255 lines
5.9 KiB
HTML
255 lines
5.9 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 LISTEN</TITLE>
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</HEAD><BODY>
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<H1>LISTEN</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2020-02-09<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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listen - listen for connections on a socket
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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/sys/types.h">sys/types.h</A>></B> /* See NOTES */
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<B>#include <<A HREF="file:///usr/include/sys/socket.h">sys/socket.h</A>></B>
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<B>int listen(int </B><I>sockfd</I><B>, int </B><I>backlog</I><B>);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>listen</B>()
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marks the socket referred to by
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<I>sockfd</I>
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as a passive socket, that is, as a socket that will
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be used to accept incoming connection requests using
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<B><A HREF="/cgi-bin/man/man2html?2+accept">accept</A></B>(2).
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<P>
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The
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<I>sockfd</I>
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argument is a file descriptor that refers to a socket of type
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<B>SOCK_STREAM</B>
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or
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<B>SOCK_SEQPACKET</B>.
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<P>
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The
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<I>backlog</I>
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argument defines the maximum length
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to which the queue of pending connections for
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<I>sockfd</I>
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may grow.
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If a connection request arrives when the queue is full, the client
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may receive an error with an indication of
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<B>ECONNREFUSED</B>
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or, if the underlying protocol supports retransmission, the request may be
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ignored so that a later reattempt at connection succeeds.
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<A NAME="lbAE"> </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="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="1"><B>EADDRINUSE</B>
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<DD>
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Another socket is already listening on the same port.
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<DT id="2"><B>EADDRINUSE</B>
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<DD>
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(Internet domain sockets)
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The socket referred to by
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<I>sockfd</I>
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had not previously been bound to an address and,
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upon attempting to bind it to an ephemeral port,
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it was determined that all port numbers in the ephemeral port range
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are currently in use.
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See the discussion of
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<I>/proc/sys/net/ipv4/ip_local_port_range</I>
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in
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<B><A HREF="/cgi-bin/man/man2html?7+ip">ip</A></B>(7).
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<DT id="3"><B>EBADF</B>
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<DD>
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The argument
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<I>sockfd</I>
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is not a valid file descriptor.
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<DT id="4"><B>ENOTSOCK</B>
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<DD>
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The file descriptor
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<I>sockfd</I>
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does not refer to a socket.
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<DT id="5"><B>EOPNOTSUPP</B>
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<DD>
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The socket is not of a type that supports the
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<B>listen</B>()
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operation.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008, 4.4BSD
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(<B>listen</B>()
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first appeared in 4.2BSD).
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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To accept connections, the following steps are performed:
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<DL COMPACT><DT id="6"><DD>
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<DL COMPACT>
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<DT id="7">1.<DD>
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A socket is created with
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<B><A HREF="/cgi-bin/man/man2html?2+socket">socket</A></B>(2).
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<DT id="8">2.<DD>
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The socket is bound to a local address using
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<B><A HREF="/cgi-bin/man/man2html?2+bind">bind</A></B>(2),
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so that other sockets may be
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<B><A HREF="/cgi-bin/man/man2html?2+connect">connect</A></B>(2)ed
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to it.
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<DT id="9">3.<DD>
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A willingness to accept incoming connections and a queue limit for incoming
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connections are specified with
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<B>listen</B>().
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<DT id="10">4.<DD>
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Connections are accepted with
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<B><A HREF="/cgi-bin/man/man2html?2+accept">accept</A></B>(2).
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</DL>
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</DL>
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<P>
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POSIX.1 does not require the inclusion of
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<I><<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></I>,
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and this header file is not required on Linux.
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However, some historical (BSD) implementations required this header
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file, and portable applications are probably wise to include it.
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<P>
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The behavior of the
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<I>backlog</I>
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argument on TCP sockets changed with Linux 2.2.
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Now it specifies the queue length for
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<I>completely</I>
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established sockets waiting to be accepted,
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instead of the number of incomplete connection requests.
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The maximum length of the queue for incomplete sockets
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can be set using
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<I>/proc/sys/net/ipv4/tcp_max_syn_backlog</I>.
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When syncookies are enabled there is no logical maximum
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length and this setting is ignored.
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See
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<B><A HREF="/cgi-bin/man/man2html?7+tcp">tcp</A></B>(7)
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for more information.
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<P>
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If the
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<I>backlog</I>
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argument is greater than the value in
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<I>/proc/sys/net/core/somaxconn</I>,
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then it is silently truncated to that value.
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Since Linux 5.4, the default in this file is 4096;
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in earlier kernels, the default value is 128.
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In kernels before 2.4.25, this limit was a hard coded value,
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<B>SOMAXCONN</B>,
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with the value 128.
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<A NAME="lbAI"> </A>
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<H2>EXAMPLE</H2>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+bind">bind</A></B>(2).
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+accept">accept</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+bind">bind</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+connect">connect</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+socket">socket</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+socket">socket</A></B>(7)
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<A NAME="lbAK"> </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="11"><A HREF="#lbAB">NAME</A><DD>
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<DT id="12"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="13"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="14"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="15"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="16"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="17"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="18"><A HREF="#lbAI">EXAMPLE</A><DD>
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<DT id="19"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="20"><A HREF="#lbAK">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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