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No response from service, node won't close either

asked 2018-02-25 10:15:17 -0500

Pigskin gravatar image

I am running into an issue with calling a service in the callback function of a listener. I couldn't find a way to call the service as the initialization of the client stood in the main, but moving that to the function as well gave a whole different series of errors, so i tried to implement a work-around

#include "ros/ros.h"
#include "std_msgs/String.h"
#include "final/add_duplicates.h"
#include <string>
int message = 0;
std::string text;

void preparefortrouble(const std_msgs::String::ConstPtr& msg)
{
text = msg->data.c_str();
message = 1;
}

int main(int argc, char **argv)
{
    ros::init(argc, argv, "add_duplicates_client");
    ros::init(argc, argv, "print_twice");
    ros::NodeHandle n;  
    ros::Subscriber sub = n.subscribe("print_twice", 1, preparefortrouble);
    ros::ServiceClient client = n.serviceClient<final::add_duplicates>("add_duplicates");

    while (1){
        while (message == 1)
        {
            final::add_duplicates srv;
            srv.request.text = text.c_str();
            srv.request.nrOfDuplicates = 2;
            if (client.call(srv) == false)
            {
                ROS_ERROR("Orga het is stuk");
            }
            message = 0;
        }
    }
  ros::spin();

  return 0;
}

The server has been tested and works fine, the topic is called in the terminal. However, when i use this node, it cannot be close by crtl-c and sending a message to the topic results in nothing.

i try to call it with

$ rostopic pub /print_twice std_msgs/string “hello"

and my srv file is as following:

string text
int32 nrOfDuplicates
---
string multitext

thanks for the support.

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Comments

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I couldn't find a way to call the service as the initialization of the client stood in the main

I don't really understand what you write here, so I've ignored that for now.

What are you trying to do really?

gvdhoorn gravatar imagegvdhoorn ( 2018-02-25 10:50:06 -0500 )edit

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answered 2018-02-25 10:46:05 -0500

gvdhoorn gravatar image

updated 2018-02-25 11:08:18 -0500

and sending a message to the topic results in nothing

This is most likely caused by the fact that your node does not handle any incoming events (ie: msgs on topics).

To remedy that you'd need to either call ros::spin() or periodically call ros::spinOnce().

Your node does the former, but only after it has progressed through the while-loops. And this is the problem, as in order to get through the loops, your node needs to handle incoming events (otherwise message will never be set to 1). This seems to be a deadlock situation.

I would suggest to avoid the two nested while-loops, as I believe they are not needed and only complicate things.

What are you actually trying to do and what are the two while-loops supposed to achieve?

Some other things I noticed:

  • there is no ros::Rate::sleep() in the body of either of your while-loops (busy-wait, 100% cpu usage)
  • you have two calls to ros::init(..) in there, I'm not sure that is going to work (or at the very least: it's not needed)
  • avoid using final for anything (unless you want to declare something as final). It is a C++11 keyword: final specifier (since C++11)

However, when i use this node, it cannot be close by crtl-c

if you need to run a while-loop, make sure to use something like while (ros::ok()) { .. }. This makes your node check whether it has been requested to exit.

See also wiki/roscpp/Overview/Callbacks and Spinning.


Edit: (nested) while-loops are not necessarily a good idea with an event-based, asynchronous framework such as ROS. You may want to re-evaluate your approach and possibly restructure your node.

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Asked: 2018-02-25 10:15:17 -0500

Seen: 250 times

Last updated: Feb 25 '18