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Socket Programming HOWTO
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:Author: Gordon McMillan
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Sockets are used nearly everywhere, but are one of the most severely
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misunderstood technologies around. This is a 10,000 foot overview of sockets.
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It's not really a tutorial - you'll still have work to do in getting things
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operational. It doesn't cover the fine points (and there are a lot of them), but
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I hope it will give you enough background to begin using them decently.
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I'm only going to talk about INET (i.e. IPv4) sockets, but they account for at least 99% of
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the sockets in use. And I'll only talk about STREAM (i.e. TCP) sockets - unless you really
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know what you're doing (in which case this HOWTO isn't for you!), you'll get
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better behavior and performance from a STREAM socket than anything else. I will
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try to clear up the mystery of what a socket is, as well as some hints on how to
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work with blocking and non-blocking sockets. But I'll start by talking about
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blocking sockets. You'll need to know how they work before dealing with
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Part of the trouble with understanding these things is that "socket" can mean a
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number of subtly different things, depending on context. So first, let's make a
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distinction between a "client" socket - an endpoint of a conversation, and a
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"server" socket, which is more like a switchboard operator. The client
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application (your browser, for example) uses "client" sockets exclusively; the
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web server it's talking to uses both "server" sockets and "client" sockets.
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Of the various forms of :abbr:`IPC (Inter Process Communication)`,
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sockets are by far the most popular. On any given platform, there are
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likely to be other forms of IPC that are faster, but for
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cross-platform communication, sockets are about the only game in town.
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They were invented in Berkeley as part of the BSD flavor of Unix. They spread
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like wildfire with the Internet. With good reason --- the combination of sockets
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with INET makes talking to arbitrary machines around the world unbelievably easy
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(at least compared to other schemes).
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Roughly speaking, when you clicked on the link that brought you to this page,
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your browser did something like the following::
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# create an INET, STREAMing socket
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s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
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# now connect to the web server on port 80 - the normal http port
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s.connect(("www.python.org", 80))
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When the ``connect`` completes, the socket ``s`` can be used to send
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in a request for the text of the page. The same socket will read the
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reply, and then be destroyed. That's right, destroyed. Client sockets
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are normally only used for one exchange (or a small set of sequential
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What happens in the web server is a bit more complex. First, the web server
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creates a "server socket"::
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# create an INET, STREAMing socket
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serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
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# bind the socket to a public host, and a well-known port
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serversocket.bind((socket.gethostname(), 80))
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# become a server socket
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serversocket.listen(5)
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A couple things to notice: we used ``socket.gethostname()`` so that the socket
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would be visible to the outside world. If we had used ``s.bind(('localhost',
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80))`` or ``s.bind(('127.0.0.1', 80))`` we would still have a "server" socket,
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but one that was only visible within the same machine. ``s.bind(('', 80))``
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specifies that the socket is reachable by any address the machine happens to
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A second thing to note: low number ports are usually reserved for "well known"
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services (HTTP, SNMP etc). If you're playing around, use a nice high number (4
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Finally, the argument to ``listen`` tells the socket library that we want it to
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queue up as many as 5 connect requests (the normal max) before refusing outside
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connections. If the rest of the code is written properly, that should be plenty.
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Now that we have a "server" socket, listening on port 80, we can enter the
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mainloop of the web server::
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# accept connections from outside
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(clientsocket, address) = serversocket.accept()
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# now do something with the clientsocket
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# in this case, we'll pretend this is a threaded server
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ct = client_thread(clientsocket)
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There's actually 3 general ways in which this loop could work - dispatching a
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thread to handle ``clientsocket``, create a new process to handle
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``clientsocket``, or restructure this app to use non-blocking sockets, and
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mulitplex between our "server" socket and any active ``clientsocket``\ s using
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``select``. More about that later. The important thing to understand now is
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this: this is *all* a "server" socket does. It doesn't send any data. It doesn't
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receive any data. It just produces "client" sockets. Each ``clientsocket`` is
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created in response to some *other* "client" socket doing a ``connect()`` to the
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host and port we're bound to. As soon as we've created that ``clientsocket``, we
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go back to listening for more connections. The two "clients" are free to chat it
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up - they are using some dynamically allocated port which will be recycled when
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the conversation ends.
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If you need fast IPC between two processes on one machine, you should look into
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pipes or shared memory. If you do decide to use AF_INET sockets, bind the
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"server" socket to ``'localhost'``. On most platforms, this will take a
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shortcut around a couple of layers of network code and be quite a bit faster.
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The :mod:`multiprocessing` integrates cross-platform IPC into a higher-level
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The first thing to note, is that the web browser's "client" socket and the web
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server's "client" socket are identical beasts. That is, this is a "peer to peer"
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conversation. Or to put it another way, *as the designer, you will have to
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decide what the rules of etiquette are for a conversation*. Normally, the
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``connect``\ ing socket starts the conversation, by sending in a request, or
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perhaps a signon. But that's a design decision - it's not a rule of sockets.
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Now there are two sets of verbs to use for communication. You can use ``send``
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and ``recv``, or you can transform your client socket into a file-like beast and
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use ``read`` and ``write``. The latter is the way Java presents its sockets.
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I'm not going to talk about it here, except to warn you that you need to use
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``flush`` on sockets. These are buffered "files", and a common mistake is to
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``write`` something, and then ``read`` for a reply. Without a ``flush`` in
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there, you may wait forever for the reply, because the request may still be in
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Now we come to the major stumbling block of sockets - ``send`` and ``recv`` operate
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on the network buffers. They do not necessarily handle all the bytes you hand
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them (or expect from them), because their major focus is handling the network
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buffers. In general, they return when the associated network buffers have been
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filled (``send``) or emptied (``recv``). They then tell you how many bytes they
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handled. It is *your* responsibility to call them again until your message has
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been completely dealt with.
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When a ``recv`` returns 0 bytes, it means the other side has closed (or is in
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the process of closing) the connection. You will not receive any more data on
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this connection. Ever. You may be able to send data successfully; I'll talk
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more about this later.
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A protocol like HTTP uses a socket for only one transfer. The client sends a
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request, then reads a reply. That's it. The socket is discarded. This means that
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a client can detect the end of the reply by receiving 0 bytes.
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But if you plan to reuse your socket for further transfers, you need to realize
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that *there is no* :abbr:`EOT (End of Transfer)` *on a socket.* I repeat: if a socket
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``send`` or ``recv`` returns after handling 0 bytes, the connection has been
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broken. If the connection has *not* been broken, you may wait on a ``recv``
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forever, because the socket will *not* tell you that there's nothing more to
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read (for now). Now if you think about that a bit, you'll come to realize a
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fundamental truth of sockets: *messages must either be fixed length* (yuck), *or
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be delimited* (shrug), *or indicate how long they are* (much better), *or end by
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shutting down the connection*. The choice is entirely yours, (but some ways are
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righter than others).
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Assuming you don't want to end the connection, the simplest solution is a fixed
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"""demonstration class only
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- coded for clarity, not efficiency
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def __init__(self, sock=None):
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self.sock = socket.socket(
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socket.AF_INET, socket.SOCK_STREAM)
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def connect(self, host, port):
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self.sock.connect((host, port))
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def mysend(self, msg):
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while totalsent < MSGLEN:
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sent = self.sock.send(msg[totalsent:])
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raise RuntimeError("socket connection broken")
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totalsent = totalsent + sent
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while bytes_recd < MSGLEN:
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chunk = self.sock.recv(min(MSGLEN - bytes_recd, 2048))
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raise RuntimeError("socket connection broken")
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bytes_recd = bytes_recd + len(chunk)
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return b''.join(chunks)
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The sending code here is usable for almost any messaging scheme - in Python you
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send strings, and you can use ``len()`` to determine its length (even if it has
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embedded ``\0`` characters). It's mostly the receiving code that gets more
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complex. (And in C, it's not much worse, except you can't use ``strlen`` if the
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message has embedded ``\0``\ s.)
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The easiest enhancement is to make the first character of the message an
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indicator of message type, and have the type determine the length. Now you have
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two ``recv``\ s - the first to get (at least) that first character so you can
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look up the length, and the second in a loop to get the rest. If you decide to
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go the delimited route, you'll be receiving in some arbitrary chunk size, (4096
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or 8192 is frequently a good match for network buffer sizes), and scanning what
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you've received for a delimiter.
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One complication to be aware of: if your conversational protocol allows multiple
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messages to be sent back to back (without some kind of reply), and you pass
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``recv`` an arbitrary chunk size, you may end up reading the start of a
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following message. You'll need to put that aside and hold onto it, until it's
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Prefixing the message with its length (say, as 5 numeric characters) gets more
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complex, because (believe it or not), you may not get all 5 characters in one
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``recv``. In playing around, you'll get away with it; but in high network loads,
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your code will very quickly break unless you use two ``recv`` loops - the first
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to determine the length, the second to get the data part of the message. Nasty.
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This is also when you'll discover that ``send`` does not always manage to get
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rid of everything in one pass. And despite having read this, you will eventually
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In the interests of space, building your character, (and preserving my
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competitive position), these enhancements are left as an exercise for the
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reader. Lets move on to cleaning up.
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It is perfectly possible to send binary data over a socket. The major problem is
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that not all machines use the same formats for binary data. For example, a
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Motorola chip will represent a 16 bit integer with the value 1 as the two hex
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bytes 00 01. Intel and DEC, however, are byte-reversed - that same 1 is 01 00.
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Socket libraries have calls for converting 16 and 32 bit integers - ``ntohl,
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htonl, ntohs, htons`` where "n" means *network* and "h" means *host*, "s" means
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*short* and "l" means *long*. Where network order is host order, these do
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nothing, but where the machine is byte-reversed, these swap the bytes around
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In these days of 32 bit machines, the ascii representation of binary data is
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frequently smaller than the binary representation. That's because a surprising
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amount of the time, all those longs have the value 0, or maybe 1. The string "0"
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would be two bytes, while binary is four. Of course, this doesn't fit well with
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fixed-length messages. Decisions, decisions.
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Strictly speaking, you're supposed to use ``shutdown`` on a socket before you
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``close`` it. The ``shutdown`` is an advisory to the socket at the other end.
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Depending on the argument you pass it, it can mean "I'm not going to send
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anymore, but I'll still listen", or "I'm not listening, good riddance!". Most
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socket libraries, however, are so used to programmers neglecting to use this
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piece of etiquette that normally a ``close`` is the same as ``shutdown();
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close()``. So in most situations, an explicit ``shutdown`` is not needed.
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One way to use ``shutdown`` effectively is in an HTTP-like exchange. The client
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sends a request and then does a ``shutdown(1)``. This tells the server "This
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client is done sending, but can still receive." The server can detect "EOF" by
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a receive of 0 bytes. It can assume it has the complete request. The server
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sends a reply. If the ``send`` completes successfully then, indeed, the client
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Python takes the automatic shutdown a step further, and says that when a socket
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is garbage collected, it will automatically do a ``close`` if it's needed. But
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relying on this is a very bad habit. If your socket just disappears without
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doing a ``close``, the socket at the other end may hang indefinitely, thinking
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you're just being slow. *Please* ``close`` your sockets when you're done.
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Probably the worst thing about using blocking sockets is what happens when the
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other side comes down hard (without doing a ``close``). Your socket is likely to
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hang. TCP is a reliable protocol, and it will wait a long, long time
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before giving up on a connection. If you're using threads, the entire thread is
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essentially dead. There's not much you can do about it. As long as you aren't
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doing something dumb, like holding a lock while doing a blocking read, the
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thread isn't really consuming much in the way of resources. Do *not* try to kill
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the thread - part of the reason that threads are more efficient than processes
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is that they avoid the overhead associated with the automatic recycling of
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resources. In other words, if you do manage to kill the thread, your whole
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process is likely to be screwed up.
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If you've understood the preceding, you already know most of what you need to
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know about the mechanics of using sockets. You'll still use the same calls, in
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much the same ways. It's just that, if you do it right, your app will be almost
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In Python, you use ``socket.setblocking(0)`` to make it non-blocking. In C, it's
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more complex, (for one thing, you'll need to choose between the BSD flavor
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``O_NONBLOCK`` and the almost indistinguishable Posix flavor ``O_NDELAY``, which
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is completely different from ``TCP_NODELAY``), but it's the exact same idea. You
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do this after creating the socket, but before using it. (Actually, if you're
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nuts, you can switch back and forth.)
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The major mechanical difference is that ``send``, ``recv``, ``connect`` and
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``accept`` can return without having done anything. You have (of course) a
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number of choices. You can check return code and error codes and generally drive
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yourself crazy. If you don't believe me, try it sometime. Your app will grow
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large, buggy and suck CPU. So let's skip the brain-dead solutions and do it
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In C, coding ``select`` is fairly complex. In Python, it's a piece of cake, but
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it's close enough to the C version that if you understand ``select`` in Python,
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you'll have little trouble with it in C::
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ready_to_read, ready_to_write, in_error = \
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You pass ``select`` three lists: the first contains all sockets that you might
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want to try reading; the second all the sockets you might want to try writing
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to, and the last (normally left empty) those that you want to check for errors.
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You should note that a socket can go into more than one list. The ``select``
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call is blocking, but you can give it a timeout. This is generally a sensible
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thing to do - give it a nice long timeout (say a minute) unless you have good
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reason to do otherwise.
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In return, you will get three lists. They contain the sockets that are actually
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readable, writable and in error. Each of these lists is a subset (possibly
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empty) of the corresponding list you passed in.
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If a socket is in the output readable list, you can be
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as-close-to-certain-as-we-ever-get-in-this-business that a ``recv`` on that
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socket will return *something*. Same idea for the writable list. You'll be able
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to send *something*. Maybe not all you want to, but *something* is better than
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nothing. (Actually, any reasonably healthy socket will return as writable - it
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just means outbound network buffer space is available.)
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If you have a "server" socket, put it in the potential_readers list. If it comes
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out in the readable list, your ``accept`` will (almost certainly) work. If you
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have created a new socket to ``connect`` to someone else, put it in the
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potential_writers list. If it shows up in the writable list, you have a decent
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chance that it has connected.
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Actually, ``select`` can be handy even with blocking sockets. It's one way of
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determining whether you will block - the socket returns as readable when there's
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something in the buffers. However, this still doesn't help with the problem of
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determining whether the other end is done, or just busy with something else.
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**Portability alert**: On Unix, ``select`` works both with the sockets and
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files. Don't try this on Windows. On Windows, ``select`` works with sockets
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only. Also note that in C, many of the more advanced socket options are done
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differently on Windows. In fact, on Windows I usually use threads (which work
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very, very well) with my sockets.
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