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Creating custom ROS 2 msg and srv files

Goal: Define custom interface files (.msg and .srv) and use them with Python and C++ nodes.

Tutorial level: Beginner

Time: 20 minutes

Background

In previous tutorials you utilized message and service interfaces to learn about topics, services, and simple publisher/subscriber (C++/Python) and service/client (C++/Python) nodes. The interfaces you used were predefined in those cases.

While it’s good practice to use predefined interface definitions, you will probably need to define your own messages and services sometimes as well. This tutorial will introduce you to the simplest method of creating custom interface definitions.

Prerequisites

You should have a ROS 2 workspace.

This tutorial also uses the packages created in the publisher/subscriber (C++ and Python) and service/client (C++ and Python) tutorials to try out the new custom messages.

Tasks

1 Create a new package

For this tutorial you will be creating custom .msg and .srv files in their own package, and then utilizing them in a separate package. Both packages should be in the same workspace.

Since we will use the pub/sub and service/client packages created in earlier tutorials, make sure you are in the same workspace as those packages (dev_ws/src), and then run the following command to create a new package:

ros2 pkg create --build-type ament_cmake tutorial_interfaces

tutorial_interfaces is the name of the new package. Note that it is a CMake package; there currently isn’t a way to generate a .msg or .srv file in a pure Python package. You can create a custom interface in a CMake package, and then use it in a Python node, which will be covered in the last section.

It is good practice to keep .msg and .srv files in their own directories within a package. Create the directories in dev_ws/src/tutorial_interfaces:

mkdir msg

mkdir srv

2 Create custom definitions

2.1 msg definition

In the tutorial_interfaces/msg directory you just created, make a new file called Num.msg with one line of code declaring its data structure:

int64 num

This is your custom message that transfers a single 64-bit integer called num.

2.2 srv definition

Back in the tutorial_interfaces/srv directory you just created, make a new file called AddThreeInts.srv with the following request and response structure:

int64 a
int64 b
int64 c
---
int64 sum

This is your custom service that requests three integers named a, b, and c, and responds with an integer called sum.

3 CMakeLists.txt

To convert the interfaces you defined into language-specific code (like C++ and Python) so that they can be used in those languages, add the following lines to CMakeLists.txt:

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
  "srv/AddThreeInts.srv"
 )

4 package.xml

Because the interfaces rely on rosidl_default_generators for generating language-specific code, you need to declare a dependency on it. Add the following lines to package.xml

<build_depend>rosidl_default_generators</build_depend>

<exec_depend>rosidl_default_runtime</exec_depend>

<member_of_group>rosidl_interface_packages</member_of_group>

5 Build the tutorial_interfaces package

Now that all the parts of your custom interfaces package are in place, you can build the package. In the root of your workspace (~/dev_ws), run the following command:

colcon build --packages-select tutorial_interfaces

Now the interfaces will be discoverable by other ROS 2 packages.

6 Confirm msg and srv creation

In a new terminal, run the following command from within your workspace (dev_ws) to source it:

. install/setup.bash

Now you can confirm that your interface creation worked by using the ros2 interface show command:

ros2 msg show tutorial_interfaces/msg/Num

should return:

int64 num

And

ros2 srv show tutorial_interfaces/srv/AddThreeInts

should return:

int64 a
int64 b
int64 c
---
int64 sum

7 Test the new interfaces

For this step you can use the packages you created in previous tutorials. A few simple modifications to the nodes, CMakeLists and package files will allow you to use your new interfaces.

7.1 Testing Num.msg with pub/sub

With some slight modifications to the publisher/subscriber package created in a previous tutorial (C++ or Python), you can see Num.msg in action. Since you’ll be changing the standard string msg to a numerical one, the output will be slightly different.

Publisher:

#include <chrono>
#include <memory>

#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/num.hpp"     // CHANGE

using namespace std::chrono_literals;

class MinimalPublisher : public rclcpp::Node
{
public:
  MinimalPublisher()
  : Node("minimal_publisher"), count_(0)
  {
    publisher_ = this->create_publisher<tutorial_interfaces::msg::Num>("topic", 10);    // CHANGE
    timer_ = this->create_wall_timer(
      500ms, std::bind(&MinimalPublisher::timer_callback, this));
  }

private:
  void timer_callback()
  {
    auto message = tutorial_interfaces::msg::Num();                               // CHANGE
    message.num = this->count_++;                                        // CHANGE
    RCLCPP_INFO(this->get_logger(), "Publishing: '%d'", message.num);    // CHANGE
    publisher_->publish(message);
  }
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<tutorial_interfaces::msg::Num>::SharedPtr publisher_;         // CHANGE
  size_t count_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalPublisher>());
  rclcpp::shutdown();
  return 0;
}

Subscriber:

#include <memory>

#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/msg/num.hpp"     // CHANGE
using std::placeholders::_1;

class MinimalSubscriber : public rclcpp::Node
{
public:
  MinimalSubscriber()
  : Node("minimal_subscriber")
  {
    subscription_ = this->create_subscription<tutorial_interfaces::msg::Num>(          // CHANGE
      "topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
  }

private:
  void topic_callback(const tutorial_interfaces::msg::Num::SharedPtr msg) const       // CHANGE
  {
    RCLCPP_INFO(this->get_logger(), "I heard: '%d'", msg->num);              // CHANGE
  }
  rclcpp::Subscription<tutorial_interfaces::msg::Num>::SharedPtr subscription_;       // CHANGE
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalSubscriber>());
  rclcpp::shutdown();
  return 0;
}

CMakeLists.txt:

Add the following lines (C++ only):

...

find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(tutorial_interfaces REQUIRED)                         # CHANGE

add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp tutorial_interfaces)         # CHANGE

add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp tutorial_interfaces)     # CHANGE

install(TARGETS
  talker
  listener
  DESTINATION lib/${PROJECT_NAME})

ament_package()

package.xml:

Add the following line:

<depend>tutorial_interfaces</depend>

After making the above edits and saving all the changes, build the package:

colcon build --packages-select cpp_pubsub

Then open two new terminals, source dev_ws in each, and run:

ros2 run cpp_pubsub talker

ros2 run cpp_pubsub listener

Since Num.msg relays only an integer, the talker should only be publishing integer values, as opposed to the string it published previously:

[INFO] [minimal_publisher]: Publishing: '0'
[INFO] [minimal_publisher]: Publishing: '1'
[INFO] [minimal_publisher]: Publishing: '2'

7.2 Testing AddThreeInts.srv with service/client

With some slight modifications to the service/client package created in a previous tutorial (C++ or Python), you can see AddThreeInts.srv in action. Since you’ll be changing the original two integer request srv to a three integer request srv, the output will be slightly different.

Service:

#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/srv/add_three_ints.hpp"     // CHANGE

#include <memory>

void add(const std::shared_ptr<tutorial_interfaces::srv::AddThreeInts::Request> request,     // CHANGE
          std::shared_ptr<tutorial_interfaces::srv::AddThreeInts::Response>       response)  // CHANGE
{
  response->sum = request->a + request->b + request->c;                                       // CHANGE
  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Incoming request\na: %ld" " b: %ld" " c: %ld",   // CHANGE
                request->a, request->b, request->c);                                          // CHANGE
  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "sending back response: [%ld]", (long int)response->sum);
}

int main(int argc, char **argv)
{
  rclcpp::init(argc, argv);

  std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_three_ints_server");  // CHANGE

  rclcpp::Service<tutorial_interfaces::srv::AddThreeInts>::SharedPtr service =                 // CHANGE
    node->create_service<tutorial_interfaces::srv::AddThreeInts>("add_three_ints",  &add);     // CHANGE

  RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Ready to add three ints.");      // CHANGE

  rclcpp::spin(node);
  rclcpp::shutdown();
}

Client:

#include "rclcpp/rclcpp.hpp"
#include "tutorial_interfaces/srv/add_three_ints.hpp"        // CHANGE

#include <chrono>
#include <cstdlib>
#include <memory>

using namespace std::chrono_literals;

int main(int argc, char **argv)
{
  rclcpp::init(argc, argv);

  if (argc != 4) { // CHANGE
      RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "usage: add_three_ints_client X Y Z");      // CHANGE
      return 1;
  }

  std::shared_ptr<rclcpp::Node> node = rclcpp::Node::make_shared("add_three_ints_client"); // CHANGE
  rclcpp::Client<tutorial_interfaces::srv::AddThreeInts>::SharedPtr client =                        // CHANGE
    node->create_client<tutorial_interfaces::srv::AddThreeInts>("add_three_ints");                  // CHANGE

  auto request = std::make_shared<tutorial_interfaces::srv::AddThreeInts::Request>();               // CHANGE
  request->a = atoll(argv[1]);
  request->b = atoll(argv[2]);
  request->c = atoll(argv[3]);               // CHANGE

  while (!client->wait_for_service(1s)) {
    if (!rclcpp::ok()) {
      RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Interrupted while waiting for the service. Exiting.");
      return 0;
    }
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "service not available, waiting again...");
  }

  auto result = client->async_send_request(request);
  // Wait for the result.
  if (rclcpp::spin_until_future_complete(node, result) ==
    rclcpp::executor::FutureReturnCode::SUCCESS)
  {
    RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "Sum: %ld", result.get()->sum);
  } else {
    RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Failed to call service add_three_ints");    // CHANGE
  }

  rclcpp::shutdown();
  return 0;
}

CMakeLists.txt:

Add the following lines (C++ only):

...

find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(tutorial_interfaces REQUIRED)        # CHANGE

add_executable(server src/add_two_ints_server.cpp)
ament_target_dependencies(server
  rclcpp tutorial_interfaces)                      #CHANGE

add_executable(client src/add_two_ints_client.cpp)
ament_target_dependencies(client
  rclcpp tutorial_interfaces)                      #CHANGE

install(TARGETS
  server
  client
  DESTINATION lib/${PROJECT_NAME})

ament_package()

package.xml:

Add the following line:

<depend>tutorial_interfaces</depend>

After making the above edits and saving all the changes, build the package:

colcon build --packages-select cpp_srvcli

Then open two new terminals, source dev_ws in each, and run:

ros2 run cpp_srvcli server

ros2 run cpp_srvcli client 2 3 1

Summary

In this tutorial, you learned how to create custom interfaces in their own package and how to utilize those interfaces from within other packages.

This is a simple method of interface creation and utilization. You can learn more about interfaces here.

Next steps

The next tutorial covers more ways to use interfaces in ROS 2.