Possibility of data race in subscriber callback(s)?

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Short answer: no, there is not a memory race (in terms of two threads writing to the same memory address at the same time). By default, ROS doesn't start any threads when you make subscribers/callbacks. When you call ros::spin() or ros::spinOnce(), it will call each of the subscribers you have added sequentially, in the same thread as the spin.

Note about your code: if the while(ros::ok()) _effort = this->getEffort(); is running in the main loop, then your subscribers will never get called, since (as mentioned above), in a single threaded program, subscribers are only called when ros::spin() or ros::spinOnce() is called. A slight modification to your above code that would call the subscriber would be:

while(ros::ok()) {
    ros::spinOnce();
    _effort = this->getEffort();
}

Now if you are creating a different thread to run the while(ros::ok()) _effort = this->getEffort();, then yes, there will be a data race, as ROS doesn't handle memory locks for you. For some more resources on this, look around ROS answers some more (here's one).

Source: mostly from http://wiki.ros.org/roscpp/Overview/C... .