At Sat, 17 Jul 2010 15:18:44 +0200, Thomas Chust wrote: > 2010/7/17 gabor papp <[email protected]>: > > [...] > > What I'm trying to do is passing a boxed variable to the FFI module, and > > when the C library changes the variable it calls the set callback, in which > > I set the content of the box. > > > > I would like to pass the box through the client-data pointer, so the > > callback can do this, but I'm not sure how to do it. Is it possible to > > achieve something like this with FFI? > > [...] > > Hello Gabor, > > nearly everything is possible ;-) > > I think you should consider encapsulating the type of values that can > be passed around and the box to store them in the closure of the > getter and setter callbacks, which may be much more convenient than > creating a C data structure that can hold this information.
I agree. The main problem with passing a Racket box as the `client-data' is that the GC cannot see it. So, you'd have to use an immobile cell created with `malloc-immobile-cell' and explicitly free it `free-immobile-cell'. It's simpler to use a closure and ignore the `client-data' argument at the C level. That is, the foreign library supports `client-data' to effectively implement closures, but you already have closures in Racket. So just pass `#f' for `client-data' and ignore the second argument in the callback, and use a function that closure over whatever needs to be set. A catch with the closure approach is that you have to make sure that the closure is not GC'ed while it is registered with the foreign library. If you just have a few callbacks that are `define'd at a module top level (as in Thomas's code), then it's not problem, since the callbacks won't be GC'ed. If there's no particular reason that the callabcks avoid GC, though, see the `#:keep' argument of `_fun' and `_closure' for some other options. _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
