155f15118a
Linux standardized what we call CLOCK_{REALTIME,MONOTONIC}_FAST as
CLOCK_{REALTIME,MONOTONIC}_COARSE. In addition, Linux spells
CLOCK_UPTIME as CLOCK_BOOTTIME.
Add aliases to time.h and document these new aliases in
clock_gettime(2).
Reviewed by: vangyzen, kib (prior), dchagin (prior)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30988
197 lines
5.6 KiB
Groff
197 lines
5.6 KiB
Groff
.\" $OpenBSD: clock_gettime.2,v 1.4 1997/05/08 20:21:16 kstailey 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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.\" $FreeBSD$
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.\"
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.Dd July 1, 2021
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.Dt CLOCK_GETTIME 2
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.Os
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.Sh NAME
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.Nm clock_gettime ,
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.Nm clock_settime ,
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.Nm clock_getres
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.Nd get/set/calibrate date and time
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In time.h
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.Ft int
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.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp"
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.Ft int
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.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp"
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.Ft int
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.Fn clock_getres "clockid_t clock_id" "struct timespec *tp"
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.Sh DESCRIPTION
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The
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.Fn clock_gettime
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and
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.Fn clock_settime
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system calls allow the calling process to retrieve or set the value
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used by a clock which is specified by
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.Fa clock_id .
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.Pp
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The
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.Fa clock_id
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argument can be a value obtained from
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.Xr clock_getcpuclockid 3
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or
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.Xr pthread_getcpuclockid 3
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as well as the following values:
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.Pp
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.Bl -tag -width indent -compact
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.It Dv CLOCK_REALTIME
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.It Dv CLOCK_REALTIME_PRECISE
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.It Dv CLOCK_REALTIME_FAST
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.It Dv CLOCK_REALTIME_COARSE
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Increments as a wall clock should.
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.It Dv CLOCK_MONOTONIC
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.It Dv CLOCK_MONOTONIC_PRECISE
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.It Dv CLOCK_MONOTONIC_FAST
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.It Dv CLOCK_MONOTONIC_COARSE
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Increments in SI seconds.
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.It Dv CLOCK_UPTIME
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.It Dv CLOCK_UPTIME_PRECISE
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.It Dv CLOCK_UPTIME_FAST
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.It Dv CLOCK_BOOTTIME
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Starts at zero when the kernel boots and increments
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monotonically in SI seconds while the machine is running.
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.It Dv CLOCK_VIRTUAL
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Increments only when
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the CPU is running in user mode on behalf of the calling process.
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.It Dv CLOCK_PROF
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Increments when the CPU is running in user or kernel mode.
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.It Dv CLOCK_SECOND
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Returns the current second without performing a full time counter
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query, using an in-kernel cached value of the current second.
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.It Dv CLOCK_PROCESS_CPUTIME_ID
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Returns the execution time of the calling process.
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.It Dv CLOCK_THREAD_CPUTIME_ID
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Returns the execution time of the calling thread.
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.El
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.Pp
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The clock IDs
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.Fa CLOCK_REALTIME_FAST ,
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.Fa CLOCK_MONOTONIC_FAST ,
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.Fa CLOCK_UPTIME_FAST
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are analogs of corresponding IDs without _FAST suffix but do not perform
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a full time counter query, so their accuracy is one timer tick.
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Similarly,
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.Fa CLOCK_REALTIME_PRECISE ,
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.Fa CLOCK_MONOTONIC_PRECISE ,
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.Fa CLOCK_UPTIME_PRECISE
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are used to get the most exact value as possible, at the expense of
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execution time.
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The clock IDs
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.Fa CLOCK_REALTIME_COARSE ,
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.Fa CLOCK_MONOTONIC_COARSE
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are aliases of corresponding IDs with _FAST suffix for compatibility with other
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systems.
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Finally,
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.Dv CLOCK_BOOTTIME
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is an alias for
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.Dv CLOCK_UPTIME
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for compatibility with other systems.
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.Pp
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The structure pointed to by
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.Fa tp
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is defined in
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.In sys/timespec.h
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as:
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.Bd -literal
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struct timespec {
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time_t tv_sec; /* seconds */
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long tv_nsec; /* and nanoseconds */
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};
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.Ed
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.Pp
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Only the super-user may set the time of day, using only
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.Fa CLOCK_REALTIME .
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If the system securelevel is greater than 1 (see
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.Xr init 8 ) ,
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the time may only be advanced.
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This limitation is imposed to prevent a malicious super-user
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from setting arbitrary time stamps on files.
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The system time can still be adjusted backwards using the
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.Xr adjtime 2
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system call even when the system is secure.
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.Pp
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The resolution (granularity) of a clock is returned by the
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.Fn clock_getres
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system call.
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This value is placed in a (non-NULL)
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.Fa *tp .
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.Sh RETURN VALUES
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.Rv -std
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.Sh ERRORS
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The following error codes may be set in
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.Va errno :
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The
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.Fa clock_id
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or
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.Fa timespec
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argument
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was not a valid value.
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.It Bq Er EPERM
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A user other than the super-user attempted to set the time.
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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_getcpuclockid 3 ,
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.Xr ctime 3 ,
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.Xr pthread_getcpuclockid 3
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.Sh STANDARDS
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The
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.Fn clock_gettime ,
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.Fn clock_settime ,
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and
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.Fn clock_getres
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system calls conform to
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.St -p1003.1b-93 .
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The clock IDs
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.Fa CLOCK_REALTIME_FAST ,
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.Fa CLOCK_REALTIME_PRECISE ,
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.Fa CLOCK_MONOTONIC_FAST ,
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.Fa CLOCK_MONOTONIC_PRECISE ,
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.Fa CLOCK_UPTIME ,
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.Fa CLOCK_UPTIME_FAST ,
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.Fa CLOCK_UPTIME_PRECISE ,
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.Fa CLOCK_SECOND
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are FreeBSD extensions to the POSIX interface.
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.Sh HISTORY
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The
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.Fn clock_gettime ,
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.Fn clock_settime ,
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and
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.Fn clock_getres
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system calls first appeared in
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.Fx 3.0 .
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