Aliases and symbolic links are lightweight references to files and folders. Aliases are associated with Mac OS Standard (HFS) and Mac OS Extended (HFS+) volume formats. Symbolic links are a feature of HFS+ and UFS file systems. Both aliases and symbolic links allow multiple references to files and folders without requiring multiple copies of these items. Prior to Mac OS X v10.2, aliases and symbolic links behaved very differently when a referenced file or folder moved or changed.
On HFS and HFS+ file systems, each file and folder has a unique, persistent identity. Aliases use this identity along with pathname information to find files and folders on the same volume.
In versions of Mac OS X before 10.2, aliases located a file or folder using its unique identity first and its pathname second. Beginning with Mac OS X 10.2, aliases reversed this search order by using the pathname first and unique identity second. This means that if you move a file and replace it with an identically named file, aliases to the original file now point to the new file. Similarly, if you move a file on the same volume (without replacing it), aliases use the unique identity information to locate the file.
When a file or folder moves, the alias may update either its path information or unique identity information to account for the change. If a file moves somewhere on the same volume, the alias updates its internal record with the new path information for the file. Similarly, if the original file is replaced by a file with the same name, but a different unique identity, the alias updates its internal record with the unique identity of the new file.
Because aliases use a file system path to resolve a file’s location initially, they now offer a similar behavior to symbolic links. Symbolic links rely exclusively on path information to locate a file. If you move a file somewhere on the same volume without replacing it, symbolic links to the file break while aliases do not. The only way to fix a symbolic link is to delete it and create a new one.
The Finder and other system applications now use aliases with this pathname-first behavior. However, applications can still resolve aliases by unique identity first using the methods of the Alias Manager.
If your application supports versions of Mac OS X prior to Mac OS X v10.2, you should follow certain guidelines when modifying files. First, when editing a file, modify the existing file. Second, if you need to replace a file transparently with a new version, use FSExchangeObjects to swap the new file for the old one. The NSDocument class already uses similar techniques to update the document file, thus maintaining aliases whenever possible.
Last updated: 2009-08-14