.\" Copyright (c) 1983, 1991 The Regents of the University of California. .\" and Copyright (C) 2007, Michael Kerrisk .\" All rights reserved. .\" .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB) .\" 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. .\" %%%LICENSE_END .\" .\" $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 .\" Modified 950727 by aeb, following a suggestion by Urs Thuermann .\" .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond .\" Modified 1998 by Andi Kleen .\" Modified 11 May 2001 by Sam Varshavchik .\" .\" .TH LISTEN 2 2017-09-15 "Linux" "Linux Programmer's Manual" .SH NAME listen \- listen for connections on a socket .SH SYNOPSIS .nf .BR "#include " " /* See NOTES */" .B #include .PP .BI "int listen(int " sockfd ", int " backlog ); .fi .SH DESCRIPTION .BR listen () marks the socket referred to by .I sockfd as a passive socket, that is, as a socket that will be used to accept incoming connection requests using .BR accept (2). .PP The .I sockfd argument is a file descriptor that refers to a socket of type .B SOCK_STREAM or .BR SOCK_SEQPACKET . .PP The .I backlog argument defines the maximum length to which the queue of pending connections for .I sockfd may grow. If a connection request arrives when the queue is 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 a later reattempt at connection succeeds. .SH RETURN VALUE On success, zero is returned. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EADDRINUSE Another socket is already listening on the same port. .TP .B EADDRINUSE (Internet domain sockets) The socket referred to by .I sockfd had not previously been bound to an address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers in the ephemeral port range are currently in use. See the discussion of .I /proc/sys/net/ipv4/ip_local_port_range in .BR ip (7). .TP .B EBADF The argument .I sockfd is not a valid file descriptor. .TP .B ENOTSOCK The file descriptor .I sockfd does not refer to a socket. .TP .B EOPNOTSUPP The socket is not of a type that supports the .BR listen () operation. .SH CONFORMING TO POSIX.1-2001, POSIX.1-2008, 4.4BSD .RB ( listen () first appeared in 4.2BSD). .SH NOTES To accept connections, the following steps are performed: .RS 4 .IP 1. 4 A socket is created with .BR socket (2). .IP 2. The socket is bound to a local address using .BR bind (2), so that other sockets may be .BR connect (2)ed to it. .IP 3. A willingness to accept incoming connections and a queue limit for incoming connections are specified with .BR listen (). .IP 4. Connections are accepted with .BR accept (2). .RE .PP POSIX.1 does not require the inclusion of .IR , and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it. .PP The behavior of the .I backlog argument 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 .IR /proc/sys/net/ipv4/tcp_max_syn_backlog . When syncookies are enabled there is no logical maximum length and this setting is ignored. See .BR tcp (7) for more information. .PP If the .I backlog argument is greater than the value in .IR /proc/sys/net/core/somaxconn , then it is silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, .BR SOMAXCONN , with the value 128. .\" The following is now rather historic information (MTK, Jun 05) .\" Don't rely on this value in portable applications since BSD .\" (and some BSD-derived systems) limit the backlog to 5. .SH EXAMPLE See .BR bind (2). .SH SEE ALSO .BR accept (2), .BR bind (2), .BR connect (2), .BR socket (2), .BR socket (7) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.