Question

I’ve been trying to code a simple chat server in Python, my code is as follows:

import socket
import select

port = 11222
serverSocket = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
serverSocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1024)
serverSocket.bind(('',port))
serverSocket.listen(5)

sockets=[serverSocket]
print 'Server is started on port' , port,'\n'

def acceptConn():
    newsock, addr = serverSocket.accept()
    sockets.append(newsock)
    newsock.send('You are now connected to the chat server\n')
    msg = 'Client joined',addr.__str__(),
    broadcast(msg, newsock)

def broadcast(msg, sourceSocket):
    for s in sockets:
        if (s != serverSocket and s != sourceSocket):
            s.send(msg)
    print msg,


while True:
    (sread, swrite, sexec)=select.select(sockets,[],[])
    for s in sread:
        if s == serverSocket:
            acceptConn()
        else:
            msg=s.recv(100) 
            if msg.rstrip() == "quit":
                host,port=socket.getpeername()
                msg = 'Client left' , (host,port)
                broadcast(msg,s)
                s.close()
                sockets.remove(s)
                del s
            else:
                host,port=s.getpeername()
                msg = s.recv(1024)
                broadcast(msg,s)
                continue

After running the server and connecting via telnet, the server reads single character and skips the next one. Example if I type Hello in telnet, server reads H l o.
Any help please ?! 🙂

Answer 1

You call recv twice.

First:

msg=s.recv(100)

Then, if that’s not “quit”, you read and broadcast another message:

msg = s.recv(1024)
broadcast(msg,s)

So the original message is lost.

Because you’re using telnet as the client, you get one character at a time, so you see every other character. If you used, say, nc instead, you’d get different results—but still the same basic problem of every other read being thrown away.

There are a few other problems here:

• You’re expecting clients to send “quit” before quitting—you should be handling EOF or error from recv and/or passing sockets in the x as well as the r.
• You’re assuming that “quit” will always appear in a single message, and an entire message all to itself. This is not a reasonable assumption with TCP. You may get four 1-byte reads of “q”, “u”, “i”, and “t”, or you may get a big read of “OK, bye everyone\nquit\n”, neither of which will match.
• The “Client left” and “Client joined” messages are tuples, not strings, and they’re formed differently, so you’re going to see (‘Client joined’, “(‘127.0.0.1’, 56564)”) (‘Client left’, (‘127.0.0.1’, 56564)).
• You’re relying on the clients to send newlines between their messages. First, as mentioned above, even if they did, there’s no guarantee that you’ll get complete/discrete messages. Second, your “system” messages don’t have newlines.

Here’s a modified version of your sample that fixes most of the problems, except for requiring “quit” to be in a single message alone and relying on the clients to send newlines:

#!/usr/bin/python

import socket
import select
import sys

port = 11222
serverSocket = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
serverSocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1024)
serverSocket.bind(('',port))
serverSocket.listen(5)

sockets=[serverSocket]
print 'Server is started on port' , port,'\n'

def acceptConn():
    newsock, addr = serverSocket.accept()
    sockets.append(newsock)
    newsock.send('You are now connected to the chat server\n')
    msg = 'Client joined: %s:%d\n' % addr
    broadcast(msg, newsock)

def broadcast(msg, sourceSocket):
    for s in sockets:
        if (s != serverSocket and s != sourceSocket):
            s.send(msg)
    sys.stdout.write(msg)
    sys.stdout.flush()


while True:
    (sread, swrite, sexec)=select.select(sockets,[],[])
    for s in sread:
        if s == serverSocket:
            acceptConn()
        else:
            msg=s.recv(100)
            if not msg or msg.rstrip() == "quit":
                host,port=s.getpeername()
                msg = 'Client left: %s:%d\n' % (host,port)
                broadcast(msg,s)
                s.close()
                sockets.remove(s)
                del s
            else:
                host,port=s.getpeername()
                broadcast(msg,s)
                continue

To fix the ‘quit’ problem, you’re going to have to keep a buffer for each client, and do something like this:

buffers[s] += msg
if '\nquit\n' in buffers[s]:
   # do quit stuff
lines = buffers[s].split('\n')[-1]
buffers[s] = ('\n' if len(lines) > 1 else '') + lines[-1]

But you’ve still got the newline problem. Imagine that user1 logs in and types “abc\n” while user2 logs in and types “def\n”; you may get something like “abClient joined: 127.0.0.1:56881\ndec\nf\n”.

If you want a line-based protocol, you have to rewrite your code to do the echoing on a line-by-line instead of read-by-read basis.