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XXCOPY TECHNICAL BULLETIN #08


From:    Kan Yabumoto           tech@xxcopy.com
To:      XXCOPY user
Subject: Short names made by Win95/98 and by WinNT/2K/XP
Date:    2003-10-14 (Revised)
===============================================================================

In our earlier article, XXTB #03, we discussed the subtle problem
of long and short filenames commonly encountered by Windows users.
The problem has been widely known and various solutions have been
devised to handle most such cases.  XXCOPY can handle such cases
correctly in most cases.

Unfortunately, when you mix the two types of OS in operation, one
more potential problem may arise.


About the various Windows OS.

   Although all Windows operating systems except early ones (Win 3.x)
   support long filenames, there is a subtle difference in the
   algorithm of alias synthesis by the various OSes.  The difference
   may adversely affect XXCOPY's effort in preserving the short
   filename.

   First, let us examine the difference.  Windows 95 and 98 use a
   straightforward scheme in picking up the short name alias, whereas
   Windows NT4, 2000 and XP add a little twist in the way the numeric
   tail is generated.

   It is easy for anyone to observe the filename related idiosyncrasies.
   Just pick a file and repeatedly copy it in a directory under a series
   of long names having the same beginning.  For example:

        mkdir c:\temp
        copy  c:\autoexec.bat   c:\temp\mylongname1
        copy  c:\autoexec.bat   c:\temp\mylongname2
        copy  c:\autoexec.bat   c:\temp\mylongname3
        copy  c:\autoexec.bat   c:\temp\mylongname4
        copy  c:\autoexec.bat   c:\temp\mylongname5
        copy  c:\autoexec.bat   c:\temp\mylongname6
        copy  c:\autoexec.bat   c:\temp\mylongname7
        copy  c:\autoexec.bat   c:\temp\mylongname8
        copy  c:\autoexec.bat   c:\temp\mylongname9

   Then, using an appropriate tool, examine the destination directory.
   The following table shows the result of the copy operations in
   Windows 95/98 and the Windows NT/2000/XP variations.

        Longname         Windows 95/98/ME  Windows NT4/2K/XP
       ----------------------------------------------------------
        mylongname1          MYLONG~1          MYLONG~1
        mylongname2          MYLONG~2          MYLONG~2
        mylongname3          MYLONG~3          MYLONG~3
        mylongname4          MYLONG~4          MYLONG~4
        mylongname5          MYLONG~5          MYA476~1
        mylongname6          MYLONG~6          MYA486~1
        mylongname7          MYLONG~7          MYA496~1
        mylongname8          MYLONG~8          MYA4A6~1
        mylongname9          MYLONG~9          MYA4B6~1

   Whereas the Win9x OSes use a simple decimal number in the numeric
   tail, the WinNT family OSes follow the same pattern only in the
   first four cases.  After that, they start using a 4-digit
   hexadecimal number in the middle of the 8.3 name as the
   distinguishing part (only the first two letters remains the same).

   The hexadecimal value is probably a hash value for the string to
   supposedly shorten the filename matching operation which could be
   very time consuming.  Microsoft programmers chose to keep the first
   four match done numerically for the sake of compatibility to the
   Win9X systems.  What they failed to realize is that they allowed
   only the first four such names for compatibility.


The adverse effect of the two alias algorithms.

   As long as you use XXCOPY in a homogeneous environment where
   the files are generated under one OS, XXCOPY can preserve the
   short name.  But, when you have a mixed environment (e.g., with a
   dual-boot system), you may copy a directory with many similar
   names which were created under one OS and then perform a directory
   copy operation using XXCOPY under the other OS, XXCOPY may not be
   able to preserve the short name.

   One consolation is that the great majority of files will receive
   the correct short name alias using XXCOPY since the numeric tail of
   the first four aliases in a directory are common in all Windows OSes.
   Therefore the problem should be quite rare.  Ironically, if your
   system has many Microsoft software packages, you will find quite
   a few directories which all start like:

     "\Program Files\Microsoft xxxxx"

   One can argue that exactly because it is so rare, this phenomenon
   would not be widely known.  Therefore, when the rare problem occurs,
   it will take many agonizing hours for the user to discover the
   cause of the problem.


Then, what happens with XXCOPY?

   Since XXCOPY does not access the raw directory contents in the
   process of preserving the short name alias, its magic does not work
   all the time.  We cannot do much about this problem without making
   the XXCOPY utility breaking the rules of good programming practice.
   At least we can list the situations where the potential problem may
   arise and how you can avoid it.

   1.  Stay within the 8.3 naming convention if possible.
   2.  Stay away from a dual-boot system if possible.
   3.  If you have a dual-boot system and you switch the OS from
       time to time, make sure a directory is written exclusively
       under one OS if possible.
   4.  Finally, when you have to copy a directory, use XXCOPY and
       perform the copy operation under the same OS in which the
       files in the directory were created.
   5.  In a networked environment with different types of OS between
       the source and the destination directories, expect the worst.

   Note that although it is ideal that when you copy a directory,
   all the files in the destination should be identical to those in
   the source, in most case, it is not the end of the world.  As a
   matter of fact, in most cases, the discrepancies in short names
   do not cause any problem.  As long as the files and directories
   that are affected by this problem is "non-system" files, the
   problem would be benign if present at all.

   In the case of Windows 95/98, the most well documented problem
   which are associated with the unmatched short name is a few
   directory and filenames which are stored in their short name
   alias in a few critical cases in the system registry.  Since
   Windows 95/98 references these files at the system initialization
   time (locating and loading VxD files using short names), the
   difference would be life and death (that is, success and failure
   in system initialization).


A final advice with XXCOPY:

   If you have a large number of collided aliases (where the short
   name distinctions are made by the synthesized numeric tail only),
   and you are copying across OSes (from Win95/98 to WinNT/2000/XP
   or vice versa), we suggest you turn off XXCOPY's short name
   preservation switch by /NX0 in order to avoid the time consuming
   and futile effort by XXCOPY.



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