path: root/lib/stdlib/doc/src/gen_statem.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>gen_statem</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_statem</module>
  <modulesummary>Generic state machine behavior.</modulesummary>
  <description>
    <p>
      This behavior module provides a state machine. Two
      <seealso marker="#type-callback_mode"><em>callback modes</em></seealso>
      are supported:
    </p>
    <list type="bulleted">
      <item>
        <p>One for finite-state machines
          (<seealso marker="gen_fsm"><c>gen_fsm</c></seealso> like),
          which requires the state to be an atom and uses that state as
          the name of the current callback function
        </p>
      </item>
      <item>
        <p>One without restriction on the state data type
          that uses one callback function for all states
        </p>
      </item>
    </list>
    <note>
      <p>
        This is a new behavior in Erlang/OTP 19.0.
        It has been thoroughly reviewed, is stable enough
        to be used by at least two heavy OTP applications, and is here to stay.
        Depending on user feedback, we do not expect
        but can find it necessary to make minor
        not backward compatible changes into Erlang/OTP 20.0.
      </p>
    </note>
    <p>
      The <c>gen_statem</c> behavior is intended to replace
      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso> for new code.
      It has the same features and adds some really useful:
    </p>
    <list type="bulleted">
      <item>State code is gathered.</item>
      <item>The state can be any term.</item>
      <item>Events can be postponed.</item>
      <item>Events can be self-generated.</item>
      <item>Automatic state entry events can be generated.</item>
      <item>A reply can be sent from a later state.</item>
      <item>There can be multiple <c>sys</c> traceable replies.</item>
    </list>
    <p>
      The callback model(s) for <c>gen_statem</c> differs from
      the one for <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>,
      but it is still fairly easy to rewrite
      from <c>gen_fsm</c> to <c>gen_statem</c>.
    </p>
    <p>
      A generic state machine process (<c>gen_statem</c>) implemented
      using this module has a standard set of interface functions
      and includes functionality for tracing and error reporting.
      It also fits into an OTP supervision tree. For more information, see
      <seealso marker="doc/design_principles:statem">OTP Design Principles</seealso>.
    </p>
    <p>
      A <c>gen_statem</c> assumes all specific parts to be located in a
      callback module exporting a predefined set of functions.
      The relationship between the behavior functions and the callback
      functions is as follows:</p>
    <pre>
gen_statem module            Callback module
-----------------            ---------------
gen_statem:start
gen_statem:start_link -----> Module:init/1

Server start or code change
                      -----> Module:callback_mode/0

gen_statem:stop       -----> Module:terminate/3

gen_statem:call
gen_statem:cast
erlang:send
erlang:'!'            -----> Module:StateName/3
                             Module:handle_event/4

-                     -----> Module:terminate/3

-                     -----> Module:code_change/4</pre>
    <p>
      Events are of different
      <seealso marker="#type-event_type">types</seealso>,
      so the callback functions can know the origin of an event
      and how to respond.
    </p>
    <p>
      If a callback function fails or returns a bad value,
      the <c>gen_statem</c> terminates, unless otherwise stated.
      However, an exception of class
      <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
      is not regarded as an error but as a valid return
      from all callback functions.
    </p>
    <marker id="state_function"/>
    <p>
      The "<em>state function</em>" for a specific
      <seealso marker="#type-state">state</seealso>
      in a <c>gen_statem</c> is the callback function that is called
      for all events in this state. It is selected depending on which
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      that the callback module defines with the callback function
      <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>state_functions</c>, the state must be an atom and
      is used as the state function name; see
      <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>.
      This gathers all code for a specific state
      in one function as the <c>gen_statem</c> engine
      branches depending on state name.
      Notice the fact that there is a mandatory callback function
      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
      makes the state name <c>terminate</c> unusable in this mode.
    </p>
    <p>
      When the
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      is <c>handle_event_function</c>, the state can be any term
      and the state function name is
      <seealso marker="#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>.
      This makes it easy to branch depending on state or event as you desire.
      Be careful about which events you handle in which
      states so that you do not accidentally postpone an event
      forever creating an infinite busy loop.
    </p>
    <p>
      The <c>gen_statem</c> enqueues incoming events in order of arrival
      and presents these to the
      <seealso marker="#state_function">state function</seealso>
      in that order. The state function can postpone an event
      so it is not retried in the current state.
      After a state change the queue restarts with the postponed events.
    </p>
    <p>
      The <c>gen_statem</c> event queue model is sufficient
      to emulate the normal process message queue with selective receive.
      Postponing an event corresponds to not matching it
      in a receive statement, and changing states corresponds
      to entering a new receive statement.
    </p>
    <p>
      The <seealso marker="#state_function">state function</seealso>
      can insert events using the
      <seealso marker="#type-action"><c>action()</c></seealso>
      <c>next_event</c>
      and such an event is inserted as the next to present
      to the state function. That is, as if it is
      the oldest incoming event. A dedicated
      <seealso marker="#type-event_type"><c>event_type()</c></seealso>
      <c>internal</c> can be used for such events making them impossible
      to mistake for external events.
    </p>
    <p>
      Inserting an event replaces the trick of calling your own
      state handling functions that you often would have to
      resort to in, for example,
      <seealso marker="gen_fsm"><c>gen_fsm</c></seealso>
      to force processing an inserted event before others.
    </p>
    <p>
      The <c>gen_statem</c> engine can automatically insert
      a special event whenever a new state is entered; see 
      <seealso marker="#type-state_entry_mode"><c>state_entry_mode()</c></seealso>.
      This makes it easy to handle code common to all state entries.
    </p>
    <note>
      <p>If you in <c>gen_statem</c>, for example, postpone
        an event in one state and then call another state function
        of yours, you have not changed states and hence the postponed event
        is not retried, which is logical but can be confusing.
      </p>
    </note>
    <p>
      For the details of a state transition, see type
      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>.
    </p>
    <p>
      A <c>gen_statem</c> handles system messages as described in
      <seealso marker="sys"><c>sys</c></seealso>.
      The <c>sys</c> module can be used for debugging a <c>gen_statem</c>.
    </p>
    <p>
      Notice that a <c>gen_statem</c> does not trap exit signals
      automatically, this must be explicitly initiated in
      the callback module (by calling
      <seealso marker="erts:erlang#process_flag/2"><c>process_flag(trap_exit, true)</c></seealso>.
    </p>
    <p>
      Unless otherwise stated, all functions in this module fail if
      the specified <c>gen_statem</c> does not exist or
      if bad arguments are specified.
    </p>
    <p>
      The <c>gen_statem</c> process can go into hibernation; see
      <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>.
      It is done when a
      <seealso marker="#state_function">state function</seealso> or
      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
      specifies <c>hibernate</c> in the returned
      <seealso marker="#type-action"><c>Actions</c></seealso>
      list. This feature can be useful to reclaim process heap memory
      while the server is expected to be idle for a long time.
      However, use this feature with care,
      as hibernation can be too costly
      to use after every event; see
      <seealso marker="erts:erlang#hibernate/3"><c>erlang:hibernate/3</c></seealso>.
    </p>
  </description>

  <section>
    <title>Example</title>
    <p>
      The following example shows a simple pushbutton model
      for a toggling pushbutton implemented with
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      <c>state_functions</c>.
      You can push the button and it replies if it went on or off,
      and you can ask for a count of how many times it has been
      pushed to switch on.
    </p>
    <p>The following is the complete callback module file
      <c>pushbutton.erl</c>:</p>
    <code type="erl">
-module(pushbutton).
-behaviour(gen_statem).

-export([start/0,push/0,get_count/0,stop/0]).
-export([terminate/3,code_change/4,init/1,callback_mode/0]).
-export([on/3,off/3]).

name() -> pushbutton_statem. % The registered server name

%% API.  This example uses a registered name name()
%% and does not link to the caller.
start() ->
    gen_statem:start({local,name()}, ?MODULE, [], []).
push() ->
    gen_statem:call(name(), push).
get_count() ->
    gen_statem:call(name(), get_count).
stop() ->
    gen_statem:stop(name()).

%% Mandatory callback functions
terminate(_Reason, _State, _Data) ->
    void.
code_change(_Vsn, State, Data, _Extra) ->
    {ok,State,Data}.
init([]) ->
    %% Set the initial state + data.  Data is used only as a counter.
    State = off, Data = 0,
    {ok,State,Data}.
callback_mode() -> state_functions.

%%% State function(s)

off({call,From}, push, Data) ->
    %% Go to 'on', increment count and reply
    %% that the resulting status is 'on'
    {next_state,on,Data+1,[{reply,From,on}]};
off(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data).

on({call,From}, push, Data) ->
    %% Go to 'off' and reply that the resulting status is 'off'
    {next_state,off,Data,[{reply,From,off}]};
on(EventType, EventContent, Data) ->
    handle_event(EventType, EventContent, Data).

%% Handle events common to all states
handle_event({call,From}, get_count, Data) ->
    %% Reply with the current count
    {keep_state,Data,[{reply,From,Data}]};
handle_event(_, _, Data) ->
    %% Ignore all other events
    {keep_state,Data}.
    </code>
    <p>The following is a shell session when running it:</p>
    <pre>
1> pushbutton:start().
{ok,&lt;0.36.0>}
2> pushbutton:get_count().
0
3> pushbutton:push().
on
4> pushbutton:get_count().
1
5> pushbutton:push().
off
6> pushbutton:get_count().
1
7> pushbutton:stop().
ok
8> pushbutton:push().
** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
     in function  gen:do_for_proc/2 (gen.erl, line 261)
     in call from gen_statem:call/3 (gen_statem.erl, line 386)
    </pre>
    <p>
      To compare styles, here follows the same example using
      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
      <c>state_functions</c>, or rather the code to replace
      after function <c>init/1</c> of the <c>pushbutton.erl</c>
      example file above:
    </p>
    <code type="erl">
callback_mode() -> handle_event_function.

%%% State function(s)

handle_event({call,From}, push, off, Data) ->
    %% Go to 'on', increment count and reply
    %% that the resulting status is 'on'
    {next_state,on,Data+1,[{reply,From,on}]};
handle_event({call,From}, push, on, Data) ->
    %% Go to 'off' and reply that the resulting status is 'off'
    {next_state,off,Data,[{reply,From,off}]};
%%
%% Event handling common to all states
handle_event({call,From}, get_count, State, Data) ->
    %% Reply with the current count
    {next_state,State,Data,[{reply,From,Data}]};
handle_event(_, _, State, Data) ->
    %% Ignore all other events
    {next_state,State,Data}.
    </code>
  </section>

  <datatypes>
    <datatype>
      <name name="server_name"/>
      <desc>
	<p>
	  Name specification to use when starting
	  a <c>gen_statem</c> server. See
	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>
	  and
	  <seealso marker="#type-server_ref"><c>server_ref()</c></seealso>
	  below.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="server_ref"/>
      <desc>
	<p>
	  Server specification to use when addressing
	  a <c>gen_statem</c> server.
	  See <seealso marker="#call/2"><c>call/2</c></seealso> and
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  above.
	</p>
	<p>It can be:</p>
	<taglist>
	  <tag><c>pid() | LocalName</c></tag>
	  <item>
            <p>
              The <c>gen_statem</c> is locally registered.
            </p>
          </item>
	  <tag><c>{Name,Node}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is locally registered
	      on another node.
            </p>
	  </item>
	  <tag><c>{global,GlobalName}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is globally registered in
	      <seealso marker="kernel:global"><c>global</c></seealso>.
            </p>
	  </item>
	  <tag><c>{via,RegMod,ViaName}</c></tag>
	  <item>
            <p>
	      The <c>gen_statem</c> is registered in
	      an alternative process registry.
	      The registry callback module <c>RegMod</c>
	      is to export functions
	      <c>register_name/2</c>, <c>unregister_name/1</c>,
	      <c>whereis_name/1</c>, and <c>send/2</c>,
	      which are to behave like the corresponding functions in
	      <seealso marker="kernel:global"><c>global</c></seealso>.
	      Thus, <c>{via,global,GlobalName}</c> is the same as
	      <c>{global,GlobalName}</c>.
            </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="debug_opt"/>
      <desc>
	<p>
	  Debug option that can be used when starting
	  a <c>gen_statem</c> server through,
	  <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>.
	</p>
	<p>
	  For every entry in <c><anno>Dbgs</anno></c>,
	  the corresponding function in
	  <seealso marker="sys"><c>sys</c></seealso> is called.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_opt"/>
      <desc>
	<p>
	  Options that can be used when starting
	  a <c>gen_statem</c> server through, for example,
	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="start_ret"/>
      <desc>
	<p>
	  Return value from the start functions, for example,
	  <seealso marker="#start_link/3"><c>start_link/3</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="from"/>
      <desc>
	<p>
	  Destination to use when replying through, for example, the
	  <seealso marker="#type-action"><c>action()</c></seealso>
	  <c>{reply,From,Reply}</c>
	  to a process that has called the <c>gen_statem</c> server using
	  <seealso marker="#call/2"><c>call/2</c></seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state"/>
      <desc>
	<p>
	  After a state change (<c>NextState =/= State</c>),
	  all postponed events are retried.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_name"/>
      <desc>
	<p>
	  If the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>,
	  the state must be of this type.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="data"/>
      <desc>
	<p>
	  A term in which the state machine implementation
	  is to store any server data it needs. The difference between
	  this and the <seealso marker="#type-state"><c>state()</c></seealso>
	  itself is that a change in this data does not cause
	  postponed events to be retried. Hence, if a change
	  in this data would change the set of events that
	  are handled, then that data item is to be made
	  a part of the state.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_type"/>
      <desc>
	<p>
	  External events are of three types:
	  <c>{call,<anno>From</anno>}</c>, <c>cast</c>, or <c>info</c>.
	  <seealso marker="#call/2">Calls</seealso>
	  (synchronous) and
	  <seealso marker="#cast/2">casts</seealso>
	  originate from the corresponding API functions.
	  For calls, the event contains whom to reply to.
	  Type <c>info</c> originates from regular process messages sent
	  to the <c>gen_statem</c>. Also, the state machine
	  implementation can generate events of types
	  <c>timeout</c>, <c>enter</c> and <c>internal</c> to itself.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="callback_mode"/>
      <desc>
	<p>
	  The <em>callback mode</em> is selected when starting the
	  <c>gen_statem</c> and after code change
	  using the return value from
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
	</p>
	<taglist>
	  <tag><c>state_functions</c></tag>
	  <item>
	    <p>
	      The state must be of type
	      <seealso marker="#type-state_name"><c>state_name()</c></seealso>
	      and one callback function per state, that is,
	      <seealso marker="#Module:StateName/3"><c>Module:StateName/3</c></seealso>,
	      is used.
	    </p>
	  </item>
	  <tag><c>handle_event_function</c></tag>
	  <item>
	    <p>
	      The state can be any term and the callback function
	      <seealso marker="#Module:handle_event/4"><c>Module:handle_event/4</c></seealso>
	      is used for all states.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="state_entry_mode"/>
      <desc>
	<p>
	  The <em>state entry mode</em> is selected when starting the
	  <c>gen_statem</c> and after code change
	  using the return value from
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  returns a list containing <c>state_entry_events</c>,
	  the <c>gen_statem</c> engine will, at every state change,
	  insert an event of type
	  <seealso marker="#type-event_type">enter</seealso>
	  with content <c>OldState</c>.  This event will be inserted
	  before all other events such as those generated by
	  <seealso marker="#type-action"><c>action()</c></seealso>
	  <c>next_event</c>.
	</p>
	<p>
	  If 
	  <seealso marker="#Module:callback_mode/0"><c>Module:callback_mode/0</c></seealso>
	  does not return such a list, no state entry events are inserted.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="transition_option"/>
      <desc>
	<p>
	  Transition options can be set by
	  <seealso marker="#type-action">actions</seealso>
	  and they modify the following in how
	  the state transition is done:
	</p>
	<list type="ordered">
	  <item>
	    <p>
	      All
              <seealso marker="#type-action">actions</seealso>
	      are processed in order of appearance.
            </p>
	  </item>
	  <item>
            <p>
	      If
              <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	      is <c>true</c>,
	      the current event is postponed.
	    </p>
	  </item>
	  <item>
	    <p>
	      If the state changes, the queue of incoming events
              is reset to start with the oldest postponed.
	    </p>
	  </item>
	  <item>
            <p>
	      All events stored with
	      <seealso marker="#type-action"><c>action()</c></seealso>
	      <c>next_event</c>
	      are inserted in the queue to be processed before
	      all other events.
            </p>
	  </item>
	  <item>
            <p>
	      If the state changes or is the initial state, and the 
	      <seealso marker="#type-state_entry_mode"><em>state entry mode</em></seealso>
	      is <c>state_entry_events</c>, an event of type
	      <seealso marker="#type-event_type">enter</seealso>
	      with content <c>OldState</c> is inserted
	      to be processed before all other events including those above.
            </p>
	  </item>
	  <item>
	    <p>
	      If an
              <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	      is set through
	      <seealso marker="#type-action"><c>action()</c></seealso>
	      <c>timeout</c>,
	      an event timer can be started or a time-out zero event
	      can be enqueued.
	    </p>
	  </item>
	  <item>
	    <p>
	      The (possibly new)
	      <seealso marker="#state_function">state function</seealso>
	      is called with the oldest enqueued event if there is any,
	      otherwise the <c>gen_statem</c> goes into <c>receive</c>
	      or hibernation
	      (if
	      <seealso marker="#type-hibernate"><c>hibernate()</c></seealso>
	      is <c>true</c>)
	      to wait for the next message. In hibernation the next
	      non-system event awakens the <c>gen_statem</c>, or rather
	      the next incoming message awakens the <c>gen_statem</c>,
	      but if it is a system event
	      it goes right back into hibernation.
	    </p>
	  </item>
	</list>
      </desc>
    </datatype>
    <datatype>
      <name name="postpone"/>
      <desc>
	<p>
	  If <c>true</c>, postpones the current event and retries
	  it when the state changes
	  (<c>NextState =/= State</c>).
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="hibernate"/>
      <desc>
	<p>
	  If <c>true</c>, hibernates the <c>gen_statem</c>
	  by calling
	  <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>
	  before going into <c>receive</c>
	  to wait for a new external event.
	  If there are enqueued events,
	  to prevent receiving any new event, an
	  <seealso marker="erts:erlang#garbage_collect/0"><c>erlang:garbage_collect/0</c></seealso>
	  is done instead to simulate
	  that the <c>gen_statem</c> entered hibernation
	  and immediately got awakened by the oldest enqueued event.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="event_timeout"/>
      <desc>
	<p>
	  Generates an event of
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>timeout</c>
	  after this time (in milliseconds) unless another
	  event arrives in which case this time-out is cancelled.
	  Notice that a retried or inserted event
	  counts like a new in this respect.
	</p>
	<p>
	  If the value is <c>infinity</c>, no timer is started, as
	  it never triggers anyway.
	</p>
	<p>
	  If the value is <c>0</c>, the time-out event is immediately enqueued
	  unless there already are enqueued events, as the
	  time-out is then immediately cancelled.
	  This is a feature ensuring that a time-out <c>0</c> event
	  is processed before any not yet received external event.
	</p>
	<p>
	  Notice that it is not possible or needed to cancel this time-out,
	  as it is cancelled automatically by any other event.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="action"/>
      <desc>
	<p>
	  These state transition actions can be invoked by
	  returning them from the
	  <seealso marker="#state_function">state function</seealso>, from
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  or by giving them to
	  <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>.
	</p>
	<p>
	  Actions are executed in the containing list order.
	</p>
	<p>
	  Actions that set
	  <seealso marker="#type-transition_option">transition options</seealso>
	  override any previous of the same type,
	  so the last in the containing list wins.
	  For example, the last
	  <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	  overrides any other <c>event_timeout()</c> in the list.
	</p>
	<taglist>
	  <tag><c>postpone</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-postpone"><c>postpone()</c></seealso>
	      for this state transition.
	      This action is ignored when returned from
	      <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	      or given to
	      <seealso marker="#enter_loop/5"><c>enter_loop/5,6</c></seealso>,
	      as there is no event to postpone in those cases.
	    </p>
	  </item>
	  <tag><c>hibernate</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-hibernate"><c>hibernate()</c></seealso>
	      for this state transition.
	    </p>
	  </item>
	  <tag><c>Timeout</c></tag>
	  <item>
	    <p>
	      Short for <c>{timeout,Timeout,Timeout}</c>, that is,
	      the time-out message is the time-out time.
	      This form exists to make the
	      <seealso marker="#state_function">state function</seealso>
	      return value <c>{next_state,NextState,NewData,Timeout}</c>
	      allowed like for <c>gen_fsm</c>'s
	      <seealso marker="gen_fsm#Module:StateName/2"><c>Module:StateName/2</c></seealso>.
	    </p>
	  </item>
	  <tag><c>timeout</c></tag>
	  <item>
	    <p>
	      Sets the
	      <seealso marker="#type-transition_option"><c>transition_option()</c></seealso>
	      <seealso marker="#type-event_timeout"><c>event_timeout()</c></seealso>
	      to <c><anno>Time</anno></c> with <c><anno>EventContent</anno></c>.
	    </p>
	  </item>
	  <tag><c>reply_action()</c></tag>
	  <item>
	    <p>
              Replies to a caller.
	    </p>
          </item>
	  <tag><c>next_event</c></tag>
	  <item>
	    <p>
	      Stores the specified <c><anno>EventType</anno></c>
	      and <c><anno>EventContent</anno></c> for insertion after all
	      actions have been executed.
	    </p>
	    <p>
	      The stored events are inserted in the queue as the next to process
	      before any already queued events. The order of these stored events
	      is preserved, so the first <c>next_event</c> in the containing
	      list becomes the first to process.
	    </p>
	    <p>
	      An event of type
	      <seealso marker="#type-event_type"><c>internal</c></seealso>
	      is to be used when you want to reliably distinguish
	      an event inserted this way from any external event.
	    </p>
	  </item>
	</taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="reply_action"/>
      <desc>
	<p>
	  Replies to a caller waiting for a reply in
	  <seealso marker="#call/2"><c>call/2</c></seealso>.
	  <c><anno>From</anno></c> must be the term from argument
	  <seealso marker="#type-event_type"><c>{call,<anno>From</anno>}</c></seealso>
	  to the
	  <seealso marker="#state_function">state function</seealso>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="state_function_result"/>
      <desc>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> does a state transition to
	      <c><anno>NextStateName</anno></c>
	      (which can be the same as the current state),
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	    </p>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="handle_event_result"/>
      <desc>
	<taglist>
	  <tag><c>next_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> does a state transition to
	      <c><anno>NextState</anno></c>
	      (which can be the same as the current state),
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	    </p>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="common_state_callback_result"/>
      <desc>
	<taglist>
	  <tag><c>stop</c></tag>
	  <item>
	    <p>
	      Terminates the <c>gen_statem</c> by calling
	      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
	      with <c>Reason</c> and
	      <c><anno>NewData</anno></c>, if specified.
	    </p>
	  </item>
	  <tag><c>stop_and_reply</c></tag>
	  <item>
	    <p>
	      Sends all <c><anno>Replies</anno></c>,
	      then terminates the <c>gen_statem</c> by calling
	      <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
	      with <c>Reason</c> and
	      <c><anno>NewData</anno></c>, if specified.
	    </p>
	  </item>
	  <tag><c>keep_state</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state, or
	      does a state transition to the current state if you like,
	      sets <c><anno>NewData</anno></c>,
	      and executes all <c><anno>Actions</anno></c>.
	      This is the same as
	      <c>{next_state,CurrentState,<anno>NewData</anno>,<anno>Actions</anno>}</c>.
	    </p>
	  </item>
	  <tag><c>keep_state_and_data</c></tag>
	  <item>
	    <p>
	      The <c>gen_statem</c> keeps the current state or
	      does a state transition to the current state if you like,
	      keeps the current server data,
	      and executes all <c><anno>Actions</anno></c>.
	      This is the same as
	      <c>{next_state,CurrentState,CurrentData,<anno>Actions</anno>}</c>.
	    </p>
	  </item>
	</taglist>
	<p>
	  All these terms are tuples or atoms and this property
	  will hold in any future version of <c>gen_statem</c>.
	</p>
      </desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="call" arity="2"/>
      <name name="call" arity="3"/>
      <fsummary>Make a synchronous call to a <c>gen_statem</c>.</fsummary>
      <desc>
        <p>
	  Makes a synchronous call to the <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  by sending a request
	  and waiting until its reply arrives.
	  The <c>gen_statem</c> calls the
	  <seealso marker="#state_function">state function</seealso> with
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>{call,From}</c> and event content
	  <c><anno>Request</anno></c>.
	</p>
	<p>
	  A <c><anno>Reply</anno></c> is generated when a
	  <seealso marker="#state_function">state function</seealso>
	  returns with
	  <c>{reply,From,<anno>Reply</anno>}</c> as one
	  <seealso marker="#type-action"><c>action()</c></seealso>,
	  and that <c><anno>Reply</anno></c> becomes the return value
	  of this function.
	</p>
	<p>
	  <c><anno>Timeout</anno></c> is an integer &gt; 0,
	  which specifies how many milliseconds to wait for a reply,
	  or the atom <c>infinity</c> to wait indefinitely,
	  which is the default. If no reply is received within
	  the specified time, the function call fails.
	</p>
	<note>
	  <p>
	    For <c><anno>Timeout</anno> &lt; infinity</c>,
	    to avoid getting a late reply in the caller's
	    inbox if the caller should catch exceptions,
	    this function spawns a proxy process that
	    does the call. A late reply gets delivered to the
	    dead proxy process, hence gets discarded. This is
	    less efficient than using
	    <c><anno>Timeout</anno> == infinity</c>.
	  </p>
	</note>
	<p>
	  <c><anno>Timeout</anno></c> can also be a tuple
	  <c>{clean_timeout,<anno>T</anno>}</c> or
	  <c>{dirty_timeout,<anno>T</anno>}</c>, where
	  <c><anno>T</anno></c> is the timeout time.
	  <c>{clean_timeout,<anno>T</anno>}</c> works like
	  just <c>T</c> described in the note above
	  and uses a proxy process for <c>T &lt; infinity</c>,
	  while <c>{dirty_timeout,<anno>T</anno>}</c>
	  bypasses the proxy process which is more lightweight.
	</p>
	<note>
	  <p>
	    If you combine catching exceptions from this function
	    with <c>{dirty_timeout,<anno>T</anno>}</c>
	    to avoid that the calling process dies when the call
	    times out, you will have to be prepared to handle
	    a late reply.
	    So why not just allow the calling process to die?
	  </p>
	</note>
	<p>
	  The call can also fail, for example, if the <c>gen_statem</c>
	  dies before or during this function call.
	</p>
      </desc>
    </func>

    <func>
      <name name="cast" arity="2"/>
      <fsummary>Send an asynchronous event to a <c>gen_statem</c>.</fsummary>
      <desc>
	<p>
	  Sends an asynchronous event to the <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  and returns <c>ok</c> immediately,
	  ignoring if the destination node or <c>gen_statem</c>
	  does not exist.
	  The <c>gen_statem</c> calls the
	  <seealso marker="#state_function">state function</seealso> with
	  <seealso marker="#type-event_type"><c>event_type()</c></seealso>
	  <c>cast</c> and event content
	  <c><anno>Msg</anno></c>.
	</p>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="4"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  The same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  with <c>Actions = []</c> except that no
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  must have been registered.  This creates an anonymous server.
	</p>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="5"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  If <c><anno>Server_or_Actions</anno></c> is a <c>list()</c>,
	  the same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  except that no
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  must have been registered and
	  <c>Actions = <anno>Server_or_Actions</anno></c>.
	  This creates an anonymous server.
	</p>
	<p>
	  Otherwise the same as
	  <seealso marker="#enter_loop/6"><c>enter_loop/6</c></seealso>
	  with
	  <c>Server = <anno>Server_or_Actions</anno></c> and
	  <c>Actions = []</c>.
	</p>
      </desc>
    </func>

    <func>
      <name name="enter_loop" arity="6"/>
      <fsummary>Enter the <c>gen_statem</c> receive loop.</fsummary>
      <desc>
	<p>
	  Makes the calling process become a <c>gen_statem</c>.
	  Does not return, instead the calling process enters
	  the <c>gen_statem</c> receive loop and becomes
	  a <c>gen_statem</c> server.
	  The process <em>must</em> have been started
	  using one of the start functions in
	  <seealso marker="proc_lib"><c>proc_lib</c></seealso>.
	  The user is responsible for any initialization of the process,
	  including registering a name for it.
	</p>
	<p>
	  This function is useful when a more complex initialization
	  procedure is needed than
	  the <c>gen_statem</c> behavior provides.
	</p>
	<p>
	  <c><anno>Module</anno></c>, <c><anno>Opts</anno></c>
	  have the same meaning as when calling
	  <seealso marker="#start_link/3"><c>start[_link]/3,4</c></seealso>.
	</p>
	<p>
	  If <c><anno>Server</anno></c> is <c>self()</c> an anonymous
	  server is created just as when using 
	  <seealso marker="#start_link/3"><c>start[_link]/3</c></seealso>.
	  If <c><anno>Server</anno></c> is a
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  a named server is created just as when using
	  <seealso marker="#start_link/4"><c>start[_link]/4</c></seealso>.
	  However, the
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  name must have been registered accordingly
	  <em>before</em> this function is called.
	</p>
        <p>
	  <c><anno>State</anno></c>, <c><anno>Data</anno></c>,
	  and <c><anno>Actions</anno></c>
	  have the same meanings as in the return value of
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
	  Also, the callback module does not need to export a
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  function.
	</p>
        <p>
	  The function fails if the calling process was not started by a
	  <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  start function, or if it is not registered
	  according to
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name name="reply" arity="1"/>
      <name name="reply" arity="2"/>
      <fsummary>Reply to a caller.</fsummary>
      <desc>
        <p>
	  This function can be used by a <c>gen_statem</c>
	  to explicitly send a reply to a process that waits in
	  <seealso marker="#call/2"><c>call/2</c></seealso>
	  when the reply cannot be defined in
	  the return value of a
	  <seealso marker="#state_function">state function</seealso>.
	</p>
	<p>
	  <c><anno>From</anno></c> must be the term from argument
	  <seealso marker="#type-event_type"><c>{call,<anno>From</anno>}</c></seealso>
	  to the
	  <seealso marker="#state_function">state function</seealso>.
	  <c><anno>From</anno></c> and <c><anno>Reply</anno></c>
	  can also be specified using a
	  <seealso marker="#type-reply_action"><c>reply_action()</c></seealso>
	  and multiple replies with a list of them.
	</p>
	<note>
	  <p>
	    A reply sent with this function is not visible
	    in <seealso marker="sys"><c>sys</c></seealso> debug output.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name name="start" arity="3"/>
      <name name="start" arity="4"/>
      <fsummary>Create a standalone <c>gen_statem</c> process.</fsummary>
      <desc>
        <p>
	  Creates a standalone <c>gen_statem</c> process according to
	  OTP design principles (using
	  <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  primitives).
	  As it does not get linked to the calling process,
	  this start function cannot be used by a supervisor
	  to start a child.
	</p>
	<p>
	  For a description of arguments and return values, see
	  <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name name="start_link" arity="3"/>
      <name name="start_link" arity="4"/>
      <fsummary>Create a linked <c>gen_statem</c> process.</fsummary>
      <desc>
        <p>
	  Creates a <c>gen_statem</c> process according
	  to OTP design principles
	  (using
	  <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	  primitives)
	  that is linked to the calling process.
	  This is essential when the <c>gen_statem</c> must be part of
	  a supervision tree so it gets linked to its supervisor.
	</p>
	<p>
	  The <c>gen_statem</c> process calls
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  to initialize the server. To ensure a synchronized startup
	  procedure, <c>start_link/3,4</c> does not return until
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  has returned.
	</p>
	<p>
	  <c><anno>ServerName</anno></c> specifies the
	  <seealso marker="#type-server_name"><c>server_name()</c></seealso>
	  to register for the <c>gen_statem</c>.
	  If the <c>gen_statem</c> is started with <c>start_link/3</c>,
	  no <c><anno>ServerName</anno></c> is provided and
	  the <c>gen_statem</c> is not registered.
	</p>
        <p><c><anno>Module</anno></c> is the name of the callback module.</p>
        <p>
	  <c><anno>Args</anno></c> is an arbitrary term that is passed as
	  the argument to
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
	</p>
        <list type="bulleted">
          <item>
	    <p>
	      If option <c>{timeout,Time}</c> is present in
	      <c><anno>Opts</anno></c>, the <c>gen_statem</c>
	      is allowed to spend <c>Time</c> milliseconds initializing
	      or it terminates and the start function returns
	      <seealso marker="#type-start_ret"><c>{error,timeout}</c></seealso>.
            </p>
          </item>
          <item>
            <p>
              If option
              <seealso marker="#type-debug_opt"><c>{debug,Dbgs}</c></seealso>
              is present in <c><anno>Opts</anno></c>, debugging through
              <seealso marker="sys"><c>sys</c></seealso> is activated.
            </p>
          </item>
          <item>
            <p>
              If option <c>{spawn_opt,SpawnOpts}</c> is present in
              <c><anno>Opts</anno></c>, <c>SpawnOpts</c> is passed
              as option list to
	      <seealso marker="erts:erlang#spawn_opt/2"><c>erlang:spawn_opt/2</c></seealso>,
              which is used to spawn the <c>gen_statem</c> process.
            </p>
          </item>
        </list>
        <note>
	  <p>
	    Using spawn option <c>monitor</c> is not
            allowed, it causes this function to fail with reason
            <c>badarg</c>.
	  </p>
        </note>
	<p>
	  If the <c>gen_statem</c> is successfully created
	  and initialized, this function returns
	  <seealso marker="#type-start_ret"><c>{ok,Pid}</c></seealso>,
	  where <c>Pid</c> is the <c>pid()</c>
	  of the <c>gen_statem</c>.
	  If a process with the specified <c><anno>ServerName</anno></c>
	  exists already, this function returns
	  <seealso marker="#type-start_ret"><c>{error,{already_started,Pid}}</c></seealso>,
	  where <c>Pid</c> is the <c>pid()</c> of that process.
	</p>
	<p>
	  If <c>Module:init/1</c> fails with <c>Reason</c>,
	  this function returns
	  <seealso marker="#type-start_ret"><c>{error,Reason}</c></seealso>.
	  If <c>Module:init/1</c> returns
	  <seealso marker="#type-start_ret"><c>{stop,Reason}</c></seealso>
	  or
	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
	  the process is terminated and this function
	  returns
	  <seealso marker="#type-start_ret"><c>{error,Reason}</c></seealso>
	  or
	  <seealso marker="#type-start_ret"><c>ignore</c></seealso>,
	  respectively.
	</p>
      </desc>
    </func>

    <func>
      <name name="stop" arity="1"/>
      <fsummary>Synchronously stop a generic server.</fsummary>
      <desc>
	<p>
	  The same as
	  <seealso marker="#stop/3"><c>stop(<anno>ServerRef</anno>, normal, infinity)</c></seealso>.
	</p>
      </desc>
    </func>

    <func>
      <name name="stop" arity="3"/>
      <fsummary>Synchronously stop a generic server.</fsummary>
      <desc>
	<p>
	  Orders the <c>gen_statem</c>
	  <seealso marker="#type-server_ref"><c><anno>ServerRef</anno></c></seealso>
	  to exit with the specified <c><anno>Reason</anno></c>
	  and waits for it to terminate.
	  The <c>gen_statem</c> calls
	  <seealso marker="#Module:terminate/3"><c>Module:terminate/3</c></seealso>
	  before exiting.
	</p>
	<p>
	  This function returns <c>ok</c> if the server terminates
	  with the expected reason. Any other reason than <c>normal</c>,
	  <c>shutdown</c>, or <c>{shutdown,Term}</c> causes an
	  error report to be issued through
	  <seealso marker="kernel:error_logger#format/2"><c>error_logger:format/2</c></seealso>.
	  The default <c><anno>Reason</anno></c> is <c>normal</c>.
	</p>
	<p>
	  <c><anno>Timeout</anno></c> is an integer &gt; 0,
	  which specifies how many milliseconds to wait for the server to
	  terminate, or the atom <c>infinity</c> to wait indefinitely.
	  Defaults to <c>infinity</c>.
	  If the server does not terminate within the specified time,
	  a <c>timeout</c> exception is raised.
	</p>
	<p>
	  If the process does not exist, a <c>noproc</c> exception
	  is raised.
	</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Callback Functions</title>
    <p>
      The following functions are to be exported from a
      <c>gen_statem</c> callback module.
    </p>
  </section>

  <funcs>
    <func>
      <name>Module:callback_mode() -> CallbackMode</name>
      <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
      <type>
	<v>
	  CallbackMode =
	  <seealso marker="#type-callback_mode">callback_mode()</seealso> |
	  [ <seealso marker="#type-callback_mode">callback_mode()</seealso>
	  | state_entry_events ]
	</v>
      </type>
      <desc>
        <p>
	  This function is called by a <c>gen_statem</c>
	  when it needs to find out the
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  of the callback module.  The value is cached by <c>gen_statem</c>
	  for efficiency reasons, so this function is only called
	  once after server start and after code change,
	  but before the first
	  <seealso marker="#state_function">state function</seealso>
	  is called.  More occasions may be added in future versions
	  of <c>gen_statem</c>.
	</p>
	<p>
	  Server start happens either when
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  returns or when
	  <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>
	  is called.  Code change happens when
	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
	  returns.
	</p>
	<p>
	  The <c>CallbackMode</c> is either just
	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
	  or a list containing
	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
	  and possibly the atom
	  <seealso marker="#type-state_entry_mode"><c>state_entry_events</c></seealso>.
	</p>
	<p>
	  If the atom <c>state_entry_events</c> is present in the list,
	  the <c>gen_statem</c> engine will, at every state change,
	  insert an event of type
	  <seealso marker="#type-event_type">enter</seealso>
	  with content <c>OldState</c>.  This event will be inserted
	  before all other events such as those generated by
	  <seealso marker="#type-action"><c>action()</c></seealso>
	  <c>next_event</c>.
	</p>
	<p>
	  No state entry event will be inserted after a
	  <seealso marker="#Module:code_change/4"><c>Module:code_change/4</c></seealso>
	  since transforming the state to a newer version is regarded
	  as staying in the same state even if the newer version state
	  should have a different name.
	</p>
	<note>
	  <p>
	    If this function's body does not return an inline constant
	    value the callback module is doing something strange.
	  </p>
	</note>
      </desc>
    </func>

    <func>
      <name>Module:code_change(OldVsn, OldState, OldData, Extra) ->
        Result
      </name>
      <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
      <type>
        <v>OldVsn = Vsn | {down,Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>OldState = NewState = term()</v>
        <v>Extra = term()</v>
	<v>Result = {ok,NewState,NewData} | Reason</v>
	<v>
	  OldState = NewState =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  OldData = NewData =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <p>
	  This function is called by a <c>gen_statem</c> when it is to
	  update its internal state during a release upgrade/downgrade,
	  that is, when the instruction <c>{update,Module,Change,...}</c>,
	  where <c>Change={advanced,Extra}</c>, is specified in the
	  <seealso marker="sasl:appup"><c>appup</c></seealso>
	  file. For more information, see
	  <seealso marker="doc/design_principles:release_handling#instr">OTP Design Principles</seealso>.
	</p>
	<p>
	  For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
	  for a downgrade, <c>OldVsn</c> is
	  <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
	  attribute(s) of the old version of the callback module
	  <c>Module</c>. If no such attribute is defined, the version
	  is the checksum of the Beam file.
	</p>
        <p>
	  <c>OldState</c> and <c>OldData</c> is the internal state
	  of the <c>gen_statem</c>.
	</p>
	<p>
	  <c>Extra</c> is passed "as is" from the <c>{advanced,Extra}</c>
	  part of the update instruction.
	</p>
	<p>
	  If successful, the function must return the updated
	  internal state in an
	  <c>{ok,NewState,NewData}</c> tuple.
	</p>
	<p>
	  If the function returns a failure <c>Reason</c>, the ongoing
	  upgrade fails and rolls back to the old release.
	  Note that <c>Reason</c> can not be an <c>{ok,_,_}</c> tuple
	  since that will be regarded as a
	  <c>{ok,NewState,NewData}</c> tuple,
	  and that a tuple matching <c>{ok,_}</c>
	  is an also invalid failure <c>Reason</c>.
	  It is recommended to use an atom as <c>Reason</c> since
	  it will be wrapped in an <c>{error,Reason}</c> tuple.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>
	Optional function for initializing process and internal state.
      </fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result = {ok,State,Data}</v>
	<v>&nbsp;| {ok,State,Data,Actions}</v>
        <v>&nbsp;| {stop,Reason} | ignore</v>
	<v>State = <seealso marker="#type-state">state()</seealso></v>
	<v>
	  Data = <seealso marker="#type-data">data()</seealso>
	</v>
	<v>
	  Actions =
	  [<seealso marker="#type-action">action()</seealso>] |
	  <seealso marker="#type-action">action()</seealso>
	</v>
	<v>Reason = term()</v>
      </type>
      <desc>
	<marker id="Module:init-1"/>
	<p>
	  Whenever a <c>gen_statem</c> is started using
	  <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>
	  or
	  <seealso marker="#start/3"><c>start/3,4</c></seealso>,
	  this optional function is called by the new process to initialize
	  the implementation state and server data.
	</p>
	<p>
	  <c>Args</c> is the <c>Args</c> argument provided to the start
	  function.
	</p>
	<p>
	  If the initialization is successful, the function is to
	  return <c>{ok,State,Data}</c> or
	  <c>{ok,State,Data,Actions}</c>.
	  <c>State</c> is the initial
	  <seealso marker="#type-state"><c>state()</c></seealso>
	  and <c>Data</c> the initial server
	  <seealso marker="#type-data"><c>data()</c></seealso>.
	</p>
	<p>
	  The <seealso marker="#type-action"><c>Actions</c></seealso>
	  are executed when entering the first
	  <seealso marker="#type-state">state</seealso> just as for a
	  <seealso marker="#state_function">state function</seealso>.
	</p>
	<p>
	  If the initialization fails,
          the function is to return <c>{stop,Reason}</c>
	  or <c>ignore</c>; see
          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.
	</p>
        <note>
	  <p>
	    This callback is optional, so a callback module does not need
	    to export it, but most do.  If this function is not exported,
	    the <c>gen_statem</c> should be started through
	    <seealso marker="proc_lib"><c>proc_lib</c></seealso>
	    and
	    <seealso marker="#enter_loop/4"><c>enter_loop/4-6</c></seealso>.
	  </p>
        </note>
      </desc>
    </func>

    <func>
      <name>Module:format_status(Opt, [PDict,State,Data]) ->
        Status
      </name>
      <fsummary>Optional function for providing a term describing the
        current <c>gen_statem</c> status.</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>Key = term()</v>
	<v>Value = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
	  <p>
	    This callback is optional, so a callback module does not need
	    to export it. The <c>gen_statem</c> module provides a default
	    implementation of this function that returns
	    <c>{State,Data}</c>.
	  </p>
	  <p>
	    If this callback is exported but fails,
	    to hide possibly sensitive data,
	    the default function will instead return <c>{State,Info}</c>,
	    where <c>Info</c> says nothing but the fact that
	    <c>format_status/2</c> has crashed.
	  </p>
        </note>
        <p>This function is called by a <c>gen_statem</c> process when
          any of the following apply:</p>
        <list type="bulleted">
	  <item>
	    One of
	    <seealso marker="sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
	    is invoked to get the <c>gen_statem</c> status. <c>Opt</c> is set
	    to the atom <c>normal</c> for this case.
	  </item>
	  <item>
	    The <c>gen_statem</c> terminates abnormally and logs an error.
	    <c>Opt</c> is set to the atom <c>terminate</c> for this case.
	  </item>
	</list>
        <p>
	  This function is useful for changing the form and
          appearance of the <c>gen_statem</c> status for these cases. A
          callback module wishing to change the
          <seealso marker="sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
	  return value and how
          its status appears in termination error logs exports an
          instance of <c>format_status/2</c>, which returns a term
          describing the current status of the <c>gen_statem</c>.
	</p>
	<p>
	  <c>PDict</c> is the current value of the process dictionary
          of the <c>gen_statem</c>.
	</p>
	<p>
	  <seealso marker="#type-state"><c>State</c></seealso>
	  is the internal state of the <c>gen_statem</c>.
	</p>
	<p>
	  <seealso marker="#type-data"><c>Data</c></seealso>
	  is the internal server data of the <c>gen_statem</c>.
	</p>
	<p>
	  The function is to return <c>Status</c>, a term that
          changes the details of the current state and status of
          the <c>gen_statem</c>. There are no restrictions on the
          form <c>Status</c> can take, but for the
          <seealso marker="sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
	  case (when <c>Opt</c>
          is <c>normal</c>), the recommended form for
          the <c>Status</c> value is <c>[{data, [{"State",
          Term}]}]</c>, where <c>Term</c> provides relevant details of
          the <c>gen_statem</c> state. Following this recommendation is not
          required, but it makes the callback module status
          consistent with the rest of the
          <seealso marker="sys#get_status/1"><c>sys:get_status/1,2</c></seealso>
	  return value.
	</p>
	<p>
	  One use for this function is to return compact alternative
          state representations to avoid having large state terms
          printed in log files. Another use is to hide sensitive data from
	  being written to the error log.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:StateName(EventType, EventContent, Data) ->
        StateFunctionResult
      </name>
      <name>Module:handle_event(EventType, EventContent,
        State, Data) -> HandleEventResult
      </name>
      <fsummary>Handle an event.</fsummary>
      <type>
	<v>
	  EventType =
	  <seealso marker="#type-event_type">event_type()</seealso>
	</v>
	<v>EventContent = term()</v>
	<v>
	  State =
	  <seealso marker="#type-state">state()</seealso>
	</v>
	<v>
	  Data = NewData =
	  <seealso marker="#type-data">data()</seealso>
	</v>
	<v>
	  StateFunctionResult =
	  <seealso marker="#type-state_function_result">state_function_result()</seealso>
	</v>
	<v>
	  HandleEventResult =
	  <seealso marker="#type-handle_event_result">handle_event_result()</seealso>
	</v>
      </type>
      <desc>
	<p>
	  Whenever a <c>gen_statem</c> receives an event from
	  <seealso marker="#call/2"><c>call/2</c></seealso>,
	  <seealso marker="#cast/2"><c>cast/2</c></seealso>, or
	  as a normal process message, one of these functions is called. If
	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
	  is <c>state_functions</c>, <c>Module:StateName/3</c> is called,
	  and if it is <c>handle_event_function</c>,
	  <c>Module:handle_event/4</c> is called.
	</p>
	<p>
	  If <c>EventType</c> is
	  <seealso marker="#type-event_type"><c>{call,From}</c></seealso>,
	  the caller waits for a reply. The reply can be sent
	  from this or from any other
	  <seealso marker="#state_function">state function</seealso>
	  by returning with <c>{reply,From,Reply}</c> in
	  <seealso marker="#type-action"><c>Actions</c></seealso>, in
	  <seealso marker="#type-reply_action"><c>Replies</c></seealso>,
	  or by calling
	  <seealso marker="#reply/2"><c>reply(From, Reply)</c></seealso>.
	</p>
	<p>
	  If this function returns with a next state that
	  does not match equal (<c>=/=</c>) to the current state,
	  all postponed events are retried in the next state.
	</p>
	<p>
	  The only difference between <c>StateFunctionResult</c> and
	  <c>HandleEventResult</c> is that for <c>StateFunctionResult</c>
	  the next state must be an atom, but for <c>HandleEventResult</c>
	  there is no restriction on the next state.
	</p>
	<p>
	  For options that can be set and actions that can be done
	  by <c>gen_statem</c> after returning from this function,
	  see <seealso marker="#type-action"><c>action()</c></seealso>.
	</p>
	<p>
	  Note the fact that you can use
	  <seealso marker="erts:erlang#throw/1"><c>throw</c></seealso>
	  to return the result, which can be useful.
	  For example to bail out with <c>throw(keep_state_and_data)</c>
	  from deep within complex code that is in no position to
	  return <c>{next_state,State,Data}</c>.
	</p>
      </desc>
    </func>

    <func>
      <name>Module:terminate(Reason, State, Data) -> Ignored</name>
      <fsummary>Clean up before termination.</fsummary>
      <type>
        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
	<v>State = <seealso marker="#type-state">state()</seealso></v>
	<v>Data = <seealso marker="#type-data">data()</seealso></v>
	<v>Ignored = term()</v>
      </type>
      <desc>
	<p>
	  This function is called by a <c>gen_statem</c>
	  when it is about to terminate. It is to be the opposite of
	  <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
	  and do any necessary cleaning up. When it returns,
	  the <c>gen_statem</c> terminates with <c>Reason</c>. The return
	  value is ignored.</p>
	<p>
	  <c>Reason</c> is a term denoting the stop reason and
	  <seealso marker="#type-state"><c>State</c></seealso>
	  is the internal state of the <c>gen_statem</c>.
	</p>
	<p>
	  <c>Reason</c> depends on why the <c>gen_statem</c>
	  is terminating.
	  If it is because another callback function has returned, a
	  stop tuple <c>{stop,Reason}</c> in
	  <seealso marker="#type-action"><c>Actions</c></seealso>,
	  <c>Reason</c> has the value specified in that tuple.
	  If it is because of a failure, <c>Reason</c> is the error reason.
	</p>
        <p>
	  If the <c>gen_statem</c> is part of a supervision tree and is
          ordered by its supervisor to terminate, this function is
          called with <c>Reason = shutdown</c> if both the following
          conditions apply:</p>
        <list type="bulleted">
          <item>
            <p>
	      The <c>gen_statem</c> has been set
	      to trap exit signals.
            </p>
	  </item>
          <item>
            <p>
	      The shutdown strategy as defined in the supervisor's
              child specification is an integer time-out value, not
              <c>brutal_kill</c>.
            </p>
	  </item>
        </list>
        <p>
	  Even if the <c>gen_statem</c> is <em>not</em>
	  part of a supervision tree, this function is called
	  if it receives an <c>'EXIT'</c> message from its parent.
	  <c>Reason</c> is the same as
	  in the <c>'EXIT'</c> message.
	</p>
        <p>
	  Otherwise, the <c>gen_statem</c> is immediately terminated.
	</p>
        <p>
	  Notice that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c>,
	  the <c>gen_statem</c> is assumed to terminate because of an error
	  and an error report is issued using
          <seealso marker="kernel:error_logger#format/2"><c>error_logger:format/2</c></seealso>.
	</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p>
      <seealso marker="gen_event"><c>gen_event(3)</c></seealso>,
      <seealso marker="gen_fsm"><c>gen_fsm(3)</c></seealso>,
      <seealso marker="gen_server"><c>gen_server(3)</c></seealso>,
      <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>,
      <seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
      <seealso marker="sys"><c>sys(3)</c></seealso>.
    </p>
  </section>
</erlref>