83 lines
3.3 KiB
Text
83 lines
3.3 KiB
Text
NAME
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time2posix, posix2time - convert seconds since the Epoch
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SYNOPSIS
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#include <sys/types.h>
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#include <time.h>
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time_t time2posix(t)
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time_t t
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time_t posix2time(t)
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time_t t
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cc ... -ltz
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DESCRIPTION
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IEEE Standard 1003.1 (POSIX) legislates that a time_t value
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of 536457599 shall correspond to "Wed Dec 31 23:59:59 UTC
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1986." This effectively implies that POSIX time_t's cannot
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include leap seconds and, therefore, that the system time
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must be adjusted as each leap occurs.
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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 time_t
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values continue to increase over leap events (as a true
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`seconds since...' value). This means that these values
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will differ from those required by POSIX by the net number
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of leap seconds inserted since the Epoch.
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Typically this is not a problem as the type time_t is
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intended to be (mostly) opaque-time_t values should only be
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obtained-from and passed-to functions such as time(2),
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localtime(3), mktime(3), and difftime(3). However, POSIX
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gives an arithmetic expression for directly computing a
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time_t value from a given date/time, and the same
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relationship is assumed by some (usually older)
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applications. Any programs creating/dissecting time_t's
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using such a relationship will typically not handle
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intervals over leap seconds correctly.
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The time2posix and posix2time functions are provided to
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address this time_t mismatch by converting between local
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time_t values and their POSIX equivalents. This is done by
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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
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inserted or deleted. These converted values can then be
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used in lieu of correcting the older applications, or when
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communicating with POSIX-compliant systems.
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Time2posix is single-valued. That is, every local time_t
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corresponds to a single POSIX time_t. Posix2time is less
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well-behaved: for a positive leap second hit the result is
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not unique, and for a negative leap second hit the
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corresponding POSIX time_t doesn't exist so an adjacent
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value is returned. Both of these are good indicators of the
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inferiority of the POSIX representation.
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The following table summarizes the relationship between a
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time T and it's conversion to, and back from, the POSIX
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representation over the leap second inserted at the end of
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June, 1993.
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DATE TIME T X=time2posix(T) posix2time(X)
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93/06/30 23:59:59 A+0 B+0 A+0
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93/06/30 23:59:60 A+1 B+1 A+1 or A+2
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93/07/01 00:00:00 A+2 B+1 A+1 or A+2
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93/07/01 00:00:01 A+3 B+2 A+3
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A leap second deletion would look like...
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DATE TIME T X=time2posix(T) posix2time(X)
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??/06/30 23:59:58 A+0 B+0 A+0
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??/07/01 00:00:00 A+1 B+2 A+1
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??/07/01 00:00:01 A+2 B+3 A+2
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[Note: posix2time(B+1) => A+0 or A+1]
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If leap-second support is not enabled, local time_t's and
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POSIX time_t's are equivalent, and both time2posix and
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posix2time degenerate to the identity function.
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SEE ALSO
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difftime(3), localtime(3), mktime(3), time(2)
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