[ros-users] questions about nodelets

[Jack O'Quin]

On Tue, Nov 9, 2010 at 1:18 PM, Josh Faust <jfaust at willowgarage.com> wrote:

>>  * Are the callbacks invoked in the nodelet thread (by default),
>> similar to a node? (I assume "yes".)
>
> This depend on which nodehandle you use.  The one returned by
> get(Private)NodeHandle() guarantees that callbacks will not happen in
> multiple threads at once, but the thread they're called from may be
> different from call to call.
> The one returned by getMT(Private)NodeHandle() allows callbacks in multiple
> threads at the same time.

That helps. The documentation could use a bit more detail about the
threading model:

  http://www.ros.org/doc/api/nodelet/html/classnodelet_1_1Nodelet.html

>>  * Is there any special hook for shutting down a nodelet? I don't see
>> anything like an onExit() method.
>
> The destructor.

Sure. I assume the callbacks just don't get called any more.  But, a
device thread (see below) needs to know when to terminate. Does it
still check ros::ok() or ros::shutdown()?

>>  * Can a device driver run its entire main loop in the onInit() method
>> using "while (ros::ok()) { ... }"?
>
> No, onInit() needs to return.  If you need to do some kind of polling loop
> you need to start your own thread.

OK, thanks. That seems to explain most of the problems I am seeing.

Presumably, this thread will *not* be serialized with respect to
callbacks. So, I will need my own locks in some situations.
-- 
 joq