/Doc/howto/sockets.rst
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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 (possibly
- 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
- potential_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.
- 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``.