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