Message-ID: <4CCB42AF.2090907@redhat.com>
Date: Fri, 29 Oct 2010 15:54:55 -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 03:44 PM, Ken Brown wrote:
> While trying to debug a timezone problem in the Cygwin build of emacs, I'=
ve come across a difference between Cygwin and Linux in the behavior of loc=
altime with respect to TZ.  Suppose I set TZ, call localtime, unset TZ, and=
 call localtime again.  On Cygwin, the second call to localtime re-uses the=
 previous value of TZ.  On Linux, localtime reverts to giving local informa=
tion, just as if TZ had never been set.  Here's a Simple Test Case:
>=20
> #include <time.h>
> #include <stdio.h>
>=20
> extern char **environ;
>=20
> void
> unset_TZ (void)
> {
>   char **from, **to;
>   for (to =3D from =3D environ; (*to =3D *from); from++)
>     if (! (to[0][0] =3D=3D 'T' && to[0][1] =3D=3D 'Z' && to[0][2] =3D=3D =
'=3D'))
>       to++;
> }

Messing directly with environ is your problem.  POSIX says that it is
only portable to traverse (but not modify) environ's contents, or to
completely assign a new array to environ.  By going behind cygwin's
back, and not using unsetenv(), you have violated POSIX and can't expect
sane results.

>=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);
>   unset_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);
>   unset_TZ ();
>   printf ("TZ unset again; hour =3D %d\n", localtime (&now)->tm_hour);
> }

If properly using unsetenv("TZ") still causes problems, then we should
investigate further.

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


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