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LINK(2) System Calls Manual LINK(2)

link — make a hard file link

library “libc”

#include <unistd.h>

int
link(const char *name1, const char *name2);

#include <fcntl.h>

int
linkat(int fd1, const char *name1, int fd2, const char *name2, int flags);

The () function call atomically creates the specified directory entry (hard link) name2 with the attributes of the underlying object pointed at by name1. If the link is successful: the link count of the underlying object is incremented; name1 and name2 share equal access and rights to the underlying object.

If name1 is removed, the file name2 is not deleted and the link count of the underlying object is decremented.

name1 must exist for the hard link to succeed and both name1 and name2 must be in the same file system. name1 may not be a directory unless the caller is the super-user and the file system containing it supports linking to directories.

() works the same way as link() except if name1 (resp. name2) is relative. In that case, it is looked up from a directory whose file descriptor was passed as fd1 (resp. fd2). Search permission is required on the directories named by fd1 and fd2. fd1 or fd2 can be set to AT_FDCWD in order to specify the current directory.

When operating on a symlink, () resolves the symlink and creates a hard link on the target. linkat() will do the same if AT_SYMLINK_FOLLOW is set in flags, but it will link on the symlink itself if the flag is clear.

The link() and linkat() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

link() and linkat() will fail and no link will be created if:

[]
A component of either path prefix denies search permission, or the requested link requires writing in a directory with a mode that denies write permission.
[]
The directory in which the entry for the new link is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted.
[]
The link named by name2 does exist.
[]
One of the pathnames specified is outside the process's allocated address space.
[]
An I/O error occurred while reading from or writing to the file system to make the directory entry.
[]
Too many symbolic links were encountered in translating one of the pathnames.
The link count of the file named by name1 would exceed {LINK_MAX}.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
A component of either path prefix does not exist, or the file named by name1 does not exist.
[]
The directory in which the entry for the new link is being placed cannot be extended because there is no space left on the file system containing the directory.
[]
A component of either path prefix is not a directory.
[]
The file system containing the file named by name1 does not support links.
[]
The file named by name1 is a directory and the effective user ID is not super-user, or the file system containing the file does not permit the use of link() on a directory.
[]
The requested link requires writing in a directory on a read-only file system.
[]
The link named by name2 and the file named by name1 are on different file systems.

In addition, linkat() will fail if:

[]
name1 or name2 does not specify an absolute path and fd1 or fd2, respectively, is neither AT_FDCWD nor a valid file descriptor open for reading or searching.
[]
name1 or name2 is not an absolute path and fd1 or fd2, respectively, is a file descriptor associated with a non-directory file.

symlink(2), unlink(2)

The link() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”). linkat() conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The link() function appeared in Version 1 AT&T UNIX.

September 1, 2019 NetBSD-9.2