Re: [musl] Broken mktime calculations when crossing DST boundary

[Rich Felker <dalias@libc.org>]

On Mon, Mar 25, 2024 at 12:55:28PM +0000, Alexander Weps wrote:
> See below.
> 
> AW
> 
> On Monday, March 25th, 2024 at 13:21, Rich Felker <dalias@libc.org> wrote:
> 
> > On Mon, Mar 25, 2024 at 11:52:00AM +0000, Alexander Weps wrote:
> >
> > > This is the simplest and most obvious example how broken the
> > > calculation in musl is:
> > >
> > > void test10()
> > > {
> > > time_t t = 0;
> > > struct tm tm = {0};
> > > char buf[64];
> > >
> > > tm.tm_year = 2011 - 1900;
> > > tm.tm_mon = 12 - 1;
> > > tm.tm_mday = 29;
> > > tm.tm_hour = 0;
> > > tm.tm_min = 0;
> > > tm.tm_sec = 0;
> > > tm.tm_isdst = 0;
> > >
> > > strftime(buf, sizeof buf, "%F %T %Z", &tm);
> > > printf("before: %s %ld %ld\n", buf, t, calc(&tm));
> > >
> > > t = mktime(&tm);
> > >
> > > strftime(buf, sizeof buf, "%F %T %Z", &tm);
> > > printf("after1: %s %ld %ld\n", buf, t, calc(&tm));
> > >
> > > tm.tm_mday += 1;
> > > t = mktime(&tm);
> > >
> > > strftime(buf, sizeof buf, "%F %T %Z", &tm);
> > > printf("after2: %s %ld %ld\n", buf, t, calc(&tm));
> > > }
> > >
> > > TZ=Pacific/Apia
> > > Year is greater than 1970.
> > >
> > > Input:
> > > 2011-12-29 01:00:00 -10
> > >
> > > Add a day:
> > > tm.tm_mday += 1;
> > > t = mktime(&tm);
> > >
> > > Output:
> > > 2011-12-29 01:00:00 -10
> > >
> > > Musl cannot reliably increment date by a day. Incrementing struct tm
> > > representing 2011-12-29 01:00:00 -10 by one day leads to the same
> > > date.
> > >
> > > Causing a program to loop or stack overflow.
> >
> >
> > I thought you had found a real bug here, and spent some time working
> > out the math by hand on paper because local time is so headbangingly
> > awful and confusing. In the end, the conclusion I'm left with is that
> > it's working just as expected.
> 
> It isn't.
> 
> Output from musl:
> 
> 2011-12-29 01:00:00 -10
> 
>     tm.tm_mday += 1;
>     t = mktime(&tm);
> 
> 2011-12-29 01:00:00 -10 <-- date is the same after incrementing
> 
>     tm.tm_mday -= 1;
>     t = mktime(&tm);

Read my mail again. I'm talking about starting from 2011-12-31 and
decrementing.

Rich