time2posix.3   [plain text]

.\" $FreeBSD: src/lib/libc/stdtime/time2posix.3,v 1.13 2001/10/01 16:08:59 ru Exp $
.Dd May 1, 1996
.Nm time2posix ,
.Nm posix2time
.Nd convert seconds since the Epoch
.Lb libc
.In time.h
.Ft time_t
.Fn time2posix "time_t t"
.Ft time_t
.Fn posix2time "time_t t"
.St -p1003.1-88
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,
that the system time must be adjusted as each leap occurs.
If the time package is configured with leap-second support
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
opaque\(emtime_t values should only be obtained-from and
passed-to functions such as
.Xr time 3 ,
.Xr localtime 3 ,
.Xr mktime 3
.Xr difftime 3 .
.St -p1003.1-88
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)
Any programs creating/dissecting time_t's
using such a relationship will typically not handle intervals
over leap seconds correctly.
.Fn time2posix
.Fn 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
or when communicating with POSIX-compliant systems.
.Fn time2posix
function is single-valued.
That is,
every local time_t
corresponds to a single POSIX time_t.
.Fn 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,
.Bl -column "93/06/30" "23:59:59" "A+0" "X=time2posix(T)"
.It Sy "DATE	TIME	T	X=time2posix(T)	posix2time(X)"
.It "93/06/30	23:59:59	A+0	B+0	A+0"
.It "93/06/30	23:59:60	A+1	B+1	A+1 or A+2"
.It "93/07/01	00:00:00	A+2	B+1	A+1 or A+2"
.It "93/07/01	00:00:01	A+3	B+2	A+3"
A leap second deletion would look like...
.Bl -column "??/06/30" "23:59:58" "A+0" "X=time2posix(T)"
.It Sy "DATE	TIME	T	X=time2posix(T)	posix2time(X)"
.It "??/06/30	23:59:58	A+0	B+0	A+0"
.It "??/07/01	00:00:00	A+1	B+2	A+1"
.It "??/07/01	00:00:01	A+2	B+3	A+2"
.D1 No "[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
.Fn time2posix
.Fn posix2time
degenerate to the identity function.
.Xr difftime 3 ,
.Xr localtime 3 ,
.Xr mktime 3 ,
.Xr time 3