TIME2POSIX(3) BSD 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'') 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
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 corre-
sponding POSIX time_t doesn't exist so an adjacent value is returned. Both of these are good indicators of the inferiority of the POSIX rep-
resentation.
The ``z'' variants of the two functions behave exactly like their counterparts, but they operate in the given tz argument which was previ-
ously 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)
BSD
December 4, 2010 BSD