1.\" $NetBSD: time.9,v 1.1 1995/11/25 21:24:53 perry Exp $ 2.\" 3.\" Copyright (c) 1994 Christopher G. Demetriou 4.\" 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by Christopher G. Demetriou 17.\" for the NetBSD Project. 18.\" 3. The name of the author may not be used to endorse or promote products 19.\" derived from this software without specific prior written permission 20.\" 21.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 22.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 23.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 24.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 25.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 26.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 27.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 28.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 29.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 30.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 31.\" 32.\" $FreeBSD: src/share/man/man9/time.9,v 1.3.2.5 2001/08/17 13:08:54 ru Exp $ 33.\" 34.Dd March 22, 1997 35.Dt TIME 9 36.Os 37.Sh NAME 38.Nm boottime , 39.Nm mono_time , 40.Nm runtime , 41.Nm time 42.Nd system time variables 43.Sh SYNOPSIS 44.Bl -item -compact 45.It 46.Va extern struct timeval boottime ; 47.It 48.Va extern volatile struct timeval mono_time ; 49.It 50.Va extern struct timeval runtime ; 51.It 52.Va extern struct timeval time ; 53.El 54.Sh DESCRIPTION 55The 56.Va time 57variable is the system's 58.Dq wall time 59clock. 60It is set at boot by 61.Xr inittodr 9 , 62and is updated by the 63.Xr settimeofday 2 64system call and by periodic clock interrupts. 65.Pp 66The 67.Va boottime 68variable holds the system boot time. It is set from 69.Va time 70at system boot, and is updated when the system time is adjusted 71with 72.Xr settimeofday 2 . 73.Pp 74The 75.Va runtime 76variable holds the time that the system switched to the 77current process. 78It is set after each context switch, 79and is updated when the system time is adjusted with 80.Xr settimeofday 2 . 81Because 82.Va runtime 83is used for system accounting, it is set with the high-resolution 84.Xr microtime 9 85function, rather than being copied from 86.Va time . 87.Pp 88The 89.Va mono_time 90variable is a monotonically increasing system clock. It is set 91from 92.Va time 93at boot, and is updated by the periodic timer interrupt. (It is 94not updated by 95.Xr settimeofday 2 . ) 96.Pp 97All of these variables contain times 98expressed in seconds and microseconds since midnight (0 hour), 99January 1, 1970. 100.Pp 101Clock interrupts should be blocked 102when reading or writing 103.Va time 104or 105.Va mono_time , 106because those variables are updated by 107.Fn hardclock . 108The 109.Xr gettime 9 110function can be used to read the 111.Va time 112variable in an atomic manner. There is no equivalent 113function for accessing 114.Va mono_time . 115The 116.Va boottime 117and 118.Va runtime 119variables may be read and written without special precautions. 120.Pp 121It should be noted that due to the differences in how the time values 122returned by 123.Xr gettime 9 124and 125.Xr microtime 9 126are updated, comparing the results of the two routines 127may result in a time value that appears to go backwards. 128This can be avoided by consistently using one function or the other 129for use in any given context. 130.Sh SEE ALSO 131.Xr settimeofday 2 , 132.Xr gettime 9 , 133.\" .Xr hardclock 9 , 134.\" .Xr hz 9 , 135.Xr inittodr 9 , 136.Xr microtime 9 137.Sh BUGS 138The notion of having a single 139.Va runtime 140variable obviously would not be appropriate in multiprocessor systems. 141