Filenames

With VisionFS, each file or directory effectively has three names:

• the UNIX filename

• a long name, reported to Windows 95 and Windows NT

• a short name, reported to all versions of Windows

When a long or short name is used in Windows, VisionFS uses the correct UNIX filename.

Potentially, there are fewer long names than UNIX filenames, and fewer short names than long names. This means that the process of converting UNIX filenames to long names, then to short names, might result in two unique UNIX filenames losing their uniqueness. VisionFS ensures that this can't happen by adding unique suffixes to the basename where a clash would occur.

Converting UNIX filenames to long names

This involves the following steps:

1. Remove trailing dots.

2. Remove the characters not allowed in long names:
   
   ? " / \ < > * | :

3. Prefix the DOS reserved basenames with ``_''. These basenames are:
   
   aux com1 com2 com3 com4 con lpt1 lpt2 lpt3 nul prn clock$

4. Add unique suffixes to the basename to avoid name clashes, as described below. A name would clash if it's the same as another name, ignoring case.

Converting UNIX filenames to short names

This involves the following steps:

1. Remove all dots but the last.

2. Remove the additional characters not allowed in short names:
   
   [ ] ; = + ,

3. Convert to uppercase.

4. Truncate the basename to 8 characters and the extension to 3 characters.

5. Add unique suffixes to the basename to avoid name clashes, as described below. A unique suffix is also added if there's no basename -- for example, one would be added for the UNIX filename .cshrc.

Making filenames unique

To make a set of clashing long or short names unique, VisionFS first generates a 32-bit checksum for each, based on the original UNIX filename. The checksum is converted to a printable form.

Then, a suffix is formed for each clashing name, using the special character for the share (by default, ~) and the last character of the printable checksum (or however many characters are needed to form unique suffixes for all clashing names).

Finally, the suffixes are added to the basenames of the clashing names, replacing the last 2 (or more) characters to ensure the basenames don't exceed 8 characters in length.

By default, if basenames are unique when truncated to 8.3 format, then no changes are made -- a unique suffix isn't needed. In some cases, you might want VisionFS to always use a suffix when truncating. The Profile Editor lets you do this, using the ``Always use in truncated filenames'' box on the Files tab of a share's properties.