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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 1.5 2007/05/12 21:22:10 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
79 and the introduction of the syncache, the
81 parameter also determined the length of the incomplete
82 connection queue, which held TCP sockets in the process
83 of completing TCP's 3-way handshake. These incomplete connections
84 are now held entirely in the syncache, which is unaffected by
85 queue lengths. Inflated
87 values to help handle denial
88 of service attacks are no longer necessary.
93 .Dq Va kern.ipc.somaxconn
94 specifies a hard limit on
96 if a value greater than
97 .Va kern.ipc.somaxconn
98 or less than zero is specified,
100 is silently forced to
101 .Va kern.ipc.somaxconn .
102 .Sh INTERACTION WITH ACCEPT FILTERS
103 When accept filtering is used on a socket, a second queue will
104 be used to hold sockets that have connected, but have not yet
105 met their accept filtering criteria. Once the criteria has been
106 met, these sockets will be moved over into the completed connection
107 queue to be accept()ed. 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