ROS course 3: Simple Subcribe/Publish

1. Environment

This post is tested under the following Environment:

Item	Value
CPU	AMD Ryzen 5 2600
Ubuntu	18.04
ROS	Melodic

For installation, see 02 Install. The source code of all parts is upload of github, see 11 git for more detail.

Summary

Our example has the following structure. talker send message to chatter topic. listener read message from chatter topic and display into console.

Source code is as follow :

src/basic_lecture/
├── launch
├── msg
└── src
    ├── basic_simple_talker.cpp
    ├── basic_simple_listener.cpp        
├── package.xml
├── CMakeLists.txt

Procedure

Create ros package

mkdir -p ~/catkin_ws/src
mkdir ros_lecture
cd ~/catkin_ws/src/ros_lecture

# catkin_create_pkg [package] [dependency 1] [dependency 2] ...
# create package basic_lecture with msgs, roscpp, rospy as dependencies
catkin_create_pkg basic_lecture std_msgs rospy roscpp
cd ~/catkin_ws/src/ros_lecture/basic_lecture/src
touch basic_simple_talker.cpp
touch basic_simple_listener.cpp

Code of Publisher

#include "ros/ros.h"
#include "std_msgs/String.h"

int main(int argc, char **argv){
  ros::init(argc, argv, "simple_talker");
  ros::NodeHandle n;
  ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 10);
  ros::Rate loop_rate(10);

  while (ros::ok()){
    std_msgs::String msg;
    msg.data = "hello world!";
    ROS_INFO("%s", msg.data.c_str());
    chatter_pub.publish(msg);

    ros::spinOnce();
    loop_rate.sleep();
  }
  return 0;
}

Code of Subcriber

#include "ros/ros.h"
#include "std_msgs/String.h"

void chatterCallback(const std_msgs::String& msg){
  ROS_INFO("I heard: [%s]", msg.data.c_str());
}

int main(int argc, char **argv){
  ros::init(argc, argv, "listener");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("chatter", 10, chatterCallback);
  ros::Rate loop_rate(10);

  ros::spin();
  return 0;
}

Edit build file

ROS use catkin as build tool. Catkin is based on CMake. in this example, we want to build basic_simple_talker.cpp and basic_simple_listener.cpp. We add the following 4 lines to CMakeList.txt

add_executable(basic_simple_talker src/basic_simple_talker.cpp)
add_executable(basic_simple_listener src/basic_simple_listener.cpp)

target_link_libraries(basic_simple_talker ${catkin_LIBRARIES})
target_link_libraries(basic_simple_listener ${catkin_LIBRARIES})

Build

cd ~/catkin_ws
catkin_make

Execution

# in 1st terminal
roscore
# in 2nd terminal
rosrun basic_lecture basic_simple_talker
# in 3rd terminal
rosrun basic_lecture basic_simple_listener

Result

Reference

1. ROS wiki: publish/subcribe

6. Table of Contents

ROS TOC