Date: Thu, 31 Aug 2017 17:56:53 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: strftime %Z returning empty string On Thu, Aug 31, 2017 at 05:12:23PM -0400, jacob@...shcomputing.com wrote: > On Thu, 31 Aug 2017, A. Wilcox wrote: > > >%Z is a glibc extension. I don't believe musl supports it. > > http://pubs.opengroup.org/onlinepubs/9699919799/functions/strftime.html > > Z Replaced by the timezone name or abbreviation, or by no bytes > if no timezone information exists. [tm_isdst] > > That "if no timezone information exists" is rather useless in my > eyes. Exists where? Later text suggests that the TZ environment > variable is relevant. In order for %Z to reliably be able to print something meaningful, the struct tm must contain the extension field tm_zone reflecting the zone to which it applies. Otherwise, there are at least two sources of ambiguity: there's no indication whether the struct tm contains UTC or local time, and in the latter case, tm_isdst is insufficient to disambiguate a zone name, since the TZ (for zoneinfo style TZ) may contain multiple different historical zones that apply to the territory rather than just a single daylight and single normal zone name. The latter ambiguity could probably be dealt with by trying to interpret the struct tm fields to obtain an absolute time, then determine according to the current TZ what zone name was in effect at that time. But this doesn't disambiguate UTC vs local time. If further consideration of the issue by standards committees (mainly Austin Group) determines that there is some behavior that should be followed to resolve the ambiguity, even if it's not a very nice behavior, I'll see what we can do in musl to conform to the standard. But right now it's underspecified and there's really no way to get meaningful information from a zero-filled struct tm. Rich
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