77 lines
3.5 KiB
Plaintext
77 lines
3.5 KiB
Plaintext
TIME2POSIX(3) Library Functions Manual TIME2POSIX(3)
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NAME
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time2posix, posix2time - convert seconds since the Epoch
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SYNOPSIS
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#include <time.h>
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time_t time2posix(time_t t);
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time_t posix2time(time_t t);
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cc ... -ltz
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DESCRIPTION
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IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to
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stand for 1986-12-31 23:59:59 UTC. This effectively implies that POSIX
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time_t values cannot include leap seconds and, therefore, that the
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system time must be adjusted as each leap occurs.
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If the time package is configured with leap-second support enabled,
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however, no such adjustment is needed and time_t values continue to
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increase over leap events (as a true "seconds since..." value). This
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means that these values will differ from those required by POSIX by the
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net number of leap seconds inserted since the Epoch.
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Typically this is not a problem as the type time_t is intended to be
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(mostly) opaque - time_t values should only be obtained-from and
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passed-to functions such as time(2), localtime(3), mktime(3), and
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difftime(3). However, POSIX gives an arithmetic expression for
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directly computing a time_t value from a given date/time, and the same
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relationship is assumed by some (usually older) applications. Any
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programs creating/dissecting time_t's using such a relationship will
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typically not handle intervals over leap seconds correctly.
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The time2posix and posix2time functions are provided to address this
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time_t mismatch by converting between local time_t values and their
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POSIX equivalents. This is done by accounting for the number of time-
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base changes that would have taken place on a POSIX system as leap
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seconds were 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 corresponds
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to a single POSIX time_t. Posix2time is less well-behaved: for a
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positive leap second hit the result is not unique, and for a negative
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leap second hit the corresponding POSIX time_t doesn't exist so an
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adjacent 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 time T and
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it's conversion to, and back from, the POSIX representation over the
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leap second inserted at the end of 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 POSIX
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time_t's are equivalent, and both time2posix and posix2time degenerate
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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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TIME2POSIX(3)
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