time2posix, posix2time  convert seconds since the Epoch
Standard C Library (libc, lc)
#include <time.h>
time_t
time2posix(time_t t);
time_t
posix2time(time_t t);
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 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 obtainedfrom 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 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() is singlevalued. That is, every local time_t corresponds to
a single POSIX time_t. posix2time() is less wellbehaved: 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 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.
difftime(3), localtime(3), mktime(3), time(3)
BSD April 1, 2001 BSD
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