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32 .\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
33 .\" $FreeBSD: src/lib/libc/sys/listen.2,v 1.12.2.9 2002/05/09 02:24:40 silby Exp $
34 .\" $DragonFly: src/lib/libc/sys/listen.2,v 1.4 2006/03/21 04:24:28 swildner Exp $
41 .Nd listen for connections on a socket
48 .Fn listen "int s" "int backlog"
50 To accept connections, a socket
53 a willingness to accept incoming connections and
54 a queue limit for incoming connections are specified with
56 and then the connections are
61 call applies only to sockets of type
68 parameter defines the maximum length the queue of
69 pending connections may grow to.
71 request arrives with the queue full the client may
72 receive an error with an indication of
74 or, in the case of TCP, the connection will be
77 Note that before FreeBSD 4.5 and the introduction of the syncache,
80 parameter also determined the length of the incomplete
81 connection queue, which held TCP sockets in the process
82 of completing TCP's 3-way handshake. These incomplete connections
83 are now held entirely in the syncache, which is unaffected by
84 queue lengths. Inflated
86 values to help handle denial
87 of service attacks are no longer necessary.
92 .Dq Va kern.ipc.somaxconn
93 specifies a hard limit on
95 if a value greater than
96 .Va kern.ipc.somaxconn
97 or less than zero is specified,
100 .Va kern.ipc.somaxconn .
101 .Sh INTERACTION WITH ACCEPT FILTERS
102 When accept filtering is used on a socket, a second queue will
103 be used to hold sockets that have connected, but have not yet
104 met their accept filtering criteria. Once the criteria has been
105 met, these sockets will be moved over into the completed connection
106 queue to be accept()ed. If this secondary queue is full and a
107 new connection comes in, the oldest socket which has not yet met
108 its accept filter criteria will be terminated.
110 This secondary queue, like the primary listen queue, is sized
123 is not a valid descriptor.
129 The socket is not of a type that supports the operation
142 function call appeared in
144 The ability to configure the maximum
146 at run-time, and to use a negative
148 to request the maximum allowable value, was introduced in