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diff --git a/time2posix.3.txt b/time2posix.3.txt new file mode 100644 index 000000000000..d9db97010ebe --- /dev/null +++ b/time2posix.3.txt @@ -0,0 +1,76 @@ +TIME2POSIX(3) Library Functions Manual TIME2POSIX(3) + +NAME + time2posix, posix2time - convert seconds since the Epoch + +SYNOPSIS + #include <time.h> + + time_t time2posix(time_t t); + + time_t posix2time(time_t t); + + cc ... -ltz + +DESCRIPTION + IEEE Standard 1003.1 (POSIX) requires the time_t value 536457599 to + stand for 1986-12-31 23:59:59 UTC. This effectively implies that POSIX + time_t values 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 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 + (mostly) opaque - time_t values should only be obtained-from and + passed-to functions such as time(2), localtime(3), mktime(3), and + difftime(3). However, POSIX 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) applications. Any + programs creating/dissecting time_t's using such a relationship 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 + communicating with POSIX-compliant systems. + + Time2posix is single-valued. That is, every local time_t corresponds + to a single POSIX time_t. Posix2time 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 a time T and + it's 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 degenerate + to the identity function. + +SEE ALSO + difftime(3), localtime(3), mktime(3), time(2) + + TIME2POSIX(3) |