std chrono time synchronization

Question

I need to send some data over network with timestamps, this time should have high precision. Looking at std::chrono clocks I found out that std::chrono::*_clock::now() returns time_point which is depends at clock's epoch. I failed to find out which epoch is used in each clock and which of them can be used safely when sended wia network. For example on windows high_resolution_clock is wrapper around QueryPerformanceCounter it has good precision, but, I think, useless as timestamp for network tasks.

So question is how to "synchronize" high resolution clock over network?

Answer 1

std::chrono::system_clock's epoch is currently unspecified. However it is portably a measure of time since 1970-01-01 00:00:00 UTC, neglecting leap seconds. This is consistent with what is called Unix Time.

I am currently working to standardize this existing practice. I have private, unofficial assurances from the implementors of std::chrono::system_clock that they will not change their existing practice in the interim.

The other two std-defined chrono clocks: high_resolution_clock, and steady_clock, do not have portably defined epochs.

Note that system_clock, though it has a (de-facto) portable epoch, does not have portable precision. On clang/libc++ the precision is microseconds. On gcc the precision is nanoseconds, and on Windows the precision is 100ns. So you might time_point_cast<microseconds>(system_clock::now()) to obtain portable precision.

Answer 2

If you want "high precision" (you should specify better) synchronization you would need first of all to make all your network devices to regularly synchronize with the same NTP server. You need to configure the machines in order to adjust the clocks every few minutes (or seconds), because after updated (with some network delays which will make them not precise at the millisecond) , the clocks will start to drift again.