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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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LINK(2) Linux Programmer's Manual LINK(2)
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".
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 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().
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.
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)
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)
Pages that refer to this page: fcntl(2), open(2), rename(2), symlink(2), syscalls(2), unlink(2), remove(3), capabilities(7), inode(7), inotify(7), signal-safety(7), symlink(7)
Copyright and license for this manual page
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