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1 time2posix(3) Library Functions Manual time2posix(3) 2 3 NAME 4 time2posix, posix2time - convert seconds since the Epoch 5 6 SYNOPSIS 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 15 DESCRIPTION 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 values using such a relationship 34 will 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 its 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 and POSIX time_t 70 values are equivalent, and both time2posix and posix2time degenerate to 71 the identity function. 72 73 SEE ALSO 74 difftime(3), localtime(3), mktime(3), time(2) 75 76 time2posix(3)