service_client.cpp

#include "ros/ros.h"                          
#include "ros_tutorials_service/SrvTutorial.h"
#include <cstdlib>                            

int main(int argc, char **argv)            
{
  ros::init(argc, argv, "service_client");    

  if (argc != 3) 
  {
    ROS_INFO("cmd : rosrun ros_tutorials_service service_client arg0 arg1");
    ROS_INFO("arg0: double number, arg1: double number");
    return 1;
  }

  ros::NodeHandle nh;
  ros::ServiceClient ros_tutorials_service_client = nh.serviceClient<ros_tutorials_service::SrvTutorial>("ros_tutorial_srv");
  ros_tutorials_service::SrvTutorial srv;

  srv.request.a = atoll(argv[1]);
  srv.request.b = atoll(argv[2]);
  
  int count = 0;
  while (ros_tutorials_service_client.call(srv) && ++count < 100)
  {
    ROS_INFO("send srv, srv.Request.a and b: %ld, %ld", (long int)srv.request.a, (long int)srv.request.b);
    ROS_INFO("receive srv, srv.Response.result: %ld", (long int)srv.response.result);

    srv.request.a = srv.request.b;
    srv.request.b = srv.response.result;
  }
  ROS_INFO("end!");

  return 0;
}

service_server.cpp

#include "ros/ros.h"
#include "ros_tutorials_service/SrvTutorial.h"

bool calculation(ros_tutorials_service::SrvTutorial::Request &req,
                  ros_tutorials_service::SrvTutorial::Response &res)
{
    res.result = req.a + req.b;

    ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
    ROS_INFO("sending back response: %ld", (long int)res.result);

    return true;
} 

int main(int argc, char **argv) {
    ros::init(argc, argv, "service_server");
    ros::NodeHandle nh;

    ros::ServiceServer ros_tutorials_service_server = nh.advertiseService("ros_tutorial_srv", calculation);

    ROS_INFO("ready srv server!");
    ros::spin();
    return 0;
}