mirror of https://github.com/mkerrisk/man-pages
436 lines
10 KiB
Groff
436 lines
10 KiB
Groff
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
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.\" and Copyright (C) 2006, 2014 Michael Kerrisk
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 1994-08-21 by Michael Haardt
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.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 2005-04-04, as per suggestion by Michael Hardt for rename.2
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.\"
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.TH LINK 2 2020-12-21 "Linux" "Linux Programmer's Manual"
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.SH NAME
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link, linkat \- make a new name for a file
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "int link(const char *" oldpath ", const char *" newpath );
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.PP
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.BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
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.B #include <unistd.h>
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.PP
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.BI "int linkat(int " olddirfd ", const char *" oldpath ,
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.BI " int " newdirfd ", const char *" newpath ", int " flags );
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.fi
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR linkat ():
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.nf
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Since glibc 2.10:
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_POSIX_C_SOURCE\ >=\ 200809L
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Before glibc 2.10:
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_ATFILE_SOURCE
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.fi
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.SH DESCRIPTION
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.BR link ()
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creates a new link (also known as a hard link) to an existing file.
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.PP
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If
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.I newpath
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exists, it will
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.I not
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be overwritten.
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.PP
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This new name may be used exactly as the old one for any operation;
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both names refer to the same file (and so have the same permissions
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and ownership) and it is impossible to tell which name was the
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"original".
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.SS linkat()
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The
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.BR linkat ()
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system call operates in exactly the same way as
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.BR link (),
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except for the differences described here.
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.PP
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If the pathname given in
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.I oldpath
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is relative, then it is interpreted relative to the directory
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referred to by the file descriptor
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.I olddirfd
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(rather than relative to the current working directory of
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the calling process, as is done by
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.BR link ()
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for a relative pathname).
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.PP
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If
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.I oldpath
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is relative and
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.I olddirfd
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is the special value
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.BR AT_FDCWD ,
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then
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.I oldpath
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is interpreted relative to the current working
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directory of the calling process (like
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.BR link ()).
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.PP
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If
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.I oldpath
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is absolute, then
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.I olddirfd
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is ignored.
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.PP
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The interpretation of
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.I newpath
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is as for
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.IR oldpath ,
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except that a relative pathname is interpreted relative
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to the directory referred to by the file descriptor
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.IR newdirfd .
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.PP
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The following values can be bitwise ORed in
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.IR flags :
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.TP
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.BR AT_EMPTY_PATH " (since Linux 2.6.39)"
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.\" commit 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3
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If
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.I oldpath
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is an empty string, create a link to the file referenced by
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.IR olddirfd
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(which may have been obtained using the
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.BR open (2)
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.B O_PATH
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flag).
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In this case,
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.I olddirfd
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can refer to any type of file except a directory.
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This will generally not work if the file has a link count of zero (files
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created with
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.BR O_TMPFILE
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and without
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.BR O_EXCL
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are an exception).
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The caller must have the
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.BR CAP_DAC_READ_SEARCH
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capability in order to use this flag.
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This flag is Linux-specific; define
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.B _GNU_SOURCE
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.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed
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to obtain its definition.
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.TP
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.BR AT_SYMLINK_FOLLOW " (since Linux 2.6.18)"
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By default,
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.BR linkat (),
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does not dereference
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.I oldpath
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if it is a symbolic link (like
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.BR link ()).
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The flag
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.B AT_SYMLINK_FOLLOW
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can be specified in
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.I flags
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to cause
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.I oldpath
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to be dereferenced if it is a symbolic link.
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If procfs is mounted,
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this can be used as an alternative to
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.BR AT_EMPTY_PATH ,
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like this:
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.IP
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.in +4n
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.EX
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linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
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newname, AT_SYMLINK_FOLLOW);
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.EE
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.in
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.PP
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Before kernel 2.6.18, the
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.I flags
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argument was unused, and had to be specified as 0.
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.PP
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See
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.BR openat (2)
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for an explanation of the need for
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.BR linkat ().
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.SH RETURN VALUE
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On success, zero is returned.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EACCES
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Write access to the directory containing
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.I newpath
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is denied, or search permission is denied for one of the directories
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in the path prefix of
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.I oldpath
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or
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.IR newpath .
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(See also
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.BR path_resolution (7).)
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.TP
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.B EDQUOT
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The user's quota of disk blocks on the filesystem has been exhausted.
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.TP
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.B EEXIST
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.I newpath
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already exists.
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.TP
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.B EFAULT
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.IR oldpath " or " newpath " points outside your accessible address space."
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.TP
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.B EIO
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An I/O error occurred.
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.TP
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.B ELOOP
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Too many symbolic links were encountered in resolving
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.IR oldpath " or " newpath .
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.TP
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.B EMLINK
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The file referred to by
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.I oldpath
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already has the maximum number of links to it.
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For example, on an
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.BR ext4 (5)
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filesystem that does not employ the
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.I dir_index
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feature, the limit on the number of hard links to a file is 65,000; on
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.BR btrfs (5),
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the limit is 65,535 links.
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.TP
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.B ENAMETOOLONG
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.IR oldpath " or " newpath " was too long."
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.TP
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.B ENOENT
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A directory component in
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.IR oldpath " or " newpath
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does not exist or is a dangling symbolic link.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.TP
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.B ENOSPC
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The device containing the file has no room for the new directory
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entry.
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.TP
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.B ENOTDIR
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A component used as a directory in
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.IR oldpath " or " newpath
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is not, in fact, a directory.
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.TP
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.B EPERM
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.I oldpath
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is a directory.
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.TP
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.B EPERM
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The filesystem containing
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.IR oldpath " and " newpath
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does not support the creation of hard links.
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.TP
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.BR EPERM " (since Linux 3.6)"
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The caller does not have permission to create a hard link to this file
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(see the description of
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.IR /proc/sys/fs/protected_hardlinks
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in
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.BR proc (5)).
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.TP
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.B EPERM
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.I oldpath
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is marked immutable or append-only.
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(See
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.BR ioctl_iflags (2).)
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.TP
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.B EROFS
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The file is on a read-only filesystem.
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.TP
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.B EXDEV
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.IR oldpath " and " newpath
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are not on the same mounted filesystem.
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(Linux permits a filesystem to be mounted at multiple points, but
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.BR link ()
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does not work across different mount points,
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even if the same filesystem is mounted on both.)
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.PP
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The following additional errors can occur for
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.BR linkat ():
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.TP
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.B EBADF
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.I olddirfd
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or
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.I newdirfd
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is not a valid file descriptor.
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.TP
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.B EINVAL
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An invalid flag value was specified in
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.IR flags .
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.TP
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.B ENOENT
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.B AT_EMPTY_PATH
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was specified in
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.IR flags ,
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but the caller did not have the
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.B CAP_DAC_READ_SEARCH
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capability.
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.TP
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.B ENOENT
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An attempt was made to link to the
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.I /proc/self/fd/NN
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file corresponding to a file descriptor created with
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.IP
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.in +4n
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.EX
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open(path, O_TMPFILE | O_EXCL, mode);
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.EE
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.in
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.IP
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See
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.BR open (2).
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.TP
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.B ENOENT
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An attempt was made to link to a
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.I /proc/self/fd/NN
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file corresponding to a file that has been deleted.
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.TP
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.B ENOENT
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.I oldpath
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is a relative pathname and
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.I olddirfd
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refers to a directory that has been deleted,
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or
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.I newpath
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is a relative pathname and
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.I newdirfd
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refers to a directory that has been deleted.
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.TP
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.B ENOTDIR
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.I oldpath
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is relative and
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.I olddirfd
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is a file descriptor referring to a file other than a directory;
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or similar for
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.I newpath
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and
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.I newdirfd
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.TP
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.B EPERM
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.BR AT_EMPTY_PATH
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was specified in
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.IR flags ,
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.I oldpath
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is an empty string, and
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.IR olddirfd
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refers to a directory.
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.SH VERSIONS
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.BR linkat ()
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was added to Linux in kernel 2.6.16;
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library support was added to glibc in version 2.4.
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.SH CONFORMING TO
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.BR link ():
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SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
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.\" SVr4 documents additional ENOLINK and
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.\" EMULTIHOP error conditions; POSIX.1 does not document ELOOP.
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.\" X/OPEN does not document EFAULT, ENOMEM or EIO.
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.PP
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.BR linkat ():
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POSIX.1-2008.
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.SH NOTES
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Hard links, as created by
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.BR link (),
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cannot span filesystems.
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Use
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.BR symlink (2)
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if this is required.
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.PP
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POSIX.1-2001 says that
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.BR link ()
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should dereference
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.I oldpath
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if it is a symbolic link.
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However, since kernel 2.0,
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.\" more precisely: since kernel 1.3.56
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Linux does not do so: if
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.I oldpath
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is a symbolic link, then
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.I newpath
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is created as a (hard) link to the same symbolic link file
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(i.e.,
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.I newpath
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becomes a symbolic link to the same file that
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.I oldpath
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refers to).
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Some other implementations behave in the same manner as Linux.
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.\" For example, the default Solaris compilation environment
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.\" behaves like Linux, and contributors to a March 2005
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.\" thread in the Austin mailing list reported that some
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.\" other (System V) implementations did/do the same -- MTK, Apr 05
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POSIX.1-2008 changes the specification of
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.BR link (),
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making it implementation-dependent whether or not
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.I oldpath
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is dereferenced if it is a symbolic link.
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For precise control over the treatment of symbolic links when
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creating a link, use
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.BR linkat ().
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.SS Glibc notes
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On older kernels where
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.BR linkat ()
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is unavailable, the glibc wrapper function falls back to the use of
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.BR link (),
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unless the
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.B AT_SYMLINK_FOLLOW
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is specified.
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When
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.I oldpath
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and
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.I newpath
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are relative pathnames,
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glibc constructs pathnames based on the symbolic links in
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.IR /proc/self/fd
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that correspond to the
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.I olddirfd
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and
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.IR newdirfd
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arguments.
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.SH BUGS
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On NFS filesystems, the return code may be wrong in case the NFS server
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performs the link creation and dies before it can say so.
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Use
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.BR stat (2)
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to find out if the link got created.
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.SH SEE ALSO
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.BR ln (1),
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.BR open (2),
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.BR rename (2),
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.BR stat (2),
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.BR symlink (2),
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.BR unlink (2),
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.BR path_resolution (7),
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.BR symlink (7)
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