LINK(2) Linux Programmer's Manual LINK(2)
NAME
link, linkat - make a new name for a file
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 dif-
ferences 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 inter-
preted 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 inter-
preted 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, old-
dirfd 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 kernel 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 appropriately.
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 sym-
bolic 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 ioctl_iflags(2).)
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 dif-
ferent mount points, even if the same filesystem is mounted on both.)
The following additional errors can occur for linkat():
EBADF olddirfd or newdirfd is not 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 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
linkat() was added to Linux in kernel 2.6.16; library support was added to glibc in ver-
sion 2.4.
CONFORMING TO
link(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
linkat(): POSIX.1-2008.
NOTES
Hard links, as created by link(), cannot span filesystems. Use symlink(2) if this is
required.
POSIX.1-2001 says that link() should dereference oldpath if it is a symbolic link. How-
ever, since kernel 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 sym-
bolic 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 notes
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.
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)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 LINK(2)
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