I’m reading the book Programming Erlang and on page 253 there is an example for running a client/server connection. The code below suggests to run the following commands in two separate erlang console windows from which I get the following error.
2> socket_examples:nano_client_eval("list_to_tuple([2+3*4,10+20])").
** exception error: no match of right hand side value {error,econnrefused}
in function socket_examples:nano_client_eval/1 (socket_examples.erl, line 28)
the command for the server side:
socket_examples:start_nano_server().
the command for the client side:
socket_examples:nano_client_eval("list_to_tuple([2+3*4,10+20])").
I don’t have my firewall on but that shouldn’t be an issue since its not online.
%% ---
%% Excerpted from "Programming Erlang",
%%---
-module(socket_examples).
-compile(export_all).
-import(lists, [reverse/1]).
nano_get_url() ->
nano_get_url("www.google.com").
nano_get_url(Host) ->
{ok,Socket} = gen_tcp:connect(Host,80,[binary, {packet, 0}]), %% (1)
ok = gen_tcp:send(Socket, "GET / HTTP/1.0\r\n\r\n"), %% (2)
receive_data(Socket, []).
receive_data(Socket, SoFar) ->
receive
{tcp,Socket,Bin} -> %% (3)
receive_data(Socket, [Bin|SoFar]);
{tcp_closed,Socket} -> %% (4)
list_to_binary(reverse(SoFar)) %% (5)
end.
nano_client_eval(Str) ->
{ok, Socket} =
gen_tcp:connect("localhost", 2345,
[binary, {packet, 4}]),
ok = gen_tcp:send(Socket, term_to_binary(Str)),
receive
{tcp,Socket,Bin} ->
io:format("Client received binary = ~p~n",[Bin]),
Val = binary_to_term(Bin),
io:format("Client result = ~p~n",[Val]),
gen_tcp:close(Socket)
end.
start_nano_server() ->
{ok, Listen} = gen_tcp:listen(2345, [binary, {packet, 4}, %% (6)
{reuseaddr, true},
{active, true}]),
{ok, Socket} = gen_tcp:accept(Listen), %% (7)
gen_tcp:close(Listen), %% (8)
loop(Socket).
loop(Socket) ->
receive
{tcp, Socket, Bin} ->
io:format("Server received binary = ~p~n",[Bin]),
Str = binary_to_term(Bin), %% (9)
io:format("Server (unpacked) ~p~n",[Str]),
Reply = lib_misc:string2value(Str), %% (10)
io:format("Server replying = ~p~n",[Reply]),
gen_tcp:send(Socket, term_to_binary(Reply)), %% (11)
loop(Socket);
{tcp_closed, Socket} ->
io:format("Server socket closed~n")
end.
error_test() ->
spawn(fun() -> error_test_server() end),
lib_misc:sleep(2000),
{ok,Socket} = gen_tcp:connect("localhost",4321,[binary, {packet, 2}]),
io:format("connected to:~p~n",[Socket]),
gen_tcp:send(Socket, <<"123">>),
receive
Any ->
io:format("Any=~p~n",[Any])
end.
error_test_server() ->
{ok, Listen} = gen_tcp:listen(4321, [binary,{packet,2}]),
{ok, Socket} = gen_tcp:accept(Listen),
error_test_server_loop(Socket).
error_test_server_loop(Socket) ->
receive
{tcp, Socket, Data} ->
io:format("received:~p~n",[Data]),
atom_to_list(Data),
error_test_server_loop(Socket)
end.
The message you receive ({error,econnrefused}) shows that the server is down.
As Muzaaya said, the {packet,N} should be consistent in the system. If not the server will fail in the binary_to_term conversion and crash (I didn’t dig into that but i guess this is linked to the way the information is transported).
The next requests will get then the connection refused message.
with the change suggested by Muzaaya the server is able to execute binary_to_term, and everything works until the call to b_misc:string2value… which is not included in the post.
I notice something weird then:
I cannot understand what is going on.
Edit:
Thanks Muzaaya, I forgot to look at the client code…
I post here the code modified, in addition to the packet definition, I made some changes in the server and the client. I must say that I didn’t look deeply in the documentation, but it seems that there were some trouble in the original code with opening, closing and accepting. I am not sure that the code here is correct, and I know it misses something to close properly the socket, and above all that this code doesn’t serve multiple requests in parallel.