Erlang - Ports

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In Erlang, ports are used for communication between different programs. A socket is a communication endpoint that allows machines to communicate over the Internet by using the Internet Protocol (IP).

Types of Protocols Used in Ports

There are 2 types of protocols available for communication. One is UDP and the other is TCP. UDP lets applications send short messages (called datagrams) to each other, but there is no guarantee of delivery for these messages. They can also arrive out of order. TCP, on the other hand, provides a reliable stream of bytes that are delivered in order as long as the connection is established.

Let’s look at a simple example of opening a port using UDP.

Example

-module(helloworld). 
-export([start/0]). 

start() ->
   {ok, Socket} = gen_udp:open(8789), 
   io:fwrite("~p",[Socket]), 
   io:fwrite("~p",[gen_udp:send 
   (Socket,"localhost",8789,"Hello")]).

Output

The output of the above program will be as follows.

#Port<0.376>ok

Receiving a Message on the Port

Once the port has been opened a message can also be received on the port. This is done via the recv method. Let’s look at the syntax and the following example.

Syntax

recv(Socket, length)

Parameters

• Socket − This is the socket created with the gen_udp:open command.

• Length − This is the length of the message which needs to be received.

Return Values

An ok message is returned if the message was sent properly.

For example

-module(helloworld). 
-export([start/0]). 

start() ->
   {ok, Socket} = gen_udp:open(8789), 
   io:fwrite("~p",[Socket]), 
   io:fwrite("~p",[gen_udp:send(Socket,"localhost",8789,"Hello")]),
   io:fwrite("~p",[gen_udp:recv(Socket, 20)]).

The Complete Program

Now obviously we cannot have the same send and receive message in the same program. You need to have them defined in different programs. So let create the following code which creates a server component that listens to messages and a client component which sends messages.

Example

-module(helloworld). 
-export([start/0,client/1]). 

start() -> 
   spawn(fun() -> server(4000) end).

server(Port) ->
   {ok, Socket} = gen_udp:open(Port, [binary, {active, false}]), 
   io:format("server opened socket:~p~n",[Socket]), 
   loop(Socket). 

loop(Socket) ->
   inet:setopts(Socket, [{active, once}]), 
   receive 
      {udp, Socket, Host, Port, Bin} -> 
      io:format("server received:~p~n",[Bin]), 
      gen_udp:send(Socket, Host, Port, Bin), 
      loop(Socket) 
   end. 

client(N) -> 
   {ok, Socket} = gen_udp:open(0, [binary]), 
   io:format("client opened socket=~p~n",[Socket]), 
   ok = gen_udp:send(Socket, "localhost", 4000, N), Value = receive 
      {udp, Socket, _, _, Bin} ->
         io:format("client received:~p~n",[Bin]) after 2000 ->
      0 
   end, 
   
gen_udp:close(Socket), 
Value.

The following things need to be noted about the above program.

• We define 2 functions, the first is server. This will be used to listen on the port 4000. The second is the client which will be used to send the message “Hello” to the server component.

• The receive loop is used to read the messages sent within a define loop.

Output

Now you need to run the program from 2 windows. The first window will be used to run the server component by running the following code in the erl command line window.

helloworld:start().

This will display the following output in the command line window.

server opened socket:#Port<0.2314>

Now in the second erl command line window, run the following command.

Helloworld:client(“<<Hello>>”).

When you issue this command, the following output will be displayed in the first command line window.

server received:<<"Hello">>