Python socket not receiving without sending

Question

I copied the echo server example from the python documentation and it's working fine. But when I edit the code, so it wont send the data back to the client, the socket.recv() method doesn't return when it's called the second time.

import socket

HOST = ''
PORT = 50007 
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)

conn, addr = s.accept()
print('Connected by', addr)

while True:
    data = conn.recv(1024)
    if not data: break
conn.sendall(b'ok')
conn.close()

In the original version from the python documentation the while loop is slightly different:

while True:
    data = conn.recv(1024)
    if not data: break
    conn.sendall(data)

Client's code:

import socket

HOST = 'localhost'
PORT = 50007
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall(b'Hello, world')
data = s.recv(1024)
s.close()
print('Received', repr(data))

Where's the client's source code? Are you just connecting directly with nc or telnet? — Kevin
The receive won't return until the client closes the connection. But the client won't close the connection until it receives something. — tdelaney
I first want to close the connection after the client received an OK by the server... how can I do this in my case? — no0by5
Can you explain why you are calling recv a second time? Also, why does the client close the socket without ensuring it has received everything the server intends to send? — David Schwartz
For the case the client is sending larger data and the buffer of 1024 isn't big enough. — no0by5

tdelaney tdelaney · Accepted Answer · 2017-04-14T23:28:00

TCP sockets are streams of data. There is no one-to-one correlation between send calls on one side and receive calls on the other. There is a higher level correlation based on the protocol you implement. In the original code, the rule was that the server would send exactly what it received until the client closed the incoming side of the connection. Then the server closed the socket.

With your change, the rules changed. Now the server keeps receiving and discarding data until the client closes the incoming side of the connection. Then the server sends "ok" and closes the socket.

A client using the first rule hangs because its expecting data before it closes the socket. If it wants to work with this new server rule, it has to close its outgoing side of the socket to tell the server its done, and then it can get the return data.

I've updated the client and server to shutdown parts of the connection and also have the client do multiple recv's in case the incoming data is fragmented. Less complete implementations seem to work for small payloads because you are unlikely to get fragmentation, but break horribly in real production code.

server

import socket

HOST = ''
PORT = 50007 
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((HOST, PORT))
s.listen(1)

conn, addr = s.accept()
print('Connected by', addr)

while True:
    data = conn.recv(1024)
    if not data: break
conn.sendall(b'ok')
conn.shutdown(socket.SHUT_WR)
conn.close()

client

import socket

HOST = 'localhost'
PORT = 50007
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall(b'Hello, world')
s.shutdown(socket.SHUT_WR)
data = b''
while True:
    buf = s.recv(1024)
    if not buf:
        break
    data += buf
s.close()
print('Received', repr(data))

Python socket not receiving without sending

2 Answers