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334 lines
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HTML
334 lines
12 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<meta name="description" content="LuaSocket: Introduction to the core">
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<meta name="keywords" content="Lua, LuaSocket, TCP, UDP, Network,
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Library, Support">
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<title>LuaSocket: Introduction to the core</title>
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<link rel="stylesheet" href="reference.css" type="text/css">
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</head>
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<body>
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<!-- header +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<div class=header>
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<hr>
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<center>
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<table summary="LuaSocket logo">
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<tr><td align=center><a href="http://www.lua.org">
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<img width=128 height=128 border=0 alt="LuaSocket" src="luasocket.png">
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</a></td></tr>
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<tr><td align=center valign=top>Network support for the Lua language
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</td></tr>
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</table>
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<p class=bar>
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<a href="home.html">home</a> ·
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<a href="home.html#download">download</a> ·
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<a href="installation.html">installation</a> ·
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<a href="introduction.html">introduction</a> ·
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<a href="reference.html">reference</a>
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</p>
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</center>
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<hr>
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</div>
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<!-- introduction +++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h2>Introduction</h2>
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<p>
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LuaSocket is a <a href="http://www.lua.org">Lua</a> extension library
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that is composed by two parts: a C core that provides support for the TCP
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and UDP transport layers, and a set of Lua modules that add support for
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the SMTP (sending e-mails), HTTP (WWW access) and FTP (uploading and
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downloading files) protocols and other functionality commonly needed by
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applications that deal with the Internet. This introduction is about the C
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core.
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</p>
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<p>
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Communication in LuaSocket is performed via I/O objects. These can
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represent different network domains. Currently, support is provided for TCP
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and UDP, but nothing prevents other developers from implementing SSL, Local
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Domain, Pipes, File Descriptors etc. I/O objects provide a standard
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interface to I/O across different domains and operating systems.
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</p>
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<p>
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The API design had two goals in mind. First, users
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experienced with the C API to sockets should feel comfortable using LuaSocket.
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Second, the simplicity and the feel of the Lua language should be
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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.
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</p>
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<p>
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One of the simplifications is the receive pattern capability.
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Applications can read data from stream domains (such as TCP)
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line by line, block by block, or until the connection is closed.
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All I/O reads are buffered and the performance differences between
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different receive patterns are negligible.
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</p>
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<p>
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Another advantage is the flexible timeout control
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mechanism. As in C, all I/O operations are blocking by default. For
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example, the <a href=tcp.html#send><tt>send</tt></a>,
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<a href=tcp.html#receive><tt>receive</tt></a> and
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<a href=tcp.html#accept><tt>accept</tt></a> methods
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of the TCP domain will block the caller application until
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the operation is completed (if ever!). However, with a call to the
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<a href=tcp.html#settimeout><tt>settimeout</tt></a>
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method, an application can specify upper limits on
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the time it can be blocked by LuaSocket (the "<tt>total</tt>" timeout), on
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the time LuaSocket can internally be blocked by any OS call (the
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"<tt>block</tt>" timeout) or a combination of the two. Each LuaSocket
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call might perform several OS calls, so that the two timeout values are
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<em>not</em> equivalent.
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</p>
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<p>
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Finally, the host name resolution is transparent, meaning that most
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functions and methods accept both IP addresses and host names. In case a
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host name is given, the library queries the system's resolver and
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tries the main IP address returned. Note that direct use of IP addresses
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is more efficient, of course. The
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<a href=dns.html#toip><tt>toip</tt></a>
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and <a href=dns.html#tohostname><tt>tohostname</tt></a>
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functions from the DNS module are provided to convert between host names and IP addresses.
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</p>
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<p>
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Together, these changes make network programming in LuaSocket much simpler
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than it is in C, as the following sections will show.
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</p>
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<!-- tcp ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h3 id=tcp>TCP</h3>
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<p>
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TCP (Transfer Control Protocol) is reliable stream protocol. In other
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words, applications communicating through TCP can send and receive data as
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an error free stream of bytes. Data is split in one end and
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reassembled transparently on the other end. There are no boundaries in
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the data transfers. The library allows users to read data from the
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sockets in several different granularities: patterns are available for
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lines, arbitrary sized blocks or "read up to connection closed", all with
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good performance.
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</p>
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<p>
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The library distinguishes three types of TCP sockets: <em>master</em>,
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<em>client</em> and <em>server</em> sockets.
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</p>
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<p>
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Master sockets are newly created TCP sockets returned by the function
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<a href=tcp.html#tcp><tt>socket.tcp</tt></a>. A master socket is
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transformed into a server socket
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after it is associated with a <em>local</em> address by a call to the
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<a href=tcp.html#bind><tt>bind</tt></a> method followed by a call to the
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<a href=tcp.html#listen><tt>listen</tt></a>. Conversely, a master socket
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can be changed into a client socket with the method
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<a href=tcp.html#connect><tt>connect</tt></a>,
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which associates it with a <em>remote</em> address.
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</p>
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<p>
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On server sockets, applications can use the
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<a href=tcp.html#accept><tt>accept</tt></a> method
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to wait for a client connection. Once a connection is established, a
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client socket object is returned representing this connection. The
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other methods available for server socket objects are
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<a href=tcp.html#getsockname><tt>getsockname</tt></a>,
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<a href=tcp.html#setoption><tt>setoption</tt></a>,
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<a href=tcp.html#settimeout><tt>settimeout</tt></a>, and
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<a href=tcp.html#close><tt>close</tt></a>.
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</p>
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<p>
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Client sockets are used to exchange data between two applications over
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the Internet. Applications can call the methods
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<a href=tcp.html#send><tt>send</tt></a> and
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<a href=tcp.html#receive><tt>receive</tt></a>
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to send and receive data. The other methods
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available for client socket objects are
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<a href=tcp.html#getsockname><tt>getsockname</tt></a>,
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<a href=tcp.html#getpeername><tt>getpeername</tt></a>,
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<a href=tcp.html#setoption><tt>setoption</tt></a>,
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<a href=tcp.html#settimeout><tt>settimeout</tt></a>,
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<a href=tcp.html#shutdown><tt>shutdown</tt></a>, and
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<a href=tcp.html#close><tt>close</tt></a>.
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</p>
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<p>
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Example:
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</p>
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<blockquote>
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<p>
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A simple echo server, using LuaSocket. The program binds to an ephemeral
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port (one that is chosen by the operating system) on the local host and
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awaits client connections on that port. When a connection is established,
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the program reads a line from the remote end and sends it back, closing
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the connection immediately. You can test it using the telnet
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program.
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</p>
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<pre class=example>
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-- load namespace
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local socket = require("socket")
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-- create a TCP socket and bind it to the local host, at any port
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local server = assert(socket.bind("*", 0))
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-- find out which port the OS chose for us
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local ip, port = server:getsockname()
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-- print a message informing what's up
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print("Please telnet to localhost on port " .. port)
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print("After connecting, you have 10s to enter a line to be echoed")
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-- loop forever waiting for clients
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while 1 do
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-- wait for a connection from any client
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local client = server:accept()
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-- make sure we don't block waiting for this client's line
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client:settimeout(10)
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-- receive the line
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local line, err = client:receive()
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-- if there was no error, send it back to the client
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if not err then client:send(line .. "\n") end
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-- done with client, close the object
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client:close()
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end
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</pre>
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</blockquote>
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<!-- udp ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h3 id=udp>UDP</h3>
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<p>
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UDP (User Datagram Protocol) is a non-reliable datagram protocol. In
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other words, applications communicating through UDP send and receive
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data as independent blocks, which are not guaranteed to reach the other
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end. Even when they do reach the other end, they are not guaranteed to be
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error free. Data transfers are atomic, one datagram at a time. Reading
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only part of a datagram discards the rest, so that the following read
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operation will act on the next datagram. The advantages are in
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simplicity (no connection setup) and performance (no error checking or
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error correction).
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</p>
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<p>
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Note that although no guarantees are made, these days
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networks are so good that, under normal circumstances, few errors
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happen in practice.
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</p>
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<p>
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An UDP socket object is created by the
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<a href=udp.html#udp><tt>socket.udp</tt></a> function. UDP
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sockets do not need to be connected before use. The method
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<a href=udp.html#sendto><tt>sendto</tt></a>
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can be used immediately after creation to
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send a datagram to IP address and port. Host names are not allowed
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because performing name resolution for each packet would be forbiddingly
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slow. Methods
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<a href=udp.html#receive><tt>receive</tt></a> and
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<a href=udp.html#receivefrom><tt>receivefrom</tt></a>
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can be used to retrieve datagrams, the latter returning the IP and port of
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the sender as extra return values (thus being slightly less
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efficient).
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</p>
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<p>
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When communication is performed repeatedly with a single peer, an
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application should call the
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<a href=udp.html#setpeername><tt>setpeername</tt></a> method to specify a
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permanent partner. Methods
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<a href=udp.html#sendto><tt>sendto</tt></a> and
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<a href=udp.html#receivefrom><tt>receivefrom</tt></a>
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can no longer be used, but the method
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<a href=udp.html#send><tt>send</tt></a> can be used to send data
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directly to the peer, and the method
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<a href=udp.html#receive><tt>receive</tt></a>
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will only return datagrams originating
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from that peer. There is about 30% performance gain due to this practice.
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</p>
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<p>
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To associate an UDP socket with a local address, an application calls the
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<a href=udp.html#setsockname><tt>setsockname</tt></a>
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method <em>before</em> sending any datagrams. Otherwise, the socket is
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automatically bound to an ephemeral address before the first data
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transmission and once bound the local address cannot be changed.
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The other methods available for UDP sockets are
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<a href=udp.html#getpeername><tt>getpeername</tt></a>,
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<a href=udp.html#getsockname><tt>getsockname</tt></a>,
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<a href=udp.html#settimeout><tt>settimeout</tt></a>,
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<a href=udp.html#setoption><tt>setoption</tt></a> and
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<a href=udp.html#close><tt>close</tt></a>.
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</p>
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<p>
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Example:
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</p>
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<blockquote>
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<p>
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A simple daytime client, using LuaSocket. The program connects to a remote
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server and tries to retrieve the daytime, printing the answer it got or an
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error message.
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</p>
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<pre class=example>
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-- change here to the host an port you want to contact
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local host, port = "localhost", 13
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-- load namespace
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local socket = require("socket")
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-- convert host name to ip address
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local ip = assert(socket.dns.toip(host))
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-- create a new UDP object
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local udp = assert(socket.udp())
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-- contact daytime host
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assert(udp:sendto("anything", ip, port))
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-- retrieve the answer and print results
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io.write(assert(udp:receive()))
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</pre>
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</blockquote>
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<!-- More +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h3 id=more>Support modules</h3>
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<p> Although not covered in the introduction, LuaSocket offers
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much more than TCP and UDP functionality. As the library
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evolved, support for <a href=http.html>HTTP</a>, <a href=ftp.html>FTP</a>,
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and <a href=smtp.html>SMTP</a> were built on top of these. These modules
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and many others are covered by the <a href=reference.html>reference manual</a>.
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</p>
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<!-- footer +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<div class=footer>
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<hr>
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<center>
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<p class=bar>
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<a href="home.html">home</a> ·
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<a href="home.html#down">download</a> ·
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<a href="installation.html">installation</a> ·
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<a href="introduction.html">introduction</a> ·
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<a href="reference.html">reference</a>
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</p>
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<p>
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<small>
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Last modified by Diego Nehab on <br>
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Mon Nov 21 01:57:43 EST 2005
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</small>
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</p>
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</center>
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</div>
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</body>
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</html>
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