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28 .\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
29 .\" $FreeBSD: src/lib/libc/sys/listen.2,v 1.12.2.9 2002/05/09 02:24:40 silby Exp $
36 .Nd listen for connections on a socket
43 .Fn listen "int s" "int backlog"
45 To accept connections, a socket
48 a willingness to accept incoming connections and
49 a queue limit for incoming connections are specified with
51 and then the connections are
56 call applies only to sockets of type
63 parameter defines the maximum length the queue of
64 pending connections may grow to.
66 request arrives with the queue full the client may
67 receive an error with an indication of
69 or, in the case of TCP, the connection will be
74 and the introduction of the syncache, the
76 parameter also determined the length of the incomplete
77 connection queue, which held TCP sockets in the process
78 of completing TCP's 3-way handshake.
79 These incomplete connections
80 are now held entirely in the syncache, which is unaffected by
84 values to help handle denial
85 of service attacks are no longer necessary.
90 .Dq Va kern.ipc.somaxconn
91 specifies a hard limit on
93 if a value greater than
94 .Va kern.ipc.somaxconn
95 or less than zero is specified,
98 .Va kern.ipc.somaxconn .
99 .Sh INTERACTION WITH ACCEPT FILTERS
100 When accept filtering is used on a socket, a second queue will
101 be used to hold sockets that have connected, but have not yet
102 met their accept filtering criteria.
103 Once the criteria has been
104 met, these sockets will be moved over into the completed connection
107 If this secondary queue is full and a
108 new connection comes in, the oldest socket which has not yet met
109 its accept filter criteria will be terminated.
111 This secondary queue, like the primary listen queue, is sized
124 is not a valid descriptor.
130 The socket is not of a type that supports the operation
143 function call appeared in
145 The ability to configure the maximum
147 at run-time, and to use a negative
149 to request the maximum allowable value, was introduced in