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

Estevo euccastro at gmail.com
Tue Dec 24 14:48:47 EST 2013

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

(Heads up: while the file has extension .tar.gz, it's a non gzipped

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

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.

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:


[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.
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