Tutorial:Networking with UDP-TheServer

The complete source for the UDP server

local socket = require "socket"
-- begin
local udp = socket.udp()
-- normally socket reads block until they have data, or a
-- certain amount of time passes.
-- that doesn't suit us, so we tell it not to do that by setting the 
-- 'timeout' to zero
-- unlike the client, the server has to be specific about where its
-- 'bound', or the poor clients will never find it.
-- thus while we can happily let the client auto-bind to whatever it likes,
-- we have to tell the server to bind to something known.
-- the first part is which "interface" we should bind to...a bit beyond this tutorial, but '*' basically means "all of them"
-- port is simpler, the system maintains a list of up to 65535 (!) "ports"
-- ...really just numbers. point is that if you send to a particular port, 
-- then only things "listening" to that port will be able to receive it, 
-- and likewise you can only read data sent to ports you are listening too.
-- generally speaking, if an address is which machine you want to talk to, then a port is what program on that machine you want to talk to.
-- [NOTE: on some operating systems, ports between 0 and 1024 are "reserved for 
-- privileged processes". its a security precaution for those system.
-- generally speaking, just not using ports in that range avoids a lot of problems]
udp:setsockname('*', 12345)
local world = {} -- the empty world-state
-- We declare a whole bunch of local variables that we'll be using the in 
-- main server loop below. you probably recognise some of them from the
--client example, but you are also probably wondering what's with the fruity
-- names, 'msg_or_ip'? 'port_or_nil'?
-- well, we're using a slightly different function this time, you'll see when we get there.
local data, msg_or_ip, port_or_nil
local entity, cmd, parms
-- indefinite loops are probably not something you used to if you only 
-- know love, but they are quite common. and in fact love has one at its
-- heart, you just don't see it.
-- regardless, we'll be needing one for our server. and this little
-- variable lets us *stop* it :3
local running = true
-- the beginning of the loop proper...
print "Beginning server loop."
while running do
	-- this line looks familiar, I'm sure, but we're using 'receivefrom'
	-- this time. its similar to receive, but returns the data, sender's
	-- ip address, and the sender's port. (which you'll hopefully recognise
	-- as the two things we need to send messages to someone)
	-- we didn't have to do this in the client example because we just bound
	-- the socket to the server. ...but that also ignores messages from
	-- sources other than what we've bound to, which obviously won't do at
	-- all as a server.
	-- [NOTE: strictly, we could have just used receivefrom (and its 
	-- counterpart, sendto) in the client. there's nothing special about the
	-- functions to prevent it, indeed. send/receive are just convenience
	-- functions, sendto/receive from are the real workers.]
	data, msg_or_ip, port_or_nil = udp:receivefrom()
	if data then
		-- more of these funky match paterns!
		entity, cmd, parms = data:match("^(%S*) (%S*) (.*)")
		if cmd == 'move' then
			local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")
			assert(x and y) -- validation is better, but asserts will serve.
			-- don't forget, even if you matched a "number", the result is still a string!
			-- thankfully conversion is easy in lua.
			x, y = tonumber(x), tonumber(y)
			-- and finally we stash it away
			local ent = world[entity] or {x=0, y=0}
			world[entity] = {x=ent.x+x, y=ent.y+y}
		elseif cmd == 'at' then
			local x, y = parms:match("^(%-?[%d.e]*) (%-?[%d.e]*)$")
			assert(x and y) -- validation is better, but asserts will serve.
			x, y = tonumber(x), tonumber(y)
			world[entity] = {x=x, y=y}
		elseif cmd == 'update' then
			for k, v in pairs(world) do
				udp:sendto(string.format("%s %s %d %d", k, 'at', v.x, v.y), msg_or_ip,  port_or_nil)
		elseif cmd == 'quit' then
			running = false;
			print("unrecognised command:", cmd)
	elseif msg_or_ip ~= 'timeout' then
		error("Unknown network error: "..tostring(msg))
print "Thank you."
-- and that the end of the udp server example.