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Date: Fri, 29 Oct 2010 16:16:06 -0600
From: Eric Blake <eblake AT redhat DOT com>
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To: cygwin AT cygwin DOT com
Subject: Re: localtime and TZ
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On 10/29/2010 04:11 PM, Ken Brown wrote:
>=20
> Thanks, Eric.  I didn't know about any of this.  (I was using a modificat=
ion of a configure test from the emacs sources.)

Probably worth pointing it out to the emacs upstream, then :)

>  But I get the same behavior with the following revised test case:
>=20
> #include <time.h>
> #include <stdio.h>
>=20
> int
> main (void)
> {
>   time_t now =3D time ((time_t *) 0);
>   printf ("TZ is initially unset; hour =3D %d\n", localtime (&now)->tm_ho=
ur);
>   putenv ("TZ=3DGMT0");
>   printf ("TZ=3DGMT0; hour =3D %d\n", localtime (&now)->tm_hour);
>   unsetenv("TZ");
>   printf ("TZ unset; hour =3D %d\n", localtime (&now)->tm_hour);
>   putenv ("TZ=3DPST8");
>   printf ("TZ=3DPST8; hour =3D %d\n", localtime (&now)->tm_hour);
>   unsetenv("TZ");
>   printf ("TZ unset again; hour =3D %d\n", localtime (&now)->tm_hour);
> }
>=20
> So the question remains whether this difference between Cygwin and Linux =
is a bug or by design.

Apparently by design.  POSIX requires:

http://www.opengroup.org/onlinepubs/9699919799/functions/localtime.html

Local timezone information is used as though localtime() calls tzset().

http://www.opengroup.org/onlinepubs/9699919799/functions/tzset.html

The tzset() function shall use the value of the environment variable TZ
to set time conversion information used by ctime , localtime , mktime ,
and strftime . If TZ is absent from the environment,
implementation-defined default timezone information shall be used.

But not the best of design, so a patch to fix cygwin to copy Linux'
behavior is probably appreciated.

--=20
Eric Blake   eblake AT redhat DOT com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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