1.\" $OpenBSD: clock_gettime.2,v 1.4 1997/05/08 20:21:16 kstailey Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.Dd August 10, 2024 31.Dt CLOCK_GETTIME 2 32.Os 33.Sh NAME 34.Nm clock_gettime , 35.Nm clock_settime , 36.Nm clock_getres 37.Nd get/set/calibrate date and time 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In time.h 42.Ft int 43.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp" 44.Ft int 45.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp" 46.Ft int 47.Fn clock_getres "clockid_t clock_id" "struct timespec *tp" 48.Sh DESCRIPTION 49The 50.Fn clock_gettime 51and 52.Fn clock_settime 53system calls allow the calling process to retrieve or set the value 54used by a clock which is specified by 55.Fa clock_id . 56.Pp 57The 58.Fa clock_id 59argument can be a value obtained from 60.Xr clock_getcpuclockid 3 61or 62.Xr pthread_getcpuclockid 3 63as well as the following values: 64.Pp 65.Bl -tag -width indent -compact 66.It Dv CLOCK_REALTIME 67.It Dv CLOCK_REALTIME_PRECISE 68.It Dv CLOCK_REALTIME_FAST 69.It Dv CLOCK_REALTIME_COARSE 70Increments in SI seconds like a wall clock. 71It uses a 1970 epoch and implements the UTC timescale. 72The count of physical SI seconds since 1970, adjusted by subtracting the number 73of positive leap seconds and adding the number of negative leap seconds. 74Behavior during a leap second is not defined by and POSIX standard. 75.It Dv CLOCK_MONOTONIC 76.It Dv CLOCK_MONOTONIC_PRECISE 77.It Dv CLOCK_MONOTONIC_FAST 78.It Dv CLOCK_MONOTONIC_COARSE 79.It Dv CLOCK_BOOTTIME 80Increments in SI seconds, even while the system is suspended. 81Its epoch is unspecified. 82The count is not adjusted by leap seconds. 83.Fx implements 84.It Dv CLOCK_UPTIME 85.It Dv CLOCK_UPTIME_PRECISE 86.It Dv CLOCK_UPTIME_FAST 87Increments monotonically in SI seconds while the machine is running. 88The count is not adjusted by leap seconds. 89The epoch is unspecified. 90.It Dv CLOCK_VIRTUAL 91Increments only when 92the CPU is running in user mode on behalf of the calling process. 93.It Dv CLOCK_PROF 94Increments when the CPU is running in user or kernel mode. 95.It Dv CLOCK_SECOND 96Returns the current second without performing a full time counter 97query, using an in-kernel cached value of the current second. 98.It Dv CLOCK_PROCESS_CPUTIME_ID 99Returns the execution time of the calling process. 100.It Dv CLOCK_THREAD_CPUTIME_ID 101Returns the execution time of the calling thread. 102.It Dv CLOCK_TAI 103Increments in SI seconds like a wall clock. 104It uses a 1970 epoch and implements the TAI timescale. 105Similar to CLOCK_REALTIME, but without leap seconds. 106It will increase monotonically during a leap second. 107Will return EINVAL if the current offset between TAI and UTC is not known, 108which may be the case early in boot before NTP or other time daemon has 109synchronized. 110.El 111.Pp 112The clock IDs 113.Dv CLOCK_BOOTTIME , 114.Dv CLOCK_REALTIME , 115.Dv CLOCK_TAI , 116.Dv CLOCK_MONOTONIC , 117and 118.Dv CLOCK_UPTIME 119perform a full time counter query. 120The clock IDs with the _FAST suffix, i.e., 121.Dv CLOCK_REALTIME_FAST , 122.Dv CLOCK_MONOTONIC_FAST , 123and 124.Dv CLOCK_UPTIME_FAST , 125do not perform 126a full time counter query, so their accuracy is one timer tick. 127Similarly, 128.Dv CLOCK_REALTIME_PRECISE , 129.Dv CLOCK_MONOTONIC_PRECISE , 130and 131.Dv CLOCK_UPTIME_PRECISE 132are used to get the most exact value as possible, at the expense of 133execution time. 134The clock IDs 135.Dv CLOCK_REALTIME_COARSE 136and 137.Dv CLOCK_MONOTONIC_COARSE 138are aliases of corresponding IDs with _FAST suffix for compatibility with other 139systems. 140Finally, 141.Dv CLOCK_BOOTTIME 142is an alias for 143.Dv CLOCK_MONOTONIC 144for compatibility with other systems and is unrelated to the 145.Fa kern.boottime 146.Xr sysctl 8 . 147.Pp 148The structure pointed to by 149.Fa tp 150is defined in 151.In sys/timespec.h 152as: 153.Bd -literal 154struct timespec { 155 time_t tv_sec; /* seconds */ 156 long tv_nsec; /* and nanoseconds */ 157}; 158.Ed 159.Pp 160Only the super-user may set the time of day, using only 161.Dv CLOCK_REALTIME . 162If the system 163.Xr securelevel 7 164is greater than 1 (see 165.Xr init 8 ) , 166the time may only be advanced. 167This limitation is imposed to prevent a malicious super-user 168from setting arbitrary time stamps on files. 169The system time can still be adjusted backwards using the 170.Xr adjtime 2 171system call even when the system is secure. 172.Pp 173The resolution (granularity) of a clock is returned by the 174.Fn clock_getres 175system call. 176This value is placed in a (non-NULL) 177.Fa *tp . 178.Sh RETURN VALUES 179.Rv -std 180.Sh ERRORS 181The following error codes may be set in 182.Va errno : 183.Bl -tag -width Er 184.It Bq Er EINVAL 185The 186.Fa clock_id 187or 188.Fa timespec 189argument 190was not a valid value. 191.It Bq Er EPERM 192A user other than the super-user attempted to set the time. 193.El 194.Sh SEE ALSO 195.Xr date 1 , 196.Xr adjtime 2 , 197.Xr clock_getcpuclockid 3 , 198.Xr ctime 3 , 199.Xr pthread_getcpuclockid 3 200.Sh STANDARDS 201The 202.Fn clock_gettime , 203.Fn clock_settime , 204and 205.Fn clock_getres 206system calls conform to 207.St -p1003.1-2008 . 208The clock IDs 209.Dv CLOCK_BOOTTIME , 210.Dv CLOCK_MONOTONIC_FAST , 211.Dv CLOCK_MONOTONIC_PRECISE , 212.Dv CLOCK_REALTIME_FAST , 213.Dv CLOCK_REALTIME_PRECISE , 214.Dv CLOCK_SECOND , 215.Dv CLOCK_TAI , 216.Dv CLOCK_UPTIME , 217.Dv CLOCK_UPTIME_FAST , 218and 219.Dv CLOCK_UPTIME_PRECISE 220are 221.Fx 222extensions to the POSIX interface. 223.Pp 224UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions. 225However, the 226.Vt time_t 227type is a simple count that does not provide a unique encoding for leap seconds, 228nor a specification for what values should be used to encode a leap second. 229.Pp 230.Sh HISTORY 231The 232.Fn clock_gettime , 233.Fn clock_settime , 234and 235.Fn clock_getres 236system calls first appeared in 237.Fx 3.0 . 238