ctypes and how to copy data passed to callback

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  • waldek
    #1

    ctypes and how to copy data passed to callback

    Hi,

    I'm trying to handle data passed to Py Callback which is called from
    C dll. Callback passes data to another thread using Queue module and
    there the data are printed out.

    If data is printed out in a callback itself it's ok. If I put on
    queue and next get from queue in another thread script prints some
    trash. Looks like the data is released when callback returned. I tired
    to make d = copy.deepcopy(d ata), but it does not work - I got nothing.
    Any idea why it's happening ?

    --------------------- main thread --------
    def callback(data, size):
    myqueue.put((da ta, size))

    mydll = cdll.MyDLL
    cbproto = CFUNCTYPE(c_int , POINTER(c_char) , c_int)
    mycallback = cbproto(callbac k)

    mydll.RegisterC allback(mycallb ack)

    ---------------------------------- thread listener
    ----------------------

    while True:
    data, size = myqueue.get()
    print "***", data[:size]

    ------------------------------------------------------------------------------
    Tags: None
    • Diez B. Roggisch
      #2
      Re: ctypes and how to copy data passed to callback

      waldek wrote:
      Hi,
      >
      I'm trying to handle data passed to Py Callback which is called from
      C dll. Callback passes data to another thread using Queue module and
      there the data are printed out.
      >
      If data is printed out in a callback itself it's ok. If I put on
      queue and next get from queue in another thread script prints some
      trash. Looks like the data is released when callback returned. I tired
      to make d = copy.deepcopy(d ata), but it does not work - I got nothing.
      Any idea why it's happening ?
      >
      --------------------- main thread --------
      def callback(data, size):
      myqueue.put((da ta, size))
      >
      mydll = cdll.MyDLL
      cbproto = CFUNCTYPE(c_int , POINTER(c_char) , c_int)
      mycallback = cbproto(callbac k)
      >
      mydll.RegisterC allback(mycallb ack)
      >
      ---------------------------------- thread listener
      ----------------------
      >
      while True:
      data, size = myqueue.get()
      print "***", data[:size]
      >
      ------------------------------------------------------------------------------
      You need to allocate e.g. a bytebuffer using ctypes and then copy the memory
      area you get passed into that buffer.

      Otherwise I presume whoever invokes the callback releases the originaly
      memory block after the callback terminated.

      Diez

        Comment

        • Thomas Heller
          #3
          Re: ctypes and how to copy data passed to callback

          waldek schrieb:
          Hi,
          >
          I'm trying to handle data passed to Py Callback which is called from
          C dll. Callback passes data to another thread using Queue module and
          there the data are printed out.
          >
          If data is printed out in a callback itself it's ok. If I put on
          queue and next get from queue in another thread script prints some
          trash. Looks like the data is released when callback returned. I tired
          to make d = copy.deepcopy(d ata), but it does not work - I got nothing.
          Any idea why it's happening ?
          >
          --------------------- main thread --------
          def callback(data, size):
          myqueue.put((da ta, size))
          >
          mydll = cdll.MyDLL
          cbproto = CFUNCTYPE(c_int , POINTER(c_char) , c_int)
          mycallback = cbproto(callbac k)
          >
          mydll.RegisterC allback(mycallb ack)
          >
          ---------------------------------- thread listener
          ----------------------
          >
          while True:
          data, size = myqueue.get()
          print "***", data[:size]
          >
          ------------------------------------------------------------------------------
          I guess your code would work if you change it in this way:
          def callback(data, size):
          myqueue.put(dat a[:size])
          while True:
          data = myqueue.get()
          print "***", data
          Thomas

            Comment

            • waldek
              #4
              Re: ctypes and how to copy data passed to callback

              On Jul 28, 4:03 pm, Thomas Heller <thel...@python .netwrote:
              waldek schrieb:
              >
              >
              >
              Hi,
              >
              I'm trying to handle data passed to Py Callback which is called from
              C dll. Callback passes data to another thread using Queue module and
              there the data are printed out.
              >
              If data is printed out in a callback itself it's ok. If I put on
              queue and next get from queue in another thread script prints some
              trash. Looks like the data is released when callback returned. I tired
              to make d = copy.deepcopy(d ata), but it does not work - I got nothing.
              Any idea why it's happening ?
              >
              --------------------- main thread --------
              def callback(data, size):
              myqueue.put((da ta, size))
              >
              mydll = cdll.MyDLL
              cbproto = CFUNCTYPE(c_int , POINTER(c_char) , c_int)
              mycallback = cbproto(callbac k)
              >
              mydll.RegisterC allback(mycallb ack)
              >
              ---------------------------------- thread listener
              ----------------------
              >
              while True:
              data, size = myqueue.get()
              print "***", data[:size]
              >
              ------------------------------------------------------------------------------
              >
              I guess your code would work if you change it in this way:
              >
              def callback(data, size):
              myqueue.put(dat a[:size])
              while True:
              data = myqueue.get()
              print "***", data
              >
              Thomas
              Both solutions work fine. The secon is nicer :)

              Thanks

                Comment

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