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 <cori...@vinschen.de>
* localtime.cc (tzset): Call tzsetwall only if it hasn't been
called before.
2007-08-01 Corinna Vinschen <cori...@vinschen.de>
* localtime.cc (tzsetwall): Don't set TZ.
Ken
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