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Getting the rclcpp::Duration in microseconds

asked 2022-04-21 15:56:36 -0500

Hello everyone!

I am trying to calculate the time it takes for a single message from one Node to get to the other Node.

The way I calculate the difference is by substituting the timestamp of the received message from the present time of the node (rclcpp::Node::now()) and store it as rclcpp::Duration. Next, I convert the rclcpp::Duration to std::chrono::duration of microseconds, so that I can store it later in uint64_t. There is a method to achieve this as described here.

However, I seem to not understand, will this conversion convert only the nanoseconds? (assuming the seconds() represent amount of seconds since epoch, and nanoseconds() represent nanoseconds from seconds). Should I then separately convert seconds and nanoseconds, convert them to microseconds and then sum up?

Or, the to_chrono() converts both seconds and nanoseconds (ie total time since epoch) in nanoseconds?

The code is below:

void inputCallback(const digital_twin_msgs::msg::SupplyInput::SharedPtr msg)
      if(msg->seq_id == p_input_sub->next_id) {
        p_input_sub->recv_counter += 1;
        rclcpp::Duration diff = rclcpp::Node::now() - msg->stamp;
        auto num_of_us = diff.to_chrono<std::chrono::duration<uint64_t, std::micro>>(); // <- is this total time since epoch in microseconds?
        uint64_t var = num_of_us.count();
        p_input_sub->lost_count += 1;
      p_input_sub->next_id = msg->seq_id + 1;

Thank you in advance!

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answered 2022-04-21 17:03:19 -0500

aprotyas gravatar image

Yes, what you're getting is the time in microseconds. May I suggest one QoL change to that line:

auto num_of_us = diff.to_chrono<std::chrono::microseconds>();
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To answer the rest of the question. No, the nanoseconds function won't only convert the nanoseconds. nanoseconds returns the full time period represented by the rclcpp::Duration object in nanoseconds, as does seconds in seconds. There's usually a lot of accuracy lost with the latter, so I'd suggest sticking to the former, or better, to to_chrono.

aprotyas gravatar image aprotyas  ( 2022-04-21 17:06:04 -0500 )edit

Great! Thank you for the QoL change and for explanation, that is exactly what I need :) . I wish I had the experience to know these kinds of little details regarding std::chrono types. It indeed looks much clearer.

sejego gravatar image sejego  ( 2022-04-22 01:24:26 -0500 )edit

No worries. Slight tangent, but if you want to learn about std::chronospecifically, this video will take you a long way:

aprotyas gravatar image aprotyas  ( 2022-04-22 13:23:49 -0500 )edit

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Asked: 2022-04-21 15:56:36 -0500

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Last updated: Apr 21