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 September 26, 2016 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 62can be one of the following values: 63.Dv CLOCK_REALTIME , 64.Dv CLOCK_REALTIME_PRECISE , 65and 66.Dv CLOCK_REALTIME_FAST 67for time that increments as a wall clock should; 68.Dv CLOCK_MONOTONIC , 69.Dv CLOCK_MONOTONIC_PRECISE , 70and 71.Dv CLOCK_MONOTONIC_FAST 72which increments in SI seconds; 73.Dv CLOCK_UPTIME , 74.Dv CLOCK_UPTIME_PRECISE , 75and 76.Dv CLOCK_UPTIME_FAST 77which starts at zero when the kernel boots and increments 78monotonically in SI seconds while the machine is running; 79.Dv CLOCK_VIRTUAL 80for time that increments only when 81the CPU is running in user mode on behalf of the calling process; 82.Dv CLOCK_PROF 83for time that increments when the CPU is running in user or 84kernel mode; 85.Dv CLOCK_SECOND 86which returns the current second without performing a full time counter 87query, using in-kernel cached value of current second; 88.Dv CLOCK_PROCESS_CPUTIME_ID 89which returns the CPU-time clock of the calling process; or 90.Dv CLOCK_THREAD_CPUTIME_ID 91which returns the CPU-time clock of the calling thread. 92.Pp 93The clock IDs 94.Dv CLOCK_REALTIME_FAST , 95.Dv CLOCK_MONOTONIC_FAST , 96and 97.Dv CLOCK_UPTIME_FAST 98are analogs of corresponding IDs without _FAST suffix but do not perform 99a full time counter query, so their accuracy is one timer tick. 100Similarly, 101.Dv CLOCK_REALTIME_PRECISE , 102.Dv CLOCK_MONOTONIC_PRECISE , 103and 104.Dv CLOCK_UPTIME_PRECISE 105are used to get the most exact value as possible, at the expense of 106execution time. 107.Pp 108The clock IDs 109.Dv CLOCK_UPTIME_FAST , 110.Dv CLOCK_MONOTONIC_FAST , 111.Dv CLOCK_REALTIME_FAST , 112and 113.Dv CLOCK_SECOND 114make use of 115.Xr kpmap 4 116and do not incur any system call overhead after a certain amount of calls. 117.Pp 118The structure pointed to by 119.Fa tp 120is defined in 121.In sys/_timespec.h 122as: 123.Bd -literal 124struct timespec { 125 time_t tv_sec; /* seconds */ 126 long tv_nsec; /* and nanoseconds */ 127}; 128.Ed 129.Pp 130Only the super-user may set the time of day, using only 131.Dv CLOCK_REALTIME . 132If the system securelevel is greater than 1 (see 133.Xr init 8 ) , 134the time may only be advanced. 135This limitation is imposed to prevent a malicious super-user 136from setting arbitrary time stamps on files. 137The system time can still be adjusted backwards using the 138.Xr adjtime 2 139system call even when the system is secure. 140.Pp 141The resolution (granularity) of a clock is returned by the 142.Fn clock_getres 143system call. 144This value is placed in a (non-NULL) 145.Fa *tp . 146.Sh RETURN VALUES 147.Rv -std 148.Sh ERRORS 149The following error codes may be set in 150.Va errno : 151.Bl -tag -width Er 152.It Bq Er EINVAL 153The 154.Fa clock_id 155or 156.Fa tp 157argument was not a valid value. 158.It Bq Er EPERM 159A user other than the super-user attempted to set the time. 160.El 161.Sh SEE ALSO 162.Xr date 1 , 163.Xr adjtime 2 , 164.Xr ctime 3 , 165.Xr timed 8 166.Sh STANDARDS 167The 168.Fn clock_gettime , 169.Fn clock_settime , 170and 171.Fn clock_getres 172system calls conform to 173.St -p1003.1b-93 . 174The clock IDs 175.Dv CLOCK_REALTIME_FAST , 176.Dv CLOCK_REALTIME_PRECISE , 177.Dv CLOCK_MONOTONIC_FAST , 178.Dv CLOCK_MONOTONIC_PRECISE , 179.Dv CLOCK_UPTIME , 180.Dv CLOCK_UPTIME_FAST , 181.Dv CLOCK_UPTIME_PRECISE , 182and 183.Dv CLOCK_SECOND 184are 185.Fx 186extensions to the POSIX interface. 187