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@(#)Theory 7.4
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.
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