.\"-
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.Dd July 29, 2024
.Dt ADJKERNTZ 8
.Os
.Sh NAME
.Nm adjkerntz
.Nd adjust real-time clock (rtc) and kernel timezone offset
.Sh SYNOPSIS
.Nm
.Fl i
.Nm
.Fl a Op Fl s
.Sh DESCRIPTION
The
.Nm
utility maintains the proper relationship between the kernel clock, which
is always set to UTC and the real-time clock, which may be set to local time.
The
.Nm
utility also informs the kernel about machine timezone shifts in order to
maintain proper timestamps for local time file systems such as the MS-DOS
file system.
The main purpose of maintaining these timestamps properly is to keep the
timestamps of a
.Fx
MS-DOS file system and an MS-DOS operating system synchronized when they are
installed on the same system rather than fixing broken MS-DOS file
timestamps.
If the file
.Pa /etc/wall_cmos_clock
exists, it means that the real-time clock keeps local time
.Pq MS-DOS and MS-Windows compatible mode .
If that file does not exist, it means that the real-time clock keeps UTC time.
The
.Nm
utility passes this state to the
.Va machdep.wall_cmos_clock
kernel variable.
.Pp
Adjustments may be needed at system startup and shutdown, and
whenever a time zone change occurs.
To handle these different situations,
.Nm
is invoked in two ways:
.Bl -tag -width "-a [-s]"
.It Fl i
This form handles system startups and shutdowns.
The
.Nm
utility is invoked with this option from
.Pa /etc/rc
on entry to multi-user mode, before any other daemons have been started.
The
.Nm
utility puts itself into the background.
Then, for a local time real-time clock,
.Nm
reads the local time from it
and sets the kernel clock to the corresponding UTC time.
The
.Nm
utility also stores the local time zone offset in the
.Va machdep.adjkerntz
kernel variable, for use by subsequent invocations of
.Ql adjkerntz -a
and by local time file systems.
.Pp
For a local time real-time clock
.Ql adjkerntz -i
pauses and remains inactive as a background daemon until it
receives a SIGTERM.
The SIGTERM will normally be sent by
.Xr init 8
when the system leaves multi-user mode
.Pq usually, because the system is being shut down .
After receiving the SIGTERM,
.Nm
reads the UTC kernel clock and updates the real-time clock, if necessary,
to ensure that it reflects the current local time zone.
Then
.Nm
exits.
.It Fl a Op Fl s
This form is used to update the local time real-time clock and
.Va machdep.adjkerntz
kernel variable when time zone changes occur,
e.g., when entering or leaving daylight savings time.
The
.Nm
utility uses the kernel clock's UTC time,
the previously stored time zone offset,
and the changed time zone rule to calculate a new time zone offset.
It stores the new offset into the
.Va machdep.adjkerntz
kernel variable and updates the wall real-time clock to the new local time.
If
.Ql adjkerntz -a
was started at a nonexistent time
.Pq during a timezone change ,
it exits with a warning diagnostic unless the
.Fl s
option was used, in which case
.Nm
sleeps 30 minutes and tries again.
.Pp
This form should be invoked from root's
.Xr crontab 5
every half hour between midnight and 5am,
when most modern time zone changes occur.
Warning: do not use the
.Fl s
option in a
.Xr crontab 5
command line, or multiple
.Ql adjkerntz -a
instances could conflict with each other.
.El
.Pp
The
.Nm
utility clears the kernel timezone structure and makes the kernel clock run
in the UTC time zone.
Super-user privileges are required for all operations.
.Sh ENVIRONMENT
.Bl -tag -width /etc/wall_cmos_clock
.It Ev TZ
Time zone change rule, see
.Xr tzset 3 ;
not needed when
.Xr tzsetup 8
or
.Xr zic 8
is used.
.El
.Sh FILES
.Bl -tag -width /etc/wall_cmos_clock -compact
.It Pa /etc/localtime
Current zoneinfo file, see
.Xr tzsetup 8
and
.Xr zic 8 .
.It Pa /etc/wall_cmos_clock
Empty file.
Its presence indicates that the machine's real-time clock is set to local
time, while its absence indicates a UTC real-time clock.
.El
.Sh DIAGNOSTICS
No diagnostics.
If an error occurs,
.Nm
logs an error message via
.Xr syslog 3
and exits with a nonzero return code.
.Sh SEE ALSO
.Xr tzset 3 ,
.Xr crontab 5 ,
.Xr mount_msdosfs 8 ,
.Xr ntpd 8 ,
.Xr rc 8 ,
.Xr sysctl 8 ,
.Xr tzsetup 8 ,
.Xr zic 8
.Sh HISTORY
The
.Nm
utility appeared in
.Fx 1.0 .
.Sh AUTHORS
.An Andrey A. Chernov Aq Mt ache@astral.msk.su