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roscpp overview: Callbacks and Spinning |

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roscpp does not try to specify a threading model for your application. This means that while roscpp may use threads behind the scenes to do network management, scheduling etc., it will never expose its threads to your application. roscpp does, however, allow your callbacks to be called from any number of threads if that's what you want.

The end result is that without a little bit of work from the user your subscription, service and other callbacks will never be called. The most common solution is ros::spin(), but you must use one of the options below.

Note: Callback queues/spinning do not have any effect on the internal network communication in roscpp. They only affect when user callbacks occur. They will have an effect on the subscription queue, since how fast you process your callbacks and how quickly messages are arriving determines whether or not messages will be dropped.

Single-threaded Spinning

The simplest (and most common) version of single-threaded spinning is ros::spin():

ros::init(argc, argv, "my_node"); ros::NodeHandle nh; ros::Subscriber sub = nh.subscribe(...); ... ros::spin();

In this application all user callbacks will be called from within the ros::spin() call. ros::spin() will not return until the node has been shutdown, either through a call to ros::shutdown() or a Ctrl-C.

Another common pattern is to call ros::spinOnce() periodically:

ros::Rate r(10); // 10 hz while (should_continue) { ... do some work, publish some messages, etc. ... ros::spinOnce(); r.sleep(); }

ros::spinOnce() will call all the callbacks waiting to be called at that point in time.

Implementing a spin() of our own is quite simple:

#include <ros/callback_queue.h> ros::NodeHandle n; while (ros::ok()) { ros::getGlobalCallbackQueue()->callAvailable(ros::WallDuration(0.1)); }

and spinOnce() is simply:

#include <ros/callback_queue.h> ros::getGlobalCallbackQueue()->callAvailable(ros::WallDuration(0));

Note: spin() and spinOnce() are really meant for single-threaded applications, and are not optimized for being called from multiple threads at once. See the multi-threaded spinning section for information on spinning from multiple threads.

Multi-threaded Spinning

roscpp provides some built-in support for calling callbacks from multiple threads. There are two built-in options for this:

ros::MultiThreadedSpinner

MultiThreadedSpinner is a blocking spinner, similar to ros::spin(). You can specify a number of threads in its constructor, but if unspecified (or set to 0), it will use a thread for each CPU core.

ros::MultiThreadedSpinner spinner(4); // Use 4 threads spinner.spin(); // spin() will not return until the node has been shutdown

ros::AsyncSpinner (since 0.10)

CallbackQueue::callAvailable() and callOne()

See also: CallbackQueue API docs

You can create callback queues this way:

#include <ros/callback_queue.h> ... ros::CallbackQueue my_queue;

The CallbackQueue class has two ways of invoking the callbacks inside it: callAvailable() and callOne(). callAvailable() will take everything currently in the queue and invoke all of them. callOne() will simply invoke the oldest callback on the queue.

Both callAvailable() and callOne() can take in an optional timeout, which is the amount of time they will wait for a callback to become available before returning. If this is zero and there are no callbacks in the queue the method will return immediately.

Through ROS 0.10 the default timeout has been 0.1 seconds. ROS 0.11 makes the default 0.

Advanced: Using Different Callback Queues

You may have noticed the call to ros::getGlobalCallbackQueue() in the above implementation of spin(). By default, all callbacks get assigned into that global queue, which is then processed by ros::spin() or one of the alternatives. roscpp also lets you assign custom callback queues and service them separately. This can be done in one of two granularities:

Per subscribe(), advertise(), advertiseService(), etc.

Per NodeHandle

(1) is possible using the advanced versions of those calls that take a *Options structure. See the API docs for those calls for more information.

(2) is the more common way:

ros::NodeHandle nh; nh.setCallbackQueue(&my_callback_queue);