121 lines
3.3 KiB
Groff
121 lines
3.3 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd May 1, 1996
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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 "const time_t *t"
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.Ft time_t
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.Fn posix2time "const 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 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 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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