1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 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.\" 3. 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. 15.\" 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 26.\" SUCH DAMAGE. 27.\" 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.\" 31.Dd November 3, 1995 32.Dt LISTEN 2 33.Os 34.Sh NAME 35.Nm listen 36.Nd listen for connections on a socket 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/types.h 41.In sys/socket.h 42.Ft int 43.Fn listen "int s" "int backlog" 44.Sh DESCRIPTION 45To accept connections, a socket 46is first created with 47.Xr socket 2 , 48a willingness to accept incoming connections and 49a queue limit for incoming connections are specified with 50.Fn listen , 51and then the connections are 52accepted with 53.Xr accept 2 . 54The 55.Fn listen 56call applies only to sockets of type 57.Dv SOCK_STREAM 58or 59.Dv SOCK_SEQPACKET . 60.Pp 61The 62.Fa backlog 63parameter defines the maximum length the queue of 64pending connections may grow to. 65If a connection 66request arrives with the queue full the client may 67receive an error with an indication of 68.Er ECONNREFUSED , 69or, in the case of TCP, the connection will be 70silently dropped. 71.Pp 72Note that before 73.Fx 4.5 74and the introduction of the syncache, the 75.Fa backlog 76parameter also determined the length of the incomplete 77connection queue, which held TCP sockets in the process 78of completing TCP's 3-way handshake. 79These incomplete connections 80are now held entirely in the syncache, which is unaffected by 81queue lengths. 82Inflated 83.Fa backlog 84values to help handle denial 85of service attacks are no longer necessary. 86.Pp 87The 88.Xr sysctl 3 89MIB variable 90.Dq Va kern.ipc.somaxconn 91specifies a hard limit on 92.Fa backlog ; 93if a value greater than 94.Va kern.ipc.somaxconn 95or less than zero is specified, 96.Fa backlog 97is silently forced to 98.Va kern.ipc.somaxconn . 99.Sh INTERACTION WITH ACCEPT FILTERS 100When accept filtering is used on a socket, a second queue will 101be used to hold sockets that have connected, but have not yet 102met their accept filtering criteria. 103Once the criteria has been 104met, these sockets will be moved over into the completed connection 105queue to be 106.Fn accept Ns ed. 107If this secondary queue is full and a 108new connection comes in, the oldest socket which has not yet met 109its accept filter criteria will be terminated. 110.Pp 111This secondary queue, like the primary listen queue, is sized 112according to the 113.Fa backlog 114parameter. 115.Sh RETURN VALUES 116.Rv -std listen 117.Sh ERRORS 118.Fn Listen 119will fail if: 120.Bl -tag -width Er 121.It Bq Er EBADF 122The argument 123.Fa s 124is not a valid descriptor. 125.It Bq Er ENOTSOCK 126The argument 127.Fa s 128is not a socket. 129.It Bq Er EOPNOTSUPP 130The socket is not of a type that supports the operation 131.Fn listen . 132.El 133.Sh SEE ALSO 134.Xr accept 2 , 135.Xr connect 2 , 136.Xr socket 2 , 137.Xr sysctl 3 , 138.Xr sysctl 8 , 139.Xr accept_filter 9 140.Sh HISTORY 141The 142.Fn listen 143function call appeared in 144.Bx 4.2 . 145The ability to configure the maximum 146.Fa backlog 147at run-time, and to use a negative 148.Fa backlog 149to request the maximum allowable value, was introduced in 150.Fx 2.2 . 151