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Date: Mon, 25 Mar 2024 13:48:00 +0000
From: Alexander Weps <exander77@...me>
To: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
For a change you requested 31 - 1:
tm.tm_mday = 31 - 1;
Complete code to run:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
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 = 31 - 1; // <-- here is the change
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\n", buf, t);
t = mktime(&tm);
strftime(buf, sizeof buf, "%F %T %Z", &tm);
printf("after1: %s %ld\n", buf, t);
tm.tm_mday -= 1;
t = mktime(&tm);
strftime(buf, sizeof buf, "%F %T %Z", &tm);
printf("after2: %s %ld\n", buf, t);
tm.tm_mday += 1;
t = mktime(&tm);
strftime(buf, sizeof buf, "%F %T %Z", &tm);
printf("after3: %s %ld\n", buf, t);
}
int main(int argc, char *argv[])
{
test10();
return 0;
}
$ musl-gcc foo.c -o foo && TZ=Pacific/Apia ./foo
before: 2011-12-30 00:00:00 0
after1: 2011-12-29 01:00:00 -10 1325156400
after2: 2011-12-28 01:00:00 -10 1325070000
after3: 2011-12-29 01:00:00 -10 1325156400
$ gcc foo.c -o foo && TZ=Pacific/Apia ./foo
before: 2011-12-30 00:00:00 +13 0
after1: 2011-12-31 00:00:00 +14 1325239200
after2: 2011-12-30 00:00:00 +14 -1
after3: 2011-12-31 00:00:00 +14 1325239200
So this is a also a bug in struct tm interpretation.
Behavior is consistent with tm_mday = 31;
AW
On Monday, March 25th, 2024 at 14:42, Rich Felker <dalias@...c.org> wrote:
> On Mon, Mar 25, 2024 at 01:24:57PM +0000, Alexander Weps wrote:
>
> > See below.
> >
> > AW
> >
> > On Monday, March 25th, 2024 at 14:13, Rich Felker dalias@...c.org wrote:
> >
> > > On Mon, Mar 25, 2024 at 12:55:28PM +0000, Alexander Weps wrote:
> > >
> > > > > If you take your test program and switch it to initialize with
> > > > > tm_mday=31, then do -=1 instead of +=1, you'll find that it gives
> > > > > 2011-12-29 01:00:00 -10 as well, only now it seems like the correct,
> > > > > expected thing to happen. Any change to "fix" the case you're
> > > > > complaining about would necessarily break this case.
> > > >
> > > > So (- day, +day):
> > > >
> > > > Musl:
> > > > 2011-12-31 01:00:00 +14
> > > > 2011-12-29 01:00:00 -10
> > > > 2011-12-29 01:00:00 -10
> > > >
> > > > Glibc:
> > > > 2012-01-01 01:00:00 +14
> > > > 2011-12-31 01:00:00 +14
> > > > 2012-01-01 01:00:00 +14
> > > >
> > > > Seems like musl doesn't even interpret the initial struct tm
> > > > correctly in that case. It is off by day.
> > > >
> > > > Because December only had 30 days, 31s day after normalization is
> > > > January 1st.
> > >
> > > This is nonsense. December has a day 31, which you can clearly see
> > > from the glibc output. For this particular year in this zone, with the
> > > zone rule change, there are "only 30 days" in December, but they are
> > > numbered 1-29 and 31, not 1-30.
> >
> > You confuse day of month which is represented in tm_mday with
> > calendar day that is interpreted by strftime.
> >
> > You said to set tm_mday = 31, which would be January 1st after normalization.
> > December 31s is 30th day of month represented as tm_mday = 30.
>
>
> OK, I meant tm_mday=31-1.
>
> > > What did you do that got glibc to output 2012-01-01? I guess you wrote
> > > code to do some wacky arithmetic after the original code you already
> > > had, rather than changing the code to start with 2011-12-31 as I
> > > suggested to get a look at what's happening.
> > >
> > > > > In any case, the core issue you're hitting here is that time zones are
> > > > > HARD to work with and that there is inherent complexity that libc
> > > > > cannot save you from. You only got lucky that what you were trying to
> > > > > do "worked" with glibc because you were iterating days forward; if you
> > > > > were doing reverse, it would break exactly the same way.
> > > >
> > > > I am not really commenting on this, until you sort out the above
> > > > inconsistencies.
> > >
> > > I already have but you refuse to look.
> >
> > It was addressed, do didn't scroll at the end of the e-mail.
>
>
> Run the attached passing the starting date to check as the first/only
> argument, and these test dates:
>
> - "2011-12-29 00:00:00"
> - "2011-12-31 00:00:00"
>
> Hopefully that will clarify things for you. On musl you will see:
>
> normalized input: 2011-12-29 00:00:00 -10
> +1day per mktime: 2011-12-29 00:00:00 -10
> +1day via time_t: 2011-12-31 00:00:00 +14
> -1day per mktime: 2011-12-28 00:00:00 -10
> -1day via time_t: 2011-12-28 00:00:00 -10
>
> normalized input: 2011-12-31 00:00:00 +14
> +1day per mktime: 2012-01-01 00:00:00 +14
> +1day via time_t: 2012-01-01 00:00:00 +14
> -1day per mktime: 2011-12-29 00:00:00 -10
> -1day via time_t: 2011-12-29 00:00:00 -10
>
> You can see what you get on glibc.
>
> Rich
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