How do I pack an unsigned int as a binary string in nodejs?

Question

I wonder if anyone can help me. I'm new to nodejs and I've been trying to send a message to a server using nodejs as the client. The server is written in C and looking at a PHP installation it makes use of the pack('N',len) to send the length of the string to the server. I have tried to implement a similar thing in javascript but am hitting some problems. I wonder if you can point out where I am going wrong (credit to phpjs from where I copied the packing string code).

My client nodejs javascript code is:

   var net = require('net');
     var strs = Array("<test1>1</test1>",` 
        "<test_2>A STRING</test_2>", "<test_3>0</test_3>", 
        "<test_4></test_4>", "<test_5></test_5>",
        "<test_6></test_6>", "<test_7_></test_7>",
        "<test_8></test_8>", "<test_9>10</test_9>",
        "<test_10></test_10>", "<test_11></test11>",
        "<test_12></test_12>", "<test_13></test_13>",
        "<test_14></test_14>");

        hmsg = strs[0] + strs[1] + strs[2] + strs[3] + strs[4] + strs[5] + strs[6];
        console.log(hmsg.length);
        msg =  hmsg + "<test_20></test_20>"`

     msglen = hmsg.length;
     astr = '';
     astr += String.fromCharCode((msglen >>> 24) && 0xFF);
     astr += String.fromCharCode((msglen >>> 16) && 0xFF);
     astr += String.fromCharCode((msglen >>> 8) & 0xFF);
     astr += String.fromCharCode((msglen >>> 0) & 0xFF);
     var pmsg = astr + msg;
     console.log(pmsg);
     var client = net.createConnection({host: 'localhost', port: 1250});
     console.log("client connected");
     client.write(pmsg);
     client.end();

Running 'node testApp' prints out the correct length of the header string. If I look at what the server is receiving I can see that as long as the header string is < 110 chars it decodes the correct length, but if the header string is > 110 (by adding strs[6] or more to the hmsg) the decoded length is incorrect. Including strs[6] I get a string of length 128 on the client side and 194 on the server side.

I am clearly doing something wrong in packing the integer, but I'm not familiar with packing bits and am not sure where I am going wrong. Can anyone point out where my error is? Many thanks!

Update Thanks to Fedor Indutny on the nodejs mailing list the following worked for me:

console.log(hmsg.length, msg.length);
var msglen = hmsg.length;
var buf = new Buffer(msg.length+4);
mslen = buf.writeUInt32BE(msglen, 0);
mslen = buf.write(msg, 4);

var client = net.createConnection({host: 'localhost', port: 8190});
console.log("client connected");
client.write(buf);
client.end();

I.e. using the Buffer's writeUInt32 was all that was needed for the length of the header message. I'm posting here in the hope it may help someone else.

Answer 1

I see that you managed to get it working on your own, but for a much easier fix you could just use the binary module.

https://github.com/substack/node-binary

Install with npm install binary