<?xml version="1.0" encoding="utf-8" ?> <!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>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> <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,<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> 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="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 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 > 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> =/= infinity</c>, to avoid getting a late reply in the caller's inbox, 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> The call can 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 > 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> </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> <note> <p> If this function's body does not consist of solely one of two possible <seealso marker="#type-callback_mode">atoms</seealso> 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> 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> | {ok,State,Data,Actions}</v> <v> | {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>