[Discuss-gnuradio] Simples case of message passing

[Daniel Franch]

Earlier today, I've posted a question on how to do frame detection and extraction in GNU Radio (http://gnuradio.4.n7.nabble.com/Simple-Frame-Detection-td52233.html). I have a feeling that the best way to accomplish what I need is using the message passing system. 

But I simply can't find an easy enough example to understand how message passing works. So I tried to create two simple blocks in order to better understand this system. I simply wanted a block that would receive a stream (not neccesarily tagged) of bytes and then post them as a message. The other one would receive this message and then output the same stream of bytes. I know there are message source/sink blocks that do this, but as I understand, they work on a previous message passing system. 

My blocks were written this way: 

Message writing: 
message_write_b_impl::message_write_b_impl() 
      : gr::sync_block("message_write_b", 
              gr::io_signature::make(1, 1, sizeof(char)), 
              gr::io_signature::make(0, 0, 0)) 
    { 
        message_port_register_out(pmt::mp("out")); 
        buffer = new char[128]; 
        counter = 0; 
    } 

    /* 
     * Our virtual destructor. 
     */ 
    message_write_b_impl::~message_write_b_impl() 
    { 
        delete[] buffer; 
    } 
    
    void 
    message_write_b_impl::send_message() 
    { 
        pmt::pmt_t meta = pmt::make_dict(); 
        pmt::pmt_t payload = pmt::from_long(long(buffer[counter++])); 
        message_port_pub(pmt::mp("out"), pmt::cons(meta, payload)); 
        std::memset(buffer, 0, sizeof(buffer)); 
    } 

    int 
    message_write_b_impl::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]; 

        for(int i = 0; i < noutput_items; i++) 
        { 
            buffer[counter] = in[i]; 
            this->send_message(); 
            
            if(counter >= 128) 
            { 
                counter = 0; 
            }             
        } 
        consume_each(noutput_items); 
        
        return 0; 
    } 

Message read: 
message_read_b_impl::message_read_b_impl() 
      : gr::sync_block("message_read_b", 
              gr::io_signature::make(0, 0, 0), 
              gr::io_signature::make(1, 1, sizeof(char))) 
    { 
        message_port_register_in(pmt::mp("in")); 
        set_msg_handler(pmt::mp("in"), 
        boost::bind(&message_read_b_impl::in_handling, this, _1)); 
        
        buffer = new char[128]; 
        counter = 0; 
        rx_check = false; 
    } 

    /* 
     * Our virtual destructor. 
     */ 
    message_read_b_impl::~message_read_b_impl() 
    { 
        delete[] buffer; 
    } 
    
    void 
    message_read_b_impl::in_handling(pmt::pmt_t msg) 
    { 
        if(pmt::is_integer(msg)) 
        { 
            buffer[counter++] = char(pmt::from_long(msg)); 
            rx_check = true; 
        } 
        if(counter == 128) 
        { 
            counter = 0; 
            rx_check = false; 
        } 
    } 

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

        for(int i = 0; i < noutput_items; i++) 
        {       
            while(!rx_check); 
            out[i] = buffer[i]; 
        } 
        return noutput_items; 
    } 

As far as I understand, this could be the simplest case to send bytes through messages. But I feel there is something wrong, as the pmt doesn't seem to have a method to deal with chars, so I have to do lots of conversions. Still, this didn't seem to be a reason of bigger concerns. 

My python QA block simply does a message connect between their message ports. But when I try to run the test, the tb simply freezes and I have to finish with ctrl+C. 

Is there any easier way to implement message passing? 

Thanks in advance, 
Dan Franch