link(2) System Calls Manual link(2)
NAME
link, linkat - make a new name for a file
LIBRARY
Standard C library (libc, -lc)
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 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 refer-
enced 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_TMP-
FILE 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 spec-
ified 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 to indicate the error.
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_resolu-
tion(7).)
EDQUOT The user's quota of disk blocks on the filesystem has been ex-
hausted.
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/pro-
tected_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 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 corre-
sponding 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 corre-
sponding to a file that has been deleted.
ENOENT oldpath is a relative pathname and olddirfd refers to a direc-
tory 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
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 old-
path 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 be-
have in the same manner as Linux. POSIX.1-2008 changes the specifica-
tion of link(), making it implementation-dependent whether or not old-
path 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 func-
tion falls back to the use of link(), unless the AT_SYMLINK_FOLLOW is
specified. When oldpath and newpath are relative pathnames, glibc con-
structs pathnames based on the symbolic links in /proc/self/fd that
correspond to the olddirfd and newdirfd arguments.
STANDARDS
link() POSIX.1-2008.
HISTORY
link() SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).
linkat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
NOTES
Hard links, as created by link(), cannot span filesystems. Use sym-
link(2) if this is required.
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_resolu-
tion(7), symlink(7)
Linux man-pages 6.7 2023-10-31 link(2)
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