Callback from subscriber does not work

Question

Hi. This is probably an insignificant problem, but I just can't see it.

I used the publisher/subscriber tutorial to make communication in my own program. The publisher publish well in a topic, and the subscriber is subscribe to it, or 'rostopic info' says so, but the callback function does not even start. Whatever I have inside (a variable change, a ROS_INFO, etc.), nothing seems to happen.

This is my code (without the lines of my own program, which are not important):

Publisher:

#include <ros/ros.h>
#include "std_msgs/String.h"
#include <sstream>
#include <iostream>

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

  ros::NodeHandle nh;

  std_msgs::String msg;

  ros::Publisher chatter_pub = nh.advertise<std_msgs::String>("chatter", 10);


  ros::Rate rate(10.0);

  while (nh.ok()){
   
    msg.data = "hello world";
    chatter_pub.publish(msg);


    ros::spinOnce();

    rate.sleep();
  }
  return 0;
};

Subscriber:

#include <ros/ros.h>
#include <ros/console.h>
#include <std_msgs/String.h>
#include <iostream>
#include <sstream>

int message=0;

void cb(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO_STREAM("callback");
  message=1;

}


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

ros::NodeHandle n;

  ros::Rate rate(10.0);
  while (node.ok()){
  
 ros::Subscriber sub = n.subscribe ("/chatter", 10, cb);

 ROS_INFO_STREAM(message);
    rate.sleep();
  }

  ros::spinOnce();
  return 0;
};

Everything else from the program works, and it executes well, it is just the callback function wich seems not to run.

Thanks in advance for the answers.

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Originally posted by alexjs on ROS Answers with karma: 3 on 2014-11-18

Post score: 0

Answer 1

The ros::spinOnce(), where the callback is actually called, is not in your main while loop! Further the subscriber should be set up only once before the while loop. Should be like:

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

ros::NodeHandle n;

 ros::Subscriber sub = n.subscribe ("/chatter", 10, cb);

  ros::Rate rate(10.0);
  while (node.ok()){


 ROS_INFO_STREAM(message);
    ros::spinOnce(); // this is where the magic happens!!
    rate.sleep();
  }

  return 0;

};

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Originally posted by Wolf with karma: 7555 on 2014-11-18

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by alexjs on 2014-11-20:
Thank yo so much, that was the problem!

Comment by nemesis on 2015-08-07:
@Wolf - I am facing a similar problem but I the above recommendation still doesn't work for me. Could you have a look? http://answers.ros.org/question/214749/subscriber-callback-function-not-called-when-trying-to-capture-data-using-kinect/