On Sat, May 27, 2017 at 11:46 PM, Jeff King <p...@peff.net> wrote: > On Sat, May 27, 2017 at 06:57:08PM +0200, Ævar Arnfjörð Bjarmason wrote: > >> There's another test which breaks if we just s/gmtime/localtime/g. As >> far as I can tell to make the non-local case work we'd need to do a >> whole dance where we set the TZ variable to e.g. UTC$offset, then call >> strftime(), then call it again. Maybe there's some way to just specify >> the tz offset, but I didn't find any in a quick skimming of time.h. > > There isn't. At least on _some_ platforms, the zone information is > embedded in "struct tm" and stored by gmtime() and localtime(), but the > fields aren't publicly accessible. Which is why your patch worked for > format-local (it swaps out gmtime() for localtime() which sets those > fields behind the scenes). But: > > - I'm not sure that's guaranteed by the standard; strftime() might get > its zone information elsewhere (if it needs to reliably distinguish > between gmtime() and localtime() results it has to at least set a > bit in the "struct tm", but that bit may not be the full zone info). > > - Even if it does work, you're stuck with only the local timezone. In > theory you could temporarily tweak the process's timezone, call > localtime(), and then tweak it back. I was never able to get that to > work (links below). > > On glibc, at least, you can access the zone fields in "struct tm" by > compiling with _DEFAULT_SOURCE. > > So I think the best we could do is probably to have a feature macro like > TM_HAS_GMTOFF, and set tm->tm_gmtoff and tm->tm_zone on platforms that > support it. I'm not sure what we'd put in the latter, though; we don't > actually have the timezone name at all (we just have "+0200" or whatever > we parsed from the git object, but that would be better than nothing). > > That leaves other platforms still broken, but like I said, I don't think > there's a portable solution. > > Here are some links to past explorations: > > http://public-inbox.org/git/20160208152858.ga17...@sigill.intra.peff.net/ > > http://public-inbox.org/git/87vb2d37ea....@web.de/
There's a third and possibly least shitty option that isn't covered in those threads; We could just make a pass over the strftime format ourselves and replace %z and %Z with the timezone (as done for DATE_ISO8601_STRICT in date.c), then hand the rest off to strftime(). I'm not going to pursue it though, Ulrich, are you maybe interested in hacking on that?
