Publish one message without while infinite loop

Question

Hi all, I am using ROS Noetic. I tried to publish one message without "while infinite loop" in the node that I created. In fact, I need just to publish one message from the topic to socketCAN but it doesn't work. I know that the node will be killed but it should publish one message?!! Anybody know why ? Thank you.

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Originally posted by mros on ROS Answers with karma: 23 on 2020-12-10

Post score: 0

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Original comments

Comment by Akhil Kurup on 2020-12-10:
We don't know what your code looks like and what message you are trying to publish and where. Please post your code, at least a snippet, so we can get context. Also, please be more detailed in your question. What have you tried so far and what errors you are facing (if any)!

Answer 1

It most likely is the case that your publisher is publishing your message before any subscribers to that message have been connected. It takes a "non-zero" amount of time for the "hand-shaking" to occur through the ROS master and between the publisher/subscriber nodes. You can write a section of code that will wait for a subscriber, publish the message (to that subscriber) then shut down the node. If you immediately shutdown a node after calling publish on a publisher it is not guaranteed that the message was sent and received by the subscriber.

A convenient overload to the advertise function provided by the NodeHandle class allows a client to specify a SubscriberStatusCallback function pointer that gets called when a new subscriber connects to the publisher see the rosdocs.

Here is a simple code example with a std_msgs/String topic:

#include <std_msgs/String.h>

#include <ros/ros.h>

void subscriberConnectCallback(const ros::SingleSubscriberPublisher& publisher)
{
  ROS_INFO("Super cool I got a new subscriber [%s] on the %s topic!", publisher.getSubscriberName().c_str(),
           publisher.getTopic().c_str());

  // Build the message and publish
  std_msgs::String msg;
  msg.data = "super cool";
  publisher.publish(msg);

  // Signal shutdown
  ros::shutdown();
}

int main(int argc, char* argv[])
{
  ros::init(argc, argv, "test_node");

  // Create publisher with private node handle and bind callback to handle subscriber connection
  ros::NodeHandle pnh("~");
  ros::Publisher publisher =
      pnh.advertise<std_msgs::String>("chatter", 1, (ros::SubscriberStatusCallback)subscriberConnectCallback);

  // Spin this node until a shutdown is triggered
  ros::spin();

  return 0;
}

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Originally posted by danambrosio with karma: 106 on 2020-12-10

This answer was ACCEPTED on the original site

Post score: 0

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Original comments

Comment by mros on 2020-12-11:
Hello, Thank you so much for your help, it works well but I had to remove ("~").

Comment by jarvisschultz on 2020-12-11:
@mros If this is the correct answer and it solved your issue, please mark this answer as correct.

Comment by danambrosio on 2020-12-11:
Just for your information, the private node handle is commonly used for publishers and would require a rosrun argument remap or launch remap.

The ability to remap topics/services/actions is useful to prevent hardcoded topics/service names in a node.