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The exact answer of my question is

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

ros::Publisher vel_pub;


void negotiatorCallback(const geometry_msgs::Twist::ConstPtr& msg)
{
        vel_pub.publish(msg);

}


int main(int argc, char **argv)
{
    flag = 0;
    ros::init(argc, argv, "negotiator");
    ros::NodeHandle p;
    vel_pub = p.advertise<geometry_msgs::Twist>("cmd_vel",1000);

    ros::NodeHandle n;
    ros::Subscriber sub = n.subscribe<geometry_msgs::Twist>("tele_cmd_vel", 0, negotiatorCallback);

    ros::NodeHandle vel_sub;
    ros::Subscriber tele_cmd_vel_sub = vel_sub.subscribe<geometry_msgs::Twist>("move_base_cmd_vel", 1000, negotiatorCallback);
    ros::spin();
    return 0;
}