shared variable between callbacks

Question

Hi all, I have a ros node that is supposed to accept two sources (IMU and PointCloud) with two different frequecies (Imu is ~200 Hz, PointCloud is ~30 Hz). So expect around 7 IMU messages for every cloud message.

If I use a message filter, I can collect in a single callback the two closest signals.

Instead, I would like to independently collect all the occurences of IMU between two point cloud messages, so that I can use them to get a better interpolation of the IMU signal to synchronize the two signals.

Would two callbacks + a global variable (a std::vector) suffice? Should I implement some blocking mechanism to prevent the global variable from being modified when I'm inside the cloud callback?

Are the callbacks sequential? In that case, how can I make them concurrent?

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Originally posted by mark_vision on ROS Answers with karma: 275 on 2015-05-05

Post score: 0

Answer 1

The answer depends on whether you are using roscpp or rospy. It seems you are using roscpp because you mentioned std::vector. If you are simply using ros::spin or ros::spinOnce there is only a single thread handling callbacks so there should be no need for any sort of mutex.

If you'd like to use a multi-threaded approach to processing your callbacks, roscpp includes the ros::MultiThreadedSpinner and ros::AsyncSpinner. If using one of these spinners, you should take care to prevent access collisions.

This information was pulled from the Callbacks and Spinning roscpp Overview page.

EDIT

IMO, it can be a bit difficult to find documentation on the rospy threading model (and I'm apparently too lazy to write a wiki page about it), but this ROS answers post has a clear and concise description.

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Originally posted by jarvisschultz with karma: 9031 on 2015-05-05

This answer was ACCEPTED on the original site

Post score: 2

Answer 2

In a typical ros node without Asynch/Multithreaded Spinners or something you do not have to take care about thread safety, i. e. you don't need blocking mechanisms. Any of your callbacks can only be called when your main process loop is calling ros::spin() or ros::spinOnce() and only one of our callbacks will be called at the same time (no parallel callbacks).

If you need parallel callbacks you should at this: http://wiki.ros.org/roscpp/Overview/Callbacks%20and%20Spinning

Keep in mind that if you use asynch/multithreaded spinning your callbacks might be executed parallel, i. e. you need blocking mechanisms, mutexes, condition variables and stuff.

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Originally posted by Wolf with karma: 7555 on 2015-05-05

This answer was NOT ACCEPTED on the original site

Post score: 1