time2posix, posix2time -- convert seconds since the Epoch
Standard C Library (libc, -lc)
#include <time.h>
time_t
time2posix(const time_t *t);
time_t
posix2time(const time_t *t);
IEEE Std 1003.1-1988 (``POSIX.1'') legislates that a time_t value of
536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 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), mktime(3) and difftime(3). However,
IEEE Std 1003.1-1988 (``POSIX.1'') 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
POSIX-compliant systems.
The time2posix() function is single-valued. That is, every local time_t
corresponds to a single POSIX time_t. The posix2time() function 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 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.
difftime(3), localtime(3), mktime(3), time(3)
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