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32 .\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93
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 system call applies only to sockets of type
67 argument defines the maximum length the queue of
68 pending connections may grow to.
69 The real maximum queue length will be 1.5 times more than the value
75 system call on the listening socket allows the caller to change the maximum
76 queue length using a new
80 request arrives with the queue full the client may
81 receive an error with an indication of
83 or, in the case of TCP, the connection will be
86 Current queue lengths of listening sockets can be queried using
92 and the introduction of the syncache,
95 argument also determined the length of the incomplete
96 connection queue, which held TCP sockets in the process
97 of completing TCP's 3-way handshake.
98 These incomplete connections
99 are now held entirely in the syncache, which is unaffected by
103 values to help handle denial
104 of service attacks are no longer necessary.
109 .Dq Va kern.ipc.somaxconn
110 specifies a hard limit on
112 if a value greater than
113 .Va kern.ipc.somaxconn
114 or less than zero is specified,
116 is silently forced to
117 .Va kern.ipc.somaxconn .
118 .Sh INTERACTION WITH ACCEPT FILTERS
119 When accept filtering is used on a socket, a second queue will
120 be used to hold sockets that have connected, but have not yet
121 met their accept filtering criteria.
122 Once the criteria has been
123 met, these sockets will be moved over into the completed connection
126 If this secondary queue is full and a
127 new connection comes in, the oldest socket which has not yet met
128 its accept filter criteria will be terminated.
130 This secondary queue, like the primary listen queue, is sized
145 is not a valid descriptor.
147 The socket is already connected, or in the process of being connected.
153 The socket is not of a type that supports the operation
167 system call appeared in
169 The ability to configure the maximum
171 at run-time, and to use a negative
173 to request the maximum allowable value, was introduced in