[PATCH 2/2 v2] compat/timegm: new compat function for systems lacking timegm()

[Jason at zx2c4.com (Jason A. Donenfeld)]

On Thu, Apr 4, 2013 at 4:46 PM, John Keeping <john at keeping.me.uk> wrote:
>
> I decided not to do that based on this message:
>
>
> http://lists.samba.org/archive/samba-technical/2002-November/025578.html
>
> but of course it doesn't provide any actual reason for why that version
> doesn't work.
>

Aye yie yie. Time handling functions are a mess. I just went looking at how
glibc [1] and uclibc [2] actually go about doing things, and it's a real
mess in there. Looks like the replacement function in the patch might not
be completely correct.

[1] http://git.uclibc.org/uClibc/tree/libc/misc/time/time.c#n2330
[2]
http://sourceware.org/git/?p=glibc.git;a=blob;f=time/mktime.c;h=e75132c2e457af4b5629eef302c0d6dac15fef5c;hb=HEAD#l348
(uclibc gets bonus points for using cgit)




>
> We only use timegm in week calculations, where we do:
>
>     time_t t = timegm(tm);
>     t = <new value>;
>     gmtime_r(&t, tm);
>
> so I wonder whether it would be equivalent to use mktime/localtime_r
> instead of timegm/gmtime_r.
>


gmtime(timegm(tm)) == tm

and

localtime(mktime(tm)) == tm


So I can't imagine there'd be a problem with just replacing those functions.