
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 leapsecond support enabled, however, no such adjust
ment 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 obtainedfrom and passedto 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 han
dle 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 timebase 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 systems.
time2posix() and time2posix_z() are singlevalued. That is, every local time_t corresponds
to a single POSIX time_t. posix2time() and posix2time() are less wellbehaved: for a posi
tive 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 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 leapsecond 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 
