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28. February 2012



Picking up where our last section left off, by the end of this chapter you will be able to send a high score across the wire and process it in node.  In order to do this, we need to create a common format both our client and server understand.  In this particular case we are going to use JSON.  If you have done any recent web program, you are probably familiar with JSON already.  Basically JSON is a micro-format designed for transferring data on the web with a lighter footprint than XML.  All told it is a pretty simple format, here is the JSON we are going to use for storing high scores:



{ "Scores" : [ {"Name" : "Mike", "Score" : 2}, {"Name" :"Bob", "Score" : 14}, {"Name" :"Steve", "Score" : 12}, {"Name" :"John", "Score" : 10}, {"Name" :"Henry", "Score" : 8} ] }

Click here to download Highscores.txt


This JSON represents an object named “Scores” composed of an array of 5 objects that in turn are made up of a string field “Name” and a integer field “Score”.  Javascript and JSON go together like peanut butter and jam, but what about C++?  Well, you could encode your data into a string with very little effort ( one of the big advantageous of JSON ), but “little effort” is still effort, and I’m a lazy guy!  Therefore we are going to use an existing library.  I wanted a light weight and extremely simple JSON library, so I went with the aptly named SimpleJSON.  Installation really couldn’t be simple, just add the 4 cpp files ( 2 headers, 2 source ) to your project and you are done.



Now lets take a look at our SFML client.  It is going to be a simple command line utility for now, from a dos prompt simply pass in the name and high score as parameters, and it will send them across to the node server.  Lets take a look at Scoreboard.cpp:


#include "SFML/Network.hpp" #include "JSON.h" #include <iostream> int main(int argc, char* argv[]) { if(argc != 3) { std::cout << "Invalid usage, proper format is player name then score, for example:" << std::endl; std::cout << "Scoreboard \"Player Name\" 42" << std::endl; return -1; } sf::IPAddress ip(""); sf::SocketUDP socket; sf::Packet packet; JSONObject data; data[L"action"] = new JSONValue(L"AddScore"); data[L"name"] = new JSONValue(std::wstring(argv[1],argv[1] + strlen(argv[1]))); data[L"score"] = new JSONValue(atof(argv[2])); JSONValue * val = new JSONValue(data); data.clear(); std::wstring dataString = val->Stringify(); delete val; std::string notSoWide; notSoWide.assign(dataString.begin(),dataString.end()); packet.Append(notSoWide.c_str(),notSoWide.length()); unsigned short port = 1000; if(socket.Send(packet,ip,port) != sf::Socket::Done) { std::cout << "An error ocurred sending packet" << std::endl; } socket.Close(); return 0; }

Click here to download scoreboard.cpp



One annoyance of the library I chose for JSON is it works with UTF-8 wide strings, but the string that we send we want encoded as standard ascii, so there is a bit of gunk as we create the JSON object using wide character strings, then after turning it into a JSON string, we encode it back to ascii.  Otherwise the code is quite straight forward.



First we verify we got the proper number of command line arguments, declare our various SFML and JSON related variables.  We are setting the ip address to, which is the loopback address, or the equivalent of saying “this machine”.  Next we build up our JSON string.  If you have worked with XML before, the process will be very familiar. We create a JSONObject named data, which is essentially a map of key value pairs of other JSONValues.  When then populate it with our data, then in turn use it as the parameter in creating a new JSONValue.  All the heavy lifting is done in JSONValue’s constructor.  Stringify() is the method that does the actual re-encoding returning a std::wstring.  Of course, we actually want a std:string, so we create one.  Obviously in time sensitive code, we would alter SimpleJSON to use std::string instead.  Our end result is a JSON string that looks like this:


{"action":"AddScore","name":"Bob Dole","score":23}



Now that we have our data in JSON encoded string format, it’s time to send it.  We simply append our string data to our packet and send it using our Socket.  If an error occurs, report it.  Otherwise, we are done, close our Socket and exit.  If you strip away all the wide character string annoyances, the process is actually quite straight forward.



Now lets take a look at the Node side of things.  The code is fairly long, so instead of walking through it I have simply commented it.  If you have any questions not covered by the comments, fire away.  So here is the contents of Server.js


var dgram = require('dgram'), fileSystem = require('fs'), highScores, server; //Load high scores from file fileSystem.readFile("HighScores.txt", 'utf8', function(err,data){ if(err){ //Error occurred loading file, spit out error message then die console.log("Error occurred loading file"); process.exit(); } console.log("Loading high scores from file"); try{ // use JSON to turn file contents back into a Javascript array object highScores = JSON.parse(data); }catch(e) { // Exception occurred parsing file contents as JSON, error out and die. console.log("Exception occured parsing data"); process.exit(); } // Now sort the high scores by score, high to low highScores.Scores.sort(function(a,b){ return b.Score - a.Score; }); // Display the sorted high scores to the console console.log(highScores); }); //Alternative way to read file in NodeJS //file.on("error",function(exception){ // process.exit(); // } //); //file.on("data",function(data){ // fileData = data; // } //); //file.on("close",function(){ // highScores = JSON.parse(fileData); //}); //Create a UDP socket server = dgram.createSocket('udp4'); console.log("Socket created"); // Add a handler for incoming traffic on the socket. This will be called each time something connects to the socket server.on("message",function (msg,rinfo) { //console.log(parseInt(msg).toString()); console.log(rinfo); // SFML sends two packets, one with the size of the following packet ( as a 4 byte number ) // We don't need it, so as a crude-hack, we ignore any 4 byte packets if(rinfo.size != 4) { console.log("Received message:" + msg.toString()); // Socket data comes in as a JSON encoded array of objects, turn back into a JS object var jsonData,i; try{ jsonData = JSON.parse(msg); } catch( exception ) { console.log("Invalid JSON request received"); return; // Non lethal error, just stop processing packet } // The action parameter determines what you should do with this packet switch(jsonData.action) { // action==AddScore, add the score to the highscore array if it's higher than an existing score case "AddScore": console.log("AddScore called\n"); // Make sure highscore has been initialize... order can be a weird thing in node if(highScores != undefined){ // Loop through current highScores ( which should be sorted ) // and insert score if a lower match found for(i=0;i < highScores.Scores.length;++i) { if(highScores.Scores[i].Score < jsonData.score){ highScores.Scores.splice(i,0,{"Name" :, "Score" : jsonData.score}); console.log("Inserted highscore by: " +; break; // match found, stop looping } } } // Display newly created highscore array console.log(highScores.Scores); break; } } // // }); // Called when socket starts listening for packets. besides logging, currently serves no purpose server.on("listening", function () { var address = server.address(); console.log("server listening " + address.address + ":" + address.port); }); // Finally, bind the server to port 1000. 1000 was randomly chosen. Think of this as saying GO! // Now we are listening for UDP connections on port 1000 server.bind(1000);

Click here to download server.js



Now start the server at the command line ( node server.js ) and run the client from a different command line.  It should look like this:





As you can see, we are successfully sending data from SFML over a socket to our node based server.  In the next part, we will look at sending data the other way.



You can download the complete project right here.  The scripts are in a sub-folder named NodeJS.



Click for part 3

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