Question

I have a roscpp service callback that, upon request, does a few things and then publishes a message on a topic. Here is an oversimplified example:

void someServiceCb(std_srvs::Trigger::Request& req,
                   std_srvs::Trigger::Response& resp) {

  if (something)
  {
    std_msgs::Empty msg;
    pub.publish(msg);
    resp.success = true;
    ros::spinOnce();  // Do I need this? Can it cause problems?
    doSomeOtherStuff();
  }
  else
  {
    resp.success = false;
  }
}

The intent is to get the message out as soon as possible.

Do I need that ros::spinOnce() in order to get the message out as quickly as possible?
Can calling ros::spinOnce() inside a service callback cause problems?

Asked by spmaniato on 2017-01-14 11:08:24 UTC

Answer 1

You don't need a call back to publish a message. It will publish at "pub.publish(msg);". As to calling ros::spin from a callabck, I have no idea what happens. I've never used in that fashion. Call backs are typically used from the main() to enter a callback when new data arrives on a subscribed topic.

Asked by billy on 2017-01-14 14:26:11 UTC

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Thanks @billy. I've been doing some tests and comparing against the docs. It looks like on certain occasions calling ros::spinOnce() might be a problem (as it takes us back to the callback queue). Will try to formulate a full answer later today.

Asked by spmaniato on 2017-01-14 15:14:23 UTC

Yep; calling publish puts the message in a transmit queue, where it's picked up and sent by a background thread that isn't tied to spinOnce.

Asked by ahendrix on 2017-01-14 23:01:33 UTC

spinOnce does process message and service callbacks, so if there's another pending service request it can cause your service callback to be called a second time, from within the spinOnce in the first service callback.

Asked by ahendrix on 2017-01-14 23:02:14 UTC

Calling spinOnce inside a service callback that publishes a message

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