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1TIME2POSIX(3)              Library Functions Manual              TIME2POSIX(3)
2
3NAME
4       time2posix, posix2time - convert seconds since the Epoch
5
6SYNOPSIS
7       #include <time.h>
8
9       time_t time2posix(time_t t);
10
11       time_t posix2time(time_t t);
12
13       cc ... -ltz
14
15DESCRIPTION
16       IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to
17       stand for 1986-12-31 23:59:59 UTC.  This effectively implies that POSIX
18       time_t values cannot include leap seconds and, therefore, that the
19       system time must be adjusted as each leap occurs.
20
21       If the time package is configured with leap-second support enabled,
22       however, no such adjustment is needed and time_t values continue to
23       increase over leap events (as a true "seconds since..."  value).  This
24       means that these values will differ from those required by POSIX by the
25       net number of leap seconds inserted since the Epoch.
26
27       Typically this is not a problem as the type time_t is intended to be
28       (mostly) opaque - time_t values should only be obtained-from and
29       passed-to functions such as time(2), localtime(3), mktime(3), and
30       difftime(3).  However, POSIX gives an arithmetic expression for
31       directly computing a time_t value from a given date/time, and the same
32       relationship is assumed by some (usually older) applications.  Any
33       programs creating/dissecting time_t's using such a relationship will
34       typically not handle intervals over leap seconds correctly.
35
36       The time2posix and posix2time functions are provided to address this
37       time_t mismatch by converting between local time_t values and their
38       POSIX equivalents.  This is done by accounting for the number of time-
39       base changes that would have taken place on a POSIX system as leap
40       seconds were inserted or deleted.  These converted values can then be
41       used in lieu of correcting the older applications, or when
42       communicating with POSIX-compliant systems.
43
44       The time2posix function is single-valued.  That is, every local time_t
45       corresponds to a single POSIX time_t.  The posix2time function is less
46       well-behaved: for a positive leap second hit the result is not unique,
47       and for a negative leap second hit the corresponding POSIX time_t
48       doesn't exist so an adjacent value is returned.  Both of these are good
49       indicators of the inferiority of the POSIX representation.
50
51       The following table summarizes the relationship between a time T and
52       it's conversion to, and back from, the POSIX representation over the
53       leap second inserted at the end of June, 1993.
54       DATE     TIME     T   X=time2posix(T) posix2time(X)
55       93/06/30 23:59:59 A+0 B+0             A+0
56       93/06/30 23:59:60 A+1 B+1             A+1 or A+2
57       93/07/01 00:00:00 A+2 B+1             A+1 or A+2
58       93/07/01 00:00:01 A+3 B+2             A+3
59
60       A leap second deletion would look like...
61
62       DATE     TIME     T   X=time2posix(T) posix2time(X)
63       ??/06/30 23:59:58 A+0 B+0             A+0
64       ??/07/01 00:00:00 A+1 B+2             A+1
65       ??/07/01 00:00:01 A+2 B+3             A+2
66
67                            [Note: posix2time(B+1) => A+0 or A+1]
68
69       If leap-second support is not enabled, local time_t's and POSIX
70       time_t's are equivalent, and both time2posix and posix2time degenerate
71       to the identity function.
72
73SEE ALSO
74       difftime(3), localtime(3), mktime(3), time(2)
75
76                                                                 TIME2POSIX(3)
77