aboutsummaryrefslogblamecommitdiffstats
path: root/lib/stdlib/doc/src/gen_fsm.xml
blob: 739cd0bffd35780914c0936ff04069453ed19e0d (plain) (tree)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731


























































































































































































































































































































































































































































































































































































































































































































































                                                                                                               





























                                                                             











                                                           
<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>gen_fsm</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_fsm</module>
  <modulesummary>Generic Finite State Machine Behaviour</modulesummary>
  <description>
    <p>A behaviour module for implementing a finite state machine.
      A generic finite state machine process (gen_fsm) implemented
      using this module will have a standard set of interface functions
      and include functionality for tracing and error reporting. It will
      also fit into an OTP supervision tree. Refer to
      <seealso marker="doc/design_principles:fsm">OTP Design Principles</seealso> for more information.</p>
    <p>A gen_fsm assumes all specific parts to be located in a callback
      module exporting a pre-defined set of functions. The relationship
      between the behaviour functions and the callback functions can be
      illustrated as follows:</p>
    <pre>
gen_fsm module                    Callback module
--------------                    ---------------
gen_fsm:start_link                -----> Module:init/1

gen_fsm:send_event                -----> Module:StateName/2

gen_fsm:send_all_state_event      -----> Module:handle_event/3

gen_fsm:sync_send_event           -----> Module:StateName/3

gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4

-                                 -----> Module:handle_info/3

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

-                                 -----> Module:code_change/4</pre>
    <p>If a callback function fails or returns a bad value, the gen_fsm
      will terminate.</p>
    <p>A gen_fsm handles system messages as documented in
       <seealso marker="sys">sys(3)</seealso>. The <c>sys</c> module
       can be used for debugging a gen_fsm.</p>
    <p>Note that a gen_fsm does not trap exit signals automatically,
      this must be explicitly initiated in the callback module.</p>
    <p>Unless otherwise stated, all functions in this module fail if
      the specified gen_fsm does not exist or if bad arguments are
      given.</p>
    <p>The gen_fsm process can go into hibernation 
    (see <seealso marker="erts:erlang#erlang:hibernate/3">erlang(3)</seealso>) if a callback
    function specifies <c>'hibernate'</c> instead of a timeout value. This
    might be useful if the server is expected to be idle for a long
    time. However this feature should be used with care as hibernation
    implies at least two garbage collections (when hibernating and
    shortly after waking up) and is not something you'd want to do
    between each call to a busy state machine.</p>

  </description>
  <funcs>
    <func>
      <name>start_link(Module, Args, Options) -> Result</name>
      <name>start_link(FsmName, Module, Args, Options) -> Result</name>
      <fsummary>Create a gen_fsm process in a supervision tree.</fsummary>
      <type>
        <v>FsmName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics</v>
        <v>&nbsp;&nbsp;&nbsp;&nbsp;| {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [SOpt]</v>
        <v>&nbsp;&nbsp;&nbsp;SOpt - see erlang:spawn_opt/2,3,4,5</v>
        <v>Result = {ok,Pid} | ignore | {error,Error}</v>
        <v>&nbsp;Pid = pid()</v>
        <v>&nbsp;Error = {already_started,Pid} | term()</v>
      </type>
      <desc>
        <p>Creates a gen_fsm process as part of a supervision tree.
          The function should be called, directly or indirectly, by
          the supervisor. It will, among other things, ensure that
          the gen_fsm is linked to the supervisor.</p>
        <p>The gen_fsm process calls <c>Module:init/1</c> to
          initialize. To ensure a synchronized start-up procedure,
          <c>start_link/3,4</c> does not return until
          <c>Module:init/1</c> has returned.</p>
        <p>If <c>FsmName={local,Name}</c>, the gen_fsm is registered
          locally as <c>Name</c> using <c>register/2</c>.
          If <c>FsmName={global,GlobalName}</c>, the gen_fsm is
          registered globally as <c>GlobalName</c> using
          <c>global:register_name/2</c>. If no name is provided,
          the gen_fsm is not registered.</p>
        <p><c>Module</c> is the name of the callback module.</p>
        <p><c>Args</c> is an arbitrary term which is passed as
          the argument to <c>Module:init/1</c>.</p>
        <p>If the option <c>{timeout,Time}</c> is present, the gen_fsm
          is allowed to spend <c>Time</c> milliseconds initializing
          or it will be terminated and the start function will return
          <c>{error,timeout}</c>.</p>
        <p>If the option <c>{debug,Dbgs}</c> is present,
          the corresponding <c>sys</c> function will be called for each
          item in <c>Dbgs</c>. See
          <seealso marker="sys">sys(3)</seealso>.</p>
        <p>If the option <c>{spawn_opt,SOpts}</c> is present,
          <c>SOpts</c> will be passed as option list to
          the <c>spawn_opt</c> BIF which is used to spawn the gen_fsm
          process. See
          <seealso marker="erts:erlang#spawn_opt/2">erlang(3)</seealso>.</p>
        <note>
          <p>Using the spawn option <c>monitor</c> is currently not
            allowed, but will cause the function to fail with reason
            <c>badarg</c>.</p>
        </note>
        <p>If the gen_fsm is successfully created and initialized
          the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is
          the pid of the gen_fsm. If there already exists a process with
          the specified <c>FsmName</c>, the function returns
          <c>{error,{already_started,Pid}}</c> where <c>Pid</c> is
          the pid of that process.</p>
        <p>If <c>Module:init/1</c> fails with <c>Reason</c>,
          the function returns <c>{error,Reason}</c>. If
          <c>Module:init/1</c> returns <c>{stop,Reason}</c> or
          <c>ignore</c>, the process is terminated and the function
          returns <c>{error,Reason}</c> or <c>ignore</c>, respectively.</p>
      </desc>
    </func>
    <func>
      <name>start(Module, Args, Options) -> Result</name>
      <name>start(FsmName, Module, Args, Options) -> Result</name>
      <fsummary>Create a stand-alone gen_fsm process.</fsummary>
      <type>
        <v>FsmName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics</v>
        <v>&nbsp;&nbsp;&nbsp;&nbsp;| {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [term()]</v>
        <v>Result = {ok,Pid} | ignore | {error,Error}</v>
        <v>&nbsp;Pid = pid()</v>
        <v>&nbsp;Error = {already_started,Pid} | term()</v>
      </type>
      <desc>
        <p>Creates a stand-alone gen_fsm process, i.e. a gen_fsm which
          is not part of a supervision tree and thus has no supervisor.</p>
        <p>See <seealso marker="#start_link/3">start_link/3,4</seealso>
          for a description of arguments and return values.</p>
      </desc>
    </func>
    <func>
      <name>send_event(FsmRef, Event) -> ok</name>
      <fsummary>Send an event asynchronously to a generic FSM.</fsummary>
      <type>
        <v>FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Event = term()</v>
      </type>
      <desc>
        <p>Sends an event asynchronously to the gen_fsm <c>FsmRef</c>
          and returns <c>ok</c> immediately. The gen_fsm will call
          <c>Module:StateName/2</c> to handle the event, where
          <c>StateName</c> is the name of the current state of
          the gen_fsm.</p>
        <p><c>FsmRef</c> can be:</p>
        <list type="bulleted">
          <item>the pid,</item>
          <item><c>Name</c>, if the gen_fsm is locally registered,</item>
          <item><c>{Name,Node}</c>, if the gen_fsm is locally
           registered at another node, or</item>
          <item><c>{global,GlobalName}</c>, if the gen_fsm is globally
           registered.</item>
        </list>
        <p><c>Event</c> is an arbitrary term which is passed as one of
          the arguments to <c>Module:StateName/2</c>.</p>
      </desc>
    </func>
    <func>
      <name>send_all_state_event(FsmRef, Event) -> ok</name>
      <fsummary>Send an event asynchronously to a generic FSM.</fsummary>
      <type>
        <v>FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Event = term()</v>
      </type>
      <desc>
        <p>Sends an event asynchronously to the gen_fsm <c>FsmRef</c>
          and returns <c>ok</c> immediately. The gen_fsm will call
          <c>Module:handle_event/3</c> to handle the event.</p>
        <p>See <seealso marker="#send_event/2">send_event/2</seealso>
          for a description of the arguments.</p>
        <p>The difference between <c>send_event</c> and
          <c>send_all_state_event</c> is which callback function is
          used to handle the event. This function is useful when
          sending events that are handled the same way in every state,
          as only one <c>handle_event</c> clause is needed to handle
          the event instead of one clause in each state name function.</p>
      </desc>
    </func>
    <func>
      <name>sync_send_event(FsmRef, Event) -> Reply</name>
      <name>sync_send_event(FsmRef, Event, Timeout) -> Reply</name>
      <fsummary>Send an event synchronously to a generic FSM.</fsummary>
      <type>
        <v>FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Event = term()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>Sends an event to the gen_fsm <c>FsmRef</c> and waits until a
          reply arrives or a timeout occurs. The gen_fsm will call
          <c>Module:StateName/3</c> to handle the event, where
          <c>StateName</c> is the name of the current state of
          the gen_fsm.</p>
        <p>See <seealso marker="#send_event/2">send_event/2</seealso>
          for a description of <c>FsmRef</c> and <c>Event</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero which
          specifies how many milliseconds to wait for a reply, or
          the atom <c>infinity</c> to wait indefinitely. Default value
          is 5000. If no reply is received within the specified time,
          the function call fails.</p>
        <p>The return value <c>Reply</c> is defined in the return value
          of <c>Module:StateName/3</c>.</p>
        <p>The ancient behaviour of sometimes consuming the server
          exit message if the server died during the call while
          linked to the client has been removed in OTP R12B/Erlang 5.6.</p>
      </desc>
    </func>
    <func>
      <name>sync_send_all_state_event(FsmRef, Event) -> Reply</name>
      <name>sync_send_all_state_event(FsmRef, Event, Timeout) -> Reply</name>
      <fsummary>Send an event synchronously to a generic FSM.</fsummary>
      <type>
        <v>FsmRef = Name | {Name,Node} | {global,GlobalName} | pid()</v>
        <v>&nbsp;Name = Node = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Event = term()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>Sends an event to the gen_fsm <c>FsmRef</c> and waits until a
          reply arrives or a timeout occurs. The gen_fsm will call
          <c>Module:handle_sync_event/4</c> to handle the event.</p>
        <p>See <seealso marker="#send_event/2">send_event/2</seealso>
          for a description of <c>FsmRef</c> and <c>Event</c>. See
          <seealso marker="#sync_send_event/3">sync_send_event/3</seealso>
          for a description of <c>Timeout</c> and <c>Reply</c>.</p>
        <p>See
          <seealso marker="#send_all_state_event/2">send_all_state_event/2</seealso>
          for a discussion about the difference between
          <c>sync_send_event</c> and <c>sync_send_all_state_event</c>.</p>
      </desc>
    </func>
    <func>
      <name>reply(Caller, Reply) -> true</name>
      <fsummary>Send a reply to a caller.</fsummary>
      <type>
        <v>Caller - see below</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>This function can be used by a gen_fsm to explicitly send a
          reply to a client process that called
          <seealso marker="#sync_send_event/2">sync_send_event/2,3</seealso>
          or
          <seealso marker="#sync_send_all_state_event/2">sync_send_all_state_event/2,3</seealso>,
          when the reply cannot be defined in the return value of
          <c>Module:State/3</c> or <c>Module:handle_sync_event/4</c>.</p>
        <p><c>Caller</c> must be the <c>From</c> argument provided to
          the callback function. <c>Reply</c> is an arbitrary term,
          which will be given back to the client as the return value of
          <c>sync_send_event/2,3</c> or
          <c>sync_send_all_state_event/2,3</c>.</p>
      </desc>
    </func>
    <func>
      <name>send_event_after(Time, Event) -> Ref</name>
      <fsummary>Send a delayed event internally in a generic FSM.</fsummary>
      <type>
        <v>Time = integer()</v>
        <v>Event = term()</v>
        <v>Ref = reference()</v>
      </type>
      <desc>
        <p>Sends a delayed event internally in the gen_fsm that calls
          this function after <c>Time</c> ms. Returns immediately a
          reference that can be used to cancel the delayed send using
          <seealso marker="#cancel_timer/1">cancel_timer/1</seealso>.</p>
        <p>The gen_fsm will call <c>Module:StateName/2</c> to handle
          the event, where <c>StateName</c> is the name of the current
          state of the gen_fsm at the time the delayed event is
          delivered.</p>
        <p><c>Event</c> is an arbitrary term which is passed as one of
          the arguments to <c>Module:StateName/2</c>.</p>
      </desc>
    </func>
    <func>
      <name>start_timer(Time, Msg) -> Ref</name>
      <fsummary>Send a timeout event internally in a generic FSM.</fsummary>
      <type>
        <v>Time = integer()</v>
        <v>Msg = term()</v>
        <v>Ref = reference()</v>
      </type>
      <desc>
        <p>Sends a timeout event internally in the gen_fsm that calls
          this function after <c>Time</c> ms. Returns immediately a
          reference that can be used to cancel the timer using
          <seealso marker="#cancel_timer/1">cancel_timer/1</seealso>.</p>
        <p>The gen_fsm will call <c>Module:StateName/2</c> to handle
          the event, where <c>StateName</c> is the name of the current
          state of the gen_fsm at the time the timeout message is
          delivered.</p>
        <p><c>Msg</c> is an arbitrary term which is passed in the
          timeout message, <c>{timeout, Ref, Msg}</c>, as one of
          the arguments to <c>Module:StateName/2</c>.</p>
      </desc>
    </func>
    <func>
      <name>cancel_timer(Ref) -> RemainingTime | false</name>
      <fsummary>Cancel an internal timer in a generic FSM.</fsummary>
      <type>
        <v>Ref = reference()</v>
        <v>RemainingTime = integer()</v>
      </type>
      <desc>
        <p>Cancels an internal timer referred by <c>Ref</c> in the
          gen_fsm that calls this function.</p>
        <p><c>Ref</c> is a reference returned from
          <seealso marker="#send_event_after/2">send_event_after/2</seealso>
          or
          <seealso marker="#start_timer/2">start_timer/2</seealso>.</p>
        <p>If the timer has already timed out, but the event not yet
          been delivered, it is cancelled as if it had <em>not</em>
          timed out, so there will be no false timer event after
          returning from this function.</p>
        <p>Returns the remaining time in ms until the timer would
          have expired if <c>Ref</c> referred to an active timer,
          <c>false</c> otherwise.</p>
      </desc>
    </func>
    <func>
      <name>enter_loop(Module, Options, StateName, StateData)</name>
      <name>enter_loop(Module, Options, StateName, StateData, FsmName)</name>
      <name>enter_loop(Module, Options, StateName, StateData, Timeout)</name>
      <name>enter_loop(Module, Options, StateName, StateData, FsmName, Timeout)</name>
      <fsummary>Enter the gen_fsm receive loop</fsummary>
      <type>
        <v>Module = atom()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics</v>
        <v>&nbsp;&nbsp;&nbsp;&nbsp;| {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>StateName = atom()</v>
        <v>StateData = term()</v>
        <v>FsmName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = term()</v>
        <v>Timeout = int() | infinity</v>
      </type>
      <desc>
        <p>Makes an existing process into a gen_fsm. Does not return,
          instead the calling process will enter the gen_fsm receive
          loop and become a gen_fsm process. The process <em>must</em>
          have been started using one of the start functions in
          <c>proc_lib</c>, see
          <seealso marker="proc_lib">proc_lib(3)</seealso>. The user is
          responsible for any initialization of the process, including
          registering a name for it.</p>
        <p>This function is useful when a more complex initialization
          procedure is needed than the gen_fsm behaviour provides.</p>
        <p><c>Module</c>, <c>Options</c> and <c>FsmName</c> have
          the same meanings as when calling
          <seealso marker="#start_link/3">start[_link]/3,4</seealso>.
          However, if <c>FsmName</c> is specified, the process must have
          been registered accordingly <em>before</em> this function is
          called.</p>
        <p><c>StateName</c>, <c>StateData</c> and <c>Timeout</c> have
          the same meanings as in the return value of
          <seealso marker="#Moduleinit">Module:init/1</seealso>.
          Also, the callback module <c>Module</c> does not need to
          export an <c>init/1</c> function.</p>
        <p>Failure: If the calling process was not started by a
          <c>proc_lib</c> start function, or if it is not registered
          according to <c>FsmName</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>CALLBACK FUNCTIONS</title>
    <p>The following functions should be exported from a <c>gen_fsm</c>
      callback module.</p>
    <p>In the description, the expression <em>state name</em> is used to
      denote a state of the state machine. <em>state data</em> is used
      to denote the internal state of the Erlang process which
      implements the state machine.</p>
    <p></p>
  </section>
  <funcs>
    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>Initialize process and internal state name and state data.</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Return = {ok,StateName,StateData} | {ok,StateName,StateData,Timeout}</v>
	<v>&nbsp;&nbsp;| {ok,StateName,StateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason} | ignore</v>
        <v>&nbsp;StateName = atom()</v>
        <v>&nbsp;StateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <marker id="Moduleinit"></marker>
        <p>Whenever a gen_fsm is started using
          <seealso marker="#start/3">gen_fsm:start/3,4</seealso> or
          <seealso marker="#start_link/3">gen_fsm:start_link/3,4</seealso>,
          this function is called by the new process to initialize.</p>
        <p><c>Args</c> is the <c>Args</c> argument provided to the start
          function.</p>
        <p>If initialization is successful, the function should return
          <c>{ok,StateName,StateData}</c>, 
          <c>{ok,StateName,StateData,Timeout}</c> or <c>{ok,StateName,StateData,hibernate}</c>, 
	  where <c>StateName</c>
          is the initial state name and <c>StateData</c> the initial
          state data of the gen_fsm.</p>
        <p>If an integer timeout value is provided, a timeout will occur
          unless an event or a message is received within <c>Timeout</c>
          milliseconds. A timeout is represented by the atom
          <c>timeout</c> and should be handled by
          the <c>Module:StateName/2</c> callback functions. The atom
          <c>infinity</c> can be used to wait indefinitely, this is
          the default value.</p>
	<p>If <c>hibernate</c> is specified instead of a timeout value, the process will go 
	   into hibernation when waiting for the next message to arrive (by calling 
	   <seealso marker="proc_lib#hibernate/3">proc_lib:hibernate/3</seealso>).</p>
         <p>If something goes wrong during the initialization
          the function should return <c>{stop,Reason}</c>, where
          <c>Reason</c> is any term, or <c>ignore</c>.</p>
      </desc>
    </func>
    <func>
      <name>Module:StateName(Event, StateData) -> Result</name>
      <fsummary>Handle an asynchronous event.</fsummary>
      <type>
        <v>Event = timeout | term()</v>
        <v>StateData = term()</v>
        <v>Result = {next_state,NextStateName,NewStateData} </v>
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,Timeout}</v>
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewStateData}</v>
        <v>&nbsp;NextStateName = atom()</v>
        <v>&nbsp;NewStateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>There should be one instance of this function for each
          possible state name. Whenever a gen_fsm receives an event
          sent using
          <seealso marker="#send_event/2">gen_fsm:send_event/2</seealso>,
          the instance of this function with the same name as
          the current state name <c>StateName</c> is called to handle
          the event. It is also called if a timeout occurs.</p>
        <p><c>Event</c> is either the atom <c>timeout</c>, if a timeout
          has occurred, or the <c>Event</c> argument provided to
          <c>send_event/2</c>.</p>
        <p><c>StateData</c> is the state data of the gen_fsm.</p>
        <p>If the function returns
          <c>{next_state,NextStateName,NewStateData}</c>, 
          <c>{next_state,NextStateName,NewStateData,Timeout}</c> or
	  <c>{next_state,NextStateName,NewStateData,hibernate}</c>,
          the gen_fsm will continue executing with the current state
          name set to <c>NextStateName</c> and with the possibly
          updated state data <c>NewStateData</c>. See
          <c>Module:init/1</c> for a description of <c>Timeout</c> and <c>hibernate</c>.</p>
        <p>If the function returns <c>{stop,Reason,NewStateData}</c>,
          the gen_fsm will call
          <c>Module:terminate(Reason,NewStateData)</c> and terminate.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_event(Event, StateName, StateData) -> Result</name>
      <fsummary>Handle an asynchronous event.</fsummary>
      <type>
        <v>Event = term()</v>
        <v>StateName = atom()</v>
        <v>StateData = term()</v>
        <v>Result = {next_state,NextStateName,NewStateData} </v>
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,Timeout}</v>
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewStateData}</v>
        <v>&nbsp;NextStateName = atom()</v>
        <v>&nbsp;NewStateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a gen_fsm receives an event sent using
          <seealso marker="#send_all_state_event/2">gen_fsm:send_all_state_event/2</seealso>,
          this function is called to handle the event.</p>
        <p><c>StateName</c> is the current state name of the gen_fsm.</p>
        <p>See <c>Module:StateName/2</c> for a description of the other
          arguments and possible return values.</p>
      </desc>
    </func>
    <func>
      <name>Module:StateName(Event, From, StateData) -> Result</name>
      <fsummary>Handle a synchronous event.</fsummary>
      <type>
        <v>Event = term()</v>
        <v>From = {pid(),Tag}</v>
        <v>StateData = term()</v>
        <v>Result = {reply,Reply,NextStateName,NewStateData}</v> 
	<v>&nbsp;&nbsp;| {reply,Reply,NextStateName,NewStateData,Timeout}</v> 
	<v>&nbsp;&nbsp;| {reply,Reply,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData}</v> 
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,Timeout}</v> 
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;NextStateName = atom()</v>
        <v>&nbsp;NewStateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <p>There should be one instance of this function for each
          possible state name. Whenever a gen_fsm receives an event
          sent using
          <seealso marker="#sync_send_event/2">gen_fsm:sync_send_event/2,3</seealso>,
          the instance of this function with the same name as
          the current state name <c>StateName</c> is called to handle
          the event.</p>
        <p><c>Event</c> is the <c>Event</c> argument provided to
          <c>sync_send_event</c>.</p>
        <p><c>From</c> is a tuple <c>{Pid,Tag}</c> where <c>Pid</c> is
          the pid of the process which called <c>sync_send_event/2,3</c>
          and <c>Tag</c> is a unique tag.</p>
        <p><c>StateData</c> is the state data of the gen_fsm.</p>
        <p>If the function returns
          <c>{reply,Reply,NextStateName,NewStateData}</c>, 
          <c>{reply,Reply,NextStateName,NewStateData,Timeout}</c> or 
	  <c>{reply,Reply,NextStateName,NewStateData,hibernate}</c>,
          <c>Reply</c> will be given back to <c>From</c> as the return
          value of <c>sync_send_event/2,3</c>. The gen_fsm then
          continues executing with the current state name set to
          <c>NextStateName</c> and with the possibly updated state data
          <c>NewStateData</c>. See <c>Module:init/1</c> for a
          description of <c>Timeout</c> and <c>hibernate</c>.</p>
        <p>If the function returns
          <c>{next_state,NextStateName,NewStateData}</c>, 
          <c>{next_state,NextStateName,NewStateData,Timeout}</c> or
          <c>{next_state,NextStateName,NewStateData,hibernate}</c>,
          the gen_fsm will continue executing in <c>NextStateName</c>
          with <c>NewStateData</c>. Any reply to <c>From</c> must be
          given explicitly using
          <seealso marker="#reply/2">gen_fsm:reply/2</seealso>.</p>
        <p>If the function returns
          <c>{stop,Reason,Reply,NewStateData}</c>, <c>Reply</c> will be
          given back to <c>From</c>. If the function returns
          <c>{stop,Reason,NewStateData}</c>, any reply to <c>From</c>
          must be given explicitly using <c>gen_fsm:reply/2</c>.
          The gen_fsm will then call
          <c>Module:terminate(Reason,NewStateData)</c> and terminate.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_sync_event(Event, From, StateName, StateData) -> Result</name>
      <fsummary>Handle a synchronous event.</fsummary>
      <type>
        <v>Event = term()</v>
        <v>From = {pid(),Tag}</v>
        <v>StateName = atom()</v>
        <v>StateData = term()</v>
        <v>Result = {reply,Reply,NextStateName,NewStateData}</v> 
	<v>&nbsp;&nbsp;| {reply,Reply,NextStateName,NewStateData,Timeout}</v> 
	<v>&nbsp;&nbsp;| {reply,Reply,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData}</v> 
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,Timeout}</v> 
	<v>&nbsp;&nbsp;| {next_state,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;NextStateName = atom()</v>
        <v>&nbsp;NewStateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a gen_fsm receives an event sent using
          <seealso marker="#sync_send_all_state_event/2">gen_fsm:sync_send_all_state_event/2,3</seealso>,
          this function is called to handle the event.</p>
        <p><c>StateName</c> is the current state name of the gen_fsm.</p>
        <p>See <c>Module:StateName/3</c> for a description of the other
          arguments and possible return values.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_info(Info, StateName, StateData) -> Result</name>
      <fsummary>Handle an incoming message.</fsummary>
      <type>
        <v>Info = term()</v>
        <v>StateName = atom()</v>
        <v>StateData = term()</v>
        <v>Result = {next_state,NextStateName,NewStateData}</v> 
	<v>&nbsp;>&nbsp;| {next_state,NextStateName,NewStateData,Timeout}</v> 
	<v>&nbsp;>&nbsp;| {next_state,NextStateName,NewStateData,hibernate}</v>
        <v>&nbsp;>&nbsp;| {stop,Reason,NewStateData}</v>
        <v>&nbsp;NextStateName = atom()</v>
        <v>&nbsp;NewStateData = term()</v>
        <v>&nbsp;Timeout = int()>0 | infinity</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_fsm when it receives any
          other message than a synchronous or asynchronous event (or a
          system message).</p>
        <p><c>Info</c> is the received message.</p>
        <p>See <c>Module:StateName/2</c> for a description of the other
          arguments and possible return values.</p>
      </desc>
    </func>
    <func>
      <name>Module:terminate(Reason, StateName, StateData)</name>
      <fsummary>Clean up before termination.</fsummary>
      <type>
        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
        <v>StateName = atom()</v>
        <v>StateData = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_fsm when it is about to
          terminate. It should be the opposite of <c>Module:init/1</c>
          and do any necessary cleaning up. When it returns, the gen_fsm
          terminates with <c>Reason</c>. The return value is ignored.</p>
        <p><c>Reason</c> is a term denoting the stop reason,
          <c>StateName</c> is the current state name, and
          <c>StateData</c> is the state data of the gen_fsm.</p>
        <p><c>Reason</c> depends on why the gen_fsm is terminating. If
          it is because another callback function has returned a stop
          tuple <c>{stop,..}</c>, <c>Reason</c> will have the value
          specified in that tuple. If it is due to a failure,
          <c>Reason</c> is the error reason.</p>
        <p>If the gen_fsm is part of a supervision tree and is ordered
          by its supervisor to terminate, this function will be called
          with <c>Reason=shutdown</c> if the following conditions apply:</p>
        <list type="bulleted">
          <item>the gen_fsm has been set to trap exit signals, and</item>
          <item>the shutdown strategy as defined in the supervisor's
           child specification is an integer timeout value, not
          <c>brutal_kill</c>.</item>
        </list>
        <p>Even if the gen_fsm is <em>not</em> part of a supervision tree,
	   this function will be called if it receives an <c>'EXIT'</c>
	   message from its parent. <c>Reason</c> will be the same as in
	   the <c>'EXIT'</c> message.</p>
        <p>Otherwise, the gen_fsm will be immediately terminated.</p>
        <p>Note that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> the gen_fsm is
	  assumed to terminate due to an error and
	  an error report is issued using
          <seealso marker="kernel:error_logger#format/2">error_logger:format/2</seealso>.</p>
      </desc>
    </func>
    <func>
      <name>Module:code_change(OldVsn, StateName, StateData, Extra) -> {ok, NextStateName, NewStateData}</name>
      <fsummary>Update the internal state data during upgrade/downgrade.</fsummary>
      <type>
        <v>OldVsn = Vsn | {down,  Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>StateName = NextStateName = atom()</v>
        <v>StateData = NewStateData = term()</v>
        <v>Extra = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_fsm when it should update
          its internal state data during a release upgrade/downgrade,
          i.e. when the instruction <c>{update,Module,Change,...}</c>
          where <c>Change={advanced,Extra}</c> is given in
          the <c>appup</c> file. See
          <seealso marker="doc/design_principles:release_handling#instr">OTP Design Principles</seealso>.</p>
        <p>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          in the case of a downgrade, <c>OldVsn</c> is
          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
          attribute(s) of the old version of the callback module
          <c>Module</c>. If no such attribute is defined, the version is
          the checksum of the BEAM file.</p>
        <p><c>StateName</c> is the current state name and
          <c>StateData</c> the internal state data of the gen_fsm.</p>
        <p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
          part of the update instruction.</p>
        <p>The function should return the new current state name and
          updated internal data.</p>
      </desc>
    </func>
    <func>
      <name>Module:format_status(normal, [PDict, StateData]) -> Status</name>
      <fsummary>Optional function for providing a term describing the
        current gen_fsm status.</fsummary>
      <type>
        <v>PDict = [{Key, Value}]</v>
        <v>StateData = term()</v>
        <v>Status = [term()]</v>
      </type>
      <desc>
        <p><em>This callback is optional, so callback modules need not
          export it. The gen_fsm module provides a default
          implementation of this function that returns the callback
          module state data.</em></p>
        <p>This function is called by a gen_fsm process when one
          of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
          is invoked to get the gen_fsm status. A callback module
          wishing to customise the <c>sys:get_status/1,2</c> return
          value exports an instance of <c>format_status/2</c> that
          returns a term describing the current status of the
          gen_fsm.</p>
        <p><c>PDict</c> is the current value of the gen_fsm's
          process dictionary.</p>
        <p><c>StateData</c> is the internal state data of the
          gen_fsm.</p>
        <p>The function should return <c>Status</c>, a list of one or
          more terms that customise the details of the current state
          and status of the gen_fsm.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="gen_event">gen_event(3)</seealso>,
      <seealso marker="gen_server">gen_server(3)</seealso>,
      <seealso marker="supervisor">supervisor(3)</seealso>,
      <seealso marker="proc_lib">proc_lib(3)</seealso>,
      <seealso marker="sys">sys(3)</seealso></p>
  </section>
</erlref>