### 14.4 Hard Links

In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.

To add a name to a file, use the `link` function. (The new name is also called a hard link to the file.) Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file's existing name or names.

One file can have names in several directories, so the organization of the file system is not a strict hierarchy or tree.

In most implementations, it is not possible to have hard links to the same file in multiple file systems. `link` reports an error if you try to make a hard link to the file from another file system when this cannot be done.

The prototype for the `link` function is declared in the header file `unistd.h`.

— Function: int link (const char *oldname, const char *newname)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> The `link` function makes a new link to the existing file named by oldname, under the new name newname.

> This function returns a value of `0` if it is successful and `-1` on failure. In addition to the usual file name errors (see File Name Errors) for both oldname and newname, the following `errno` error conditions are defined for this function:

> `EACCES`
>   You are not allowed to write to the directory in which the new link is to be written.
> `EEXIST`
>   There is already a file named newname. If you want to replace this link with a new link, you must remove the old link explicitly first.
> `EMLINK`
>   There are already too many links to the file named by oldname. (The maximum number of links to a file is `LINK_MAX`; see Limits for Files.)
> `ENOENT`
>   The file named by oldname doesn't exist. You can't make a link to a file that doesn't exist.
> `ENOSPC`
>   The directory or file system that would contain the new link is full and cannot be extended.
> `EPERM`
>   On GNU/Linux and GNU/Hurd systems and some others, you cannot make links to directories. Many systems allow only privileged users to do so. This error is used to report the problem.
> `EROFS`
>   The directory containing the new link can't be modified because it's on a read-only file system.
> `EXDEV`
>   The directory specified in newname is on a different file system than the existing file.
> `EIO`
>   A hardware error occurred while trying to read or write the to filesystem.