Skip to main content

Instant Messaging, Threading and Sockets

So we were given an assignment for software today. It's going to be used to teach us promala and spin and good multi-threaded software practices. The overall goal is to make a simple GUI based multi-user instant messager system in C++.
I haven't used allot of concurrency before so I thought I would investigate by trying some stuff out in python. Also I haven't used sockets directly before - so wanted to have a look at them.

Firstly taking a read of the Socket Programming HOWTO guide - there is plenty of information there. And also handy is the Socket documentation for python.

For now I just want a echoing server that can service multiple clients at once.

So the server will be the difficult part - might as well dive in there!

Firstly creating a socket and echoing any data it recieves is pretty easy with this loop:

while True:
           data = self.conn.recv(1024)
           if not data:
Now wraping this up in a thread than can be started at any time is straighforward. And for good measure I'll add some logging in at the same time.

class AsyncEcho(threading.Thread):
    def __init__(self, conn, addr):
        self.conn = conn
        self.addr = repr(addr) # Note just storing string for identification purposes

    def run(self):"Starting to run thread for client: %s" % self.addr)
        while True:
            logging.debug("server waiting to receive from client: %s" % self.addr)
            data = self.conn.recv(1024)
            if not data:
                logging.debug("Connection to client '%s' complete. Breaking connection" % self.addr)
            logging.debug('Server received: "%s" from client "%s", sending back...' % (data,self.addr))
            self.conn.send(data)'Finished background servicing of client: %s' % self.addr)

 And the rest of the script that actually starts these threads, minus the imports:"Server started")
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
logging.debug("Created a socket on the server")
s.bind((HOST, PORT))
logging.debug("Server Socket has been bound. Server is ready to accept connections")
while True:
    s.listen(5) # We want to queue up at most 5 requests
    logging.debug("Main thread of server listening for new connections")
    conn, addr = s.accept()'New connection by <%s>' %  repr(addr))
    logging.debug("Socket Object: %s" % repr(conn))
    background = AsyncEcho(conn, addr)"Created a new thread object to service this client, about to start it.")
    background.start()'Started the thread, the main program continues to run in foreground.')

 Now that wasn't too painful at all, the client is even easier. It needs to establish a connection via the socket with the server, then pass and receive data. Easy as:

clientName = raw_input("Client Name: ")

HOST = 'localhost'        # The remote host, could be differant to server...

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
logging.debug("Created a socket on the client")
s.connect((HOST, PORT))
logging.debug("Client socket connected")

logging.debug("Sending Client name")
logging.debug("Sent data, trying to receive now")
data = s.recv(1024)
logging.debug("Data received '%s'" % data)
Now we might as well have a loop for the client so it acts more like a command line:"Start a loop. 'quit' will quit")
while True:
    data = raw_input(">")
    if data == "quit":
    data2 = s.recv(1024)


So starting up the server:

brian@brian-hitlab:~/projects/python/instantmess/src$ python
INFO:root:Logger enabled
INFO:root:Server started
DEBUG:root:Created a socket on the server
DEBUG:root:Server Socket has been bound. Server is ready to accept connections
DEBUG:root:Main thread of server listening for new connections

At this point the server just waits for a client to be started, so lets do that in another terminal:

brian@brian-hitlab:~/projects/python/instantmess/src$ python
INFO:root:Logger enabled
INFO:root:Client started
Client Name: client1
DEBUG:root:Created a socket on the client
DEBUG:root:Client socket connected
DEBUG:root:Sending Client name
DEBUG:root:Sent data, trying to receive now
DEBUG:root:Data received 'client1'

And then the loop:

INFO:root:Start a loop. 'quit' will quit
>this is a test
this is a test
DEBUG:root:Closing socket
Received 'quit'

During all this the server was spitting out lots as well:

DEBUG:root:server waiting to receive from client: ('', 50535)
DEBUG:root:Server received: "Hi" from client "('', 50535)", sending back...
DEBUG:root:server waiting to receive from client: ('', 50535)
DEBUG:root:Server received: "this is a test" from client "('', 50535)", sending back...
DEBUG:root:server waiting to receive from client: ('', 50535)
DEBUG:root:Server received: "sweet" from client "('', 50535)", sending back...
DEBUG:root:server waiting to receive from client: ('', 50535)
DEBUG:root:Connection to client '('', 50535)' complete. Breaking connection
INFO:root:Finished background servicing of client: ('', 50535)

Okay time for lunch.


Popular posts from this blog

Matplotlib in Django

The official django tutorial is very good, it stops short of displaying
data with matplotlib - which could be very handy for dsp or automated
testing. This is an extension to the tutorial. So first you must do the
official tutorial!
Complete the tutorial (as of writing this up to part 4).

Adding an image to a view

To start with we will take a static image from the hard drive and
display it on the polls index page.
Usually if it really is a static image this would be managed by the
webserver eg apache. For introduction purposes we will get django to
serve the static image. To do this we first need to change the

Change the template
At the moment poll_list.html probably looks something like this:

<h1>Django test app - Polls</h1> {% if object_list %} <ul> {% for object in object_list %} <li><a href="/polls/{{}}">{{ object.question }}</a></li> {% endfor %} </ul> {% else %} <p>No polls are available.</p> …

Homomorphic encryption using RSA

I recently had cause to briefly look into Homomorphic Encryption, the process of carrying out computations on encrypted data. This technique allows for privacy preserving computation. Fully homomorphic encryption (FHE) allows both addition and multiplication, but is (currently) impractically slow.

Partially homomorphic encryption just has to meet one of these criteria and can be much more efficient.
An unintended, but well-known, malleability in the common RSA algorithm means that the multiplication of ciphertexts is equal to the multiplication of the original messages. So unpadded RSA is a partially homomorphic encryption system.

RSA is beautiful in how simple it is. See wikipedia to see how to generate the public (e, m) and private keys (d, m).

Given a message x it is encrypted with the public keys it to get the ciphertext C(x)with:

To decrypt a ciphertext

Bluetooth with Python 3.3

Since about version 3.3 Python supports Bluetooth sockets natively. To put this to the test I got hold of an iRacer from sparkfun. To send to New Zealand the cost was $60. The toy has an on-board Bluetooth radio that supports the RFCOMM transport protocol.

The drive protocol is dead easy, you send single byte instructions when a direction or speed change is required. The bytes are broken into two nibbles: 0xXY where X is the direction and Y is the speed. For example the byte 0x16 means forwards at mid-speed. I was surprised to note the car continues carrying out the last given demand!

I let pairing get dealt with by the operating system. The code to create a Car object that is drivable over Bluetooth is very straight forward in pure Python: