Why can't Linux hard links link directories?

Reason: Introducing a hard connection to the directory may introduce a loop in the directory. When the directory is traversed, the system will fall into an infinite loop, which will result in the inability to locate the access directory. The directory structure of Linux is a tree with "/directory" as the root node. If custom hard links are allowed, it is likely to destroy this structure and even form a loop; and once a loop is formed, for commands that need to traverse the directory tree , is fatal. Therefore, in order to avoid damaging the directory tree structure, Linux does not allow users to customize hard links on the directory.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

LINUX hard links cannot be linked to the directory because the introduction of a hard link to the directory may introduce a loop in the directory. When the directory is traversed, the system will fall into an infinite loop, which will result in the inability to locate the directory. Visit catalog.

In the Linux system, each file (directory is also a file) corresponds to an inode structure. The inode data structure contains information about the file type (directory, ordinary file, symbolic link file, etc.), and That is to say, the operating system can determine the symbolic link when traversing the directory. Since it can determine the symbolic link, of course it can take some measures to prevent entering an excessive loop. The system stops traversing after encountering 8 symbolic links in a row. This This is why directory symbolic connections do not enter an infinite loop. However, for hard connections, due to the limitations of the data structure and algorithm used in the operating system, it is currently impossible to prevent such an infinite loop.

The essence of hard connection in linux

Before discussing the problem, let’s talk about the essence of hard connection.

The hard link is actually the content of the block pointed to by the inode of the directory. Each record in the directory block is a hard link. The directory itself is a type of file in Linux (directory file, symbol d), and the file content of the "directory file" is a hard link.

For example (note the change in the number of directory connections in the picture):

The diagram before and after the change in the number of tmp directory connections is as follows (the red line indicates the source of the connection number) ):

Through the above example, we can find that the number of hard links when a directory is just created is 2, one of which comes from the parent directory and the other from itself. of".". If you create a subdirectory for the directory, the number of hard connections will change from 1 to 3. This is because there is an additional ".." connection from the subdirectory.

Some people may ask here, doesn’t it mean that hard links cannot be used for directories? What's going on with the demo above?

A concept needs to be clarified here, "hard links cannot be used for directories", which means that hard links to directories cannot be customized by users, but can only be maintained by the operating system. In fact, directories and hard links are inseparable and integrated with each other. The "hard link" itself is actually the way a directory maintains its sub-file names & sub-directory names.

The nightmare of custom hard connections: loops

After talking about the nature of hard connections, we can return to today’s topic: why not Allow user-defined hard links to directories?

We know that the directory structure of Linux is a tree with "/directory" as the root node. If custom hard links are allowed, it is likely to destroy this structure or even form a loop. For example, the following command If it can be executed successfully:

Once a loop is formed, it is fatal for commands that need to traverse the directory tree (for example, from top to bottom The du command that traverses from bottom to top, and the pwd command that traverses from bottom to top). Therefore, in order to avoid damaging the directory tree structure, Linux does not allow users to customize hard links on the directory. The reason why soft links do not have this restriction is that soft link files have a special file type that can be recognized by the system, while "hard link files" are no different from normal files of the system and cannot be judged.

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