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: head/lib/libc/sys/clock_gettime.2 292777 2015-12-27 15:37:07Z dchagin $ 31.\" 32.Dd June 17, 2019 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 sys/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 71Increments as a wall clock should. 72.It Dv CLOCK_MONOTONIC 73.It Dv CLOCK_MONOTONIC_PRECISE 74.It Dv CLOCK_MONOTONIC_FAST 75Increments in SI seconds. 76.It Dv CLOCK_UPTIME 77.It Dv CLOCK_UPTIME_PRECISE 78.It Dv CLOCK_UPTIME_FAST 79Starts at zero when the kernel boots and increments 80monotonically in SI seconds while the machine is running. 81.It Dv CLOCK_VIRTUAL 82Increments only when 83the CPU is running in user mode on behalf of the calling process. 84.It Dv CLOCK_PROF 85Increments when the CPU is running in user or kernel mode. 86.It Dv CLOCK_SECOND 87Returns the current second without performing a full time counter 88query, using an in-kernel cached value of the current second. 89.It Dv CLOCK_PROCESS_CPUTIME_ID 90Returns the execution time of the calling process. 91.It Dv CLOCK_THREAD_CPUTIME_ID 92Returns the execution time of the calling thread. 93.El 94.Pp 95The clock IDs 96.Dv CLOCK_REALTIME_FAST , 97.Dv CLOCK_MONOTONIC_FAST , 98and 99.Dv CLOCK_UPTIME_FAST 100are analogs of corresponding IDs without _FAST suffix but do not perform 101a full time counter query, so their accuracy is one timer tick. 102Similarly, 103.Dv CLOCK_REALTIME_PRECISE , 104.Dv CLOCK_MONOTONIC_PRECISE , 105and 106.Dv CLOCK_UPTIME_PRECISE 107are used to get the most exact value as possible, at the expense of 108execution time. 109.Pp 110The clock IDs 111.Dv CLOCK_UPTIME_FAST , 112.Dv CLOCK_MONOTONIC_FAST , 113.Dv CLOCK_REALTIME_FAST , 114and 115.Dv CLOCK_SECOND 116make use of 117.Xr kpmap 4 118and do not incur any system call overhead after a certain amount of calls. 119.Pp 120The structure pointed to by 121.Fa tp 122is defined in 123.In sys/_timespec.h 124as: 125.Bd -literal 126struct timespec { 127 time_t tv_sec; /* seconds */ 128 long tv_nsec; /* and nanoseconds */ 129}; 130.Ed 131.Pp 132Only the super-user may set the time of day, using only 133.Dv CLOCK_REALTIME . 134If the system securelevel is greater than 1 (see 135.Xr init 8 ) , 136the time may only be advanced. 137This limitation is imposed to prevent a malicious super-user 138from setting arbitrary time stamps on files. 139The system time can still be adjusted backwards using the 140.Xr adjtime 2 141system call even when the system is secure. 142.Pp 143The resolution (granularity) of a clock is returned by the 144.Fn clock_getres 145system call. 146This value is placed in a (non-NULL) 147.Fa *tp . 148.Sh RETURN VALUES 149.Rv -std 150.Sh ERRORS 151The following error codes may be set in 152.Va errno : 153.Bl -tag -width Er 154.It Bq Er EINVAL 155The 156.Fa clock_id 157or 158.Fa tp 159argument was not a valid value. 160.It Bq Er EPERM 161A user other than the super-user attempted to set the time. 162.El 163.Sh SEE ALSO 164.Xr date 1 , 165.Xr adjtime 2 , 166.Xr clock_getcpuclockid 3 , 167.Xr ctime 3 , 168.Xr pthread_getcpuclockid 3 169.Sh STANDARDS 170The 171.Fn clock_gettime , 172.Fn clock_settime , 173and 174.Fn clock_getres 175system calls conform to 176.St -p1003.1b-93 . 177The clock IDs 178.Dv CLOCK_REALTIME_FAST , 179.Dv CLOCK_REALTIME_PRECISE , 180.Dv CLOCK_MONOTONIC_FAST , 181.Dv CLOCK_MONOTONIC_PRECISE , 182.Dv CLOCK_UPTIME , 183.Dv CLOCK_UPTIME_FAST , 184.Dv CLOCK_UPTIME_PRECISE , 185and 186.Dv CLOCK_SECOND 187are 188.Fx 189extensions to the POSIX interface. 190