| 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
Contents
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.
beginner_tutorials/AddTwoInts.h is the header file generated from the srv file that we created earlier.
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.
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.
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:
Could not fetch external code from 'https://raw.github.com/ros/catkin_tutorials/master/create_package_srvclient/catkin_ws/src/beginner_tutorials/CMakeLists.txt': HTTP Error 429: Too Many Requests
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:
make sure you have followed the directions in the previous tutorial: creating the AddTwoInts.srv.
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.
Running the nodes
Running nodes requires you have a ROS core started. Open a new shell, and type:
roscore
If all goes well, you should see an output that looks something like this:
... logging to /u/takayama/.ros/logs/83871c9c-934b-11de-a451- 001d927076eb/roslaunch-ads-31831.log ... loading XML file [/wg/stor1a/rosbuild/shared_installation/ros/tools/roslaunch/roscore.xml] Added core node of type [rosout/rosout] in namespace [/] started roslaunch server http://ads:54367/ SUMMARY ====== NODES changing ROS_MASTER_URI to [http://ads:11311/] for starting master locally starting new master (master configured for auto start) process[master]: started with pid [31874] ROS_MASTER_URI=http://ads:11311/ setting /run_id to 83871c9c-934b-11de-a451-001d927076eb +PARAM [/run_id] by /roslaunch +PARAM [/roslaunch/uris/ads:54367] by /roslaunch process[rosout-1]: started with pid [31889] started core service [/rosout] +SUB [/time] /rosout http://ads:33744/ +SERVICE [/rosout/get_loggers] /rosout http://ads:33744/ +SERVICE [/rosout/set_logger_level] /rosout http://ads:33744/ +PUB [/rosout_agg] /rosout http://ads:33744/ +SUB [/rosout] /rosout http://ads:33744/
Now everything is set to run server and client.
Running the Server
Start by running the server. Open a new shell and type:
rosrun beginner_tutorials add_two_ints_server
You should see something similar to:
Ready to add two ints.
Running the Client
Now let's run the client with the necessary arguments, in another shell:
$ rosrun beginner_tutorials add_two_ints_client 1 3
In the client's shell, you should see something similar to:
Sum: 4
In the server's shell, instead, you should see something similar to:
request: x=1, y=3 sending back response: [4]
Now that you have written a simple service and client, let's examine the simple service and client.
Video Tutorial
The following video presents a small tutorial on ROS services.
