PX4-Autopilot/src/modules/micrortps_bridge/hello_world.md

Hello world

As a basic example we go to explain how implement a simple use case what sends information of some sensors (sensor_combined uORB topic) in the direction PX4-Fast RTPS and receives information as a text message (log_message uORB topic) sent in the other direction.

Creating the code

$ cd /path/to/PX4/Firmware
$ python Tools/generate_microRTPS_bridge.py --send msg/sensor_combined.msg --receive msg/sensor_combined.msg msg/log_message.msg -u src/examples/micrortps_client

NOTE: It may be needed specify other different arguments, as the path to the Fast RTPS bin installation folder if it was installed in other path different to default one (-f /path/to/fastrtps/installation/bin). For more information, click this link.

That generates and installs the PX4 side of the code (the client) in src/examples/micrortps_client and the Fast RPS side (the agent) in src/modules/micrortps_bridge/micrortps_agent.

To see the message received in the client one time each second (src/examples/micrortps_client/microRTPS_client.cpp), we change the default value of the sleep to 1000 and we will add this printf to the code under the orb_publish line for log_message topic (line 274):

...
#define SLEEP_MS 1000
...
...
...
case 36:
{
    deserialize_log_message(&log_message_data, data_buffer, &microCDRReader);
    if (!log_message_pub)
        log_message_pub = orb_advertise(ORB_ID(log_message), &log_message_data);
    else
        orb_publish(ORB_ID(log_message), log_message_pub, &log_message_data);
    printf("%s\n", log_message_data.text);
    ++received;
}
...

To enable the compilation of the example client we need to modify the micrortps_client line in the cmake of our platform (cmake/configs) in that way:

set(config_module_list
  ...
  #src/modules/micrortps_bridge/micrortps_client
  src/examples/micrortps_client
  ...
)

Also we need to a add basic CMakeLists.txt like that:

px4_add_module(
	MODULE examples__micrortps_client
	MAIN micrortps_client
	STACK_MAIN 4096
	SRCS
		microRTPS_transport.cxx
		microRTPS_client.cpp
	DEPENDS
		platforms__common
	)

Now, we change the agent in order to send a log message each time we receive a Fast RTPS message with the info of the uORB topic sensor_combined (in the Fast RTPS world this topic will be called sensor_combined_PubSubTopic).

• In the src/modules/micrortps_bridge/micrortps_agent/RtpsTopic.cxx file we will change the RtpsTopics::getMsg function to return a log_message for each sensor_combined with the text "The temperature is XX.XXXXXXX celsius degrees":

 bool RtpsTopics::getMsg(const char topic_ID, eprosima::fastcdr::Cdr &scdr)
 {
     bool ret = false;
     switch (topic_ID)
     {
         case 58: // sensor_combined
             if (_sensor_combined_sub.hasMsg())
             {
                 sensor_combined_ msg = _sensor_combined_sub.getMsg();
                 log_message_ log_msg = {};
                 std::string message = "The temperature is " + std::to_string(msg.baro_temp_celcius()) + " celsius degrees";
                 std::memcpy(log_msg.text().data(), message.c_str(), message.length());
                 log_msg.serialize(scdr);
                 ret = true;
             }
         break;
         default:
             printf("Unexpected topic ID to getMsg\n");
         break;
     }

     return ret;
 }

• In the src/micrortps_bridge/micrortps_agent/microRTPS_agent.cxx we will change the topic ID of the received message to the topic ID of the log_message (36) that is really the topic we are handling:

  ...
  if (topics.getMsg(topic_ID, scdr))
  {
      topic_ID = 36;
      length = scdr.getSerializedDataLength();
      if (0 < (length = transport_node->write(topic_ID, scdr.getBufferPointer(), length)))
      {
          total_sent += length;
          ++sent;
      }
  }
  ...

Result

After compiling and launching both the client and the agent we obtain this kind of messages in the client shell window (showing the message created in the agent with info from temperature sensor in the PX4 side):

  ...
  The temperature is: 47.779999 celsius
  ...