.net qr code NETWORK DATA AND NETWORK ERRORS in Font

Maker QR-Code in Font NETWORK DATA AND NETWORK ERRORS

CHAPTER 5 NETWORK DATA AND NETWORK ERRORS
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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 format = struct.Struct('!I') # for messages up to 2**32 - 1 in length def 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 get(sock): lendata = recvall(sock, format.size) (length,) = format.unpack(lendata) return recvall(sock, length)
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def put(sock, message): sock.send(format.pack(len(message)) + message) 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) while True: message = get(sc) if not message: break print 'Message says:', repr(message) sc.close() s.close() elif sys.argv[1:] == ['client']: s.connect((HOST, PORT)) s.shutdown(socket.SHUT_RD) put(s, 'Beautiful is better than ugly.') put(s, 'Explicit is better than implicit.') put(s, 'Simple is better than complex.') put(s, '') s.close() else: print >>sys.stderr, 'usage: streamer.py server|client [host]' Note how careful we have to be! Even though four bytes of length is such a tiny amount of data that we cannot imagine recv() not returning it all at once, our code is still correct only if we carefully wrap
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CHAPTER 5 NETWORK DATA AND NETWORK ERRORS
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recv() in a loop that just in case will keep demanding more data until all four bytes have arrived. This is the kind of caution that will serve you well when writing network code. It is also the kind of fiddly little detail that makes most people glad that they can deal just with higher-level protocols, and not have to learn to talk with sockets in the first place! So those are six good options for dividing up an unending stream of data into digestible chunks so that clients and servers know when a message is complete and they can turn around and respond. Note that many modern protocols mix them together, and that you are free to do the same thing. A good example is the HTTP protocol, which we will learn more about in Part 2 of this book. It uses a delimiter the blank line '\r\n\r\n' to signal when its headers are complete. Because the headers are text, line endings can safely be treated as special characters. But since the actual payload can be pure binary data, like an image or compressed file, the Content-Length provided in the headers is used to determine how much more data to read off of the socket. Thus HTTP mixes the fourth and fifth patterns we have looked at here. In fact, it can also use our sixth option: if a server is streaming a response whose length it cannot predict, then it can use a chunked encoding, which sends several blocks that are each prefixed with their length. A zero length marks the end of the transmission, just as it does in Listing 5 2.
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Pickles and Self-Delimiting Formats
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Note that some kinds of data that you might send across the network already include some form of delimiting built-in. If you are transmitting such data, then you might not have to impose your own framing atop what the data is already doing. Consider Python pickles, for example, the native form of serialization that comes with the Standard Library. Using a quirky mix of text commands and data, a pickle stores the contents of a Python data structure so that you can reconstruct it later or on a different machine: >>> import pickle >>> pickle.dumps([5, 6, 7]) '(lp0\nI5\naI6\naI7\na.' The interesting thing about the format is the '.' character that you see at the end of the foregoing string it is the format's way of marking the end of a pickle. Upon encountering it, the loader can stop and return the value without reading any further. Thus we can take the foregoing pickle, stick some ugly data on the end, and see that loads() will completely ignore it and give us our original list back: >>> pickle.loads('(lp0\nI5\naI6\naI7\na.UjJGdVpHRnNaZz09') [5, 6, 7] Of course, using loads() this way is not useful for network data, since it does not tell us how many bytes it processed in order to reload the pickle; we still do not know how much of our string is pickle data. But if we switch to reading from a file and using the pickle load() function, then the file pointer will be left right at the end of the pickle data, and we can start reading from there if we want to read what came after the pickle: >>> from StringIO import StringIO >>> f = StringIO('(lp0\nI5\naI6\naI7\na.UjJGdVpHRnNaZz09') >>> pickle.load(f) [5, 6, 7] >>> f.pos 18 >>> f.read() 'UjJGdVpHRnNaZz09' Alternately, we could create a protocol that just consisted of sending pickles back and forth between two Python programs. Note that we would not need the kind of loop that we put into the recvall()
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