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<title>Filename Namespace - Concept - NTFS Documentation</title>
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<h1>Concept - Filename Namespace</h1>
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Old versions of the FAT filesystem had strict limits on filenames.
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Many characters were forbidden, and the length was restricted to
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11 characters (a small namespace). Newer versions of FAT allowed
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more characters and longer filenames. NTFS has almost no restrictions.
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Filenames are given a flag to show which namespace the name belongs
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to. In order to support old applications, NTFS allocates a short
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DOS-friendly name to any file with an DOS-incompatible name.
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<h2>Possible Namespaces</h2>
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This is the largest namespace. It is case sensitive and
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allows all Unicode characters except for NULL (0) and
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Forward Slash '/'. The maximum name length is 255 characters.
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N.B. There are some characters, e.g. Colon ':', which are
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valid in NTFS, but Windows will not allow you to use.
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Win32 is a subset of the POSIX namespace and is case insensitive.
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It uses all the Unicode characters, except:
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'"' '*' '/' ':' '<' '>' '?' '\' '|'
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N.B. Names cannot end with Dot '.', or Space ' '.
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DOS is a subset of the Win32 namespace, allowing only 8 bit upper
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case characters, greater than Space ' ', and excluding:
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'"' '*' '+' ',' '/' ':' ';' '<' '=' '>' '?' '\'.
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N.B. Names must match the following pattern:
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1 to 8 characters, then '.', then 1 to 3 characters.
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<dt>3: Win32 & DOS</dt>
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This namespace means that both the Win32 and the DOS filenames are
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identical and hence have been saved in this single filename record.
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To convert a POSIX or Win32 filename to a DOS-friendly filename, follow these steps:
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<li>Remove all Unicode characters</li>
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<li>Remove all '.' but the last one if <em>it is not the first character</em></li>
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<li>Uppercase all letters</li>
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<li>Remove forbidden characters</li>
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<li>Truncate everything before the potential '.' to 6 characters, and add the string "~1"</li>
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<li>Truncate everything after the potential '.' to 3 characters</li>
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<li>While the name already exists, increment the string "~1"</li>
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N.B. Step 7 means that although the generated DOS name is unique,
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it is impossible to deduce it from the Win32 name only.
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