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 106.Dv CLOCK_REALTIME , 107but without leap seconds. 108It will increase monotonically during a leap second. 109Will return 110.Er EINVAL 111if the current offset between TAI and UTC is not known, 112which may be the case early in boot before NTP or other time daemon has 113synchronized. 114.El 115.Pp 116The clock IDs 117.Dv CLOCK_BOOTTIME , 118.Dv CLOCK_REALTIME , 119.Dv CLOCK_TAI , 120.Dv CLOCK_MONOTONIC , 121and 122.Dv CLOCK_UPTIME 123perform a full time counter query. 124The clock IDs with the _FAST suffix, i.e., 125.Dv CLOCK_REALTIME_FAST , 126.Dv CLOCK_MONOTONIC_FAST , 127and 128.Dv CLOCK_UPTIME_FAST , 129do not perform 130a full time counter query, so their accuracy is one timer tick. 131Similarly, 132.Dv CLOCK_REALTIME_PRECISE , 133.Dv CLOCK_MONOTONIC_PRECISE , 134and 135.Dv CLOCK_UPTIME_PRECISE 136are used to get the most exact value as possible, at the expense of 137execution time. 138The clock IDs 139.Dv CLOCK_REALTIME_COARSE 140and 141.Dv CLOCK_MONOTONIC_COARSE 142are aliases of corresponding IDs with _FAST suffix for compatibility with other 143systems. 144Finally, 145.Dv CLOCK_BOOTTIME 146is an alias for 147.Dv CLOCK_MONOTONIC 148for compatibility with other systems and is unrelated to the 149.Fa kern.boottime 150.Xr sysctl 8 . 151.Pp 152The structure pointed to by 153.Fa tp 154is defined in 155.In sys/timespec.h 156as: 157.Bd -literal 158struct timespec { 159 time_t tv_sec; /* seconds */ 160 long tv_nsec; /* and nanoseconds */ 161}; 162.Ed 163.Pp 164Only the super-user may set the time of day, using only 165.Dv CLOCK_REALTIME . 166If the system 167.Xr securelevel 7 168is greater than 1 (see 169.Xr init 8 ) , 170the time may only be advanced. 171This limitation is imposed to prevent a malicious super-user 172from setting arbitrary time stamps on files. 173The system time can still be adjusted backwards using the 174.Xr adjtime 2 175system call even when the system is secure. 176.Pp 177The resolution (granularity) of a clock is returned by the 178.Fn clock_getres 179system call. 180This value is placed in a (non-NULL) 181.Fa *tp . 182.Sh RETURN VALUES 183.Rv -std 184.Sh ERRORS 185The following error codes may be set in 186.Va errno : 187.Bl -tag -width Er 188.It Bq Er EINVAL 189The 190.Fa clock_id 191or 192.Fa timespec 193argument 194was not a valid value. 195.It Bq Er EPERM 196A user other than the super-user attempted to set the time. 197.El 198.Sh SEE ALSO 199.Xr date 1 , 200.Xr adjtime 2 , 201.Xr clock_getcpuclockid 3 , 202.Xr ctime 3 , 203.Xr pthread_getcpuclockid 3 204.Sh STANDARDS 205The 206.Fn clock_gettime , 207.Fn clock_settime , 208and 209.Fn clock_getres 210system calls conform to 211.St -p1003.1-2008 . 212The clock IDs 213.Dv CLOCK_BOOTTIME , 214.Dv CLOCK_MONOTONIC_FAST , 215.Dv CLOCK_MONOTONIC_PRECISE , 216.Dv CLOCK_REALTIME_FAST , 217.Dv CLOCK_REALTIME_PRECISE , 218.Dv CLOCK_SECOND , 219.Dv CLOCK_TAI , 220.Dv CLOCK_UPTIME , 221.Dv CLOCK_UPTIME_FAST , 222and 223.Dv CLOCK_UPTIME_PRECISE 224are 225.Fx 226extensions to the POSIX interface. 227.Pp 228UTC is defined by ITU-R TF.460-6, Standard-frequency and time-signal emissions. 229However, the 230.Vt time_t 231type is a simple count that does not provide a unique encoding for leap seconds, 232nor a specification for what values should be used to encode a leap second. 233.Pp 234.Sh HISTORY 235The 236.Fn clock_gettime , 237.Fn clock_settime , 238and 239.Fn clock_getres 240system calls first appeared in 241.Fx 3.0 . 242