> You should have run the attached test_mktime_changes_tzname.c ...

I did run it and tried several times to understand what it does.  It did not help.

If you claim that calling mktime() changes the value of the global tzname variable, you should be able to demonstrate it in five lines of code:

extern char **tzname;
tzset();
printf("%s %s\n", tzname[0], tzname[1]);
mktime(&tt);
printf("%s %s\n", tzname[0], tzname[1]);

(plus a few lines to initialize tt)


If the code above prints two different lines - file a bug report with your C library vendor.