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  Socket Programming HOWTO  

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: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``.