Thanks, this is super helpful. :) On Monday, September 17, 2018 at 9:58:06 AM UTC-7, Matthew Flatt wrote: > > At Mon, 17 Sep 2018 09:34:46 -0700 (PDT), Stephen Foster wrote: > > But when I have two bodies that share the same velocity_func, I get a > > segfault: > > The problem is that a callback is retained (by default) as long as the > function that it wraps is retained, but only a single callback is > retained for a given function. So, creating a second callback loses the > link between the function and the first callback, and the first > callback can be is reclaimed by the GC. > > This is often a difficult problem to solve. > > > *Things that didn't work* > > > > 1) Wrapping the function f in separate lambdas still gives a segfault: > > > > (set-cpBody-velocity_func! (chipmunk-body body1) > > (lambda(b g d dt) > > (f b d g dt))) > > > > (set-cpBody-velocity_func! (chipmunk-body body2) > > (lambda(b g d dt) > > (f b d g dt))) > > This change makes things slightly worse, because nothing retains either > of those individual lambdas. > > > 2) Storing a reference to the function f in a stab-in-the-dark attempt > to > > "trick the system" or "prevent some kind of mysterious garbage > collection" > > still segfaults: > > > > (define probably-silly > > (list > > (lambda(b g d dt) > > (f b d g dt)) > > (lambda(b g d dt) > > (f b d g dt)))) > > > > (set-cpBody-velocity_func! (chipmunk-body body1) > > (list-ref probably-silly 0)) > > > > (set-cpBody-velocity_func! (chipmunk-body body2) > > (list-ref probably-silly 1)) > > Honestly, I'm not clear on why this fails. The `probably-silly` list > should be retained via the module's namespace, and so both of those > `lambda`s should be preserved, and so both callbacks should be > preserved. I'm stumped. > > > > 3) Storing the function f in two separate variables also segfaults: > > > > (define (f body gravity damping dt) > > (ffi:cpointer-push-tag! body 'cpBody) > > (cpBodySetVelocity body (cpv 0 -10)) > > > > ffi:_void) > > > > (define f2 f) > > This does nothing, since `f` and `f2` refer to the same value. > > > > *Things that do work but feel dumb* > > > > 1) Explicitly duplicating the function verbatim: > > > > (define (f body gravity damping dt) > > (ffi:cpointer-push-tag! body 'cpBody) > > (cpBodySetVelocity body (cpv 0 -10)) > > > > ffi:_void) > > > > (define (f2 body gravity damping dt) > > (ffi:cpointer-push-tag! body 'cpBody) > > (cpBodySetVelocity body (cpv 0 -10)) > > > > ffi:_void) > > > > (set-cpBody-velocity_func! (chipmunk-body body1) > > f) > > > > (set-cpBody-velocity_func! (chipmunk-body body2) > > f2) > > This works because each callbacks is associated to a separate function, > and the separate functions are retained through the module's namespace. > > > 2) Duplicating the function with a macro lets me generalize the above > > workaround for more than two bodies, but it still feels gross. > > Yes. > > > > If someone has an explanation or a better workaround, that would be > great. > > Use `#:keep` in the `_fun` for the velocity callback. > > To tie the callback's lifetime to the enclosing record, you can > probably use a weak hashtable mapping `cpBody` pointers to callbacks: > > (define callbacks (make-weak-hasheq)) > (define current-cpBody (make-parameter #f)) > > (define _cpBodyVelocityFunc > (_fun #:keep (lambda (cb) (hash-set! callbacks (current-cpBody) cb)) > _pointer _cpVect _cpFloat _cpFloat -> _void)) > > where `current-cpBody` is a way of communicating the record where the > pointer is being stored: > > (let ([body (chipmunk-body body1)]) > (parameterize ([current-cpBody body])) > (set-cpBody-velocity_func! body > f)) > > Beware that `callbacks` ties the callback to a cpointer record, not to > the actual pointer value. Since `chipmunk-body` returns the same > Cpointer every time for a given `chipmunk` Racket structure, I think > that's ok --- but it won't be ok if you get the same body pointer from > Chipmunk (via the FFI) multiple times. > >
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