CODEWITHSUNDEEP
pythonsocketsnetwork-programming
Pradyot
Pradyot

Reputation: 3059

Can the python client tcp socket be used for multiple sends?

I tried using a socket for 2 sends. The first one succeeds and the next one does not. From the http://docs.python.org/howto/sockets.html it would appear that multiple sends should be allowed. For Better or worse, I don't really need to read from the socket. I have used twisted, but for the present purpose, I would like to stick to a socket, if I can help it(partly because I am using it within an application already using twisted to communicate.. this is a seperate connection).

"When the connect completes, the socket s can be used to send in a request for the text of the page. The same socket will read the reply, and then be destroyed. That’s right, destroyed. Client sockets are normally only used for one exchange (or a small set of sequential exchanges)."

return value for the send that succeeds = 35 return value for the send that FAILS = 32

code with some minor editing to remove any business logic.

self._commandSock = socket.socket(socket.AF_INET,socket.SOCK_STREAM)


def sendPrereqs(self,id,prereqs):
    self._commandSock.connect(self._commandConnection)
    #parse prereqs
    temp = prereqs.split(',')
    for pair in temp:
        tup = pair.partition(':')
        try:
            command = 'some command'
            logging.info('sending command: ' + command)
            ret = self._commandSock.send(command)
            if ret == None:
                logging.info('send called successfully: ' + command)
            else:
                logging.info('socket returned non-None: ' + str(ret))
        except:

            print 'Unexpected Exception ', sys.exc_info()[0]()
            print sys.exc_info()
            #logging.info('Unexpected Exception '+ str(sys.exc_info()[0]()))
            #logging.info(' ' + str(sys.exc_info()))
    self._commandSock.close()`

Upvotes: 0

Views: 3902

Answers (2)

abarnert
abarnert

Reputation: 365717

return value for the send that succeeds = 35 return value for the send that FAILS = 32

Documentation says that successful send should return None.

No it doesn't. Documentation says:

Returns the number of bytes sent. Applications are responsible for checking that all data has been sent; if only some of the data was transmitted, the application needs to attempt delivery of the remaining data. For further information on this concept, consult the Socket Programming HOWTO.

You still haven't explained what you mean by "FAILS". The send call is returning successfully, and it's almost certainly placed 32 bytes into the socket write buffer.

If the only reason you think it's failing is that it returns the correct value, then the answer is obviously that it's not failing.

If something else is going wrong, there are all kinds of things that could be wrong at a higher level. One likely one is this: The server (especially if it was coded by someone who doesn't understand sockets well) is coded to expect one recv() of 35 bytes, and one recv() of 32 bytes. But it's actually getting a single recv() of 67 bytes, and waiting forever for the second, which never comes. There is no rule guaranteeing that each send() on a stream (TCP) socket corresponds to one recv() on the other side. You need to create some kind of stream protocol that demarcates the separate messages.

Meanwhile, the quote you're referring to is irrelevant. It's describing how client sockets are used by simple web browsers: They make a connection, do one send, receive all the data, then destroy the connection. I can see why it misled you, and you may want to file a documentation bug to get it improved. But for now, just ignore it.

If you want to make absolutely sure that the client is sending the right data, and the problem is in the server, there are two easy ways to do that:

  1. Use netcat as a trivial substitute server (e.g., nc -kl 6000, replacing the "6000" with the actual port) and making sure it logs what you think the server should be seeing.

  2. Use Wireshark to watch the connection between the client and server.

Once you've verified that the problem is on the server side, you need to debug the server. If you need help with that, that's probably best done in a new question, where you can post the server code instead of the client, and explain (with a link here) that you're sure the client is sending the right information.

Upvotes: 2

phihag
phihag

Reputation: 287845

The documentation is only referring to a common scenario. You can call send, sendall, and sendto on all sockets as often as you want - as long as the socket is not closed.

Note that these methods return the number of bytes sent, 32 and 35 simply mean you sent 32 bytes the first and 35 bytes the second time.

The fact that socket.send returns without an exception means that the data got handed to the operating system, but not that it actually reached the endpoint (or has been read correctly by an application there).

Upvotes: 0

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