[gambit-list] Anyone else working in a schemey wrapper for c structs/unions/types?

Mikael mikael.rcv at gmail.com
Tue Dec 24 15:44:06 EST 2013

2013/12/24 Estevo <euccastro at gmail.com>

> Hi Frederick,
> If I'm not wrong you can take a similar approach to the one I took in
>> my OpenAL wrapper on the Dumping Grounds page of the Gambit Wiki
>> (http://dynamo.iro.umontreal.ca/wiki/images/2/2c/OpenAL.tar.gz). When
>> I allocate a foreign struct, I attach a will to the object and in the
>> will free the underlying C pointer when the foreign object is garbage
>> collected. The implementation is at the bottom of the file labelled
>> 'foreign-macros.scm' in that tarball.
> Thanks a lot for pointing me to this!  I think it will be of great help.
> If anyone knows about any other references to code by people that have
> worked at wrapping C structs/unions/types before I'd be very thankful to
> know.
> (Heads up: while the file has extension .tar.gz, it's a non gzipped
> tarball.)
> While I'll look into your approach in more depth, some preliminary
> comments:
> IIUC, the use case you talk about (handling a single struct in isolation)
> is handled extremely well by the default Gambit struct/union/type
> finalizer.  Even for arrays I'd just write a C finalizer that frees their
> memory; I explain below why.  I'm not sure why you are using wills here
> instead.
> In any event, I'm convinced that if a library relies, to trigger
> finalization, on wills on foreign objects that are exposed to code written
> by others, then the library code can never really know for sure when the
> object will never be reachable again.  Consider:
> > (define o (get-foreign-wrapper ...))  ; a will has been created for o,
> which will delete the underlying C data
> > (define t (make-table weak-values: #t))
> > (table-set! t 'o o)
> > (set! o #f)
> > (##gc)
> At this point your foreign wrapper is not strongly reachable anymore.  The
> garbage collection has triggered your will, which has freed the C object
> wrapped by o.  But you can still
> > (println (table-ref t 'o))
> And get an access to invalid memory or a segmentation fault.

Wait, what is returned by get-foreign-wrapper and are other references to
it sticking around at that GC?

> Note that the Gambit's ffi finalizers don't have this problem, since they
> only get called when your object is actually reclaimed.  Nobody can attach
> new finalizers to your foreign objects, so at the time they get called you
> know that nobody else will be messing with those[1].  For more discussion
> on this, see:
> https://mercure.iro.umontreal.ca/pipermail/gambit-list/2013-December/007322.html
> [1] Well, you can do all sort of things from C code, but that is true for
> vanilla Scheme objects too.  I'm concerned about users getting weird
> crashes by doing perfectly valid things in Scheme code.
> _______________________________________________
> Gambit-list mailing list
> Gambit-list at iro.umontreal.ca
> https://webmail.iro.umontreal.ca/mailman/listinfo/gambit-list
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.iro.umontreal.ca/pipermail/gambit-list/attachments/20131224/8146cc50/attachment.htm>

More information about the Gambit-list mailing list