[tz] offtime_r

[Paul Eggert]

On 2023-04-26 02:18, Dag-Erling Smørgrav wrote:

> - although the same effect can be achieved with localtime_rz(), it
>    requires quite a bit of setup.

Although localtime_rz requires setup, I'm not sure I'd call it "quite a 
bit" of setup. Using localtime_rz:

   timezone_t tz = tzalloc("MST7");
   localtime_rz(tz, &timestamp, &tmstruct);

is not that much more complicated than using offtime_r would be:

   offtime_r(&timestamp, -7 * 60 * 60, &tmstruct);

although admittedly the localtime_rz case should eventually call tzfree.

Plus, offtime_r would share offtime's error-proneness with 
abbreviations, as it would set tm.tm_zone to a wild abbreviation whereas 
localtime_rz sets it to the more-sensible "MST".


> - offtime() is behind STD_INSPIRED, while localtime_rz() is behind
>    NETBSD_INSPIRED, so it might not necessarily be available.  FreeBSD
>    for instance builds tzcode with STD_INSPIRED but not NETBSD_INSPIRED.

How about if we instead change FreeBSD to define NETBSD_INSPIRED? That 
would help portability between FreeBSD and NetBSD, and would speed up 
applications that deal with multiple timezones. As Eric S. Raymond 
writes in 
<http://www.catb.org/esr/time-programming/#_berkeley_multiple_timezone_api>, 
"This is how it should have been done in the first place...".