[Vm-dev] classic VM: timezone correction in sqUnixUtcWithOffset()

stes@PANDORA.BE stes at telenet.be
Sun Apr 25 18:09:56 UTC 2021


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Hi,

Here's another fix for sqUnixMain.c in the SVN (subversion) repository.

This is for timezone correction in sqUnixUtcWithOffset().

Basically there is code for the Linux/glibc case which is correct:

 #if defined(HAVE_TM_GMTOFF)
   *offset= localtime(&seconds)->tm_gmtoff;
 #else

The above code is correct but it depends on tm_gmtoff (glibc) as on Linux.

The incorrect code is for platforms that use the plain standard C library,
without the glibc extension (the #else clause).

The timezone (HAVE_TIMEZONE) is itself also extension which is enabled
by default for Solaris because it is _XOPEN_SOURCE but not _STRICT_STDC.

- -  {
- -    struct tm *local= localtime(&seconds);
- -    struct tm *gmt= gmtime(&seconds);
- -    int d= local->tm_yday - gmt->tm_yday;
- -    int h= ((d < -1 ? 24 : 1 < d ? -24 : d * 24) + local->tm_hour - gmt->tm_hour);
- -    int m= h * 60 + local->tm_min - gmt->tm_min;
- -    *offset= m * 60;
- -  }

The above is incorrect because localtime() and gmtime() will overwrite the
same buffer, so the gmtime() call erases the value of localtime().

I have a patch in attach that uses the same logic as plugins/LocalePlugin/sqLocalePlugin.c and as in convertToSqueakTime() which is doing it right.

+#ifdef HAVE_TIMEZONE
+  *offset = ((daylight) * 60 * 60) - timezone;
+#else
+#error: cannot determine timezone correction
 #endif
+#endif
   return 0;
 }
 
Please apply the patch in attach to subversion ...

I think it is safe to apply this patch because (1) it does not change
the #if HAVE_TM_GMTOFF case (the Linux case is unchanged) and (2) it is
consistent with plugins/LocalePlugin/sqLocalePlugin.c and convertToSqueakTime() and (3) the code with the difference between localtime() and gmtime() is certainly wrong.

Thanks,
David Stes

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