The simple server from the Overview chapter, implemented using sys and proc_lib so it fits into a supervision tree:

-module(ch4).
-export([start_link/0]).
-export([alloc/0, free/1]).
-export([init/1]).
-export([system_continue/3, system_terminate/4,
         write_debug/3,
         system_get_state/1, system_replace_state/2]).

start_link() ->
    proc_lib:start_link(ch4, init, [self()]).

alloc() ->
    ch4 ! {self(), alloc},
    receive
        {ch4, Res} ->
            Res
    end.

free(Ch) ->
    ch4 ! {free, Ch},
    ok.

init(Parent) ->
    register(ch4, self()),
    Chs = channels(),
    Deb = sys:debug_options([]),
    proc_lib:init_ack(Parent, {ok, self()}),
    loop(Chs, Parent, Deb).

loop(Chs, Parent, Deb) ->
    receive
        {From, alloc} ->
            Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                    ch4, {in, alloc, From}),
            {Ch, Chs2} = alloc(Chs),
            From ! {ch4, Ch},
            Deb3 = sys:handle_debug(Deb2, fun ch4:write_debug/3,
                                    ch4, {out, {ch4, Ch}, From}),
            loop(Chs2, Parent, Deb3);
        {free, Ch} ->
            Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                    ch4, {in, {free, Ch}}),
            Chs2 = free(Ch, Chs),
            loop(Chs2, Parent, Deb2);

        {system, From, Request} ->
            sys:handle_system_msg(Request, From, Parent,
                                  ch4, Deb, Chs)
    end.

system_continue(Parent, Deb, Chs) ->
    loop(Chs, Parent, Deb).

system_terminate(Reason, _Parent, _Deb, _Chs) ->
    exit(Reason).

system_get_state(Chs) ->
    {ok, Chs}.

system_replace_state(StateFun, Chs) ->
    NChs = StateFun(Chs),
    {ok, NChs, NChs}.

write_debug(Dev, Event, Name) ->
    io:format(Dev, "~p event = ~p~n", [Name, Event]).

Example on how the simple debugging functions in sys can be used for ch4 as well:

% erl
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

Eshell V5.2.3.6  (abort with ^G)
1> ch4:start_link().
{ok,<0.30.0>}
2> sys:statistics(ch4, true).
ok
3> sys:trace(ch4, true).
ok
4> ch4:alloc().
ch4 event = {in,alloc,<0.25.0>}
ch4 event = {out,{ch4,ch1},<0.25.0>}
ch1
5> ch4:free(ch1).
ch4 event = {in,{free,ch1}}
ok
6> sys:statistics(ch4, get).
{ok,[{start_time,{{2003,6,13},{9,47,5}}},
     {current_time,{{2003,6,13},{9,47,56}}},
     {reductions,109},
     {messages_in,2},
     {messages_out,1}]}
7> sys:statistics(ch4, false).
ok
8> sys:trace(ch4, false).
ok
9> sys:get_status(ch4).
{status,<0.30.0>,
        {module,ch4},
        [[{'$ancestors',[<0.25.0>]},{'$initial_call',{ch4,init,[<0.25.0>]}}],
         running,<0.25.0>,[],
         [ch1,ch2,ch3]]}

A function in the proc_lib module should be used to start the process. There are several possible functions, for example spawn_link/3,4 for asynchronous start and start_link/3,4,5 for synchronous start.

A process started using one of these functions will store information that is needed for a process in a supervision tree, for example about the ancestors and initial call.

Also, if the process terminates with another reason than normal or shutdown, a crash report (see SASL User's Guide) is generated.

In the example, synchronous start is used. The process is started by calling ch4:start_link():

ch4:start_link calls the function proc_lib:start_link. This function takes a module name, a function name and an argument list as arguments and spawns and links to a new process. The new process starts by executing the given function, in this case ch4:init(Pid), where Pid is the pid (self()) of the first process, that is the parent process.

In init, all initialization including name registration is done. The new process must also acknowledge that it has been started to the parent:

proc_lib:start_link is synchronous and does not return until proc_lib:init_ack has been called.

To support the debug facilites in sys, we need a debug structure, a term Deb which is initialized using sys:debug_options/1:

sys:debug_options/1 takes a list of options as argument. Here the list is empty, which means no debugging is enabled initially. See sys(3) for information about possible options.

Then for each system event that we want to be logged or traced, the following function should be called.

handle_debug returns an updated debug structure Deb1.

In the example, handle_debug is called for each incoming and outgoing message. The format function Func is the function ch4:write_debug/3 which prints the message using io:format/3.

System messages are received as:

The content and meaning of these messages do not need to be interpreted by the process. Instead the following function should be called:

This function does not return. It will handle the system message and then call:

if process execution should continue, or:

if the process should terminate. Note that a process in a supervision tree is expected to terminate with the same reason as its parent.

If the process should return its state handle_system_msg will call:

or if the process should replace its state using the fun StateFun:

In the example:

If the special process is set to trap exits, note that if the parent process terminates, the expected behavior is to terminate with the same reason: