<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=us-ascii"><meta name=Generator content="Microsoft Word 14 (filtered medium)"><style><!--
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</o:shapelayout></xml><![endif]--></head><body lang=NL-BE link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span lang=EN-GB>Hi,<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>I am forking an FFI library for physics simulation on Github (<a href="https://github.com/samvv/Rhipmunk-Physics">https://github.com/samvv/Rhipmunk-Physics</a>) but I got stuck with storing racket objects in the library’s native structs. From the library’s website: <a href="http://chipmunk-physics.net/release/Chipmunk-6.x/Chipmunk-6.1.2-API-Reference/group__cp_body.html">http://chipmunk-physics.net/release/Chipmunk-6.x/Chipmunk-6.1.2-API-Reference/group__cp_body.html</a>. Here’s the part of the code that needs to be changed:<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB> <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(define _cpDataPointer _racket)<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(define-cstruct _cpBody<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB> …<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB> [data _cpDataPointer] ; user-defined data, like a game object<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB> …)<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>In the following test everything seems to work as expected:<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>(define body1 (cpBodyNew 1.0 1.0))<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(set-cpBody-data! body1 "This is a test")<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(display (cpBody-data body1))<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>> “This is a test”<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(set-cpBody-data! body1 (cons 1 2))<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(display (cpBody-data body1))<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>> (1 . 2)<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>However, when I run the following code:<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>(collect-garbage)<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB>(display (cpBody-data body1))<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>Something goes wrong internally. Sometimes DrRacket crashes, sometimes it returns values like #<bad-value>. I have looked into (malloc-immobile-cell v) and (free-immobile-cell cptr) but I am not sure that fiddling with the memory is such a good idea. Could you please give me any guidelines or point me to a procedure I can use to provide this functionality?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>Thanks,<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-GB><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-GB>Sam</span><span lang=EN-GB><o:p></o:p></span></p></div></body></html>