man2/listen.2

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  33 .\"     $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
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  35 .\" Modified Fri Jul 23 22:07:54 1993 by Rik Faith <faith@cs.unc.edu>
  36 .\" Modified 950727 by aeb, following a suggestion by Urs Thuermann
  37 .\" <urs@isnogud.escape.de>
  38 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  39 .\" Modified 1998 by Andi Kleen
  40 .\" Modified 11 May 2001 by Sam Varshavchik <mrsam@courier-mta.com>
  41 .\"
  42 .\"
  43 .TH LISTEN 2 2007-12-28 "Linux" "Linux Programmer's Manual"
  44 .SH NAME
  45 listen \- listen for connections on a socket
  46 .SH SYNOPSIS
  47 .nf
  48 .BR "#include <sys/types.h>" "          /* See NOTES */"
  49 .br
  50 .B #include <sys/socket.h>
  51 .sp
  52 .BI "int listen(int " sockfd ", int " backlog );
  53 .fi
  54 .SH DESCRIPTION
  55 .BR listen ()
  56 marks the socket referred to by
  57 .I sockfd
  58 as a passive socket, that is, as a socket that will
  59 be used to accept incoming connection requests using
  60 .BR accept (2).
  61
  62 The
  63 .I sockfd
  64 argument is a file descriptor that refers to a socket of type
  65 .B SOCK_STREAM
  66 or
  67 .BR SOCK_SEQPACKET .
  68
  69 The
  70 .I backlog
  71 argument defines the maximum length
  72 to which the queue of pending connections for
  73 .I sockfd
  74 may grow.
  75 If a connection request arrives when the queue is full, the client
  76 may receive an error with an indication of
  77 .B ECONNREFUSED
  78 or, if the underlying protocol supports retransmission, the request may be
  79 ignored so that a later reattempt at connection succeeds.
  80 .SH "RETURN VALUE"
  81 On success, zero is returned.
  82 On error, \-1 is returned, and
  83 .I errno
  84 is set appropriately.
  85 .SH ERRORS
  86 .TP
  87 .B EADDRINUSE
  88 Another socket is already listening on the same port.
  89 .TP
  90 .B EBADF
  91 The argument
  92 .I sockfd
  93 is not a valid descriptor.
  94 .TP
  95 .B ENOTSOCK
  96 The argument
  97 .I sockfd
  98 is not a socket.
  99 .TP
 100 .B EOPNOTSUPP
 101 The socket is not of a type that supports the
 102 .BR listen ()
 103 operation.
 104 .SH "CONFORMING TO"
 105 4.4BSD, POSIX.1-2001.
 106 The
 107 .BR listen ()
 108 function call first appeared in 4.2BSD.
 109 .SH NOTES
 110 To accept connections, the following steps are performed:
 111 .RS 4
 112 .IP 1. 4
 113 A socket is created with
 114 .BR socket (2).
 115 .IP 2.
 116 The socket is bound to a local address using
 117 .BR bind (2),
 118 so that other sockets may be
 119 .BR connect (2)ed
 120 to it.
 121 .IP 3.
 122 A willingness to accept incoming connections and a queue limit for incoming
 123 connections are specified with
 124 .BR listen ().
 125 .IP 4.
 126 Connections are accepted with
 127 .BR accept (2).
 128 .RE
 129 .PP
 130 POSIX.1-2001 does not require the inclusion of
 131 .IR <sys/types.h> ,
 132 and this header file is not required on Linux.
 133 However, some historical (BSD) implementations required this header
 134 file, and portable applications are probably wise to include it.
 135
 136 The behavior of the
 137 .I backlog
 138 argument on TCP sockets changed with Linux 2.2.
 139 Now it specifies the queue length for
 140 .I completely
 141 established sockets waiting to be accepted,
 142 instead of the number of incomplete connection requests.
 143 The maximum length of the queue for incomplete sockets
 144 can be set using the
 145 .I tcp_max_syn_backlog
 146 sysctl.
 147 When syncookies are enabled there is no logical maximum
 148 length and this sysctl setting is ignored.
 149 See
 150 .BR tcp (7)
 151 for more information.
 152
 153 If the
 154 .I backlog
 155 argument is greater than the value in
 156 .IR /proc/sys/net/core/somaxconn ,
 157 then it is silently truncated to that value;
 158 the default value in this file is 128.
 159 In kernels before 2.4.25, this limit was a hard coded value,
 160 .BR SOMAXCONN ,
 161 with the value 128.
 162 .\" The following is now rather historic information (MTK, Jun 05)
 163 .\" Don't rely on this value in portable applications since BSD
 164 .\" (and some BSD-derived systems) limit the backlog to 5.
 165 .SH EXAMPLE
 166 See
 167 .BR bind (2).
 168 .SH "SEE ALSO"
 169 .BR accept (2),
 170 .BR bind (2),
 171 .BR connect (2),
 172 .BR socket (2)