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)):
- Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
link() creates a new link (also known as a hard link) to an existing file.
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).
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()).
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
The following values can be bitwise ORed in
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.
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.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
Write access to the directory containing
newpath is denied, or search permission is denied for one of the directories in the path prefix of
newpath. (See also path_resolution(7).)
The user's quota of disk blocks on the filesystem has been exhausted.
newpath already exists.
newpath points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in resolving
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.
newpath was too long.
A directory component in
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 entry.
A component used as a directory in
newpath is not, in fact, a directory.
oldpath is a directory.
The filesystem containing
newpath does not support the creation of hard links.
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)).
oldpath is marked immutable or append-only. (See ioctl_iflags(2).)
The file is on a read-only filesystem.
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():
newdirfd is not a valid file descriptor.
An invalid flag value was specified in
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);
An attempt was made to link to a
/proc/self/fd/NN file corresponding to a file that has been deleted.
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
AT_EMPTY_PATH was specified in
oldpath is an empty string, and
olddirfd refers to a directory.
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
newpath are relative pathnames, glibc constructs pathnames based on the symbolic links in
/proc/self/fd that correspond to the
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.
This page is part of release 5.10 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/.