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.\" 4. 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 December 27, 2015 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 62can be one of the following values: 63.Dv CLOCK_REALTIME , 64.Dv CLOCK_REALTIME_PRECISE , 65.Dv CLOCK_REALTIME_FAST 66for time that increments as 67a wall clock should; 68.Dv CLOCK_MONOTONIC , 69.Dv CLOCK_MONOTONIC_PRECISE , 70.Dv CLOCK_MONOTONIC_FAST 71which increments in SI seconds; 72.Dv CLOCK_UPTIME , 73.Dv CLOCK_UPTIME_PRECISE , 74.Dv CLOCK_UPTIME_FAST 75which starts at zero when the kernel boots and increments 76monotonically in SI seconds while the machine is running; 77.Dv CLOCK_VIRTUAL 78for time that increments only when 79the CPU is running in user mode on behalf of the calling process; 80.Dv CLOCK_PROF 81for time that increments when the CPU is running in user or 82kernel mode; or 83.Dv CLOCK_SECOND 84which returns the current second without performing a full time counter 85query, using in-kernel cached value of current second. 86.Pp 87The clock IDs 88.Fa CLOCK_REALTIME_FAST , 89.Fa CLOCK_MONOTONIC_FAST , 90.Fa CLOCK_UPTIME_FAST 91are analogs of corresponding IDs without _FAST suffix but do not perform 92a full time counter query, so their accuracy is one timer tick. 93Similarly, 94.Fa CLOCK_REALTIME_PRECISE , 95.Fa CLOCK_MONOTONIC_PRECISE , 96.Fa CLOCK_UPTIME_PRECISE 97are used to get the most exact value as possible, at the expense of 98execution time. 99.Pp 100The structure pointed to by 101.Fa tp 102is defined in 103.In sys/timespec.h 104as: 105.Bd -literal 106struct timespec { 107 time_t tv_sec; /* seconds */ 108 long tv_nsec; /* and nanoseconds */ 109}; 110.Ed 111.Pp 112Only the super-user may set the time of day, using only 113.Fa CLOCK_REALTIME . 114If the system securelevel is greater than 1 (see 115.Xr init 8 ) , 116the time may only be advanced. 117This limitation is imposed to prevent a malicious super-user 118from setting arbitrary time stamps on files. 119The system time can still be adjusted backwards using the 120.Xr adjtime 2 121system call even when the system is secure. 122.Pp 123The resolution (granularity) of a clock is returned by the 124.Fn clock_getres 125system call. 126This value is placed in a (non-NULL) 127.Fa *tp . 128.Sh RETURN VALUES 129.Rv -std 130.Sh ERRORS 131The following error codes may be set in 132.Va errno : 133.Bl -tag -width Er 134.It Bq Er EINVAL 135The 136.Fa clock_id 137or 138.Fa timespec 139argument 140was not a valid value. 141.It Bq Er EPERM 142A user other than the super-user attempted to set the time. 143.El 144.Sh SEE ALSO 145.Xr date 1 , 146.Xr adjtime 2 , 147.Xr ctime 3 , 148.Xr timed 8 149.Sh STANDARDS 150The 151.Fn clock_gettime , 152.Fn clock_settime , 153and 154.Fn clock_getres 155system calls conform to 156.St -p1003.1b-93 . 157The clock IDs 158.Fa CLOCK_REALTIME_FAST , 159.Fa CLOCK_REALTIME_PRECISE , 160.Fa CLOCK_MONOTONIC_FAST , 161.Fa CLOCK_MONOTONIC_PRECISE , 162.Fa CLOCK_UPTIME , 163.Fa CLOCK_UPTIME_FAST , 164.Fa CLOCK_UPTIME_PRECISE , 165.Fa CLOCK_SECOND 166are FreeBSD extensions to the POSIX interface. 167