1gen_statem(3) Erlang Module Definition gen_statem(3)
2
6 gen_statem - Generic state machine behavior.
7
9 gen_statem provides a generic state machine behaviour and replaces its
10 predecessor gen_fsm since Erlang/OTP 20.0.
11 This reference manual describes types generated from the types in the
13 gen_statem source code, so they are correct. However, the generated
14 descriptions also reflect the type hiearchy, which makes them kind of
15 hard to read.
16 To get an overview of the concepts and operation of gen_statem, do read
18 the gen_statem Behaviour in OTP Design Principles which frequently
19 links back to this reference manual to avoid containing detailed facts
20 that may rot by age.
21 Note:
23 This behavior appeared in Erlang/OTP 19.0. In OTP 19.1 a backwards
24 incompatible change of the return tuple from Module:init/1 was made and
25 the mandatory callback function Module:callback_mode/0 was introduced.
26 In OTP 20.0 the generic timeouts were added.
27 gen_statem has got the same features that gen_fsm had and adds some
30 really useful:
31 * Co-located state code
33 * Arbitrary term state
35 * Event postponing
37 * Self-generated events
39 * State time-out
41 * Multiple generic named time-outs
43 * Absolute time-out time
45 * Automatic state enter calls
47 *
49 Reply from other state than the request, sys traceable
50 * Multiple sys traceable replies
52 Two callback modes are supported:
54 * One for finite-state machines (gen_fsm like), which requires the
56 state to be an atom and uses that state as the name of the current
57 callback function
58 * One without restriction on the state data type that uses one call‐
60 back function for all states
61 The callback model(s) for gen_statem differs from the one for gen_fsm,
63 but it is still fairly easy to rewrite from gen_fsm to gen_statem.
64 A generic state machine process (gen_statem) implemented using this
66 module has a standard set of interface functions and includes function‐
67 ality for tracing and error reporting. It also fits into an OTP super‐
68 vision tree. For more information, see OTP Design Principles.
69 A gen_statem assumes all specific parts to be located in a callback
71 module exporting a predefined set of functions. The relationship
72 between the behavior functions and the callback functions is as fol‐
73 lows:
74 gen_statem module Callback module
76 ----------------- ---------------
77 gen_statem:start
78 gen_statem:start_link -----> Module:init/1
79 Server start or code change
81 -----> Module:callback_mode/0
82 gen_statem:stop -----> Module:terminate/3
84 gen_statem:call
86 gen_statem:cast
87 erlang:send
88 erlang:'!' -----> Module:StateName/3
89 Module:handle_event/4
90 - -----> Module:terminate/3
92 - -----> Module:code_change/4
94 Events are of different types, so the callback functions can know the
96 origin of an event and how to respond.
97 If a callback function fails or returns a bad value, the gen_statem
99 terminates, unless otherwise stated. However, an exception of class
100 throw is not regarded as an error but as a valid return from all call‐
101 back functions.
102 The "state callback" for a specific state in a gen_statem is the call‐
104 back function that is called for all events in this state. It is
105 selected depending on which callback mode that the callback module
106 defines with the callback function Module:callback_mode/0.
107 When the callback mode is state_functions, the state must be an atom
109 and is used as the state callback name; see Module:StateName/3. This
110 co-locates all code for a specific state in one function as the
111 gen_statem engine branches depending on state name. Note the fact that
112 the callback function Module:terminate/3 makes the state name terminate
113 unusable in this mode.
114 When the callback mode is handle_event_function, the state can be any
116 term and the state callback name is Module:handle_event/4. This makes
117 it easy to branch depending on state or event as you desire. Be careful
118 about which events you handle in which states so that you do not acci‐
119 dentally postpone an event forever creating an infinite busy loop.
120 The gen_statem enqueues incoming events in order of arrival and
122 presents these to the state callback in that order. The state callback
123 can postpone an event so it is not retried in the current state. After
124 a state change the queue restarts with the postponed events.
125 The gen_statem event queue model is sufficient to emulate the normal
127 process message queue with selective receive. Postponing an event cor‐
128 responds to not matching it in a receive statement, and changing states
129 corresponds to entering a new receive statement.
130 The state callback can insert events using the action() next_event and
132 such an event is inserted as the next to present to the state callback.
133 That is, as if it is the oldest incoming event. A dedicated
134 event_type() internal can be used for such events making them impossi‐
135 ble to mistake for external events.
136 Inserting an event replaces the trick of calling your own state han‐
138 dling functions that you often would have to resort to in, for example,
139 gen_fsm to force processing an inserted event before others.
140 The gen_statem engine can automatically make a specialized call to the
142 state callback whenever a new state is entered; see state_enter(). This
143 is for writing code common to all state entries. Another way to do it
144 is to insert an event at the state transition, and/or to use a dedi‐
145 cated state transition function, but that is something you will have to
146 remember at every state transition to the state(s) that need it.
147 Note:
149 If you in gen_statem, for example, postpone an event in one state and
150 then call another state callback of yours, you have not changed states
151 and hence the postponed event is not retried, which is logical but can
152 be confusing.
153 For the details of a state transition, see type transition_option().
156 A gen_statem handles system messages as described in sys. The sys mod‐
158 ule can be used for debugging a gen_statem.
159 Notice that a gen_statem does not trap exit signals automatically, this
161 must be explicitly initiated in the callback module (by calling
162 process_flag(trap_exit, true).
163 Unless otherwise stated, all functions in this module fail if the spec‐
165 ified gen_statem does not exist or if bad arguments are specified.
166 The gen_statem process can go into hibernation; see proc_lib:hiber‐
168 nate/3. It is done when a state callback or Module:init/1 specifies
169 hibernate in the returned Actions list. This feature can be useful to
170 reclaim process heap memory while the server is expected to be idle for
171 a long time. However, use this feature with care, as hibernation can be
172 too costly to use after every event; see erlang:hibernate/3.
173 There is also a server start option {hibernate_after, Timeout} for
175 start/3,4 or start_link/3,4 that may be used to automatically hibernate
176 the server.
177
179 The following example shows a simple pushbutton model for a toggling
180 pushbutton implemented with callback mode state_functions. You can push
181 the button and it replies if it went on or off, and you can ask for a
182 count of how many times it has been pushed to switch on.
183 The following is the complete callback module file pushbutton.erl:
185 -module(pushbutton).
187 -behaviour(gen_statem).
188 -export([start/0,push/0,get_count/0,stop/0]).
190 -export([terminate/3,code_change/4,init/1,callback_mode/0]).
191 -export([on/3,off/3]).
192 name() -> pushbutton_statem. % The registered server name
194 %% API. This example uses a registered name name()
196 %% and does not link to the caller.
197 start() ->
198 gen_statem:start({local,name()}, ?MODULE, [], []).
199 push() ->
200 gen_statem:call(name(), push).
201 get_count() ->
202 gen_statem:call(name(), get_count).
203 stop() ->
204 gen_statem:stop(name()).
205 %% Mandatory callback functions
207 terminate(_Reason, _State, _Data) ->
208 void.
209 code_change(_Vsn, State, Data, _Extra) ->
210 {ok,State,Data}.
211 init([]) ->
212 %% Set the initial state + data. Data is used only as a counter.
213 State = off, Data = 0,
214 {ok,State,Data}.
215 callback_mode() -> state_functions.
216 %%% state callback(s)
218 off({call,From}, push, Data) ->
220 %% Go to 'on', increment count and reply
221 %% that the resulting status is 'on'
222 {next_state,on,Data+1,[{reply,From,on}]};
223 off(EventType, EventContent, Data) ->
224 handle_event(EventType, EventContent, Data).
225 on({call,From}, push, Data) ->
227 %% Go to 'off' and reply that the resulting status is 'off'
228 {next_state,off,Data,[{reply,From,off}]};
229 on(EventType, EventContent, Data) ->
230 handle_event(EventType, EventContent, Data).
231 %% Handle events common to all states
233 handle_event({call,From}, get_count, Data) ->
234 %% Reply with the current count
235 {keep_state,Data,[{reply,From,Data}]};
236 handle_event(_, _, Data) ->
237 %% Ignore all other events
238 {keep_state,Data}.
239 The following is a shell session when running it:
242 1> pushbutton:start().
244 {ok,<0.36.0>}
245 2> pushbutton:get_count().
246 0
247 3> pushbutton:push().
248 on
249 4> pushbutton:get_count().
250 1
251 5> pushbutton:push().
252 off
253 6> pushbutton:get_count().
254 1
255 7> pushbutton:stop().
256 ok
257 8> pushbutton:push().
258 ** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
259 in function gen:do_for_proc/2 (gen.erl, line 261)
260 in call from gen_statem:call/3 (gen_statem.erl, line 386)
261 To compare styles, here follows the same example using callback mode
264 handle_event_function, or rather the code to replace after function
265 init/1 of the pushbutton.erl example file above:
266 callback_mode() -> handle_event_function.
268 %%% state callback(s)
270 handle_event({call,From}, push, off, Data) ->
272 %% Go to 'on', increment count and reply
273 %% that the resulting status is 'on'
274 {next_state,on,Data+1,[{reply,From,on}]};
275 handle_event({call,From}, push, on, Data) ->
276 %% Go to 'off' and reply that the resulting status is 'off'
277 {next_state,off,Data,[{reply,From,off}]};
278 %%
279 %% Event handling common to all states
280 handle_event({call,From}, get_count, State, Data) ->
281 %% Reply with the current count
282 {next_state,State,Data,[{reply,From,Data}]};
283 handle_event(_, _, State, Data) ->
284 %% Ignore all other events
285 {next_state,State,Data}.
286
289 server_name() =
290 {global, GlobalName :: term()} |
291 {via, RegMod :: module(), Name :: term()} |
292 {local, atom()}
293 Name specification to use when starting a gen_statem server. See
295 start_link/3 and server_ref() below.
296 server_ref() =
298 pid() |
299 (LocalName :: atom()) |
300 {Name :: atom(), Node :: atom()} |
301 {global, GlobalName :: term()} |
302 {via, RegMod :: module(), ViaName :: term()}
303 Server specification to use when addressing a gen_statem server.
305 See call/2 and server_name() above.
306 It can be:
308 pid() | LocalName:
310 The gen_statem is locally registered.
311 {Name,Node}:
313 The gen_statem is locally registered on another node.
314 {global,GlobalName}:
316 The gen_statem is globally registered in global.
317 {via,RegMod,ViaName}:
319 The gen_statem is registered in an alternative process reg‐
320 istry. The registry callback module RegMod is to export
321 functions register_name/2, unregister_name/1,
322 whereis_name/1, and send/2, which are to behave like the
323 corresponding functions in global. Thus, {via,global,Global‐
324 Name} is the same as {global,GlobalName}.
325 debug_opt() =
327 {debug,
328 Dbgs ::
329 [trace | log | statistics | debug | {logfile, string()}]}
330 Debug option that can be used when starting a gen_statem server
332 through, enter_loop/4-6.
333 For every entry in Dbgs, the corresponding function in sys is
335 called.
336 hibernate_after_opt() =
338 {hibernate_after, HibernateAfterTimeout :: timeout()}
339 hibernate_after option that can be used when starting a
341 gen_statem server through, enter_loop/4-6.
342 If option{hibernate_after,HibernateAfterTimeout} is present, the
344 gen_statem process awaits any message for HibernateAfterTimeout
345 milliseconds and if no message is received, the process goes
346 into hibernation automatically (by calling proc_lib:hiber‐
347 nate/3).
348 start_opt() =
350 debug_opt() |
351 {timeout, Time :: timeout()} |
352 hibernate_after_opt() |
353 {spawn_opt, [proc_lib:spawn_option()]}
354 Options that can be used when starting a gen_statem server
356 through, for example, start_link/3.
357 start_ret() = {ok, pid()} | ignore | {error, term()}
359 Return value from the start functions, for example,
361 start_link/3.
362 from() = {To :: pid(), Tag :: term()}
364 Destination to use when replying through, for example, the
366 action() {reply,From,Reply} to a process that has called the
367 gen_statem server using call/2.
368 state() = state_name() | term()
370 If the callback mode is handle_event_function, the state can be
372 any term. After a state change (NextState =/= State), all post‐
373 poned events are retried.
374 state_name() = atom()
376 If the callback mode is state_functions, the state must be of
378 this type. After a state change (NextState =/= State), all post‐
379 poned events are retried.
380 data() = term()
382 A term in which the state machine implementation is to store any
384 server data it needs. The difference between this and the
385 state() itself is that a change in this data does not cause
386 postponed events to be retried. Hence, if a change in this data
387 would change the set of events that are handled, then that data
388 item is to be made a part of the state.
389 event_type() =
391 external_event_type() | timeout_event_type() | internal
392 There are 3 categories of events: external, timeout, and inter‐
394 nal.
395 internal events can only be generated by the state machine
397 itself through the state transition action next_event.
398 external_event_type() = {call, From :: from()} | cast | info
400 External events are of 3 types: {call,From}, cast, or info.
402 Calls (synchronous) and casts originate from the corresponding
403 API functions. For calls, the event contains whom to reply to.
404 Type info originates from regular process messages sent to the
405 gen_statem.
406 timeout_event_type() =
408 timeout | {timeout, Name :: term()} | state_timeout
409 There are 3 types of timeout events that the state machine can
411 generate for itself with the corresponding timeout_action()s.
412 callback_mode_result() =
414 callback_mode() | [callback_mode() | state_enter()]
415 This is the return type from Module:callback_mode/0 and selects
417 callback mode and whether to do state enter calls, or not.
418 callback_mode() = state_functions | handle_event_function
420 The callback mode is selected when starting the gen_statem and
422 after code change using the return value from Module:call‐
423 back_mode/0.
424 state_functions:
426 The state must be of type state_name() and one callback
427 function per state, that is, Module:StateName/3, is used.
428 handle_event_function:
430 The state can be any term and the callback function Mod‐
431 ule:handle_event/4 is used for all states.
432 state_enter() = state_enter
434 Whether the state machine should use state enter calls or not is
436 selected when starting the gen_statem and after code change
437 using the return value from Module:callback_mode/0.
438 If Module:callback_mode/0 returns a list containing state_enter,
440 the gen_statem engine will, at every state change, call the
441 state callback with arguments (enter, OldState, Data). This may
442 look like an event but is really a call performed after the pre‐
443 vious state callback returned and before any event is delivered
444 to the new state callback. See Module:StateName/3 and Mod‐
445 ule:handle_event/4. Such a call can be repeated by returning a
446 repeat_state or repeat_state_and_data tuple from the state call‐
447 back.
448 If Module:callback_mode/0 does not return such a list, no state
450 enter calls are done.
451 If Module:code_change/4 should transform the state, it is
453 regarded as a state rename and not a state change, which will
454 not cause a state enter call.
455 Note that a state enter call will be done right before entering
457 the initial state even though this formally is not a state
458 change. In this case OldState will be the same as State, which
459 can not happen for a subsequent state change, but will happen
460 when repeating the state enter call.
461 transition_option() =
463 postpone() |
464 hibernate() |
465 event_timeout() |
466 generic_timeout() |
467 state_timeout()
468 Transition options can be set by actions and modify the state
470 transition. Here are the sequence of steps for a state transi‐
471 tion:
472 * If state enter calls are used, and either: the state
474 changes, it is the initial state, or one of the callback
475 results repeat_state or repeat_state_and_data is used; the
476 gen_statem calls the new state callback with arguments
477 (enter, OldState, Data).
478 Any actions returned from this call are handled as if they
480 were appended to the actions returned by the state callback
481 that caused the state entry.
482 Should this state enter call return any of the mentioned
484 repeat_* callback results it is repeated again, with the
485 updated Data.
486 * All actions are processed in order of appearance.
488 * If postpone() is true, the current event is postponed.
490 * If the state changes, the queue of incoming events is reset
492 to start with the oldest postponed.
493 * All events stored with action() next_event are inserted to
495 be processed before previously queued events.
496 * Time-out timers event_timeout(), generic_timeout() and
498 state_timeout() are handled. Time-outs with zero time are
499 guaranteed to be delivered to the state machine before any
500 external not yet received event so if there is such a time-
501 out requested, the corresponding time-out zero event is
502 enqueued as the newest received event; that is after already
503 queued events such as inserted and postponed events.
504 Any event cancels an event_timeout() so a zero time event
506 time-out is only generated if the event queue is empty.
507 A state change cancels a state_timeout() and any new transi‐
509 tion option of this type belongs to the new state.
510 * If there are enqueued events the state callback for the pos‐
512 sibly new state is called with the oldest enqueued event,
513 and we start again from the top of this list.
514 * Otherwise the gen_statem goes into receive or hibernation
516 (if hibernate() is true) to wait for the next message. In
517 hibernation the next non-system event awakens the
518 gen_statem, or rather the next incoming message awakens the
519 gen_statem, but if it is a system event it goes right back
520 into hibernation. When a new message arrives the state call‐
521 back is called with the corresponding event, and we start
522 again from the top of this sequence.
523 postpone() = boolean()
525 If true, postpones the current event and retries it when the
527 state changes (NextState =/= State).
528 hibernate() = boolean()
530 If true, hibernates the gen_statem by calling proc_lib:hiber‐
532 nate/3 before going into receive to wait for a new external
533 event.
534 Note:
536 If there are enqueued events to process when hibrnation is
537 requested, this is optimized by not hibernating but instead
538 calling erlang:garbage_collect/0 to simulate that the gen_statem
539 entered hibernation and immediately got awakened by an enqueued
540 event.
541 event_timeout() = timeout() | integer()
544 Starts a timer set by enter_action() timeout. When the timer
546 expires an event of event_type() timeout will be generated. See
547 erlang:start_timer/4 for how Time and Options are interpreted.
548 Future erlang:start_timer/4 Options will not necessarily be sup‐
549 ported.
550 Any event that arrives cancels this time-out. Note that a
552 retried or inserted event counts as arrived. So does a state
553 time-out zero event, if it was generated before this time-out is
554 requested.
555 If Time is infinity, no timer is started, as it never would
557 expire anyway.
558 If Time is relative and 0 no timer is actually started, instead
560 the the time-out event is enqueued to ensure that it gets pro‐
561 cessed before any not yet received external event, but after
562 already queued events.
563 Note that it is not possible nor needed to cancel this time-out,
565 as it is cancelled automatically by any other event.
566 generic_timeout() = timeout() | integer()
568 Starts a timer set by enter_action() {timeout,Name}. When the
570 timer expires an event of event_type() {timeout,Name} will be
571 generated. See erlang:start_timer/4 for how Time and Options are
572 interpreted. Future erlang:start_timer/4 Options will not neces‐
573 sarily be supported.
574 If Time is infinity, no timer is started, as it never would
576 expire anyway.
577 If Time is relative and 0 no timer is actually started, instead
579 the the time-out event is enqueued to ensure that it gets pro‐
580 cessed before any not yet received external event.
581 Setting a timer with the same Name while it is running will
583 restart it with the new time-out value. Therefore it is possible
584 to cancel a specific time-out by setting it to infinity.
585 state_timeout() = timeout() | integer()
587 Starts a timer set by enter_action() state_timeout. When the
589 timer expires an event of event_type() state_timeout will be
590 generated. See erlang:start_timer/4 for how Time and Options are
591 interpreted. Future erlang:start_timer/4 Options will not neces‐
592 sarily be supported.
593 If Time is infinity, no timer is started, as it never would
595 expire anyway.
596 If Time is relative and 0 no timer is actually started, instead
598 the the time-out event is enqueued to ensure that it gets pro‐
599 cessed before any not yet received external event.
600 Setting this timer while it is running will restart it with the
602 new time-out value. Therefore it is possible to cancel this
603 time-out by setting it to infinity.
604 timeout_option() = {abs, Abs :: boolean()}
606 If Abs is true an absolute timer is started, and if it is false
608 a relative, which is the default. See erlang:start_timer/4 for
609 details.
610 action() =
612 postpone |
613 {postpone, Postpone :: postpone()} |
614 {next_event,
615 EventType :: event_type(),
616 EventContent :: term()} |
617 enter_action()
618 These state transition actions can be invoked by returning them
620 from the state callback when it is called with an event, from
621 Module:init/1 or by giving them to enter_loop/5,6.
622 Actions are executed in the containing list order.
624 Actions that set transition options override any previous of
626 the same type, so the last in the containing list wins. For
627 example, the last postpone() overrides any previous postpone()
628 in the list.
629 postpone:
631 Sets the transition_option() postpone() for this state tran‐
632 sition. This action is ignored when returned from Mod‐
633 ule:init/1 or given to enter_loop/5,6, as there is no event
634 to postpone in those cases.
635 next_event:
637 This action does not set any transition_option() but instead
638 stores the specified EventType and EventContent for inser‐
639 tion after all actions have been executed.
640 The stored events are inserted in the queue as the next to
642 process before any already queued events. The order of these
643 stored events is preserved, so the first next_event in the
644 containing list becomes the first to process.
645 An event of type internal is to be used when you want to
647 reliably distinguish an event inserted this way from any
648 external event.
649 enter_action() =
651 hibernate |
652 {hibernate, Hibernate :: hibernate()} |
653 timeout_action() |
654 reply_action()
655 These state transition actions can be invoked by returning them
657 from the state callback, from Module:init/1 or by giving them to
658 enter_loop/5,6.
659 Actions are executed in the containing list order.
661 Actions that set transition options override any previous of the
663 same type, so the last in the containing list wins. For example,
664 the last event_timeout() overrides any previous event_timeout()
665 in the list.
666 hibernate:
668 Sets the transition_option() hibernate() for this state
669 transition.
670 timeout_action() =
672 (Time :: event_timeout()) |
673 {timeout, Time :: event_timeout(), EventContent :: term()} |
674 {timeout,
675 Time :: event_timeout(),
676 EventContent :: term(),
677 Options :: timeout_option() | [timeout_option()]} |
678 {{timeout, Name :: term()},
679 Time :: generic_timeout(),
680 EventContent :: term()} |
681 {{timeout, Name :: term()},
682 Time :: generic_timeout(),
683 EventContent :: term(),
684 Options :: timeout_option() | [timeout_option()]} |
685 {state_timeout,
686 Time :: state_timeout(),
687 EventContent :: term()} |
688 {state_timeout,
689 Time :: state_timeout(),
690 EventContent :: term(),
691 Options :: timeout_option() | [timeout_option()]}
692 These state transition actions can be invoked by returning them
694 from the state callback, from Module:init/1 or by giving them to
695 enter_loop/5,6.
696 These timeout actions sets timeout transition options.
698 Time:
700 Short for {timeout,Time,Time}, that is, the time-out message
701 is the time-out time. This form exists to make the state
702 callback return value {next_state,NextState,NewData,Time}
703 allowed like for gen_fsm.
704 timeout:
706 Sets the transition_option() event_timeout() to Time with
707 EventContent and time-out options Options.
708 {timeout,Name}:
710 Sets the transition_option() generic_timeout() to Time for
711 Name with EventContent and time-out options Options.
712 state_timeout:
714 Sets the transition_option() state_timeout() to Time with
715 EventContent and time-out options Options.
716 reply_action() = {reply, From :: from(), Reply :: term()}
718 This state transition action can be invoked by returning it from
720 the state callback, from Module:init/1 or by giving it to
721 enter_loop/5,6.
722 It does not set any transition_option() but instead replies to a
724 caller waiting for a reply in call/2. From must be the term from
725 argument {call,From} in a call to a state callback.
726 Note that using this action from Module:init/1 or enter_loop/5,6
728 would be weird on the border of witchcraft since there has been
729 no earlier call to a state callback in this server.
730 init_result(StateType) =
732 {ok, State :: StateType, Data :: data()} |
733 {ok,
734 State :: StateType,
735 Data :: data(),
736 Actions :: [action()] | action()} |
737 ignore |
738 {stop, Reason :: term()}
739 For a succesful initialization, State is the initial state() and
741 Data the initial server data() of the gen_statem.
742 The Actions are executed when entering the first state just as
744 for a state callback, except that the action postpone is forced
745 to false since there is no event to postpone.
746 For an unsuccesful initialization, {stop,Reason} or ignore
748 should be used; see start_link/3,4.
749 state_enter_result(State) =
751 {next_state, State, NewData :: data()} |
752 {next_state,
753 State,
754 NewData :: data(),
755 Actions :: [enter_action()] | enter_action()} |
756 state_callback_result(enter_action())
757 State is the current state and it can not be changed since the
759 state callback was called with a state enter call.
760 next_state:
762 The gen_statem does a state transition to State, which has
763 to be the current state, sets NewData, and executes all
764 Actions.
765 event_handler_result(StateType) =
767 {next_state, NextState :: StateType, NewData :: data()} |
768 {next_state,
769 NextState :: StateType,
770 NewData :: data(),
771 Actions :: [action()] | action()} |
772 state_callback_result(action())
773 StateType is state_name() if callback mode is state_functions,
775 or state() if callback mode is handle_event_function.
776 next_state:
778 The gen_statem does a state transition to NextState (which
779 can be the same as the current state), sets NewData, and
780 executes all Actions.
781 state_callback_result(ActionType) =
783 {keep_state, NewData :: data()} |
784 {keep_state,
785 NewData :: data(),
786 Actions :: [ActionType] | ActionType} |
787 keep_state_and_data |
788 {keep_state_and_data, Actions :: [ActionType] | ActionType} |
789 {repeat_state, NewData :: data()} |
790 {repeat_state,
791 NewData :: data(),
792 Actions :: [ActionType] | ActionType} |
793 repeat_state_and_data |
794 {repeat_state_and_data, Actions :: [ActionType] | ActionType} |
795 stop |
796 {stop, Reason :: term()} |
797 {stop, Reason :: term(), NewData :: data()} |
798 {stop_and_reply,
799 Reason :: term(),
800 Replies :: [reply_action()] | reply_action()} |
801 {stop_and_reply,
802 Reason :: term(),
803 Replies :: [reply_action()] | reply_action(),
804 NewData :: data()}
805 ActionType is enter_action() if the state callback was called
807 with a state enter call and action() if the state callback was
808 called with an event.
809 keep_state:
811 The gen_statem keeps the current state, or does a state
812 transition to the current state if you like, sets NewData,
813 and executes all Actions. This is the same as
814 {next_state,CurrentState,NewData,Actions}.
815 keep_state_and_data:
817 The gen_statem keeps the current state or does a state tran‐
818 sition to the current state if you like, keeps the current
819 server data, and executes all Actions. This is the same as
820 {next_state,CurrentState,CurrentData,Actions}.
821 repeat_state:
823 The gen_statem keeps the current state, or does a state
824 transition to the current state if you like, sets NewData,
825 and executes all Actions. If the gen_statem runs with state
826 enter calls, the state enter call is repeated, see type
827 transition_option(), otherwise repeat_state is the same as
828 keep_state.
829 repeat_state_and_data:
831 The gen_statem keeps the current state and data, or does a
832 state transition to the current state if you like, and exe‐
833 cutes all Actions. This is the same as {repeat_state,Cur‐
834 rentData,Actions}. If the gen_statem runs with state enter
835 calls, the state enter call is repeated, see type transi‐
836 tion_option(), otherwise repeat_state_and_data is the same
837 as keep_state_and_data.
838 stop:
840 Terminates the gen_statem by calling Module:terminate/3 with
841 Reason and NewData, if specified.
842 stop_and_reply:
844 Sends all Replies, then terminates the gen_statem by calling
845 Module:terminate/3 with Reason and NewData, if specified.
846 All these terms are tuples or atoms and this property will hold
848 in any future version of gen_statem.
849
851 call(ServerRef :: server_ref(), Request :: term()) ->
852 Reply :: term()
853 call(ServerRef :: server_ref(),
855 Request :: term(),
856 Timeout ::
857 timeout() |
858 {clean_timeout, T :: timeout()} |
859 {dirty_timeout, T :: timeout()}) ->
860 Reply :: term()
861 Makes a synchronous call to the gen_statem ServerRef by sending
863 a request and waiting until its reply arrives. The gen_statem
864 calls the state callback with event_type() {call,From} and event
865 content Request.
866 A Reply is generated when a state callback returns with
868 {reply,From,Reply} as one action(), and that Reply becomes the
869 return value of this function.
870 Timeout is an integer > 0, which specifies how many milliseconds
872 to wait for a reply, or the atom infinity to wait indefinitely,
873 which is the default. If no reply is received within the speci‐
874 fied time, the function call fails.
875 Note:
877 For Timeout < infinity, to avoid getting a late reply in the
878 caller's inbox if the caller should catch exceptions, this func‐
879 tion spawns a proxy process that does the call. A late reply
880 gets delivered to the dead proxy process, hence gets discarded.
881 This is less efficient than using Timeout == infinity.
882 Timeout can also be a tuple {clean_timeout,T} or {dirty_time‐
885 out,T}, where T is the time-out time. {clean_timeout,T} works
886 like just T described in the note above and uses a proxy process
887 while {dirty_timeout,T} bypasses the proxy process which is more
888 lightweight.
889 Note:
891 If you combine catching exceptions from this function with
892 {dirty_timeout,T} to avoid that the calling process dies when
893 the call times out, you will have to be prepared to handle a
894 late reply. Note that there is an odd chance to get a late reply
895 even with {dirty_timeout,infinity} or infinity for example in
896 the event of network problems. So why not just let the calling
897 process die by not catching the exception?
898 The call can also fail, for example, if the gen_statem dies
901 before or during this function call.
902 cast(ServerRef :: server_ref(), Msg :: term()) -> ok
904 Sends an asynchronous event to the gen_statem ServerRef and
906 returns ok immediately, ignoring if the destination node or
907 gen_statem does not exist. The gen_statem calls the state call‐
908 back with event_type() cast and event content Msg.
909 enter_loop(Module :: module(),
911 Opts :: [debug_opt() | hibernate_after_opt()],
912 State :: state(),
913 Data :: data()) ->
914 no_return()
915 The same as enter_loop/6 with Actions = [] except that no
917 server_name() must have been registered. This creates an anony‐
918 mous server.
919 enter_loop(Module :: module(),
921 Opts :: [debug_opt() | hibernate_after_opt()],
922 State :: state(),
923 Data :: data(),
924 Server_or_Actions :: server_name() | pid() | [action()]) ->
925 no_return()
926 If Server_or_Actions is a list(), the same as enter_loop/6
928 except that no server_name() must have been registered and
929 Actions = Server_or_Actions. This creates an anonymous server.
930 Otherwise the same as enter_loop/6 with Server =
932 Server_or_Actions and Actions = [].
933 enter_loop(Module :: module(),
935 Opts :: [debug_opt() | hibernate_after_opt()],
936 State :: state(),
937 Data :: data(),
938 Server :: server_name() | pid(),
939 Actions :: [action()] | action()) ->
940 no_return()
941 Makes the calling process become a gen_statem. Does not return,
943 instead the calling process enters the gen_statem receive loop
944 and becomes a gen_statem server. The process must have been
945 started using one of the start functions in proc_lib. The user
946 is responsible for any initialization of the process, including
947 registering a name for it.
948 This function is useful when a more complex initialization pro‐
950 cedure is needed than the gen_statem behavior provides.
951 Module, Opts have the same meaning as when calling
953 start[_link]/3,4.
954 If Server is self() an anonymous server is created just as when
956 using start[_link]/3. If Server is a server_name() a named
957 server is created just as when using start[_link]/4. However,
958 the server_name() name must have been registered accordingly
959 before this function is called.
960 State, Data, and Actions have the same meanings as in the return
962 value of Module:init/1. Also, the callback module does not need
963 to export a Module:init/1 function.
964 The function fails if the calling process was not started by a
966 proc_lib start function, or if it is not registered according to
967 server_name().
968 reply(Replies :: [reply_action()] | reply_action()) -> ok
970 reply(From :: from(), Reply :: term()) -> ok
972 This function can be used by a gen_statem to explicitly send a
974 reply to a process that waits in call/2 when the reply cannot be
975 defined in the return value of a state callback.
976 From must be the term from argument {call,From} to the state
978 callback. A reply or multiple replies canalso be sent using one
979 or several reply_action()s from a state callback.
980 Note:
982 A reply sent with this function is not visible in sys debug out‐
983 put.
984 start(Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
987 start_ret()
988 start(ServerName :: server_name(),
990 Module :: module(),
991 Args :: term(),
992 Opts :: [start_opt()]) ->
993 start_ret()
994 Creates a standalone gen_statem process according to OTP design
996 principles (using proc_lib primitives). As it does not get
997 linked to the calling process, this start function cannot be
998 used by a supervisor to start a child.
999 For a description of arguments and return values, see
1001 start_link/3,4.
1002 start_link(Module :: module(),
1004 Args :: term(),
1005 Opts :: [start_opt()]) ->
1006 start_ret()
1007 start_link(ServerName :: server_name(),
1009 Module :: module(),
1010 Args :: term(),
1011 Opts :: [start_opt()]) ->
1012 start_ret()
1013 Creates a gen_statem process according to OTP design principles
1015 (using proc_lib primitives) that is linked to the calling
1016 process. This is essential when the gen_statem must be part of a
1017 supervision tree so it gets linked to its supervisor.
1018 The gen_statem process calls Module:init/1 to initialize the
1020 server. To ensure a synchronized startup procedure,
1021 start_link/3,4 does not return until Module:init/1 has returned.
1022 ServerName specifies the server_name() to register for the
1024 gen_statem. If the gen_statem is started with start_link/3, no
1025 ServerName is provided and the gen_statem is not registered.
1026 Module is the name of the callback module.
1028 Args is an arbitrary term that is passed as the argument to Mod‐
1030 ule:init/1.
1031 * If option {timeout,Time} is present in Opts, the gen_statem
1033 is allowed to spend Time milliseconds initializing or it
1034 terminates and the start function returns {error,timeout}.
1035 * If option{hibernate_after,HibernateAfterTimeout} is present,
1037 the gen_statem process awaits any message for Hiber‐
1038 nateAfterTimeout milliseconds and if no message is received,
1039 the process goes into hibernation automatically (by calling
1040 proc_lib:hibernate/3).
1041 * If option {debug,Dbgs} is present in Opts, debugging through
1043 sys is activated.
1044 * If option {spawn_opt,SpawnOpts} is present in Opts,
1046 SpawnOpts is passed as option list to erlang:spawn_opt/2,
1047 which is used to spawn the gen_statem process.
1048 Note:
1050 Using spawn option monitor is not allowed, it causes this func‐
1051 tion to fail with reason badarg.
1052 If the gen_statem is successfully created and initialized, this
1055 function returns {ok,Pid}, where Pid is the pid() of the
1056 gen_statem. If a process with the specified ServerName exists
1057 already, this function returns {error,{already_started,Pid}},
1058 where Pid is the pid() of that process.
1059 If Module:init/1 fails with Reason, this function returns
1061 {error,Reason}. If Module:init/1 returns {stop,Reason} or
1062 ignore, the process is terminated and this function returns
1063 {error,Reason} or ignore, respectively.
1064 stop(ServerRef :: server_ref()) -> ok
1066 The same as stop(ServerRef, normal, infinity).
1068 stop(ServerRef :: server_ref(),
1070 Reason :: term(),
1071 Timeout :: timeout()) ->
1072 ok
1073 Orders the gen_statem ServerRef to exit with the specified Rea‐
1075 son and waits for it to terminate. The gen_statem calls Mod‐
1076 ule:terminate/3 before exiting.
1077 This function returns ok if the server terminates with the
1079 expected reason. Any other reason than normal, shutdown, or
1080 {shutdown,Term} causes an error report to be issued through log‐
1081 ger(3). The default Reason is normal.
1082 Timeout is an integer > 0, which specifies how many milliseconds
1084 to wait for the server to terminate, or the atom infinity to
1085 wait indefinitely. Defaults to infinity. If the server does not
1086 terminate within the specified time, a timeout exception is
1087 raised.
1088 If the process does not exist, a noproc exception is raised.
1090
1092 The following functions are to be exported from a gen_statem callback
1093 module.
1094
1096 Module:callback_mode() -> CallbackMode
1097 Types:
1099 CallbackMode = callback_mode() | [ callback_mode() |
1101 state_enter() ]
1102 This function is called by a gen_statem when it needs to find
1104 out the callback mode of the callback module. The value is
1105 cached by gen_statem for efficiency reasons, so this function is
1106 only called once after server start and after code change, but
1107 before the first state callback in the current code version is
1108 called. More occasions may be added in future versions of
1109 gen_statem.
1110 Server start happens either when Module:init/1 returns or when
1112 enter_loop/4-6 is called. Code change happens when Mod‐
1113 ule:code_change/4 returns.
1114 The CallbackMode is either just callback_mode() or a list con‐
1116 taining callback_mode() and possibly the atom state_enter.
1117 Note:
1119 If this function's body does not return an inline constant value
1120 the callback module is doing something strange.
1121 Module:code_change(OldVsn, OldState, OldData, Extra) -> Result
1124 Types:
1126 OldVsn = Vsn | {down,Vsn}
1128 Vsn = term()
1129 OldState = NewState = term()
1130 Extra = term()
1131 Result = {ok,NewState,NewData} | Reason
1132 OldState = NewState = state()
1133 OldData = NewData = data()
1134 Reason = term()
1135 Note:
1137 This callback is optional, so callback modules need not export
1138 it. If a release upgrade/downgrade with Change={advanced,Extra}
1139 specified in the .appup file is made when code_change/4 is not
1140 implemented the process will crash with exit reason undef.
1141 This function is called by a gen_statem when it is to update its
1144 internal state during a release upgrade/downgrade, that is, when
1145 the instruction {update,Module,Change,...}, where
1146 Change={advanced,Extra}, is specified in the appup file. For
1147 more information, see OTP Design Principles.
1148 For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is
1150 {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old
1151 version of the callback module Module. If no such attribute is
1152 defined, the version is the checksum of the Beam file.
1153 OldState and OldData is the internal state of the gen_statem.
1155 Extra is passed "as is" from the {advanced,Extra} part of the
1157 update instruction.
1158 If successful, the function must return the updated internal
1160 state in an {ok,NewState,NewData} tuple.
1161 If the function returns a failure Reason, the ongoing upgrade
1163 fails and rolls back to the old release. Note that Reason can
1164 not be an {ok,_,_} tuple since that will be regarded as a
1165 {ok,NewState,NewData} tuple, and that a tuple matching {ok,_} is
1166 an also invalid failure Reason. It is recommended to use an atom
1167 as Reason since it will be wrapped in an {error,Reason} tuple.
1168 Also note when upgrading a gen_statem, this function and hence
1170 the Change={advanced,Extra} parameter in the appup file is not
1171 only needed to update the internal state or to act on the Extra
1172 argument. It is also needed if an upgrade or downgrade should
1173 change callback mode, or else the callback mode after the code
1174 change will not be honoured, most probably causing a server
1175 crash.
1176 Module:init(Args) -> Result(StateType)
1178 Types:
1180 Args = term()
1182 Result(StateType) = init_result(StateType)
1183 Whenever a gen_statem is started using start_link/3,4 or
1185 start/3,4, this function is called by the new process to ini‐
1186 tialize the implementation state and server data.
1187 Args is the Args argument provided to that start function.
1189 Note:
1191 Note that if the gen_statem is started through proc_lib and
1192 enter_loop/4-6, this callback will never be called. Since this
1193 callback is not optional it can in that case be implemented as:
1194 init(Args) -> erlang:error(not_implemented, [Args]).
1196 Module:format_status(Opt, [PDict,State,Data]) -> Status
1199 Types:
1201 Opt = normal | terminate
1203 PDict = [{Key, Value}]
1204 State = state()
1205 Data = data()
1206 Key = term()
1207 Value = term()
1208 Status = term()
1209 Note:
1211 This callback is optional, so a callback module does not need to
1212 export it. The gen_statem module provides a default implementa‐
1213 tion of this function that returns {State,Data}.
1214 If this callback is exported but fails, to hide possibly sensi‐
1216 tive data, the default function will instead return
1217 {State,Info}, where Info says nothing but the fact that for‐
1218 mat_status/2 has crashed.
1219 This function is called by a gen_statem process when any of the
1222 following apply:
1223 *
1225 One of sys:get_status/1,2 is invoked to get the gen_statem
1226 status. Opt is set to the atom normal for this case.
1227 *
1229 The gen_statem terminates abnormally and logs an error. Opt
1230 is set to the atom terminate for this case.
1231 This function is useful for changing the form and appearance of
1233 the gen_statem status for these cases. A callback module wishing
1234 to change the sys:get_status/1,2 return value and how its status
1235 appears in termination error logs exports an instance of for‐
1236 mat_status/2, which returns a term describing the current status
1237 of the gen_statem.
1238 PDict is the current value of the process dictionary of the
1240 gen_statem.
1241 State is the internal state of the gen_statem.
1243 Data is the internal server data of the gen_statem.
1245 The function is to return Status, a term that contains the
1247 appropriate details of the current state and status of the
1248 gen_statem. There are no restrictions on the form Status can
1249 take, but for the sys:get_status/1,2 case (when Opt is normal),
1250 the recommended form for the Status value is [{data, [{"State",
1251 Term}]}], where Term provides relevant details of the gen_statem
1252 state. Following this recommendation is not required, but it
1253 makes the callback module status consistent with the rest of the
1254 sys:get_status/1,2 return value.
1255 One use for this function is to return compact alternative state
1257 representations to avoid having large state terms printed in log
1258 files. Another use is to hide sensitive data from being written
1259 to the error log.
1260 Module:StateName(enter, OldState, Data) -> StateEnterResult(StateName)
1262 Module:StateName(EventType, EventContent, Data) -> StateFunctionResult
1263 Module:handle_event(enter, OldState, State, Data) -> StateEnterRe‐
1264 sult(State)
1265 Module:handle_event(EventType, EventContent, State, Data) -> Han‐
1266 dleEventResult
1267 Types:
1269 EventType = event_type()
1271 EventContent = term()
1272 State = state()
1273 Data = NewData = data()
1274 StateEnterResult(StateName) = state_enter_result(StateName)
1275 StateFunctionResult = event_handler_result(state_name())
1276 StateEnterResult(State) = state_enter_result(State)
1277 HandleEventResult = event_handler_result(state())
1278 Whenever a gen_statem receives an event from call/2, cast/2, or
1280 as a normal process message, one of these functions is called.
1281 If callback mode is state_functions, Module:StateName/3 is
1282 called, and if it is handle_event_function, Module:han‐
1283 dle_event/4 is called.
1284 If EventType is {call,From}, the caller waits for a reply. The
1286 reply can be sent from this or from any other state callback by
1287 returning with {reply,From,Reply} in Actions, in Replies, or by
1288 calling reply(From, Reply).
1289 If this function returns with a next state that does not match
1291 equal (=/=) to the current state, all postponed events are
1292 retried in the next state.
1293 The only difference between StateFunctionResult and HandleEven‐
1295 tResult is that for StateFunctionResult the next state must be
1296 an atom, but for HandleEventResult there is no restriction on
1297 the next state.
1298 For options that can be set and actions that can be done by
1300 gen_statem after returning from this function, see action().
1301 When the gen_statem runs with state enter calls, these functions
1303 are also called with arguments (enter, OldState, ...) whenever
1304 the state changes. In this case there are some restrictions on
1305 the actions that may be returned: postpone() is not allowed
1306 since a state enter call is not an event so there is no event to
1307 postpone, and {next_event,_,_} is not allowed since using state
1308 enter calls should not affect how events are consumed and pro‐
1309 duced. You may also not change states from this call. Should you
1310 return {next_state,NextState, ...} with NextState =/= State the
1311 gen_statem crashes. Note that it is actually allowed to use
1312 {repeat_state, NewData, ...} although it makes little sense
1313 since you immediately will be called again with a new state
1314 enter call making this just a weird way of looping, and there
1315 are better ways to loop in Erlang. If you do not update NewData
1316 and have some loop termination condition, or if you use
1317 {repeat_state_and_data, _} or repeat_state_and_data you have an
1318 infinite loop! You are advised to use {keep_state,...},
1319 {keep_state_and_data,_} or keep_state_and_data since changing
1320 states from a state enter call is not possible anyway.
1321 Note the fact that you can use throw to return the result, which
1323 can be useful. For example to bail out with
1324 throw(keep_state_and_data) from deep within complex code that
1325 can not return {next_state,State,Data} because State or Data is
1326 no longer in scope.
1327 Module:terminate(Reason, State, Data) -> Ignored
1329 Types:
1331 Reason = normal | shutdown | {shutdown,term()} | term()
1333 State = state()
1334 Data = data()
1335 Ignored = term()
1336 Note:
1338 This callback is optional, so callback modules need not export
1339 it. The gen_statem module provides a default implementation
1340 without cleanup.
1341 This function is called by a gen_statem when it is about to ter‐
1344 minate. It is to be the opposite of Module:init/1 and do any
1345 necessary cleaning up. When it returns, the gen_statem termi‐
1346 nates with Reason. The return value is ignored.
1347 Reason is a term denoting the stop reason and State is the
1349 internal state of the gen_statem.
1350 Reason depends on why the gen_statem is terminating. If it is
1352 because another callback function has returned, a stop tuple
1353 {stop,Reason} in Actions, Reason has the value specified in that
1354 tuple. If it is because of a failure, Reason is the error rea‐
1355 son.
1356 If the gen_statem is part of a supervision tree and is ordered
1358 by its supervisor to terminate, this function is called with
1359 Reason = shutdown if both the following conditions apply:
1360 * The gen_statem has been set to trap exit signals.
1362 * The shutdown strategy as defined in the supervisor's child
1364 specification is an integer time-out value, not brutal_kill.
1365 Even if the gen_statem is not part of a supervision tree, this
1367 function is called if it receives an 'EXIT' message from its
1368 parent. Reason is the same as in the 'EXIT' message.
1369 Otherwise, the gen_statem is immediately terminated.
1371 Notice that for any other reason than normal, shutdown, or
1373 {shutdown,Term}, the gen_statem is assumed to terminate because
1374 of an error and an error report is issued using logger(3).
1375
1377 gen_event(3), gen_fsm(3), gen_server(3), proc_lib(3), supervisor(3),
1378 sys(3).
1379Ericsson AB stdlib 3.8.2.1 gen_statem(3)