141 lines
4.1 KiB
Groff
141 lines
4.1 KiB
Groff
.\" $NetBSD: time.9,v 1.1 1995/11/25 21:24:53 perry Exp $
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.\"
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.\" Copyright (c) 1994 Christopher G. Demetriou
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by Christopher G. Demetriou
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.\" for the NetBSD Project.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd March 22, 1997
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.Dt TIME 9
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.Os FreeBSD
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.Sh NAME
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.Nm boottime ,
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.Nm mono_time ,
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.Nm runtime ,
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.Nm time
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.Nd system time variables
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.Sh SYNOPSIS
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.Bl -item -compact
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.It
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.Va extern struct timeval boottime ;
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.It
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.Va extern volatile struct timeval mono_time ;
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.It
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.Va extern struct timeval runtime ;
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.It
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.Va extern struct timeval time ;
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.El
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.Sh DESCRIPTION
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The
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.Va time
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variable is the system's
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.Dq wall time
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clock.
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It is set at boot by
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.Xr inittodr 9 ,
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and is updated by the
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.Xr settimeofday 2
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system call and by periodic clock interrupts.
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.Pp
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The
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.Va boottime
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variable holds the system boot time. It is set from
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.Va time
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at system boot, and is updated when the system time is adjusted
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with
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.Xr settimeofday 2 .
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.Pp
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The
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.Va runtime
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variable holds the time that the system switched to the
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current process.
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It is set after each context switch,
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and is updated when the system time is adjusted with
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.Xr settimeofday 2 .
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Because
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.Va runtime
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is used for system accounting, it is set with the high-resolution
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.Xr microtime 9
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function, rather than being copied from
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.Va time .
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.Pp
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The
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.Va mono_time
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variable is a monotonically increasing system clock. It is set
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from
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.Va time
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at boot, and is updated by the periodic timer interrupt. (It is
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not updated by
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.Xr settimeofday 2 .)
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.Pp
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All of these variables contain times
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expressed in seconds and microseconds since midnight (0 hour),
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January 1, 1970.
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.Pp
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Clock interrupts should be blocked
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when reading or writing
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.Va time
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or
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.Va mono_time ,
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because those variables are updated by
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.Fn hardclock .
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The
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.Xr gettime 9
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function can be used to read the
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.Va time
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variable in an atomic manner. There is no equivalent
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function for accessing
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.Va mono_time.
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The
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.Va boottime
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and
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.Va runtime
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variables may be read and written without special precautions.
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.Pp
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It should be noted that due to the differences in how the time values
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returned by
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.Xr gettime 9
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and
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.Xr microtime 9
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are updated, comparing the results of the two routines
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may result in a time value that appears to go backwards.
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This can be avoided by consistently using one function or the other
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for use in any given context.
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.Sh SEE ALSO
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.Xr settimeofday 2 ,
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.Xr gettime 9 ,
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.\" .Xr hz 9 ,
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.\" .Xr hardclock 9 ,
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.Xr inittodr 9 ,
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.Xr microtime 9
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.Sh BUGS
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The notion of having a single
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.Va runtime
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variable obviously would not be appropriate in multiprocessor systems.
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