link
, linkat
—
make a hard file link
Standard C Library (libc, -lc)
#include <unistd.h>
int
link
(const
char *name1, const char
*name2);
int
linkat
(int fd1,
const char *name1, int fd2,
const char *name2, int
flag);
The link
() system call atomically creates the specified
directory entry (hard link) name2 with the attributes of
the underlying object pointed at by name1. If the link
is successful: the link count of the underlying object is incremented;
name1 and name2 share equal access
and rights to the underlying object.
If name1 is removed, the file
name2 is not deleted and the link count of the
underlying object is decremented.
The object pointed at by the name1 argument
must exist for the hard link to succeed and both name1
and name2 must be in the same file system. The
name1 argument may not be a directory.
The linkat
() system call is equivalent to
link except in the case where either
name1 or name2 or both are
relative paths. In this case a relative path name1 is
interpreted relative to the directory associated with the file descriptor
fd1 instead of the current working directory and
similarly for name2 and the file descriptor
fd2.
Values for flag are constructed by a
bitwise-inclusive OR of flags from the following list, defined in
<fcntl.h>
:
AT_SYMLINK_FOLLOW
- If name1 names a symbolic link, a new link for the
target of the symbolic link is created.
AT_RESOLVE_BENEATH
- Only walk paths below the directory specified by the
fd1 descriptor. See the description of the
O_RESOLVE_BENEATH
flag in the
open(2)
manual page.
AT_EMPTY_PATH
- If the name1 argument is an empty string, link the
file referenced by the descriptor fd1. The operation
requires that the calling process has the
PRIV_VFS_FHOPEN
privilege, effectively being
executed with effective user root
.
If linkat
() is passed the special value
AT_FDCWD
in the fd1 or
fd2 parameter, the current working directory is used
for the respective name argument. If both
fd1 and fd2 have value
AT_FDCWD
, the behavior is identical to a call to
link
(). Unless flag contains
the AT_SYMLINK_FOLLOW
flag, if
name1 names a symbolic link, a new link is created for
the symbolic link name1 and not its target.
The link
() function returns the value 0 if
successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
The link
() system call will fail and no link will be
created if:
- [
ENOTDIR
]
- A component of either path prefix is not a directory.
- [
ENAMETOOLONG
]
- A component of either pathname exceeded 255 characters, or entire length
of either path name exceeded 1023 characters.
- [
ENOENT
]
- A component of either path prefix does not exist.
- [
EOPNOTSUPP
]
- The file system containing the file named by name1
does not support links.
- [
EMLINK
]
- The link count of the file named by name1 would
exceed 32767.
- [
EACCES
]
- A component of either path prefix denies search permission.
- [
EACCES
]
- The requested link requires writing in a directory with a mode that denies
write permission.
- [
ELOOP
]
- Too many symbolic links were encountered in translating one of the
pathnames.
- [
ENOENT
]
- The file named by name1 does not exist.
- [
EEXIST
]
- The link named by name2 does exist.
- [
EPERM
]
- The file named by name1 is a directory.
- [
EPERM
]
- The file named by name1 has its immutable or
append-only flag set, see the
chflags(2)
manual page for more information.
- [
EPERM
]
- The parent directory of the file named by name2 has
its immutable flag set.
- [
EXDEV
]
- The link named by name2 and the file named by
name1 are on different file systems.
- [
ENOSPC
]
- The directory in which the entry for the new link is being placed cannot
be extended because there is no space left on the file system containing
the directory.
- [
EDQUOT
]
- The directory in which the entry for the new link is being placed cannot
be extended because the user's quota of disk blocks on the file system
containing the directory has been exhausted.
- [
EIO
]
- An I/O error occurred while reading from or writing to the file system to
make the directory entry.
- [
EINTEGRITY
]
- Corrupted data was detected while reading from the file system.
- [
EROFS
]
- The requested link requires writing in a directory on a read-only file
system.
- [
EFAULT
]
- One of the pathnames specified is outside the process's allocated address
space.
In addition to the errors returned by the
link
(), the linkat
() system
call may fail if:
- [
EBADF
]
- The name1 or name2 argument
does not specify an absolute path and the fd1 or
fd2 argument, respectively, is neither
AT_FDCWD
nor a valid file descriptor open for
searching.
- [
EINVAL
]
- The value of the flag argument is not valid.
- [
ENOTDIR
]
- The name1 or name2 argument is
not an absolute path and fd1 or
fd2, respectively, is neither
AT_FDCWD
nor a file descriptor associated with a
directory.
- [
ENOTCAPABLE
]
- name1 is not strictly relative to the starting
directory. For example, name1 is absolute or
includes a ".." component that escapes the directory hierarchy
specified by fd, and the process is in capability
mode or the
AT_RESOLVE_BENEATH
flag was
specified.
The link
() system call is expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”). The
linkat
() system call follows The Open Group Extended
API Set 2 specification.
The link
() function appeared in
Version 1 AT&T UNIX. The
linkat
() system call appeared in
FreeBSD 8.0.
The link
() system call traditionally
allows the super-user to link directories which corrupts the file system
coherency. This implementation no longer permits it.