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Re: localtime and TZ

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 localtime 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 information, just as if TZ had never been set.  Here's a Simple Test Case:
> #include <time.h>
> #include <stdio.h>
> extern char **environ;
> void
> unset_TZ (void)
> {
>   char **from, **to;
>   for (to = from = environ; (*to = *from); from++)
>     if (! (to[0][0] == 'T' && to[0][1] == 'Z' && to[0][2] == '='))
>       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.

> 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);
>   unset_TZ ();
>   printf ("TZ unset; hour = %d\n", localtime (&now)->tm_hour);
>   putenv ("TZ=PST8");
>   printf ("TZ=PST8; hour = %d\n", localtime (&now)->tm_hour);
>   unset_TZ ();
>   printf ("TZ unset again; hour = %d\n", localtime (&now)->tm_hour);
> }

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

Eric Blake    +1-801-349-2682
Libvirt virtualization library

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