320 lines
7.6 KiB
Groff
320 lines
7.6 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)link.2 8.3 (Berkeley) 1/12/94
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.\" $FreeBSD$
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.\"
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.Dd March 30, 2021
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.Dt LINK 2
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.Os
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.Sh NAME
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.Nm link ,
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.Nm linkat
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.Nd make a hard file link
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn link "const char *name1" "const char *name2"
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.Ft int
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.Fo linkat
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.Fa "int fd1" "const char *name1" "int fd2" "const char *name2" "int flag"
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.Fc
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.Sh DESCRIPTION
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The
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.Fn link
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system call
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atomically creates the specified directory entry (hard link)
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.Fa name2
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with the attributes of the underlying object pointed at by
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.Fa name1 .
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If the link is successful: the link count of the underlying object
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is incremented;
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.Fa name1
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and
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.Fa name2
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share equal access and rights
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to the
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underlying object.
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.Pp
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If
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.Fa name1
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is removed, the file
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.Fa name2
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is not deleted and the link count of the
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underlying object is
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decremented.
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.Pp
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The object pointed at by the
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.Fa name1
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argument
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must exist for the hard link to
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succeed and
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both
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.Fa name1
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and
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.Fa name2
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must be in the same file system.
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The
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.Fa name1
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argument
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may not be a directory.
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.Pp
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The
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.Fn linkat
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system call is equivalent to
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.Fa link
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except in the case where either
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.Fa name1
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or
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.Fa name2
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or both are relative paths.
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In this case a relative path
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.Fa name1
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is interpreted relative to
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the directory associated with the file descriptor
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.Fa fd1
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instead of the current working directory and similarly for
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.Fa name2
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and the file descriptor
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.Fa fd2 .
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.Pp
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Values for
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.Fa flag
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are constructed by a bitwise-inclusive OR of flags from the following
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list, defined in
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.In fcntl.h :
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.Bl -tag -width indent
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.It Dv AT_SYMLINK_FOLLOW
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If
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.Fa name1
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names a symbolic link, a new link for the target of the symbolic link is
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created.
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.It Dv AT_RESOLVE_BENEATH
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Only walk paths below the directory specified by the
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.Ar fd1
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descriptor.
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See the description of the
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.Dv O_RESOLVE_BENEATH
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flag in the
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.Xr open 2
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manual page.
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.It Dv AT_EMPTY_PATH
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If the
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.Fa name1
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argument is an empty string, link the file referenced by the descriptor
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.Fa fd1 .
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The operation requires that the calling process has the
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.Dv PRIV_VFS_FHOPEN
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privilege, effectively being executed with effective user
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.Dv root .
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.El
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.Pp
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If
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.Fn linkat
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is passed the special value
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.Dv AT_FDCWD
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in the
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.Fa fd1
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or
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.Fa fd2
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parameter, the current working directory is used for the respective
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.Fa name
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argument.
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If both
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.Fa fd1
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and
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.Fa fd2
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have value
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.Dv AT_FDCWD ,
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the behavior is identical to a call to
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.Fn link .
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Unless
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.Fa flag
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contains the
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.Dv AT_SYMLINK_FOLLOW
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flag, if
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.Fa name1
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names a symbolic link, a new link is created for the symbolic link
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.Fa name1
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and not its target.
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.Sh RETURN VALUES
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.Rv -std link
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.Sh ERRORS
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The
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.Fn link
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system call
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will fail and no link will be created if:
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.Bl -tag -width Er
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.It Bq Er ENOTDIR
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A component of either path prefix is not a directory.
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.It Bq Er ENAMETOOLONG
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A component of either pathname exceeded 255 characters,
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or entire length of either path name exceeded 1023 characters.
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.It Bq Er ENOENT
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A component of either path prefix does not exist.
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.It Bq Er EOPNOTSUPP
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The file system containing the file named by
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.Fa name1
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does not support links.
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.It Bq Er EMLINK
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The link count of the file named by
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.Fa name1
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would exceed 32767.
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.It Bq Er EACCES
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A component of either path prefix denies search permission.
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.It Bq Er EACCES
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The requested link requires writing in a directory with a mode
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that denies write permission.
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.It Bq Er ELOOP
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Too many symbolic links were encountered in translating one of the pathnames.
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.It Bq Er ENOENT
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The file named by
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.Fa name1
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does not exist.
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.It Bq Er EEXIST
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The link named by
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.Fa name2
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does exist.
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.It Bq Er EPERM
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The file named by
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.Fa name1
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is a directory.
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.It Bq Er EPERM
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The file named by
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.Fa name1
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has its immutable or append-only flag set, see the
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.Xr chflags 2
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manual page for more information.
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.It Bq Er EPERM
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The parent directory of the file named by
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.Fa name2
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has its immutable flag set.
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.It Bq Er EXDEV
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The link named by
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.Fa name2
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and the file named by
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.Fa name1
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are on different file systems.
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.It Bq Er ENOSPC
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The directory in which the entry for the new link is being placed
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cannot be extended because there is no space left on the file
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system containing the directory.
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.It Bq Er EDQUOT
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The directory in which the entry for the new link
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is being placed cannot be extended because the
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user's quota of disk blocks on the file system
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containing the directory has been exhausted.
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.It Bq Er EIO
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An I/O error occurred while reading from or writing to
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the file system to make the directory entry.
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.It Bq Er EINTEGRITY
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Corrupted data was detected while reading from the file system.
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.It Bq Er EROFS
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The requested link requires writing in a directory on a read-only file
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system.
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.It Bq Er EFAULT
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One of the pathnames specified
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is outside the process's allocated address space.
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.El
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.Pp
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In addition to the errors returned by the
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.Fn link ,
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the
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.Fn linkat
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system call may fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa name1
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or
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.Fa name2
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argument does not specify an absolute path and the
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.Fa fd1
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or
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.Fa fd2
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argument, respectively, is neither
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.Dv AT_FDCWD
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nor a valid file descriptor open for searching.
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.It Bq Er EINVAL
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The value of the
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.Fa flag
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argument is not valid.
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.It Bq Er ENOTDIR
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The
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.Fa name1
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or
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.Fa name2
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argument is not an absolute path and
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.Fa fd1
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or
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.Fa fd2 ,
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respectively, is neither
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.Dv AT_FDCWD
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nor a file descriptor associated with a directory.
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.It Bq Er ENOTCAPABLE
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.Fa name1
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is not strictly relative to the starting directory.
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For example,
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.Fa name1
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is absolute or includes a ".." component that escapes
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the directory hierarchy specified by
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.Fa fd ,
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and the process is in capability mode or the
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.Dv AT_RESOLVE_BENEATH
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flag was specified.
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.El
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.Sh SEE ALSO
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.Xr chflags 2 ,
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.Xr readlink 2 ,
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.Xr symlink 2 ,
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.Xr unlink 2
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.Sh STANDARDS
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The
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.Fn link
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system call is expected to conform to
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.St -p1003.1-90 .
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The
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.Fn linkat
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system call follows The Open Group Extended API Set 2 specification.
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.Sh HISTORY
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The
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.Fn link
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function appeared in
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.At v1 .
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The
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.Fn linkat
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system call appeared in
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.Fx 8.0 .
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.Pp
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The
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.Fn link
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system call traditionally allows the super-user to link directories which
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corrupts the file system coherency.
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This implementation no longer permits it.
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