1.\" $OpenBSD: listen.2,v 1.11 2010/04/14 22:43:34 millert Exp $ 2.\" $NetBSD: listen.2,v 1.7 1996/02/16 20:38:45 phil Exp $ 3.\" 4.\" Copyright (c) 1983, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)listen.2 8.2 (Berkeley) 12/11/93 32.\" 33.Dd $Mdocdate: April 14 2010 $ 34.Dt LISTEN 2 35.Os 36.Sh NAME 37.Nm listen 38.Nd listen for connections on a socket 39.Sh SYNOPSIS 40.Fd #include <sys/types.h> 41.Fd #include <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, if the underlying protocol supports retransmission, 70the request may be ignored so that retries may succeed. 71.Sh RETURN VALUES 72A 0 return value indicates success; \-1 indicates an error. 73.Sh ERRORS 74.Fn listen 75will fail if: 76.Bl -tag -width Er 77.It Bq Er EBADF 78The argument 79.Fa s 80is not a valid descriptor. 81.It Bq Er ENOTSOCK 82The argument 83.Fa s 84is not a socket. 85.It Bq Er EOPNOTSUPP 86The socket is not of a type that supports the operation 87.Fn listen . 88.El 89.Sh SEE ALSO 90.Xr accept 2 , 91.Xr connect 2 , 92.Xr socket 2 , 93.Xr sysctl 8 94.Sh HISTORY 95The 96.Fn listen 97function call appeared in 98.Bx 4.2 . 99.Sh BUGS 100The 101.Fa backlog 102is currently limited (silently) to the value of the kern.somaxconn 103sysctl, which defaults to 128. 104