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Creating custom 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 (ros2_ws/src), and then run the following command to create a new package:

ros2 pkg create --build-type ament_cmake --license Apache-2.0 tutorial_interfaces

tutorial_interfaces is the name of the new package. Note that it is, and can only be, a CMake package, but this doesn’t restrict in which type of packages you can use your messages and services. You can create your own custom interfaces in a CMake package, and then use it in a C++ or Python node, which will be covered in the last section.

The .msg and .srv files are required to be placed in directories called msg and srv respectively. Create the directories in ros2_ws/src/tutorial_interfaces:

mkdir msg 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 a custom message that transfers a single 64-bit integer called num.

Also in the tutorial_interfaces/msg directory you just created, make a new file called Sphere.msg with the following content:

geometry_msgs/Point center
float64 radius

This custom message uses a message from another message package (geometry_msgs/Point in this case).

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(geometry_msgs REQUIRED)
find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Num.msg"
  "msg/Sphere.msg"
  "srv/AddThreeInts.srv"
  DEPENDENCIES geometry_msgs # Add packages that above messages depend on, in this case geometry_msgs for Sphere.msg
)

Note

The first argument (library name) in the rosidl_generate_interfaces must match ${PROJECT_NAME} (see https://github.com/ros2/rosidl/issues/441#issuecomment-591025515).

4 `package.xml`

Because the interfaces rely on rosidl_default_generators for generating language-specific code, you need to declare a build tool dependency on it. rosidl_default_runtime is a runtime or execution-stage dependency, needed to be able to use the interfaces later. The rosidl_interface_packages is the name of the dependency group that your package, tutorial_interfaces, should be associated with, declared using the <member_of_group> tag.

Add the following lines within the <package> element of package.xml:

<depend>geometry_msgs</depend>
<buildtool_depend>rosidl_default_generators</buildtool_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 (~/ros2_ws), run the following command:

colcon build --packages-select tutorial_interfaces

colcon build --packages-select tutorial_interfaces

colcon build --merge-install --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 (ros2_ws) to source it:

source install/setup.bash

. install/setup.bash

call install/setup.bat

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

ros2 interface show tutorial_interfaces/msg/Num

should return:

int64 num

And

ros2 interface show tutorial_interfaces/msg/Sphere

should return:

geometry_msgs/Point center
        float64 x
        float64 y
        float64 z
float64 radius

And

ros2 interface 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.txt and package.xml files will allow you to use your new interfaces.

7.1 Testing `Num.msg` with pub/sub

With a few 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

    auto timer_callback = [this](){
      auto message = tutorial_interfaces::msg::Num();                                   // CHANGE
      message.num = this->count_++;                                                     // CHANGE
      RCLCPP_INFO_STREAM(this->get_logger(), "Publishing: '" << message.num << "'");    // CHANGE
      publisher_->publish(message);
    };
    timer_ = this->create_wall_timer(500ms, timer_callback);
  }

private:
  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;
}

import rclpy
from rclpy.executors import ExternalShutdownException
from rclpy.node import Node

from tutorial_interfaces.msg import Num                            # CHANGE


class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(Num, 'topic', 10)  # CHANGE
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = Num()                                                # CHANGE
        msg.num = self.i                                           # CHANGE
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%d"' % msg.num)       # CHANGE
        self.i += 1


def main(args=None):
    try:
        with rclpy.init(args=args):
            minimal_publisher = MinimalPublisher()

            rclpy.spin(minimal_publisher)
    except (KeyboardInterrupt, ExternalShutdownException):
        pass


if __name__ == '__main__':
    main()

Subscriber

#include <functional>
#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")
  {
    auto topic_callback = [this](const tutorial_interfaces::msg::Num & msg){     // CHANGE
      RCLCPP_INFO_STREAM(this->get_logger(), "I heard: '" << msg.num << "'");    // CHANGE
    };
    subscription_ = this->create_subscription<tutorial_interfaces::msg::Num>(    // CHANGE
      "topic", 10, topic_callback);
  }

private:
  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;
}

import rclpy
from rclpy.executors import ExternalShutdownException
from rclpy.node import Node

from tutorial_interfaces.msg import Num                        # CHANGE


class MinimalSubscriber(Node):

    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            Num,                                               # CHANGE
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning

    def listener_callback(self, msg):
        self.get_logger().info('I heard: "%d"' % msg.num)  # CHANGE


def main(args=None):
    try:
        with rclpy.init(args=args):
            minimal_subscriber = MinimalSubscriber()

            rclpy.spin(minimal_subscriber)
    except (KeyboardInterrupt, ExternalShutdownException):
        pass


if __name__ == '__main__':
    main()

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>

<exec_depend>tutorial_interfaces</exec_depend>

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

On Linux/macOS:

colcon build --packages-select cpp_pubsub

On Windows:

colcon build --merge-install --packages-select cpp_pubsub

On Linux/macOS:

colcon build --packages-select py_pubsub

On Windows:

colcon build --merge-install --packages-select py_pubsub

Then open two new terminals, source ros2_ws in each, and run:

ros2 run cpp_pubsub talker

ros2 run cpp_pubsub listener

ros2 run py_pubsub talker

ros2 run py_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 a few 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();
}

from tutorial_interfaces.srv import AddThreeInts                                                           # CHANGE

import rclpy
from rclpy.executors import ExternalShutdownException
from rclpy.node import Node


class MinimalService(Node):

    def __init__(self):
        super().__init__('minimal_service')
        self.srv = self.create_service(AddThreeInts, 'add_three_ints', self.add_three_ints_callback)       # CHANGE

    def add_three_ints_callback(self, request, response):
        response.sum = request.a + request.b + request.c                                                   # CHANGE
        self.get_logger().info('Incoming request\na: %d b: %d c: %d' % (request.a, request.b, request.c))  # CHANGE

        return response

def main(args=None):
    try:
        with rclpy.init(args=args):
            minimal_service = MinimalService()

            rclpy.spin(minimal_service)
    except (KeyboardInterrupt, ExternalShutdownException):
        pass


if __name__ == '__main__':
    main()

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 = 41;
  request->b = 1;
  request->c = 1;                                                                           // 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::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;
}

from tutorial_interfaces.srv import AddThreeInts                            # CHANGE

import rclpy
from rclpy.executors import ExternalShutdownException
from rclpy.node import Node


class MinimalClientAsync(Node):

    def __init__(self):
        super().__init__('minimal_client_async')
        self.cli = self.create_client(AddThreeInts, 'add_three_ints')       # CHANGE
        while not self.cli.wait_for_service(timeout_sec=1.0):
            self.get_logger().info('service not available, waiting again...')
        self.req = AddThreeInts.Request()                                   # CHANGE

    def send_request(self):
        self.req.a = 41
        self.req.b = 1
        self.req.c = 1                                                      # CHANGE
        return self.cli.call_async(self.req)


def main(args=None):
    try:
        with rclpy.init(args=args):
            minimal_client = MinimalClientAsync()
            future = minimal_client.send_request()
            rclpy.spin_until_future_complete(minimal_client, future)
            response = future.result()
            minimal_client.get_logger().info(
                'Result of add_three_ints: for %d + %d + %d = %d' %                                # CHANGE
                (minimal_client.req.a, minimal_client.req.b, minimal_client.req.c, response.sum))  # CHANGE
    except (KeyboardInterrupt, ExternalShutdownException):
        pass


if __name__ == '__main__':
    main()

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>

<exec_depend>tutorial_interfaces</exec_depend>

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

On Linux/macOS:

colcon build --packages-select cpp_srvcli

On Windows:

colcon build --merge-install --packages-select cpp_srvcli

On Linux/macOS:

colcon build --packages-select py_srvcli

On Windows:

colcon build --merge-install --packages-select py_srvcli

Then open two new terminals, source ros2_ws in each, and run:

ros2 run cpp_srvcli server

ros2 run cpp_srvcli client

ros2 run py_srvcli service

ros2 run py_srvcli client

Summary 

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

This tutorial only scratches the surface about defining custom interfaces. You can learn more about it in About ROS 2 interfaces.

Next steps 

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

Creating custom msg and srv files

2.1 msg definition

2.2 srv definition

7.1 Testing Num.msg with pub/sub

7.2 Testing AddThreeInts.srv with service/client

7.1 Testing `Num.msg` with pub/sub

7.2 Testing `AddThreeInts.srv` with service/client