link, linkat - make a new name for a file
int link(const char *oldpath, const char *newpath);
#include <fcntl.h> /* Definition of AT_* constants */
int linkat(int olddirfd, const char *oldpath,
int newdirfd, const char *newpath, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
() creates a new link (also known as a hard link) to an existing
exists, it will not
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".
() system call operates in exactly the same way as
(), 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).
is relative and olddirfd
is the special value
, then oldpath
is interpreted relative to the current
working directory of the calling process (like link
is absolute, then olddirfd
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
- 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,
Before kernel 2.6.18, the flags
argument was unused, and had to be
specified as 0.
(2) for an explanation of the need for linkat
On success, zero is returned. On error, -1 is returned, and errno
- 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
- The user's quota of disk blocks on the filesystem has been exhausted.
- newpath already exists.
- oldpath or newpath points outside your accessible address
- An I/O error occurred.
- Too many symbolic links were encountered in resolving oldpath or
- 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
- 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.
- 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
- oldpath is marked immutable or append-only. (See
- 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 mount points, even if the
same filesystem is mounted on both.)
The following additional errors can occur for linkat
- olddirfd or newdirfd is not 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).
- 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
- AT_EMPTY_PATH was specified in flags, oldpath is an
empty string, and olddirfd refers to a directory.
() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
Hard links, as created by link
(), cannot span filesystems. Use
(2) if this is required.
POSIX.1-2001 says that link
() should dereference oldpath
if it is
a symbolic link. However, since kernel 2.0, Linux does not do so: if
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
is dereferenced if it is a symbolic link. For precise
control over the treatment of symbolic links when creating a link, use
On older kernels where linkat
() is unavailable, the glibc wrapper
function falls back to the use of link
(), unless the
is specified. When oldpath
are relative pathnames, glibc constructs pathnames based on the symbolic links
that correspond to the olddirfd
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.