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Date: Tue, 7 Aug 2018 22:14:58 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] strftime: fix %z sign for small negative time
 zone offsets

On Thu, Aug 02, 2018 at 07:29:23PM -0400, Rich Felker wrote:
> On Wed, Jul 25, 2018 at 12:11:42PM +0200, Rafał Miłecki wrote:
> > From: Rafał Miłecki <rafal@...ecki.pl>
> > 
> > Using + printf flag for printing plus/minus sign isn't reliable for
> > negative offsets greater than -3600. In such cases the first two digits
> > are zero but offset still should be printed with a leading minus char.
> > 
> > Existing implementation results in code:
> > 	struct tm tm = { .tm_gmtoff = -1800, };
> > 	char buf[255];
> > 	strftime(buf, sizeof(buf), "%z", &tm);
> > 	puts(buf);
> > printing +0030 instead of -0030.
> > 
> > This patch fixes it by handling sign character manually and checking the
> > __tm_gmtoff value (instead of __tm_gmtoff / 3600).
> > 
> > Signed-off-by: Rafał Miłecki <rafal@...ecki.pl>
> > ---
> >  src/time/strftime.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/time/strftime.c b/src/time/strftime.c
> > index 0a256970..6716ad4b 100644
> > --- a/src/time/strftime.c
> > +++ b/src/time/strftime.c
> > @@ -181,7 +181,8 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
> >  			*l = 0;
> >  			return "";
> >  		}
> > -		*l = snprintf(*s, sizeof *s, "%+.2ld%.2d",
> > +		*l = snprintf(*s, sizeof *s, "%c%.2ld%.2d",
> > +			tm->__tm_gmtoff >= 0 ? '+' : '-',
> >  			(tm->__tm_gmtoff)/3600,
> >  			abs(tm->__tm_gmtoff%3600)/60);
> >  		return *s;
> > -- 
> 
> I think this is incorrect. tm->__tm_gmtoff/3600 can still be negative,
> and then the - sign is printed twice.
> 
> I suspect your patch is correct if we add abs() around that expression
> too, but maybe it makes more sense to assemble the value to be printed
> as a single integer:
> 
> 	tm->__tm_gmtoff/3600*100 + tm->__tm_gmtoff%3600/60
> 
> I didn't spend a lot of time working out that this is correct, but it
> looks right and checks out with a few test cases.

I've thrown various values at it and it seems to work fine, so I'll
commit this version.

Rich

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