[Bug libc/13401] New: calling localtime() or mktime() with a time far in the past corrupts cached timezone info

["pitrou at free dot fr" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13401

             Bug #: 13401
           Summary: calling localtime() or mktime() with a time far in the
                    past corrupts cached timezone info
           Product: glibc
           Version: 2.12
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: pitrou@free.fr
    Classification: Unclassified


See the following snippet:

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

int main() {
    time_t t;
    struct tm tmp;
    char str[200];

    t = time(NULL);
    tmp = *gmtime(&t);
    tmp.tm_gmtoff = 0;
    tmp.tm_zone = NULL;

    strftime(str, sizeof(str), "%Z", &tmp);
    puts(str);

    t = -2461446500;
    localtime(&t);

    t = time(NULL);
    tmp = *gmtime(&t);
    tmp.tm_gmtoff = 0;
    tmp.tm_zone = NULL;

    strftime(str, sizeof(str), "%Z", &tmp);
    puts(str);

    return 0;
}


Output here is:
CET
PMT

After a bit of digging, it seems the external variable "tzname" gets (wrongly)
mutated by the localtime() call. The "%Z" format specifier just reuses that
info.

This issue got reproduced on several distributions, see
http://bugs.python.org/issue13309

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