The mount_nullfs utility creates a null layer, duplicating a sub-tree of the file system name space under another part of the global file system namespace. This allows existing files and directories to be accessed using a different pathname.
The primary differences between a virtual copy of the file system and a symbolic link are that the getcwd(3) functions work correctly in the virtual copy, and that other file systems may be mounted on the virtual copy without affecting the original. A different device number for the virtual copy is returned by stat(2), but in other respects it is indistinguishable from the original.
The mount_nullfs file system differs from a traditional loopback file system in two respects: it is implemented using a stackable layers techniques, and its "null-nodes" stack above all lower-layer vnodes, not just over directory vnodes.
The options are as follows: