323e727a4c
Most systems (Linux, OpenBSD) adjust 0.5 ms per second, or 0.5 us per 1 ms tick. That is, the clock is sped up or slowed down by at most 0.05%. This means adjusting the clock by 1 s takes 2000 s, and the clock an be adjusted by at most 1.8 s per hour. FreeBSD adjusts 5 ms per second if the remaining time adjustment is >= 1 s (0.5%) , else it adjusts by 0.5 ms as well. This allows adjusting by (almost) 18 s per hour. Since Serenity OS can lose more than 22 s per hour (#3429), this picks an adjustment rate up to 1% for now. This allows us to adjust up to 36s per hour, which should be sufficient to adjust the clock fast enough to keep up with how much time the clock currently loses. Once we have a fancier NTP implementation that can adjust tick rate in addition to offset, we can think about reducing this. adjtime is a bit old-school and most current POSIX-y OSs instead implement adjtimex/ntp_adjtime, but a) we have to start somewhere b) ntp_adjtime() is a fairly gnarly API. OpenBSD's adjfreq looks like it might provide similar functionality with a nicer API. But before worrying about all this, it's probably a good idea to get to a place where the kernel APIs are (barely) good enough so that we can write an ntp service, and once we have that we should write a way to automatically evaluate how well it keeps the time adjusted, and only then should we add improvements ot the adjustment mechanism.
1.2 KiB
1.2 KiB
Name
adjtime - gradually adjust system clock
Synopsis
#include <sys/time.h>
int adjtime(const struct timeval* delta, struct timeval* old_delta);
Description
adjtime() gradually increments the system time by delta, if it is non-null.
Serenity OS slows down or speeds up the system clock by at most 1%, so adjusting the time by N seconds takes 100 * n seconds to complete.
Calling settimeofday() or clock_settime() cancels in-progress time adjustments done by adjtime.
If delta is not null, adjtime can only called by the superuser.
If old_delta is not null, it returns the currently remaining time adjustment. Querying the remaining time adjustment does not need special permissions.
Pledge
In pledged programs, the settime promise is required when delta is not null.
Errors
EFAULT:deltaand/orold_deltaare not null and not in readable memory.EINVAL:deltais not null and has atv_nsecfield that's less than 0 or larger or equal to10^6. Negative deltas should have a negativetv_secfield but atv_nsecthat's larger or equal zero. For example, a delta of -0.5 s is represented by{-1, 500'000}.EPERM:deltais not null but geteuid() is not 0.