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Problem with data from subscriber not going anywhere

asked 2017-12-27 04:55:33 -0600

hanjuku-joshi gravatar image

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In my code there are two subscriber functions. I want to pass the shared pointer data to other data defined in my file. My code is below:

    std::vector<double> imu_, euler_t;
    ros::Publisher escs_pub;

    ros::Subscriber imu_sub;
    ros::Subscriber DOF_sub;
    double I_cm_xx, I_cm_yy, I_cm_zz, P_q, P_w;

    void KeyboardCallback(const maple::Euler_Thrust::ConstPtr& msg){

    void imuCallback(const sensor_msgs::Imu::ConstPtr &imu) {

    int main(int argc, char **argv){
        ros::init(argc, argv, "P_2");
            ros::NodeHandle nh;
            ros::NodeHandle pnh("~");
            pnh.param("I_cm_xx", I_cm_xx, 0.0118);
            pnh.param("I_cm_yy", I_cm_yy, 0.0118);
            pnh.param("I_cm_zz", I_cm_zz, 0.0181);
            pnh.param("P_q", P_q, 2.27);
            pnh.param("P_w", P_w, 0.85);
            escs_pub = nh.advertise<maple::ESCs>("ESCs",1);
            DOF_sub = nh.subscribe("KeyboardTeleop", 1, KeyboardCallback);
            imu_sub = nh.subscribe("imu", 1, imuCallback);
            my_pkg::std_msg msg;

            double q_ref_w = cos(euler_t[0]/2)*cos(euler_t[1]/2)*cos(euler_t[2]/2); //THIS IS WHERE THE SEGMENTATION FAULT OCCURS, BECAUSE THE EULER_T VECTOR IS EMPTY OR UNPOPULATED!!!!!!

                  //more math  


        return 0;

I'm so lost... When excluding any operations with the vectors doing ROS_INFO inside the subscriber functions shows that they do indeed get the data too. I made this code as simple as it can be and it's still not working. I have a feeling it's a problem with the vectors not being passed the values in the right order and I tried that but nothing works. Please, any help would be massively appreciated

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answered 2017-12-27 07:39:02 -0600

There are a few issues here (some of which you could probably avoid if you go through the ROS tutorials carefully ;) )

  • Advertise and subscribe should be done outside the while loop as otherwise publishers and subscribers get re-initalized every time.
  • When running a while ros::ok() loop, you should call ros::spinOnce() instead of spin() as the latter is a blocking call.
  • The euler_t vector only gets resized in the subscriber callback. As callbacks only get serviced when a spin (or spinOnce, see above) has been called, it is perfectly normal that your code trying to access elements of the vector crashes.
  • Even if you would call spin correctly, there is no guarantee that the callback filling euler_t is called before you try accessing elements of it in the while-loop. For this reason (and many others) it's a good idea to check if the vector really has the correct size before trying to access it's elements directly.
  • Instead of filling vectors in the callbacks you could also store the shared pointer to the last received message and then check if that one is non-zero wherever you try to use the data.
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Excellent advice. I actually just ended up solving the problem using arrays instead of vectors, but you certainly have me plenty information and advice to work on for the future. Legend :D

hanjuku-joshi gravatar image hanjuku-joshi  ( 2017-12-27 08:15:13 -0600 )edit

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Asked: 2017-12-27 04:55:33 -0600

Seen: 63 times

Last updated: Dec 27 '17