dfc79e892f
Upgrade of the tzcode from 2004a to 2009e. Changes are numerous, but include... - New format of the output of zic, which supports both 32 and 64 bit time_t formats. - zdump on 64 bit platforms will actually produce some output instead of doing nothing for a looooooooong time. - linux_base-fX, with X >= at least 8, will work without problems related to the local time again. The original patch, based on the 2008e, has been running for a long time on both my laptop and desktop machine and have been tested by other people. After the installation of this code and the running of zic(8), you need to run tzsetup(8) again to install the new datafile. Approved by: wollman@ for usr.sbin/zic MFC after: 1 month
124 lines
3.4 KiB
Groff
124 lines
3.4 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd September 11, 2005
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.Dt TIME2POSIX 3
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.Os
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.Sh NAME
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.Nm time2posix ,
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.Nm posix2time
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.Nd convert seconds since the Epoch
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In time.h
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.Ft time_t
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.Fn time2posix "time_t t"
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.Ft time_t
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.Fn posix2time "time_t t"
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.Sh DESCRIPTION
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.St -p1003.1-88
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legislates that a time_t value of
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536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 1986."
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This effectively implies that POSIX time_t's cannot include leap
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seconds and,
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therefore,
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that the system time must be adjusted as each leap occurs.
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.Pp
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If the time package is configured with leap-second support
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enabled,
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however,
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no such adjustment is needed and
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time_t values continue to increase over leap events
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(as a true `seconds since...' value).
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This means that these values will differ from those required by POSIX
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by the net number of leap seconds inserted since the Epoch.
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.Pp
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Typically this is not a problem as the type time_t is intended
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to be
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(mostly)
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opaque\(emtime_t values should only be obtained-from and
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passed-to functions such as
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.Xr time 3 ,
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.Xr localtime 3 ,
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.Xr mktime 3
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and
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.Xr difftime 3 .
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However,
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.St -p1003.1-88
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gives an arithmetic
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expression for directly computing a time_t value from a given date/time,
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and the same relationship is assumed by some
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(usually older)
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applications.
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Any programs creating/dissecting time_t's
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using such a relationship will typically not handle intervals
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over leap seconds correctly.
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.Pp
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The
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.Fn time2posix
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and
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.Fn posix2time
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functions are provided to address this time_t mismatch by converting
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between local time_t values and their POSIX equivalents.
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This is done by accounting for the number of time-base changes that
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would have taken place on a POSIX system as leap seconds were inserted
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or deleted.
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These converted values can then be used in lieu of correcting the older
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applications,
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or when communicating with POSIX-compliant systems.
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.Pp
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The
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.Fn time2posix
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function is single-valued.
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That is,
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every local time_t
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corresponds to a single POSIX time_t.
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The
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.Fn posix2time
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function is less well-behaved:
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for a positive leap second hit the result is not unique,
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and for a negative leap second hit the corresponding
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POSIX time_t does not exist so an adjacent value is returned.
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Both of these are good indicators of the inferiority of the
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POSIX representation.
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.Pp
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The following table summarizes the relationship between time_t
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and its conversion to,
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and back from,
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the POSIX representation over the leap second inserted at the end of June,
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1993.
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.Bl -column "93/06/30" "23:59:59" "A+0" "X=time2posix(T)"
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.It Sy "DATE TIME T X=time2posix(T) posix2time(X)"
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.It "93/06/30 23:59:59 A+0 B+0 A+0"
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.It "93/06/30 23:59:60 A+1 B+1 A+1 or A+2"
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.It "93/07/01 00:00:00 A+2 B+1 A+1 or A+2"
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.It "93/07/01 00:00:01 A+3 B+2 A+3"
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.El
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.Pp
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A leap second deletion would look like...
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.Bl -column "??/06/30" "23:59:58" "A+0" "X=time2posix(T)"
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.It Sy "DATE TIME T X=time2posix(T) posix2time(X)"
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.It "??/06/30 23:59:58 A+0 B+0 A+0"
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.It "??/07/01 00:00:00 A+1 B+2 A+1"
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.It "??/07/01 00:00:01 A+2 B+3 A+2"
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.El
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.Pp
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.D1 No "[Note: posix2time(B+1) => A+0 or A+1]"
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.Pp
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If leap-second support is not enabled,
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local time_t's and
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POSIX time_t's are equivalent,
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and both
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.Fn time2posix
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and
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.Fn posix2time
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degenerate to the identity function.
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.Sh "SEE ALSO"
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.Xr difftime 3 ,
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.Xr localtime 3 ,
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.Xr mktime 3 ,
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.Xr time 3
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.\" @(#)time2posix.3 8.1
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.\" This file is in the public domain, so clarified as of
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.\" 1996-06-05 by Arthur David Olson.
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