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[ROS2] msg stamp time difference

asked 2019-04-18 19:46:17 -0600

vadbut gravatar image

Hi,

I'm trying to assess if the msg is too old by comparing current time and the msg time stamp.

rclcpp::Time time_now = rclcpp::Clock().now();
double dt = (time_now - trajectory_buf_->header.stamp).nanoseconds();

However, the application crashes with "terminate called after throwing an instance of 'std::runtime_error' what(): can't subtract times with different time sources"

Alternatively, I tried it this way:

std_msgs::msg::Header::_stamp_type time_now = rclcpp::Clock().now();
double dt = (time_now - trajectory_buf_->header.stamp).nanoseconds();

This one doesn't compile with "no match for ‘operator-’"

What is the right way to do it?

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can't subtract times with different time sources

I believe the issue here is not that the types of time_now and trajectory_buf_->header.stamp are different, it's that the value they contain came from a different time source (see Clock and Time/Clock/ROS Time Source for some background).

gvdhoorn gravatar image gvdhoorn  ( 2019-04-19 01:40:54 -0600 )edit

the trajectory_buf message is being stamped in another node using rclcpp::Clock().now() call, that I'm also trying to use here. I don't see how the source is different. I looked through the article you mentioned, but didn't find the explanation.

vadbut gravatar image vadbut  ( 2019-04-19 12:27:01 -0600 )edit

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answered 2019-04-19 03:24:22 -0600

tfoote gravatar image

You need to get the clock from your node instance (my_node.get_clock()) , otherwise it does not have a valid time source and that's why your datatypes don't match.

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can you please explain what is the difference between "rclcpp::Clock().now()" and my_node.get_clock() ? I used the "rclcpp::Clock().now()" across all my nodes to stamp messages and it worked fine before I came across this problem. Switching to get_clock() indeed solved the problem, but I would like to figure out what is the difference between the methods.

vadbut gravatar image vadbut  ( 2019-04-19 12:36:50 -0600 )edit
1

bump. I would also like this comment answered

ateator gravatar image ateator  ( 2020-08-01 00:56:24 -0600 )edit

The static method in roscpp takes advantage of global state that's setup by roscpp:::init. This useage of global state has been removed in ROS 2 allowing truly running separate nodes in the same process. But as such you need to explicitly connect things like clocks to the Node instance that you care about.

tfoote gravatar image tfoote  ( 2020-08-01 01:08:09 -0600 )edit

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Asked: 2019-04-18 19:46:17 -0600

Seen: 2,264 times

Last updated: Apr 19 '19