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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 $
40 .Nd listen for connections on a socket
47 .Fn listen "int s" "int backlog"
49 To accept connections, a socket
52 a willingness to accept incoming connections and
53 a queue limit for incoming connections are specified with
55 and then the connections are
60 call applies only to sockets of type
67 parameter defines the maximum length the queue of
68 pending connections may grow to.
70 request arrives with the queue full the client may
71 receive an error with an indication of
73 or, in the case of TCP, the connection will be
76 Note that before FreeBSD 4.5 and the introduction of the syncache,
79 parameter also determined the length of the incomplete
80 connection queue, which held TCP sockets in the process
81 of completing TCP's 3-way handshake. These incomplete connections
82 are now held entirely in the syncache, which is unaffected by
83 queue lengths. Inflated
85 values to help handle denial
86 of service attacks are no longer necessary.
91 .Dq Va kern.ipc.somaxconn
92 specifies a hard limit on
94 if a value greater than
95 .Va kern.ipc.somaxconn
96 or less than zero is specified,
99 .Va kern.ipc.somaxconn .
100 .Sh INTERACTION WITH ACCEPT FILTERS
101 When accept filtering is used on a socket, a second queue will
102 be used to hold sockets that have connected, but have not yet
103 met their accept filtering criteria. Once the criteria has been
104 met, these sockets will be moved over into the completed connection
105 queue to be accept()ed. If this secondary queue is full and a
106 new connection comes in, the oldest socket which has not yet met
107 its accept filter criteria will be terminated.
109 This secondary queue, like the primary listen queue, is sized
122 is not a valid descriptor.
128 The socket is not of a type that supports the operation
133 .Xr accept_filter 9 ,
141 function call appeared in
143 The ability to configure the maximum
145 at run-time, and to use a negative
147 to request the maximum allowable value, was introduced in