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Provide a clock through clock_gettime() that returns the current TAI time (UTC without leap seconds) as a complement to CLOCK_REALTIME. This provides compatibility with Linux, which also provides a CLOCK_TAI since kernel 2.6.26, and this seems to be becoming the standard way to acquire TAI time. Unlike Linux, this code will return EINVAL if the TAI offset (set by ntpd, ptpd, etc.) is not known since it seems pathological for CLOCK_TAI to silently give the wrong (UTC) time if the offset is not known as it does on Linux. Reviewed by: imp Differential Revision: https://reviews.freebsd.org/D46268
237 lines
6.9 KiB
Groff
237 lines
6.9 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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.Dd August 10, 2024
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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 in SI seconds like a wall clock.
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It uses a 1970 epoch and implements the UTC timescale.
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The count of physical SI seconds since 1970, adjusted by subtracting the number
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of positive leap seconds and adding the number of negative leap seconds.
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Behavior during a leap second is not defined by and POSIX standard.
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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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.It Dv CLOCK_BOOTTIME
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Increments in SI seconds, even while the system is suspended.
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Its epoch is unspecified.
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The count is not adjusted by leap seconds.
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.Fx implements
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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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Increments monotonically in SI seconds while the machine is running.
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The count is not adjusted by leap seconds.
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The epoch is unspecified.
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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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.It Dv CLOCK_TAI
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Increments in SI seconds like a wall clock.
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It uses a 1970 epoch and implements the TAI timescale.
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Similar to CLOCK_REALTIME, but without leap seconds.
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It will increase monotonically during a leap second.
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Will return EINVAL if the current offset between TAI and UTC is not known,
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which may be the case early in boot before NTP or other time daemon has
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synchronized.
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.El
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.Pp
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The clock IDs
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.Dv CLOCK_BOOTTIME ,
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.Dv CLOCK_REALTIME ,
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.Dv CLOCK_TAI ,
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.Dv CLOCK_MONOTONIC ,
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and
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.Dv CLOCK_UPTIME
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perform a full time counter query.
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The clock IDs with the _FAST suffix, i.e.,
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.Dv CLOCK_REALTIME_FAST ,
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.Dv CLOCK_MONOTONIC_FAST ,
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and
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.Dv CLOCK_UPTIME_FAST ,
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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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.Dv CLOCK_REALTIME_PRECISE ,
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.Dv CLOCK_MONOTONIC_PRECISE ,
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and
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.Dv 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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.Dv CLOCK_REALTIME_COARSE
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and
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.Dv 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_MONOTONIC
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for compatibility with other systems and is unrelated to the
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.Fa kern.boottime
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.Xr sysctl 8 .
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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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.Dv CLOCK_REALTIME .
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If the system
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.Xr securelevel 7
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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 -p1003.1-2008 .
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The clock IDs
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.Dv CLOCK_BOOTTIME ,
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.Dv CLOCK_MONOTONIC_FAST ,
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.Dv CLOCK_MONOTONIC_PRECISE ,
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.Dv CLOCK_REALTIME_FAST ,
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.Dv CLOCK_REALTIME_PRECISE ,
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.Dv CLOCK_SECOND ,
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.Dv CLOCK_TAI ,
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.Dv CLOCK_UPTIME ,
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.Dv CLOCK_UPTIME_FAST ,
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and
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.Dv CLOCK_UPTIME_PRECISE
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are
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.Fx
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extensions to the POSIX interface.
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.Pp
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UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions.
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However, the
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.Vt time_t
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type is a simple count that does not provide a unique encoding for leap seconds,
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nor a specification for what values should be used to encode a leap second.
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.Pp
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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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