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 March 20, 2017 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 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.Fa CLOCK_REALTIME_FAST , 97.Fa CLOCK_MONOTONIC_FAST , 98.Fa CLOCK_UPTIME_FAST 99are analogs of corresponding IDs without _FAST suffix but do not perform 100a full time counter query, so their accuracy is one timer tick. 101Similarly, 102.Fa CLOCK_REALTIME_PRECISE , 103.Fa CLOCK_MONOTONIC_PRECISE , 104.Fa CLOCK_UPTIME_PRECISE 105are used to get the most exact value as possible, at the expense of 106execution time. 107.Pp 108The structure pointed to by 109.Fa tp 110is defined in 111.In sys/timespec.h 112as: 113.Bd -literal 114struct timespec { 115 time_t tv_sec; /* seconds */ 116 long tv_nsec; /* and nanoseconds */ 117}; 118.Ed 119.Pp 120Only the super-user may set the time of day, using only 121.Fa CLOCK_REALTIME . 122If the system securelevel is greater than 1 (see 123.Xr init 8 ) , 124the time may only be advanced. 125This limitation is imposed to prevent a malicious super-user 126from setting arbitrary time stamps on files. 127The system time can still be adjusted backwards using the 128.Xr adjtime 2 129system call even when the system is secure. 130.Pp 131The resolution (granularity) of a clock is returned by the 132.Fn clock_getres 133system call. 134This value is placed in a (non-NULL) 135.Fa *tp . 136.Sh RETURN VALUES 137.Rv -std 138.Sh ERRORS 139The following error codes may be set in 140.Va errno : 141.Bl -tag -width Er 142.It Bq Er EINVAL 143The 144.Fa clock_id 145or 146.Fa timespec 147argument 148was not a valid value. 149.It Bq Er EPERM 150A user other than the super-user attempted to set the time. 151.El 152.Sh SEE ALSO 153.Xr date 1 , 154.Xr adjtime 2 , 155.Xr clock_getcpuclockid 3 , 156.Xr ctime 3 , 157.Xr pthread_getcpuclockid 3 , 158.Xr timed 8 159.Sh STANDARDS 160The 161.Fn clock_gettime , 162.Fn clock_settime , 163and 164.Fn clock_getres 165system calls conform to 166.St -p1003.1b-93 . 167The clock IDs 168.Fa CLOCK_REALTIME_FAST , 169.Fa CLOCK_REALTIME_PRECISE , 170.Fa CLOCK_MONOTONIC_FAST , 171.Fa CLOCK_MONOTONIC_PRECISE , 172.Fa CLOCK_UPTIME , 173.Fa CLOCK_UPTIME_FAST , 174.Fa CLOCK_UPTIME_PRECISE , 175.Fa CLOCK_SECOND 176are FreeBSD extensions to the POSIX interface. 177