Жесткие ссылки в файловых системах

Directory entry (in a directory based file system) that associates a name with a file
the hyperlinking term

In computing, a hard link is a directory entry (in a directory based file system) that associates a name with a file. Thus, each file must have at least one hard link. Creating additional hard links for a file makes the contents of that file accessible via additional paths (i. e., via different names or in different directories). This causes an alias effect: a process can open the file by any one of its paths and change its content. By contrast, a soft link or “shortcut” to a file is not a direct link to the data itself, but rather a reference to a hard link or another soft link. Every directory is itself a special file on many systems, containing a list of file names instead of other data. Hence, multiple hard links to directories are possible, which could create a circular directory structure, rather than a branching structure like a tree. For that reason, some file systems forbid the creation of hard links to directories. POSIX compliant operating systems, such as Linux, Android, macOS, and the Windows NT family, support multiple hard links to the same file, depending on the file system. For instance, NTFS and ReFS support hard links, while FAT does not.

Let two hard links, named "LINK A. TXT" and "LINK B. TXT", point to the same physical data. A text editor opens "LINK A. TXT", modifies it and saves it. When the editor (or any other app) opens "LINK B. TXT", it can see those changes made to "LINK A. TXT", since both file names point to the same data. So from a user's point of view this is one file with several filenames. Editing any filename modifies "all" files, however deleting "any" filename except the last one keeps the file around. However, some editors, such as GNU Emacs, break the hard link concept. When opening a file for editing, e. g., "LINK B. TXT", emacs renames "LINK B. TXT" to "LINK B. TXT~", loads "LINK B. TXT~" into the editor, and saves the modified contents to a newly created "LINK B. TXT". Now, "LINK A. TXT" and "LINK B. TXT" no longer shares the same data. (This behavior can be changed using the emacs variable backup by copying.) Any number of hard links to the physical data may be created. To access the data, a user only needs to specify the name of any existing link; the operating system will resolve the location of the actual data. Even if the user deletes one of the hard links, the data is still accessible through any other link that remains. Once the user deletes all of the links, if no process has the file open, the operating system frees the disk space that the file once occupied.

Most file systems that support hard links use reference counting. The system stores an integer value with each logical data section that represents the total number of hard links that have been created to point to the data. When a new link is created, this value is increased by one. When a link is removed, the value is decreased by one. When the counter becomes zero, the operating system frees the logical data section. (The OS may not to do so immediately, e. g., when there are outstanding file handles open, for performance reasons, or to enable the undelete command.) This is a simple method for the file system to track the use of a given area of storage, as zero values indicate free space and nonzero values indicate used space. The maintenance of this value guarantees that there will be no dangling hard links pointing nowhere. The data section and the associated inode are preserved as long as a single hard link (directory reference) points to it or any process keeps the associated file open. On POSIX compliant operating systems, the reference count for a file or directory is returned by the stat or fstat system calls in the st nlink field of struct stat.

To prevent loops in the filesystem, and to keep the interpretation of the "" file (parent directory) consistent, operating systems do not generally allow hard links to directories. UNIX System V allowed them, but only the superuser had permission to make such links. Mac OS X v10.5 (Leopard) and newer use hard links on directories for the Time Machine backup mechanism only. Hard links can be created to files only on the same volume, i. e., within the same file system. (Different volumes may have different file systems. There is no guarantee that the target volume's file system is compatible with hard linking.) The maximum number of hard links to a single file is limited by the size of the reference counter. On Unix like systems the counter is 4,294,967,295 (on 32 bit machines) or 18,446,744,073,709,551,615 (on 64 bit machines). In some file systems, the number of hard links is limited more strictly by their on disk format. For example, as of Linux 3.11, the ext4 file system limits the number of hard links on a file to 65,000. Windows limits enforces a limit of 1024 hard links to a file on NTFS volumes. On Linux Weekly News, Neil Brown criticized hard links as high maintenance, since they complicate the design of programs that handle directory trees, including archivers and disk usage tools. These apps must take care to de duplicate files that are linked multiple times in a hierarchy. Brown notes that Plan 9 from Bell Labs, the intended successor to Unix, does not include the concept of a hard link.