link, linkat - make a new name for a file
#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:
- 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 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().
On success, zero is returned. On error, -1 is returned, and
errno is set
appropriately.
- 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
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 different 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.
linkat() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
link(): SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
linkat(): POSIX.1-2008.
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. However, 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 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.
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)