time2posix(3)

NAME

time2posix, posix2time - convert seconds since the Epoch

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <time.h>
time_t
time2posix(const time_t *t);
time_t
posix2time(const time_t *t);

DESCRIPTION

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 contin

ue 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 in

tended 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 diff

time(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) appli

cations. Any
programs creating/dissecting time_t's using such a relation

ship 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 communi

cating 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 does
not 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() de

generate to the
identity function.

SEE ALSO

difftime(3), localtime(3), mktime(3), time(3)

BSD May 1, 1996

BSD