Regarding Hard Link

Question

Can somebody please explain me why the kernel doesn\'t allow us to make a hard link to a directory. Whether it is because it breaks the rule of directed acyclic graph structure

Answer 1

This is entirely because allowing hard links to directories allows for potential loops and cycles in the directory graph without adding much value.

Answer 2

Cyclic references will break garbage collection by reference counting. Wikipedia describes the problem:

There are a variety of ways of handling the problem of detecting and collecting reference cycles. One is that a system may explicitly forbid reference cycles.

That it the way Linux does it.

Answer 3

Back in the days of 7th Edition (or Version 7) UNIX, there were no system calls mkdir(2) and rmdir(2). The mkdir(1) program was SUID root, and used the mknod(2) system call to create the directory and the link(2) system call to make the entries for . and .. in the new directory. The link(2) system call only allowed root to do that. Consequently, way back then (circa 1978), it was possible for the superuser to create links to directories, but only the superuser was permitted to do so to ensure that there were no problems with cycles or other missing links. There were diagnostic programs to pick up the pieces if the system crashed while a directory was partly created, for example.

---

You can find the Unix 7th Edition manuals at Bell Labs. Sections 2 and 3 are devoid of mkdir(2) and rmdir(2). You used the mknod(2) system call to make the directory:

NAME

mknod – make a directory or a special file

SYNOPSIS

mknod(name, mode, addr)
char *name;

DESCRIPTION

Mknod creates a new file whose name is the null-terminated string pointed to by name. The mode of the new file (including directory and special file bits) is initialized from mode. (The protection part of the mode is modified by the process’s mode mask; see umask(2)). The first block pointer of the i-node is initialized from addr. For ordinary files and directories addr is normally zero. In the case of a special file, addr specifies which special file.

Mknod may be invoked only by the super-user.

SEE ALSO

mkdir(1), mknod(1), filsys(5)

DIAGNOSTICS

Zero is returned if the file has been made; – 1 if the file already exists or if the user is not the superuser.

The entry for link(2) states:

DIAGNOSTICS

Zero is returned when a link is made; – 1 is returned when name1 cannot be found; when name2 already exists; when the directory of name2 cannot be written; when an attempt is made to link to a directory by a user other than the super-user; when an attempt is made to link to a file on another file system; when a file has too many links.

The entry for unlink(2) states:

DIAGNOSTICS

Zero is normally returned; – 1 indicates that the file does not exist, that its directory cannot be written, or that the file contains pure procedure text that is currently in use. Write permission is not required on the file itself. It is also illegal to unlink a directory (except for the super-user).

The manual page for the ln(1) command noted:

It is forbidden to link to a directory or to link across file systems.

The manual page for the mkdir(1) command notes:

Standard entries, '.', for the directory itself, and '..' for its parent, are made automatically.

This would not be worthy of comment were it not that it was possible to create directories without those links.

---

Nowadays, the mkdir(2) and rmdir(2) system calls are standard and permit any user to create and remove directories, preserving the correct semantics. There is no longer a need to permit users to create hard links to directories. This is doubly true since symbolic links were introduced - they were not in 7th Edition UNIX, but were in the BSD versions of UNIX from quite early on.

---

With normal directories, the .. entry unambiguously links back to the (single, solitary) parent directory. If you have two hard links (two names) for the same directory in different directories, where does the .. entry point? Presumably, to the original parent directory - and presumably there is no way to get to the 'other' parent directory from the linked directory. That's an asymmetry that can cause trouble. Normally, if you do:

chdir("./subdir");
chdir("..");

(where ./subdir is not a symbolic link), then you will be back in the directory you started from. If ./subdir is a hard link to a directory somewhere else, then you will be in a different directory from where you started after the second chdir(). You'd have to show that with a pair of stat() calls before and after the chdir() operations shown.

Answer 4

Because then the directory tree will cease to be a directory tree. One directory could have multiple parents.

Answer 5

In addition to the possibility of getting cycles (much like with symlinks, by the way, but these are easier to detect and handle), there is a second reason I can think of.

On UNIX, there is a common assumption in use by many programs, that will assume that all directories will have a link count of 2+number of child directories. This is due to the POSIX standard directory entries '.' and '..' which link to the directory or it's parent.

(After verification, I can say that the root (/) is not an exception).

This is especially useful as a performance optimization to detect leaf directories when recursing, but many applications will exist that have found other uses for it

Clarifying By allowing 'userdefined' hardlinks to directories, these invariants so to say will no longer hold, and any dependent applications might stop working correctly. The element of surprise is why you need root permissions (and some good design (re)thinking) in order to force creation of directory hardlinks