Hard link
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In computing, a hard link is a reference, or pointer, to the physical data on a volume. On most file systems, all named files are hard links. The name associated with the file is simply a label that refers the operating system to the actual data. As such, more than one name can be associated with the same data. Though called by different names, any changes made will affect the actual data, regardless of how the file is called at a later time. Hard links can only refer to data that exists on the same file system.
The process of unlinking disassociates a name from the data on the volume. The data is still accessible as long as at least one link that points to it still exists. When the last link is removed, the space is considered free. A process ambiguously called undeleting allows the recreation of links to data that is no longer associated with a name. However, this process is not available on all systems and is often not reliable.
Link counter
On most file systems that support hard links, an integral value is stored with each physical data section. This integer represents the total number of 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. The maintenance of this value assists users in preventing data loss. It is also the simplest way 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.
Example
Hard_Link_Illustration.png
In the figure to the right, there are two hard links named "LINK A.TXT" and "LINK B.TXT". They have both been linked—that is, made to point—to the same physical data.
If the filename "LINK A.TXT" is opened in an editor, modified and saved, then those changes will be visible even if the filename "LINK B.TXT" is opened for viewing since both filenames point to the same data. The same is true if the file were opened as "LINK B.TXT"—or any other name associated with the data.
Additional links can also be created to the physical data. The user need only specify the name of an existing link; the operating system will resolve the location of the actual data section.
If one of the links is removed, then the data is still accessible under any other links that remain. If all of the links are removed, then the space occupied by the data will be considered free. Some or all of that space could be used in the future for other files.
See also
- Symbolic link
- Ln (Unix)—The ln command, which is used to create new links on Unix-like systems.de:Harter Link