title: How to implement message passing in GNU Radio?
body: <p>I need to implement message passing, my idea is to make some sort of message source (I inherit from <code>public gr_sync_block</code>) that works as a controller for another block (it has to send a message each 6 minutes).</p>
  
  <p>I read that is necessary to inherit from <code>gnuradio::block</code>  -and by the way, installing grextras is mandatory-. In the <code>.h</code> file I added the <code>#include &lt;gnuradio/block.h&gt;</code> and inherited from block <code>class JDFM_API jdfm_control : public gr_sync_block, public gnuradio::block</code>. I know that I have redefine some things like the <code>gnuradio::block</code> constructor but I dont know what <code>msg_signature</code> is, I also don't get the relation between block's parameters and work parameter, the last thing that I am not sure is if I still can use <code>gnuradio-companion</code> if I create a block like this.</p>
  
  <p>I haven't been able to find a simple example of messages implementation. If anyone can guide me or show me an example, it would be awesome.</p>
  
  <p>Thanks in advance. </p>