Why is adding a while loop causing unexpected behavior?

Question

I have this client Python program, sending to a Java server program. My client was working fine before adding the while loop. I needed to add a while loop to keep sending user input; now the text does not appear on the other end until I kill the program (the Python client), then all the text I entered in Python will appear on the other end, but only after killing the program! What causes this?

import socket

HOST = "192.168.0.76"
PORT = 8080

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((HOST, PORT))
while (True):
    sock.sendall(raw_input(""))

You probably need a new thread to send the message. What is happening is that your program is blocking all the messages by repeating the loop before getting a chance to send anything. — OneCricketeer, May 23 '16 at 21:59
i thought of that,and i added a time sleep for 2 seconds... didn't work — aero, May 23 '16 at 22:00
That's because that only pauses the one single thread you are on. — OneCricketeer, May 23 '16 at 22:00
Sounds to me like something somewhere is buffering the data. — user2357112, May 23 '16 at 22:04
A [thread](https://en.wikipedia.org/wiki/Thread_(computing)) is a programming term. — OneCricketeer, May 23 '16 at 22:05
@user2357112 i know, it sounds like that to me too, iam just wondering why,cricket is talking about threading and maybe he's right although i don't know how that works, i'm not sure why it has to be that complex since it's just a while loop. — aero, May 23 '16 at 22:12
Sockets are a streaming protocol, so you can't use TCP for this. There is no EOT (end-of-transmission) on one. The only way to ensure all the data has been sent would be to close the socket. See the [Using a Socket](https://docs.python.org/2/howto/sockets.html#using-a-socket) section in the [Socket Programming HOWTO](https://docs.python.org/2/howto/sockets.html#using-a-socket). — martineau, May 24 '16 at 01:05

score 0 · Answer 1 · edited May 23 '17 at 12:19

0

Taking a quick look at the socket docs, I noticed that there is a setsockopt() function. If you look at the docs (man setsockopt), there's a SO_SNDBUF flag, which controls the buffer size while sending. If you didn't send enough, it probably doesn't flush until there's more data.

However, there doesn't appear to be a way to flush sockets (due to TCP frame sizes), and this related answer might give you some more hints: Python Socket Flush

edited May 23 '17 at 12:19

Community

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answered May 23 '16 at 22:33

wli

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On a related note, have you considered using websockets instead? It should give you quick communications of small data bits without you having to deal with this lowlevel stuff. – wli May 23 '16 at 22:35

score -1 · Answer 2 · answered May 23 '16 at 22:42

-1

change the loop to:

while True:
    input = raw_input("")
    sock.send(input)

answered May 23 '16 at 22:42

Danny

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`sock.send` does not guarantee to send all of the given buffer, the number of bytes sent must be explicitly checked, this approach will cause data loss. – donkopotamus May 23 '16 at 22:43

Why is adding a while loop causing unexpected behavior?

2 Answers2