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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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