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Date: Sat, 30 Oct 2010 08:58:56 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin AT cygwin DOT com
Subject: Re: localtime and TZ
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On 10/30/2010 8:43 AM, Ken Brown wrote:
> On 10/29/2010 7:17 PM, Ken Brown wrote:
>> On 10/29/2010 6:16 PM, Eric Blake wrote:
>>> On 10/29/2010 04:11 PM, Ken Brown wrote:
>>>>
>>>> Thanks, Eric.  I didn't know about any of this.  (I was using a modification 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:
>>>>
>>>> #include<time.h>
>>>> #include<stdio.h>
>>>>
>>>> int
>>>> main (void)
>>>> {
>>>>      time_t now = time ((time_t *) 0);
>>>>      printf ("TZ is initially unset; hour = %d\n", localtime (&now)->tm_hour);
>>>>      putenv ("TZ=GMT0");
>>>>      printf ("TZ=GMT0; hour = %d\n", localtime (&now)->tm_hour);
>>>>      unsetenv("TZ");
>>>>      printf ("TZ unset; hour = %d\n", localtime (&now)->tm_hour);
>>>>      putenv ("TZ=PST8");
>>>>      printf ("TZ=PST8; hour = %d\n", localtime (&now)->tm_hour);
>>>>      unsetenv("TZ");
>>>>      printf ("TZ unset again; hour = %d\n", localtime (&now)->tm_hour);
>>>> }
>>>>
>>>> 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.
>>
>> Wouldn't you interpret this as meaning that the implementation-defined
>> default timezone information should be the same every time localtime is
>> called with TZ unset?  If not, what should a program do to get the
>> "standard" default timezone information that it would get if TZ had
>> never been set in the first place?
>
> I've looked at Cygwin's localtime.cc, and the behavior I'm complaining
> about is caused by the following code at the beginning of tzset:
>
> 	const char *	name = getenv("TZ");
>
> 	if (name == NULL) {
> 		if (!lcl_is_set)
> 			tzsetwall();
> 		goto out;
> 	}
>
> So getting rid of 'if (!lcl_is_set)' would solve the problem.  But this
> would be inefficient, because it would mean that tzsetwall gets called
> every time tzset is called if TZ is never set.  To get around that, one
> could have tzsetwall set TZ.
>
> It seems that tzset and tzsetwall used to behave the way I'm proposing
> before the following two changes were made:
>
> 2007-12-11  Corinna Vinschen<corinna AT vinschen DOT de>
>
> 	* localtime.cc (tzset): Call tzsetwall only if it hasn't been
> 	called before.
>
> 2007-08-01  Corinna Vinschen<corinna AT vinschen DOT de>
>
> 	* localtime.cc (tzsetwall): Don't set TZ.

I've just found the reason for the 2007-08-01 change:

   http://www.cygwin.com/ml/cygwin/2007-08/msg00041.html

So I don't know what should be done.

Ken

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