The simple server from Overview, 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 the sys module can also be used for ch4:

% 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 is to be used to start the process. Several functions are available, 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 stores information (for example, about the ancestors and initial call) that is needed for a process in a supervision tree.

If the process terminates with another reason than normal or shutdown, a crash report is generated. For more information about the crash report, see the SASL User's Guide.

In the example, synchronous start is used. The process starts 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, spawns, and links to a new process. The new process starts by executing the given function, here ch4:init(Pid), where Pid is the pid (self()) of the first process, which is the parent process.

All initialization, including name registration, is done in init. 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, a debug structure is needed. The Deb term 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. For information about the possible options, see the sys(3) manual page in STDLIB.

Then, for each system event to be logged or traced, the following function is to be called.

Here:

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 is to be called:

This function does not return. It handles the system message and then either calls the following if process execution is to continue:

Or calls the following if the process is to terminate:

A process in a supervision tree is expected to terminate with the same reason as its parent.

If the process is to return its state, handle_system_msg calls:

If the process is to replace its state using the fun StateFun, handle_system_msg calls:

In the example:

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