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)