Creates a TCP (stream) or UDP (datagram) socket to the host and port specified. The return value is a socket object of class stream-usocket, or datagram-usocket.

protocol should be :stream (default) or :datagram, which means TCP or UDP (Start from USOCKET 0.5)
element-type argument is used in the construction of the associated stream, i.e. 'character or '(unsigned-byte 8), only used by TCP.
timeout is a integer, it represents the socket option SO_RCVTIMEO (read timeout), in seconds.
deadline is only supported in Clozure CL and Digitool MCL, look up their documents please.
local-host and local-port, when specified, will cause the socket calling bind() on local address. This is useful for selecting interfaces to send, or listening on UDP port. Note: use only one of them are allowed when reasonable (listen on wildcard address, or bind to random free port).

nodelay Allows to disable/enable Nagle's algorithm (http://en.wikipedia.org/wiki/Nagle%27s_algorithm).
If this parameter is omitted, the behaviour is inherited from the CL implementation (in most cases, Nagle's algorithm is enabled by default, but for example in ACL it is disabled).
If the parmeter is specified, one of these three values is possible:

• T - Disable Nagle's algorithm; signals an UNSUPPORTED condition if the implementation does not support explicit manipulation with that option.
• NIL - Leave Nagle's algorithm enabled on the socket; signals an UNSUPPORTED condition if the implementation does not support explicit manipulation with that option.
• :IF-SUPPORTED - Disables Nagle's algorithm if the implementation allow this, otherwises just ignore this option.

Creates and returns a passive ("server") socket associated with host and port. The object returned is of subtype stream-server-usocket.

host names a local interface.
port names a local port, or 0 (zero) to request a random free port.
reuse-address is a boolean (t, nil) value signalling reuse of the address is requested (or not).
backlog is the length of the queue containing connections which haven't actually been accepted yet.
element-type is the default element type used for sockets created by socket-accept. character is the default when it's not explicitly provided.

Creates and returns an active ("connected") stream socket new-socket from the socket passed. The return value is a socket object of class stream-usocket.

element-type is the element type used to construct the associated stream. If it's not specified, the element-type of socket (as used when it was created by the call to socket-listen) is used.

Flushes the stream associated with the socket and closes the socket connection.

Returns the local address and/or port information of socket.

Returns the remote address and/or port information of socket. The socket passed to this function must be a connected socket.

Send a (unsigned-byte 8) data buffer to a datagram socket, and return the number of bytes sent. (Start from USOCKET 0.5)

socket should be a datagram-usocket.
buffer is a Lisp vector, type of (simple-array (unsigned-byte 8) *).
length is used to tell socket-send the actual useful length of data buffer for sending to socket.
host and port are used for unconnected datagram sockets, for sending to specific destination.

Receive data from a datagram socket, and return 4 values: return-buffer, return-length, remote-host, and remove-port. If the datagram socket was created by socket-connect with a timeout keyword argument, this function will block at most that timeout value (in seconds). (Start from USOCKET 0.5)

socket should be a datagram-usocket.
buffer is a Lisp vector, type of (simple-array (unsigned-byte 8) *). Using nil here is also allowed, new buffer will be created to hold data.
length is used to specify the length of a exist buffer for receiving at most these data. Using nil here is allowed, and the actual length of buffer will be used; when buffer is also nil, a default maximum length (65507) will be used. 

The socket-shutdown call causes all or part of a full-duplex connection on the socket associated with sockfd to be shut down. If direction is :input, further receptions will be disallowed. If direction is :output, further transmissions will be disallowed. If direction is :io, further receptions and transmissions will be disallowed. (Starting from USOCKET 0.6.4)

socket should be a datagram-usocket.
direction should be either :input, :output or :io.

Waiting on one or multiple sockets for given time, and returns once some of them are available of reading data. This is like UNIX's "select" function.

It returns two values: the first is the list of sockets which are readable (or in case of server sockets acceptable). nil may be returned for this value either when waiting timed out or when it was interrupted (EINTR).  The second value is a real number indicating the time remaining within the timeout period or nil if none.

Without the ready-only argument, WAIT-FOR-INPUT will return all sockets in the original list you passed it. This prevents a new list from being consed up. Some users of USOCKET were reluctant to use it if it wouldn't behave that way, expecting it to cost significant performance to do the associated garbage collection.

Without the ready-only arg, you need to check the socket STATE slot for the values documented in usocket class.

Create a simple TCP or UDP socket server. (Start from USOCKET 0.5)

host names a local interface,
port names a local port,
function names a function object, which is used to handle TCP or UDP connections, the actual API of this function will be explained later.
arguments is a list used for passing extra arguments to user-defined function.
in-new-thread is a boolean, default is nil. When it's T, the server will be created in a new thread and socket-server returns immediately in current thread.
protocol could be either :stream (default) or :datagram, which decide the socket server is TCP server or UDP server.
timeout is UDP only, it provides the internal socket-receive call (in UDP event loop of the socket server) a read timeout, default value is 1 (second).
max-buffer-size is UDP only, it's the max UDP data buffer size when handling UDP packets, default value is 65507.
element-type is TCP only, it's element-type of the stream provided for user-defined function,
reuse-address is TCP only, it's a boolean option for internal call of socket-listen in the socket server,
multi-threading is TCP only, it's a boolean, default value is nil. When it's T, each client connection will cause a new thread being created to handle that client, so that the TCP server could handle multiple clients at the same time. (Note: since UDP server is connectionless, it can always handle multiple clients, as long as the handler function run fast enough)

The handler function for TCP is stream-based. A template function is this one:

(defun default-tcp-handler (stream) ; null
  (declare (type stream stream))
  (terpri stream))

Note: 1. you don't need to close the stream as socket-server will do that for you. 2. More function arguments can be defined, and these extra arguments must be feeded as the optional arguments of socket-server.

The handler function for UDP is buffer-based, that is, you receive a buffer of data as input, and you return another buffer for output. A template function is a simple UDP echo server:

(defun default-udp-handler (buffer) ; echo
  (declare (type (simple-array (unsigned-byte 8) *) buffer))
  buffer)

Note: 1. data length is the length of the whole buffer. 2. Sometimes you may want to know the client's IP address and sending port, these informations are specially bounded on variables *remote-host* and *remote-port* when handler function is running.