code: def _src_input_callback(cb_data, data):
      
      cb_data = ffi.from_handle(cb_data)
      ret = cb_data['callback']()
      if ret is None:
          cb_data['last_input'] = None
          return 0  # No frames supplied
      input_data = _np.require(ret, requirements='C', dtype=_np.float32)
      input_frames, channels = _check_data(input_data)
  
      # Check whether the correct number of channels is supplied by user.
      if cb_data['channels'] != channels:
          raise ValueError('Invalid number of channels in callback.')
  
      # Store a reference of the input data to ensure it is still alive when
      # accessed by libsamplerate.
      cb_data['last_input'] = input_data
  
      data[0] = ffi.cast('float*', ffi.from_buffer(input_data))
      return input_frames
docs: Internal callback function to be used with the callback API.
  
      Pulls the Python callback function from the handle contained in `cb_data`
      and calls it to fetch frames. Frames are converted to the format required by
      the API (float, interleaved channels). A reference to these data is kept
      internally.
  
      Returns
      -------
      frames : int
          The number of frames supplied.