I came across a c library for opening files given a Unicode filename. Before opening the file, it first converts the filename to a path by prepending "\\?\". Is there any reason to do this other than to increase the maximum number of characters allowed in the path, per this msdn article?
It looks like these "\\?\" paths require the Unicode versions of the Windows API and standard library.
I've been writing Windows code since 1995, and although I'm aware of that prefix, I've never found any reason to use it. Increasing the path length beyond MAX_PATH seems to be the only reason for it, and neither I nor any of my programs' customers have ever done so, to my knowledge.
Yes, it's just for that purpose. However, you will likely see compatibility problems if you decide to creating paths over MAX_PATH length. For example, the explorer shell and the command prompt (at least on XP, I don't know about Vista) can't handle paths over that length and will return errors.
I think the first thing to note is that "\\?\" does not make the path a UNC path. You were more accurate the second time when you called it a UNC-style path. But even then, the similarity only comes from having two backslashes at the start. It really has nothing to do with UNC. That's backed up by the fact that you have to use even more characters to get a UNC path with the "\\?\" prefix.
I think you've got the entire reason for using that prefix. It lifts the maximum-length limit as described in the article you cited. And it only applies to Unicode paths; non-Unicode paths don't get to avoid the limit by using that prefix.
One thing to note is that the prefix is not allowed for relative paths, only for absolute ones. You might want to double-check that your C library honors that restriction.
The best use for this method is probably not to create new files, but to manage existing files, which someone else may have created.
I managed a file server which routinely would get files with path_length > MAX_PATH. You see, the users saw the files as H:\myfile.txt, but on the server it was actually H:\users\username\myfile.txt. So if a user created a file with exactly MAX_PATH characters, on the server it was MAX_PATH+len("users\username").
(Creating a file with MAX_PATH characters is not so uncommon, since when you save a web page on Internet Explorer it uses the page title as the filename, which can be quite long for some pages).
Also, sharing a drive (via network or usb) with a Mac or a Linux machine, you can find yourself with files with names like con, prn or lpt1. And again, the prefix lets you and your scripts handle those files.