link(2) System Calls Manual link(2)

link, linkat - make a new name for a file

Standard C library (libc, -lc)

#include <unistd.h>
int link(const char *oldpath, const char *newpath);
#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>
int linkat(int olddirfd, const char *oldpath,
           int newdirfd, const char *newpath, int flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

linkat():

    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _ATFILE_SOURCE

link() creates a new link (also known as a hard link) to an existing file.

If newpath exists, it will not be overwritten.

This new name may be used exactly as the old one for any operation; both names refer to the same file (and so have the same permissions and ownership) and it is impossible to tell which name was the "original".

The linkat() system call operates in exactly the same way as link(), except for the differences described here.

If the pathname given in oldpath is relative, then it is interpreted relative to the directory referred to by the file descriptor olddirfd (rather than relative to the current working directory of the calling process, as is done by link() for a relative pathname).

If oldpath is relative and olddirfd is the special value AT_FDCWD, then oldpath is interpreted relative to the current working directory of the calling process (like link()).

If oldpath is absolute, then olddirfd is ignored.

The interpretation of newpath is as for oldpath, except that a relative pathname is interpreted relative to the directory referred to by the file descriptor newdirfd.

The following values can be bitwise ORed in flags:

If oldpath is an empty string, create a link to the file referenced by olddirfd (which may have been obtained using the open(2) O_PATH flag). In this case, olddirfd can refer to any type of file except a directory. This will generally not work if the file has a link count of zero (files created with O_TMPFILE and without O_EXCL are an exception). The caller must have the CAP_DAC_READ_SEARCH capability in order to use this flag. This flag is Linux-specific; define _GNU_SOURCE to obtain its definition.
By default, linkat(), does not dereference oldpath if it is a symbolic link (like link()). The flag AT_SYMLINK_FOLLOW can be specified in flags to cause oldpath to be dereferenced if it is a symbolic link. If procfs is mounted, this can be used as an alternative to AT_EMPTY_PATH, like this:

linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
       newname, AT_SYMLINK_FOLLOW);

Before Linux 2.6.18, the flags argument was unused, and had to be specified as 0.

See openat(2) for an explanation of the need for linkat().

On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

Write access to the directory containing newpath is denied, or search permission is denied for one of the directories in the path prefix of oldpath or newpath. (See also path_resolution(7).)
The user's quota of disk blocks on the filesystem has been exhausted.
newpath already exists.
oldpath or newpath points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in resolving oldpath or newpath.
The file referred to by oldpath already has the maximum number of links to it. For example, on an ext4(5) filesystem that does not employ the dir_index feature, the limit on the number of hard links to a file is 65,000; on btrfs(5), the limit is 65,535 links.
oldpath or newpath was too long.
A directory component in oldpath or newpath does not exist or is a dangling symbolic link.
Insufficient kernel memory was available.
The device containing the file has no room for the new directory entry.
A component used as a directory in oldpath or newpath is not, in fact, a directory.
oldpath is a directory.
The filesystem containing oldpath and newpath does not support the creation of hard links.
The caller does not have permission to create a hard link to this file (see the description of /proc/sys/fs/protected_hardlinks in proc(5)).
oldpath is marked immutable or append-only. (See ioctl_iflags(2).)
The file is on a read-only filesystem.
oldpath and newpath are not on the same mounted filesystem. (Linux permits a filesystem to be mounted at multiple points, but link() does not work across different mounts, even if the same filesystem is mounted on both.)

The following additional errors can occur for linkat():

oldpath (newpath) is relative but olddirfd (newdirfd) is neither AT_FDCWD nor a valid file descriptor.
An invalid flag value was specified in flags.
AT_EMPTY_PATH was specified in flags, but the caller did not have the CAP_DAC_READ_SEARCH capability.
An attempt was made to link to the /proc/self/fd/NN file corresponding to a file descriptor created with

open(path, O_TMPFILE | O_EXCL, mode);

See open(2).
An attempt was made to link to a /proc/self/fd/NN file corresponding to a file that has been deleted.
oldpath is a relative pathname and olddirfd refers to a directory that has been deleted, or newpath is a relative pathname and newdirfd refers to a directory that has been deleted.
oldpath is relative and olddirfd is a file descriptor referring to a file other than a directory; or similar for newpath and newdirfd
AT_EMPTY_PATH was specified in flags, oldpath is an empty string, and olddirfd refers to a directory.

POSIX.1-2001 says that link() should dereference oldpath if it is a symbolic link. However, since Linux 2.0, Linux does not do so: if oldpath is a symbolic link, then newpath is created as a (hard) link to the same symbolic link file (i.e., newpath becomes a symbolic link to the same file that oldpath refers to). Some other implementations behave in the same manner as Linux. POSIX.1-2008 changes the specification of link(), making it implementation-dependent whether or not oldpath is dereferenced if it is a symbolic link. For precise control over the treatment of symbolic links when creating a link, use linkat().

On older kernels where linkat() is unavailable, the glibc wrapper function falls back to the use of link(), unless the AT_SYMLINK_FOLLOW is specified. When oldpath and newpath are relative pathnames, glibc constructs pathnames based on the symbolic links in /proc/self/fd that correspond to the olddirfd and newdirfd arguments.

POSIX.1-2008.

SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).
POSIX.1-2008. Linux 2.6.16, glibc 2.4.

Hard links, as created by link(), cannot span filesystems. Use symlink(2) if this is required.

On NFS filesystems, the return code may be wrong in case the NFS server performs the link creation and dies before it can say so. Use stat(2) to find out if the link got created.

ln(1), open(2), rename(2), stat(2), symlink(2), unlink(2), path_resolution(7), symlink(7)

2023-10-31 Linux man-pages 6.7