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6169 :: How do I pass Nim functions to C code as callbacks?

• Keithcat1 (orginal) [2020-04-07T17:30:35+02:00] view original

  H. I've looked around and I can't answer this. I want to be able to pass a Nim function / proc to C as a function pointer and get C to call it. I don't have a specific reason I want to do this, but I might need it soon. Thanks

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• Araq (orginal) [2020-04-07T19:21:18+02:00] view original

  proc callback(arg: cint; arg2: cstring) {.cdecl.} = setupForeignThreadGc() # ... more code here toC(callback)

  Run

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• mantielero (orginal) [2020-04-07T19:59:43+02:00] view original

  I was just looking into this at this very moment and I am not sure if I use it well.

  I was reading the Thread coordination section and this forum entry.

  So I was implementing the following:

  proc callback( userData: pointer,
              f: ptr VSFrameRef,
              n: cint,
              a4: ptr VSNodeRef,
              errorMsg: cstring) {.cdecl.} =
  setupForeignThreadGc()     #<---------------------
  API.freeFrame( f )
  tearDownForeignThreadGc()    #<---------------------
  
  
  proc NullAsync*(vsmap:ptr VSMap):int =
  let node = getFirstNode(vsmap)
  #API.freeMap(vsmap)
  let vinfo = API.getVideoInfo(node) # video info pointer
  let nframes = vinfo.numFrames
  for i in 0..<nframes:
      API.getFrameAsync( i, node, callback, nil)     # getFrameAsync is part of the API and calls the callback.
  
  API.freeMap(vsmap)
  API.freeNode(node)
  return nframes
  Run

  Right now it fails. I am compiling with --threads:on.

  I am not sure if I am not using it as I should or if I am not using properly the VapourSynth's API.

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• Araq (orginal) [2020-04-07T20:08:29+02:00] view original

  Do not call tearDownForeignThreadGc.

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• Keithcat1 (orginal) [2020-04-07T20:20:51+02:00] view original

  OK, here is an example. logic.c: typedef int (adder)(int, int);

  int run(adder fn, int a, int b) {

  return fn(a, b);

  }

  logic.nim: {. compile: "logic.c" .} type adder {. importc .} = proc(a, b: cint): cint proc run(fn: adder, a, b: cint): cint {. importc, cdecl, nodecl .} proc add(a, b: cint): cint {. cdecl .} = a+b

  var res=int(run(add, cint(100), cint(5125))) echo $res

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