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time2posix(3) Library Functions Manual time2posix(3)
NAME
time2posix, posix2time - convert seconds since the Epoch
SYNOPSIS
#include <time.h>
time_t time2posix(time_t t);
time_t posix2time(time_t t);
cc ... -ltz
DESCRIPTION
IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to
stand for 1986-12-31 23:59:59 UTC. 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(2), 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 values 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 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 and POSIX time_t
values are equivalent, and both time2posix and posix2time degenerate to
the identity function.
SEE ALSO
difftime(3), localtime(3), mktime(3), time(2)
Time Zone Database time2posix(3)
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