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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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link(2) System Calls Manual link(2)
link, linkat - make a new name for a file
Standard C library (libc, -lc)
#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 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().
On success, zero is returned. On error, -1 is returned, and
errno is set to indicate the error.
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.
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.
link() POSIX.1-2008.
link() SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).
linkat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
Hard links, as created by link(), cannot span filesystems. Use
symlink(2) if this is required.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-06-13 link(2)
Pages that refer to this page: link(1), ln(1), sshfs(1), fcntl(2), io_uring_enter2(2), io_uring_enter(2), open(2), rename(2), symlink(2), syscalls(2), unlink(2), io_uring_prep_link(3), io_uring_prep_linkat(3), remove(3), capabilities(7), inode(7), inotify(7), signal-safety(7), symlink(7), mount(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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