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++;
}
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);
}
I run the program with TZ unset. The output on Cygwin is
TZ is initially unset; hour = 17
TZ=GMT0; hour = 21
TZ unset; hour = 21
TZ=PST8; hour = 13
TZ unset again; hour = 13
The output on Linux is
TZ is initially unset; hour = 17
TZ=GMT0; hour = 21
TZ unset; hour = 17
TZ=PST8; hour = 13
TZ unset again; hour = 17
Is this a bug in Cygwin, or is the difference with Linux by design?
Ken
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