

(define x (gen-c)) ; Generate closure

(print x " ")

(store! x) ; Store closure in C world, however still keep strong reference in |x|

(##gc) ; All non-___STILL/___FIXED allocated objects will change ___SCMOBJ, thus invalidating them

(define y (get)) ; We pick up the ___SCMOBJ from the C world and assign it to |y|. |x| above must be maintained as a strong reference up to the completion of this step.

; |x| is no longer needed for (get) to work, here, since we have a separate strong reference to the object now, in |y|.

(set! x #f) (##gc)

; All fine.

(print y "\n")

The above example breaks, if the (##still-copy part is removed, in which case the closure will be ___MOVABLE, the ___SCMOBJ will be invalidated at GC, and therefore accessing the result of (get) produces SIGSEGV or similar unknown behavior, in my test it would print out #<unknown>.

Marc please confirm that what I do above here is a best practice and is all correct.

Phil _______________________________________________ Gambit-list mailing list Gambit-list@iro.umontreal.ca https://mailman.iro.umontreal.ca/cgi-bin/mailman/listinfo/gambit-list