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How to debug errors in Erlang?

I have one error when starting my gen server, I want to know how to debug it, thanks!

I run "example: add_listener (self ()," 127.0.0.1 ", 10999)." after start_link.

Error:

=ERROR REPORT==== 11-May-2011::13:41:57 === ** Generic server <0.37.0> terminating ** Last message in was {'EXIT',<0.35.0>, {{timeout, {gen_server,call, [<0.35.0>, {add_listener,"127.0.0.1",10999}]}}, [{gen_server,call,2}, {erl_eval,do_apply,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]}} ** When Server state == {state,example, {dict,0,16,16,8,80,48, {[],[],[],[],[],[],[],[],[],[],[],[],[], [],[],[]}, {{[],[],[],[],[],[],[],[],[],[],[],[],[], [],[],[]}}}, {dict,0,16,16,8,80,48, {[],[],[],[],[],[],[],[],[],[],[],[],[], [],[],[]}, {{[],[],[],[],[],[],[],[],[],[],[],[],[], [],[],[]}}}, []} ** Reason for termination == ** {{timeout,{gen_server,call,[<0.35.0>,{add_listener,"127.0.0.1",10999}]}}, [{gen_server,call,2}, {erl_eval,do_apply,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]} ** exception exit: {timeout,{gen_server,call, [<0.35.0>,{add_listener,"127.0.0.1",10999}]}} in function gen_server:call/2

My code is:

 -module(test_ess_tcp). -export([start_link/0, add_listener/3, remove_listener/3]). -export([init/2, handle_call/3, handle_cast/2, handle_info/2]). -export([terminate/2, sock_opts/0, new_connection/4]). -behavior(ess_tcp). start_link() -> ess_tcp:start_link(?MODULE, []). add_listener(Pid, IpAddr, Port) -> gen_server:call(Pid, {add_listener, IpAddr, Port}). remove_listener(Pid, IpAddr, Port) -> gen_server:call(Pid, {remove_listener, IpAddr, Port}). init([], State) -> %% Example storing callback module specific state %% This modifies the server state {ok, ess_tcp:store_cb_state([], State)}. handle_call({add_listener, IpAddr, Port}, _From, State) -> %% Example of getting callback module state io:format("Not used here, but just an example"), [] = ess_tcp:get_cb_state(State), case ess_tcp:add_listen_socket({IpAddr, Port}, State) of {ok, State1} -> {reply, ok, State1}; Error -> {reply, Error, State} end; handle_call({remove_listener, IpAddr, Port}, _From, State) -> case ess_tcp:remove_listen_socket({IpAddr, Port}, State) of {ok, State1} -> {reply, ok, State1}; Error -> {reply, Error, State} end; handle_call(_Msg, _From, State) -> {reply, ignored, State}. handle_cast(_Msg, State) -> {noreply, State}. handle_info({tcp, Sock, Data}, State) -> Me = self(), P = spawn(fun() -> worker(Me, Sock, Data) end), gen_tcp:controlling_process(Sock, P), {noreply, State}; handle_info(_Msg, State) -> {noreply, State}. terminate(_Reason, _State) -> ok. sock_opts() -> [binary, {active, once}, {packet, 0}]. new_connection(_IpAddr, _Port, Sock, State) -> Me = self(), P = spawn(fun() -> worker(Me, Sock) end), gen_tcp:controlling_process(Sock, P), {ok, State}. worker(Owner, Sock) -> gen_tcp:send(Sock, "Hello\n"), inet:setopts(Sock, [{active, once}]), gen_tcp:controlling_process(Sock, Owner). worker(Owner, Sock, Data) -> gen_tcp:send(Sock, Data), inet:setopts(Sock, [{active, once}]), gen_tcp:controlling_process(Sock, Owner).
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2 answers

Well, your gen_server: the call gets a timeout on the call. This means that gen_server either takes longer than the default 3-second timeout for the call, or it is blocked somewhere.

Using trace to debug behavior is ideal. For example, if you enter this into the shell before running the test:

 dbg:tracer(),dbg:p(all,c),dbg:tpl(ess_tcp, x).

you follow all the functions in ess_tcp to see what happens there. For more information on dbg, see http://www.erlang.org/doc/man/dbg.html

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In fact, the best debugging tools in erlang are the io: format format statements. Put io: formats where you have doubts, and see if you get the expected values. As for the above, it is mostly stuck somewhere !!

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