def config_descriptor(index)
      ppConfig = FFI::MemoryPointer.new :pointer
      res = Call.libusb_get_config_descriptor(@pDev, index, ppConfig)
      LIBUSB.raise_error res, "in libusb_get_config_descriptor" if res!=0
      pConfig = ppConfig.read_pointer
      config = Configuration.new(self, pConfig)
      config
    end
def config_descriptor(index)
      ppConfig = FFI::MemoryPointer.new :pointer
      res = Call.libusb_get_config_descriptor(@pDev, index, ppConfig)
      LIBUSB.raise_error res, "in libusb_get_config_descriptor" if res!=0
      pConfig = ppConfig.read_pointer
      config = Configuration.new(self, pConfig)
      config
    end
Obtain a config descriptor of the device.

 @param [Fixnum] index  number of the config descriptor
 @return Configuration