Socket programming in python - Timestamps

Question

from socket import *
from time import ctime


HOST = 'localhost'
PORT = 21567
BUFSIZ = 1024
ADDR = (HOST, PORT)

tcpSerSock = socket(AF_INET, SOCK_STREAM)
tcpSerSock.bind(ADDR)
tcpSerSock.listen(5)

while True:
    print 'waiting for connection...'
    tcpCliSock, addr = tcpSerSock.accept()
    print "...connected from:", addr

    while True:
        data = tcpCliSock.recv(BUFSIZ)
        if not data:
            break
        tcpCliSock.send("[%s] %s" % (ctime(), data))

    tcpCliSock.close()
tcpSerSock.close()

I got quite some errors cleared them. I am following the Core Python Application book. I need to know what happens in the loop?

tcpCliSock, addr = tcpSerSock.accept()

what does this line of code do exactly? 'waiting for connection?' with whom and how? there are no other parameters given to connect with something! How exactly does this listen() work? Explain the program in short. Thank you

Read the docs: docs.python.org/2/library/socket.html#socket.socket.accept — Blender
– Blender, Commented Sep 15, 2013 at 18:49

Mark R. Wilkins · Accepted Answer · 2013-09-15 19:05:39Z

1

What you want is described in the Python Sockets HOWTO in the documentation. Here's the Python 2 version:

http://docs.python.org/2/howto/sockets.html

The short version is that the code you posted is a simple server that is configured to respond to requests from a separate process, possibly on a different machine, that sends a small chunk of data to it. This server sends back the data with a time stamp in front.

socket.listen() is used to set up a socket to receive incoming connections. The connection requests go into a short queue and are handled in order. socket.accept() checks to see if there's an incoming connection in that listen queue, and if so, it opens up a new socket to the other machine just to handle that one connection. When the connection ends, the new socket to the first client closes and the server socket moves on to the next connection in the queue to handle it.

To make this server do anything, you'd need to use some client program to open the correct port and send a message to it. For something this simple, you might try using telnet if it's installed on your machine. Run this server, then try:

telnet localhost 21567

on the same machine and start typing some text. You should see responses. My output looked like this:

$ telnet localhost 21567
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
j
[Sun Sep 15 11:58:27 2013] j
jkl
[Sun Sep 15 11:58:29 2013] jkl
fdjksla;
[Sun Sep 15 11:58:30 2013] fdjksla;
fdjkasl;fjdksal;fjkdsa;
[Sun Sep 15 11:58:32 2013] fdjkasl;fjdksal;fjkdsa;

NOTE: You will have to put a hostname into your HOST field for this to work. Try 'localhost' and see if that helps. Also, I believe if you use localhost as your HOST parameter you may not be able to receive connections from other machines. In that instance, you'll need to use your own local hostname or IP address on the network.

I've edited the OP with fixed indenting and 'localhost' for the HOST parameter, and tested that this works with telnet in another shell as I've described here.

I notice also that this code never reaches the close() for the server socket. You'd want a break somewhere in your outer while loop if you wanted to close your server and exit in an orderly way.

edited Sep 15, 2013 at 19:05

answered Sep 15, 2013 at 18:51

Mark R. Wilkins

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2 Comments

Foon Over a year ago

Note that if you use "" for the HOST, this will listen on all interfaces (which is normally your ethernet/wireless card(s) and then "loopback" which is a virtual interface only accessible from within the server); by doing this, you can then use telnet (or another application) on another box to connect to your server. Note that if you're behind a wireless or other router that is doing NAT (i.e. you have an ip address like 192.168.1.2), you'll need to do extra stuff generally outside the scope of this comment to allow folks to reach your server from outside your network.

Mark R. Wilkins Over a year ago

I think that the OP was trying to use "" for HOST but may have added a few spaces in there when putting the code up on Stack Overflow.

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