1 .\" Copyright (c) 1983, 1991, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
7 .\" 1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\" 2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\" 4. Neither the name of the University nor the names of its contributors
13 .\" may be used to endorse or promote products derived from this software
14 .\" without specific prior written permission.
16 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
17 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
20 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 .\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
29 .\" $FreeBSD: src/lib/libc/sys/listen.2,v 1.12.2.9 2002/05/09 02:24:40 silby Exp $
30 .\" $DragonFly: src/lib/libc/sys/listen.2,v 1.5 2007/05/12 21:22:10 swildner Exp $
37 .Nd listen for connections on a socket
44 .Fn listen "int s" "int backlog"
46 To accept connections, a socket
49 a willingness to accept incoming connections and
50 a queue limit for incoming connections are specified with
52 and then the connections are
57 call applies only to sockets of type
64 parameter defines the maximum length the queue of
65 pending connections may grow to.
67 request arrives with the queue full the client may
68 receive an error with an indication of
70 or, in the case of TCP, the connection will be
75 and the introduction of the syncache, the
77 parameter also determined the length of the incomplete
78 connection queue, which held TCP sockets in the process
79 of completing TCP's 3-way handshake. These incomplete connections
80 are now held entirely in the syncache, which is unaffected by
81 queue lengths. Inflated
83 values to help handle denial
84 of service attacks are no longer necessary.
89 .Dq Va kern.ipc.somaxconn
90 specifies a hard limit on
92 if a value greater than
93 .Va kern.ipc.somaxconn
94 or less than zero is specified,
97 .Va kern.ipc.somaxconn .
98 .Sh INTERACTION WITH ACCEPT FILTERS
99 When accept filtering is used on a socket, a second queue will
100 be used to hold sockets that have connected, but have not yet
101 met their accept filtering criteria. Once the criteria has been
102 met, these sockets will be moved over into the completed connection
103 queue to be accept()ed. If this secondary queue is full and a
104 new connection comes in, the oldest socket which has not yet met
105 its accept filter criteria will be terminated.
107 This secondary queue, like the primary listen queue, is sized
120 is not a valid descriptor.
126 The socket is not of a type that supports the operation
139 function call appeared in
141 The ability to configure the maximum
143 at run-time, and to use a negative
145 to request the maximum allowable value, was introduced in