link(2) - Linux manual page (original) (raw)
link(2) System Calls Manual link(2)
NAME top
link, linkat - make a new name for a fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_SOURCEDESCRIPTION top
**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)](../man2/open.2.html) **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)](../man2/openat.2.html) for an explanation of the need for **linkat**().RETURN VALUE top
On success, zero is returned. On error, -1 is returned, and _[errno](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
**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)](../man7/path%5Fresolution.7.html).)
**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)](../man5/ext4.5.html)
filesystem that does not employ the _dirindex_ 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/protectedhardlinks_ in [proc(5)](../man5/proc.5.html)).
**EPERM** _oldpath_ is marked immutable or append-only. (See
[FS_IOC_SETFLAGS(2const)](../man2/FS%5FIOC%5FSETFLAGS.2const.html).)
**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)](../man2/open.2.html).
**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.VERSIONS top
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.
STANDARDS top
**link**() POSIX.1-2008.HISTORY top
**link**() SVr4, 4.3BSD, POSIX.1-2001 (but see VERSIONS).
**linkat**()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.NOTES top
Hard links, as created by **link**(), cannot span filesystems. Use
[symlink(2)](../man2/symlink.2.html) if this is required.BUGS top
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)](../man2/stat.2.html) to find out if the link got created.SEE ALSO top
[ln(1)](../man1/ln.1.html), [open(2)](../man2/open.2.html), [rename(2)](../man2/rename.2.html), [stat(2)](../man2/stat.2.html), [symlink(2)](../man2/symlink.2.html), [unlink(2)](../man2/unlink.2.html),
[path_resolution(7)](../man7/path%5Fresolution.7.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)