Publisher and subscriber in one ROS node error

asked 2021-10-03 07:25:09 -0500

Astronaut
302 ●177 ●206 ●214

I`m doing some underwater UUV Gazebo simulation and would like to use the

hector_pose_estimation

package to fuse IMU and Fluid pressure sensors input for pose estimation.

So my Pressure Sensor has sensor_msgs/FluidPressure but the package wants pressure_height (geometry_msgs/PointStamped) as input. So I created a ROS C++ node to republish sensor_msgs/FluidPressure as the geometry_msgs/PointStamped. The code compiled with no error, but when run it I got error. Firstly, this is the code

#include "ros/ros.h"
#include <sensor_msgs/FluidPressure.h>
#include <std_msgs/Float64.h>
#include <geometry_msgs/PointStamped.h>


double current_pressure;
double depth;
ros::Publisher pub;
geometry_msgs::PointStamped pressure_height;
void pressureCallback(const sensor_msgs::FluidPressure::ConstPtr& msg)
{

     current_pressure= msg->fluid_pressure;

     depth=current_pressure / (997.04 * 9.8);

     //geometry_msgs::PointStamped::ConstPtr& output;

     pressure_height.point.z = depth;
     //positions_pub.publish(output_msg); 

     std::cout << "Pressure: " << pressure_height << std::endl;

      //pressure_height = output.point.z;
      pub.publish(pressure_height);

}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "pressure_republisher");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("/rexrov2/pressure", 10, pressureCallback);
  ros::Publisher pub = n.advertise<geometry_msgs::PointStamped>("pressure_height", 100);
  ros::spin();
  return 0;
}

And when run it this it the ERROR:

[FATAL] [1633258406.824329181, 173.610000000]: ASSERTION FAILED
    file = /opt/ros/melodic/include/ros/publisher.h
    line = 106
    cond = false
    message = 
[FATAL] [1633258406.826511155, 173.612000000]: Call to publish() on an invalid Publisher
[FATAL] [1633258406.826551511, 173.612000000]:

Any help?

answered 2021-10-03 08:37:42 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2021-10-03 08:53:02 -0500

This is a C++ problem.

You have:

#include "ros/ros.h"
...

ros::Publisher pub;

...

int main(int argc, char **argv)
{
  ...
  ros::Publisher pub = n.advertise<geometry_msgs::PointStamped>("pressure_height", 100);
  ...
}

The pub outside your main(..) (ie: in the global scope) is not the same ros::Publisher as the one you create and initialise inside main(..). The latter is a new variable, which happens to have the same name (it shadows the pub in the global scope).

In your pressureCallback(..) you then try to use the pub in the global scope, which hasn't been initialised yet. This is an error, as you can't use a ros::Publisher which hasn't been initialised, hence the fatal error message .

You should be able to do this:

int main(int argc, char **argv)
{
  ...
  pub = n.advertise<geometry_msgs::PointStamped>("pressure_height", 100);
  ...
}

Having written that, the following might be easier, and still results in the conversion being performed:

rosrun topic_tools transform /rexrov2/pressure /pressure_height geometry_msgs/PointStamped \
  'geometry_msgs.msg.PointStamped( \
    header=m.header, \
    point=geometry_msgs.msg.Point( \
      x=0.0, \
      y=0.0, \
      z=(m.fluid_pressure / (997.04 * 9.8)) \
    )' \
   --import geometry_msgs

This uses topic_tools/transform, which avoids the need to write your own node entirely.

Note 1: this ignores the variance in the FluidPressure message.

Note 2: I haven't tested this, so there may be a syntax error hidden there.

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