lib/libsys/link.2

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.Dd March 30, 2021
.Dt LINK 2
.Os
.Sh NAME
.Nm link ,
.Nm linkat
.Nd make a hard file link
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn link "const char *name1" "const char *name2"
.Ft int
.Fo linkat
.Fa "int fd1" "const char *name1" "int fd2" "const char *name2" "int flag"
.Fc
.Sh DESCRIPTION
The
.Fn link
system call
atomically creates the specified directory entry (hard link)
.Fa name2
with the attributes of the underlying object pointed at by
.Fa name1 .
If the link is successful: the link count of the underlying object
is incremented;
.Fa name1
and
.Fa name2
share equal access and rights
to the
underlying object.
.Pp
If
.Fa name1
is removed, the file
.Fa name2
is not deleted and the link count of the
underlying object is
decremented.
.Pp
The object pointed at by the
.Fa name1
argument
must exist for the hard link to
succeed and
both
.Fa name1
and
.Fa name2
must be in the same file system.
The
.Fa name1
argument
may not be a directory.
.Pp
The
.Fn linkat
system call is equivalent to
.Fa link
except in the case where either
.Fa name1
or
.Fa name2
or both are relative paths.
In this case a relative path
.Fa name1
is interpreted relative to
the directory associated with the file descriptor
.Fa fd1
instead of the current working directory and similarly for
.Fa name2
and the file descriptor
.Fa fd2 .
.Pp
Values for
.Fa flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.In fcntl.h :
.Bl -tag -width indent
.It Dv AT_SYMLINK_FOLLOW
If
.Fa name1
names a symbolic link, a new link for the target of the symbolic link is
created.
.It Dv AT_RESOLVE_BENEATH
Only walk paths below the directory specified by the
.Ar fd1
descriptor.
See the description of the
.Dv O_RESOLVE_BENEATH
flag in the
.Xr open 2
manual page.
.It Dv AT_EMPTY_PATH
If the
.Fa name1
argument is an empty string, link the file referenced by the descriptor
.Fa fd1 .
The operation requires that the calling process has the
.Dv PRIV_VFS_FHOPEN
privilege, effectively being executed with effective user
.Dv root .
.El
.Pp
If
.Fn linkat
is passed the special value
.Dv AT_FDCWD
in the
.Fa fd1
or
.Fa fd2
parameter, the current working directory is used for the respective
.Fa name
argument.
If both
.Fa fd1
and
.Fa fd2
have value
.Dv AT_FDCWD ,
the behavior is identical to a call to
.Fn link .
Unless
.Fa flag
contains the
.Dv AT_SYMLINK_FOLLOW
flag, if
.Fa name1
names a symbolic link, a new link is created for the symbolic link
.Fa name1
and not its target.
.Sh RETURN VALUES
.Rv -std link
.Sh ERRORS
The
.Fn link
system call
will fail and no link will be created if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of either path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of either pathname exceeded 255 characters,
or entire length of either path name exceeded 1023 characters.
.It Bq Er ENOENT
A component of either path prefix does not exist.
.It Bq Er EOPNOTSUPP
The file system containing the file named by
.Fa name1
does not support links.
.It Bq Er EMLINK
The link count of the file named by
.Fa name1
would exceed 32767.
.It Bq Er EACCES
A component of either path prefix denies search permission.
.It Bq Er EACCES
The requested link requires writing in a directory with a mode
that denies write permission.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating one of the pathnames.
.It Bq Er ENOENT
The file named by
.Fa name1
does not exist.
.It Bq Er EEXIST
The link named by
.Fa name2
does exist.
.It Bq Er EPERM
The file named by
.Fa name1
is a directory.
.It Bq Er EPERM
The file named by
.Fa name1
has its immutable or append-only flag set, see the
.Xr chflags 2
manual page for more information.
.It Bq Er EPERM
The parent directory of the file named by
.Fa name2
has its immutable flag set.
.It Bq Er EXDEV
The link named by
.Fa name2
and the file named by
.Fa name1
are on different file systems.
.It Bq Er 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.
.It Bq Er 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.
.It Bq Er EIO
An I/O error occurred while reading from or writing to
the file system to make the directory entry.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er EROFS
The requested link requires writing in a directory on a read-only file
system.
.It Bq Er EFAULT
One of the pathnames specified
is outside the process's allocated address space.
.El
.Pp
In addition to the errors returned by the
.Fn link ,
the
.Fn linkat
system call may fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa name1
or
.Fa name2
argument does not specify an absolute path and the
.Fa fd1
or
.Fa fd2
argument, respectively, is neither
.Dv AT_FDCWD
nor a valid file descriptor open for searching.
.It Bq Er EINVAL
The value of the
.Fa flag
argument is not valid.
.It Bq Er ENOTDIR
The
.Fa name1
or
.Fa name2
argument is not an absolute path and
.Fa fd1
or
.Fa fd2 ,
respectively, is neither
.Dv AT_FDCWD
nor a file descriptor associated with a directory.
.It Bq Er ENOTCAPABLE
.Fa name1
is not strictly relative to the starting directory.
For example,
.Fa name1
is absolute or includes a ".." component that escapes
the directory hierarchy specified by
.Fa fd ,
and the process is in capability mode or the
.Dv AT_RESOLVE_BENEATH
flag was specified.
.El
.Sh SEE ALSO
.Xr chflags 2 ,
.Xr readlink 2 ,
.Xr symlink 2 ,
.Xr unlink 2
.Sh STANDARDS
The
.Fn link
system call is expected to conform to
.St -p1003.1-90 .
The
.Fn linkat
system call follows The Open Group Extended API Set 2 specification.
.Sh HISTORY
The
.Fn link
function appeared in
.At v1 .
The
.Fn linkat
system call appeared in
.Fx 8.0 .
.Pp
The
.Fn link
system call traditionally allows the super-user to link directories which
corrupts the file system coherency.
This implementation no longer permits it.