$OpenBSD: Theory,v 1.3 1996/08/19 08:34:46 tholo Exp $ These time and date functions are much like the System V Release 2.0 (SVR2) time and date functions; there are a few additions and changes to extend the usefulness of the SVR2 functions: * In SVR2, time display in a process is controlled by the environment variable TZ, which "must be a three-letter time zone name, followed by a number representing the difference between local time and Greenwich Mean Time in hours, followed by an optional three-letter name for a daylight time zone;" when the optional daylight time zone is present, "standard U.S.A. Daylight Savings Time conversion is applied." This means that SVR2 can't deal with other (for example, Australian) daylight savings time rules, or situations where more than two time zone abbreviations are used in an area. * In SVR2, time conversion information is compiled into each program that does time conversion. This means that when time conversion rules change (as in the United States in 1987), all programs that do time conversion must be recompiled to ensure proper results. * In SVR2, time conversion fails for near-minimum or near-maximum time_t values when doing conversions for places that don't use GMT. * In SVR2, there's no tamper-proof way for a process to learn the system's best idea of local wall clock. (This is important for applications that an administrator wants used only at certain times-- without regard to whether the user has fiddled the "TZ" environment variable. While an administrator can "do everything in GMT" to get around the problem, doing so is inconvenient and precludes handling daylight savings time shifts--as might be required to limit phone calls to off-peak hours.) * These functions can account for leap seconds, thanks to Bradley White (bww@k.cs.cmu.edu). These are the changes that have been made to the SVR2 functions: * The "TZ" environment variable is used in generating the name of a file from which time zone information is read (or is interpreted a la POSIX); "TZ" is no longer constrained to be a three-letter time zone name followed by a number of hours and an optional three-letter daylight time zone name. The daylight saving time rules to be used for a particular time zone are encoded in the time zone file; the format of the file allows U.S., Australian, and other rules to be encoded, and allows for situations where more than two time zone abbreviations are used. It was recognized that allowing the "TZ" environment variable to take on values such as "America/New_York" might cause "old" programs (that expect "TZ" to have a certain form) to operate incorrectly; consideration was given to using some other environment variable (for example, "TIMEZONE") to hold the string used to generate the time zone information file name. In the end, however, it was decided to continue using "TZ": it is widely used for time zone purposes; separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance; and systems where "new" forms of "TZ" might cause problems can simply use TZ values such as "EST5EDT" which can be used both by "new" programs (a la POSIX) and "old" programs (as zone names and offsets). * To handle places where more than two time zone abbreviations are used, the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] (where "tmp" is the value the function returns) to the time zone abbreviation to be used. This differs from SVR2, where the elements of tzname are only changed as a result of calls to tzset. * Since the "TZ" environment variable can now be used to control time conversion, the "daylight" and "timezone" variables are no longer needed or supported. (You can use a compile-time option to cause these variables to be defined and to be set by "tzset"; however, their values will not be used by "localtime.") * The "localtime" function has been set up to deliver correct results for near-minimum or near-maximum time_t values. (A comment in the source code tells how to get compatibly wrong results). * A function "tzsetwall" has been added to arrange for the system's best approximation to local wall clock time to be delivered by subsequent calls to "localtime." Source code for portable applications that "must" run on local wall clock time should call "tzsetwall();" if such code is moved to "old" systems that don't provide tzsetwall, you won't be able to generate an executable program. (These time zone functions also arrange for local wall clock time to be used if tzset is called--directly or indirectly--and there's no "TZ" environment variable; portable applications should not, however, rely on this behavior since it's not the way SVR2 systems behave.) Points of interest to folks with Version 7 or BSD systems: * The BSD "timezone" function is not present in this package; it's impossible to reliably map timezone's arguments (a "minutes west of GMT" value and a "daylight saving time in effect" flag) to a time zone abbreviation, and we refuse to guess. Programs that in the past used the timezone function may now examine tzname[localtime(&clock)->tm_isdst] to learn the correct time zone abbreviation to use. Alternatively, use localtime(&clock)->tm_zone if this has been enabled. * The BSD gettimeofday function is not used in this package; this lets users control the time zone used in doing time conversions. Users who don't try to control things (that is, users who do not set the environment variable TZ) get the time conversion specified in the file "/etc/zoneinfo/localtime"; see the time zone compiler writeup for information on how to initialize this file. The functions that are conditionally compiled if STD_INSPIRED is defined should, at this point, be looked on primarily as food for thought. They are not in any sense "standard compatible"--some are not, in fact, specified in *any* standard. They do, however, represent responses of various authors to standardization proposals. Other time conversion proposals, in particular the one developed by folks at Hewlett Packard, offer a wider selection of functions that provide capabilities beyond those provided here. The absence of such functions from this package is not meant to discourage the development, standardization, or use of such functions. Rather, their absence reflects the decision to make this package close to SVR2 (with the exceptions outlined above) to ensure its broad acceptability. If more powerful time conversion functions can be standardized, so much the better.