Based on the documentation I found and the CP_ARBITER_GET_SHAPES macro,
it looks like the double-pointer arguments are just used for multiple
outputs. So you should use the following type for the function:
(_fun _cpArbiter-pointer
(out1 : (_ptr o _cpShape-pointer))
(out2 : (_ptr o _cpShape-pointer))
-> _void
-> (values out1 out2))
The FFI takes care of allocating the necessary temporary space that the
function writes into. The Racket-side function takes just one argument
(for the arbiter), and it returns two _cpShape-pointer values. So you
would use it like this:
(let-values ([(water poly) (cpArbiterGetShapes arb)])
___
(let ([count (cpPolyShapeGetNumVerts poly)])
___))
Does that work?
Ryan
On 12/02/2012 07:59 PM, Vince Kuyatt wrote:
So I recently started to poke at my humble little bindings to the
Chipmunk library again. (someone sent me an email asking for help,
which reminded me that I hadn't worked on it in a while) The problem
is that I'm still stumbling over the same block that made me stop in
the first place a number of months back. One of the function in the
Chipmunk library (cpArbiterGetShapes) takes two double pointers to a
cpShape. (cpShape**) So far, I have been unable to figure out how to
make this work without a future function that accesses the cpShape
crashing the entire Racket runtime, including the IDE window I have
open.
So the function definition for cpArbiterGetShapes is:
static inline void cpArbiterGetShapes(const cpArbiter *arb, cpShape
**a, cpShape **b)
And my current binding in Racket is:
(_fun _cpArbiter-pointer
(_ptr io _cpShape-pointer)
(_ptr io _cpShape-pointer)
-> _void)
And the (pertinent) code where I use it is:
(let*
([water (cast (malloc _cpShape-pointer) _pointer _cpShape-pointer)]
[poly (cast (malloc _cpShape-pointer) _pointer _cpShape-pointer)]
[water-ptr (cast (malloc _cpShape-pointer) _pointer _cpShape-pointer)]
[poly-ptr (cast (malloc _cpShape-pointer) _pointer _cpShape-pointer)])
(memcpy water-ptr water (ctype-sizeof _cpShape-pointer))
(memcpy poly-ptr poly (ctype-sizeof _cpShape-pointer))
(cpArbiterGetShapes arb water-ptr poly-ptr)
(let*
(...
[count (cpPolyShapeGetNumVerts poly)] ;Note that it crashes
on this line
Which is following the buoyancy demo included in Chipmunk by default
as close as possible. Also note that cpPolyShapeGetNumVerts is
supposed to take a _cpShape-pointer. I'm really not sure how to do
what I need in Racket given this situation, and all I've really seen
in the Racket documentation is just single pointers. I've also tried
just allocating water and poly with not water-ptr and poly-ptr and
just using those, but that crashes Racket just the same. I would
really, really, really appreciate any help with this. It's bugging me
that I can't get this solved and I really want this to work.
Thank you so much in advance for any assistance provided. (also I
apologize if this ends up badly formatted. I'm not sure of formatting
rules for a listserv when sending through email)
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