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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta name="description" content="LuaSocket: Introduction to the core">
<meta name="keywords" content="Lua, LuaSocket, TCP, UDP, Network,
Library, Support">
<title>LuaSocket: Introduction to the core</title>
<link rel="stylesheet" href="reference.css" type="text/css">
</head>
<body>
<!-- header +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<div class=header>
<hr>
<center>
<table summary="LuaSocket logo">
<tr><td align=center><a href="http://www.lua.org">
<img width=128 height=128 border=0 alt="LuaSocket" src="luasocket.png">
</a></td></tr>
<tr><td align=center valign=top>Network support for the Lua language
</td></tr>
</table>
<p class=bar>
<a href="index.html">home</a> ·
<a href="index.html#download">download</a> ·
<a href="installation.html">installation</a> ·
<a href="introduction.html">introduction</a> ·
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</p>
</center>
<hr>
</div>
<!-- introduction +++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<h2>Introduction</h2>
<p>
LuaSocket is a <a href="http://www.lua.org">Lua</a> extension library
that is composed by two parts: a C core that provides support for the TCP
and UDP transport layers, and a set of Lua modules that add support for
the SMTP (sending e-mails), HTTP (WWW access) and FTP (uploading and
downloading files) protocols and other functionality commonly needed by
applications that deal with the Internet. This introduction is about the C
core.
</p>
<p>
Communication in LuaSocket is performed via I/O objects. These can
represent different network domains. Currently, support is provided for TCP
and UDP, but nothing prevents other developers from implementing SSL, Local
Domain, Pipes, File Descriptors etc. I/O objects provide a standard
interface to I/O across different domains and operating systems.
</p>
<p>
The API design had two goals in mind. First, users
experienced with the C API to sockets should feel comfortable using LuaSocket.
Second, the simplicity and the feel of the Lua language should be
preserved. To achieve these goals, the LuaSocket API keeps the function names and semantics the C API whenever possible, but their usage in Lua has been greatly simplified.
</p>
<p>
One of the simplifications is the receive pattern capability.
Applications can read data from stream domains (such as TCP)
line by line, block by block, or until the connection is closed.
All I/O reads are buffered and the performance differences between
different receive patterns are negligible.
</p>
<p>
Another advantage is the flexible timeout control
mechanism. As in C, all I/O operations are blocking by default. For
example, the <a href=tcp.html#send><tt>send</tt></a>,
<a href=tcp.html#receive><tt>receive</tt></a> and
<a href=tcp.html#accept><tt>accept</tt></a> methods
of the TCP domain will block the caller application until
the operation is completed (if ever!). However, with a call to the
<a href=tcp.html#settimeout><tt>settimeout</tt></a>
method, an application can specify upper limits on
the time it can be blocked by LuaSocket (the "<tt>total</tt>" timeout), on
the time LuaSocket can internally be blocked by any OS call (the
"<tt>block</tt>" timeout) or a combination of the two. Each LuaSocket
call might perform several OS calls, so that the two timeout values are
<em>not</em> equivalent.
</p>
<p>
Finally, the host name resolution is transparent, meaning that most
functions and methods accept both IP addresses and host names. In case a
host name is given, the library queries the system's resolver and
tries the main IP address returned. Note that direct use of IP addresses
is more efficient, of course. The
<a href=dns.html#toip><tt>toip</tt></a>
and <a href=dns.html#tohostname><tt>tohostname</tt></a>
functions from the DNS module are provided to convert between host names and IP addresses.
</p>
<p>
Together, these changes make network programming in LuaSocket much simpler
than it is in C, as the following sections will show.
</p>
<!-- tcp ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<h3 id=tcp>TCP</h3>
<p>
TCP (Transfer Control Protocol) is reliable stream protocol. In other
words, applications communicating through TCP can send and receive data as
an error free stream of bytes. Data is split in one end and
reassembled transparently on the other end. There are no boundaries in
the data transfers. The library allows users to read data from the
sockets in several different granularities: patterns are available for
lines, arbitrary sized blocks or "read up to connection closed", all with
good performance.
</p>
<p>
The library distinguishes three types of TCP sockets: <em>master</em>,
<em>client</em> and <em>server</em> sockets.
</p>
<p>
Master sockets are newly created TCP sockets returned by the function
<a href=tcp.html#tcp><tt>socket.tcp</tt></a>. A master socket is
transformed into a server socket
after it is associated with a <em>local</em> address by a call to the
<a href=tcp.html#bind><tt>bind</tt></a> method followed by a call to the
<a href=tcp.html#listen><tt>listen</tt></a>. Conversely, a master socket
can be changed into a client socket with the method
<a href=tcp.html#connect><tt>connect</tt></a>,
which associates it with a <em>remote</em> address.
</p>
<p>
On server sockets, applications can use the
<a href=tcp.html#accept><tt>accept</tt></a> method
to wait for a client connection. Once a connection is established, a
client socket object is returned representing this connection. The
other methods available for server socket objects are
<a href=tcp.html#getsockname><tt>getsockname</tt></a>,
<a href=tcp.html#setoption><tt>setoption</tt></a>,
<a href=tcp.html#settimeout><tt>settimeout</tt></a>, and
<a href=tcp.html#close><tt>close</tt></a>.
</p>
<p>
Client sockets are used to exchange data between two applications over
the Internet. Applications can call the methods
<a href=tcp.html#send><tt>send</tt></a> and
<a href=tcp.html#receive><tt>receive</tt></a>
to send and receive data. The other methods
available for client socket objects are
<a href=tcp.html#getsockname><tt>getsockname</tt></a>,
<a href=tcp.html#getpeername><tt>getpeername</tt></a>,
<a href=tcp.html#setoption><tt>setoption</tt></a>,
<a href=tcp.html#settimeout><tt>settimeout</tt></a>,
<a href=tcp.html#shutdown><tt>shutdown</tt></a>, and
<a href=tcp.html#close><tt>close</tt></a>.
</p>
<p>
Example:
</p>
<blockquote>
<p>
A simple echo server, using LuaSocket. The program binds to an ephemeral
port (one that is chosen by the operating system) on the local host and
awaits client connections on that port. When a connection is established,
the program reads a line from the remote end and sends it back, closing
the connection immediately. You can test it using the telnet
program.
</p>
<pre class=example>
-- load namespace
local socket = require("socket")
-- create a TCP socket and bind it to the local host, at any port
local server = assert(socket.bind("*", 0))
-- find out which port the OS chose for us
local ip, port = server:getsockname()
-- print a message informing what's up
print("Please telnet to localhost on port " .. port)
print("After connecting, you have 10s to enter a line to be echoed")
-- loop forever waiting for clients
while 1 do
-- wait for a connection from any client
local client = server:accept()
-- make sure we don't block waiting for this client's line
client:settimeout(10)
-- receive the line
local line, err = client:receive()
-- if there was no error, send it back to the client
if not err then client:send(line .. "\n") end
-- done with client, close the object
client:close()
end
</pre>
</blockquote>
<!-- udp ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<h3 id=udp>UDP</h3>
<p>
UDP (User Datagram Protocol) is a non-reliable datagram protocol. In
other words, applications communicating through UDP send and receive
data as independent blocks, which are not guaranteed to reach the other
end. Even when they do reach the other end, they are not guaranteed to be
error free. Data transfers are atomic, one datagram at a time. Reading
only part of a datagram discards the rest, so that the following read
operation will act on the next datagram. The advantages are in
simplicity (no connection setup) and performance (no error checking or
error correction).
</p>
<p>
Note that although no guarantees are made, these days
networks are so good that, under normal circumstances, few errors
happen in practice.
</p>
<p>
An UDP socket object is created by the
<a href=udp.html#udp><tt>socket.udp</tt></a> function. UDP
sockets do not need to be connected before use. The method
<a href=udp.html#sendto><tt>sendto</tt></a>
can be used immediately after creation to
send a datagram to IP address and port. Host names are not allowed
because performing name resolution for each packet would be forbiddingly
slow. Methods
<a href=udp.html#receive><tt>receive</tt></a> and
<a href=udp.html#receivefrom><tt>receivefrom</tt></a>
can be used to retrieve datagrams, the latter returning the IP and port of
the sender as extra return values (thus being slightly less
efficient).
</p>
<p>
When communication is performed repeatedly with a single peer, an
application should call the
<a href=udp.html#setpeername><tt>setpeername</tt></a> method to specify a
permanent partner. Methods
<a href=udp.html#sendto><tt>sendto</tt></a> and
<a href=udp.html#receivefrom><tt>receivefrom</tt></a>
can no longer be used, but the method
<a href=udp.html#send><tt>send</tt></a> can be used to send data
directly to the peer, and the method
<a href=udp.html#receive><tt>receive</tt></a>
will only return datagrams originating
from that peer. There is about 30% performance gain due to this practice.
</p>
<p>
To associate an UDP socket with a local address, an application calls the
<a href=udp.html#setsockname><tt>setsockname</tt></a>
method <em>before</em> sending any datagrams. Otherwise, the socket is
automatically bound to an ephemeral address before the first data
transmission and once bound the local address cannot be changed.
The other methods available for UDP sockets are
<a href=udp.html#getpeername><tt>getpeername</tt></a>,
<a href=udp.html#getsockname><tt>getsockname</tt></a>,
<a href=udp.html#settimeout><tt>settimeout</tt></a>,
<a href=udp.html#setoption><tt>setoption</tt></a> and
<a href=udp.html#close><tt>close</tt></a>.
</p>
<p>
Example:
</p>
<blockquote>
<p>
A simple daytime client, using LuaSocket. The program connects to a remote
server and tries to retrieve the daytime, printing the answer it got or an
error message.
</p>
<pre class=example>
-- change here to the host an port you want to contact
local host, port = "localhost", 13
-- load namespace
local socket = require("socket")
-- convert host name to ip address
local ip = assert(socket.dns.toip(host))
-- create a new UDP object
local udp = assert(socket.udp())
-- contact daytime host
assert(udp:sendto("anything", ip, port))
-- retrieve the answer and print results
io.write(assert(udp:receive()))
</pre>
</blockquote>
<!-- More +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<h3 id=more>Support modules</h3>
<p> Although not covered in the introduction, LuaSocket offers
much more than TCP and UDP functionality. As the library
evolved, support for <a href=http.html>HTTP</a>, <a href=ftp.html>FTP</a>,
and <a href=smtp.html>SMTP</a> were built on top of these. These modules
and many others are covered by the <a href=reference.html>reference manual</a>.
</p>
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<a href="index.html">home</a> ·
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</p>
<p>
<small>
Last modified by Diego Nehab on <br>
Thu Apr 20 00:25:36 EDT 2006
</small>
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