First of all, let's take a Ubuntu 18.04 LTS computer and install **ROS 2 Dashing Diademata**:

Once you have a **ROS 2** installation in the computer, follow these steps to install the micro-ROS build system:

source /opt/ros/$ROS_DISTRO/setup.bash

mkdir microros_ws

These instructions will setup a workspace with a ready-to-use micro-ROS build system. This build system is in charge of downloading the required cross-compilation tools and building the apps for the required platforms.

micro-ROS apps for Linux are located at `src/uros/micro-ROS-demos/rcl/`. In order to create a new application, create a new folder containing two files: the app code and the CMake file. You can check the complete content of these file [here](https://github.com/micro-ROS/micro-ROS-demos/tree/dashing/rcl/ping_pong).

pushd src/uros/micro-ROS-demos/rcl/

mkdir ping_pong

cd ping_pong

touch app.c CMakeLists.txt

Don't forget to register your app in `src/uros/micro-ROS-demos/rcl/CMakeLists.txt` by adding the following line:

For this example we are going to create a ping pong app where a node sends a ping package with a unique identifier using a publisher and the same package is received by a pong subscriber. The node will also answer to pings received from other nodes with a pong message:

The app logic of this demo is contained in [`app.c`](https://github.com/micro-ROS/micro-ROS-demos/blob/dashing/rcl/ping_pong/main.c). A thorough review of this file can show how to create a micro-ROS publisher or subscriber, as shown below.

**For more in depth learning about RCL and RCLC programming [check this section](https://micro-ros.github.io/docs/tutorials/core/programming_rcl_rclc/).**

...

// Create a reliable ping publisher

rcl_publisher_options_t ping_publisher_ops = rcl_publisher_get_default_options();

rcl_publisher_t ping_publisher = rcl_get_zero_initialized_publisher();

rcl_publisher_init(&ping_publisher, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/ping", &ping_publisher_ops);

...

...

// Create a best effort pong subscriber

rcl_subscription_options_t pong_subscription_ops = rcl_subscription_get_default_options();

pong_subscription_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;

rcl_subscription_t pong_subscription = rcl_get_zero_initialized_subscription();

rcl_subscription_init(&pong_subscription, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/pong", &pong_subscription_ops);

...

...

// Check if some pong message is received

if (wait_set.subscriptions[index_pong_subscription]) {

rc = rcl_take(wait_set.subscriptions[index_pong_subscription], &rcv_msg, NULL, NULL);

if(rc == RCL_RET_OK) {

// Subscription app logic

}

}

...

// Reset the pong count and publish the ping message

rcl_publish(&ping_publisher, (const void*)&msg, NULL);

...

When the all client and agent packages are ready, just build them all:

Then run the agent:

Then run the app in another command line (remember sourcing ROS 2 and micro-ROS installation):

And finally, let's check that everything is working in another command line. We are going to listen to ping topic to check whether the Ping Pong node is publishing its own pings:

On another command line, let's subscribe to the pong topic

At this point, we know that our app is publishing pings. Let's check if it also answers to someone else pings in a new command line:

Now, we should see on the ping subscriber our fake ping along with the board pings:

And in the pong subscriber, we should see the board's answer to our fake ping:

One of the advantages of having an Linux micro-ROS app is that you don't need to buy a bunch of hardware in order to test some multi-node micro-ROS apps. So, with the same micro-ROS agent of the last section, let's open four different command lines and run the following on each:

ros2 run micro_ros_demos_rcl my_brand_new_app