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.\" $FreeBSD$ 31.\" 32.Dd July 8, 2022 33.Dt CLOCK_GETTIME 2 34.Os 35.Sh NAME 36.Nm clock_gettime , 37.Nm clock_settime , 38.Nm clock_getres 39.Nd get/set/calibrate date and time 40.Sh LIBRARY 41.Lb libc 42.Sh SYNOPSIS 43.In time.h 44.Ft int 45.Fn clock_gettime "clockid_t clock_id" "struct timespec *tp" 46.Ft int 47.Fn clock_settime "clockid_t clock_id" "const struct timespec *tp" 48.Ft int 49.Fn clock_getres "clockid_t clock_id" "struct timespec *tp" 50.Sh DESCRIPTION 51The 52.Fn clock_gettime 53and 54.Fn clock_settime 55system calls allow the calling process to retrieve or set the value 56used by a clock which is specified by 57.Fa clock_id . 58.Pp 59The 60.Fa clock_id 61argument can be a value obtained from 62.Xr clock_getcpuclockid 3 63or 64.Xr pthread_getcpuclockid 3 65as well as the following values: 66.Pp 67.Bl -tag -width indent -compact 68.It Dv CLOCK_REALTIME 69.It Dv CLOCK_REALTIME_PRECISE 70.It Dv CLOCK_REALTIME_FAST 71.It Dv CLOCK_REALTIME_COARSE 72Increments as a wall clock should. 73.It Dv CLOCK_MONOTONIC 74.It Dv CLOCK_MONOTONIC_PRECISE 75.It Dv CLOCK_MONOTONIC_FAST 76.It Dv CLOCK_MONOTONIC_COARSE 77Increments in SI seconds. 78.It Dv CLOCK_UPTIME 79.It Dv CLOCK_UPTIME_PRECISE 80.It Dv CLOCK_UPTIME_FAST 81.It Dv CLOCK_BOOTTIME 82Starts at zero when the kernel boots and increments 83monotonically in SI seconds while the machine is running. 84.It Dv CLOCK_VIRTUAL 85Increments only when 86the CPU is running in user mode on behalf of the calling process. 87.It Dv CLOCK_PROF 88Increments when the CPU is running in user or kernel mode. 89.It Dv CLOCK_SECOND 90Returns the current second without performing a full time counter 91query, using an in-kernel cached value of the current second. 92.It Dv CLOCK_PROCESS_CPUTIME_ID 93Returns the execution time of the calling process. 94.It Dv CLOCK_THREAD_CPUTIME_ID 95Returns the execution time of the calling thread. 96.El 97.Pp 98The clock IDs 99.Fa CLOCK_REALTIME , 100.Fa CLOCK_MONOTONIC , 101and 102.Fa CLOCK_UPTIME 103perform a full time counter query. 104The clock IDs with the _FAST suffix, i.e., 105.Fa CLOCK_REALTIME_FAST , 106.Fa CLOCK_MONOTONIC_FAST , 107and 108.Fa CLOCK_UPTIME_FAST , 109do not perform 110a full time counter query, so their accuracy is one timer tick. 111Similarly, 112.Fa CLOCK_REALTIME_PRECISE , 113.Fa CLOCK_MONOTONIC_PRECISE , 114and 115.Fa CLOCK_UPTIME_PRECISE 116are used to get the most exact value as possible, at the expense of 117execution time. 118The clock IDs 119.Fa CLOCK_REALTIME_COARSE 120and 121.Fa CLOCK_MONOTONIC_COARSE 122are aliases of corresponding IDs with _FAST suffix for compatibility with other 123systems. 124Finally, 125.Dv CLOCK_BOOTTIME 126is an alias for 127.Dv CLOCK_UPTIME 128for compatibility with other systems. 129.Pp 130The structure pointed to by 131.Fa tp 132is defined in 133.In sys/timespec.h 134as: 135.Bd -literal 136struct timespec { 137 time_t tv_sec; /* seconds */ 138 long tv_nsec; /* and nanoseconds */ 139}; 140.Ed 141.Pp 142Only the super-user may set the time of day, using only 143.Fa CLOCK_REALTIME . 144If the system 145.Xr securelevel 7 146is greater than 1 (see 147.Xr init 8 ) , 148the time may only be advanced. 149This limitation is imposed to prevent a malicious super-user 150from setting arbitrary time stamps on files. 151The system time can still be adjusted backwards using the 152.Xr adjtime 2 153system call even when the system is secure. 154.Pp 155The resolution (granularity) of a clock is returned by the 156.Fn clock_getres 157system call. 158This value is placed in a (non-NULL) 159.Fa *tp . 160.Sh RETURN VALUES 161.Rv -std 162.Sh ERRORS 163The following error codes may be set in 164.Va errno : 165.Bl -tag -width Er 166.It Bq Er EINVAL 167The 168.Fa clock_id 169or 170.Fa timespec 171argument 172was not a valid value. 173.It Bq Er EPERM 174A user other than the super-user attempted to set the time. 175.El 176.Sh SEE ALSO 177.Xr date 1 , 178.Xr adjtime 2 , 179.Xr clock_getcpuclockid 3 , 180.Xr ctime 3 , 181.Xr pthread_getcpuclockid 3 182.Sh STANDARDS 183The 184.Fn clock_gettime , 185.Fn clock_settime , 186and 187.Fn clock_getres 188system calls conform to 189.St -p1003.1b-93 . 190The clock IDs 191.Fa CLOCK_REALTIME_FAST , 192.Fa CLOCK_REALTIME_PRECISE , 193.Fa CLOCK_MONOTONIC_FAST , 194.Fa CLOCK_MONOTONIC_PRECISE , 195.Fa CLOCK_UPTIME , 196.Fa CLOCK_UPTIME_FAST , 197.Fa CLOCK_UPTIME_PRECISE , 198.Fa CLOCK_SECOND 199are 200.Fx 201extensions to the POSIX interface. 202.Sh HISTORY 203The 204.Fn clock_gettime , 205.Fn clock_settime , 206and 207.Fn clock_getres 208system calls first appeared in 209.Fx 3.0 . 210