[Bug libc/601] New: mktime(3) unreliable after calling tzset(3) when TZ is NULL or bogus

["kreckel at ginac dot de" <sourceware-bugzilla at sources dot redhat dot com>]

#include <time.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
    unsigned sec;
    struct tm t;
    time_t t1, t2;
    const char* currTz;
    t.tm_year = 100; // 2000
    t.tm_mon = 0; // Jan
    t.tm_mday = 1;
    t.tm_hour = 0;
    t.tm_min = 0;
    t.tm_sec = 0;
    t.tm_isdst = 0;
    t1 = mktime( &t );
    for ( sec = 0; sec < 24*60*60; ++sec ) {
        t.tm_hour = sec / ( 60 * 60 );
        t.tm_min = ( sec / ( 60 ) ) % 60;
        t.tm_sec = sec % ( 60 );
        printf("%02d:%02d:%02d\t",t.tm_hour,t.tm_min,t.tm_sec);

        currTz = getenv( "TZ" );
        //setenv( "TZ", "#!@+*%&
NULLCannotBeUsedBecauseOfABugWhenLinkingWithLibPthreadStringCannotBeUsedEither",
1 );  // leads to errors
        setenv( "TZ", NULL, 1 );  // leads to errors
        //setenv( "TZ", "UTC", 1 );  // seems to work
        tzset();

        t2 = mktime( &t );

        if ( currTz )
            setenv( "TZ", currTz, 1 );
        else
            unsetenv( "TZ" );

        printf("%d\n",t2-t1);
        t1 = t2;
    }
}

The above program should print:
00:00:00        3600
00:00:01        1
00:00:02        1
00:00:03        1

The left column should always be the time difference to the previous line or, in
the case of the first line, the difference of the local time to UTC.  However,
it sometimes prints:
00:00:00        3601
00:00:01        0
00:00:02        1
00:00:03        1
[...]

or, even stranger things like
[...]
00:01:15        1
00:01:16        7201
00:01:17        -7199
00:01:18        1
[...]

This happens only if I set TZ to NULL or to some bogus (see code) value *inside*
the loop. However, according to tzset(3): "If the TZ variable does appear in the
environment but its value is NULL or its value cannot be interpreted using any 
of the formats specified below, Coordinated Universal Time (UTC) is used."

I have reproduced this bug with the glibc shipped with SuSE Professional Linux
8.1, SuSE Linux Linux 9.2 and Debian 3.0, so I assume it must be from upstream.

-- 
           Summary: mktime(3) unreliable after calling tzset(3) when TZ is
                    NULL or bogus
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: gotom at debian dot or dot jp
        ReportedBy: kreckel at ginac dot de
                CC: glibc-bugs at sources dot redhat dot com


http://sources.redhat.com/bugzilla/show_bug.cgi?id=601

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