120 lines
3.5 KiB
Groff
120 lines
3.5 KiB
Groff
.\" $NetBSD: time2posix.3,v 1.11 2002/02/07 07:00:34 ross Exp $
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.Dd April 1, 2001
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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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.Fd #include \*[Lt]time.h\*[Gt]
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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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.Va time_t
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value of 536457599 shall correspond to
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.Dl Wed Dec 31 23:59:59 UTC 1986 .
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This effectively implies that POSIX
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.Va time_t Ns 's
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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, however, no such adjustment is needed and
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.Va time_t
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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
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.Va time_t
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is intended to be (mostly)
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opaque \(em
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.Va 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, POSIX gives an arithmetic expression for directly computing a
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.Va time_t
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value from a given date/time, and the same relationship is assumed by
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some (usually older) applications. Any programs creating/dissecting
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.Va time_t Ns 's
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using such a relationship will typically not handle intervals over
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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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.Va time_t
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mismatch by converting between local
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.Va time_t
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values and their POSIX equivalents. This is done by accounting for the
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number of time-base changes that would have taken place on a POSIX
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system as leap seconds were inserted or deleted. These converted
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values can then be used in lieu of correcting the older 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. That is, every local
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.Va time_t
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corresponds to a single POSIX
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.Va time_t .
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.Fn posix2time
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is less well-behaved: for a positive leap second hit the result is not
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unique, and for a negative leap second hit the corresponding POSIX
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.Va time_t
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doesn't exist so an adjacent value is returned. Both of these are good
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indicators of the inferiority of the POSIX representation.
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.Pp
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The following table summarizes the relationship between a
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.Va time_t
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and its conversion to, and back from, the POSIX representation over
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the leap second inserted at the end of June, 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 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)" "posix2time(X) -offset indent
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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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[Note: posix2time(B+1) =\*[Gt] 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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.Va time_t Ns 's
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and POSIX
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.Va time_t Ns 's
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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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.\" @(#)time2posix.3 7.7
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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 (arthur_david_olson@nih.gov).
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