206 lines
4.9 KiB
Groff
206 lines
4.9 KiB
Groff
.\" $OpenBSD: gettimeofday.2,v 1.33 2022/03/31 17:27:16 naddy Exp $
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.\"
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)gettimeofday.2 8.2 (Berkeley) 5/26/95
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.\"
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.Dd $Mdocdate: March 31 2022 $
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.Dt GETTIMEOFDAY 2
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.Os
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.Sh NAME
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.Nm gettimeofday ,
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.Nm settimeofday
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.Nd get or set the time of day
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.Sh SYNOPSIS
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.In sys/time.h
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.Ft int
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.Fn gettimeofday "struct timeval *now" "struct timezone *tz"
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.Ft int
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.Fn settimeofday "const struct timeval *now" "const struct timezone *tz"
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.Sh DESCRIPTION
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The
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.Fn gettimeofday
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function writes the absolute value of the system's Coordinated Universal Time
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.Pq UTC
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clock to
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.Fa now
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unless
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.Fa now
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is
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.Dv NULL .
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.Pp
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The UTC clock's absolute value is the time elapsed since
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Jan 1 1970 00:00:00 +0000
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.Pq the Epoch .
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The clock normally advances continuously,
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though it may jump discontinuously if a process calls
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.Fn settimeofday
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or
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.Xr clock_settime 2 .
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For this reason,
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.Fn gettimeofday
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is not generally suitable for measuring elapsed time.
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Whenever possible,
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use
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.Xr clock_gettime 2
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to measure elapsed time with one of the system's monotonic clocks instead.
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.Pp
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The
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.Fn settimeofday
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function sets the system's UTC clock to the absolute value
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.Fa now
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unless
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.Fa now
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is
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.Dv NULL .
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Only the superuser may set the clock.
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If the system
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.Xr securelevel 7
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is 2 or greater, the clock may only be advanced.
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This limitation prevents a malicious superuser
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from setting arbitrary timestamps on files.
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Setting the clock cancels any ongoing
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.Xr adjtime 2
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adjustment.
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.Pp
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The structure pointed to by
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.Fa now
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is defined in
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.In sys/time.h
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as:
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.Bd -literal
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struct timeval {
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time_t tv_sec; /* seconds */
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suseconds_t tv_usec; /* and microseconds */
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};
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.Ed
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.Pp
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The
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.Fa tz
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argument is historical:
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the system no longer maintains timezone information in the kernel.
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The
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.Fa tz
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argument should always be
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.Dv NULL .
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.Fn gettimeofday
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zeroes
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.Fa tz
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if it is not
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.Dv NULL .
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.Fn settimeofday
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ignores the contents of
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.Fa tz
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if it is not
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.Dv NULL .
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.Sh RETURN VALUES
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.Rv -std
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.Sh ERRORS
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.Fn gettimeofday
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and
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.Fn settimeofday
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EFAULT
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.Fa now
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or
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.Fa tz
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are not
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.Dv NULL
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and reference invalid memory.
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.El
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.Pp
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.Fn settimeofday
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will also fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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.Fa now
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specifies a microsecond value less than zero or greater than or equal to
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one million.
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.It Bq Er EPERM
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The caller is not the superuser.
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.It Bq Er EPERM
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The system
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.Xr securelevel 7
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is 2 or greater and
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.Fa now
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specifies a time in the past.
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.El
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.Sh SEE ALSO
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.Xr date 1 ,
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.Xr adjtime 2 ,
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.Xr clock_gettime 2 ,
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.Xr getitimer 2 ,
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.Xr ctime 3 ,
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.Xr time 3 ,
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.Xr timeradd 3
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.Sh STANDARDS
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The
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.Fn gettimeofday
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function conforms to
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.St -p1003.1-2008 .
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.Pp
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The
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.Fn settimeofday
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function is non-standard,
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though many systems offer it.
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.Sh HISTORY
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As predecessors of these functions,
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former system calls
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.Fn time
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and
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.Fn stime
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first appeared in
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.At v1 ,
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and
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.Fn ftime
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first appeared in
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.At v7 .
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The
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.Fn gettimeofday
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and
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.Fn settimeofday
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system calls first appeared in
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.Bx 4.1c .
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.Sh CAVEATS
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Setting the time with
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.Fn settimeofday
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is dangerous; if possible use
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.Xr adjtime 2
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instead.
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Many daemon programming techniques utilize time-delta techniques
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using the results from
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.Fn gettimeofday
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instead of from
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.Xr clock_gettime 2
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on the
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.Dv CLOCK_MONOTONIC
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clock.
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Time jumps can cause these programs to malfunction in unexpected ways.
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If the time must be set, consider rebooting the machine for safety.
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