multithreading - Python Server Client program error: "OSError: [WinError 10048]"

Question

So I'm studying Python from the book Fundamentals of Python by Kenneth Lambert and I'm having trouble with an error from one of the programs in the book.

Here in chapter 10 talks about Clients and Servers. My professor asked us to key those programs in Python to see how they work. The first programs worked perfectly, but in a program I'm getting an error which seems to be a Windows error instead of a Python one.

This is the program in page 339:

from socket import *
from time import ctime
from threading import Thread

class ClientHandler(Thread):
    """Handles a client request."""
    def __init__(self, client):
        Thread.__init__(self)
        self._client = client

    def run(self):
        self._client.send(bytes(ctime() + '
Have a nice day!' , 'ascii'))
        self._client.close()

HOST = "localhost"
PORT = 5000
BUFSIZE = 1024
ADDRESS = (HOST, PORT)

server = socket(AF_INET, SOCK_STREAM)
server.bind(ADDRESS)
server.listen(5)

# The server now just waits for connections from clients
# and hands sockets off to client handlers
while True:
    print('Waiting for connection')
    client, address = server.accept()
    print('...connected from:', address)
    handler = ClientHandler(client)
    handler.start()

When I run this program, it displays the "Waiting for connection" message in the Shell. However, when I try to connect to the program using the Command Prompt, it displays the following error:

C:Python33>python multi-client-server.py
Traceback (most recent call last):
  File "multi-client-server.py", line 30, in <module>
    server.bind(ADDRESS)
OSError: [WinError 10048] Only one usage of each socket address (protocol/networ
k address/port) is normally permitted

We haven't studied this in class a lot. So I'm just wondering why this happens and how to fix it.

Thanks!

Answer 1

So, as per your questions:

We haven't studied this in class a lot. So I'm just wondering why this happens and how to fix it.

Why: You are trying to run the same code snippet from two different CMD on a Windows OS. So, when you initially execute the code snippet, the server starts listening on the port number 5000, then when you execute the same code snippet from the second CMD wndow it conflicts with the socket that is already being used by the first one. I tested this on Windows 8.

How to fix: To fix this issue, you have to simply use a different port number when you execute the code snippet for the second time, so that the socket(IP+port) doesn't conflicts with the previous one. Simply edit your code and put PORT = 15200 and save this file with a different name.(I have provided the code below too.) Now try executing the first code snippet file from a CMD windows and then execute the second code snippet file that you created right now from the second CMD window. The issue will be solved!

Code:

from socket import *
from time import ctime
from threading import Thread

class ClientHandler(Thread):
    """Handles a client request."""
    def __init__(self, client):
        Thread.__init__(self)
        self._client = client

    def run(self):
        self._client.send(bytes(ctime() + '
Have a nice day!' , 'ascii'))
        self._client.close()

HOST = "localhost"
PORT = 15200 # Port number was changed here
BUFSIZE = 1024
ADDRESS = (HOST, PORT)

server = socket(AF_INET, SOCK_STREAM)
server.bind(ADDRESS)
server.listen(5)

# The server now just waits for connections from clients
# and hands sockets off to client handlers
while True:
    print('Waiting for connection')
    client, address = server.accept()
    print('...connected from:', address)
    handler = ClientHandler(client)
    handler.start()

If you prefer then have a look here for the basic client-server issues.