Re: [PATCH] strftime: fix %z sign for small negative time zone offsets

[Rich Felker <dalias@libc.org>]

On Wed, Jul 25, 2018 at 12:11:42PM +0200, Rafał Miłecki wrote:
> From: Rafał Miłecki <rafal@milecki.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@milecki.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.

Rich