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.\"     @(#)adjtime.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: releng/12.0/lib/libc/sys/adjtime.2 314436 2017-02-28 23:42:47Z imp $
.\"
.Dd June 4, 1993
.Dt ADJTIME 2
.Os
.Sh NAME
.Nm adjtime
.Nd "correct the time to allow synchronization of the system clock"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/time.h
.Ft int
.Fn adjtime "const struct timeval *delta" "struct timeval *olddelta"
.Sh DESCRIPTION
The
.Fn adjtime
system call
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 olddelta
is not a null pointer,
the structure pointed to will contain, upon return, the
number of microseconds still to be corrected
from the earlier call.
.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
The
.Fn adjtime
system call
is restricted to the super-user.
.Sh RETURN VALUES
.Rv -std adjtime
.Sh ERRORS
The
.Fn adjtime
system call will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
An argument points outside the process's allocated address space.
.It Bq Er EPERM
The process's effective user ID is not that of the super-user.
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr gettimeofday 2 ,
.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
system call appeared in
.Bx 4.3 .