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Revision 1.13, Wed Apr 16 13:34:58 2003 UTC (17 years, 6 months ago) by wiz
Branch: MAIN
CVS Tags: yamt-pf42-baseX, yamt-pf42-base4, yamt-pf42-base3, yamt-pf42-base2, yamt-pf42-base, yamt-pf42, wrstuden-revivesa-base-3, wrstuden-revivesa-base-2, wrstuden-revivesa-base-1, wrstuden-revivesa-base, wrstuden-revivesa, wrstuden-fixsa-newbase, wrstuden-fixsa-base-1, wrstuden-fixsa-base, wrstuden-fixsa, netbsd-5-base, netbsd-5-2-RELEASE, netbsd-5-2-RC1, netbsd-5-2-3-RELEASE, netbsd-5-2-2-RELEASE, netbsd-5-2-1-RELEASE, netbsd-5-2, netbsd-5-1-RELEASE, netbsd-5-1-RC4, netbsd-5-1-RC3, netbsd-5-1-RC2, netbsd-5-1-RC1, netbsd-5-1-5-RELEASE, netbsd-5-1-4-RELEASE, netbsd-5-1-3-RELEASE, netbsd-5-1-2-RELEASE, netbsd-5-1-1-RELEASE, netbsd-5-1, netbsd-5-0-RELEASE, netbsd-5-0-RC4, netbsd-5-0-RC3, netbsd-5-0-RC2, netbsd-5-0-RC1, netbsd-5-0-2-RELEASE, netbsd-5-0-1-RELEASE, netbsd-5-0, netbsd-5, netbsd-4-base, netbsd-4-0-RELEASE, netbsd-4-0-RC5, netbsd-4-0-RC4, netbsd-4-0-RC3, netbsd-4-0-RC2, netbsd-4-0-RC1, netbsd-4-0-1-RELEASE, netbsd-4-0, netbsd-4, netbsd-3-base, netbsd-3-1-RELEASE, netbsd-3-1-RC4, netbsd-3-1-RC3, netbsd-3-1-RC2, netbsd-3-1-RC1, netbsd-3-1-1-RELEASE, netbsd-3-1, netbsd-3-0-RELEASE, netbsd-3-0-RC6, netbsd-3-0-RC5, netbsd-3-0-RC4, netbsd-3-0-RC3, netbsd-3-0-RC2, netbsd-3-0-RC1, netbsd-3-0-3-RELEASE, netbsd-3-0-2-RELEASE, netbsd-3-0-1-RELEASE, netbsd-3-0, netbsd-3, netbsd-2-base, netbsd-2-1-RELEASE, netbsd-2-1-RC6, netbsd-2-1-RC5, netbsd-2-1-RC4, netbsd-2-1-RC3, netbsd-2-1-RC2, netbsd-2-1-RC1, netbsd-2-1, netbsd-2-0-base, netbsd-2-0-RELEASE, netbsd-2-0-RC5, netbsd-2-0-RC4, netbsd-2-0-RC3, netbsd-2-0-RC2, netbsd-2-0-RC1, netbsd-2-0-3-RELEASE, netbsd-2-0-2-RELEASE, netbsd-2-0-1-RELEASE, netbsd-2-0, netbsd-2, mjf-devfs2-base, mjf-devfs2, matt-nb5-pq3-base, matt-nb5-pq3, matt-nb5-mips64-u2-k2-k4-k7-k8-k9, matt-nb5-mips64-u1-k1-k5, matt-nb5-mips64-premerge-20101231, matt-nb5-mips64-premerge-20091211, matt-nb5-mips64-k15, matt-nb5-mips64, matt-nb4-mips64-k7-u2a-k9b, matt-mips64-base2, matt-mips64-base, matt-mips64, matt-armv6-prevmlocking, matt-armv6-nbase, matt-armv6-base, matt-armv6, keiichi-mipv6-base, keiichi-mipv6, hpcarm-cleanup-nbase, hpcarm-cleanup-base, hpcarm-cleanup, cube-autoconf-base, cube-autoconf, christos-time_t-nbase, christos-time_t-base, christos-time_t, abandoned-netbsd-4-base, abandoned-netbsd-4
Branch point for: jym-xensuspend
Changes since 1.12: +2 -2 lines

Use
.In header.h
instead of
.Fd #include \*[Lt]header.h\*[Gt]
Much easier to read and write, and supported by groff for ages.
Okayed by ross.

.\"	$NetBSD: time2posix.3,v 1.13 2003/04/16 13:34:58 wiz Exp $
.Dd April 1, 2001
.Dt TIME2POSIX 3
.Os
.Sh NAME
.Nm time2posix ,
.Nm posix2time
.Nd convert seconds since the Epoch
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft time_t
.Fn time2posix "time_t t"
.Ft time_t
.Fn posix2time "time_t t"
.Sh DESCRIPTION
.St -p1003.1
legislates that a
.Va time_t
value of 536457599 shall correspond to
.Dl Wed Dec 31 23:59:59 UTC 1986 .
This effectively implies that POSIX
.Va time_t Ns 's
cannot include leap seconds and, therefore,
that the system time must be adjusted as each leap occurs.
.Pp
If the time package is configured with leap-second support
enabled, however, no such adjustment is needed and
.Va time_t
values continue to increase over leap events
(as a true `seconds since...' value).
This means that these values will differ from those required by POSIX
by the net number of leap seconds inserted since the Epoch.
.Pp
Typically this is not a problem as the type
.Va time_t
is intended to be (mostly)
opaque \(em
.Va time_t
values should only be obtained-from and
passed-to functions such as
.Xr time 3 ,
.Xr localtime 3 ,
.Xr mktime 3 ,
and
.Xr difftime 3 .
However, POSIX gives an arithmetic expression for directly computing a
.Va time_t
value from a given date/time, and the same relationship is assumed by
some (usually older) applications.
Any programs creating/dissecting
.Va time_t Ns 's
using such a relationship will typically not handle intervals over
leap seconds correctly.
.Pp
The
.Fn time2posix
and
.Fn posix2time
functions are provided to address this
.Va time_t
mismatch by converting between local
.Va time_t
values and their POSIX equivalents.
This is done by accounting for the number of time-base changes that would
have taken place on a POSIX system as leap seconds were inserted or deleted.
These converted values can then be used in lieu of correcting the
older applications, or when communicating with POSIX-compliant systems.
.Pp
.Fn time2posix
is single-valued.
That is, every local
.Va time_t
corresponds to a single POSIX
.Va time_t .
.Fn posix2time
is less well-behaved: for a positive leap second hit the result is not
unique, and for a negative leap second hit the corresponding POSIX
.Va time_t
doesn't exist so an adjacent value is returned.
Both of these are good indicators of the inferiority of the POSIX
representation.
.Pp
The following table summarizes the relationship between a
.Va time_t
and its conversion to, and back from, the POSIX representation over
the leap second inserted at the end of June, 1993.
.Bl -column "93/06/30" "23:59:59" "A+0" "X=time2posix(T)" "posix2time(X) -offset indent
.It Sy DATE	TIME	T	X=time2posix(T)	posix2time(X)
.It 93/06/30	23:59:59	A+0	B+0	A+0
.It 93/06/30	23:59:60	A+1	B+1	A+1 or A+2
.It 93/07/01	00:00:00	A+2	B+1	A+1 or A+2
.It 93/07/01	00:00:01	A+3	B+2	A+3
.El
.Pp
A leap second deletion would look like...
.Bl -column "??/06/30" "23:59:58" "A+0" "X=time2posix(T)" "posix2time(X) -offset indent
.It Sy DATE	TIME	T	X=time2posix(T)	posix2time(X)
.It ??/06/30	23:59:58	A+0	B+0	A+0
.It ??/07/01	00:00:00	A+1	B+2	A+1
.It ??/07/01	00:00:01	A+2	B+3	A+2
.El
[Note: posix2time(B+1) =\*[Gt] A+0 or A+1]
.Pp
If leap-second support is not enabled, local
.Va time_t Ns 's
and POSIX
.Va time_t Ns 's
are equivalent, and both
.Fn time2posix
and
.Fn posix2time
degenerate to the identity function.
.Sh SEE ALSO
.Xr difftime 3 ,
.Xr localtime 3 ,
.Xr mktime 3 ,
.Xr time 3
.\" @(#)time2posix.3	7.7
.\" This file is in the public domain, so clarified as of
.\" 1996-06-05 by Arthur David Olson (arthur_david_olson@nih.gov).