.net qr code NETWORK DATA AND NETWORK ERRORS in Font

Making QR Code ISO/IEC18004 in Font NETWORK DATA AND NETWORK ERRORS

CHAPTER 5 NETWORK DATA AND NETWORK ERRORS
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which also supplies calls like socket() and bind(). I suggest that you ignore these awkward functions, and use the struct module instead; it is more flexible, more general, and produces more readable code.
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Framing and Quoting
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If you are using UDP datagrams for communication, then the protocol itself takes the trouble to deliver your data in discrete and identifiable chunks and you have to reorder and re-transmit them yourself if anything goes wrong on the network, as outlined in 2. But if you have made the far more common option of using a TCP stream for communication, then you will face the issue of framing of how to delimit your messages so that the receiver can tell where one message ends and the next begins. Since the data you supply to sendall() might be broken up into several packets, the program that receives your message might have to make several recv() calls before your whole message has been read. The issue of framing asks the question: when is it safe for the receiver to finally stop calling recv() and respond to your message As you might imagine, there are several approaches. First, there is a pattern that can be used by extremely simple network protocols that involve only the delivery of data no response is expected, so there never has to come a time when the receiver decides Enough! and turns around to send a response. In this case, the sender can loop until all of the outgoing data has been passed to sendall() and then close() the socket. The receiver need only call recv() repeatedly until the call finally returns an empty string, indicating that the sender has finally closed the socket. You can see this pattern in Listing 5 1. Listing 5 1. Sending a Single Stream of Data #!/usr/bin/env python # Foundations of Python Network Programming - 5 - streamer.py # Client that sends data then closes the socket, not expecting a reply. import socket, sys s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) HOST = sys.argv.pop() if len(sys.argv) == 3 else '127.0.0.1' PORT = 1060 if sys.argv[1:] == ['server']: s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) s.bind((HOST, PORT)) s.listen(1) print 'Listening at', s.getsockname() sc, sockname = s.accept() print 'Accepted connection from', sockname sc.shutdown(socket.SHUT_WR) message = '' while True: more = sc.recv(8192) # arbitrary value of 8k if not more: # socket has closed when recv() returns '' break message += more print 'Done receiving the message; it says:' print message sc.close()
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CHAPTER 5 NETWORK DATA AND NETWORK ERRORS
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s.close()
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elif sys.argv[1:] == ['client']: s.connect((HOST, PORT)) s.shutdown(socket.SHUT_RD) s.sendall('Beautiful is better than ugly.\n') s.sendall('Explicit is better than implicit.\n') s.sendall('Simple is better than complex.\n') s.close() else: print >>sys.stderr, 'usage: streamer.py server|client [host]' If you run this script as a server and then, at another command prompt, run the client version, you will see that all of the client's data makes it intact to the server, with the end-of-file event generated by the client closing the socket serving as the only framing that is necessary: $ python streamer.py server Listening at ('127.0.0.1', 1060) Accepted connection from ('127.0.0.1', 52039) Done receiving the message; it says: Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Note the nicety that, since this socket is not intended to receive any data, the client and server both go ahead and shut down communication in the other direction. This prevents any accidental use of the socket in the other direction use that could eventually queue up enough unread data to produce deadlock, as we saw in Listing 3-2. It is really only necessary for either the client or server to call shutdown() on the socket; it is redundant for both of them to do so. But since you someday might be programming only one end of such a connection, I thought you might want to see how the shutdown looks from both directions. A second pattern is a variant on the first: streaming in both directions. The socket is initially left open in both directions. First, data is streamed in one direction exactly as shown in Listing 5 1 and then that direction alone is shut down. Second, data is then streamed in the other direction, and the socket is finally closed. Again, Listing 3-2 provides an important warning: always finish the data transfer in one direction before turning around to stream data back in the other, or you could produce a client and server that are deadlocked. A third pattern, which we have already seen, is to use fixed-length messages, as illustrated in Listing 3-1. You can use the Python sendall() method to keep sending parts of a string until the whole thing has been transmitted, and then use a recv() loop of our own devising to make sure that you receive the whole message: def recvall(sock, length): data = '' while len(data) < length: more = sock.recv(length - len(data)) if not more: raise EOFError('socket closed %d bytes into a %d-byte message' % (len(data), length)) data += more return data
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