<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html xmlns:fn="http://www.w3.org/2005/02/xpath-functions"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <link rel="stylesheet" href="../otp_doc.css" type="text/css"> <title>Erlang -- Concurrent Programming</title> </head> <body bgcolor="white" text="#000000" link="#0000ff" vlink="#ff00ff" alink="#ff0000"><div id="container"> <script id="js" type="text/javascript" language="JavaScript" src="../js/flipmenu/flipmenu.js"></script><script id="js2" type="text/javascript" src="../js/erlresolvelinks.js"></script><script language="JavaScript" type="text/javascript"> <!-- function getWinHeight() { var myHeight = 0; if( typeof( window.innerHeight ) == 'number' ) { //Non-IE myHeight = window.innerHeight; } else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) { //IE 6+ in 'standards compliant mode' myHeight = document.documentElement.clientHeight; 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By concurrency we mean programs which can handle several threads of execution at the same time. For example, modern operating systems would allow you to use a word processor, a spreadsheet, a mail client and a print job all running at the same time. Of course each processor (CPU) in the system is probably only handling one thread (or job) at a time, but it swaps between the jobs a such a rate that it gives the illusion of running them all at the same time. It is easy to create parallel threads of execution in an Erlang program and it is easy to allow these threads to communicate with each other. In Erlang we call each thread of execution a <strong>process</strong>.</p> <p>(Aside: the term "process" is usually used when the threads of execution share no data with each other and the term "thread" when they share data in some way. Threads of execution in Erlang share no data, that's why we call them processes).</p> <p>The Erlang BIF <span class="code">spawn</span> is used to create a new process: <span class="code">spawn(Module, Exported_Function, List of Arguments)</span>. Consider the following module:</p> <div class="example"><pre> -module(tut14). -export([start/0, say_something/2]). say_something(What, 0) -> done; say_something(What, Times) -> io:format("~p~n", [What]), say_something(What, Times - 1). start() -> spawn(tut14, say_something, [hello, 3]), spawn(tut14, say_something, [goodbye, 3]).</pre></div> <div class="example"><pre> 5> <span class="bold_code">c(tut14).</span> {ok,tut14} 6> <span class="bold_code">tut14:say_something(hello, 3).</span> hello hello hello done</pre></div> <p>We can see that function <span class="code">say_something</span> writes its first argument the number of times specified by second argument. Now look at the function <span class="code">start</span>. It starts two Erlang processes, one which writes "hello" three times and one which writes "goodbye" three times. Both of these processes use the function <span class="code">say_something</span>. Note that a function used in this way by <span class="code">spawn</span> to start a process must be exported from the module (i.e. in the <span class="code">-export</span> at the start of the module).</p> <div class="example"><pre> 9> <span class="bold_code">tut14:start().</span> hello goodbye <0.63.0> hello goodbye hello goodbye</pre></div> <p>Notice that it didn't write "hello" three times and then "goodbye" three times, but the first process wrote a "hello", the second a "goodbye", the first another "hello" and so forth. But where did the <0.63.0> come from? The return value of a function is of course the return value of the last "thing" in the function. The last thing in the function <span class="code">start</span> is</p> <div class="example"><pre> spawn(tut14, say_something, [goodbye, 3]).</pre></div> <p><span class="code">spawn</span> returns a <strong>process identifier</strong>, or <strong>pid</strong>, which uniquely identifies the process. So <0.63.0> is the pid of the <span class="code">spawn</span> function call above. We will see how to use pids in the next example.</p> <p>Note as well that we have used ~p instead of ~w in <span class="code">io:format</span>. To quote the manual: "~p Writes the data with standard syntax in the same way as ~w, but breaks terms whose printed representation is longer than one line into many lines and indents each line sensibly. It also tries to detect lists of printable characters and to output these as strings".</p> <h3><a name="id2263966">3.2 Message Passing</a></h3> <p>In the following example we create two processes which send messages to each other a number of times.</p> <div class="example"><pre> -module(tut15). -export([start/0, ping/2, pong/0]). ping(0, Pong_PID) -> Pong_PID ! finished, io:format("ping finished~n", []); ping(N, Pong_PID) -> Pong_PID ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_PID). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start() -> Pong_PID = spawn(tut15, pong, []), spawn(tut15, ping, [3, Pong_PID]).</pre></div> <div class="example"><pre> 1> <span class="bold_code">c(tut15).</span> {ok,tut15} 2> <span class="bold_code">tut15: start().</span> <0.36.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished</pre></div> <p>The function <span class="code">start</span> first creates a process, let's call it "pong":</p> <div class="example"><pre> Pong_PID = spawn(tut15, pong, [])</pre></div> <p>This process executes <span class="code">tut15:pong()</span>. <span class="code">Pong_PID</span> is the process identity of the "pong" process. The function <span class="code">start</span> now creates another process "ping".</p> <div class="example"><pre> spawn(tut15, ping, [3, Pong_PID]),</pre></div> <p>this process executes</p> <div class="example"><pre> tut15:ping(3, Pong_PID)</pre></div> <p><0.36.0> is the return value from the <span class="code">start</span> function.</p> <p>The process "pong" now does:</p> <div class="example"><pre> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end.</pre></div> <p>The <span class="code">receive</span> construct is used to allow processes to wait for messages from other processes. It has the format:</p> <div class="example"><pre> receive pattern1 -> actions1; pattern2 -> actions2; .... patternN actionsN end.</pre></div> <p>Note: no ";" before the <span class="code">end</span>.</p> <p>Messages between Erlang processes are simply valid Erlang terms. I.e. they can be lists, tuples, integers, atoms, pids etc.</p> <p>Each process has its own input queue for messages it receives. New messages received are put at the end of the queue. When a process executes a <span class="code">receive</span>, the first message in the queue is matched against the first pattern in the <span class="code">receive</span>, if this matches, the message is removed from the queue and the actions corresponding to the the pattern are executed.</p> <p>However, if the first pattern does not match, the second pattern is tested, if this matches the message is removed from the queue and the actions corresponding to the second pattern are executed. If the second pattern does not match the third is tried and so on until there are no more pattern to test. If there are no more patterns to test, the first message is kept in the queue and we try the second message instead. If this matches any pattern, the appropriate actions are executed and the second message is removed from the queue (keeping the first message and any other messages in the queue). If the second message does not match we try the third message and so on until we reach the end of the queue. If we reach the end of the queue, the process blocks (stops execution) and waits until a new message is received and this procedure is repeated.</p> <p>Of course the Erlang implementation is "clever" and minimizes the number of times each message is tested against the patterns in each <span class="code">receive</span>.</p> <p>Now back to the ping pong example.</p> <p>"Pong" is waiting for messages. If the atom <span class="code">finished</span> is received, "pong" writes "Pong finished" to the output and as it has nothing more to do, terminates. If it receives a message with the format:</p> <div class="example"><pre> {ping, Ping_PID}</pre></div> <p>it writes "Pong received ping" to the output and sends the atom <span class="code">pong</span> to the process "ping":</p> <div class="example"><pre> Ping_PID ! pong</pre></div> <p>Note how the operator "!" is used to send messages. The syntax of "!" is:</p> <div class="example"><pre> Pid ! Message</pre></div> <p>I.e. <span class="code">Message</span> (any Erlang term) is sent to the process with identity <span class="code">Pid</span>.</p> <p>After sending the message <span class="code">pong</span>, to the process "ping", "pong" calls the <span class="code">pong</span> function again, which causes it to get back to the <span class="code">receive</span> again and wait for another message. Now let's look at the process "ping". Recall that it was started by executing:</p> <div class="example"><pre> tut15:ping(3, Pong_PID)</pre></div> <p>Looking at the function <span class="code">ping/2</span> we see that the second clause of <span class="code">ping/2</span> is executed since the value of the first argument is 3 (not 0) (first clause head is <span class="code">ping(0,Pong_PID)</span>, second clause head is <span class="code">ping(N,Pong_PID)</span>, so <span class="code">N</span> becomes 3).</p> <p>The second clause sends a message to "pong":</p> <div class="example"><pre> Pong_PID ! {ping, self()},</pre></div> <p><span class="code">self()</span> returns the pid of the process which executes <span class="code">self()</span>, in this case the pid of "ping". (Recall the code for "pong", this will land up in the variable <span class="code">Ping_PID</span> in the <span class="code">receive</span> previously explained).</p> <p>"Ping" now waits for a reply from "pong":</p> <div class="example"><pre> receive pong -> io:format("Ping received pong~n", []) end,</pre></div> <p>and writes "Ping received pong" when this reply arrives, after which "ping" calls the <span class="code">ping</span> function again.</p> <div class="example"><pre> ping(N - 1, Pong_PID)</pre></div> <p><span class="code">N-1</span> causes the first argument to be decremented until it becomes 0. When this occurs, the first clause of <span class="code">ping/2</span> will be executed:</p> <div class="example"><pre> ping(0, Pong_PID) -> Pong_PID ! finished, io:format("ping finished~n", []);</pre></div> <p>The atom <span class="code">finished</span> is sent to "pong" (causing it to terminate as described above) and "ping finished" is written to the output. "Ping" then itself terminates as it has nothing left to do.</p> <h3><a name="id2264351">3.3 Registered Process Names</a></h3> <p>In the above example, we first created "pong" so as to be able to give the identity of "pong" when we started "ping". I.e. in some way "ping" must be able to know the identity of "pong" in order to be able to send a message to it. Sometimes processes which need to know each others identities are started completely independently of each other. Erlang thus provides a mechanism for processes to be given names so that these names can be used as identities instead of pids. This is done by using the <span class="code">register</span> BIF:</p> <div class="example"><pre> register(some_atom, Pid)</pre></div> <p>We will now re-write the ping pong example using this and giving the name <span class="code">pong</span> to the "pong" process:</p> <div class="example"><pre> -module(tut16). -export([start/0, ping/1, pong/0]). ping(0) -> pong ! finished, io:format("ping finished~n", []); ping(N) -> pong ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start() -> register(pong, spawn(tut16, pong, [])), spawn(tut16, ping, [3]).</pre></div> <div class="example"><pre> 2> <span class="bold_code">c(tut16).</span> {ok, tut16} 3> <span class="bold_code">tut16:start().</span> <0.38.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong ping finished Pong finished</pre></div> <p>In the <span class="code">start/0</span> function,</p> <div class="example"><pre> register(pong, spawn(tut16, pong, [])),</pre></div> <p>both spawns the "pong" process and gives it the name <span class="code">pong</span>. In the "ping" process we can now send messages to <span class="code">pong</span> by:</p> <div class="example"><pre> pong ! {ping, self()},</pre></div> <p>so that <span class="code">ping/2</span> now becomes <span class="code">ping/1</span> as we don't have to use the argument <span class="code">Pong_PID</span>.</p> <h3><a name="id2264468">3.4 Distributed Programming</a></h3> <p>Now let's re-write the ping pong program with "ping" and "pong" on different computers. Before we do this, there are a few things we need to set up to get this to work. The distributed Erlang implementation provides a basic security mechanism to prevent unauthorized access to an Erlang system on another computer (*manual*). Erlang systems which talk to each other must have the same <strong>magic cookie</strong>. The easiest way to achieve this is by having a file called <span class="code">.erlang.cookie</span> in your home directory on all machines which on which you are going to run Erlang systems communicating with each other (on Windows systems the home directory is the directory where pointed to by the $HOME environment variable - you may need to set this. On Linux or Unix you can safely ignore this and simply create a file called <span class="code">.erlang.cookie</span> in the directory you get to after executing the command <span class="code">cd</span> without any argument). The <span class="code">.erlang.cookie</span> file should contain on line with the same atom. For example on Linux or Unix in the OS shell:</p> <div class="example"><pre> $ <span class="bold_code">cd</span> $ <span class="bold_code">cat > .erlang.cookie</span> this_is_very_secret $ <span class="bold_code">chmod 400 .erlang.cookie</span></pre></div> <p>The <span class="code">chmod</span> above make the <span class="code">.erlang.cookie</span> file accessible only by the owner of the file. This is a requirement.</p> <p>When you start an Erlang system which is going to talk to other Erlang systems, you must give it a name, eg: </p> <div class="example"><pre> $ <span class="bold_code">erl -sname my_name</span></pre></div> <p>We will see more details of this later (*manual*). If you want to experiment with distributed Erlang, but you only have one computer to work on, you can start two separate Erlang systems on the same computer but give them different names. Each Erlang system running on a computer is called an Erlang node.</p> <p>(Note: <span class="code">erl -sname</span> assumes that all nodes are in the same IP domain and we can use only the first component of the IP address, if we want to use nodes in different domains we use <span class="code">-name</span> instead, but then all IP address must be given in full (*manual*).</p> <p>Here is the ping pong example modified to run on two separate nodes:</p> <div class="example"><pre> -module(tut17). -export([start_ping/1, start_pong/0, ping/2, pong/0]). ping(0, Pong_Node) -> {pong, Pong_Node} ! finished, io:format("ping finished~n", []); ping(N, Pong_Node) -> {pong, Pong_Node} ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_Node). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start_pong() -> register(pong, spawn(tut17, pong, [])). start_ping(Pong_Node) -> spawn(tut17, ping, [3, Pong_Node]).</pre></div> <p>Let us assume we have two computers called gollum and kosken. We will start a node on kosken called ping and then a node on gollum called pong.</p> <p>On kosken (on a Linux/Unix system):</p> <div class="example"><pre> kosken> <span class="bold_code">erl -sname ping</span> Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (ping@kosken)1></pre></div> <p>On gollum:</p> <div class="example"><pre> gollum> <span class="bold_code">erl -sname pong</span> Erlang (BEAM) emulator version 5.2.3.7 [hipe] [threads:0] Eshell V5.2.3.7 (abort with ^G) (pong@gollum)1></pre></div> <p>Now we start the "pong" process on gollum:</p> <div class="example"><pre> (pong@gollum)1> <span class="bold_code">tut17:start_pong().</span> true</pre></div> <p>and start the "ping" process on kosken (from the code above you will see that a parameter of the <span class="code">start_ping</span> function is the node name of the Erlang system where "pong" is running):</p> <div class="example"><pre> (ping@kosken)1> <span class="bold_code">tut17:start_ping(pong@gollum).</span> <0.37.0> Ping received pong Ping received pong Ping received pong ping finished</pre></div> <p>Here we see that the ping pong program has run, on the "pong" side we see:</p> <div class="example"><pre> (pong@gollum)2> Pong received ping Pong received ping Pong received ping Pong finished (pong@gollum)2></pre></div> <p>Looking at the <span class="code">tut17</span> code we see that the <span class="code">pong</span> function itself is unchanged, the lines:</p> <div class="example"><pre> {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong,</pre></div> <p>work in the same way irrespective of on which node the "ping" process is executing. Thus Erlang pids contain information about where the process executes so if you know the pid of a process, the "!" operator can be used to send it a message if the process is on the same node or on a different node.</p> <p>A difference is how we send messages to a registered process on another node:</p> <div class="example"><pre> {pong, Pong_Node} ! {ping, self()},</pre></div> <p>We use a tuple <span class="code">{registered_name,node_name}</span> instead of just the <span class="code">registered_name</span>.</p> <p>In the previous example, we started "ping" and "pong" from the shells of two separate Erlang nodes. <span class="code">spawn</span> can also be used to start processes in other nodes. The next example is the ping pong program, yet again, but this time we will start "ping" in another node:</p> <div class="example"><pre> -module(tut18). -export([start/1, ping/2, pong/0]). ping(0, Pong_Node) -> {pong, Pong_Node} ! finished, io:format("ping finished~n", []); ping(N, Pong_Node) -> {pong, Pong_Node} ! {ping, self()}, receive pong -> io:format("Ping received pong~n", []) end, ping(N - 1, Pong_Node). pong() -> receive finished -> io:format("Pong finished~n", []); {ping, Ping_PID} -> io:format("Pong received ping~n", []), Ping_PID ! pong, pong() end. start(Ping_Node) -> register(pong, spawn(tut18, pong, [])), spawn(Ping_Node, tut18, ping, [3, node()]).</pre></div> <p>Assuming an Erlang system called ping (but not the "ping" process) has already been started on kosken, then on gollum we do:</p> <div class="example"><pre> (pong@gollum)1> <span class="bold_code">tut18:start(ping@kosken).</span> <3934.39.0> Pong received ping Ping received pong Pong received ping Ping received pong Pong received ping Ping received pong Pong finished ping finished</pre></div> <p>Notice we get all the output on gollum. This is because the io system finds out where the process is spawned from and sends all output there.</p> <h3><a name="id2264779">3.5 A Larger Example</a></h3> <p>Now for a larger example. We will make an extremely simple "messenger". The messenger is a program which allows users to log in on different nodes and send simple messages to each other.</p> <p>Before we start, let's note the following:</p> <ul> <li> <p>This example will just show the message passing logic no attempt at all has been made to provide a nice graphical user interface - this can of course also be done in Erlang - but that's another tutorial.</p> </li> <li> <p>This sort of problem can be solved more easily if you use the facilities in OTP, which will also provide methods for updating code on the fly etc. But again, that's another tutorial.</p> </li> <li> <p>The first program we write will contain some inadequacies as regards handling of nodes which disappear, we will correct these in a later version of the program.</p> </li> </ul> <p>We will set up the messenger by allowing "clients" to connect to a central server and say who and where they are. I.e. a user won't need to know the name of the Erlang node where another user is located to send a message.</p> <p>File <span class="code">messenger.erl</span>:</p> <a name="ex"></a> <div class="example"><pre> %%% Message passing utility. %%% User interface: %%% logon(Name) %%% One user at a time can log in from each Erlang node in the %%% system messenger: and choose a suitable Name. If the Name %%% is already logged in at another node or if someone else is %%% already logged in at the same node, login will be rejected %%% with a suitable error message. %%% logoff() %%% Logs off anybody at at node %%% message(ToName, Message) %%% sends Message to ToName. Error messages if the user of this %%% function is not logged on or if ToName is not logged on at %%% any node. %%% %%% One node in the network of Erlang nodes runs a server which maintains %%% data about the logged on users. The server is registered as "messenger" %%% Each node where there is a user logged on runs a client process registered %%% as "mess_client" %%% %%% Protocol between the client processes and the server %%% ---------------------------------------------------- %%% %%% To server: {ClientPid, logon, UserName} %%% Reply {messenger, stop, user_exists_at_other_node} stops the client %%% Reply {messenger, logged_on} logon was successful %%% %%% To server: {ClientPid, logoff} %%% Reply: {messenger, logged_off} %%% %%% To server: {ClientPid, logoff} %%% Reply: no reply %%% %%% To server: {ClientPid, message_to, ToName, Message} send a message %%% Reply: {messenger, stop, you_are_not_logged_on} stops the client %%% Reply: {messenger, receiver_not_found} no user with this name logged on %%% Reply: {messenger, sent} Message has been sent (but no guarantee) %%% %%% To client: {message_from, Name, Message}, %%% %%% Protocol between the "commands" and the client %%% ---------------------------------------------- %%% %%% Started: messenger:client(Server_Node, Name) %%% To client: logoff %%% To client: {message_to, ToName, Message} %%% %%% Configuration: change the server_node() function to return the %%% name of the node where the messenger server runs -module(messenger). -export([start_server/0, server/1, logon/1, logoff/0, message/2, client/2]). %%% Change the function below to return the name of the node where the %%% messenger server runs server_node() -> messenger@bill. %%% This is the server process for the "messenger" %%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...] server(User_List) -> receive {From, logon, Name} -> New_User_List = server_logon(From, Name, User_List), server(New_User_List); {From, logoff} -> New_User_List = server_logoff(From, User_List), server(New_User_List); {From, message_to, To, Message} -> server_transfer(From, To, Message, User_List), io:format("list is now: ~p~n", [User_List]), server(User_List) end. %%% Start the server start_server() -> register(messenger, spawn(messenger, server, [[]])). %%% Server adds a new user to the user list server_logon(From, Name, User_List) -> %% check if logged on anywhere else case lists:keymember(Name, 2, User_List) of true -> From ! {messenger, stop, user_exists_at_other_node}, %reject logon User_List; false -> From ! {messenger, logged_on}, [{From, Name} | User_List] %add user to the list end. %%% Server deletes a user from the user list server_logoff(From, User_List) -> lists:keydelete(From, 1, User_List). %%% Server transfers a message between user server_transfer(From, To, Message, User_List) -> %% check that the user is logged on and who he is case lists:keysearch(From, 1, User_List) of false -> From ! {messenger, stop, you_are_not_logged_on}; {value, {From, Name}} -> server_transfer(From, Name, To, Message, User_List) end. %%% If the user exists, send the message server_transfer(From, Name, To, Message, User_List) -> %% Find the receiver and send the message case lists:keysearch(To, 2, User_List) of false -> From ! {messenger, receiver_not_found}; {value, {ToPid, To}} -> ToPid ! {message_from, Name, Message}, From ! {messenger, sent} end. %%% User Commands logon(Name) -> case whereis(mess_client) of undefined -> register(mess_client, spawn(messenger, client, [server_node(), Name])); _ -> already_logged_on end. logoff() -> mess_client ! logoff. message(ToName, Message) -> case whereis(mess_client) of % Test if the client is running undefined -> not_logged_on; _ -> mess_client ! {message_to, ToName, Message}, ok end. %%% The client process which runs on each server node client(Server_Node, Name) -> {messenger, Server_Node} ! {self(), logon, Name}, await_result(), client(Server_Node). client(Server_Node) -> receive logoff -> {messenger, Server_Node} ! {self(), logoff}, exit(normal); {message_to, ToName, Message} -> {messenger, Server_Node} ! {self(), message_to, ToName, Message}, await_result(); {message_from, FromName, Message} -> io:format("Message from ~p: ~p~n", [FromName, Message]) end, client(Server_Node). %%% wait for a response from the server await_result() -> receive {messenger, stop, Why} -> % Stop the client io:format("~p~n", [Why]), exit(normal); {messenger, What} -> % Normal response io:format("~p~n", [What]) end.</pre></div> <p>To use this program you need to:</p> <ul> <li>configure the <span class="code">server_node()</span> function</li> <li>copy the compiled code (<span class="code">messenger.beam</span>) to the directory on each computer where you start Erlang.</li> </ul> <p>In the following example of use of this program, I have started nodes on four different computers, but if you don't have that many machines available on your network, you could start up several nodes on the same machine.</p> <p>We start up four Erlang nodes, messenger@super, c1@bilbo, c2@kosken, c3@gollum.</p> <p>First we start up a the server at messenger@super:</p> <div class="example"><pre> (messenger@super)1> <span class="bold_code">messenger:start_server().</span> true</pre></div> <p>Now Peter logs on at c1@bilbo:</p> <div class="example"><pre> (c1@bilbo)1> <span class="bold_code">messenger:logon(peter).</span> true logged_on</pre></div> <p>James logs on at c2@kosken:</p> <div class="example"><pre> (c2@kosken)1> <span class="bold_code">messenger:logon(james).</span> true logged_on</pre></div> <p>and Fred logs on at c3@gollum:</p> <div class="example"><pre> (c3@gollum)1> <span class="bold_code">messenger:logon(fred).</span> true logged_on</pre></div> <p>Now Peter sends Fred a message:</p> <div class="example"><pre> (c1@bilbo)2> <span class="bold_code">messenger:message(fred, "hello").</span> ok sent</pre></div> <p>And Fred receives the message and sends a message to Peter and logs off:</p> <div class="example"><pre> Message from peter: "hello" (c3@gollum)2> <span class="bold_code">messenger:message(peter, "go away, I'm busy").</span> ok sent (c3@gollum)3> <span class="bold_code">messenger:logoff().</span> logoff</pre></div> <p>James now tries to send a message to Fred:</p> <div class="example"><pre> (c2@kosken)2> <span class="bold_code">messenger:message(fred, "peter doesn't like you").</span> ok receiver_not_found</pre></div> <p>But this fails as Fred has already logged off.</p> <p>First let's look at some of the new concepts we have introduced.</p> <p>There are two versions of the <span class="code">server_transfer</span> function, one with four arguments (<span class="code">server_transfer/4</span>) and one with five (<span class="code">server_transfer/5</span>). These are regarded by Erlang as two separate functions.</p> <p>Note how we write the <span class="code">server</span> function so that it calls itself, <span class="code">server(User_List)</span> and thus creates a loop. The Erlang compiler is "clever" and optimizes the code so that this really is a sort of loop and not a proper function call. But this only works if there is no code after the call, otherwise the compiler will expect the call to return and make a proper function call. This would result in the process getting bigger and bigger for every loop.</p> <p>We use functions in the <span class="code">lists</span> module. This is a very useful module and a study of the manual page is recommended (<span class="code">erl -man lists</span>). <span class="code">lists:keymember(Key,Position,Lists)</span> looks through a list of tuples and looks at <span class="code">Position</span> in each tuple to see if it is the same as <span class="code">Key</span>. The first element is position 1. If it finds a tuple where the element at <span class="code">Position</span> is the same as Key, it returns <span class="code">true</span>, otherwise <span class="code">false</span>.</p> <div class="example"><pre> 3> <span class="bold_code">lists:keymember(a, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]).</span> true 4> <span class="bold_code">lists:keymember(p, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]).</span> false</pre></div> <p><span class="code">lists:keydelete</span> works in the same way but deletes the first tuple found (if any) and returns the remaining list:</p> <div class="example"><pre> 5> <span class="bold_code">lists:keydelete(a, 2, [{x,y,z},{b,b,b},{b,a,c},{q,r,s}]).</span> [{x,y,z},{b,b,b},{q,r,s}]</pre></div> <p><span class="code">lists:keysearch</span> is like <span class="code">lists:keymember</span>, but it returns <span class="code">{value,Tuple_Found}</span> or the atom <span class="code">false</span>.</p> <p>There are a lot more very useful functions in the <span class="code">lists</span> module.</p> <p>An Erlang process will (conceptually) run until it does a <span class="code">receive</span> and there is no message which it wants to receive in the message queue. I say "conceptually" because the Erlang system shares the CPU time between the active processes in the system.</p> <p>A process terminates when there is nothing more for it to do, i.e. the last function it calls simply returns and doesn't call another function. Another way for a process to terminate is for it to call <span class="code">exit/1</span>. The argument to <span class="code">exit/1</span> has a special meaning which we will look at later. In this example we will do <span class="code">exit(normal)</span> which has the same effect as a process running out of functions to call.</p> <p>The BIF <span class="code">whereis(RegisteredName)</span> checks if a registered process of name <span class="code">RegisteredName</span> exists and return the pid of the process if it does exist or the atom <span class="code">undefined</span> if it does not.</p> <p>You should by now be able to understand most of the code above so I'll just go through one case: a message is sent from one user to another.</p> <p>The first user "sends" the message in the example above by:</p> <div class="example"><pre> messenger:message(fred, "hello")</pre></div> <p>After testing that the client process exists:</p> <div class="example"><pre> whereis(mess_client) </pre></div> <p>and a message is sent to <span class="code">mess_client</span>:</p> <div class="example"><pre> mess_client ! {message_to, fred, "hello"}</pre></div> <p>The client sends the message to the server by:</p> <div class="example"><pre> {messenger, messenger@super} ! {self(), message_to, fred, "hello"},</pre></div> <p>and waits for a reply from the server.</p> <p>The server receives this message and calls:</p> <div class="example"><pre> server_transfer(From, fred, "hello", User_List),</pre></div> <p>which checks that the pid <span class="code">From</span> is in the <span class="code">User_List</span>:</p> <div class="example"><pre> lists:keysearch(From, 1, User_List) </pre></div> <p>If <span class="code">keysearch</span> returns the atom <span class="code">false</span>, some sort of error has occurred and the server sends back the message:</p> <div class="example"><pre> From ! {messenger, stop, you_are_not_logged_on}</pre></div> <p>which is received by the client which in turn does <span class="code">exit(normal)</span> and terminates. If <span class="code">keysearch</span> returns <span class="code">{value,{From,Name}}</span> we know that the user is logged on and is his name (peter) is in variable <span class="code">Name</span>. We now call:</p> <div class="example"><pre> server_transfer(From, peter, fred, "hello", User_List)</pre></div> <p>Note that as this is <span class="code">server_transfer/5</span> it is not the same as the previous function <span class="code">server_transfer/4</span>. We do another <span class="code">keysearch</span> on <span class="code">User_List</span> to find the pid of the client corresponding to fred:</p> <div class="example"><pre> lists:keysearch(fred, 2, User_List)</pre></div> <p>This time we use argument 2 which is the second element in the tuple. If this returns the atom <span class="code">false</span> we know that fred is not logged on and we send the message:</p> <div class="example"><pre> From ! {messenger, receiver_not_found};</pre></div> <p>which is received by the client, if <span class="code">keysearch</span> returns:</p> <div class="example"><pre> {value, {ToPid, fred}}</pre></div> <p>we send the message:</p> <div class="example"><pre> ToPid ! {message_from, peter, "hello"}, </pre></div> <p>to fred's client and the message:</p> <div class="example"><pre> From ! {messenger, sent} </pre></div> <p>to peter's client.</p> <p>Fred's client receives the message and prints it:</p> <div class="example"><pre> {message_from, peter, "hello"} -> io:format("Message from ~p: ~p~n", [peter, "hello"])</pre></div> <p>and peter's client receives the message in the <span class="code">await_result</span> function.</p> </div> <div class="footer"> <hr> <p>Copyright © 1996-2010 Ericsson AB. All Rights Reserved.</p> </div> </div> </div></body> </html>