.\"	$OpenBSD: adjtime.2,v 1.25 2019/03/26 16:02:54 cheloha Exp $
.\"	$NetBSD: adjtime.2,v 1.5 1995/10/12 15:40:44 jtc Exp $
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.\"     @(#)adjtime.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: March 26 2019 $
.Dt ADJTIME 2
.Os
.Sh NAME
.Nm adjtime
.Nd correct the time to allow synchronization of the system clock
.Sh SYNOPSIS
.In sys/time.h
.Ft int
.Fn adjtime "const struct timeval *delta" "struct timeval *olddelta"
.Sh DESCRIPTION
.Fn adjtime
makes small adjustments to the system time, as returned by
.Xr gettimeofday 2 ,
advancing or retarding it by the time specified by the timeval
.Fa delta .
If
.Fa delta
is negative, the clock is slowed down by incrementing it more slowly
than normal until the correction is complete.
If
.Fa delta
is positive, a larger increment than normal is used.
The skew used to perform the correction is generally a fraction of one percent.
Thus, the time is always a monotonically increasing function.
A time correction from an earlier call to
.Fn adjtime
may not be finished when
.Fn adjtime
is called again.
If
.Fa delta
is
.Dv NULL ,
no adjustment is done.
If
.Fa olddelta
is
.Pf non- Dv NULL ,
the number of microseconds still to be corrected from the earlier call
is stored into
.Pf * Fa olddelta .
Setting the time with
.Xr settimeofday 2
cancels any in-progress time adjustment.
.Pp
This call may be used by time servers that synchronize the clocks
of computers in a local area network.
Such time servers would slow down the clocks of some machines
and speed up the clocks of others to bring them to the average network time.
.Pp
Only the superuser may adjust the time using the
.Fn adjtime
function.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn adjtime
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
Either of the arguments point outside the process's allocated address space.
.It Bq Er EINVAL
The
.Fa delta
argument is
.Pf non- Dv NULL
and specifies a microsecond value less than zero or
greater than or equal to one million.
.It Bq Er EINVAL
The
.Fa delta
argument is
.Pf non- Dv NULL
and represents an adjustment greater than
.Dv INT64_MAX
microseconds or less than
.Dv INT64_MIN
microseconds.
.It Bq Er EPERM
The
.Fa delta
argument is
.Pf non- Dv NULL
and the process's effective user ID is not that of the superuser.
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr adjfreq 2 ,
.Xr gettimeofday 2 ,
.Xr ntpd 8
.Sh HISTORY
The
.Fn adjtime
function call appeared in
.Bx 4.3 .
.Sh CAVEATS
Other operating systems restrict calling
.Fn adjtime
to the superuser and might not allow requesting the current
correction without specifying a new value.