.\"	$OpenBSD: adjtime.2,v 1.18 2009/06/05 15:17:02 ckuethe 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: June 5 2009 $
.Dt ADJTIME 2
.Os
.Sh NAME
.Nm adjtime
.Nd "correct the time to allow synchronization of the system clock"
.Sh SYNOPSIS
.Fd #include <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 null, no adjustment is done.
If
.Fa olddelta
is non-null, the structure pointed to will contain, upon return, the
number of microseconds still to be corrected from the earlier call.
Setting the time with
.Xr settimeofday 2
will cancel 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
A return value of 0 indicates that the call succeeded.
A return value of \-1 indicates that an error occurred, and in this
case an error code is stored in the global variable
.Va errno .
.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 EPERM
The
.Fn delta
argument is non-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 ,
.Xr timed 8 ,
.Xr timedc 8
.Rs
.%T "TSP: The Time Synchronization Protocol for UNIX 4.3BSD"
.%A R. Gusella
.%A S. Zatti
.Re
.Sh HISTORY
The
.Fn adjtime
function call appeared in
.Bx 4.3 .
.Sh CAVEATS
Other operating systems restrict calling
.Nm
to the superuser and might not allow requesting the current
correction without specifying a new value.