Erlang: How to receive a message only from a specific sender? - erlang

In Erlang, how can I receive a message sent from a specific pid?

Here are a couple of ways:
1) Send the target process a secret code, which it can send back with all its messages:
-module(my).
-compile(export_all).
worker(TargetPid, Msg, SecretId) ->
TargetPid ! {Msg, SecretId}.
get_msg(SecretId) ->
receive
{Msg, SecretId} -> Msg
end.
test() ->
SecretId = "A1!cd!",
spawn(my, worker, [self(), hello, "Xy*d1"]),
spawn(my, worker, [self(), goodbye, SecretId]),
io:format("~w~n", [get_msg(SecretId)]).
In the shell:
6> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
7> my:test().
goodbye
ok
8>
2) Instead of matching against the secret code in the receive clause, have the target process send a message tagged with its Pid:
-module(my).
-compile(export_all).
get_msg(TargetPid) ->
receive
{Msg, TargetPid} -> Msg
end.
worker(TargetPid, Msg) ->
TargetPid ! {Msg, self()}.
test() ->
spawn(my, worker, [self(), hello]),
Pid = spawn(my, worker, [self(), goodbye]),
io:format("~w~n", [get_msg(Pid)]).
In the shell:
3> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
4> my:test().
goodbye
ok
5>

You need to include the sender's pid in the message and match against it in the receiving process.

Related

Erlang beginnings: moving a function from an escript into OTP

There is a simple implementation of the factorial function in an 'escript' in the Erlang docs. The factorial function is given as:
fac(0) -> 1;
fac(N) -> N * fac(N-1).
That's all fine, I can get this to work, no problem.
I would however like to know how I can implement this same, simple factorial function in an 'OTP way' using rebar3?
Just to be clear, my questions are:
Where does the code go?
How would I call it from the shell?
Could I also run it from the command line like I do via the escript example?
FYI, I have gotten started with rebar3. Here is where I am at:
rebar3 new app factorial
creates a few files but specifically the code is in 3 files in a src directory. I can see that a supervisor is being used, seems fine.
I can interact with this project from the shell:
$ rebar3 shell
1> application:which_applications().
[{factorial,"An OTP application","0.1.0"},
{inets,"INETS CXC 138 49","7.0.3"},
{ssl,"Erlang/OTP SSL application","9.1.1"},
{public_key,"Public key infrastructure","1.6.4"},
{asn1,"The Erlang ASN1 compiler version 5.0.8","5.0.8"},
{crypto,"CRYPTO","4.4"},
{stdlib,"ERTS CXC 138 10","3.7"},
{kernel,"ERTS CXC 138 10","6.2"}]
2> application:stop(factorial).
=INFO REPORT==== 21-Jan-2019::12:42:07.484244 ===
application: factorial
exited: stopped
type: temporary
ok
3> application:start(factorial).
ok
Where does the code go?
To 'call code in the OTP way', you can put it behind a gen_server.
For this simple factorial function, I added a new file factorial.erl within the src directory which is pretty much a standard gen_server skeleton with my factorial function as one of the callbacks:
% factorial.erl
-module(factorial).
-behaviour(gen_server).
-export([start_link/0, stop/0, calc/1]).
<boilerplate gen_server stuff here, like init, etc.>
calc(N) ->
{ok, Result} = gen_server:call(?SERVER, {calc, N}),
{ok, Result}.
handle_call({calc, N}, _From, State) ->
Factorial = factorial(N),
Reply = {ok, Factorial},
{reply, Reply, State};
factorial(0) ->
1;
factorial(N) ->
N * factorial(N-1).
Since my rebar3 new app factorial created a supervisor, I modified the supervisor's init so that it calls my factorial module:
% factorial_sup.erl
<skeleton supervisor stuff here>
init([]) ->
Server = {factorial, {factorial, start_link, []},
permanent, 2000, worker, [factorial]},
Children = [Server],
RestartStrategy = {one_for_one, 0, 1},
{ok, {RestartStrategy, Children}}.
How do I call it from the shell?
$ rebar3 shell
<Enter>
1> factorial:calc(5).
{ok,120}
Since this is running under a supervisor, we can still stop and restart it:
2> application:stop(factorial).
=INFO REPORT==== 22-Jan-2019::13:31:29.243520 ===
application: factorial
exited: stopped
type: temporary
ok
3> factorial:calc(5).
** exception exit: {noproc,{gen_server,call,[factorial,{calc,5}]}}
in function gen_server:call/2 (gen_server.erl, line 215)
in call from factorial:calc/1 (/Users/robert/git/factorial/src/factorial.erl, line 32)
4> application:start(factorial).
ok
5> factorial:calc(5).
{ok,120}
How do I create an executable?
Work in progress :-).

Erlang with multiple client shells - spawned process hangs in 'io' module

working through Joe's book, got stuck on Chapter 12 exercise 1. That exercise is asking one to write a function start(AnAtom,Fun) that would register AnAtom as spawn(Fun). I've decided to try something seemingly easier - took the chapter's finished 'area_server' module, and modified it's start/0 function like this:
start() ->
Pid = spawn(ex1, loop, []),
io:format("Spawned ~p~n",[Pid]),
register(area, Pid).
so in place of a process executing the arbitrary Fun, I am registering the 'loop', which is a function in the area_server module doing all the work:
loop() ->
receive
{From, {rectangle, Width, Ht}} ->
io:format("Computing for rectangle...~n"),
From ! {self(), Width*Ht},
loop();
{From, {square, Side}} ->
io:format("Computing for square...~n"),
From ! {self(), Side*Side},
loop();
{From, Other} ->
io:format("lolwut?~n"),
From ! {self(), {error, Other}},
loop()
end.
It seems to be working just fine:
1> c("ex1.erl").
{ok,ex1}
2> ex1:start().
Spawned <0.68.0>
true
3>
3> area ! {self(), hi}.
lolwut?
{<0.61.0>,hi}
4> flush().
Shell got {<0.68.0>,{error,hi}}
ok
5> area ! {self(), {square, 7}}.
Computing for square...
{<0.61.0>,{square,7}}
6> flush().
Shell got {<0.68.0>,49}
ok
Thing went bad when I've tried to test that multiple processes can talk to the registered "server". (CTRL-G, s, c 2)
I'm in a new shell, running alongside the first - but the moment I send a message from this new shell to my 'area' registered process, something nasty happens - when querying process_info(whereis(area)), process moves from this state:
{current_function,{ex1,loop,0}},
{initial_call,{ex1,loop,0}},
to this one:
{current_function,{io,execute_request,2}},
{initial_call,{ex1,loop,0}},
while the message queue starts to grow, messages not getting processed. Hanging in module io, huh! Something is blocked on the io operations? Apparently the process is moved from my ex1:loop/0 into io:execute_request/2 (whatever that is)... are my silly prints causing the problem?
Your processes are doing what you expect with the exception of handling who has control over STDOUT at what moment. And yes, this can cause weird seeming behaviors in the shell.
So let's try something like this without any IO commands that are implied to go to STDOUT and see what happens. Below is a shell session where I define a loop that accumulates messages until I ask it to send me the messages it has accumulated. We can see from this example (which does not get hung up on who is allowed to talk to the single output resource) that the processes behave as expected.
One thing to take note of is that you do not need multiple shells to talk to or from multiple processes.
Note the return value of flush/0 in the shell -- it is a special shell command that dumps the shell's mailbox to STDOUT.
Eshell V9.0 (abort with ^G)
1> Loop =
1> fun L(History) ->
1> receive
1> halt ->
1> exit(normal);
1> {Sender, history} ->
1> Sender ! History,
1> L([]);
1> Message ->
1> NewHistory = [Message | History],
1> L(NewHistory)
1> end
1> end.
#Fun<erl_eval.30.87737649>
2> {Pid1, Ref1} = spawn_monitor(fun() -> Loop([]) end).
{<0.64.0>,#Ref<0.1663562856.2369257474.102541>}
3> {Pid2, Ref2} = spawn_monitor(fun() -> Loop([]) end).
{<0.66.0>,#Ref<0.1663562856.2369257474.102546>}
4> Pid1 ! "blah".
"blah"
5> Pid1 ! "blee".
"blee"
6> Pid1 ! {self(), history}.
{<0.61.0>,history}
7> flush().
Shell got ["blee","blah"]
ok
8> Pid1 ! "Message from shell 1".
"Message from shell 1"
9> Pid2 ! "Message from shell 1".
"Message from shell 1"
10>
User switch command
--> s
--> j
1 {shell,start,[init]}
2* {shell,start,[]}
--> c 2
Eshell V9.0 (abort with ^G)
1> Shell1_Pid1 = pid(0,64,0).
<0.64.0>
2> Shell1_Pid2 = pid(0,66,0).
<0.66.0>
3> Shell1_Pid1 ! "Message from shell 2".
"Message from shell 2"
4> Shell1_Pid2 ! "Another message from shell 2".
"Another message from shell 2"
5> Shell1_Pid1 ! {self(), history}.
{<0.77.0>,history}
6> flush().
Shell got ["Message from shell 2","Message from shell 1"]
ok
7>
User switch command
--> c 1
11> Pid2 ! {self(), history}.
{<0.61.0>,history}
12> flush().
Shell got ["Another message from shell 2","Message from shell 1"]
ok

Erlang - use of lists when the size is consistently changing over time

I am new to Erlang and I am learning by building a really small chat program in pure Erlang.
I would like for clients to be able to connect to a server and then send messages to each other. But this is all done on a local machine and not over a network just for learning.
I have a list of all the clients that have connected to the server.
If client A sends a message to client B, I get the desired output in client A's terminal but I can't work out how to get the message from client A to be displayed on client B's terminal.
Or do I have to set each client with it's own mini server
-module(server).
-export([start/0]).
-export([server/1]).
-export([connect/0]).
-export([sendMessage/2]).
%%
%% The Server
%%
start() ->
EmptyList = [],
Pid = spawn(server, server, [EmptyList]),
register(chatServe, Pid).
server(ListOfClients) ->
receive
{Client, connect} ->
Client ! {chatServe, connected},
List = clientList(ListOfClients, Client),
server(List);
{Client, message, MessageBody} ->
List = ListOfClients,
lists:foreach(fun(X) -> X ! {chatServe, new_message, MessageBody} end, List),
Client ! {chatServe, received},
server(List)
end.
%%
%% The client will call rpc:call(server#local, server, connect, [])
%% to connect
%%
connect() ->
chatServe ! {self(), connect},
receive
{chatServe, connected} -> connected
end.
%%
%% The send message method takes two args
%%
%%
sendMessage(SendTo, MessageBody) ->
chatServe ! {self(), message, MessageBody},
receive
{chatServe, received} -> received
end.
receiveMessage(SendTo, SendFrom, MessageBody) ->
receive
{}
end.
%%
%% Some helper functions
%%
clientList(List, Client) when length(List) =:= 0 ->
io:format("List Size = 1~n"),
[Client];
clientList(List, Client) ->
io:format("List size = ~p~n", [length(List) + 1]),
[Client | List].
forwardMessage(SendTo, SentFrom, MessageBody, [H | T]) when H =:= SendTo ->
SendTo ! {SentFrom, message, MessageBody};
forwardMessage(SendTo, SentFrom, MessageBody, [H | T]) ->
forwardMessage(SendTo, SentFrom, MessageBody, T);
forwardMessage(SendTo, SentFrom, MessageBody, []) -> [].
In the terminal of the client I am calling
rpc:call(host, mod, function, args).
So my question is how can I get Client A to send a message to Client B through server C, with Client A displaying success, and Client B displaying the message that was sent?
Thanks in advance
In this example my server is an Erlang node and every client is an erlang node too.
Code:
-module(test).
-export([server_start/1, client_start/2]).
-export([server_new_message/2, client_new_message/2]).
server_start(ServerName) ->
{ok, _Pid} = net_kernel:start([ServerName, shortnames]),
erlang:register(server, erlang:self()),
io:format("Server '~p' started.~nMessages: ~n ~n", [erlang:node()]),
server_loop().
server_loop() ->
receive
{msg, Name, Text} ->
io:format("~p: ~p~n", [Name, Text]),
Receivers = lists:delete(Name, erlang:nodes()),
rpc:multicall(Receivers, ?MODULE, client_new_message, [Name, Text]),
server_loop()
end.
%% Server runs this function in client's node.
%% 'client' process in client's node will receive this message and print it
client_new_message(Name, Text) ->
client ! {msg, Name, Text}.
client_start(ServerName, ClientName) ->
{ok, _Pid} = net_kernel:start([ClientName, shortnames]),
pong = net_adm:ping(ServerName),
timer:sleep(1000), % wait for updating erlang:nodes()
Other = lists:delete(ServerName, erlang:nodes()),
io:format("Client '~p' connected to server '~p'.~nOnline users: ~p~n", [erlang:node(), ServerName, Other]),
erlang:register(client, spawn_link(fun print/0)),
client_loop(ServerName).
print() ->
receive
{msg, Name, Text} ->
io:format("~p: ~p~n", [Name, Text]),
print()
end.
client_loop(ServerName) ->
rpc:call(ServerName, ?MODULE, server_new_message, [erlang:node(), io:get_line(">>> ")]),
client_loop(ServerName).
%% Clients run this function in server's node
%% 'server' process will receive messages and print them and broadcast them
server_new_message(Name, Text) ->
server ! {msg, Name, Text}.
Run:
I open 3 Erlang shells.
In shell 1 i run server:
p#jahanbakhsh ~/Desktop $ erl
Erlang/OTP 19 [erts-8.2.2] [source-1ca84a4] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.2.2 (abort with ^G)
1> test:server_start(local_chat_server).
Server 'local_chat_server#jahanbakhsh' started.
Messages:
Server is waiting for messages.
In shell 2 i run client 1:
p#jahanbakhsh ~/Desktop $ erl
Erlang/OTP 19 [erts-8.2.2] [source-1ca84a4] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.2.2 (abort with ^G)
1> test:client_start('local_chat_server#jahanbakhsh', client_1).
Client 'client_1#jahanbakhsh' connected to server 'local_chat_server#jahanbakhsh'.
Online users: []
>>>
Now i can send message from this terminal, but wait.
I run client 2 in shell 3:
p#jahanbakhsh ~/Desktop $ erl
Erlang/OTP 19 [erts-8.2.2] [source-1ca84a4] [64-bit] [smp:4:4] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.2.2 (abort with ^G)
1> test:client_start('local_chat_server#jahanbakhsh', client_2).
Client 'client_2#jahanbakhsh' connected to server 'local_chat_server#jahanbakhsh'.
Online users: [client_1#jahanbakhsh]
>>>
I send a message ("Test message") from shell 2 or client 1.
In shell 1 or server i have:
client_1#jahanbakhsh: "Test message\n" - sent to [client_2#jahanbakhsh]
In shell 3 or client 2 i have:
client_1#jahanbakhsh: "Test message\n"
>>>

Gen Server Error noproc

I get the following error when I try to run my program through shell using erlang.mk.
=INFO REPORT==== 5-May-2016::05:47:57 ===
application: rad
exited: {bad_return,
{{rad_app,start,[normal,[]]},
{'EXIT',
{noproc,{gen_server,call,[rad_config,{lookup,port}]}}}}}
type: permanent
Eshell V6.4 (abort with ^G)
(rad#127.0.0.1)1> {"Kernel pid terminated",application_controller,"{application_start_failure,rad,{bad_return,{{rad_app,start,[normal,[]]},{'EXIT',{noproc,{gen_server,call,[rad_config,{lookup,port}]}}}}}}"}
Kernel pid terminated (application_controller) ({application_start_failure,rad,{bad_return,{{rad_app,start,[normal,[]]},{'EXIT',{noproc,{gen_server,call,[radheart: Thu May 5 05:47:58 2016: _coErlang is crashing .. (waiting for crash dump file)nf
ig,{lookup,porheart: Thu May 5 05:47:58 2016: tWould reboot. Terminating.}
]}}}}}})
make: *** [run] Error 1
rad.app.src
{application, rad,
[
{description, "Awesome server written in Erlang"},
{vsn, "0.0.1"},
{registered, [rad_sup, rad_config]},
{modules, []},
{applications, [
kernel,
stdlib,
cowboy
]},
{mod, {rad_app, []}},
{env, []}
]}.
rad_config.erl
-module(rad_config).
-behaviour(gen_server).
%% API
-export([start_link/0]).
%% Gen Server Callbacks
-export([init/1 , handle_call/3 , handle_cast/2 , handle_info/2 , terminate/2 , code_change/3]).
-export([lookup/1]).
-define(SERVER, ?MODULE).
-record(state, {conf}).
start_link() ->
gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
init([]) ->
{ok, Conf} = file:consult("config/rad.cfg"),
{ok, #state{conf = Conf}}.
handle_call({lookup, Tag} , _From , State) ->
Reply = case lists:keyfind(Tag, 1, State#state.conf) of
{Tag, Value} ->
Value;
false ->
{error, noinstance}
end,
{reply, Reply, State};
handle_call(_Request, _From, State) ->
Reply = ok,
{reply, Reply, State}.
handle_cast(_Msg , State) ->
{noreply, State}.
handle_info(_Info , State) ->
{noreply, State}.
terminate(Reason , _State) ->
io:format("~n Server shutdown. Reason: ~s.~n", [Reason]),
ok.
code_change(_OldVsn , State , _Extra) ->
{ok, State}.
lookup(Tag) ->
gen_server:call(?SERVER, {lookup, Tag}).
rad_app.erl
-module(rad_app).
-behaviour(application).
-export([start/2]).
-export([stop/1]).
%% First we need to define and compile the dispatch list, a list of routes
%% that Cowboy will use to map requests to handler modules. Then we tell
%% Cowboy to listen for connections.
start(_Type, _Args) ->
Port = rad_config:lookup(port),
Dispatch = cowboy_router:compile([
{'_', [{"/", route_handler, []}]}
]),
{ok, _} = cowboy:start_http(rad_http_listener, 100,
[{port, Port}], [{env, [{dispatch, Dispatch}]}]),
%% Start the supervisor
rad_sup:start_link().
stop(_State) ->
ok.
rad_sup.erl
-module(rad_sup).
-behaviour(supervisor).
%% API
-export([start_link/0]).
%% Supervisor callbacks
-export([init/1, shutdown/0]).
-define(SERVER, ?MODULE).
%% Helper macro for declaring children of supervisor
-define(CHILD(I, Type), {I, {I, start_link, []}, permanent, 5000, Type, [I]}).
%% ===================================================================
%% API functions
%% ===================================================================
start_link() ->
supervisor:start_link({local, ?SERVER}, ?MODULE, []).
%% ===================================================================
%% Supervisor callbacks
%% ===================================================================
init([]) ->
RestartStrategy = simple_one_for_one,
MaxRestarts = 10,
MaxSecondsBwRestarts = 5,
SupFlag = {RestartStrategy, MaxRestarts, MaxSecondsBwRestarts},
Processes = [?CHILD(rad_config, worker)],
{ok, {SupFlag, Processes}}.
%% Supervisor can be shutdown by calling exit(SupPid,shutdown)
%% or, if it's linked to its parent, by parent calling exit/1.
shutdown() ->
exit(whereis(?MODULE), shutdown).
So basically I've two questions related to the error that is thrown here:
Is this error thrown because my gen_server is not able to start?
The line in rad_config corresponding to file:consult/1, I want to ask from where does this function fetches the file as in the parameter that I've passed to it is config/rad.cfg but all the .erl files are stored in src folder. And both these folders src and config are at the same directory level. So, the parameter that I've passed to file:consult/1, is it correct? Although I've tried to pass the parameter as ../config/rad.cfg also. I still get the same error.
Please help me out. I'm new to Erlang and I'm trying to solve this error for quite some time. Btw, I using Erlang 17.5.
First, it seems like when you run rad_app.erl your rad_config server is not yet started. so when your get to this line:
Port = rad_config:lookup(port)
You are actually calling:
lookup(Tag) ->
gen_server:call(?SERVER, {lookup, Tag}).
And the gen_server is not started so you are getting a noproc error.
In addition to this, even if the server was started already, you are not able to make a gen_server:call to your self. The best way to handle a case that you want to send yourself an event is to open a new process using spawn and from inside the spawned process make the call.
Your should read more about gen_server and OTP.

Empty Process Mail box in Erlang

when you send a message to the shell process, you can flush all messages out by calling: c:flush().
C:\Windows\System32>erl
Eshell V5.9 (abort with ^G)
1> self() ! josh.
josh
2> self() ! me.
me
3> self() ! you.
you
4> flush().
Shell got josh
Shell got me
Shell got you
ok
5>
In my thinking , this empties the mail box of the shell process.
What is the equivalent way of emptying the mailbox of any erlang process ?
This function should flush all messages from mailbox (in any process where you call it):
flush() ->
receive
_ -> flush()
after
0 -> ok
end.

Resources