How to signal a time zone change?

[Peter Steele]

We have a suite of applications with a Java GUI controlling everything.
One of the actions the user can perform is to set the time zone. We do
this through our Java application and update the /etc/localtime as
required. We also make an API call to tell the JVM that the time zone as
changed, and from the perspective of the Java app, the time zone is
changed correctly (the timestamps for example in our log files reflect
the change). Likewise, after the user performs this action, running
"date" on one of our systems shows that the time zone has been changed
as requested. 

 

The problem is with our C applications. They continue to operate with
the old time zone, so things like timestamps in log files are not in
sync with the timestamps in the Java app log files. If we stop and
restart the C apps they pick up the time zone change. However, we don't
want to take this extreme approach. We want the Java app to signal to
the C applications that the time zone has changed. However, I've
experimented with the various time zone related calls and I cannot
figure out what call is needed to make the C applications pick up the
time zone change. I've tried setting the environment variable TZ to the
new time zone and this doesn't seem to work, and I've tried calling

tzset() and tzsetwall(). In each case after I make these calls the
function "localtime()" does not return the same time base as the Java
application.

 

Based on what I've read, I would think that the following steps would do
the trick on the C side after the Java app changes time zone and updates

/etc/localtime:

 

                time_t date = time(NULL);

                unsetenv("TZ");

                tzset();

                printf("time zone is %s/%s", tzname[0], tzname[1]);

                struct tm* locTime = localtime(&date);

                printf("%02d:%02d:%02d", locTime->tm_hour,
locTime->tm_min, locTime->tm_sec);

 

The time printed in this example however is still based on the old time
zone. The tzname variable that is set by tzset() still shows for example
EDT even if I have just changed the time zone to PDT. If I stop and
restart the C app, the time is correct, and tzname is then PDT instead
of EDT.

 

I'm very puzzled on what I'm supposed to do to kick start the time zone
change in C. We do not want to have to restart our C apps for something
as trivial as this. I posted this originally to the questions list but
didn't get much traction. I'm hoping someone on this list can point me
in the right direction.