First, we are going to learn how to create a service and how to use it in ROS. Our service will calculate the sum of three numbers. We need two nodes: a server and a client.

In the chapter2_tutorials package, create two new nodes with the following names: example2_a.cpp and example2_b.cpp. Remember to put the files in thesrc folder.

In the first file, example2_a.cpp, add the following code:

#include "ros/ros.h" 
#include "chapter2_tutorials/chapter2_srv1.h" 
 
bool add(chapter2_tutorials::chapter2_srv1::Request  &req, 
         chapter2_tutorials::chapter2_srv1::Response &res) 
{ 
  res.sum = req.A + req.B + req.C; 
  ROS_INFO("request: A=%ld, B=%ld C=%ld", (int)req.A, (int)req.B, 
     (int)req.C); 
  ROS_INFO("sending back response: [%ld]", (int)res.sum); 
  return true; 
} 
 
int main(int argc, char **argv) 
{ 
  ros::init(argc, argv, "add_3_ints_server"); 
  ros::NodeHandle n; 
  ros::ServiceServer service = n.advertiseService("add_3_ints", 
     add); 
  ROS_INFO("Ready to add 3 ints."); 
  ros::spin(); 
 
  return 0; 
} 

Let's explain the code. These lines include the necessary headers and the srv file that we created:

#include "ros/ros.h" 
#include "chapter2_tutorials/chapter2_srv1.h" 

This function will add three variables and send the result to the other node:

bool add(chapter2_tutorials::chapter2_srv1::Request  &req, 
         chapter2_tutorials::chapter2_srv1::Response &res) 

Here, the service is created and advertised over ROS:

ros::ServiceServer service = n.advertiseService("add_3_ints", 
   add);

In the second file, example2_b.cpp, add this code:

#include "ros/ros.h" 
#include "chapter2_tutorials/chapter2_srv1.h" 
#include <cstdlib> 
 
int main(int argc, char **argv) 
{ 
  ros::init(argc, argv, "add_3_ints_client"); 
  if (argc != 4) 
  { 
    ROS_INFO("usage: add_3_ints_client A B C "); 
    return 1; 
  } 
 
  ros::NodeHandle n; 
  ros::ServiceClient client = 
     n.serviceClient<chapter2_tutorials::chapter2_srv1>("add_3_ints"); 
  chapter2_tutorials::chapter2_srv1 srv; 
  srv.request.A = atoll(argv[1]); 
  srv.request.B = atoll(argv[2]); 
  srv.request.C = atoll(argv[3]); 
  if (client.call(srv)) 
  { 
    ROS_INFO("Sum: %ld", (long int)srv.response.sum); 
  } 
  else 
  { 
    ROS_ERROR("Failed to call service add_3_ints"); 
    return 1; 
  } 
  return 0; 
} 

As usual, let's explain the code. Create a client for the service with the nameadd_3_ints:

ros::ServiceClient client = 
 n.serviceClient<chapter2_tutorials::chapter2_srv1>("add_3_ints");

Here, we create an instance of our srv request type and fill all the values to be sent.
If you remember, the message has three fields:

chapter2_tutorials::chapter2_srv1 srv; 
srv.request.A = atoll(argv[1]); 
srv.request.B = atoll(argv[2]); 
srv.request.C = atoll(argv[3]); 

With this line, the service is called and the data is sent. If the call succeeds, call() will return true, and if not, call() will return false:

if (client.call(srv)) 

To build the new nodes, edit CMakeList.txt and add the following lines:

add_executable(example2_a src/example2_a.cpp) 
add_executable(example2_b src/example2_b.cpp) 
 
add_dependencies(example2_a 
 chapter2_tutorials_generate_messages_cpp)add_dependencies(example2_b 
 chapter2_tutorials_generate_messages_cpp) 
target_link_libraries(example2_a ${catkin_LIBRARIES}) 
target_link_libraries(example2_b ${catkin_LIBRARIES}) 

Now execute the following command:

    $ cd ~/dev/catkin_ws
    $ catkin_make  

To start the nodes, execute the following command lines:

    $ rosrun chapter2_tutorials example2_a
    $ rosrun chapter2_tutorials example2_b 1 2 3  

You should see something similar to this output:

    Node example2_a
    [ INFO] [1355256113.014539262]: Ready to add 3 ints.
    [ INFO] [1355256115.792442091]: request: A=1, B=2 C=3
    [ INFO] [1355256115.792607196]: sending back response: [6]
    Node example2_b
    [ INFO] [1355256115.794134975]: Sum: 6  

Now we are going to create nodes with our custom msg file. The example is the same, that is, example1_a.cpp and example1_b.cpp, but with the new message, chapter2_msg1.msg.

The following code snippet is present in the example3_a.cpp file:

#include "ros/ros.h" 
#include "chapter2_tutorials/chapter2_msg1.h" 
#include <sstream> 
 
int main(int argc, char **argv) 
{ 
  ros::init(argc, argv, "example3_a"); 
  ros::NodeHandle n; 
  ros::Publisher pub = 
     n.advertise<chapter2_tutorials::chapter2_msg1>("message", 1000); 
  ros::Rate loop_rate(10); 
  while (ros::ok()) 
  { 
    chapter2_tutorials::chapter2_msg1 msg; 
    msg.A = 1; 
    msg.B = 2; 
    msg.C = 3; 
    pub.publish(msg); 
    ros::spinOnce(); 
    loop_rate.sleep(); 
  } 
  return 0; 
} 

The following code snippet is present in the example3_b.cpp file:

#include "ros/ros.h" 
#include "chapter2_tutorials/chapter2_msg1.h" 
 
void messageCallback(const 
 chapter2_tutorials::chapter2_msg1::ConstPtr& msg) 
{ 
  ROS_INFO("I heard: [%d] [%d] [%d]", msg->A, msg->B, msg->C); 
} 
 
int main(int argc, char **argv) 
{ 
  ros::init(argc, argv, "example3_b"); 
  ros::NodeHandle n; 
  ros::Subscriber sub = n.subscribe("message", 1000, 
     messageCallback); 
  ros::spin(); 
  return 0; 
} 

If we run both nodes now, we will see something similar to the following output:

    ...
    [ INFO] [1355270835.920368620]: I heard: [1] [2] [3]
    [ INFO] [1355270836.020326372]: I heard: [1] [2] [3]
    [ INFO] [1355270836.120367449]: I heard: [1] [2] [3]
    [ INFO] [1355270836.220266466]: I heard: [1] [2] [3]
    ...