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 that for new code
10 replaces its predecessor gen_fsm since Erlang/OTP 20.0. The gen_fsm be‐
11 haviour remains in OTP "as is".
12 Note:
14 If you are new to gen_statem and want an overview of concepts and oper‐
15 ation the section gen_statem Behaviour located in the User's Guide
16 OTP Design Principles is recommended to read before this reference
17 manual, possibly after the Description section you are reading here.
18 This reference manual contains type descriptions generated from types
21 in the gen_statem source code, so they are correct. However, the gener‐
22 ated descriptions also reflect the type hierarchy, which sometimes
23 makes it hard to get a good overview. If so, see the section gen_statem
24 Behaviour in the OTP Design Principles User's Guide.
25 Note:
27 * This behavior appeared in Erlang/OTP 19.0.
29 *
31 In OTP 19.1 a backwards incompatible change of the return tuple
32 from Module:init/1 was made and the mandatory callback function
33 Module:callback_mode/0 was introduced.
34 *
36 In OTP 20.0 generic time-outs were added.
37 *
39 In OTP 22.1 time-out content update and explicit time-out cancel
40 were added.
41 *
43 In OTP 22.3 the possibility to change the callback module with
44 actions change_callback_module, push_callback_module and pop_call‐
45 back_module, was added.
46 gen_statem has got the same features that gen_fsm had and adds some
48 really useful:
49 * Co-located state code
51 * Arbitrary term state
53 * Event postponing
55 * Self-generated events
57 * State time-out
59 * Multiple generic named time-outs
61 * Absolute time-out time
63 * Automatic state enter calls
65 *
67 Reply from other state than the request, sys traceable
68 * Multiple sys traceable replies
70 * Changing the callback module
72 Two callback modes are supported:
74 * One for finite-state machines (gen_fsm like), which requires the
76 state to be an atom and uses that state as the name of the current
77 callback function.
78 * One that allows the state to be any term and that uses one callback
80 function for all states.
81 The callback model(s) for gen_statem differs from the one for gen_fsm,
83 but it is still fairly easy to rewrite from gen_fsm to gen_statem.
84 A generic state machine server process (gen_statem) implemented using
86 this module has a standard set of interface functions and includes
87 functionality for tracing and error reporting. It also fits into an OTP
88 supervision tree. For more information, see OTP Design Principles.
89 A gen_statem assumes all specific parts to be located in a callback
91 module exporting a predefined set of functions. The relationship
92 between the behavior functions and the callback functions is as fol‐
93 lows:
94 gen_statem module Callback module
96 ----------------- ---------------
97 gen_statem:start
98 gen_statem:start_link -----> Module:init/1
99 Server start or code change
101 -----> Module:callback_mode/0
102 gen_statem:stop -----> Module:terminate/3
104 gen_statem:call
106 gen_statem:cast
107 erlang:send
108 erlang:'!' -----> Module:StateName/3
109 Module:handle_event/4
110 - -----> Module:terminate/3
112 - -----> Module:code_change/4
114 Events are of different types, so the callback functions can know the
116 origin of an event and how to respond.
117 If a callback function fails or returns a bad value, the gen_statem
119 terminates, unless otherwise stated. However, an exception of class
120 throw is not regarded as an error but as a valid return from all call‐
121 back functions.
122 The state callback for a specific state in a gen_statem is the callback
124 function that is called for all events in this state. It is selected
125 depending on which callback mode that the callback module defines with
126 the callback function Module:callback_mode/0.
127 When the callback mode is state_functions, the state must be an atom
129 and is used as the state callback name; see Module:StateName/3. This
130 co-locates all code for a specific state in one function as the
131 gen_statem engine branches depending on state name. Note the fact that
132 the callback function Module:terminate/3 makes the state name terminate
133 unusable in this mode.
134 When the callback mode is handle_event_function, the state can be any
136 term and the state callback name is Module:handle_event/4. This makes
137 it easy to branch depending on state or event as you desire. Be careful
138 about which events you handle in which states so that you do not acci‐
139 dentally postpone an event forever creating an infinite busy loop.
140 When gen_statem receives a process message it is converted into an
142 event and the state callback is called with the event as two arguments:
143 type and content. When the state callback has processed the event it
144 returns to gen_statem which does a state transition. If this state
145 transition is to a different state, that is: NextState =/= State, it is
146 a state change.
147 The state callback may return transition actions for gen_statem to exe‐
149 cute during the state transition, for example to reply to a
150 gen_statem:call/2,3.
151 One of the possible transition actions is to postpone the current
153 event. Then it is not retried in the current state. The gen_statem
154 engine keeps a queue of events divided into the postponed events and
155 the events still to process. After a state change the queue restarts
156 with the postponed events.
157 The gen_statem event queue model is sufficient to emulate the normal
159 process message queue with selective receive. Postponing an event cor‐
160 responds to not matching it in a receive statement, and changing states
161 corresponds to entering a new receive statement.
162 The state callback can insert events using the transition actions
164 next_event and such an event is inserted in the event queue as the next
165 to call the state callback with. That is, as if it is the oldest incom‐
166 ing event. A dedicated event_type() internal can be used for such
167 events making them impossible to mistake for external events.
168 Inserting an event replaces the trick of calling your own state han‐
170 dling functions that you often would have to resort to in, for example,
171 gen_fsm to force processing an inserted event before others.
172 The gen_statem engine can automatically make a specialized call to the
174 state callback whenever a new state is entered; see state_enter(). This
175 is for writing code common to all state entries. Another way to do it
176 is to explicitly insert an event at the state transition, and/or to use
177 a dedicated state transition function, but that is something you will
178 have to remember at every state transition to the state(s) that need
179 it.
180 Note:
182 If you in gen_statem, for example, postpone an event in one state and
183 then call another state callback of yours, you have not done a state
184 change and hence the postponed event is not retried, which is logical
185 but can be confusing.
186 For the details of a state transition, see type transition_option().
189 A gen_statem handles system messages as described in sys. The sys mod‐
191 ule can be used for debugging a gen_statem.
192 Notice that a gen_statem does not trap exit signals automatically, this
194 must be explicitly initiated in the callback module (by calling
195 process_flag(trap_exit, true).
196 Unless otherwise stated, all functions in this module fail if the spec‐
198 ified gen_statem does not exist or if bad arguments are specified.
199 The gen_statem process can go into hibernation; see proc_lib:hiber‐
201 nate/3. It is done when a state callback or Module:init/1 specifies
202 hibernate in the returned Actions list. This feature can be useful to
203 reclaim process heap memory while the server is expected to be idle for
204 a long time. However, use this feature with care, as hibernation can be
205 too costly to use after every event; see erlang:hibernate/3.
206 There is also a server start option {hibernate_after, Timeout} for
208 start/3,4, start_link/3,4 or enter_loop/4,5,6, that may be used to
209 automatically hibernate the server.
210
212 The following example shows a simple pushbutton model for a toggling
213 pushbutton implemented with callback mode state_functions. You can push
214 the button and it replies if it went on or off, and you can ask for a
215 count of how many times it has been pushed to switch on.
216 The following is the complete callback module file pushbutton.erl:
218 -module(pushbutton).
220 -behaviour(gen_statem).
221 -export([start/0,push/0,get_count/0,stop/0]).
223 -export([terminate/3,code_change/4,init/1,callback_mode/0]).
224 -export([on/3,off/3]).
225 name() -> pushbutton_statem. % The registered server name
227 %% API. This example uses a registered name name()
229 %% and does not link to the caller.
230 start() ->
231 gen_statem:start({local,name()}, ?MODULE, [], []).
232 push() ->
233 gen_statem:call(name(), push).
234 get_count() ->
235 gen_statem:call(name(), get_count).
236 stop() ->
237 gen_statem:stop(name()).
238 %% Mandatory callback functions
240 terminate(_Reason, _State, _Data) ->
241 void.
242 code_change(_Vsn, State, Data, _Extra) ->
243 {ok,State,Data}.
244 init([]) ->
245 %% Set the initial state + data. Data is used only as a counter.
246 State = off, Data = 0,
247 {ok,State,Data}.
248 callback_mode() -> state_functions.
249 %%% state callback(s)
251 off({call,From}, push, Data) ->
253 %% Go to 'on', increment count and reply
254 %% that the resulting status is 'on'
255 {next_state,on,Data+1,[{reply,From,on}]};
256 off(EventType, EventContent, Data) ->
257 handle_event(EventType, EventContent, Data).
258 on({call,From}, push, Data) ->
260 %% Go to 'off' and reply that the resulting status is 'off'
261 {next_state,off,Data,[{reply,From,off}]};
262 on(EventType, EventContent, Data) ->
263 handle_event(EventType, EventContent, Data).
264 %% Handle events common to all states
266 handle_event({call,From}, get_count, Data) ->
267 %% Reply with the current count
268 {keep_state,Data,[{reply,From,Data}]};
269 handle_event(_, _, Data) ->
270 %% Ignore all other events
271 {keep_state,Data}.
272 The following is a shell session when running it:
275 1> pushbutton:start().
277 {ok,<0.36.0>}
278 2> pushbutton:get_count().
279 0
280 3> pushbutton:push().
281 on
282 4> pushbutton:get_count().
283 1
284 5> pushbutton:push().
285 off
286 6> pushbutton:get_count().
287 1
288 7> pushbutton:stop().
289 ok
290 8> pushbutton:push().
291 ** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
292 in function gen:do_for_proc/2 (gen.erl, line 261)
293 in call from gen_statem:call/3 (gen_statem.erl, line 386)
294 To compare styles, here follows the same example using callback mode
297 handle_event_function, or rather the code to replace after function
298 init/1 of the pushbutton.erl example file above:
299 callback_mode() -> handle_event_function.
301 %%% state callback(s)
303 handle_event({call,From}, push, off, Data) ->
305 %% Go to 'on', increment count and reply
306 %% that the resulting status is 'on'
307 {next_state,on,Data+1,[{reply,From,on}]};
308 handle_event({call,From}, push, on, Data) ->
309 %% Go to 'off' and reply that the resulting status is 'off'
310 {next_state,off,Data,[{reply,From,off}]};
311 %%
312 %% Event handling common to all states
313 handle_event({call,From}, get_count, State, Data) ->
314 %% Reply with the current count
315 {next_state,State,Data,[{reply,From,Data}]};
316 handle_event(_, _, State, Data) ->
317 %% Ignore all other events
318 {next_state,State,Data}.
319
322 server_name() =
323 {global, GlobalName :: term()} |
324 {via, RegMod :: module(), Name :: term()} |
325 {local, atom()}
326 Name specification to use when starting a gen_statem server. See
328 start_link/3 and server_ref() below.
329 server_ref() =
331 pid() |
332 (LocalName :: atom()) |
333 {Name :: atom(), Node :: atom()} |
334 {global, GlobalName :: term()} |
335 {via, RegMod :: module(), ViaName :: term()}
336 Server specification to use when addressing a gen_statem server.
338 See call/2 and server_name() above.
339 It can be:
341 pid() | LocalName:
343 The gen_statem is locally registered.
344 {Name,Node}:
346 The gen_statem is locally registered on another node.
347 {global,GlobalName}:
349 The gen_statem is globally registered in global.
350 {via,RegMod,ViaName}:
352 The gen_statem is registered in an alternative process reg‐
353 istry. The registry callback module RegMod is to export
354 functions register_name/2, unregister_name/1,
355 whereis_name/1, and send/2, which are to behave like the
356 corresponding functions in global. Thus, {via,global,Global‐
357 Name} is the same as {global,GlobalName}.
358 start_opt() =
360 {timeout, Time :: timeout()} |
361 {spawn_opt, [proc_lib:spawn_option()]} |
362 enter_loop_opt()
363 Options that can be used when starting a gen_statem server
365 through, for example, start_link/3.
366 start_ret() = {ok, pid()} | ignore | {error, term()}
368 Return value from the start functions, for example,
370 start_link/3.
371 enter_loop_opt() =
373 {hibernate_after, HibernateAfterTimeout :: timeout()} |
374 {debug, Dbgs :: [sys:debug_option()]}
375 Options that can be used when starting a gen_statem server
377 through, enter_loop/4-6.
378 hibernate_after:
380 HibernateAfterTimeout specifies that the gen_statem process
381 awaits any message for HibernateAfterTimeout milliseconds
382 and if no message is received, the process goes into hiber‐
383 nation automatically (by calling proc_lib:hibernate/3).
384 debug:
386 For every entry in Dbgs, the corresponding function in sys
387 is called.
388 from() = {To :: pid(), Tag :: term()}
390 Destination to use when replying through, for example, the
392 action() {reply,From,Reply} to a process that has called the
393 gen_statem server using call/2.
394 state() = state_name() | term()
396 If the callback mode is handle_event_function, the state can be
398 any term. After a state change (NextState =/= State), all post‐
399 poned events are retried.
400 state_name() = atom()
402 If the callback mode is state_functions, the state must be an
404 atom. After a state change (NextState =/= State), all postponed
405 events are retried. Note that the state terminate is not possi‐
406 ble to use since it would collide with the optional callback
407 function Module:terminate/3.
408 data() = term()
410 A term in which the state machine implementation is to store any
412 server data it needs. The difference between this and the
413 state() itself is that a change in this data does not cause
414 postponed events to be retried. Hence, if a change in this data
415 would change the set of events that are handled, then that data
416 item is to be made a part of the state.
417 event_type() =
419 external_event_type() | timeout_event_type() | internal
420 There are 3 categories of events: external, timeout, and inter‐
422 nal.
423 internal events can only be generated by the state machine
425 itself through the transition action next_event.
426 external_event_type() = {call, From :: from()} | cast | info
428 External events are of 3 types: {call,From}, cast, or info.
430 Calls (synchronous) and casts originate from the corresponding
431 API functions. For calls, the event contains whom to reply to.
432 Type info originates from regular process messages sent to the
433 gen_statem.
434 timeout_event_type() =
436 timeout | {timeout, Name :: term()} | state_timeout
437 There are 3 types of time-out events that the state machine can
439 generate for itself with the corresponding timeout_action()s.
440 callback_mode_result() =
442 callback_mode() | [callback_mode() | state_enter()]
443 This is the return type from Module:callback_mode/0 and selects
445 callback mode and whether to do state enter calls, or not.
446 callback_mode() = state_functions | handle_event_function
448 The callback mode is selected with the return value from Mod‐
450 ule:callback_mode/0:
451 state_functions:
453 The state must be of type state_name() and one callback
454 function per state, that is, Module:StateName/3, is used.
455 handle_event_function:
457 The state can be any term and the callback function Mod‐
458 ule:handle_event/4 is used for all states.
459 The function Module:callback_mode/0 is called when starting the
461 gen_statem, after code change and after changing the callback
462 module with any of the actions change_callback_module,
463 push_callback_module or pop_callback_module. The result is
464 cached for subsequent calls to state callbacks.
465 state_enter() = state_enter
467 Whether the state machine should use state enter calls or not is
469 selected when starting the gen_statem and after code change
470 using the return value from Module:callback_mode/0.
471 If Module:callback_mode/0 returns a list containing state_enter,
473 the gen_statem engine will, at every state change, call the
474 state callback with arguments (enter, OldState, Data) or (enter,
475 OldState, State, Data), depending on the callback mode. This may
476 look like an event but is really a call performed after the pre‐
477 vious state callback returned and before any event is delivered
478 to the new state callback. See Module:StateName/3 and Mod‐
479 ule:handle_event/4. Such a call can be repeated by returning a
480 repeat_state or repeat_state_and_data tuple from the state call‐
481 back.
482 If Module:callback_mode/0 does not return such a list, no state
484 enter calls are done.
485 If Module:code_change/4 should transform the state, it is
487 regarded as a state rename and not a state change, which will
488 not cause a state enter call.
489 Note that a state enter call will be done right before entering
491 the initial state even though this actually is not a state
492 change. In this case OldState =:= State, which cannot happen for
493 a subsequent state change, but will happen when repeating the
494 state enter call.
495 transition_option() =
497 postpone() |
498 hibernate() |
499 event_timeout() |
500 generic_timeout() |
501 state_timeout()
502 Transition options can be set by actions and modify the state
504 transition. The state transition takes place when the state
505 callback has processed an event and returns. Here are the
506 sequence of steps for a state transition:
507 * All returned actions are processed in order of appearance.
509 In this step all replies generated by any reply_action() are
510 sent. Other actions set transition_option()s that come into
511 play in subsequent steps.
512 * If state enter calls are used, and either it is the initial
514 state or one of the callback results repeat_state_and_data
515 or repeat_state_and_data is used the gen_statem engine calls
516 the current state callback with arguments (enter, State,
517 Data) or (enter, State, State, Data) (depending on callback
518 mode) and when it returns starts again from the top of this
519 sequence.
520 If state enter calls are used, and the state changes the
522 gen_statem engine calls the new state callback with argu‐
523 ments (enter, OldState, Data) or (enter, OldState, State,
524 Data) (depending on callback mode) and when it returns
525 starts again from the top of this sequence.
526 * If postpone() is true, the current event is postponed.
528 * If this is a state change, the queue of incoming events is
530 reset to start with the oldest postponed.
531 * All events stored with action() next_event are inserted to
533 be processed before previously queued events.
534 * Time-out timers event_timeout(), generic_timeout() and
536 state_timeout() are handled. Time-outs with zero time are
537 guaranteed to be delivered to the state machine before any
538 external not yet received event so if there is such a time-
539 out requested, the corresponding time-out zero event is
540 enqueued as the newest received event; that is after already
541 queued events such as inserted and postponed events.
542 Any event cancels an event_timeout() so a zero time event
544 time-out is only generated if the event queue is empty.
545 A state change cancels a state_timeout() and any new transi‐
547 tion option of this type belongs to the new state, that is;
548 a state_timeout() applies to the state the state machine
549 enters.
550 * If there are enqueued events the state callback for the pos‐
552 sibly new state is called with the oldest enqueued event,
553 and we start again from the top of this sequence.
554 * Otherwise the gen_statem goes into receive or hibernation
556 (if hibernate() is true) to wait for the next message. In
557 hibernation the next non-system event awakens the
558 gen_statem, or rather the next incoming message awakens the
559 gen_statem, but if it is a system event it goes right back
560 into hibernation. When a new message arrives the state call‐
561 back is called with the corresponding event, and we start
562 again from the top of this sequence.
563 postpone() = boolean()
565 If true, postpones the current event and retries it after a
567 state change (NextState =/= State).
568 hibernate() = boolean()
570 If true, hibernates the gen_statem by calling proc_lib:hiber‐
572 nate/3 before going into receive to wait for a new external
573 event.
574 Note:
576 If there are enqueued events to process when hibrnation is
577 requested, this is optimized by not hibernating but instead
578 calling erlang:garbage_collect/0 to simulate that the gen_statem
579 entered hibernation and immediately got awakened by an enqueued
580 event.
581 event_timeout() = timeout() | integer()
584 Starts a timer set by enter_action() timeout. When the timer
586 expires an event of event_type() timeout will be generated. See
587 erlang:start_timer/4 for how Time and Options are interpreted.
588 Future erlang:start_timer/4 Options will not necessarily be sup‐
589 ported.
590 Any event that arrives cancels this time-out. Note that a
592 retried or inserted event counts as arrived. So does a state
593 time-out zero event, if it was generated before this time-out is
594 requested.
595 If Time is infinity, no timer is started, as it never would
597 expire anyway.
598 If Time is relative and 0 no timer is actually started, instead
600 the the time-out event is enqueued to ensure that it gets pro‐
601 cessed before any not yet received external event, but after
602 already queued events.
603 Note that it is not possible nor needed to cancel this time-out,
605 as it is cancelled automatically by any other event.
606 generic_timeout() = timeout() | integer()
608 Starts a timer set by enter_action() {timeout,Name}. When the
610 timer expires an event of event_type() {timeout,Name} will be
611 generated. See erlang:start_timer/4 for how Time and Options are
612 interpreted. Future erlang:start_timer/4 Options will not neces‐
613 sarily be supported.
614 If Time is infinity, no timer is started, as it never would
616 expire anyway.
617 If Time is relative and 0 no timer is actually started, instead
619 the the time-out event is enqueued to ensure that it gets pro‐
620 cessed before any not yet received external event.
621 Setting a timer with the same Name while it is running will
623 restart it with the new time-out value. Therefore it is possible
624 to cancel a specific time-out by setting it to infinity.
625 state_timeout() = timeout() | integer()
627 Starts a timer set by enter_action() state_timeout. When the
629 timer expires an event of event_type() state_timeout will be
630 generated. See erlang:start_timer/4 for how Time and Options are
631 interpreted. Future erlang:start_timer/4 Options will not neces‐
632 sarily be supported.
633 If Time is infinity, no timer is started, as it never would
635 expire anyway.
636 If Time is relative and 0 no timer is actually started, instead
638 the the time-out event is enqueued to ensure that it gets pro‐
639 cessed before any not yet received external event.
640 Setting this timer while it is running will restart it with the
642 new time-out value. Therefore it is possible to cancel this
643 time-out by setting it to infinity.
644 timeout_option() = {abs, Abs :: boolean()}
646 If Abs is true an absolute timer is started, and if it is false
648 a relative, which is the default. See erlang:start_timer/4 for
649 details.
650 action() =
652 postpone |
653 {postpone, Postpone :: postpone()} |
654 {next_event,
655 EventType :: event_type(),
656 EventContent :: term()} |
657 {change_callback_module, NewModule :: module()} |
658 {push_callback_module, NewModule :: module()} |
659 pop_callback_module |
660 enter_action()
661 These transition actions can be invoked by returning them from
663 the state callback when it is called with an event, from Mod‐
664 ule:init/1 or by giving them to enter_loop/5,6.
665 Actions are executed in the containing list order.
667 Actions that set transition options override any previous of
669 the same type, so the last in the containing list wins. For
670 example, the last postpone() overrides any previous postpone()
671 in the list.
672 postpone:
674 Sets the transition_option() postpone() for this state tran‐
675 sition. This action is ignored when returned from Mod‐
676 ule:init/1 or given to enter_loop/5,6, as there is no event
677 to postpone in those cases.
678 next_event:
680 This action does not set any transition_option() but instead
681 stores the specified EventType and EventContent for inser‐
682 tion after all actions have been executed.
683 The stored events are inserted in the queue as the next to
685 process before any already queued events. The order of these
686 stored events is preserved, so the first next_event in the
687 containing list becomes the first to process.
688 An event of type internal is to be used when you want to
690 reliably distinguish an event inserted this way from any
691 external event.
692 change_callback_module:
694 Changes the callback module to NewModule which will be used
695 when calling all subsequent state callbacks.
696 The gen_statem engine will find out the callback mode of
698 NewModule by calling NewModule:callback_mode/0 before the
699 next state callback.
700 Changing the callback module does not affect the state tran‐
702 sition in any way, it only changes which module that handles
703 the events. Be aware that all relevant callback functions in
704 NewModule such as the state callback, NewMod‐
705 ule:code_change/4, NewModule:format_status/2 and NewMod‐
706 ule:terminate/3 must be able to handle the state and data
707 from the old module.
708 push_callback_module
710 : Pushes the current callback module to the top of an inter‐
711 nal stack of callback modules and changes the callback mod‐
712 ule to NewModule. Otherwise like {change_callback_module,
713 NewModule} above.
714 pop_callback_module:
716 Pops the top module from the internal stack of callback
717 modules and changes the callback module to be the popped
718 module. If the stack is empty the server fails. Otherwise
719 like {change_callback_module, NewModule} above.
720 enter_action() =
722 hibernate |
723 {hibernate, Hibernate :: hibernate()} |
724 timeout_action() |
725 reply_action()
726 These transition actions can be invoked by returning them from
728 the state callback, from Module:init/1 or by giving them to
729 enter_loop/5,6.
730 Actions are executed in the containing list order.
732 Actions that set transition options override any previous of the
734 same type, so the last in the containing list wins. For example,
735 the last event_timeout() overrides any previous event_timeout()
736 in the list.
737 hibernate:
739 Sets the transition_option() hibernate() for this state
740 transition.
741 timeout_action() =
743 (Time :: event_timeout()) |
744 {timeout, Time :: event_timeout(), EventContent :: term()} |
745 {timeout,
746 Time :: event_timeout(),
747 EventContent :: term(),
748 Options :: timeout_option() | [timeout_option()]} |
749 {{timeout, Name :: term()},
750 Time :: generic_timeout(),
751 EventContent :: term()} |
752 {{timeout, Name :: term()},
753 Time :: generic_timeout(),
754 EventContent :: term(),
755 Options :: timeout_option() | [timeout_option()]} |
756 {state_timeout,
757 Time :: state_timeout(),
758 EventContent :: term()} |
759 {state_timeout,
760 Time :: state_timeout(),
761 EventContent :: term(),
762 Options :: timeout_option() | [timeout_option()]} |
763 timeout_cancel_action() |
764 timeout_update_action()
765 These transition actions can be invoked by returning them from
767 the state callback, from Module:init/1 or by giving them to
768 enter_loop/5,6.
769 These time-out actions sets time-out transition options.
771 Time:
773 Short for {timeout,Time,Time}, that is, the time-out message
774 is the time-out time. This form exists to make the state
775 callback return value {next_state,NextState,NewData,Time}
776 allowed like for gen_fsm.
777 timeout:
779 Sets the transition_option() event_timeout() to Time with
780 EventContent and time-out options Options.
781 {timeout,Name}:
783 Sets the transition_option() generic_timeout() to Time for
784 Name with EventContent and time-out options Options.
785 state_timeout:
787 Sets the transition_option() state_timeout() to Time with
788 EventContent and time-out options Options.
789 timeout_cancel_action() =
791 {timeout, cancel} |
792 {{timeout, Name :: term()}, cancel} |
793 {state_timeout, cancel}
794 This is a shorter and clearer form of timeout_action() with
796 Time = infinity which cancels a time-out.
797 timeout_update_action() =
799 {timeout, update, EventContent :: term()} |
800 {{timeout, Name :: term()}, update, EventContent :: term()} |
801 {state_timeout, update, EventContent :: term()}
802 Updates a time-out with a new EventContent. See time‐
804 out_action() for how to start a time-out.
805 If no time-out of the same type is active instead insert the
807 time-out event just like when starting a time-out with relative
808 Time = 0.
809 reply_action() = {reply, From :: from(), Reply :: term()}
811 This transition action can be invoked by returning it from the
813 state callback, from Module:init/1 or by giving it to
814 enter_loop/5,6.
815 It does not set any transition_option() but instead replies to a
817 caller waiting for a reply in call/2. From must be the term from
818 argument {call,From} in a call to a state callback.
819 Note that using this action from Module:init/1 or enter_loop/5,6
821 would be weird on the border of witchcraft since there has been
822 no earlier call to a state callback in this server.
823 init_result(StateType) =
825 {ok, State :: StateType, Data :: data()} |
826 {ok,
827 State :: StateType,
828 Data :: data(),
829 Actions :: [action()] | action()} |
830 ignore |
831 {stop, Reason :: term()}
832 For a succesful initialization, State is the initial state() and
834 Data the initial server data() of the gen_statem.
835 The Actions are executed when entering the first state just as
837 for a state callback, except that the action postpone is forced
838 to false since there is no event to postpone.
839 For an unsuccesful initialization, {stop,Reason} or ignore
841 should be used; see start_link/3,4.
842 state_enter_result(State) =
844 {next_state, State, NewData :: data()} |
845 {next_state, State,
846 NewData :: data(),
847 Actions :: [enter_action()] | enter_action()} |
848 state_callback_result(enter_action())
849 State is the current state and it cannot be changed since the
851 state callback was called with a state enter call.
852 next_state:
854 The gen_statem does a state transition to State, which has
855 to be the current state, sets NewData, and executes all
856 Actions.
857 event_handler_result(StateType) =
859 {next_state, NextState :: StateType, NewData :: data()} |
860 {next_state,
861 NextState :: StateType,
862 NewData :: data(),
863 Actions :: [action()] | action()} |
864 state_callback_result(action())
865 StateType is state_name() if callback mode is state_functions,
867 or state() if callback mode is handle_event_function.
868 next_state:
870 The gen_statem does a state transition to NextState (which
871 can be the same as the current state), sets NewData, and
872 executes all Actions. If NextState =/= CurrentState the
873 state transition is a state change.
874 state_callback_result(ActionType) =
876 {keep_state, NewData :: data()} |
877 {keep_state,
878 NewData :: data(),
879 Actions :: [ActionType] | ActionType} |
880 keep_state_and_data |
881 {keep_state_and_data, Actions :: [ActionType] | ActionType} |
882 {repeat_state, NewData :: data()} |
883 {repeat_state,
884 NewData :: data(),
885 Actions :: [ActionType] | ActionType} |
886 repeat_state_and_data |
887 {repeat_state_and_data, Actions :: [ActionType] | ActionType} |
888 stop |
889 {stop, Reason :: term()} |
890 {stop, Reason :: term(), NewData :: data()} |
891 {stop_and_reply,
892 Reason :: term(),
893 Replies :: [reply_action()] | reply_action()} |
894 {stop_and_reply,
895 Reason :: term(),
896 Replies :: [reply_action()] | reply_action(),
897 NewData :: data()}
898 ActionType is enter_action() if the state callback was called
900 with a state enter call and action() if the state callback was
901 called with an event.
902 keep_state:
904 The same as {next_state,CurrentState,NewData,Actions}.
905 keep_state_and_data:
907 The same as {keep_state,CurrentData,Actions}.
908 repeat_state:
910 If the gen_statem runs with state enter calls, the state
911 enter call is repeated, see type transition_option(), other
912 than that repeat_state is the same as keep_state.
913 repeat_state_and_data:
915 The same as {repeat_state,CurrentData,Actions}.
916 stop:
918 Terminates the gen_statem by calling Module:terminate/3 with
919 Reason and NewData, if specified.
920 stop_and_reply:
922 Sends all Replies, then terminates the gen_statem by calling
923 Module:terminate/3 with Reason and NewData, if specified.
924 All these terms are tuples or atoms and this property will hold
926 in any future version of gen_statem.
927
929 call(ServerRef :: server_ref(), Request :: term()) ->
930 Reply :: term()
931 call(ServerRef :: server_ref(),
933 Request :: term(),
934 Timeout ::
935 timeout() |
936 {clean_timeout, T :: timeout()} |
937 {dirty_timeout, T :: timeout()}) ->
938 Reply :: term()
939 Makes a synchronous call to the gen_statem ServerRef by sending
941 a request and waiting until its reply arrives. The gen_statem
942 calls the state callback with event_type() {call,From} and event
943 content Request.
944 A Reply is generated when a state callback returns with
946 {reply,From,Reply} as one action(), and that Reply becomes the
947 return value of this function.
948 Timeout is an integer > 0, which specifies how many milliseconds
950 to wait for a reply, or the atom infinity to wait indefinitely,
951 which is the default. If no reply is received within the speci‐
952 fied time, the function call fails.
953 Note:
955 For Timeout < infinity, to avoid getting a late reply in the
956 caller's inbox if the caller should catch exceptions, this func‐
957 tion spawns a proxy process that does the call. A late reply
958 gets delivered to the dead proxy process, hence gets discarded.
959 This is less efficient than using Timeout == infinity.
960 Timeout can also be a tuple {clean_timeout,T} or {dirty_time‐
963 out,T}, where T is the time-out time. {clean_timeout,T} works
964 like just T described in the note above and uses a proxy process
965 while {dirty_timeout,T} bypasses the proxy process which is more
966 lightweight.
967 Note:
969 If you combine catching exceptions from this function with
970 {dirty_timeout,T} to avoid that the calling process dies when
971 the call times out, you will have to be prepared to handle a
972 late reply. Note that there is an odd chance to get a late reply
973 even with {dirty_timeout,infinity} or infinity for example in
974 the event of network problems. So why not just let the calling
975 process die by not catching the exception?
976 The call can also fail, for example, if the gen_statem dies
979 before or during this function call.
980 cast(ServerRef :: server_ref(), Msg :: term()) -> ok
982 Sends an asynchronous event to the gen_statem ServerRef and
984 returns ok immediately, ignoring if the destination node or
985 gen_statem does not exist. The gen_statem calls the state call‐
986 back with event_type() cast and event content Msg.
987 enter_loop(Module :: module(),
989 Opts :: [enter_loop_opt()],
990 State :: state(),
991 Data :: data()) ->
992 no_return()
993 The same as enter_loop/6 with Actions = [] except that no
995 server_name() must have been registered. This creates an anony‐
996 mous server.
997 enter_loop(Module :: module(),
999 Opts :: [enter_loop_opt()],
1000 State :: state(),
1001 Data :: data(),
1002 Server_or_Actions :: server_name() | pid() | [action()]) ->
1003 no_return()
1004 If Server_or_Actions is a list(), the same as enter_loop/6
1006 except that no server_name() must have been registered and
1007 Actions = Server_or_Actions. This creates an anonymous server.
1008 Otherwise the same as enter_loop/6 with Server =
1010 Server_or_Actions and Actions = [].
1011 enter_loop(Module :: module(),
1013 Opts :: [enter_loop_opt()],
1014 State :: state(),
1015 Data :: data(),
1016 Server :: server_name() | pid(),
1017 Actions :: [action()] | action()) ->
1018 no_return()
1019 Makes the calling process become a gen_statem. Does not return,
1021 instead the calling process enters the gen_statem receive loop
1022 and becomes a gen_statem server. The process must have been
1023 started using one of the start functions in proc_lib. The user
1024 is responsible for any initialization of the process, including
1025 registering a name for it.
1026 This function is useful when a more complex initialization pro‐
1028 cedure is needed than the gen_statem behavior provides.
1029 Module, Opts have the same meaning as when calling
1031 start[_link]/3,4.
1032 If Server is self() an anonymous server is created just as when
1034 using start[_link]/3. If Server is a server_name() a named
1035 server is created just as when using start[_link]/4. However,
1036 the server_name() name must have been registered accordingly
1037 before this function is called.
1038 State, Data, and Actions have the same meanings as in the return
1040 value of Module:init/1. Also, the callback module does not need
1041 to export a Module:init/1 function.
1042 The function fails if the calling process was not started by a
1044 proc_lib start function, or if it is not registered according to
1045 server_name().
1046 reply(Replies :: [reply_action()] | reply_action()) -> ok
1048 reply(From :: from(), Reply :: term()) -> ok
1050 This function can be used by a gen_statem to explicitly send a
1052 reply to a process that waits in call/2 when the reply cannot be
1053 defined in the return value of a state callback.
1054 From must be the term from argument {call,From} to the state
1056 callback. A reply or multiple replies canalso be sent using one
1057 or several reply_action()s from a state callback.
1058 Note:
1060 A reply sent with this function is not visible in sys debug out‐
1061 put.
1062 start(Module :: module(), Args :: term(), Opts :: [start_opt()]) ->
1065 start_ret()
1066 start(ServerName :: server_name(),
1068 Module :: module(),
1069 Args :: term(),
1070 Opts :: [start_opt()]) ->
1071 start_ret()
1072 Creates a standalone gen_statem process according to OTP design
1074 principles (using proc_lib primitives). As it does not get
1075 linked to the calling process, this start function cannot be
1076 used by a supervisor to start a child.
1077 For a description of arguments and return values, see
1079 start_link/3,4.
1080 start_link(Module :: module(),
1082 Args :: term(),
1083 Opts :: [start_opt()]) ->
1084 start_ret()
1085 start_link(ServerName :: server_name(),
1087 Module :: module(),
1088 Args :: term(),
1089 Opts :: [start_opt()]) ->
1090 start_ret()
1091 Creates a gen_statem process according to OTP design principles
1093 (using proc_lib primitives) that is linked to the calling
1094 process. This is essential when the gen_statem must be part of a
1095 supervision tree so it gets linked to its supervisor.
1096 The gen_statem process calls Module:init/1 to initialize the
1098 server. To ensure a synchronized startup procedure,
1099 start_link/3,4 does not return until Module:init/1 has returned.
1100 ServerName specifies the server_name() to register for the
1102 gen_statem. If the gen_statem is started with start_link/3, no
1103 ServerName is provided and the gen_statem is not registered.
1104 Module is the name of the callback module.
1106 Args is an arbitrary term that is passed as the argument to Mod‐
1108 ule:init/1.
1109 * If option {timeout,Time} is present in Opts, the gen_statem
1111 is allowed to spend Time milliseconds initializing or it
1112 terminates and the start function returns {error,timeout}.
1113 * If option {hibernate_after,HibernateAfterTimeout} is
1115 present, the gen_statem process awaits any message for
1116 HibernateAfterTimeout milliseconds and if no message is
1117 received, the process goes into hibernation automatically
1118 (by calling proc_lib:hibernate/3).
1119 * If option {debug,Dbgs} is present in Opts, debugging through
1121 sys is activated.
1122 * If option {spawn_opt,SpawnOpts} is present in Opts,
1124 SpawnOpts is passed as option list to erlang:spawn_opt/2,
1125 which is used to spawn the gen_statem process.
1126 Note:
1128 Using spawn option monitor is not allowed, it causes this func‐
1129 tion to fail with reason badarg.
1130 If the gen_statem is successfully created and initialized, this
1133 function returns {ok,Pid}, where Pid is the pid() of the
1134 gen_statem. If a process with the specified ServerName exists
1135 already, this function returns {error,{already_started,Pid}},
1136 where Pid is the pid() of that process.
1137 If Module:init/1 fails with Reason, this function returns
1139 {error,Reason}. If Module:init/1 returns {stop,Reason} or
1140 ignore, the process is terminated and this function returns
1141 {error,Reason} or ignore, respectively.
1142 stop(ServerRef :: server_ref()) -> ok
1144 The same as stop(ServerRef, normal, infinity).
1146 stop(ServerRef :: server_ref(),
1148 Reason :: term(),
1149 Timeout :: timeout()) ->
1150 ok
1151 Orders the gen_statem ServerRef to exit with the specified Rea‐
1153 son and waits for it to terminate. The gen_statem calls Mod‐
1154 ule:terminate/3 before exiting.
1155 This function returns ok if the server terminates with the
1157 expected reason. Any other reason than normal, shutdown, or
1158 {shutdown,Term} causes an error report to be issued through log‐
1159 ger(3). The default Reason is normal.
1160 Timeout is an integer > 0, which specifies how many milliseconds
1162 to wait for the server to terminate, or the atom infinity to
1163 wait indefinitely. Defaults to infinity. If the server does not
1164 terminate within the specified time, a timeout exception is
1165 raised.
1166 If the process does not exist, a noproc exception is raised.
1168
1170 The following functions are to be exported from a gen_statem callback
1171 module.
1172
1174 Module:callback_mode() -> CallbackMode
1175 Types:
1177 CallbackMode = callback_mode() | [ callback_mode() |
1179 state_enter() ]
1180 This function is called by a gen_statem when it needs to find
1182 out the callback mode of the callback module. The value is
1183 cached by gen_statem for efficiency reasons, so this function is
1184 only called once after server start, after code change, and
1185 after changing the callback module, but before the first state
1186 callback in the current callback module's code version is
1187 called. More occasions may be added in future versions of
1188 gen_statem.
1189 Server start happens either when Module:init/1 returns or when
1191 enter_loop/4-6 is called. Code change happens when Mod‐
1192 ule:code_change/4 returns. A change of the callback module hap‐
1193 pens when a state callback returns any of the actions
1194 change_callback_module, push_callback_module or pop_call‐
1195 back_module.
1196 The CallbackMode is either just callback_mode() or a list con‐
1198 taining callback_mode() and possibly the atom state_enter.
1199 Note:
1201 If this function's body does not return an inline constant value
1202 the callback module is doing something strange.
1203 Module:code_change(OldVsn, OldState, OldData, Extra) -> Result
1206 Types:
1208 OldVsn = Vsn | {down,Vsn}
1210 Vsn = term()
1211 OldState = NewState = term()
1212 Extra = term()
1213 Result = {ok,NewState,NewData} | Reason
1214 OldState = NewState = state()
1215 OldData = NewData = data()
1216 Reason = term()
1217 Note:
1219 This callback is optional, so callback modules need not export
1220 it. If a release upgrade/downgrade with Change =
1221 {advanced,Extra} specified in the .appup file is made when
1222 code_change/4 is not implemented the process will crash with
1223 exit reason undef.
1224 This function is called by a gen_statem when it is to update its
1227 internal state during a release upgrade/downgrade, that is, when
1228 the instruction {update,Module,Change,...}, where Change =
1229 {advanced,Extra}, is specified in the appup file. For more
1230 information, see OTP Design Principles.
1231 For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is
1233 {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old
1234 version of the callback module Module. If no such attribute is
1235 defined, the version is the checksum of the Beam file.
1236 OldState and OldData is the internal state of the gen_statem.
1238 Extra is passed "as is" from the {advanced,Extra} part of the
1240 update instruction.
1241 If successful, the function must return the updated internal
1243 state in an {ok,NewState,NewData} tuple.
1244 If the function returns a failure Reason, the ongoing upgrade
1246 fails and rolls back to the old release. Note that Reason cannot
1247 be an {ok,_,_} tuple since that will be regarded as a {ok,New‐
1248 State,NewData} tuple, and that a tuple matching {ok,_} is an
1249 also invalid failure Reason. It is recommended to use an atom as
1250 Reason since it will be wrapped in an {error,Reason} tuple.
1251 Also note when upgrading a gen_statem, this function and hence
1253 the Change = {advanced,Extra} parameter in the appup file is not
1254 only needed to update the internal state or to act on the Extra
1255 argument. It is also needed if an upgrade or downgrade should
1256 change callback mode, or else the callback mode after the code
1257 change will not be honoured, most probably causing a server
1258 crash.
1259 If the server changes callback module using any of the actions
1261 change_callback_module, push_callback_module or pop_call‐
1262 back_module, be aware that it is always the current callback
1263 module that will get this callback call. That the current call‐
1264 back module handles the current state and data update should be
1265 no surprise, but it must be able to handle even parts of the
1266 state and data that it is not familiar with, somehow.
1267 In the supervisor child specification there is a list of modules
1269 which is recommended to contain only the callback module. For a
1270 gen_statem with multiple callback modules there is no real need
1271 to list all of them, it may not even be possible since the list
1272 could change after code upgrade. If this list would contain only
1273 the start callback module, as recommended, what is important is
1274 to upgrade that module whenever a synchronized code replacement
1275 is done. Then the release handler concludes that an upgrade that
1276 upgrades that module needs to suspend, code change, and resume
1277 any server whose child specification declares that it is using
1278 that module. And again; the current callback module will get the
1279 Module:code_change/4 call.
1280 Module:init(Args) -> Result(StateType)
1282 Types:
1284 Args = term()
1286 Result(StateType) = init_result(StateType)
1287 Whenever a gen_statem is started using start_link/3,4 or
1289 start/3,4, this function is called by the new process to ini‐
1290 tialize the implementation state and server data.
1291 Args is the Args argument provided to that start function.
1293 Note:
1295 Note that if the gen_statem is started through proc_lib and
1296 enter_loop/4-6, this callback will never be called. Since this
1297 callback is not optional it can in that case be implemented as:
1298 init(Args) -> erlang:error(not_implemented, [Args]).
1300 Module:format_status(Opt, [PDict,State,Data]) -> Status
1303 Types:
1305 Opt = normal | terminate
1307 PDict = [{Key, Value}]
1308 State = state()
1309 Data = data()
1310 Key = term()
1311 Value = term()
1312 Status = term()
1313 Note:
1315 This callback is optional, so a callback module does not need to
1316 export it. The gen_statem module provides a default implementa‐
1317 tion of this function that returns {State,Data}.
1318 If this callback is exported but fails, to hide possibly sensi‐
1320 tive data, the default function will instead return
1321 {State,Info}, where Info says nothing but the fact that for‐
1322 mat_status/2 has crashed.
1323 This function is called by a gen_statem process when any of the
1326 following apply:
1327 *
1329 One of sys:get_status/1,2 is invoked to get the gen_statem
1330 status. Opt is set to the atom normal for this case.
1331 *
1333 The gen_statem terminates abnormally and logs an error. Opt
1334 is set to the atom terminate for this case.
1335 This function is useful for changing the form and appearance of
1337 the gen_statem status for these cases. A callback module wishing
1338 to change the sys:get_status/1,2 return value and how its status
1339 appears in termination error logs exports an instance of for‐
1340 mat_status/2, which returns a term describing the current status
1341 of the gen_statem.
1342 PDict is the current value of the process dictionary of the
1344 gen_statem.
1345 State is the internal state of the gen_statem.
1347 Data is the internal server data of the gen_statem.
1349 The function is to return Status, a term that contains the
1351 appropriate details of the current state and status of the
1352 gen_statem. There are no restrictions on the form Status can
1353 take, but for the sys:get_status/1,2 case (when Opt is normal),
1354 the recommended form for the Status value is [{data, [{"State",
1355 Term}]}], where Term provides relevant details of the gen_statem
1356 state. Following this recommendation is not required, but it
1357 makes the callback module status consistent with the rest of the
1358 sys:get_status/1,2 return value.
1359 One use for this function is to return compact alternative state
1361 representations to avoid having large state terms printed in log
1362 files. Another use is to hide sensitive data from being written
1363 to the error log.
1364 Module:StateName(enter, OldState, Data) -> StateEnterResult(StateName)
1366 Module:StateName(EventType, EventContent, Data) -> StateFunctionResult
1367 Module:handle_event(enter, OldState, State, Data) -> StateEnterRe‐
1368 sult(State)
1369 Module:handle_event(EventType, EventContent, State, Data) -> Han‐
1370 dleEventResult
1371 Types:
1373 EventType = event_type()
1375 EventContent = term()
1376 State = state()
1377 Data = NewData = data()
1378 StateEnterResult(StateName) = state_enter_result(StateName)
1379 StateFunctionResult = event_handler_result(state_name())
1380 StateEnterResult(State) = state_enter_result(State)
1381 HandleEventResult = event_handler_result(state())
1382 Whenever a gen_statem receives an event from call/2, cast/2, or
1384 as a normal process message, one of these functions is called.
1385 If callback mode is state_functions, Module:StateName/3 is
1386 called, and if it is handle_event_function, Module:han‐
1387 dle_event/4 is called.
1388 If EventType is {call,From}, the caller waits for a reply. The
1390 reply can be sent from this or from any other state callback by
1391 returning with {reply,From,Reply} in Actions, in Replies, or by
1392 calling reply(From, Reply).
1393 If this function returns with a next state that does not match
1395 equal (=/=) to the current state, all postponed events are
1396 retried in the next state.
1397 The only difference between StateFunctionResult and HandleEven‐
1399 tResult is that for StateFunctionResult the next state must be
1400 an atom, but for HandleEventResult there is no restriction on
1401 the next state.
1402 For options that can be set and actions that can be done by
1404 gen_statem after returning from this function, see action().
1405 When the gen_statem runs with state enter calls, these functions
1407 are also called with arguments (enter, OldState, ...) during
1408 every state change. In this case there are some restrictions on
1409 the actions that may be returned: postpone() is not allowed
1410 since a state enter call is not an event so there is no event to
1411 postpone, and {next_event,_,_} is not allowed since using state
1412 enter calls should not affect how events are consumed and pro‐
1413 duced. You may also not change states from this call. Should you
1414 return {next_state,NextState, ...} with NextState =/= State the
1415 gen_statem crashes. Note that it is actually allowed to use
1416 {repeat_state, NewData, ...} although it makes little sense
1417 since you immediately will be called again with a new state
1418 enter call making this just a weird way of looping, and there
1419 are better ways to loop in Erlang. If you do not update NewData
1420 and have some loop termination condition, or if you use
1421 {repeat_state_and_data, _} or repeat_state_and_data you have an
1422 infinite loop! You are advised to use {keep_state,...},
1423 {keep_state_and_data,_} or keep_state_and_data since changing
1424 states from a state enter call is not possible anyway.
1425 Note the fact that you can use throw to return the result, which
1427 can be useful. For example to bail out with
1428 throw(keep_state_and_data) from deep within complex code that
1429 cannot return {next_state,State,Data} because State or Data is
1430 no longer in scope.
1431 Module:terminate(Reason, State, Data) -> Ignored
1433 Types:
1435 Reason = normal | shutdown | {shutdown,term()} | term()
1437 State = state()
1438 Data = data()
1439 Ignored = term()
1440 Note:
1442 This callback is optional, so callback modules need not export
1443 it. The gen_statem module provides a default implementation
1444 without cleanup.
1445 This function is called by a gen_statem when it is about to ter‐
1448 minate. It is to be the opposite of Module:init/1 and do any
1449 necessary cleaning up. When it returns, the gen_statem termi‐
1450 nates with Reason. The return value is ignored.
1451 Reason is a term denoting the stop reason and State is the
1453 internal state of the gen_statem.
1454 Reason depends on why the gen_statem is terminating. If it is
1456 because another callback function has returned, a stop tuple
1457 {stop,Reason} in Actions, Reason has the value specified in that
1458 tuple. If it is because of a failure, Reason is the error rea‐
1459 son.
1460 If the gen_statem is part of a supervision tree and is ordered
1462 by its supervisor to terminate, this function is called with
1463 Reason = shutdown if both the following conditions apply:
1464 * The gen_statem has been set to trap exit signals.
1466 * The shutdown strategy as defined in the supervisor's child
1468 specification is an integer time-out value, not brutal_kill.
1469 Even if the gen_statem is not part of a supervision tree, this
1471 function is called if it receives an 'EXIT' message from its
1472 parent. Reason is the same as in the 'EXIT' message.
1473 Otherwise, the gen_statem is immediately terminated.
1475 Notice that for any other reason than normal, shutdown, or
1477 {shutdown,Term}, the gen_statem is assumed to terminate because
1478 of an error and an error report is issued using logger(3).
1479
1481 gen_event(3), gen_fsm(3), gen_server(3), proc_lib(3), supervisor(3),
1482 sys(3).
1483Ericsson AB stdlib 3.12.1 gen_statem(3)