Fixed wrong timestamp under Free- and OpenBSD and macOS #217
Conversation
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Using CLOCK_MONOTONIC breaks the timestamp output just as much under Linux as under BSD (but this is hidden by HAVE_SO_TIMESTAMPNS being set so the clock is always CLOCK_REALTIME). So why is CLOCK_MONOTONIC there at all? |
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Hmm, you really ought to use CLOCK_MONOTONIC for every time-delta calculation (unless you're using packet timestamps from either kernel or hardware, which is highly OS-specific). Severe weirdness happens when CLOCK_REALTIME steps around due to system administrator action (or even non-slew-mode large ntp corrections, etc). If you need human-readable timing, you timestamp CLOCK_REALTIME and CLOCK_MONOTONIC at program start (or at measurement session start, if there's such a notion in your application -- for fping they're the same), and use the CLOCK_MONOTONIC delta + CLOCK_REALTIME timestamp to calculate a more sane real time that doesn't jump around. This adds a minor, constant, and irrelevant phase error to the real time output, which is already quite uncertain anyway. |
| @@ -123,7 +123,7 @@ extern int h_errno; | |||
| #endif | |||
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| #if !defined(CLOCKID) | |||
| #if defined(CLOCK_MONOTONIC) | |||
| #if defined(CLOCK_MONOTONIC) && !defined(__APPLE__) && !defined(__OpenBSD__) && !defined(__FreeBSD__) | |||
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Could you add a comment to explain why we can't use CLOCK_MONOTONIC on these platforms?
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On FreeBSD and OpenBSD CLOCK_MONOTONIC begin with 0 after reboot or undefined positive point.
Here is my test result with FreeBSD.
CLOCK_MONOTONIC (sec): 43
CLOCK_REALTIME (sec): 1628355889
CLOCK_UPTIME (sec): 43
OpenBSD Manpage: https://man.openbsd.org/clock_gettime.2
macOS unfortunately I can not test only assume
I have another version when you don't like so many not defined in the code gsnw@2d8d292
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Thanks, I meant also a comment in the code. Maybe referencing just this PR, something like this:
// don't use CLOCK_MONOTONIC on Mac and FreeBSD because gettime returns uptime (see github PR #217)
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Ups, sorry for misunderstanding. I have add a in code comment
Wrong timestamp on Free- and OpenBSD and macOS fixed by disabling #define CLOCKID CLOCK_MONOTONIC
Issues
#203
#216