Writing a Simple Service and Client (C++)

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

目录

1. Writing a Service Node
   1. The Code
   2. The Code Explained
2. Writing the Client Node
   1. The Code
   2. The Code Explained
3. Building your nodes

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 in the earlier tutorial,creating a package::

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.

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 theadd_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 andResponse.

  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 insrv.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, seeCMakeLists Now run make:

$ 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.

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