Re: [musl] Time zone has not updated after call tzset()

[Rich Felker <dalias@libc.org>]

On Sat, Feb 11, 2023 at 03:30:58PM +0100, alice wrote:
> On Sat Feb 11, 2023 at 3:20 PM CET, Rich Felker wrote:
> > On Sat, Feb 11, 2023 at 03:10:40PM +0100, alice wrote:
> > > On Sat Feb 11, 2023 at 7:53 AM CET, zhoujingqiang (A) wrote:
> > > > Hello,
> > > >
> > > > Normally, /etc/localtime is a soft link to a file that stores time zone
> > > > information.
> > > >
> > > > Without setting the TZ environment variable, I change the time zone by
> > > > changing the file linked to /etc/localtime. After calling tzset(), I find that
> > > > the time zone does not change. The test code is as follows:
> > > 
> > > musl does not support changing the timezone while running.
> > > see https://www.openwall.com/lists/musl/2017/06/09/9 ,
> > > for a response to an identical bug report
> > > 
> > > tl;dr without semantics: you have to restart a running process to get a new
> > > timezone.
> >
> > This is not quite accurate. It does, but only via application intent,
> > in the form of changing its value of TZ. It does not re-scan files if
> > the application doesn't do that, for two important reasons:
> 
> yes, with the semantics, something like:
> 
>  setenv("TZ", "something", 0);
>  tzset();
>  unsetenv("TZ");
> 
> will update to a new /etc/localtime symlink. it's mentioned in
> https://marc.info/?l=musl&m=141374003126007&w=2
> from the above thread.
> 
> (apologies, i worded that poorly; what i mean is you cannot rely on /
> etc/localtime being read on update (without the hack above), which was the
> question. setenv("TZ") is also supported, of course.)

One small note: if you unsetenv afterwards like that, you'll never get
to use the newly selected zone since the next tz-using operation will
implicitly perform another tzset (as required by POSIX).

Rich