Note: This tutorial assumes that you have completed the previous tutorials: understanding ROS services and parameters.
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Writing a Simple Publisher and Subscriber (C++)

Description: This tutorial covers how to write a publisher and subscriber node in C++.

Tutorial Level: BEGINNER

Next Tutorial: Examining the simple publisher and subscriber

Contents

  1. Writing the Publisher Node
    1. The Code
    2. The Code Explained
  2. Writing the Subscriber Node
    1. The Code
    2. The Code Explained
  3. Building your nodes
  4. Building your nodes
  5. Additional Resources
    1. Video Tutorial

Writing the Publisher Node

"Node" is the ROS term for an executable that is connected to the ROS network. Here we'll create a publisher ("talker") node which will continually broadcast a message.

Change directory into the beginner_tutorials package, you created previously in the creating a rosbuild package tutorial: 

roscd beginner_tutorials

Change directories to your beginner_tutorials package you created in your catkin workspace previous tutorials: 

roscd beginner_tutorials

The Code

Create a src directory in the beginner_tutorials package directory: 

mkdir -p src

This directory will contain any source files for our beginner_tutorials package.

Create the src/talker.cpp file within the beginner_tutorials package and paste the following inside it:

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp':

The Code Explained

Now, let's break the code down.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': ros/ros.h is a convenience include that includes all the headers necessary to use the most common public pieces of the ROS system.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': This includes the std_msgs/String message, which resides in the std_msgs package. This is a header generated automatically from the String.msg file in that package. For more information on message definitions, see the msg page.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Initialize ROS. This allows ROS to do name remapping through the command line -- not important for now. This is also where we specify the name of our node. Node names must be unique in a running system.

The name used here must be a base name, ie. it cannot have a / in it.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Create a handle to this process' node. The first NodeHandle created will actually do the initialization of the node, and the last one destructed will cleanup any resources the node was using.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Tell the master that we are going to be publishing a message of type std_msgs/String on the topic chatter. This lets the master tell any nodes listening on chatter that we are going to publish data on that topic. The second argument is the size of our publishing queue. In this case if we are publishing too quickly it will buffer up a maximum of 1000 messages before beginning to throw away old ones.

NodeHandle::advertise() returns a ros::Publisher object, which serves two purposes: 1) it contains a publish() method that lets you publish messages onto the topic it was created with, and 2) when it goes out of scope, it will automatically unadvertise.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': A ros::Rate object allows you to specify a frequency that you would like to loop at. It will keep track of how long it has been since the last call to Rate::sleep(), and sleep for the correct amount of time.

In this case we tell it we want to run at 10Hz.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': By default roscpp will install a SIGINT handler which provides Ctrl-C handling which will cause ros::ok() to return false if that happens.

ros::ok() will return false if:

  • a SIGINT is received (Ctrl-C)
  • we have been kicked off the network by another node with the same name

  • ros::shutdown() has been called by another part of the application.

  • all ros::NodeHandles have been destroyed

Once ros::ok() returns false, all ROS calls will fail.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': We broadcast a message on ROS using a message-adapted class, generally generated from a msg file. More complicated datatypes are possible, but for now we're going to use the standard String message, which has one member: "data".

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Now we actually broadcast the message to anyone who is connected.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': ROS_INFO and friends are our replacement for printf/cout. See the rosconsole documentation for more information.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Calling ros::spinOnce() here is not necessary for this simple program, because we are not receiving any callbacks. However, if you were to add a subscription into this application, and did not have ros::spinOnce() here, your callbacks would never get called. So, add it for good measure.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp': Now we use the ros::Rate object to sleep for the time remaining to let us hit our 10Hz publish rate.

Here's the condensed version of what's going on:

  • Initialize the ROS system

  • Advertise that we are going to be publishing std_msgs/String messages on the chatter topic to the master

  • Loop while publishing messages to chatter 10 times a second

Now we need to write a node to receive the messsages.

Writing the Subscriber Node

The Code

Create the src/listener.cpp file within the beginner_tutorials package and paste the following inside it:

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/listener/listener.cpp':

The Code Explained

Now, let's break it down piece by piece, ignoring some pieces that have already been explained above.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/listener/listener.cpp': This is the callback function that will get called when a new message has arrived on the chatter topic. The message is passed in a boost shared_ptr, which means you can store it off if you want, without worrying about it getting deleted underneath you, and without copying the underlying data.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/listener/listener.cpp': Subscribe to the chatter topic with the master. ROS will call the chatterCallback() function whenever a new message arrives. The 2nd argument is the queue size, in case we are not able to process messages fast enough. In this case, if the queue reaches 1000 messages, we will start throwing away old messages as new ones arrive.

NodeHandle::subscribe() returns a ros::Subscriber object, that you must hold on to until you want to unsubscribe. When the Subscriber object is destructed, it will automatically unsubscribe from the chatter topic.

There are versions of the NodeHandle::subscribe() function which allow you to specify a class member function, or even anything callable by a Boost.Function object. The roscpp overview contains more information.

Could not fetch external code from 'https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/listener/listener.cpp': ros::spin() enters a loop, calling message callbacks as fast as possible. Don't worry though, if there's nothing for it to do it won't use much CPU. ros::spin() will exit once ros::ok() returns false, which means ros::shutdown() has been called, either by the default Ctrl-C handler, the master telling us to shutdown, or it being called manually.

There are other ways of pumping callbacks, but we won't worry about those here. The roscpp_tutorials package has some demo applications which demonstrate this. The roscpp overview also contains more information.

Again, here's a condensed version of what's going on:

  • Initialize the ROS system

  • Subscribe to the chatter topic

  • Spin, waiting for messages to arrive

  • When a message arrives, the chatterCallback() function is called

Building your nodes

roscreate-pkg will create a default Makefile and CMakeLists.txt for your package. 

$ rosed beginner_tutorials CMakeLists.txt

It should look something like this: 

  • cmake_minimum_required(VERSION 2.4.6)
include($ENV{ROS_ROOT}/core/rosbuild/rosbuild.cmake)

# Set the build type.  Options are:
#  Coverage       : w/ debug symbols, w/o optimization, w/ code-coverage
#  Debug          : w/ debug symbols, w/o optimization
#  Release        : w/o debug symbols, w/ optimization
#  RelWithDebInfo : w/ debug symbols, w/ optimization
#  MinSizeRel     : w/o debug symbols, w/ optimization, stripped binaries
#set(ROS_BUILD_TYPE RelWithDebInfo)

rosbuild_init()

#set the default path for built executables to the "bin" directory
set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)
#set the default path for built libraries to the "lib" directory
set(LIBRARY_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/lib)

#uncomment if you have defined messages
#rosbuild_genmsg()
#uncomment if you have defined services
#rosbuild_gensrv()

#common commands for building c++ executables and libraries
#rosbuild_add_library(${PROJECT_NAME} src/example.cpp)
#target_link_libraries(${PROJECT_NAME} another_library)
#rosbuild_add_boost_directories()
#rosbuild_link_boost(${PROJECT_NAME} thread)
#rosbuild_add_executable(example examples/example.cpp)
#target_link_libraries(example ${PROJECT_NAME})

Adding the following at the bottom: 

rosbuild_add_executable(talker src/talker.cpp)
rosbuild_add_executable(listener src/listener.cpp)

This will create two executables, talker and listener, which by default will go into the "bin" directory.

For more information on using CMake with ROS, see CMakeLists Now run make: 

$ make

Building your nodes

You used catkin_create_pkg in a previous tutorial which created a package.xml and a CMakeLists.txt file for you.

The generated CMakeLists.txt should look like this (with modifications from the Creating Msgs and Srvs tutorial and unused comments and examples removed):

Could not fetch external code from 'https://raw.github.com/ros/catkin_tutorials/master/create_package_modified/catkin_ws/src/beginner_tutorials/CMakeLists.txt':

Don't worry about modifying the commented (#) examples, simply add these few lines to the bottom of your CMakeLists.txt: 

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

Your resulting CMakeLists.txt file should look like this:

Could not fetch external code from 'https://raw.github.com/ros/catkin_tutorials/master/create_package_pubsub/catkin_ws/src/beginner_tutorials/CMakeLists.txt':

This will create two executables, talker and listener, which by default will go into package directory of your devel space, located by default at ~/catkin_ws/devel/lib/<package name>.

Note that you have to add dependencies for the executable targets to message generation targets: 

add_dependencies(talker beginner_tutorials_generate_messages_cpp)

This makes sure message headers of this package are generated before being used. If you use messages from other packages inside your catkin workspace, you need to add dependencies to their respective generation targets as well, because catkin builds all projects in parallel. As of *Groovy* you can use the following variable to depend on all necessary targets: 

target_link_libraries(talker ${catkin_LIBRARIES})

You can invoke executables directly or you can use rosrun to invoke them. They are not placed in '<prefix>/bin' because that would pollute the PATH when installing your package to the system. If you wish for your executable to be on the PATH at installation time, you can setup an install target, see: catkin/CMakeLists.txt

For more detailed discription of the CMakeLists.txt file see: catkin/CMakeLists.txt

Now run catkin_make: 

# In your catkin workspace
$ cd ~/catkin_ws
$ catkin_make

Note: Or if you're adding as new pkg, you may need to tell catkin to force making by --force-cmake option. See catkin/Tutorials/using_a_workspace#With_catkin_make.

Now that you have written a simple publisher and subscriber, let's examine the simple publisher and subscriber.

Additional Resources

Here are some additional resources contributed by the community:

Video Tutorial

The following video presents a small tutorial explaining how to write and test a publisher and subscriber in ROS with C++ and Python based on the talker/listener example above

Wiki: ROS/Tutorials/WritingPublisherSubscriber(c++) (last edited 2019-07-18 19:12:37 by AnisKoubaa)

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