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my nodes don't send (receive) messages

asked 2012-12-18 03:28:06 -0600

ubisum gravatar image

hello everyone
I created a class stage_listener; then, in an externally defined main() function, an object of class stage_listener is created and its function listen is invoked.
The purpose of this function is forcing the object of class stage_listener to become a listener of certain kinds of messages. Here is the code of function:

void stage_listener::listen(int c, char **v){
    while_flag = 1;
    ROS_INFO("Listening activity begun");
    ros::init(c,v, "listener");
    ros::NodeHandle n_odom, n_scan, n_end;
/*  std_msgs::String s;

    //ros::Subscriber sub_odom = n_odom.subscribe("/odom", 1000, &stage_listener::end_loop,this);
    ros::Subscriber sub_odom = n_odom.subscribe("/odom", 1000, &stage_listener::addOdomNode,this);
    ros::Subscriber sub_scan = n_scan.subscribe("/base_scan", 1000, &stage_listener::addScanNode,this);
    ros::Subscriber sub_end = n_end.subscribe("/end", 1000, &stage_listener::end_loop,this);

        //ros::Subscriber sub_odom = n_odom.subscribe("/odom", 1000, &stage_listener::end_loop,this);
    ROS_INFO("Exit from while");

The function receives, as parameters, the argc and argv of the main() where object has been created, since such parameters are requested by ros::init.
These are the functions used for listening:

void stage_listener::end_loop(const std_msgs::String mes){
    ROS_INFO("Setting flag");
    while_flag = 0;

void stage_listener::addOdomNode (const nav_msgs::Odometry mes){
    geometry_msgs::Pose robot_pose = mes.pose.pose;
    geometry_msgs::Point robot_point = robot_pose.position;

    odom_node *on = new odom_node();
    (*on).xCoord = robot_point.x;
    (*on).yCoord = robot_point.y;
    (*on).frame_id = mes.header.frame_id;

    double orientation = tf::getYaw(robot_pose.orientation);
    (*on).angle = (float)orientation;


void stage_listener::addScanNode (const sensor_msgs::LaserScan mes){
    scan_node *sn = new scan_node();
    (*sn).scan = mes.ranges;
    (*sn).frame_id = mes.header.frame_id;
    (*sn).cartesian = polar2cart(mes.ranges, mes.angle_min, mes.angle_increment, mes.range_min, mes.range_max);


After invocation of function listen, the object created begins to wait for messages. An external node, in another package, prepares and sends an std_msgs::String message:

#include "std_msgs/String.h"
#include "ros/ros.h"

int main(int argc, char **argv){
    ros::init(argc, argv,"end_bag");
    ros::NodeHandle n;

    ros::Publisher chatter_pub = n.advertise<std_msgs::string>("/end",1000);
    ros::Rate loop_rate(10);

    std_msgs::String msg; = "end of loop";


    ROS_INFO("Message sent: %s",;



The problem is that the message created is never received by the waiting object, since it remains iterating on spinOnce() instruction, never exiting the loop, a result that should be granted by reception of message.

Can you individuate any error in my code causing receiver node not to get any message? thanks for support.

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1 Answer

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answered 2012-12-18 03:36:14 -0600

dornhege gravatar image

updated 2012-12-18 03:36:28 -0600

Your sender does NOT remain iterating in the spinOnce() instruction. As the name says: It only spins once. Thus, your sender will exit before anyone has connected. Use ros::spin() or put the spinOnce in a while loop to keep the program running.

Alternatively, you can wait until chatter_pub.getNumSubscribers() > 0 to make sure someone has connected before the program quits.

In general, when testing publish/subscribe: Use rostopic echo and rostopic pub in place of a subscriber/publisher to isolate where the error comes from.

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thanks for answer. let's suppose that message sent by node has to be stored into a bag file. the process (started from a shell) waiting for messages to store in bag file, is considered a subscriber? will it increment the number of subscribers of topic?

ubisum gravatar image ubisum  ( 2012-12-18 03:58:53 -0600 )edit

Yes and yes.

dornhege gravatar image dornhege  ( 2012-12-18 04:56:59 -0600 )edit

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Asked: 2012-12-18 03:28:06 -0600

Seen: 379 times

Last updated: Dec 18 '12