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<h1 id="title-xs"><a href="index.html">The Common Lisp Cookbook</a> – TCP/UDP programming with sockets</h1>
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<h1 id="title-non-xs"><a href="index.html">The Common Lisp Cookbook</a> – TCP/UDP programming with sockets</h1>
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<div id="content"
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<p>This is a short guide to TCP/IP and UDP/IP client/server programming in Common
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Lisp using <a href="https://github.com/usocket/usocket">usockets</a>.</p>
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<h2 id="tcpip">TCP/IP</h2>
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<p>As usual, we will use quicklisp to load usocket.</p>
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<pre><code>(ql:quickload "usocket")
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</code></pre>
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<p>Now we need to create a server. There are 2 primary functions that we need
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to call. <code>usocket:socket-listen</code> and <code>usocket:socket-accept</code>.</p>
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<p><code>usocket:socket-listen</code> binds to a port and listens on it. It returns a socket
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object. We need to wait with this object until we get a connection that we
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accept. That’s where <code>usocket:socket-accept</code> comes in. It’s a blocking call
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that returns only when a connection is made. This returns a new socket object
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that is specific to that connection. We can then use that connection to
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communicate with our client.</p>
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<p>So, what were the problems I faced due to my mistakes?
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Mistake 1 - My initial understanding was that <code>socket-accept</code> would return
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a stream object. NO…. It returns a socket object. In hindsight, its correct
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and my own mistake cost me time. So, if you want to write to the socket, you
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need to actually get the corresponding stream from this new socket. The socket
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object has a stream slot and we need to explicitly use that. And how does one
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know that? <code>(describe connection)</code> is your friend!</p>
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<p>Mistake 2 - You need to close both the new socket and the server socket.
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Again this is pretty obvious but since my initial code was only closing
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the connection, I kept running into a socket in use problem. Of course
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one more option is to reuse the socket when we listen.</p>
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<p>Once you get past these mistakes, it’s pretty easy to do the rest. Close
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the connections and the server socket and boom you are done!</p>
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<pre><code class="language-lisp">(defun create-server (port)
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(let* ((socket (usocket:socket-listen "127.0.0.1" port))
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(connection (usocket:socket-accept socket :element-type 'character)))
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(unwind-protect
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(progn
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(format (usocket:socket-stream connection) "Hello World~%")
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(force-output (usocket:socket-stream connection)))
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(progn
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(format t "Closing sockets~%")
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(usocket:socket-close connection)
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(usocket:socket-close socket)))))
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</code></pre>
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<p>Now for the client. This part is easy. Just connect to the server port
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and you should be able to read from the server. The only silly mistake I
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made here was to use read and not read-line. So, I ended up seeing only a
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“Hello” from the server. I went for a walk and came back to find the issue
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and fix it.</p>
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<pre><code class="language-lisp">(defun create-client (port)
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(usocket:with-client-socket (socket stream "127.0.0.1" port :element-type 'character)
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(unwind-protect
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(progn
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(usocket:wait-for-input socket)
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(format t "Input is: ~a~%" (read-line stream)))
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(usocket:socket-close socket))))
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</code></pre>
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<p>So, how do you run this? You need two REPLs, one for the server
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and one for the client. Load this file in both REPLs. Create the
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server in the first REPL.</p>
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<pre><code>(create-server 12321)
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</code></pre>
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<p>Now you are ready to run the client on the second REPL</p>
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<pre><code>(create-client 12321)
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</code></pre>
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<p>Voilà! You should see “Hello World” on the second REPL.</p>
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<h2 id="udpip">UDP/IP</h2>
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<p>As a protocol, UDP is connection-less, and therefore there is no
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concept of binding and accepting a connection. Instead we only do a
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<code>socket-connect</code> but pass a specific set of parameters to make sure that
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we create an UDP socket that’s waiting for data on a particular port.</p>
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<p>So, what were the problems I faced due to my mistakes?
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Mistake 1 - Unlike TCP, you don’t pass host and port to <code>socket-connect</code>.
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If you do that, then you are indicating that you want to send a packet.
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Instead, you pass <code>nil</code> but you set <code>:local-host</code> and <code>:local-port</code> to the address
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and port that you want to receive data on. This part took some time to
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figure out, because the documentation didn’t cover it. Instead reading
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a bit of code from
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https://code.google.com/p/blackthorn-engine-3d/source/browse/src/examples/usocket/usocket.lisp helped a lot.</p>
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<p>Also, since UDP is connectionless, anyone can send data to it at any
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time. So, we need to know which host/port did we get data from so
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that we can respond on it. So we bind multiple values to <code>socket-receive</code>
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and use those values to send back data to our peer “client”.</p>
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<pre><code class="language-lisp">(defun create-server (port buffer)
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(let* ((socket (usocket:socket-connect nil nil
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:protocol :datagram
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:element-type '(unsigned-byte 8)
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:local-host "127.0.0.1"
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:local-port port)))
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(unwind-protect
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(multiple-value-bind (buffer size client receive-port)
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(usocket:socket-receive socket buffer 8)
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(format t "~A~%" buffer)
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(usocket:socket-send socket (reverse buffer) size
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:port receive-port
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:host client))
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(usocket:socket-close socket))))
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</code></pre>
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<p>Now for the sender/receiver. This part is pretty easy. Create a socket,
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send data on it and receive data back.</p>
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<pre><code class="language-lisp">(defun create-client (port buffer)
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(let ((socket (usocket:socket-connect "127.0.0.1" port
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:protocol :datagram
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:element-type '(unsigned-byte 8))))
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(unwind-protect
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(progn
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(format t "Sending data~%")
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(replace buffer #(1 2 3 4 5 6 7 8))
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(format t "Receiving data~%")
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(usocket:socket-send socket buffer 8)
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(usocket:socket-receive socket buffer 8)
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(format t "~A~%" buffer))
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(usocket:socket-close socket))))
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</code></pre>
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<p>So, how do you run this? You need again two REPLs, one for the server
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and one for the client. Load this file in both REPLs. Create the
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server in the first REPL.</p>
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<pre><code>(create-server 12321 (make-array 8 :element-type '(unsigned-byte 8)))
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</code></pre>
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<p>Now you are ready to run the client on the second REPL</p>
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<pre><code>(create-client 12321 (make-array 8 :element-type '(unsigned-byte 8)))
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</code></pre>
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<p>Voilà! You should see a vector <code>#(1 2 3 4 5 6 7 8)</code> on the first REPL
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and <code>#(8 7 6 5 4 3 2 1)</code> on the second one.</p>
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<h2 id="credit">Credit</h2>
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<p>This guide originally comes from https://gist.github.com/shortsightedsid/71cf34282dfae0dd2528</p>
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<p class="page-source">
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Page source: <a href="https://github.com/LispCookbook/cl-cookbook/blob/master/sockets.md">sockets.md</a>
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