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Chapter-06.1-Configuring-Packages-(C++).md

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Chapter 6.1 Configuring Packages (C++)

In the last lesson, we wrote the code for a C++ ROS2 Publisher Node which sends a string type message “Hello World : ” followed by an integer that increments each time - inside the src folder of udemy_ros2_pkg package of our ros2_cpp_udemy_tutorial workspace.

Now, to run the piece of code we need to compile/build it first and create an executable (.exe) file.

In ROS2, the default C++ build system is called ament_cmake. ament_cmake is the build system for CMake based packages in ROS 2 (in particular, it will be used for most if not all C/C++ projects). And we will use ament_cmake to build our C++ code with the colcon command line tool.

The core instructions of building our udemy_ros2_pkg package are stored inside the CMakeLists.txt file of the package folder.

  • Step 1. Open the CMakeLists.txt file of the udemy_ros2_pkg package folder in VS Code.

  • Step 2. Add the following lines of code to the CMakeLists.txt file anywhere before the last line ament_package()

    find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)

add_executable(publisher src/publisher.cpp) 
ament_target_dependencies(publisher rclcpp std_msgs)

install(TARGETS
  publisher
  DESTINATION lib/${PROJECT_NAME}
)

    CMakeLists.txt file code after adding the above lines of code:

    cmake_minimum_required(VERSION 3.8)
project(udemy_ros2_pkg)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
# uncomment the following section in order to fill in
# further dependencies manually.
# find_package(<dependency> REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)

if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  # the following line skips the linter which checks for copyrights
  # comment the line when a copyright and license is added to all source files
  set(ament_cmake_copyright_FOUND TRUE)
  # the following line skips cpplint (only works in a git repo)
  # comment the line when this package is in a git repo and when
  # a copyright and license is added to all source files
  set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()

add_executable(publisher src/publisher.cpp) 
ament_target_dependencies(publisher rclcpp std_msgs)

install(TARGETS
  publisher
  DESTINATION lib/${PROJECT_NAME}
)

ament_package()

    💡 The ROS2 modules/packages (rclcpp & std_msgs) which contain the header files (rclcpp/rclcpp.hpp & std_msgs/msg/string.hpp) that we #included in our publisher.cpp code are called dependencies.

    💡 find_package(rclcpp REQUIRED) & find_package(std_msgs REQUIRED) : We need to add the dependencies used in our publisher.cpp file inside this find_package() function in the format of
    find_package(<dependency> REQUIRED)

    💡 add_executable(publisher src/publisher.cpp) :
    - publisher = name of the executable file that we are going to create for the publisher.cpp source code file
    - src/publisher.cpp = location of the source code file.

    💡 ament_target_dependencies(publisher rclcpp std_msgs) :
    - publisher = name of the executable file that we are going to create for the publisher.cpp source code file
    - rclcpp std_msgs = dependencies of publisher.cpp source code file.

    💡 install(TARGETS <executable_file_name> DESTINATION lib/${PROJECT_NAME} )

    - lib/${PROJECT_NAME} : This gives the address/destination for storing the executable file publisher when it is created ; which in this case is a folder by the name of ${PROJECT_NAME} inside the ros2_cpp_udemy_tutorial/install/udemy_ros2_pkg/lib directory. The variable PROJECT_NAME references our package name as written at line no.2 of this CMakeLists.txt file code as project(udemy_ros2_pkg).

    💡 If you ever want to change your package name, you have to change it in 3 places:

    1. The package folder name.
    2. CmakeList.txt file Line No.2 — project(udemy_ros2_pkg)
    3. package.xml file Line No.4 — udemy_ros2_pkg

  • Step 3. Add the following code to the package.xml file of the udemy_ros2_pkg package.

    <depend>rclcpp</depend>
<depend>std_msgs</depend>

    rclcpp & std_msgs are the dependencies that our publisher.cpp file of the udemy_ros2_pkg package uses.

    package.xml file after the code addition:

    <?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>udemy_ros2_pkg</name>
  <version>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="[email protected]">pritam</maintainer>
  <license>TODO: License declaration</license>

  <buildtool_depend>ament_cmake</buildtool_depend>
  <depend>rclcpp</depend>
  <depend>std_msgs</depend>

  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>

  <export>
    <build_type>ament_cmake</build_type>
  </export>

</package>

    Save your work before proceeding.

  • Step 4. Open a new terminal from the ros2_cpp_udemy_tutorial workspace directory. Run the following commands from the terminal:

    source /opt/ros/humble/setup.bash
# For setting up the ros2 environment in the terminal.
# Not needed if you already have added this command to your bashrc file using the command : echo "source /opt/ros/humble/setup.bash" >> ~/.bashrc

colcon build
# For compiling the entire ros2_cpp_udemy_tutorial workspace directory.
# After running this command successfully, you can actually see the executable file publisher created at ros2_cpp_udemy_tutorial/build/udemy_ros2_pkg folder.
# But we will use ros2 run command from our terminal to run this publisher executable file - which will be accessed from ros2_cpp_udemy_tutorial/install/udemy_ros2_pkg/lib/udemy_ros2_pkg

source install/setup.bash
# Sourcing the setup.bash file present inside the 'install' directory of our 'ros2_cpp_udemy_tutorial' workspace - so that our terminal becomes aware of the location of our 'ros2_cpp_udemy_tutorial' workspace.
# After running this command, all the packages built inside 'ros2_cpp_udemy_tutorial' workspace will be recognised by the terminal.

# To check if our terminal is sourced with the 'ros2_cpp_udemy_tutorial' workspace
ros2 pkg list
# This will give a long list of various ros2 packages in which we can see the names of all the packages built inside 'ros2_cpp_udemy_tutorial' workspace.
# In this case there is only one package built inside 'ros2_cpp_udemy_tutorial' workspace -> the 'udemy_ros2_pkg' package.

ros2 run udemy_ros2_pkg publisher
# ros2 run <name_of_package> <name_of_executable>
# This command does not work if we have not sourced our terminal to the 'ros2_cpp_udemy_tutorial' workspace folder.
# So please ensure to always run `source install/setup.bash` command before running this command.

  • Step 5. After running the last command above we cannot see anything on the terminal because the our “Hello World” messages are broadcasted over ROS2 DDS Communication Layer and not as standard output to the terminal. To see the messages on terminal, open a second terminal in the ros2_cpp_udemy_tutorial workspace directory → Run the following commands:

    ros2 node list
# Gives list of active nodes
# /hello_world_pub_node

ros2 topic list
# Gives list of active topics
# /hello_world
# /parameter_events
# /rosout

ros2 topic echo /hello_world
# For listening to the /hello_world topic messages.

Change the name of a Node with ros2 run – at run time

You can add many arguments to the ros2 run command. Among them, there is one allowing you to directly change the node’s name at run time, without having to re-write/re-compile anything.

The first thing to add is -ros-args . Use -ros-args only once, and put all arguments after it.

To change the node’s name from “my_node” to “another_node”, use r __node:=...:

$ ros2 run ros2_tutorials_py minimal_node --ros-args -r __node:=another_node
[INFO] [1593588911.947569209] [another_node]: Node has been started.

You can see on the logline: the name of the node has been changed!

This feature will be handy when you want to launch multiple nodes with different names. For example, if you have a single_wheel_controller node, you can create a right_wheel_controller and a left_wheel_controller node, using the same code and executable.

And this is important: you can’t start 2 nodes with the same name, or else expect to see some weird behavior.