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TIME2POSIX(3)              Library Functions Manual              TIME2POSIX(3)

NAME
     time2posix, time2posix_z, posix2time, posix2time_z - convert seconds
     since the Epoch

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     time_t
     time2posix(time_t t);

     time_t
     time2posix_z(const timezone_t tz, time_t t);

     time_t
     posix2time(time_t t);

     time_t
     posix2time_z(const timezone_t tz, time_t t);

DESCRIPTION
     IEEE Std 1003.1 ("POSIX.1") requires the time_t value of 536457599 to
     stand for
           Wed Dec 31 23:59:59 UTC 1986.
     This effectively implies that POSIX time_t values cannot include leap
     seconds and, therefore, that the system time must be adjusted as each
     leap occurs.

     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 passed-
     to functions such as time(3), localtime(3), localtime_r(3),
     localtime_rz(3), mktime(3), mktime_z(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(), time2posix_z(), posix2time(), and posix2time_z()
     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 POSIX-compliant systems.

     time2posix() and time2posix_z() are single-valued.  That is, every local
     time_t corresponds to a single POSIX time_t.  posix2time() and
     posix2time() are 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 "z" variants of the two functions behave exactly like their
     counterparts, but they operate in the given tz argument which was
     previously allocated using tzalloc(3) and are re-entrant.

     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
     difftime(3), localtime(3), localtime_r(3), localtime_rz(3), mktime(3),
     mktime_z(3), time(3), tzalloc(3)

NetBSD 8.99.34                  October 6, 2014                 NetBSD 8.99.34