title: Linux Getting Started

permalink: /docs/tutorials/advanced/linux/linux_getting_started/

This tutorial aims to create a new micro-ROS application on Linux for testing purposes.

First of all, make sure that you have a **ROS 2** installation.

***TIP:** if you are familiar with Docker containers, this image may be useful: [ros:dashing](https://hub.docker.com/layers/ros/library/ros/dashing/images/sha256-b796c14ea663537129897769aa6c715a851ca08dffd4875ef2ecaa31a4dbd431?context=explore)*

On the **ROS 2** installation open a command line and follow these steps:

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

mkdir microros_ws

cd microros_ws

git clone -b $ROS_DISTRO https://github.com/micro-ROS/micro-ros-build.git src/micro-ros-build

sudo apt update && rosdep update

rosdep install --from-path src --ignore-src -y

colcon build

source install/local_setup.bash

Now, let's create a firmware workspace that targets all the required code and tools:

ros2 run micro_ros_setup create_firmware_ws.sh host

Now you have all the required tools to build micro-ROS. At this point, you must know that the micro-ROS build system is a four-step workflow:

1. **Create**: retrieves all the required packages for a specific RTOS and hardware platform.

2. **Configure**: configures the downloaded packages with options such as the micro-ROS application, the selected transport layer or the micro-ROS agent IP address (in network transports).

3. **Build**: generates a binary file ready for being loaded in the hardware.

4. **Flash**: load the micro-ROS software in the hardware.

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.

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

mkdir my_brand_new_app

cd my_brand_new_app

touch app.c CMakeLists.txt

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:


To start creating this app, the `CMakeLists.txt` file should looks like:

Meanwhile `app.c` should look like the following code:

#include <rcl/rcl.h>

#include <rcl/error_handling.h>

#include <std_msgs/msg/header.h>

#include <rmw_uros/options.h>

#include <stdio.h>

#include <unistd.h>

#include <pthread.h>

#define STRING_BUFFER_LEN 100

void * trigger_guard_condition(void *args){

rcl_guard_condition_t * guard_condition = (rcl_guard_condition_t *)args;

while(true){

rcl_trigger_guard_condition(guard_condition);

sleep(5);

}

}

int main(int argc, const char * const * argv)

{

//Init RCL options

rcl_init_options_t options = rcl_get_zero_initialized_init_options();

rcl_init_options_init(&options, rcl_get_default_allocator());

// Init RCL context

rcl_context_t context = rcl_get_zero_initialized_context();

rcl_init(0, NULL, &options, &context);

// Create a node

rcl_node_options_t node_ops = rcl_node_get_default_options();

rcl_node_t node = rcl_get_zero_initialized_node();

rcl_node_init(&node, "pingpong_node", "", &context, &node_ops);

// 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 publisher

rcl_publisher_options_t pong_publisher_ops = rcl_publisher_get_default_options();

pong_publisher_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;

rcl_publisher_t pong_publisher = rcl_get_zero_initialized_publisher();

rcl_publisher_init(&pong_publisher, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/pong", &pong_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);

// Create a best effort ping subscriber

rcl_subscription_options_t ping_subscription_ops = rcl_subscription_get_default_options();

ping_subscription_ops.qos.reliability = RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;

rcl_subscription_t ping_subscription = rcl_get_zero_initialized_subscription();

rcl_subscription_init(&ping_subscription, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Header), "/microROS/ping", &ping_subscription_ops);

// Create guard condition

rcl_guard_condition_t guard_condition = rcl_get_zero_initialized_guard_condition();

rcl_guard_condition_options_t guard_condition_options = rcl_guard_condition_get_default_options();

rcl_guard_condition_init(&guard_condition, &context, guard_condition_options);

pthread_t guard_condition_thread;

pthread_create(&guard_condition_thread, NULL, trigger_guard_condition, &guard_condition);

// Create a wait set

rcl_wait_set_t wait_set = rcl_get_zero_initialized_wait_set();

rcl_wait_set_init(&wait_set, 2, 1, 0, 0, 0, 0, &context, rcl_get_default_allocator());

// Create and allocate the pingpong publication message

std_msgs__msg__Header msg;

char msg_buffer[STRING_BUFFER_LEN];

msg.frame_id.data = msg_buffer;

msg.frame_id.capacity = STRING_BUFFER_LEN;

// Create and allocate the pingpong subscription message

std_msgs__msg__Header rcv_msg;

char rcv_buffer[STRING_BUFFER_LEN];

rcv_msg.frame_id.data = rcv_buffer;

rcv_msg.frame_id.capacity = STRING_BUFFER_LEN;

// Set device id and sequence number;

int device_id = rand();

int seq_no;

int pong_count = 0;

struct timespec ts;

rcl_ret_t rc;

do {

// Clear and set the waitset

rcl_wait_set_clear(&wait_set);

size_t index_pong_subscription;

rcl_wait_set_add_subscription(&wait_set, &pong_subscription, &index_pong_subscription);

size_t index_ping_subscription;

rcl_wait_set_add_subscription(&wait_set, &ping_subscription, &index_ping_subscription);

size_t index_guardcondition;

rcl_wait_set_add_guard_condition(&wait_set, &guard_condition, &index_guardcondition);

// Run session for 100 ms

rcl_wait(&wait_set, RCL_MS_TO_NS(100));

// 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 && strcmp(msg.frame_id.data,rcv_msg.frame_id.data) == 0) {

pong_count++;

printf("Pong for seq %s (%d)\n", rcv_msg.frame_id.data, pong_count);

}

}

// Check if some ping message is received and pong it

if (wait_set.subscriptions[index_ping_subscription]) {

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

// Dont pong my own pings

if(rc == RCL_RET_OK && strcmp(msg.frame_id.data,rcv_msg.frame_id.data) != 0){

printf("Ping received with seq %s. Answering.\n", rcv_msg.frame_id.data);

rcl_publish(&pong_publisher, (const void*)&rcv_msg, NULL);

}

}

// Check if it is time to send a ping

if (wait_set.guard_conditions[index_guardcondition]) {

// Generate a new random sequence number

seq_no = rand();

sprintf(msg.frame_id.data, "%d_%d", seq_no, device_id);

msg.frame_id.size = strlen(msg.frame_id.data);

// Fill the message timestamp

clock_gettime(CLOCK_REALTIME, &ts);

msg.stamp.sec = ts.tv_sec;

msg.stamp.nanosec = ts.tv_nsec;

// Reset the pong count and publish the ping message

pong_count = 0;

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

printf("Ping send seq %s\n", msg.frame_id.data);

}

usleep(10000);

} while (true);

}

export_executable(my_brand_new_app)

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

ros2 run micro_ros_setup build_firmware.sh

source install/local_setup.bash

Then run the agent:

ros2 run micro_ros_agent micro_ros_agent udp4 --port 8888

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

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

source install/local_setup.bash

ros2 run micro_ros_demos_rcl my_brand_new_app

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:

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

ros2 topic echo /microROS/ping

You should see the topic messages published by the Ping Pong node every 5 seconds:

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

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

ros2 topic echo /microROS/pong

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:

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

ros2 topic pub --once /microROS/ping std_msgs/msg/Header '{frame_id: "fake_ping"}'

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:

cd microros_ws

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

source install/local_setup.bash

ros2 run micro_ros_demos_rcl my_brand_new_app

As soon as all micro-ROS nodes are up and connected to the micro-ROS agent you will see them interacting: