How to use data from a callback function in another function

Question

I'm trying to take the x, y, and yaw value from the callback function and use them in a different function. Although when I run the code I have below, all the variables below return a value of 0. From my thought process, I think the global variables should be accessible for the void move() function as well, which doesn't seem to be the case.

Can someone tell what's wrong?

#include "ros/ros.h"
#include "turtlesim/Pose.h"
#include "geometry_msgs/Twist.h"

float x = 0, y = 0, yaw = 0 ;

void callback(const turtlesim::Pose::ConstPtr& msg)
{
    x = msg->x;
    y = msg->y;
    yaw = msg->theta;
    
}

void move()
{
    while (ros::ok())
    {
        ROS_INFO("x: [%f], y: [%f], yaw: [%f]", x, y, yaw);
    }
}

    int main(int argc, char **argv)
    {
        ros::init(argc, argv, "pose");
        ros::NodeHandle n;
        ros::Subscriber sub = n.subscribe("/turtle1/pose", 1000, callback);
        move();
        ros::spin();
}

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Originally posted by dj95 on ROS Answers with karma: 52 on 2020-02-12

Post score: 0

Answer 1

So going line by line here to help show you the flow of the issue:

ros::init(argc, argv, "pose");
ros::NodeHandle n;
ros::Subscriber sub = n.subscribe("/turtle1/pose", 1000, callback);

All good, initializing ROS, getting a nodehandle, creating a subscriber object and giving it the callback function to call. The spinner would then throw new messages received on that topic of that type to the function.

move() {while (ros::ok()) {...}}

Well, this is going to block infinitely. And while you're going to exit when ROS shuts down, no where in this infinite loop do you actually call a spin or spinOnce for the ROS messages building up to be processed (with a 1000 message queue size).

ros::spin();

Then you spin. This spin will only be called once your move() method exits (ie you end the program) so you're only just then starting to process messages.

What you could do instead is call ros::spinOnce() in your move() function and remove the ros::spin() from the end of your main function. That way you're processing messages in the callback and printing some values to screen.

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Originally posted by stevemacenski with karma: 8272 on 2020-02-17

This answer was ACCEPTED on the original site

Post score: 2

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Original comments

Comment by dj95 on 2020-02-17:
Would it be okay if I call ros::spin() before calling the move()? Would that produce the same result?

Comment by stevemacenski on 2020-02-17:
No. Spin is blocking and will not return until the session is ended. Think of spin() being a while loop with a spin and a sleep (its more complex than that, but essentially).

Comment by dj95 on 2020-02-18:
Thank you for the explanation.