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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 2017-09-15 "Linux" "Linux Programmer's Manual"
47 listen \- listen for connections on a socket
50 .BR "#include <sys/types.h>" " /* See NOTES */"
51 .B #include <sys/socket.h>
53 .BI "int listen(int " sockfd ", int " backlog );
57 marks the socket referred to by
59 as a passive socket, that is, as a socket that will
60 be used to accept incoming connection requests using
65 argument is a file descriptor that refers to a socket of type
72 argument defines the maximum length
73 to which the queue of pending connections for
76 If a connection request arrives when the queue is full, the client
77 may receive an error with an indication of
79 or, if the underlying protocol supports retransmission, the request may be
80 ignored so that a later reattempt at connection succeeds.
82 On success, zero is returned.
83 On error, \-1 is returned, and
89 Another socket is already listening on the same port.
92 (Internet domain sockets)
93 The socket referred to by
95 had not previously been bound to an address and,
96 upon attempting to bind it to an ephemeral port,
97 it was determined that all port numbers in the ephemeral port range
100 .I /proc/sys/net/ipv4/ip_local_port_range
107 is not a valid file descriptor.
112 does not refer to a socket.
115 The socket is not of a type that supports the
119 POSIX.1-2001, POSIX.1-2008, 4.4BSD
121 first appeared in 4.2BSD).
123 To accept connections, the following steps are performed:
126 A socket is created with
129 The socket is bound to a local address using
131 so that other sockets may be
135 A willingness to accept incoming connections and a queue limit for incoming
136 connections are specified with
139 Connections are accepted with
143 POSIX.1 does not require the inclusion of
145 and this header file is not required on Linux.
146 However, some historical (BSD) implementations required this header
147 file, and portable applications are probably wise to include it.
151 argument on TCP sockets changed with Linux 2.2.
152 Now it specifies the queue length for
154 established sockets waiting to be accepted,
155 instead of the number of incomplete connection requests.
156 The maximum length of the queue for incomplete sockets
158 .IR /proc/sys/net/ipv4/tcp_max_syn_backlog .
159 When syncookies are enabled there is no logical maximum
160 length and this setting is ignored.
163 for more information.
167 argument is greater than the value in
168 .IR /proc/sys/net/core/somaxconn ,
169 then it is silently truncated to that value;
170 the default value in this file is 128.
171 In kernels before 2.4.25, this limit was a hard coded value,
174 .\" The following is now rather historic information (MTK, Jun 05)
175 .\" Don't rely on this value in portable applications since BSD
176 .\" (and some BSD-derived systems) limit the backlog to 5.