From be62d5918223b4df209cc941633b8c5b80cc0d20 Mon Sep 17 00:00:00 2001 From: Paul Eggert Date: Mon, 28 Oct 2024 09:17:10 -0700 Subject: [PATCH] Talk a bit more about tm_isdst's obsolescence MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This is in response to a question by Florian Weimer in: https://lists.iana.org/hyperkitty/list/tz@iana.org/message/NACFDAG2G4PZEL5LKDYOOZL6U4B7WPP3/ * theory.html (POSIX features no longer needed): Update tm_isdst discussion. (Other portability notes): Don’t suggest tzname. --- theory.html | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/theory.html b/theory.html index d3573ede..b1717e7d 100644 --- a/theory.html +++ b/theory.html @@ -1201,12 +1201,15 @@ The vestigial APIs are: The tm_isdst member is almost never needed and most of its uses should be discouraged in favor of the abovementioned APIs. + It was intended as an index into the tzname variable, + but as mentioned previously that usage is obsolete. Although it can still be used in arguments to mktime to disambiguate timestamps near a DST transition when the clock jumps back on platforms lacking tm_gmtoff, this - disambiguation does not work when standard time itself jumps back, - which can occur when a location changes to a time zone with a + disambiguation works only for proleptic TZ strings; + it does not work in general for geographical timezones, + such as when a location changes to a time zone with a lesser UT offset. @@ -1223,8 +1226,8 @@ The vestigial APIs are: Programs that in the past used the timezone function may now examine localtime(&clock)->tm_zone (if TM_ZONE is defined) or - tzname[localtime(&clock)->tm_isdst] - (if HAVE_TZNAME is nonzero) to learn the correct time + use strftime with a %Z conversion specification + to learn the correct time zone abbreviation to use.
  • -- 2.43.0