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.\" Copyright (c) 1983, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
.\"
.\" Modified Fri Jul 23 22:07:54 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified 950727 by aeb, following a suggestion by Urs Thuermann
.\" <urs@isnogud.escape.de>
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998 by Andi Kleen
.\"
.TH LISTEN 2 "23 July 1993" "BSD Man Page" "Linux Programmer's Manual"
.SH NAME
listen \- listen for connections on a socket
.SH SYNOPSIS
.B #include <sys/socket.h>
.sp
.BI "int listen(int " s ", int " backlog );
.SH DESCRIPTION
To accept connections, a socket is first created with
.BR socket (2),
a willingness to accept incoming connections and a queue limit for incoming
connections are specified with
.BR listen ,
and then the connections are
accepted with
.BR accept (2).
The
.B listen
call applies only to sockets of type
.B SOCK_STREAM
or
.BR SOCK_SEQPACKET .
.PP
The
.I backlog
parameter defines the maximum length the queue of pending connections may
grow to. If a connection request arrives with the queue full the client
may receive an error with an indication of
.B ECONNREFUSED
or, if the underlying protocol supports retransmission, the request may be
ignored so that retries succeed.
.SH NOTES
The behaviour of the
.I backlog
parameter on TCP sockets changed with Linux 2.2.
Now it specifies the queue length for
.I completely
established sockets waiting to be accepted, instead of the number of incomplete
connection requests. The maximum length of the queue for incomplete sockets
can be set using the
.B tcp_max_syn_backlog
sysctl.
When syncookies are enabled there is no logical maximum
length and this sysctl setting is ignored.
See
.BR tcp (7)
for more information.
.SH "RETURN VALUE"
On success, zero is returned. On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EBADF
The argument
.I s
is not a valid descriptor.
.TP
.B ENOTSOCK
The argument
.I s
is not a socket.
.TP
.B EOPNOTSUPP
The socket is not of a type that supports the
.B listen
operation.
.SH "CONFORMING TO"
Single Unix, 4.4BSD, POSIX 1003.1g draft. The
.B listen
function call first appeared in 4.2BSD.
.SH BUGS
If the socket is of type
.BR AF_INET ,
and the
.I backlog
argument is greater
than the constant
.B SOMAXCONN
(128 in Linux 2.0 & 2.2), it is silently truncated
to
.BR SOMAXCONN .
Don't rely on this value in portable applications since BSD
(and some BSD-derived systems) limit the backlog to 5.
.SH "SEE ALSO"
.BR accept (2),
.BR connect (2),
.BR socket (2)
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