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+****************************
+  Socket Programming HOWTO  
+****************************
+
+:Author: Gordon McMillan
+
+
+.. topic:: Abstract
+
+   Sockets are used nearly everywhere, but are one of the most severely
+   misunderstood technologies around. This is a 10,000 foot overview of sockets.
+   It's not really a tutorial - you'll still have work to do in getting things
+   operational. It doesn't cover the fine points (and there are a lot of them), but
+   I hope it will give you enough background to begin using them decently.
+
+
+Sockets
+=======
+
+Sockets are used nearly everywhere, but are one of the most severely
+misunderstood technologies around. This is a 10,000 foot overview of sockets.
+It's not really a tutorial - you'll still have work to do in getting things
+working. It doesn't cover the fine points (and there are a lot of them), but I
+hope it will give you enough background to begin using them decently.
+
+I'm only going to talk about INET sockets, but they account for at least 99% of
+the sockets in use. And I'll only talk about STREAM sockets - unless you really
+know what you're doing (in which case this HOWTO isn't for you!), you'll get
+better behavior and performance from a STREAM socket than anything else. I will
+try to clear up the mystery of what a socket is, as well as some hints on how to
+work with blocking and non-blocking sockets. But I'll start by talking about
+blocking sockets. You'll need to know how they work before dealing with
+non-blocking sockets.
+
+Part of the trouble with understanding these things is that "socket" can mean a
+number of subtly different things, depending on context. So first, let's make a
+distinction between a "client" socket - an endpoint of a conversation, and a
+"server" socket, which is more like a switchboard operator. The client
+application (your browser, for example) uses "client" sockets exclusively; the
+web server it's talking to uses both "server" sockets and "client" sockets.
+
+
+History
+-------
+
+Of the various forms of IPC (*Inter Process Communication*), sockets are by far
+the most popular.  On any given platform, there are likely to be other forms of
+IPC that are faster, but for cross-platform communication, sockets are about the
+only game in town.
+
+They were invented in Berkeley as part of the BSD flavor of Unix. They spread
+like wildfire with the Internet. With good reason --- the combination of sockets
+with INET makes talking to arbitrary machines around the world unbelievably easy
+(at least compared to other schemes).
+
+
+Creating a Socket
+=================
+
+Roughly speaking, when you clicked on the link that brought you to this page,
+your browser did something like the following::
+
+   #create an INET, STREAMing socket
+   s = socket.socket(
+       socket.AF_INET, socket.SOCK_STREAM)
+   #now connect to the web server on port 80 
+   # - the normal http port
+   s.connect(("www.mcmillan-inc.com", 80))
+
+When the ``connect`` completes, the socket ``s`` can now be used to send in a
+request for the text of this 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).
+
+What happens in the web server is a bit more complex. First, the web server
+creates a "server socket". ::
+
+   #create an INET, STREAMing socket
+   serversocket = socket.socket(
+       socket.AF_INET, socket.SOCK_STREAM)
+   #bind the socket to a public host, 
+   # and a well-known port
+   serversocket.bind((socket.gethostname(), 80))
+   #become a server socket
+   serversocket.listen(5)
+
+A couple things to notice: we used ``socket.gethostname()`` so that the socket
+would be visible to the outside world. If we had used ``s.bind(('', 80))`` or
+``s.bind(('localhost', 80))`` or ``s.bind(('127.0.0.1', 80))`` we would still
+have a "server" socket, but one that was only visible within the same machine.
+
+A second thing to note: low number ports are usually reserved for "well known"
+services (HTTP, SNMP etc). If you're playing around, use a nice high number (4
+digits).
+
+Finally, the argument to ``listen`` tells the socket library that we want it to
+queue up as many as 5 connect requests (the normal max) before refusing outside
+connections. If the rest of the code is written properly, that should be plenty.
+
+OK, now we have a "server" socket, listening on port 80. Now we enter the
+mainloop of the web server::
+
+   while 1:
+       #accept connections from outside
+       (clientsocket, address) = serversocket.accept()
+       #now do something with the clientsocket
+       #in this case, we'll pretend this is a threaded server
+       ct = client_thread(clientsocket)
+       ct.run()
+
+There's actually 3 general ways in which this loop could work - dispatching a
+thread to handle ``clientsocket``, create a new process to handle
+``clientsocket``, or restructure this app to use non-blocking sockets, and
+mulitplex between our "server" socket and any active ``clientsocket``\ s using
+``select``. More about that later. The important thing to understand now is
+this: this is *all* a "server" socket does. It doesn't send any data. It doesn't
+receive any data. It just produces "client" sockets. Each ``clientsocket`` is
+created in response to some *other* "client" socket doing a ``connect()`` to the
+host and port we're bound to. As soon as we've created that ``clientsocket``, we
+go back to listening for more connections. The two "clients" are free to chat it
+up - they are using some dynamically allocated port which will be recycled when
+the conversation ends.
+
+
+IPC
+---
+
+If you need fast IPC between two processes on one machine, you should look into
+whatever form of shared memory the platform offers. A simple protocol based
+around shared memory and locks or semaphores is by far the fastest technique.
+
+If you do decide to use sockets, bind the "server" socket to ``'localhost'``. On
+most platforms, this will take a shortcut around a couple of layers of network
+code and be quite a bit faster.
+
+
+Using a Socket
+==============
+
+The first thing to note, is that the web browser's "client" socket and the web
+server's "client" socket are identical beasts. That is, this is a "peer to peer"
+conversation. Or to put it another way, *as the designer, you will have to
+decide what the rules of etiquette are for a conversation*. Normally, the
+``connect``\ ing socket starts the conversation, by sending in a request, or
+perhaps a signon. But that's a design decision - it's not a rule of sockets.
+
+Now there are two sets of verbs to use for communication. You can use ``send``
+and ``recv``, or you can transform your client socket into a file-like beast and
+use ``read`` and ``write``. The latter is the way Java presents their sockets.
+I'm not going to talk about it here, except to warn you that you need to use
+``flush`` on sockets. These are buffered "files", and a common mistake is to
+``write`` something, and then ``read`` for a reply. Without a ``flush`` in
+there, you may wait forever for the reply, because the request may still be in
+your output buffer.
+
+Now we come the major stumbling block of sockets - ``send`` and ``recv`` operate
+on the network buffers. They do not necessarily handle all the bytes you hand
+them (or expect from them), because their major focus is handling the network
+buffers. In general, they return when the associated network buffers have been
+filled (``send``) or emptied (``recv``). They then tell you how many bytes they
+handled. It is *your* responsibility to call them again until your message has
+been completely dealt with.
+
+When a ``recv`` returns 0 bytes, it means the other side has closed (or is in
+the process of closing) the connection.  You will not receive any more data on
+this connection. Ever.  You may be able to send data successfully; I'll talk
+about that some on the next page.
+
+A protocol like HTTP uses a socket for only one transfer. The client sends a
+request, the reads a reply.  That's it. The socket is discarded. This means that
+a client can detect the end of the reply by receiving 0 bytes.
+
+But if you plan to reuse your socket for further transfers, you need to realize
+that *there is no "EOT" (End of Transfer) on a socket.* I repeat: if a socket
+``send`` or ``recv`` returns after handling 0 bytes, the connection has been
+broken.  If the connection has *not* been broken, you may wait on a ``recv``
+forever, because the socket will *not* tell you that there's nothing more to
+read (for now).  Now if you think about that a bit, you'll come to realize a
+fundamental truth of sockets: *messages must either be fixed length* (yuck), *or
+be delimited* (shrug), *or indicate how long they are* (much better), *or end by
+shutting down the connection*. The choice is entirely yours, (but some ways are
+righter than others).
+
+Assuming you don't want to end the connection, the simplest solution is a fixed
+length message::
+
+   class mysocket:
+       '''demonstration class only 
+         - coded for clarity, not efficiency
+       '''
+
+       def __init__(self, sock=None):
+   	if sock is None:
+   	    self.sock = socket.socket(
+   		socket.AF_INET, socket.SOCK_STREAM)
+   	else:
+   	    self.sock = sock
+
+       def connect(self, host, port):
+   	self.sock.connect((host, port))
+
+       def mysend(self, msg):
+   	totalsent = 0
+   	while totalsent < MSGLEN:
+   	    sent = self.sock.send(msg[totalsent:])
+   	    if sent == 0:
+   		raise RuntimeError, \
+   		    "socket connection broken"
+   	    totalsent = totalsent + sent
+
+       def myreceive(self):
+   	msg = ''
+   	while len(msg) < MSGLEN:
+   	    chunk = self.sock.recv(MSGLEN-len(msg))
+   	    if chunk == '':
+   		raise RuntimeError, \
+   		    "socket connection broken"
+   	    msg = msg + chunk
+   	return msg
+
+The sending code here is usable for almost any messaging scheme - in Python you
+send strings, and you can use ``len()`` to determine its length (even if it has
+embedded ``\0`` characters). It's mostly the receiving code that gets more
+complex. (And in C, it's not much worse, except you can't use ``strlen`` if the
+message has embedded ``\0``\ s.)
+
+The easiest enhancement is to make the first character of the message an
+indicator of message type, and have the type determine the length. Now you have
+two ``recv``\ s - the first to get (at least) that first character so you can
+look up the length, and the second in a loop to get the rest. If you decide to
+go the delimited route, you'll be receiving in some arbitrary chunk size, (4096
+or 8192 is frequently a good match for network buffer sizes), and scanning what
+you've received for a delimiter.
+
+One complication to be aware of: if your conversational protocol allows multiple
+messages to be sent back to back (without some kind of reply), and you pass
+``recv`` an arbitrary chunk size, you may end up reading the start of a
+following message. You'll need to put that aside and hold onto it, until it's
+needed.
+
+Prefixing the message with it's length (say, as 5 numeric characters) gets more
+complex, because (believe it or not), you may not get all 5 characters in one
+``recv``. In playing around, you'll get away with it; but in high network loads,
+your code will very quickly break unless you use two ``recv`` loops - the first
+to determine the length, the second to get the data part of the message. Nasty.
+This is also when you'll discover that ``send`` does not always manage to get
+rid of everything in one pass. And despite having read this, you will eventually
+get bit by it!
+
+In the interests of space, building your character, (and preserving my
+competitive position), these enhancements are left as an exercise for the
+reader. Lets move on to cleaning up.
+
+
+Binary Data
+-----------
+
+It is perfectly possible to send binary data over a socket. The major problem is
+that not all machines use the same formats for binary data. For example, a
+Motorola chip will represent a 16 bit integer with the value 1 as the two hex
+bytes 00 01. Intel and DEC, however, are byte-reversed - that same 1 is 01 00.
+Socket libraries have calls for converting 16 and 32 bit integers - ``ntohl,
+htonl, ntohs, htons`` where "n" means *network* and "h" means *host*, "s" means
+*short* and "l" means *long*. Where network order is host order, these do
+nothing, but where the machine is byte-reversed, these swap the bytes around
+appropriately.
+
+In these days of 32 bit machines, the ascii representation of binary data is
+frequently smaller than the binary representation. That's because a surprising
+amount of the time, all those longs have the value 0, or maybe 1. The string "0"
+would be two bytes, while binary is four. Of course, this doesn't fit well with
+fixed-length messages. Decisions, decisions.
+
+
+Disconnecting
+=============
+
+Strictly speaking, you're supposed to use ``shutdown`` on a socket before you
+``close`` it.  The ``shutdown`` is an advisory to the socket at the other end.
+Depending on the argument you pass it, it can mean "I'm not going to send
+anymore, but I'll still listen", or "I'm not listening, good riddance!".  Most
+socket libraries, however, are so used to programmers neglecting to use this
+piece of etiquette that normally a ``close`` is the same as ``shutdown();
+close()``.  So in most situations, an explicit ``shutdown`` is not needed.
+
+One way to use ``shutdown`` effectively is in an HTTP-like exchange. The client
+sends a request and then does a ``shutdown(1)``. This tells the server "This
+client is done sending, but can still receive."  The server can detect "EOF" by
+a receive of 0 bytes. It can assume it has the complete request.  The server
+sends a reply. If the ``send`` completes successfully then, indeed, the client
+was still receiving.
+
+Python takes the automatic shutdown a step further, and says that when a socket
+is garbage collected, it will automatically do a ``close`` if it's needed. But
+relying on this is a very bad habit. If your socket just disappears without
+doing a ``close``, the socket at the other end may hang indefinitely, thinking
+you're just being slow. *Please* ``close`` your sockets when you're done.
+
+
+When Sockets Die
+----------------
+
+Probably the worst thing about using blocking sockets is what happens when the
+other side comes down hard (without doing a ``close``). Your socket is likely to
+hang. SOCKSTREAM is a reliable protocol, and it will wait a long, long time
+before giving up on a connection. If you're using threads, the entire thread is
+essentially dead. There's not much you can do about it. As long as you aren't
+doing something dumb, like holding a lock while doing a blocking read, the
+thread isn't really consuming much in the way of resources. Do *not* try to kill
+the thread - part of the reason that threads are more efficient than processes
+is that they avoid the overhead associated with the automatic recycling of
+resources. In other words, if you do manage to kill the thread, your whole
+process is likely to be screwed up.
+
+
+Non-blocking Sockets
+====================
+
+If you've understood the preceeding, you already know most of what you need to
+know about the mechanics of using sockets. You'll still use the same calls, in
+much the same ways. It's just that, if you do it right, your app will be almost
+inside-out.
+
+In Python, you use ``socket.setblocking(0)`` to make it non-blocking. In C, it's
+more complex, (for one thing, you'll need to choose between the BSD flavor
+``O_NONBLOCK`` and the almost indistinguishable Posix flavor ``O_NDELAY``, which
+is completely different from ``TCP_NODELAY``), but it's the exact same idea. You
+do this after creating the socket, but before using it. (Actually, if you're
+nuts, you can switch back and forth.)
+
+The major mechanical difference is that ``send``, ``recv``, ``connect`` and
+``accept`` can return without having done anything. You have (of course) a
+number of choices. You can check return code and error codes and generally drive
+yourself crazy. If you don't believe me, try it sometime. Your app will grow
+large, buggy and suck CPU. So let's skip the brain-dead solutions and do it
+right.
+
+Use ``select``.
+
+In C, coding ``select`` is fairly complex. In Python, it's a piece of cake, but
+it's close enough to the C version that if you understand ``select`` in Python,
+you'll have little trouble with it in C. ::
+
+   ready_to_read, ready_to_write, in_error = \
+                  select.select(
+                     potential_readers, 
+                     potential_writers, 
+                     potential_errs, 
+                     timeout)
+
+You pass ``select`` three lists: the first contains all sockets that you might
+want to try reading; the second all the sockets you might want to try writing
+to, and the last (normally left empty) those that you want to check for errors.
+You should note that a socket can go into more than one list. The ``select``
+call is blocking, but you can give it a timeout. This is generally a sensible
+thing to do - give it a nice long timeout (say a minute) unless you have good
+reason to do otherwise.
+
+In return, you will get three lists. They have the sockets that are actually
+readable, writable and in error. Each of these lists is a subset (possbily
+empty) of the corresponding list you passed in. And if you put a socket in more
+than one input list, it will only be (at most) in one output list.
+
+If a socket is in the output readable list, you can be
+as-close-to-certain-as-we-ever-get-in-this-business that a ``recv`` on that
+socket will return *something*. Same idea for the writable list. You'll be able
+to send *something*. Maybe not all you want to, but *something* is better than
+nothing.  (Actually, any reasonably healthy socket will return as writable - it
+just means outbound network buffer space is available.)
+
+If you have a "server" socket, put it in the potential_readers list. If it comes
+out in the readable list, your ``accept`` will (almost certainly) work. If you
+have created a new socket to ``connect`` to someone else, put it in the
+ptoential_writers list. If it shows up in the writable list, you have a decent
+chance that it has connected.
+
+One very nasty problem with ``select``: if somewhere in those input lists of
+sockets is one which has died a nasty death, the ``select`` will fail. You then
+need to loop through every single damn socket in all those lists and do a
+``select([sock],[],[],0)`` until you find the bad one. That timeout of 0 means
+it won't take long, but it's ugly.
+
+Actually, ``select`` can be handy even with blocking sockets. It's one way of
+determining whether you will block - the socket returns as readable when there's
+something in the buffers.  However, this still doesn't help with the problem of
+determining whether the other end is done, or just busy with something else.
+
+**Portability alert**: On Unix, ``select`` works both with the sockets and
+files. Don't try this on Windows. On Windows, ``select`` works with sockets
+only. Also note that in C, many of the more advanced socket options are done
+differently on Windows. In fact, on Windows I usually use threads (which work
+very, very well) with my sockets. Face it, if you want any kind of performance,
+your code will look very different on Windows than on Unix. (I haven't the
+foggiest how you do this stuff on a Mac.)
+
+
+Performance
+-----------
+
+There's no question that the fastest sockets code uses non-blocking sockets and
+select to multiplex them. You can put together something that will saturate a
+LAN connection without putting any strain on the CPU. The trouble is that an app
+written this way can't do much of anything else - it needs to be ready to
+shuffle bytes around at all times.
+
+Assuming that your app is actually supposed to do something more than that,
+threading is the optimal solution, (and using non-blocking sockets will be
+faster than using blocking sockets). Unfortunately, threading support in Unixes
+varies both in API and quality. So the normal Unix solution is to fork a
+subprocess to deal with each connection. The overhead for this is significant
+(and don't do this on Windows - the overhead of process creation is enormous
+there). It also means that unless each subprocess is completely independent,
+you'll need to use another form of IPC, say a pipe, or shared memory and
+semaphores, to communicate between the parent and child processes.
+
+Finally, remember that even though blocking sockets are somewhat slower than
+non-blocking, in many cases they are the "right" solution. After all, if your
+app is driven by the data it receives over a socket, there's not much sense in
+complicating the logic just so your app can wait on ``select`` instead of
+``recv``.
+