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.

Function: int linkat (int oldfd, const char *oldname, int newfd, const char *newname, int flags)

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

The linkat function is analogous to the link function, except that it identifies its source and target using a combination of a file descriptor (referring to a directory) and a pathname. If a pathnames is not absolute, it is resolved relative to the corresponding file descriptor. The special file descriptor AT_FDCWD denotes the current directory.

The flags argument is a combination of the following flags:

AT_SYMLINK_FOLLOW

If the source path identified by oldfd and oldname is a symbolic link, linkat follows the symbolic link and creates a link to its target. If the flag is not set, a link for the symbolic link itself is created; this is not supported by all file systems and linkat can fail in this case.

AT_EMPTY_PATH

If this flag is specified, oldname can be an empty string. In this case, a new link to the file denoted by the descriptor oldfd is created, which may have been opened with O_PATH or O_TMPFILE. This flag is a GNU extension.