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6173 :: Nim threading and FFI - locks

• mantielero (orginal) [2020-04-08T10:54:40+02:00] view original

  Disclaimer: I don't have much experience with multithreading (I played with it long ago in python).

  In a different forum thread, we were talking about the need of setupForeignThreadGc() on the top of a callback called by C.

  In the same context, I aiming to do this in Nim.

  Should I use threads? Should I use something else given that the threads are created elsewhere in C?

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• mantielero (orginal) [2020-04-08T12:12:13+02:00] view original

  I made it work like this:

  import locks
  
  type
  FrameRequest {.bycopy.} = object
      numFrames*: int       # Total number of frames
      nthreads*: int      # Number of threads available
      completedFrames*: int # Number of frames already processed
      requestedFrames*: int # Number of frames already requested
  
  var
  reqs:FrameRequest
  lock: Lock
  cond : Cond
  
  proc callback( reqsData: pointer,
              frame: ptr VSFrameRef,
              n: cint,
              node: ptr VSNodeRef,
              errorMsg: cstring) {.cdecl.} =
  #[
  Function of the client application called by the core when a requested frame is ready, after a call to getFrameAsync().
  
  If multiple frames were requested, they can be returned in any order. Client applications must take care of reordering them.
  
  This function is only ever called from one thread at a time.
  
  getFrameAsync() may be called from this function to request more frames.
  ]#
  setupForeignThreadGc()
  
  # Do something with the frame
  API.freeFrame( frame )
  reqs.completedFrames += 1
  
  # Once a frame is completed, we request another frame while there are available
  if reqs.requestedFrames < reqs.numFrames:
      API.getFrameAsync( reqs.requestedFrames.cint, node, callback, reqsData)
      reqs.requestedFrames += 1
  
  if (reqs.completedFrames == reqs.numFrames):
      cond.signal()
  
  proc NullAsync*(vsmap:ptr VSMap):int =
  reqs.nthreads = getNumThreads()  # Get the number of threads
  echo "Number of threads: ", reqs.nthreads
  let node = getFirstNode(vsmap)
  let vinfo = API.getVideoInfo(node) # video info pointer
  reqs.numFrames = vinfo.numFrames
  reqs.completedFrames = 0
  reqs.requestedFrames = 0
  
  let initialRequest = min(reqs.nthreads, reqs.numFrames)
  initLock(lock)
  for i in 0..<initialRequest:  #
      API.getFrameAsync( i.cint, node, callback, nil) #dataInHeap)
      reqs.requestedFrames += 1
  
  cond.wait(lock)
  API.freeMap(vsmap)
  API.freeNode(node)
  return reqs.numFrames
  Run

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