Fun with Windows paths.

June 19, 2012 § 3 Comments

I am currently attempting to implement a JavaScript library to handle file system paths in a portable manner.

Right now, I am having lots of fun with Windows paths and I wanted to share a few tidbits.

Under Windows, a path name can look like:

  1. “\\?\drivename:” followed by backslash-separated components.
    Such paths can be either relative or absolute.
    In such paths, “.”, “..” and “/” are regular file names.
  2. “\\.\drivename:” followed by backslash-separated components.
    Such paths can be either relative or absolute.
    In such paths, “.”, “..” and “/” are special names.
  3. “\\?\UNC\servername” followed by backslash-separated components.
    Such paths can only be absolute.
    In such paths, “.”, “..” and “/” are regular file names.
  4. “\\servername” followed by slash- or backslash- components.
    Such paths can only be absolute.
    In such paths, “.”, “..” and “/” are special names.
  5. “drivename:” followed by slash- or backslash- components.
    Such paths can be either relative or absolute.
    In such paths, “.”, “..” and “/” are special names.
  6. Just a series of slash- or backslash- components.
    Such paths can be either relative or absolute.
    In such paths, “.”, “..” and “/” are special names.

To simplify things further, depending on the version of Windows, a drive name can be:

  • only one letter between A and Z;
  • any sequence of letters between A an Z;
  • something that looks like Volume{41AF5D4F-04CC-4D15-9389-734BD6F52A7E}.

Also

  • if a path starts with “\\?\”, its length is limited to 32,767 chars;
  • otherwise, its length is limited to 260 chars.

Also

  • some names such as “LPT”, “COM”, etc. are reserved and cannot be used as file names;
  • … unless your path starts with “\\”.

Also

  • paths are case-insensitive;
  • … except when they are case-sensitive because of the disk format;
  • … except when they are case-sensitive because of something else.

Fortunately, the Windows APIs provides the following functions to simplify matters:

  • PathCanonicalize (completely broken);
  • GetFullPathName (broken);
  • GetLongPathName (requires access permissions just to tell you if a path is well-formatted);
  • UriCanonicalize (not sure what it does exactly, I haven’t tested it yet).

Of course, not all Windows API functions accept all schemes.

As you can imagine, I am having lots of fun.

Quick exercise given two paths A and B (either absolute or relative), how do you determine the path obtained by concatenating A and B?

If you are interested in following my progress, details are on bugzilla.

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