Windows® Internals, Sixth Edition, Part 2

Hard links are implemented in a similar way. When a hard link to a file is created, NTFS adds another file name attribute to the file’s MFT file record. The two situations differ in one regard, however. When a user deletes a file that has multiple names (hard links), the file record and the file remain in place. The file and its record are deleted only when the last file name (hard link) is deleted. If a file has both an NTFS name and an autogenerated MS-DOS name, however, a user can delete the file using either name.

Here’s the algorithm NTFS uses (the algorithm is actually implemented in the kernel function RtlGenerate8dot3Name and is also used by other drivers, such as CDFS, FAT, and third-party file systems) to generate an MS-DOS name from a long file name:

Remove from the long name any characters that are illegal in MS-DOS names, including spaces and Unicode characters. Remove preceding and trailing periods. Remove all other embedded periods, except the last one.

Truncate the string before the period (if present) to six characters (it may already be six or fewer because this algorithm is applied when any character that is illegal in MS-DOS is present in the name); if it is two or fewer characters, generate and concatenate a four-character hex checksum string. Append the string ~n (where n is a number, starting with 1, that is used to distinguish different files that truncate to the same name). Truncate the string after the period (if present) to three characters.

Put the result in uppercase letters. MS-DOS is case-insensitive, and this step guarantees that NTFS won’t generate a new name that differs from the old only in case.

If the generated name duplicates an existing name in the directory, increment the ~n string. If n is greater than 4, and a checksum was not concatenated already, truncate the string before the period to two characters and generate and concatenate a four-character hex checksum string.

Table 12-7 shows the long Windows file names from Figure 12-30 and their NTFS-generated MS-DOS versions. The current algorithm and the examples in Figure 12-30 should give you an idea of what NTFS-generated MS-DOS-style file names look like.

Note

Although not generally recommended because it can cause incompatibilities with applications that rely on them, you can disable short name generation by setting HKLM\SYSTEM\CurrentControlSet\Control\FileSystem\NtfsDisable8dot3NameCreation in the registry to a DWORD value of 1 and restarting the machine.

Tunneling

NTFS uses the concept of tunneling to allow compatibility with older programs that depend on the file system to cache certain file metadata for a period of time even after the file is gone, such as when it has been deleted or renamed. With tunneling, any new file created with the same name as the original file, and within a certain period of time, will keep some of the same metadata. The idea is to replicate behavior expected by MS-DOS programs when using the safe save programming method, in which modified data is copied to a temporary file, the original file is deleted, and then the temporary file is renamed to the original name. The expected behavior in this case is that the renamed temporary file should appear to be the same as the original file, otherwise the creation time would continuously update itself with each modification (which is how the modified time is used).

NTFS uses tunneling so that when a file name is removed from a directory, its long name and short name, as well as its creation time, are saved into a cache. When a new file is added to a directory, the cache is searched to see whether there is any tunneled data to restore. Because these operations apply to directories, each directory instance has its own cache, which is deleted if the directory is removed. NTFS will use tunneling for the following series of operations if the names used result in the deletion and re-creation of the same file name:

Delete + Create

Delete + Rename

Rename + Create

Rename + Rename

By default, NTFS keeps the tunneling cache for 15 seconds, although you can modify this timeout by creating a new value called MaximumTunnelEntryAgeInSeconds in the HKLM\SYSTEM\CurrentControlSet\Control\FileSystem registry key. Tunneling can also be completely disabled by creating a new value called MaximumTunnelEntries and setting it to 0; however, this will cause older applications to break if they rely on the compatibility behavior.

You can see tunneling in action with the following simple experiment in the command prompt:

Create a file called file1.

Wait for more than 15 seconds (the default tunnel cache timeout).

Create a file called file2.

Perform a dir /TC. Note the creation times.

Rename file1 to file.

Rename file2 to file1.

Perform a dir /TC. Note that the creation times are identical.

Table 12-7. NTFS-Generated File Names

Windows Long Name

NTFS-Generated Short Name