Re: [Discuss-gnuradio] gr_message_sink

[Eric Blossom]

On Sun, Oct 07, 2007 at 04:09:48PM -0700, seph 004 wrote:
> I was wondering if someone could perhaps clarify how the  gr_message_sink 
> works . I'm trying to make a modified version of the  fft_sink. I noticed 
> that the sample stream is turned into a vector  streasm and then sent to a 
> message sink.
>   

>  When the data is unpacked from the messages, then it seems there
>  are a varying number of fft frames in each message. How does this
>  happen? Why isn't there only one frame per message?

It builds a message that contains however many items were handed it by
work.  It could be changed such that each item was placed into it's
own message, but that didn't seem to make sense.  Consider what would
happen if the items were floats.

>  Also, what exactly are msg.arg(1) and msg.arg(2). Are they part of
>  the message payload or part a special frame? Lastly, what does
>  self.msgq.delete_head() do?

>   Regards
>   Lance

See:

  http://gnuradio.org/doc/doxygen/classgr__msg__queue.html
  http://gnuradio.org/doc/doxygen/classgr__message.html
  http://gnuradio.org/doc/doxygen/classgr__message__sink.html


A quick look at the source:

int
gr_message_sink::work(int noutput_items,
                      gr_vector_const_void_star &input_items,
                      gr_vector_void_star &output_items)
{
  const char *in = (const char *) input_items[0];

  // if we'd block, drop the data on the floor and say everything is OK
  if (d_dont_block && d_msgq->full_p())
    return noutput_items;

  // build a message to hold whatever we've got
  gr_message_sptr msg = gr_make_message(0,              // msg type
                                        d_itemsize,     // arg1 for other end
                                        noutput_items,  // arg2 for other end 
(redundant)
                                        noutput_items * d_itemsize);   // len 
of msg
  memcpy(msg->msg(), in, noutput_items * d_itemsize);

  d_msgq->handle(msg);          // send it

  return noutput_items;
}


Eric