639 lines
12 KiB
HTML
639 lines
12 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of LINK</TITLE>
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</HEAD><BODY>
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<H1>LINK</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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link, linkat - make a new name for a file
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/unistd.h">unistd.h</A>></B>
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<B>int link(const char *</B><I>oldpath</I><B>, const char *</B><I>newpath</I><B>);</B>
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<B>#include <<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>> </B>/* Definition of AT_* constants */
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<B>#include <<A HREF="file:///usr/include/unistd.h">unistd.h</A>></B>
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<B>int linkat(int </B><I>olddirfd</I><B>, const char *</B><I>oldpath</I><B>,</B>
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<B> int </B><I>newdirfd</I><B>, const char *</B><I>newpath</I><B>, int </B><I>flags</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>linkat</B>():
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<DL COMPACT><DT id="1"><DD>
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<DL COMPACT>
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<DT id="2">Since glibc 2.10:<DD>
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_POSIX_C_SOURCE >= 200809L
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<DT id="3">Before glibc 2.10:<DD>
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_ATFILE_SOURCE
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</DL>
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>link</B>()
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creates a new link (also known as a hard link) to an existing file.
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<P>
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If
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<I>newpath</I>
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exists, it will
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<I>not</I>
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be overwritten.
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<P>
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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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<A NAME="lbAE"> </A>
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<H3>linkat()</H3>
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The
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<B>linkat</B>()
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system call operates in exactly the same way as
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<B>link</B>(),
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except for the differences described here.
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<P>
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If the pathname given in
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<I>oldpath</I>
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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</I>
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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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<B>link</B>()
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for a relative pathname).
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<P>
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If
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<I>oldpath</I>
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is relative and
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<I>olddirfd</I>
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is the special value
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<B>AT_FDCWD</B>,
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then
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<I>oldpath</I>
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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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<B>link</B>()).
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<P>
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If
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<I>oldpath</I>
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is absolute, then
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<I>olddirfd</I>
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is ignored.
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<P>
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The interpretation of
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<I>newpath</I>
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is as for
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<I>oldpath</I>,
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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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<I>newdirfd</I>.
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<P>
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The following values can be bitwise ORed in
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<I>flags</I>:
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<DL COMPACT>
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<DT id="4"><B>AT_EMPTY_PATH</B> (since Linux 2.6.39)
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<DD>
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If
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<I>oldpath</I>
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is an empty string, create a link to the file referenced by
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<I>olddirfd</I>
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(which may have been obtained using the
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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<B>O_PATH</B>
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flag).
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In this case,
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<I>olddirfd</I>
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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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<B>O_TMPFILE</B>
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and without
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<B>O_EXCL</B>
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are an exception).
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The caller must have the
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<B>CAP_DAC_READ_SEARCH</B>
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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</B>
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to obtain its definition.
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<DT id="5"><B>AT_SYMLINK_FOLLOW</B> (since Linux 2.6.18)
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<DD>
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By default,
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<B>linkat</B>(),
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does not dereference
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<I>oldpath</I>
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if it is a symbolic link (like
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<B>link</B>()).
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The flag
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<B>AT_SYMLINK_FOLLOW</B>
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can be specified in
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<I>flags</I>
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to cause
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<I>oldpath</I>
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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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<B>AT_EMPTY_PATH</B>,
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like this:
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<DT id="6"><DD>
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linkat(AT_FDCWD, "/proc/self/fd/<fd>", newdirfd,
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<BR> newname, AT_SYMLINK_FOLLOW);
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</DL>
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<P>
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Before kernel 2.6.18, the
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<I>flags</I>
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argument was unused, and had to be specified as 0.
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<P>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+openat">openat</A></B>(2)
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for an explanation of the need for
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<B>linkat</B>().
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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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</I>
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is set appropriately.
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<A NAME="lbAG"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="7"><B>EACCES</B>
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<DD>
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Write access to the directory containing
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<I>newpath</I>
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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</I>
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or
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<I>newpath</I>.
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(See also
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<B><A HREF="/cgi-bin/man/man2html?7+path_resolution">path_resolution</A></B>(7).)
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<DT id="8"><B>EDQUOT</B>
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<DD>
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The user's quota of disk blocks on the filesystem has been exhausted.
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<DT id="9"><B>EEXIST</B>
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<DD>
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<I>newpath</I>
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already exists.
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<DT id="10"><B>EFAULT</B>
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<DD>
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<I>oldpath</I> or <I>newpath</I> points outside your accessible address space.
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<DT id="11"><B>EIO</B>
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<DD>
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An I/O error occurred.
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<DT id="12"><B>ELOOP</B>
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<DD>
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Too many symbolic links were encountered in resolving
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<I>oldpath</I> or <I>newpath</I>.
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<DT id="13"><B>EMLINK</B>
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<DD>
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The file referred to by
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<I>oldpath</I>
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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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<B><A HREF="/cgi-bin/man/man2html?5+ext4">ext4</A></B>(5)
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filesystem that does not employ the
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<I>dir_index</I>
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feature, the limit on the number of hard links to a file is 65,000; on
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<B><A HREF="/cgi-bin/man/man2html?5+btrfs">btrfs</A></B>(5),
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the limit is 65,535 links.
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<DT id="14"><B>ENAMETOOLONG</B>
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<DD>
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<I>oldpath</I> or <I>newpath</I> was too long.
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<DT id="15"><B>ENOENT</B>
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<DD>
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A directory component in
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<I>oldpath</I> or <I>newpath</I>
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does not exist or is a dangling symbolic link.
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<DT id="16"><B>ENOMEM</B>
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<DD>
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Insufficient kernel memory was available.
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<DT id="17"><B>ENOSPC</B>
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<DD>
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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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<DT id="18"><B>ENOTDIR</B>
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<DD>
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A component used as a directory in
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<I>oldpath</I> or <I>newpath</I>
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is not, in fact, a directory.
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<DT id="19"><B>EPERM</B>
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<DD>
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<I>oldpath</I>
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is a directory.
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<DT id="20"><B>EPERM</B>
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<DD>
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The filesystem containing
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<I>oldpath</I> and <I>newpath</I>
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does not support the creation of hard links.
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<DT id="21"><B>EPERM</B> (since Linux 3.6)
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<DD>
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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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<I>/proc/sys/fs/protected_hardlinks</I>
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in
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<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5)).
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<DT id="22"><B>EPERM</B>
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<DD>
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<I>oldpath</I>
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is marked immutable or append-only.
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(See
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<B><A HREF="/cgi-bin/man/man2html?2+ioctl_iflags">ioctl_iflags</A></B>(2).)
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<DT id="23"><B>EROFS</B>
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<DD>
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The file is on a read-only filesystem.
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<DT id="24"><B>EXDEV</B>
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<DD>
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<I>oldpath</I> and <I>newpath</I>
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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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<B>link</B>()
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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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</DL>
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<P>
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The following additional errors can occur for
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<B>linkat</B>():
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<DL COMPACT>
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<DT id="25"><B>EBADF</B>
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<DD>
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<I>olddirfd</I>
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or
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<I>newdirfd</I>
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is not a valid file descriptor.
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<DT id="26"><B>EINVAL</B>
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<DD>
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An invalid flag value was specified in
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<I>flags</I>.
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<DT id="27"><B>ENOENT</B>
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<DD>
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<B>AT_EMPTY_PATH</B>
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was specified in
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<I>flags</I>,
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but the caller did not have the
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<B>CAP_DAC_READ_SEARCH</B>
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capability.
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<DT id="28"><B>ENOENT</B>
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<DD>
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An attempt was made to link to the
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<I>/proc/self/fd/NN</I>
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file corresponding to a file descriptor created with
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<DT id="29"><DD>
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<BR> open(path, O_TMPFILE | O_EXCL, mode);
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<DT id="30"><DD>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).
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<DT id="31"><B>ENOENT</B>
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<DD>
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<I>oldpath</I>
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is a relative pathname and
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<I>olddirfd</I>
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refers to a directory that has been deleted,
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or
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<I>newpath</I>
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is a relative pathname and
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<I>newdirfd</I>
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refers to a directory that has been deleted.
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<DT id="32"><B>ENOTDIR</B>
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<DD>
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<I>oldpath</I>
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is relative and
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<I>olddirfd</I>
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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</I>
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and
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<I>newdirfd</I>
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<DT id="33"><B>EPERM</B>
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<DD>
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<B>AT_EMPTY_PATH</B>
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was specified in
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<I>flags</I>,
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<I>oldpath</I>
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is an empty string, and
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<I>olddirfd</I>
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refers to a directory.
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</DL>
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<A NAME="lbAH"> </A>
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<H2>VERSIONS</H2>
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<B>linkat</B>()
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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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<A NAME="lbAI"> </A>
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<H2>CONFORMING TO</H2>
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<B>link</B>():
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SVr4, 4.3BSD, POSIX.1-2001 (but see NOTES), POSIX.1-2008.
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<P>
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<B>linkat</B>():
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POSIX.1-2008.
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<A NAME="lbAJ"> </A>
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<H2>NOTES</H2>
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Hard links, as created by
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<B>link</B>(),
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cannot span filesystems.
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Use
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<B><A HREF="/cgi-bin/man/man2html?2+symlink">symlink</A></B>(2)
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if this is required.
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<P>
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POSIX.1-2001 says that
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<B>link</B>()
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should dereference
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<I>oldpath</I>
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if it is a symbolic link.
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However, since kernel 2.0,
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Linux does not do so: if
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<I>oldpath</I>
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is a symbolic link, then
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<I>newpath</I>
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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</I>
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becomes a symbolic link to the same file that
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<I>oldpath</I>
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refers to).
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Some other implementations behave in the same manner as Linux.
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POSIX.1-2008 changes the specification of
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<B>link</B>(),
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making it implementation-dependent whether or not
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<I>oldpath</I>
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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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<B>linkat</B>().
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<A NAME="lbAK"> </A>
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<H3>Glibc notes</H3>
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On older kernels where
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<B>linkat</B>()
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is unavailable, the glibc wrapper function falls back to the use of
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<B>link</B>(),
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unless the
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<B>AT_SYMLINK_FOLLOW</B>
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is specified.
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When
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<I>oldpath</I>
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and
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<I>newpath</I>
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are relative pathnames,
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glibc constructs pathnames based on the symbolic links in
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<I>/proc/self/fd</I>
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that correspond to the
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<I>olddirfd</I>
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and
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<I>newdirfd</I>
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arguments.
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<A NAME="lbAL"> </A>
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<H2>BUGS</H2>
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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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<B><A HREF="/cgi-bin/man/man2html?2+stat">stat</A></B>(2)
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to find out if the link got created.
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<A NAME="lbAM"> </A>
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<H2>SEE ALSO</H2>
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|
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<B><A HREF="/cgi-bin/man/man2html?1+ln">ln</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+rename">rename</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+stat">stat</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+symlink">symlink</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+unlink">unlink</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+path_resolution">path_resolution</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+symlink">symlink</A></B>(7)
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<A NAME="lbAN"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="34"><A HREF="#lbAB">NAME</A><DD>
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<DT id="35"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="36"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="37"><A HREF="#lbAE">linkat()</A><DD>
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</DL>
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<DT id="38"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DT id="39"><A HREF="#lbAG">ERRORS</A><DD>
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<DT id="40"><A HREF="#lbAH">VERSIONS</A><DD>
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<DT id="41"><A HREF="#lbAI">CONFORMING TO</A><DD>
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<DT id="42"><A HREF="#lbAJ">NOTES</A><DD>
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<DL>
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<DT id="43"><A HREF="#lbAK">Glibc notes</A><DD>
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</DL>
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<DT id="44"><A HREF="#lbAL">BUGS</A><DD>
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<DT id="45"><A HREF="#lbAM">SEE ALSO</A><DD>
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<DT id="46"><A HREF="#lbAN">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:33 GMT, March 31, 2021
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</BODY>
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</HTML>
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