man2/listen.2

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  32 .\"     $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
  33 .\"
  34 .\" Modified Fri Jul 23 22:07:54 1993 by Rik Faith <faith@cs.unc.edu>
  35 .\" Modified 950727 by aeb, following a suggestion by Urs Thuermann
  36 .\" <urs@isnogud.escape.de>
  37 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  38 .\" Modified 1998 by Andi Kleen
  39 .\" Modified 11 May 2001 by Sam Varshavchik <mrsam@courier-mta.com>
  40 .\"
  41 .TH LISTEN 2 1993-07-23 "Linux" "Linux Programmer's Manual"
  42 .SH NAME
  43 listen \- listen for connections on a socket
  44 .SH SYNOPSIS
  45 .B #include <sys/socket.h>
  46 .sp
  47 .BI "int listen(int " sockfd ", int " backlog );
  48 .SH DESCRIPTION
  49 To accept connections, a socket is first created with
  50 .BR socket (2),
  51 a willingness to accept incoming connections and a queue limit for incoming
  52 connections are specified with
  53 .BR listen (),
  54 and then the connections are
  55 accepted with
  56 .BR accept (2).
  57 The
  58 .BR listen ()
  59 call applies only to sockets of type
  60 .B SOCK_STREAM
  61 or
  62 .BR SOCK_SEQPACKET .
  63 .PP
  64 The
  65 .I backlog
  66 parameter defines the maximum length the queue of pending connections may
  67 grow to.
  68 If a connection request arrives with the queue full the client
  69 may receive an error with an indication of
  70 .B ECONNREFUSED
  71 or, if the underlying protocol supports retransmission, the request may be
  72 ignored so that retries succeed.
  73 .SH "RETURN VALUE"
  74 On success, zero is returned.
  75 On error, \-1 is returned, and
  76 .I errno
  77 is set appropriately.
  78 .SH ERRORS
  79 .TP
  80 .B EADDRINUSE
  81 Another socket is already listening on the same port.
  82 .TP
  83 .B EBADF
  84 The argument
  85 .I sockfd
  86 is not a valid descriptor.
  87 .TP
  88 .B ENOTSOCK
  89 The argument
  90 .I sockfd
  91 is not a socket.
  92 .TP
  93 .B EOPNOTSUPP
  94 The socket is not of a type that supports the
  95 .BR listen ()
  96 operation.
  97 .SH "CONFORMING TO"
  98 4.4BSD, POSIX.1-2001.
  99 The
 100 .BR listen ()
 101 function call first appeared in 4.2BSD.
 102 .SH NOTES
 103 The behaviour of the
 104 .I backlog
 105 parameter on TCP sockets changed with Linux 2.2.
 106 Now it specifies the queue length for
 107 .I completely
 108 established sockets waiting to be accepted, instead of the number of incomplete
 109 connection requests.
 110 The maximum length of the queue for incomplete sockets
 111 can be set using the
 112 .B tcp_max_syn_backlog
 113 sysctl.
 114 When syncookies are enabled there is no logical maximum
 115 length and this sysctl setting is ignored.
 116 See
 117 .BR tcp (7)
 118 for more information.
 119 .SH BUGS
 120 If the socket is of type
 121 .BR AF_INET ,
 122 and the
 123 .I backlog
 124 argument is greater
 125 than the constant
 126 .B SOMAXCONN
 127 (128 in Linux 2.0 & 2.2), it is silently truncated
 128 to
 129 .BR SOMAXCONN .
 130 .\" The following is now rather historic information (MTK, Jun 05)
 131 .\" Don't rely on this value in portable applications since BSD
 132 .\" (and some BSD-derived systems) limit the backlog to 5.
 133 .SH "SEE ALSO"
 134 .BR accept (2),
 135 .BR bind (2),
 136 .BR connect (2),
 137 .BR socket (2)