On Thu, 2010-02-11 at 23:23 -0800, Suvayu Ali wrote: > Hi everyone, > > When I tried to hardlink a directory today I ran into this, > > $ ln muse test > ln: `muse': hard link not allowed for directory > > So I did a little searching and found its not exactly a forbidden. So > far the closest to an understandable explanation/reasoning I came across > was a discussion in lwn[1]. So my question is how are hardlinks so > different from softlinks? A hard link is a directory entry that references an inode. Every property of the file is represented in the inode, including its type, ownership, permissions, size and pointers to the actual data, i.e. the directory entry is simply a (name, inode) pair. As such, there can be multiple directory entries referencing the same inode. If the inode type is "directory", reading the data gives us another level of the file hierarchy. However, consider what would happen if one of the entries in this second level pointed (directly or indirectly) back to the parent directory. Very early versions of Unix actually allowed this, until people realized that you could create a circular structure that was unreachable from the filesystem root, which would be a Bad Thing (tm). (Note the analogy to garbage collection problems in LISP and other languages). In order to avoid this, the rule was established that you can't link to directories, except when the directory itself is being created, at which time two special entries are placed in it, called ".", and "..". These are inserted directly by the system itself and are the only exceptions allowed. This forces the entire structure to be a tree and not an arbitrary graph. However, as the space of inode numbers is local to a specific filesystem, links cannot cross filesystem boundaries. Thus at a later stage of development, the BSD version of Unix introduced symbolic links as a way round this. A soft or symbolic link is just a file that contains the name of another file (or directory or whatever). It's marked as a soft link so the system can dereference it when it appears in a path. Since it's just a name, there are no restrictions on what it contains, in fact it may not point at anything that actually exists, and of course can actually create a circular structure. Unix systems prevent chaos by forbidding infinite deferencing in pathnames, i.e. the system counts how many components have been traversed while trying to get to the file, and throws an error when the count exceeds a certain number, e.g. 17. This is a kludge but it keeps things sane. Note that no file can exist which *only* has symbolic links to it. Once the last hard link is broken, the file disappears. Note also that the inode maintains a count of hard links, but not of symlinks, i.e. symlinks are in some ways second-class citizens. Soft links are similar to "aliases" on Windows. AFAIK Windows has no concept analogous to hard links. poc -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Communicate/MailingListGuidelines
