Loop through unbounded tracks array in radar_msgs/RadarTrackArray

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Hi,

I am having this programming problem:

void msgCallback(const radar_msgs::RadarTrackArray::ConstPtr& msg)
{
  for (int i=0; i < msg->tracks.size(); i++)
   {
      a.push_back(msg->tracks[i].track_id);
   }
}

int main(int argc, char** argv)
{
  ...
  ros::Subscriber radar_sub = nh.subscribe("/radarTrack",1000,msgCallback);
  ros::spin();
  return 0;
}

the topic "/radarTrack" publishes messages every 20ms and the tracks array is an unbounded array, that is, it gives different length of array at each frame. For example, msg->tracks.size() can be 420, 28, 128 and so on.. When the length of tracks array is large, the calculation time of for-loop could be larger than 20ms. In this case, Does the messages that are published by "/radarTrack" wait until the for-loop calculation is complete although its calculation time is larger than 20ms?

Any help will be much appreciated.