p.erl

-module(p).
-export([start_link/0, print_state/0, init/1, loop/1, 
         system_continue/3, system_code_change/4, system_terminate/4]).

start_link() ->
    proc_lib:start_link(?MODULE, init, [self()]).
    
print_state() ->
    {ok, Result} = gen:call(?MODULE, '$p', print_state),
    Result.
    
init(Parent) ->
    register(?MODULE, self()),
    proc_lib:init_ack(Parent, {ok, self()}),
    ?MODULE:loop(0).
    
loop(State) ->
    receive
        {'$p', {From, Mref}, print_state} ->
            io:format("state: ~p~n", [State]),
            gen:reply({From, Mref}, ok),
            ?MODULE:loop(State+1);
        {system, From, Msg} ->
            sys:handle_system_msg(Msg, From, self(), ?MODULE, [], State)
    end.

system_continue(_Parent, _NDebug, State) ->
    ?MODULE:loop(State).
    
system_code_change(State, ?MODULE, _OldVsn, _Extra) -> 
    {ok, State}.
    
system_terminate(_Reason, _Parent, _Debug, _State) ->
    ok.