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NTFS Reparse Points
One of the most interesting new capabilities added to NTFS version 5 with the release of Windows 2000 was the ability to create special file system functions and associate them with files or directories. This enables the functionality of the NTFS file system to be enhanced and extended dynamically. The feature is implemented using objects that are called reparse points.
The use of reparse points begins with applications. An application that wants to use the feature stores data specific to the application--which can be any sort of data at all--into a reparse point. The reparse point is tagged with an identifier specific to the application and stored with the file or directory. A special application-specific filter (a driver of sorts) is also associated with the reparse point tag type and made known to the file system. More than one application can store a reparse point with the same file or directory, each using a different tag. Microsoft themselves reserved several different tags for their own use.
Now, let's suppose that the user decides to access a file that has been tagged with a reparse point. When the file system goes to open the file, it notices the reparse point associated with the file. It then "reparses" the original request for the file, by finding the appropriate filter associated with the application that stored the reparse point, and passing the reparse point data to that filter. The filter can then use the data in the reparse point to do whatever is appropriate based on the reparse point functionality intended by the application. It is a very flexible system; how exactly the reparse point works is left up to the application. The really nice thing about reparse points is that they operate transparently to the user. You simply access the reparse point and the instructions are carried out automatically. This creates seamless extensions to file system functionality.
In addition to allowing reparse points to implement many types of custom capabilities, Microsoft itself uses them to implement several features within Windows 2000 itself, including the following:
These are just a few examples of how reparse points can be used. As you can see, the functionality is very flexible. Reparse points are a nice addition to NTFS: they allow the capabilities of the file system to be enhanced without requiring any changes to the file system itself.