Doc/howto/sockets.rst - platform/external/python/cpython3 - Gitiles

 .. _socket-howto:

 ****************************
   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
 =======

 I'm only going to talk about INET (i.e. IPv4) sockets, but they account for at least 99% of
 the sockets in use. And I'll only talk about STREAM (i.e. TCP) 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 :abbr:`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.python.org", 80))

 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).

 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(('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.  ``s.bind(('', 80))``
 specifies that the socket is reachable by any address the machine happens to
 have.

 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.

 Now that we have a "server" socket, listening on port 80, we can enter the
 mainloop of the web server::

    while True:
        # 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
 multiplex 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
 pipes or shared memory.  If you do decide to use AF_INET 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.

 .. seealso::
    The :mod:`multiprocessing` integrates cross-platform IPC into a higher-level
    API.


 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 its 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 to 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
 more about this later.

 A protocol like HTTP uses a socket for only one transfer. The client sends a
 request, then 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* :abbr:`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):
            chunks = []
            bytes_recd = 0
            while bytes_recd < MSGLEN:
                chunk = self.sock.recv(min(MSGLEN - bytes_recd, 2048))
                if chunk == b'':
                    raise RuntimeError("socket connection broken")
                chunks.append(chunk)
                bytes_recd = bytes_recd + len(chunk)
            return b''.join(chunks)

 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 its 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. TCP 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 preceding, 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(False)`` 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 contain the sockets that are actually
 readable, writable and in error. Each of these lists is a subset (possibly
 empty) of the corresponding list you passed in.

 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
 potential_writers list. If it shows up in the writable list, you have a decent
 chance that it has connected.

 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.
	.. _socket-howto:

	****************************
	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
	=======

	I'm only going to talk about INET (i.e. IPv4) sockets, but they account for at least 99% of
	the sockets in use. And I'll only talk about STREAM (i.e. TCP) 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 :abbr:`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.python.org", 80))

	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).

	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(('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. ``s.bind(('', 80))``
	specifies that the socket is reachable by any address the machine happens to
	have.

	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.

	Now that we have a "server" socket, listening on port 80, we can enter the
	mainloop of the web server::

	while True:
	# 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
	multiplex 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
	pipes or shared memory. If you do decide to use AF_INET 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.

	.. seealso::
	The :mod:`multiprocessing` integrates cross-platform IPC into a higher-level
	API.


	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 its 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 to 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
	more about this later.

	A protocol like HTTP uses a socket for only one transfer. The client sends a
	request, then 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 :abbr:`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):
	chunks = []
	bytes_recd = 0
	while bytes_recd < MSGLEN:
	chunk = self.sock.recv(min(MSGLEN - bytes_recd, 2048))
	if chunk == b'':
	raise RuntimeError("socket connection broken")
	chunks.append(chunk)
	bytes_recd = bytes_recd + len(chunk)
	return b''.join(chunks)

	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 its 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. TCP 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 preceding, 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(False)`` 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 contain the sockets that are actually
	readable, writable and in error. Each of these lists is a subset (possibly
	empty) of the corresponding list you passed in.

	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
	potential_writers list. If it shows up in the writable list, you have a decent
	chance that it has connected.

	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.