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35 .\" $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
37 .\" Modified Fri Jul 23 22:07:54 1993 by Rik Faith <faith@cs.unc.edu>
38 .\" Modified 950727 by aeb, following a suggestion by Urs Thuermann
39 .\" <urs@isnogud.escape.de>
40 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
41 .\" Modified 1998 by Andi Kleen
42 .\" Modified 11 May 2001 by Sam Varshavchik <mrsam@courier-mta.com>
45 .TH LISTEN 2 2014-05-10 "Linux" "Linux Programmer's Manual"
47 listen \- listen for connections on a socket
50 .BR "#include <sys/types.h>" " /* See NOTES */"
52 .B #include <sys/socket.h>
54 .BI "int listen(int " sockfd ", int " backlog );
58 marks the socket referred to by
60 as a passive socket, that is, as a socket that will
61 be used to accept incoming connection requests using
66 argument is a file descriptor that refers to a socket of type
73 argument defines the maximum length
74 to which the queue of pending connections for
77 If a connection request arrives when the queue is full, the client
78 may receive an error with an indication of
80 or, if the underlying protocol supports retransmission, the request may be
81 ignored so that a later reattempt at connection succeeds.
83 On success, zero is returned.
84 On error, \-1 is returned, and
90 Another socket is already listening on the same port.
93 (Internet domain sockets)
94 The socket referred to by
96 had not previously been bound to an address and,
97 upon attempting to bind it to an ephemeral port,
98 it was determined that all port numbers in the ephemeral port range
100 See the discussion of
101 .I /proc/sys/net/ipv4/ip_local_port_range
108 is not a valid descriptor.
116 The socket is not of a type that supports the
120 4.4BSD, POSIX.1-2001.
123 function call first appeared in 4.2BSD.
125 To accept connections, the following steps are performed:
128 A socket is created with
131 The socket is bound to a local address using
133 so that other sockets may be
137 A willingness to accept incoming connections and a queue limit for incoming
138 connections are specified with
141 Connections are accepted with
145 POSIX.1-2001 does not require the inclusion of
147 and this header file is not required on Linux.
148 However, some historical (BSD) implementations required this header
149 file, and portable applications are probably wise to include it.
153 argument on TCP sockets changed with Linux 2.2.
154 Now it specifies the queue length for
156 established sockets waiting to be accepted,
157 instead of the number of incomplete connection requests.
158 The maximum length of the queue for incomplete sockets
160 .IR /proc/sys/net/ipv4/tcp_max_syn_backlog .
161 When syncookies are enabled there is no logical maximum
162 length and this setting is ignored.
165 for more information.
169 argument is greater than the value in
170 .IR /proc/sys/net/core/somaxconn ,
171 then it is silently truncated to that value;
172 the default value in this file is 128.
173 In kernels before 2.4.25, this limit was a hard coded value,
176 .\" The following is now rather historic information (MTK, Jun 05)
177 .\" Don't rely on this value in portable applications since BSD
178 .\" (and some BSD-derived systems) limit the backlog to 5.
189 This page is part of release 3.78 of the Linux
192 A description of the project,
193 information about reporting bugs,
194 and the latest version of this page,
196 \%http://www.kernel.org/doc/man\-pages/.