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Note: This tutorial assumes that you have completed the previous tutorials: examining the simple publisher and subscriber.
(!) Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags.

Writing a Simple Service and Client (C++)

Description: This tutorial covers how to write a service and client node in C++.

Tutorial Level: BEGINNER

Next Tutorial: Examining the simple service and client

Writing a Service Node

Here we'll create the service ("add_two_ints_server") node which will receive two ints and return the sum.

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

Please make sure you have followed the directions in the previous tutorial for creating the service needed in this tutorial, creating the AddTwoInts.srv (be sure to choose the right version of build tool you're using at the top of wiki page in the link).

The Code

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

   1 #include "ros/ros.h"
   2 #include "beginner_tutorials/AddTwoInts.h"
   3 
   4 bool add(beginner_tutorials::AddTwoInts::Request  &req,
   5          beginner_tutorials::AddTwoInts::Response &res)
   6 {
   7   res.sum = req.a + req.b;
   8   ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
   9   ROS_INFO("sending back response: [%ld]", (long int)res.sum);
  10   return true;
  11 }
  12 
  13 int main(int argc, char **argv)
  14 {
  15   ros::init(argc, argv, "add_two_ints_server");
  16   ros::NodeHandle n;
  17 
  18   ros::ServiceServer service = n.advertiseService("add_two_ints", add);
  19   ROS_INFO("Ready to add two ints.");
  20   ros::spin();
  21 
  22   return 0;
  23 }

The Code Explained

Now, let's break the code down.

   1 #include "ros/ros.h"
   2 #include "beginner_tutorials/AddTwoInts.h"
   3 

beginner_tutorials/AddTwoInts.h is the header file generated from the srv file that we created earlier.

   4 bool add(beginner_tutorials::AddTwoInts::Request  &req,
   5          beginner_tutorials::AddTwoInts::Response &res)

This function provides the service for adding two ints, it takes in the request and response type defined in the srv file and returns a boolean.

   6 {
   7   res.sum = req.a + req.b;
   8   ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
   9   ROS_INFO("sending back response: [%ld]", (long int)res.sum);
  10   return true;
  11 }

Here the two ints are added and stored in the response. Then some information about the request and response are logged. Finally the service returns true when it is complete.

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

Here the service is created and advertised over ROS.

Writing the Client Node

The Code

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

   1 #include "ros/ros.h"
   2 #include "beginner_tutorials/AddTwoInts.h"
   3 #include <cstdlib>
   4 
   5 int main(int argc, char **argv)
   6 {
   7   ros::init(argc, argv, "add_two_ints_client");
   8   if (argc != 3)
   9   {
  10     ROS_INFO("usage: add_two_ints_client X Y");
  11     return 1;
  12   }
  13 
  14   ros::NodeHandle n;
  15   ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");
  16   beginner_tutorials::AddTwoInts srv;
  17   srv.request.a = atoll(argv[1]);
  18   srv.request.b = atoll(argv[2]);
  19   if (client.call(srv))
  20   {
  21     ROS_INFO("Sum: %ld", (long int)srv.response.sum);
  22   }
  23   else
  24   {
  25     ROS_ERROR("Failed to call service add_two_ints");
  26     return 1;
  27   }
  28 
  29   return 0;
  30 }

The Code Explained

Now, let's break the code down.

  15   ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");

This creates a client for the add_two_ints service. The ros::ServiceClient object is used to call the service later on.

  16   beginner_tutorials::AddTwoInts srv;
  17   srv.request.a = atoll(argv[1]);
  18   srv.request.b = atoll(argv[2]);

Here we instantiate an autogenerated service class, and assign values into its request member. A service class contains two members, request and response. It also contains two class definitions, Request and Response.

  19   if (client.call(srv))

This actually calls the service. Since service calls are blocking, it will return once the call is done. If the service call succeeded, call() will return true and the value in srv.response will be valid. If the call did not succeed, call() will return false and the value in srv.response will be invalid.

Building your nodes

Again edit the beginner_tutorials CMakeLists.txt:

$ rosed beginner_tutorials CMakeLists.txt 

and add the following at the end:

rosbuild_add_executable(add_two_ints_server src/add_two_ints_server.cpp)
rosbuild_add_executable(add_two_ints_client src/add_two_ints_client.cpp)

This will create two executables, "add_two_ints_server" and "add_two_ints_client", 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

Again edit the beginner_tutorials CMakeLists.txt located at ~/catkin_ws/src/beginner_tutorials/CMakeLists.txt and add the following at the end:

https://raw.github.com/ros/catkin_tutorials/master/create_package_srvclient/catkin_ws/src/beginner_tutorials/CMakeLists.txt

  27 add_executable(add_two_ints_server src/add_two_ints_server.cpp)
  28 target_link_libraries(add_two_ints_server ${catkin_LIBRARIES})
  29 add_dependencies(add_two_ints_server beginner_tutorials_gencpp)
  30 
  31 add_executable(add_two_ints_client src/add_two_ints_client.cpp)
  32 target_link_libraries(add_two_ints_client ${catkin_LIBRARIES})
  33 add_dependencies(add_two_ints_client beginner_tutorials_gencpp)

This will create two executables, add_two_ints_server and add_two_ints_client, which by default will go into package directory of your devel space, located by default at ~/catkin_ws/devel/lib/<package name>. 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 description of the CMakeLists.txt file see: catkin/CMakeLists.txt

Now run catkin_make:

# In your catkin workspace
cd ~/catkin_ws
catkin_make

If your build fails for some reason:

  • type rosls beginner_tutorials/srv_gen/cpp/include/beginner_tutorials/ and check that the .h file matches the include in the client and server .cpp files.

Now that you have written a simple service and client, let's examine the simple service and client.


2017-10-21 12:25