142 lines
3.6 KiB
Groff
142 lines
3.6 KiB
Groff
.\" $OpenBSD: time2posix.3,v 1.20 2015/02/23 19:05:36 bentley Exp $
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.Dd $Mdocdate: February 23 2015 $
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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 SYNOPSIS
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.In sys/types.h
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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
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legislates that a
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.Vt time_t
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value of
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536457599 shall correspond to
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.Dq Wed Dec 31 23:59:59 UTC 1986 .
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This effectively implies that a POSIX
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.Vt time_t
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cannot include leap seconds and, 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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.Vt time_t
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values continue to increase over leap events
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.Po
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as a true
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.Sq seconds since...
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value
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.Pc .
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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
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.Vt time_t
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is intended to be
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.Pq mostly
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opaque.
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.Vt time_t
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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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POSIX gives an arithmetic
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expression for directly computing a
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.Vt time_t
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value from a given date/time,
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and the same relationship is assumed by some
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.Pq usually older
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applications.
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Any programs creating/dissecting
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.Vt time_t
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values
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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
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.Vt time_t
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mismatch by converting
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between local
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.Vt time_t
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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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.Fn time2posix
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is single-valued.
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That is,
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every local
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.Vt time_t
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corresponds to a single POSIX
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.Vt time_t .
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.Fn posix2time
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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
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.Vt time_t
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doesn't 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 a time
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T 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) posix2time(X) -offset indent
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.It Em DATE Ta Em TIME Ta Em T Ta Em X=time2posix(T) Ta Em posix2time(X)
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.It 93/06/30 Ta 23:59:59 Ta A+0 Ta B+0 Ta A+0
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.It 93/06/30 Ta 23:59:60 Ta A+1 Ta B+1 Ta A+1 or A+2
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.It 93/07/01 Ta 00:00:00 Ta A+2 Ta B+1 Ta A+1 or A+2
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.It 93/07/01 Ta 00:00:01 Ta A+3 Ta B+2 Ta 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) posix2time(X) -offset indent
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.It Em DATE Ta Em TIME Ta Em T Ta Em X=time2posix(T) Ta Em posix2time(X)
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.It ??/06/30 Ta 23:59:58 Ta A+0 Ta B+0 Ta A+0
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.It ??/07/01 Ta 00:00:00 Ta A+1 Ta B+2 Ta A+1
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.It ??/07/01 Ta 00:00:01 Ta A+2 Ta B+3 Ta A+2
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.El
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.Pp
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[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, local
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.Vt time_t
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and
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POSIX
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.Vt time_t
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are equivalent, 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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.\" 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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