code: def src_callback_read(state, ratio, frames, data):
      
      data_ptr = ffi.cast('float*f', ffi.from_buffer(data))
      return _lib.src_callback_read(state, ratio, frames, data_ptr)
docs: Read up to `frames` worth of data using the callback API.
  
      Returns
      -------
      frames : int
          Number of frames read or -1 on error.