Manual Pages for UNIX Darwin command on man posix2time

TIME2POSIX(3) BSD Library Functions Manual TIME2POSIX(3)

NAME

ttiimmee22ppoossiixx, ppoossiixx22ttiimmee - convert seconds since the Epoch

LLIIBBRRAARRYY

Standard C Library (libc, -lc)

SYNOPSIS

##iinncclluuddee <>

timet ttiimmee22ppoossiixx(timet t); timet ppoossiixx22ttiimmee(timet t);

DESCRIPTION

IEEE Std 1003.1-1988 (``POSIX.1'') legislates that a timet value of

536457599 shall correspond to "Wed Dec 31 23:59:59 GMT 1986." This effectively implies that POSIX timet'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, how-

ever, no such adjustment is needed and timet 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 timet is intended to be

(mostly) opaque-timet values should only be obtained-from and passed-to

functions such as time(3), localtime(3), mktime(3) and difftime(3). How-

ever, IEEE Std 1003.1-1988 (``POSIX.1'') gives an arithmetic expression

for directly computing a timet value from a given date/time, and the same relationship is assumed by some (usually older) applications. Any

programs creating/dissecting timet's using such a relationship will typ-

ically not handle intervals over leap seconds correctly. The ttiimmee22ppoossiixx() and ppoossiixx22ttiimmee() functions are provided to address this timet mismatch by converting between local timet 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 ttiimmee22ppoossiixx() function is single-valued. That is, every local timet

corresponds to a single POSIX timet. The ppoossiixx22ttiimmee() 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 timet doesn't

exist so an adjacent value is returned. Both of these are good indica-

tors of the inferiority of the POSIX representation. The following table summarizes the relationship between timet and its

conversion to, and back from, the POSIX representation over the leap sec-

ond inserted at the end of June, 1993. DDAATTEE TTIIMMEE TT XX==ttiimmee22ppoossiixx((TT)) ppoossiixx22ttiimmee((XX)) 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... DDAATTEE TTIIMMEE TT XX==ttiimmee22ppoossiixx((TT)) ppoossiixx22ttiimmee((XX)) ??/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 timet's and POSIX timet's

are equivalent, and both ttiimmee22ppoossiixx() and ppoossiixx22ttiimmee() degenerate to the identity function.