Python Programming On Win32

Communication Between Sockets

The previous example covered the key features of how sockets get connected, but was dangerously simple. We sent a very short string, then waited a couple of seconds while we typed the call to receive a response. We got back the expected data, but were lucky: in real life, there’s more to worry about.

The send() call doesn’t instantly send a string, however long. It tries to send the string and returns the number of bytes sent. With a big string (e.g., a file-transfer application) or in bad network conditions, the send() call sends only a small part on each call. It might also return zero, which indicates that the network connection has been broken. The only safe way to send a string is to do it in a loop and check the return values to see what has actually been sent. Here’s a function to send an arbitrary string safely:

# this sends strings over sockets more safely
def safeSend(sock, message):
    msglen = len(message)
    totalsent = 0
    while totalsent < msglen:
        sent = sock.send(msg[totalsent:])
        if sent == 0:
            raise RuntimeError, 'connection broken'
        totalsent = totalsent + sent

At this point you’ve hit a fundamental problem. There is no way for the receiving socket to know how much data to expect, nor whether a message has finished arriving. You have to design your protocol so that client and handler know what to expect at every stage. There are several schemes for doing this:

• Always use fixed-size messages.

• Add a final delimiter.

• Indicate how long messages are. ...