If you ask me, and I take it as implicit by your visit that you do (sorry about that, but it’s Friday afternoon and I need a weekend), Windows is far too facist about filenames. Most notably in terms of the characters one can put into a filename, and the obstreperous way it will cough and whine at you if you dare to break the rules.

For the application I’m working on, we automatically generate filenames based on certain criteria. These filenames can contain descriptive text which we don’t necessarily want to restrict to the set of filename-legal characters (I’ll return to this point momentarily). So we have to filter the descriptive text and strip out illegal characters.

So what is the set of legal characters? I consulted my trusty internet, and after the usual few minutes throwing seaweed over it and sticking pins in effigies, it yielded the following rather useful bit of wisdom, from the Windows Platform SDK, under Win32 and COM Development, System Services, Files and I/O, SDK Documentation, Storage, Storage Overview, File Management, Creating, Deleting and Maintaining Files, Naming a File.

Now at first, as I was in a hurry, I read this as “everything except < > : ” / | is ok”.

But I forgot that characters below 32 are explicitly disallowed.

And then I noticed that (quite naturally) applications don’t let you save filenames containing the “*” key, as this is used as a wildcard.

And also filenames containing the “?” key are disallowed. This is also a wildcard character, and used to escape long paths by prepending “\?” (I always liked that little hack. Such style.)

But with a firm conviction that MSDN Does Not Lie, I checked my ASCII table to make sure I wasn’t hallucinating. Sure enough, “*” and “?” are ASCII 42 and 63 (please don’t anyone tell me they knew that from memory, or I may cry).

But, and aha, there’s another clause I hadn’t read properly: “…or any character that the file system does not allow”. So, time to work out what characters are considered illegal by FAT, FAT32 and NTFS, methought.

Around this point I came across the system structure BIGFATBOOTFSINFO, which I only mention as it’s an excellent name. But I digress.

So, I wandered towards the Platform SDK again, by way of Win32 and COM Development, Development Guides, Windows 95/98/Me Programming, Long File Names, Long File Names and the Protected-Mode FAT System. It was a long walk, but I sustained myself on the way with a coffee. Herein I found more wisdom.

So, we’re getting a little closer. At least FAT16 fesses up to not allowing “*” and “?”.

As to exactly which characters are disallowed by NTFS, this remains a mystery to me. It doesn’t fess up to disallowing “*” and “?”, and indeed googling suggests that all Unicode characters are allowed by NTFS as filenames. So it is presumably the OS level Windows, quite possibly not much deeper than the Win32 API, which disallows “*” and “?” as well as the characters listed in the first quoted section above. Could it disallow others? Since “*” and “?” are not documented explicitly, one has to presume so.

If it wasn’t for the fact that our application had to support Unicode, I would write a simple filter based on ASCII character codes, and be rather facist about it. In our case we can simply replace offending characters with underscores “_” and have done with it. Once, of course, we’ve determined what those offending characters are.

I’m sure I’ve encountered this issue before, and I seem to recall wishing then just for a simple table, for both Unicode and ASCII characters, showing which were disallowed and by what level of the system (shell, file system API, file system itself). My wish is still pending, but since I have some pressing work to do I should Alt-Tab and get on with it…