path: root/system/doc/design_principles/spec_proc.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Sys and Proc_Lib</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>spec_proc.xml</file>
  </header>
  <p>The module <c>sys</c> contains functions for simple debugging of
    processes implemented using behaviours.</p>
  <p>There are also functions that, together with functions in
    the module <c>proc_lib</c>, can be used to implement a
    <em>special process</em>, a process which comply to the OTP design
    principles without making use of a standard behaviour. They can
    also be used to implement user defined (non-standard) behaviours.</p>
  <p>Both <c>sys</c> and <c>proc_lib</c> belong to the STDLIB
    application.</p>

  <section>
    <title>Simple Debugging</title>
    <p>The module <c>sys</c> contains some functions for simple debugging
      of processes implemented using behaviours. We use the
      <c>code_lock</c> example from
      the <seealso marker="fsm#ex">gen_fsm</seealso> chapter to
      illustrate this:</p>
    <pre>
% <input>erl</input>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

Eshell V5.2.3.6  (abort with ^G)
1> <input>code_lock:start_link([1,2,3,4]).</input>
{ok,&lt;0.32.0>}
2> <input>sys:statistics(code_lock, true).</input>
ok
3> <input>sys:trace(code_lock, true).</input>
ok
4> <input>code_lock:button(4).</input>
*DBG* code_lock got event {button,4} in state closed
ok
*DBG* code_lock switched to state closed
5> <input>code_lock:button(3).</input>
*DBG* code_lock got event {button,3} in state closed
ok
*DBG* code_lock switched to state closed
6> <input>code_lock:button(2).</input>
*DBG* code_lock got event {button,2} in state closed
ok
*DBG* code_lock switched to state closed
7> <input>code_lock:button(1).</input>
*DBG* code_lock got event {button,1} in state closed
ok
OPEN DOOR
*DBG* code_lock switched to state open
*DBG* code_lock got event timeout in state open
CLOSE DOOR
*DBG* code_lock switched to state closed
8> <input>sys:statistics(code_lock, get).</input> 
{ok,[{start_time,{{2003,6,12},{14,11,40}}},
     {current_time,{{2003,6,12},{14,12,14}}},
     {reductions,333},
     {messages_in,5},
     {messages_out,0}]}
9> <input>sys:statistics(code_lock, false).</input>
ok
10> <input>sys:trace(code_lock, false).</input>     
ok
11> <input>sys:get_status(code_lock).</input>
{status,&lt;0.32.0>,
        {module,gen_fsm},
        [[{'$ancestors',[&lt;0.30.0>]},
          {'$initial_call',{gen,init_it,
                                [gen_fsm,&lt;0.30.0>,&lt;0.30.0>,
                                 {local,code_lock},
                                 code_lock,
                                 [1,2,3,4],
                                 []]}}],
         running,&lt;0.30.0>,[],
         [code_lock,closed,{[],[1,2,3,4]},code_lock,infinity]]}</pre>
  </section>

  <section>
    <title>Special Processes</title>
    <p>This section describes how to write a process which comply to
      the OTP design principles, without making use of a standard
      behaviour. Such a process should:</p>
    <list type="bulleted">
      <item>be started in a way that makes the process fit into a
       supervision tree,</item>
      <item>support the <c>sys</c> <seealso marker="#debug">debug facilities</seealso>, and</item>
      <item>take care of <seealso marker="#msg">system messages</seealso>.</item>
    </list>
    <p>System messages are messages with special meaning, used in
      the supervision tree. Typical system messages are requests for
      trace output, and requests to suspend or resume process execution
      (used during release handling). Processes implemented using
      standard behaviours automatically understand these messages.</p>

    <section>
      <title>Example</title>
      <p>The simple server from
        the <seealso marker="des_princ#ch1">Overview</seealso> chapter,
        implemented using <c>sys</c> and <c>proc_lib</c> so it fits into
        a supervision tree:</p>
      <marker id="ex"></marker>
      <pre>
-module(ch4).
-export([start_link/0]).
-export([alloc/0, free/1]).
-export([init/1]).
-export([system_continue/3, system_terminate/4,
         write_debug/3]).

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

write_debug(Dev, Event, Name) ->
    io:format(Dev, "~p event = ~p~n", [Name, Event]).</pre>
      <p>Example on how the simple debugging functions in <c>sys</c> can
        be used for <c>ch4</c> as well:</p>
      <pre>
% <input>erl</input>
Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

Eshell V5.2.3.6  (abort with ^G)
1> <input>ch4:start_link().</input>
{ok,&lt;0.30.0>}
2> <input>sys:statistics(ch4, true).</input>
ok
3> <input>sys:trace(ch4, true).</input>
ok
4> <input>ch4:alloc().</input>
ch4 event = {in,alloc,&lt;0.25.0>}
ch4 event = {out,{ch4,ch1},&lt;0.25.0>}
ch1
5> <input>ch4:free(ch1).</input>
ch4 event = {in,{free,ch1}}
ok
6> <input>sys:statistics(ch4, get).</input>
{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> <input>sys:statistics(ch4, false).</input>
ok
8> <input>sys:trace(ch4, false).</input>
ok
9> <input>sys:get_status(ch4).</input>
{status,&lt;0.30.0>,
        {module,ch4},
        [[{'$ancestors',[&lt;0.25.0>]},{'$initial_call',{ch4,init,[&lt;0.25.0>]}}],
         running,&lt;0.25.0>,[],
         [ch1,ch2,ch3]]}</pre>
    </section>

    <section>
      <title>Starting the Process</title>
      <p>A function in the <c>proc_lib</c> module should be used to
        start the process. There are several possible functions, for
        example <c>spawn_link/3,4</c> for asynchronous start and
        <c>start_link/3,4,5</c> for synchronous start.</p>
      <p>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.</p>
      <p>Also, if the process terminates with another reason than
        <c>normal</c> or <c>shutdown</c>, a crash report (see SASL
        User's Guide) is generated.</p>
      <p>In the example, synchronous start is used. The process is
        started by calling <c>ch4:start_link()</c>:</p>
      <code type="none">
start_link() ->
    proc_lib:start_link(ch4, init, [self()]).</code>
      <p><c>ch4:start_link</c> calls the function
        <c>proc_lib:start_link</c>. 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 <c>ch4:init(Pid)</c>, where
        <c>Pid</c> is the pid (<c>self()</c>) of the first process, that
        is the parent process.</p>
      <p>In <c>init</c>, all initialization including name registration
        is done. The new process must also acknowledge that it has been
        started to the parent:</p>
      <code type="none">
init(Parent) ->
    ...
    proc_lib:init_ack(Parent, {ok, self()}),
    loop(...).</code>
      <p><c>proc_lib:start_link</c> is synchronous and does not return
        until <c>proc_lib:init_ack</c> has been called.</p>
    </section>

    <section>
      <marker id="debug"></marker>
      <title>Debugging</title>
      <p>To support the debug facilites in <c>sys</c>, we need a
        <em>debug structure</em>, a term <c>Deb</c> which is
        initialized using <c>sys:debug_options/1</c>:</p>
      <code type="none">
init(Parent) ->
    ...
    Deb = sys:debug_options([]),
    ...
    loop(Chs, Parent, Deb).</code>
      <p><c>sys:debug_options/1</c> takes a list of options as argument.
        Here the list is empty, which means no debugging is enabled
        initially. See <c>sys(3)</c> for information about possible
        options.</p>
      <p>Then for each <em>system event</em> that we want to be logged
        or traced, the following function should be called.</p>
      <code type="none">
sys:handle_debug(Deb, Func, Info, Event) => Deb1</code>
      <list type="bulleted">
        <item>
          <p><c>Deb</c> is the debug structure.</p>
        </item>
        <item>
          <p><c>Func</c> is a tuple <c>{Module, Name}</c> (or a fun) and
            should specify a (user defined) function used to format
            trace output. For each system event, the format function is
            called as <c>Module:Name(Dev, Event, Info)</c>, where:</p>
          <list type="bulleted">
            <item>
              <p><c>Dev</c> is the IO device to which the output should
                be printed. See <c>io(3)</c>.</p>
            </item>
            <item>
              <p><c>Event</c> and <c>Info</c> are passed as-is from
                <c>handle_debug</c>.</p>
            </item>
          </list>
        </item>
        <item>
          <p><c>Info</c> is used to pass additional information to
            <c>Func</c>, it can be any term and is passed as-is.</p>
        </item>
        <item>
          <p><c>Event</c> is the system event. It is up to the user to
            define what a system event is and how it should be
            represented, but typically at least incoming and outgoing
            messages are considered system events and represented by
            the tuples <c>{in,Msg[,From]}</c> and <c>{out,Msg,To}</c>,
            respectively.</p>
        </item>
      </list>
      <p><c>handle_debug</c> returns an updated debug structure
        <c>Deb1</c>.</p>
      <p>In the example, <c>handle_debug</c> is called for each incoming
        and outgoing message. The format function <c>Func</c> is
        the function <c>ch4:write_debug/3</c> which prints the message
        using <c>io:format/3</c>.</p>
      <code type="none">
loop(Chs, Parent, Deb) ->
    receive
        {From, alloc} ->
            Deb2 = sys:handle_debug(Deb, {ch4, write_debug},
                                    ch4, {in, alloc, From}),
            {Ch, Chs2} = alloc(Chs),
            From ! {ch4, Ch},
            Deb3 = sys:handle_debug(Deb2, {ch4, write_debug},
                                    ch4, {out, {ch4, Ch}, From}),
            loop(Chs2, Parent, Deb3);
        {free, Ch} ->
            Deb2 = sys:handle_debug(Deb, {ch4, write_debug},
                                    ch4, {in, {free, Ch}}),
            Chs2 = free(Ch, Chs),
            loop(Chs2, Parent, Deb2);
        ...
    end.

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

    <section>
      <marker id="msg"></marker>
      <title>Handling System Messages</title>
      <p><em>System messages</em> are received as:</p>
      <code type="none">
{system, From, Request}</code>
      <p>The content and meaning of these messages do not need to be
        interpreted by the process. Instead the following function
        should be called:</p>
      <code type="none">
sys:handle_system_msg(Request, From, Parent, Module, Deb, State)</code>
      <p>This function does not return. It will handle the system
        message and then call:</p>
      <code type="none">
Module:system_continue(Parent, Deb, State)</code>
      <p>if process execution should continue, or:</p>
      <code type="none">
Module:system_terminate(Reason, Parent, Deb, State)</code>
      <p>if the process should terminate. Note that a process in a
        supervision tree is expected to terminate with the same reason as
        its parent.</p>
      <list type="bulleted">
        <item><c>Request</c> and <c>From</c> should be passed as-is from
         the system message to the call to <c>handle_system_msg</c>.</item>
        <item><c>Parent</c> is the pid of the parent.</item>
        <item><c>Module</c> is the name of the module.</item>
        <item><c>Deb</c> is the debug structure.</item>
        <item><c>State</c> is a term describing the internal state and
         is passed to <c>system_continue</c>/<c>system_terminate</c>.</item>
      </list>
      <p>In the example:</p>
      <code type="none">
loop(Chs, Parent, Deb) ->
    receive
        ...

        {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).</code>
      <p>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:</p>
      <code type="none">
init(...) ->
    ...,
    process_flag(trap_exit, true),
    ...,
    loop(...).

loop(...) ->
    receive
        ...

        {'EXIT', Parent, Reason} ->
            ..maybe some cleaning up here..
            exit(Reason);
        ...
    end.</code>
    </section>
  </section>

  <section>
    <title>User-Defined Behaviours</title>
    <p>To implement a user-defined behaviour, write code similar to
      code for a special process but calling functions in a callback
      module for handling specific tasks.</p>
    <p>If it is desired that the compiler should warn for missing callback
    functions, as it does for the OTP behaviours, add callback attributes in the
    behaviour module to describe the expected callbacks:</p>
    <code type="none">
-callback Name1(Arg1_1, Arg1_2, ..., Arg1_N1) -> Res1.
-callback Name2(Arg2_1, Arg2_2, ..., Arg2_N2) -> Res2.
...
-callback NameM(ArgM_1, ArgM_2, ..., ArgM_NM) -> ResM.</code>
    <p>where <c>NameX</c> are the names of the expected callbacks and
    <c>ArgX_Y</c>, <c>ResX</c> are types as they are described in Specifications
    for functions in <seealso marker="../reference_manual/typespec">Types and
    Function Specifications</seealso>. The whole syntax of spec attributes is
    supported by callback attributes.</p>
    <p>Alternatively you may directly implement and export the function:</p>
    <code type="none">
behaviour_info(callbacks) ->
    [{Name1,Arity1},...,{NameN,ArityN}].</code>
    <p>where each <c>{Name,Arity}</c> specifies the name and arity of a callback
    function. This function is otherwise automatically generated by the compiler
    using the callback attributes.</p>
    <p>When the compiler encounters the module attribute
    <c>-behaviour(Behaviour).</c> in a module <c>Mod</c>, it will call
    <c>Behaviour:behaviour_info(callbacks)</c> and compare the result with the
    set of functions actually exported from <c>Mod</c>, and issue a warning if
    any callback function is missing.</p>
    <p>Example:</p>
    <code type="none">
%% User-defined behaviour module
-module(simple_server).
-export([start_link/2,...]).

-callback init(State :: term()) -> 'ok'.
-callback handle_req(Req :: term(), State :: term()) -> {'ok', Reply :: term()}.
-callback terminate() -> 'ok'.

%% Alternatively you may define:
%%
%% -export([behaviour_info/1]).
%% behaviour_info(callbacks) ->
%%     [{init,1},
%%      {handle_req,2},
%%      {terminate,0}].

start_link(Name, Module) ->
    proc_lib:start_link(?MODULE, init, [self(), Name, Module]).

init(Parent, Name, Module) ->
    register(Name, self()),
    ...,
    Dbg = sys:debug_options([]),
    proc_lib:init_ack(Parent, {ok, self()}),
    loop(Parent, Module, Deb, ...).

...</code>
    <p>In a callback module:</p>
    <code type="none">
-module(db).
-behaviour(simple_server).

-export([init/0, handle_req/2, terminate/0]).

...</code>
  </section>
</chapter>