link(2)

NAME
link, linkat - make a new name for a file

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#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

DESCRIPTION
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".

  linkat()
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:

AT_EMPTY_PATH (since Linux 2.6.39)
    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.

AT_SYMLINK_FOLLOW (since Linux 2.6.18)
    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().

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

ERRORS

EACCES
    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).)

EDQUOT
    The user's quota of disk blocks on the filesystem has been exhausted.

EEXIST
    newpath already exists.

EFAULT
    oldpath or newpath points outside your accessible address space.

EIO An I/O error occurred.

ELOOP
    Too many symbolic links were encountered in resolving oldpath or newpath.

EMLINK
    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.

ENAMETOOLONG
    oldpath or newpath was too long.

ENOENT
    A directory component in oldpath or newpath does not exist or is a dangling symbolic link.

ENOMEM
    Insufficient kernel memory was available.

ENOSPC
    The device containing the file has no room for the new directory entry.

ENOTDIR
    A component used as a directory in oldpath or newpath is not, in fact, a directory.

EPERM
    oldpath is a directory.

EPERM
    The filesystem containing oldpath and newpath does not support the creation of hard links.

EPERM (since Linux 3.6)
    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)).

EPERM
    oldpath is marked immutable or append-only. (See FS_IOC_SETFLAGS(2const).)

EROFS
    The file is on a read-only filesystem.

EXDEV
    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():

EBADF
    oldpath (newpath) is relative but olddirfd (newdirfd) is neither AT_FDCWD nor a valid file descriptor.

EINVAL
    An invalid flag value was specified in flags.

ENOENT
    AT_EMPTY_PATH was specified in flags, but the caller did not have the CAP_DAC_READ_SEARCH capability.

ENOENT
    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).

ENOENT
    An attempt was made to link to a /proc/self/fd/NN file corresponding to a file that has been deleted.

ENOENT
    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.

ENOTDIR
    oldpath is relative and olddirfd is a file descriptor referring to a file other than a directory; or similar for newpath and newdirfd

EPERM
    AT_EMPTY_PATH was specified in flags, oldpath is an empty string, and olddirfd refers to a directory.

VERSIONS
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().

  glibc
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.

STANDARDS

link()
    POSIX.1-2008.

HISTORY

link()
    SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).

linkat()
    POSIX.1-2008. Linux 2.6.16, glibc 2.4.

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

BUGS
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.

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