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 16.9.2 Listening for Connections
 --------------------------------
 
 Now let us consider what the server process must do to accept
 connections on a socket.  First it must use the `listen' function to
 enable connection requests on the socket, and then accept each incoming
 connection with a call to `accept' ( Accepting Connections).
 Once connection requests are enabled on a server socket, the `select'
 function reports when the socket has a connection ready to be accepted
 ( Waiting for I/O).
 
    The `listen' function is not allowed for sockets using
 connectionless communication styles.
 
    You can write a network server that does not even start running
 until a connection to it is requested.   Inetd Servers.
 
    In the Internet namespace, there are no special protection mechanisms
 for controlling access to a port; any process on any machine can make a
 connection to your server.  If you want to restrict access to your
 server, make it examine the addresses associated with connection
 requests or implement some other handshaking or identification protocol.
 
    In the local namespace, the ordinary file protection bits control
 who has access to connect to the socket.
 
  -- Function: int listen (int SOCKET, int N)
      The `listen' function enables the socket SOCKET to accept
      connections, thus making it a server socket.
 
      The argument N specifies the length of the queue for pending
      connections.  When the queue fills, new clients attempting to
      connect fail with `ECONNREFUSED' until the server calls `accept' to
      accept a connection from the queue.
 
      The `listen' function returns `0' on success and `-1' on failure.
      The following `errno' error conditions are defined for this
      function:
 
     `EBADF'
           The argument SOCKET is not a valid file descriptor.
 
     `ENOTSOCK'
           The argument SOCKET is not a socket.
 
     `EOPNOTSUPP'
           The socket SOCKET does not support this operation.
 

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