2fe8fb192f
There is important information about booting non-ack images in docs/UPDATING. ack/aout-format images can't be built any more, and booting clang/ELF-format ones is a little different. Updating to the new boot monitor is recommended. Changes in this commit: . drop boot monitor -> allowing dropping ack support . facility to copy ELF boot files to /boot so that old boot monitor can still boot fairly easily, see UPDATING . no more ack-format libraries -> single-case libraries . some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases . drop several ack toolchain commands, but not all support commands (e.g. aal is gone but acksize is not yet). . a few libc files moved to netbsd libc dir . new /bin/date as minix date used code in libc/ . test compile fix . harmonize includes . /usr/lib is no longer special: without ack, /usr/lib plays no kind of special bootstrapping role any more and bootstrapping is done exclusively through packages, so releases depend even less on the state of the machine making them now. . rename nbsd_lib* to lib* . reduce mtree
150 lines
4.1 KiB
Groff
150 lines
4.1 KiB
Groff
.\" $NetBSD: time2posix.3,v 1.17 2010/12/17 01:30:14 wiz Exp $
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.Dd December 4, 2010
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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 time2posix_z ,
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.Nm posix2time ,
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.Nm posix2time_z ,
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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 time2posix_z "const timezone_t tz" "time_t t"
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.Ft time_t
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.Fn posix2time "time_t t"
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.Ft time_t
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.Fn posix2time_z "const timezone_t tz" "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 localtime_r 3 ,
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.Xr localtime_rz 3 ,
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.Xr mktime 3 ,
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.Xr mktime_z 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.
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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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.Fn time2posix_z ,
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.Fn posix2time ,
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and
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.Fn posix2time_z
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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.
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This is done by accounting for the number of time-base changes that would
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have taken place on a POSIX system as leap seconds were inserted or deleted.
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These converted values can then be used in lieu of correcting the
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older applications, or when communicating with POSIX-compliant systems.
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.Pp
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.Fn time2posix
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and
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.Fn time2posix_z
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are single-valued.
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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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and
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.Fn posix2time
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are 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.
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Both of these are good indicators of the inferiority of the POSIX
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representation.
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.Pp
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The
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.Dq z
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variants of the two functions behave exactly like their counterparts,
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but they operate in the given
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.Fa tz
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argument which was previously allocated using
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.Xr tzalloc 3
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and are re-entrant.
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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 localtime_r 3 ,
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.Xr localtime_rz 3 ,
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.Xr mktime 3 ,
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.Xr mktime_z 3 ,
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.Xr time 3 ,
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.Xr tzalloc 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.
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