man2/listen.2

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  35 .\"     $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
  36 .\"
  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>
  43 .\"
  44 .\"
  45 .TH LISTEN 2 2020-02-09 "Linux" "Linux Programmer's Manual"
  46 .SH NAME
  47 listen \- listen for connections on a socket
  48 .SH SYNOPSIS
  49 .nf
  50 .BR "#include <sys/types.h>" "          /* See NOTES */"
  51 .B #include <sys/socket.h>
  52 .PP
  53 .BI "int listen(int " sockfd ", int " backlog );
  54 .fi
  55 .SH DESCRIPTION
  56 .BR listen ()
  57 marks the socket referred to by
  58 .I sockfd
  59 as a passive socket, that is, as a socket that will
  60 be used to accept incoming connection requests using
  61 .BR accept (2).
  62 .PP
  63 The
  64 .I sockfd
  65 argument is a file descriptor that refers to a socket of type
  66 .B SOCK_STREAM
  67 or
  68 .BR SOCK_SEQPACKET .
  69 .PP
  70 The
  71 .I backlog
  72 argument defines the maximum length
  73 to which the queue of pending connections for
  74 .I sockfd
  75 may grow.
  76 If a connection request arrives when the queue is full, the client
  77 may receive an error with an indication of
  78 .B ECONNREFUSED
  79 or, if the underlying protocol supports retransmission, the request may be
  80 ignored so that a later reattempt at connection succeeds.
  81 .SH RETURN VALUE
  82 On success, zero is returned.
  83 On error, \-1 is returned, and
  84 .I errno
  85 is set appropriately.
  86 .SH ERRORS
  87 .TP
  88 .B EADDRINUSE
  89 Another socket is already listening on the same port.
  90 .TP
  91 .B EADDRINUSE
  92 (Internet domain sockets)
  93 The socket referred to by
  94 .I sockfd
  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
  98 are currently in use.
  99 See the discussion of
 100 .I /proc/sys/net/ipv4/ip_local_port_range
 101 in
 102 .BR ip (7).
 103 .TP
 104 .B EBADF
 105 The argument
 106 .I sockfd
 107 is not a valid file descriptor.
 108 .TP
 109 .B ENOTSOCK
 110 The file descriptor
 111 .I sockfd
 112 does not refer to a socket.
 113 .TP
 114 .B EOPNOTSUPP
 115 The socket is not of a type that supports the
 116 .BR listen ()
 117 operation.
 118 .SH CONFORMING TO
 119 POSIX.1-2001, POSIX.1-2008, 4.4BSD
 120 .RB ( listen ()
 121 first appeared in 4.2BSD).
 122 .SH NOTES
 123 To accept connections, the following steps are performed:
 124 .RS 4
 125 .IP 1. 4
 126 A socket is created with
 127 .BR socket (2).
 128 .IP 2.
 129 The socket is bound to a local address using
 130 .BR bind (2),
 131 so that other sockets may be
 132 .BR connect (2)ed
 133 to it.
 134 .IP 3.
 135 A willingness to accept incoming connections and a queue limit for incoming
 136 connections are specified with
 137 .BR listen ().
 138 .IP 4.
 139 Connections are accepted with
 140 .BR accept (2).
 141 .RE
 142 .PP
 143 POSIX.1 does not require the inclusion of
 144 .IR <sys/types.h> ,
 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.
 148 .PP
 149 The behavior of the
 150 .I backlog
 151 argument on TCP sockets changed with Linux 2.2.
 152 Now it specifies the queue length for
 153 .I completely
 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
 157 can be set using
 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.
 161 See
 162 .BR tcp (7)
 163 for more information.
 164 .PP
 165 If the
 166 .I backlog
 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 Since Linux 5.4, the default in this file is 4096;
 171 in earlier kernels, the default value is 128.
 172 In kernels before 2.4.25, this limit was a hard coded value,
 173 .BR SOMAXCONN ,
 174 with the value 128.
 175 .\" The following is now rather historic information (MTK, Jun 05)
 176 .\" Don't rely on this value in portable applications since BSD
 177 .\" (and some BSD-derived systems) limit the backlog to 5.
 178 .SH EXAMPLE
 179 See
 180 .BR bind (2).
 181 .SH SEE ALSO
 182 .BR accept (2),
 183 .BR bind (2),
 184 .BR connect (2),
 185 .BR socket (2),
 186 .BR socket (7)