man2/listen.2

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  32 .\"     $Id: listen.2,v 1.6 1999/05/18 14:10:32 freitag Exp $
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  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 "BSD Man Page" "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 " s ", 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 .B 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.  If a connection request arrives with the queue full the client
  68 may receive an error with an indication of
  69 .B ECONNREFUSED
  70 or, if the underlying protocol supports retransmission, the request may be
  71 ignored so that retries succeed.
  72 .SH NOTES
  73 The behaviour of the
  74 .I backlog
  75 parameter on TCP sockets changed with Linux 2.2.
  76 Now it specifies the queue length for
  77 .I completely
  78 established sockets waiting to be accepted, instead of the number of incomplete
  79 connection requests. The maximum length of the queue for incomplete sockets
  80 can be set using the
  81 .B tcp_max_syn_backlog
  82 sysctl.
  83 When syncookies are enabled there is no logical maximum
  84 length and this sysctl setting is ignored.
  85 See
  86 .BR tcp (7)
  87 for more information.
  88
  89 .SH "RETURN VALUE"
  90 On success, zero is returned.  On error, \-1 is returned, and
  91 .I errno
  92 is set appropriately.
  93 .SH ERRORS
  94 .TP
  95 .B EADDRINUSE
  96 Another socket is already listening on the same port.
  97 .TP
  98 .B EBADF
  99 The argument
 100 .I s
 101 is not a valid descriptor.
 102 .TP
 103 .B ENOTSOCK
 104 The argument
 105 .I s
 106 is not a socket.
 107 .TP
 108 .B EOPNOTSUPP
 109 The socket is not of a type that supports the
 110 .B listen
 111 operation.
 112 .SH "CONFORMING TO"
 113 Single Unix, 4.4BSD, POSIX 1003.1g draft. The
 114 .B listen
 115 function call first appeared in 4.2BSD.
 116 .SH BUGS
 117 If the socket is of type
 118 .BR AF_INET ,
 119 and the
 120 .I backlog
 121 argument is greater
 122 than the constant
 123 .B SOMAXCONN
 124 (128 in Linux 2.0 & 2.2), it is silently truncated
 125 to
 126 .BR SOMAXCONN .
 127 Don't rely on this value in portable applications since BSD
 128 (and some BSD-derived systems) limit the backlog to 5.
 129 .SH "SEE ALSO"
 130 .BR accept (2),
 131 .BR connect (2),
 132 .BR socket (2)