> Is a *nix file system guranteed to be acyclic?
> 
> That is no:
> 
> File-a  links to file-b and file-b links to file-a
> 
> And more important
> 
> No directory-a contains link-b where link-b points to directory-a.
> 
> Also
> 
> Directory-a contains no link-b to directory-c where directory-c contains 
> link-d to directory-a.

I think it all depends on which FS you are using.  Linux has ext2, ext3 (ext2 + 
journalling), XFS, ReiserFS, etc...