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Date: Mon, 25 Mar 2024 09:08:03 -0400
From: Rich Felker <dalias@...c.org>
To: Alexander Weps <exander77@...me>
Cc: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
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@...c.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
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