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  man pages->NetBSD man pages -> time2posix (3)              



NAME    [Toc]    [Back]

     time2posix, posix2time - convert seconds since the Epoch

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <time.h>

     time2posix(time_t t);

     posix2time(time_t t);

DESCRIPTION    [Toc]    [Back]

     IEEE Std 1003.1 (``POSIX.1'') legislates that a time_t value of 536457599
     shall correspond to
           Wed Dec 31 23:59:59 UTC 1986.
     This effectively implies that POSIX time_t's cannot include leap seconds
     and, therefore, that the system time must be adjusted as each leap

     If the time package is configured with leap-second support enabled, however,
 no such adjustment is needed and 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.

     Typically this is not a problem as the type time_t is intended to be
     (mostly) opaque -- time_t values should only be obtained-from and passedto
 functions such as time(3), localtime(3), mktime(3), and difftime(3).
     However, POSIX gives an arithmetic expression for directly computing a
     time_t value from a given date/time, and the same relationship is assumed
     by some (usually older) applications. Any programs creating/dissecting
     time_t's using such a relationship will typically not handle intervals
     over leap seconds correctly.

     The time2posix() and posix2time() functions are provided to address this
     time_t mismatch by converting between local 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 POSIXcompliant

     time2posix() is single-valued. That is, every local time_t corresponds to
     a single POSIX time_t.  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 time_t doesn't exist so an adjacent value is
     returned. Both of these are good indicators of the inferiority of the
     POSIX representation.

     The following table summarizes the relationship between a time_t and its
     conversion to, and back from, the POSIX representation over the leap second
 inserted at the end of June, 1993.

     DATE       TIME       T     X=time2posix(T)   posix2time(X)
     93/06/30   23:59:59   A+0   B+0               A+0
     93/06/30   23:59:60   A+1   B+1               A+1 or A+2
     93/07/01   00:00:00   A+2   B+1               A+1 or A+2
     93/07/01   00:00:01   A+3   B+2               A+3

     A leap second deletion would look like...

     DATE       TIME       T     X=time2posix(T)   posix2time(X)
     ??/06/30   23:59:58   A+0   B+0               A+0
     ??/07/01   00:00:00   A+1   B+2               A+1
     ??/07/01   00:00:01   A+2   B+3               A+2
     [Note: posix2time(B+1) => A+0 or A+1]

     If leap-second support is not enabled, local time_t's and POSIX time_t's
     are equivalent, and both time2posix() and posix2time() degenerate to the
     identity function.

SEE ALSO    [Toc]    [Back]

     difftime(3), localtime(3), mktime(3), time(3)

BSD                              April 1, 2001                             BSD
[ Back ]
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