The data is user-defined so that's why it might change. There's a small bug in the second part of your example but I get the idea. Using ptr-set! directly gives me the same problem with the garbage collector, probably because it isn't pointing to an immobile cell anymore. Wrapping it up with a 'malloc-immobile-cell' makes everything work. Here's a full example if anyone's interested:
(define (cpBodyGetData cpBody) ; get the contents of the old cell (ptr-ref (cpBody-data cpBody)) (define (cpBodySetData cpBody val) ; change it to a new cell (free-immobile-cell (cpBody-data cpBody)) (set-cpBody-data! cpBody (malloc-immobile-cell val))) This of course requires that an initial (empty) cell is set when make-cpBody is called, since no predicate is available to test the data pointer. Thanks again, Sam -----Oorspronkelijk bericht----- Van: Matthew Flatt [mailto:[email protected]] Verzonden: maandag 4 maart 2013 18:26 Aan: Sam Vervaeck CC: 'users' Onderwerp: Re: [racket] FFI Pointing to racket objects At Mon, 4 Mar 2013 18:06:10 +0100, "Sam Vervaeck" wrote: > Just one more question: when implementing malloc-immobile-cell and > free-immobile-cell, how do you check if a pointer has not yet been > set? Doing things like: > > (if (cpBody-data body1) > "I have some data" > "I have nothing") > > Always shows "I have nothing". I expected that the `data' field would have an immutable-cell value --- which might contain #f but not be #f itself --- so I expected "I have some data" always. You should use `ptr-ref' and `pre-set!' to get or change the value in an immobile cell. Here's an example transcript: > (define-cstruct _cpBody ([data _racket])) > (define b (make-cpBody (malloc-immobile-cell #f))) > (cpBody-data b) #<cpointer> > (ptr-ref (cpBody-data b) _racket) #f > (ptr-set! (cpBody-data b) _racket 'something) > (collect-garbage) > (ptr-ref (cpBody-data b) _racket) 'something ____________________ Racket Users list: http://lists.racket-lang.org/users
