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<tr><td align=center valign=top>Network support for the Lua language
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<h2>Introduction</h2>

<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 there is work in progress to implement SSL,
Local Domain, Pipes, File Descriptors etc. I/O objects provide a standard
interface to I/O across different domains and operating systems.
LuaSocket&nbsp;2.0 has been rewritten from scratch to simplify the future
addition of new domains.
</p>

<p>
The LuaSocket API was designed with 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 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>
Another important difference 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>
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 returned IP address. 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>
Previous versions of LuaSocket provided global functions for operating on
I/O objects. To give the library a Lua 5.0 feel, these have been eliminated
from LuaSocket 2.0. I/O operations are only available as methods of the
corresponding I/O objects.  Naturally, different I/O objects accept
different operations. The core functionality for TCP and UDP objects is
introduced in the following sections, following a few words about
initialization. 
</p>

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<h3>Initializing the library</h3>

<p>
Beginning with version 2.0 and following the Lua 5.0 trend, all LuaSocket
functionality is defined inside a table (or rather a namespace) stored with
the global name <tt>socket</tt>.  To  have this table created and its
contents made available to a Lua script, the interpreter running the script
must  be linked to the LuaSocket library, and  to whatever libraries  the
host OS  requires  for network access (Windows requires ws2_32.lib, for
instance). LuaSocket is initialized in  the
Lua state given as the argument  to  the  function
<tt>luaopen_socket</tt>, the  only  C function  exported  by  the  library.
After initialization, scripts are free to use all of the LuaSocket API. 
</p>

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<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 granularity: 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:  master, client and server 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. Conversely, it 
can be changed into a client socket with the method 
<a href=tcp.html#connect><tt>connect</tt></a>, 
that 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> 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  after. You  can  test it  using the  telnet
program. 
</p>

<pre class=example>
-- create a new TCP object
server, err = socket.tcp()
assert(server, err)
-- bind it to the local host, at any port
ret, err = server:bind("*", 0)
assert(ret, err)
-- find out which port the OS chose for us
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 conection from any client
  client, err = server:accept()
  -- make sure we don't block waiting for this client's line
  client:settimeout(10)
  -- receive the line
  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>

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<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>
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>
host = "localhost" -- change here to the host you want to contact
port = port or 13
-- convert host name to ip address
ip, err = socket.dns.toip(host)
assert(ip, err)
-- create a new UDP object
udp = socket.udp()
-- contact daytime host
nsent, err = udp:sendto("anything", ip, port)
assert(not err, err)
-- retrieve the answer
dgram, err = udp:receive()
assert(dgram, err)
-- display to user
print(dgram)
</pre>
</blockquote>

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Last modified by Diego Nehab on <br>
Sat Aug 9 01:00:41 PDT 2003
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