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Is there a list of characters which are not allowed in filenames and paths on OS X?

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Actually, both : and ? are allowed, I must be having a different problem. – Paul Biggar Oct 28 '10 at 12:18
I'm new to this telepathy thing, so I just answered what you wrote, not what you thought ;-) – Daniel Beck Oct 28 '10 at 12:38
Useful references: Wikipedia article on Filenames and Fixing Unix/Linux/POSIX Filenames. – Dennis Williamson Oct 29 '10 at 3:21
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? is allowed.

Only 0x00 (NUL) and : are forbidden for HFS+.

: however is switched in POSIX-based software to /:

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Thanks to Graham Lee who corrected me on this!

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Actually ':' is forbidden in HFS+, but '/' is forbidden in the POSIX library. So Mac OS X virtual filesystem accepts ':', rejects '/', but has to switch them around when dealing with HFS+. It's quite confusing :). – Graham Lee Oct 28 '10 at 13:33
@Graham thanks for the clarification. – Daniel Beck Oct 28 '10 at 13:41
no problem. I didn't fully understand it until I read the chapters in Mac OS X Internals about the filesystem implementation :-S. – Graham Lee Oct 28 '10 at 13:48
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As Daniel Beck said, NUL and ":" (aka "/" in POSIX contexts) are forbidden; however, it's more complicated than that. HFS+ filenames must be stored in UTF-8 format, in fully decomposed form, with composing characters stored in canonical order (see tn1150). So the answer to your question depends on what you mean by "character":

If you're interested in the sequence of bytes that make up filenames, any sequence that isn't valid UTF-8 (or not properly decomposed) is forbidden.

If you mean unicode code points, that decomposition rule still forbids any point that represents an accented character (it must be stored instead as the base letter + combining accents) (see tn1150table).

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This answer is misleading. On disk, HFS+ file names are stored as UTF-16, not UTF-8. – Chris Suter May 12 at 2:23
Also ':' characters are swapped with '/'. However, you don't really need to know any of this; the on disk implementation is irrelevant. All POSIX calls into and out of the Kernel use UTF-8 and in that context '/' is a path separator and ':' is allowed and it doesn't have to be decomposed. – Chris Suter May 12 at 2:34
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