Re: [Qemu-devel] [PATCH] qemu_timedate_diff() shouldn't modify its argument.

[Ronen Hod <rhod@redhat.com>]

On 11/06/2011 06:00 PM, Gleb Natapov wrote:
> The caller of qemu_timedate_diff() does not expect that tm it passes to
> the function will be modified, but mktime() is destructive and modifies
> its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
> will not rely on it since its value may be outdated.

I believe that the original issue was not related to outdated data at 
the moment of the daylight saving time transition.
using tmp.tm_isdst = -1 sounds good, but why use a copy of tm? The only 
significant field that will change in the tm is the tm_isdst itself that 
will be set to 0/1 (correctly).

Acked-by: Ronen Hod <rhod@redhat.com>

> Signed-off-by: Gleb Natapov<gleb@redhat.com>
> diff --git a/vl.c b/vl.c
> index 624da0f..641629b 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -460,8 +460,11 @@ int qemu_timedate_diff(struct tm *tm)
>       if (rtc_date_offset == -1)
>           if (rtc_utc)
>               seconds = mktimegm(tm);
> -        else
> -            seconds = mktime(tm);
> +        else {
> +            struct tm tmp = *tm;
> +            tmp.tm_isdst = -1; /* use timezone to figure it out */
> +            seconds = mktime(&tmp);
> +	}
>       else
>           seconds = mktimegm(tm) + rtc_date_offset;
>
> --
> 			Gleb.
>