How to debug errors in Erlang? -

i got 1 error when starting gen server, want know how debug it, thanks!

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

the error :

=error report==== 11-may-2011::13:41:57 === ** generic server <0.37.0> terminating  ** last message in {'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 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 :

-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     %% modifies 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, 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). 

well, gen_server:call getting timeout when called. means gen_server either taking longer default 3 second timeout call or blocked somewhere.

using tracing debug kind of behaviour ideal. instance if type in shell before running test:

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

you trace on functions within ess_tcp see going on in there. more info dbg see http://www.erlang.org/doc/man/dbg.html