aboutsummaryrefslogtreecommitdiffstats
path: root/lib/stdlib/doc/src/gen_server.xml
blob: 27edbc8de7fbac0f2afc46fa8363cab5519516ed (plain) (blame)
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
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

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

    </legalnotice>

    <title>gen_server</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_server</module>
  <modulesummary>Generic server behavior.</modulesummary>
  <description>
    <p>This behavior module provides the server of a client-server
      relation. A generic server process (<c>gen_server</c>) implemented using
      this module has a standard set of interface functions and
      includes functionality for tracing and error reporting. It also
      fits into an OTP supervision tree. For more information, see section
      <seealso marker="doc/design_principles:gen_server_concepts">
      gen_server Behaviour</seealso> in OTP Design Principles.</p>

    <p>A <c>gen_server</c> process assumes all specific parts to be located in
      a callback module exporting a predefined set of functions.
      The relationship between the behavior functions and the callback
      functions is as follows:</p>

    <pre>
gen_server module            Callback module
-----------------            ---------------
gen_server:start
gen_server:start_link -----> Module:init/1

gen_server:stop       -----> Module:terminate/2

gen_server:call
gen_server:multi_call -----> Module:handle_call/3

gen_server:cast
gen_server:abcast     -----> Module:handle_cast/2

-                     -----> Module:handle_info/2

-                     -----> Module:handle_continue/2

-                     -----> Module:terminate/2

-                     -----> Module:code_change/3</pre>

    <p>If a callback function fails or returns a bad value, the
      <c>gen_server</c> process terminates.</p>

    <p>A <c>gen_server</c> process handles system messages as described in
      <seealso marker="sys"><c>sys(3)</c></seealso>. The <c>sys</c> module
      can be used for debugging a <c>gen_server</c> process.</p>

    <p>Notice that a <c>gen_server</c> process 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 <c>gen_server</c> process does not exist or if bad
      arguments are specified.</p>

    <p>The <c>gen_server</c> process can go into hibernation
      (see <seealso marker="erts:erlang#hibernate/3">
      <c>erlang:hibernate/3</c></seealso>) if a callback
      function specifies <c>'hibernate'</c> instead of a time-out value. This
      can be useful if the server is expected to be idle for a long
      time. However, use this feature with care, as hibernation
      implies at least two garbage collections (when hibernating and
      shortly after waking up) and is not something you want to do
      between each call to a busy server.</p>

    <p>If the <c>gen_server</c> process needs to perform an action
      immediately after initialization or to break the execution of a
      callback into multiple steps, it can return <c>{continue,Continue}</c>
      in place of the time-out or hibernation value, which will immediately
      invoke the <c>handle_continue/2</c> callback.</p>

  </description>

  <funcs>
    <func>
      <name>abcast(Name, Request) -> abcast</name>
      <name>abcast(Nodes, Name, Request) -> abcast</name>
      <fsummary>Send an asynchronous request to many generic servers.</fsummary>
      <type>
        <v>Nodes = [Node]</v>
        <v>&nbsp;Node = atom()</v>
        <v>Name = atom()</v>
        <v>Request = term()</v>
      </type>
      <desc>
        <p>Sends an asynchronous request to the <c>gen_server</c> processes
          locally registered as <c>Name</c> at the specified nodes. The function
          returns immediately and ignores nodes that do not exist, or
          where the <c>gen_server</c> <c>Name</c> does not exist.
          The <c>gen_server</c> processes call
          <seealso marker="#Module:handle_cast/2">
          <c>Module:handle_cast/2</c></seealso> to handle the request.</p>
        <p>For a description of the arguments, see
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>call(ServerRef, Request) -> Reply</name>
      <name>call(ServerRef, Request, Timeout) -> Reply</name>
      <fsummary>Make a synchronous call to a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Request = term()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>Makes a synchronous call to the <c>ServerRef</c> of the
          <c>gen_server</c> process
          by sending a request and waiting until a reply arrives or a
          time-out occurs. The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso> to handle the request.</p>
        <p><c>ServerRef</c> can be any of the following:</p>
        <list type="bulleted">
          <item>The pid</item>
          <item><c>Name</c>, if the <c>gen_server</c> process is locally
            registered</item>
          <item><c>{Name,Node}</c>, if the <c>gen_server</c> process is locally
            registered at another node</item>
          <item><c>{global,GlobalName}</c>, if the <c>gen_server</c> process is
            globally registered</item>
          <item><c>{via,Module,ViaName}</c>, if the <c>gen_server</c> process is
            registered through an alternative process registry</item>
        </list>
        <p><c>Request</c> is any term that is passed as one of
          the arguments to <c>Module:handle_call/3</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero that
          specifies how many milliseconds to wait for a reply, or
          the atom <c>infinity</c> to wait indefinitely. Defaults to
          5000. If no reply is received within the specified time,
          the function call fails. If the caller catches the failure
          and continues running, and the server is just late with the reply,
          it can arrive at any time later into the message queue of the caller.
          The caller must in this case be prepared for this
          and discard any such garbage messages that are two element
          tuples with a reference as the first element.</p>
        <p>The return value <c>Reply</c> is defined in the return value
          of <c>Module:handle_call/3</c>.</p>
        <p>The call can fail for many reasons, including time-out and the
          called <c>gen_server</c> process dying before or during the call.</p>
      </desc>
    </func>

    <func>
      <name>cast(ServerRef, Request) -> ok</name>
      <fsummary>Send an asynchronous request to a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Request = term()</v>
      </type>
      <desc>
        <p>Sends an asynchronous request to the <c>ServerRef</c> of the
          <c>gen_server</c> process
          and returns <c>ok</c> immediately, ignoring
          if the destination node or <c>gen_server</c> process does not exist.
          The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_cast/2">
          <c>Module:handle_cast/2</c></seealso> to handle the request.</p>
        <p>For a description of <c>ServerRef</c>, see
          <seealso marker="#call/2"><c>call/2,3</c></seealso>.</p>
        <p><c>Request</c> is any term that is passed as one
          of the arguments to <c>Module:handle_cast/2</c>.</p>
      </desc>
    </func>

    <func>
      <name>enter_loop(Module, Options, State)</name>
      <name>enter_loop(Module, Options, State, ServerName)</name>
      <name>enter_loop(Module, Options, State, Timeout)</name>
      <name>enter_loop(Module, Options, State, ServerName, Timeout)</name>
      <fsummary>Enter the <c>gen_server</c> receive loop.</fsummary>
      <type>
        <v>Module = atom()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {hibernate_after,HibernateAfterTimeout}</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>State = term()</v>
        <v>ServerName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Timeout = int() | infinity</v>
      </type>
      <desc>
        <p>Makes an existing process into a <c>gen_server</c> process. Does not
          return, instead the calling process enters the <c>gen_server</c>
          process receive
          loop and becomes a <c>gen_server</c> process. The process
          <em>must</em> have been started using one of the start functions in
          <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>. The user is
          responsible for any initialization of the process, including
          registering a name for it.</p>
        <p>This function is useful when a more complex initialization procedure
          is needed than the <c>gen_server</c> process behavior provides.</p>
        <p><c>Module</c>, <c>Options</c>, and <c>ServerName</c> have
          the same meanings as when calling
          <seealso marker="#start_link/3"><c>start[_link]/3,4</c></seealso>.
          However, if <c>ServerName</c> is specified, the process must
          have been registered accordingly <em>before</em> this function
          is called.</p>
        <p><c>State</c> and <c>Timeout</c> have the same meanings as in
          the return value of
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.
          The callback module <c>Module</c> does not need to
          export an <c>init/1</c> function.</p>
        <p>The function fails if the calling process was not started by a
          <c>proc_lib</c> start function, or if it is not registered
          according to <c>ServerName</c>.</p>
      </desc>
    </func>

    <func>
      <name>multi_call(Name, Request) -> Result</name>
      <name>multi_call(Nodes, Name, Request) -> Result</name>
      <name>multi_call(Nodes, Name, Request, Timeout) -> Result</name>
      <fsummary>Make a synchronous call to many generic servers.</fsummary>
      <type>
        <v>Nodes = [Node]</v>
        <v>&nbsp;Node = atom()</v>
        <v>Name = atom()</v>
        <v>Request = term()</v>
        <v>Timeout = int()>=0 | infinity</v>
        <v>Result = {Replies,BadNodes}</v>
        <v>&nbsp;Replies = [{Node,Reply}]</v>
        <v>&nbsp;&nbsp;Reply = term()</v>
        <v>BadNodes = [Node]</v>
      </type>
      <desc>
        <p>Makes a synchronous call to all <c>gen_server</c> processes locally
          registered as <c>Name</c> at the specified nodes by first
          sending a request to every node and then waits for
          the replies. The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso> to handle the request.</p>
        <p>The function returns a tuple <c>{Replies,BadNodes}</c>, where
          <c>Replies</c> is a list of <c>{Node,Reply}</c> and
          <c>BadNodes</c> is a list of node that either did not exist,
          or where the <c>gen_server</c> <c>Name</c> did not exist or did not
          reply.</p>
        <p><c>Nodes</c> is a list of node names to which the request
          is to be sent. Default value is the list of all known nodes
          <c>[node()|nodes()]</c>.</p>
        <p><c>Name</c> is the locally registered name of each
          <c>gen_server</c> process.</p>
        <p><c>Request</c> is any term that is passed as one of
          the arguments to <c>Module:handle_call/3</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero that
          specifies how many milliseconds to wait for each reply, or
          the atom <c>infinity</c> to wait indefinitely. Defaults
          to <c>infinity</c>. If no reply is received from a node within
          the specified time, the node is added to <c>BadNodes</c>.</p>
        <p>When a reply <c>Reply</c> is received from the <c>gen_server</c>
          process at a node <c>Node</c>, <c>{Node,Reply}</c> is added to
          <c>Replies</c>. <c>Reply</c> is defined in the return value of
          <c>Module:handle_call/3</c>.</p>
        <warning>
          <p>If one of the nodes cannot process monitors, for example,
            C or Java nodes, and the <c>gen_server</c> process is not started
            when the requests are sent, but starts within 2 seconds,
            this function waits the whole <c>Timeout</c>,
            which may be infinity.</p>
          <p>This problem does not exist if all nodes are Erlang nodes.</p>
        </warning>
        <p>To prevent late answers (after the time-out) from polluting
          the message queue of the caller, a middleman process is used to
          do the calls. Late answers are then discarded
          when they arrive to a terminated process.</p>
      </desc>
    </func>

    <func>
      <name>reply(Client, Reply) -> Result</name>
      <fsummary>Send a reply to a client.</fsummary>
      <type>
        <v>Client - see below</v>
        <v>Reply = term()</v>
        <v>Result = term()</v>
      </type>
      <desc>
        <p>This function can be used by a <c>gen_server</c> process to
          explicitly send a reply to a client that called
          <seealso marker="#call/2"><c>call/2,3</c></seealso> or
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>,
          when the reply cannot be defined in the return value of
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
        <p><c>Client</c> must be the <c>From</c> argument provided to
          the callback function. <c>Reply</c> is any term
          given back to the client as the return value of
          <c>call/2,3</c> or <c>multi_call/2,3,4</c>.</p>
        <p>The return value <c>Result</c> is not further defined, and
          is always to be ignored.</p>
      </desc>
    </func>

    <func>
      <name>start(Module, Args, Options) -> Result</name>
      <name>start(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a standalone <c>gen_server</c> process.</fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {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 standalone <c>gen_server</c> process, that is, a
          <c>gen_server</c> process that is not part of a supervision tree
          and thus has no supervisor.</p>
        <p>For a description of arguments and return values, see
          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>start_link(Module, Args, Options) -> Result</name>
      <name>start_link(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a <c>gen_server</c> process in a supervision tree.
      </fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {hibernate_after,HibernateAfterTimeout} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {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 <c>gen_server</c> process as part of a supervision tree.
          This function is to be called, directly or indirectly, by
          the supervisor. For example, it ensures that
          the <c>gen_server</c> process is linked to the supervisor.</p>
        <p>The <c>gen_server</c> process calls
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso> to
          initialize. To ensure a synchronized startup procedure,
          <c>start_link/3,4</c> does not return until
          <c>Module:init/1</c> has returned.</p>
        <list type="bulleted">
          <item>
            <p>If <c>ServerName={local,Name}</c>, the <c>gen_server</c> process
              is registered locally as <c>Name</c> using <c>register/2</c>.</p>
          </item>
          <item>
            <p>If <c>ServerName={global,GlobalName}</c>, the <c>gen_server</c>
              process id registered globally as <c>GlobalName</c> using
              <seealso marker="kernel:global#register_name/2">
              <c>global:register_name/2</c></seealso> If no name is
              provided, the <c>gen_server</c> process is not registered.</p>
          </item>
          <item>
            <p>If <c>ServerName={via,Module,ViaName}</c>, the <c>gen_server</c>
              process registers with the registry represented by <c>Module</c>.
              The <c>Module</c> callback is to export the functions
              <c>register_name/2</c>, <c>unregister_name/1</c>,
              <c>whereis_name/1</c>, and <c>send/2</c>, which are to behave
              like the corresponding functions in
              <seealso marker="kernel:global"><c>global</c></seealso>.
              Thus, <c>{via,global,GlobalName}</c> is a valid reference.</p>
          </item>
        </list>
        <p><c>Module</c> is the name of the callback module.</p>
        <p><c>Args</c> is any term that is passed as
          the argument to
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
        <list type="bulleted">
          <item>
            <p>If option <c>{timeout,Time}</c> is present, the <c>gen_server</c>
              process is allowed to spend <c>Time</c> milliseconds
              initializing or it is terminated and the start function
              returns <c>{error,timeout}</c>.</p>
          </item>
          <item>
            <p>If option <c>{hibernate_after,HibernateAfterTimeout}</c> is present, the <c>gen_server</c>
              process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
              if no message is received, the process goes into hibernation automatically
              (by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).</p>
          </item>
          <item>
            <p>If option <c>{debug,Dbgs}</c> is present,
              the corresponding <c>sys</c> function is called for each
              item in <c>Dbgs</c>; see
              <seealso marker="sys"><c>sys(3)</c></seealso>.</p>
          </item>
          <item>
            <p>If option <c>{spawn_opt,SOpts}</c> is present,
              <c>SOpts</c> is passed as option list to
              the <c>spawn_opt</c> BIF, which is used to spawn
              the <c>gen_server</c> process; see
              <seealso marker="erts:erlang#spawn_opt/2">
              <c>spawn_opt/2</c></seealso>.</p>
          </item>
        </list>
        <note>
          <p>Using spawn option <c>monitor</c> is not
            allowed, it causes the function to fail with reason
            <c>badarg</c>.</p>
        </note>
        <p>If the <c>gen_server</c> process is successfully created and
          initialized, the function returns <c>{ok,Pid}</c>, where <c>Pid</c>
          is the pid of the <c>gen_server</c> process. If a process with the
          specified <c>ServerName</c> exists already, 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>stop(ServerRef) -> ok</name>
      <name>stop(ServerRef, Reason, Timeout) -> ok</name>
      <fsummary>Synchronously stop a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Reason = term()</v>
        <v>Timeout = int()>0 | infinity</v>
      </type>
      <desc>
        <p>Orders a generic server to exit with the specified <c>Reason</c>
          and waits for it to terminate. The <c>gen_server</c> process calls
          <seealso marker="#Module:terminate/2">
          <c>Module:terminate/2</c></seealso> before exiting.</p>
        <p>The function returns <c>ok</c> if the server terminates
          with the expected reason. Any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> causes an
          error report to be issued using
          <seealso marker="kernel:logger"><c>logger(3)</c></seealso>.
          The default <c>Reason</c> is <c>normal</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero that
          specifies how many milliseconds to wait for the server to
          terminate, or the atom <c>infinity</c> to wait
          indefinitely. Defaults to <c>infinity</c>. If the
          server has not terminated within the specified time, a
          <c>timeout</c> exception is raised.</p>
        <p>If the process does not exist, a <c>noproc</c> exception
          is raised.</p>
      </desc>
    </func>
  </funcs>

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

  <funcs>
    <func>
      <name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}</name>
      <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
      <type>
        <v>OldVsn = Vsn | {down, Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>State = NewState = term()</v>
        <v>Extra = term()</v>
        <v>Reason = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not export it.
            If a release upgrade/downgrade with <c>Change={advanced,Extra}</c>
            specified in the <c>appup</c> file is made when <c>code_change/3</c>
            isn't implemented the process will crash with an <c>undef</c> exit
            reason.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when it is
          to update its internal state during a release upgrade/downgrade,
          that is, when the instruction <c>{update,Module,Change,...}</c>,
          where <c>Change={advanced,Extra}</c>, is specifed in
          the <c>appup</c> file. For more information, see section
          <seealso marker="doc/design_principles:release_handling#instr">
          Release Handling Instructions</seealso> in OTP Design Principles.</p>
        <p>For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          for a downgrade, <c>OldVsn</c> is
          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
          attribute(s) of the old version of the callback module
          <c>Module</c>. If no such attribute is defined, the version
          is the checksum of the Beam file.</p>
        <p><c>State</c> is the internal state of the <c>gen_server</c>
          process.</p>
        <p><c>Extra</c> is passed "as is" from the <c>{advanced,Extra}</c>
          part of the update instruction.</p>
        <p>If successful, the function must return the updated
          internal state.</p>
        <p>If the function returns <c>{error,Reason}</c>, the ongoing
          upgrade fails and rolls back to the old release.</p>
      </desc>
    </func>

    <func>
      <name>Module:format_status(Opt, [PDict, State]) -> Status</name>
      <fsummary>Optional function for providing a term describing the
        current <c>gen_server</c> status.</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
        <v>State = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_server</c> module provides a default
            implementation of this function that returns the callback
            module state.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process in the
          following situations:</p>
        <list type="bulleted">
          <item>
            <p>One of <seealso marker="sys#get_status/1">
              <c>sys:get_status/1,2</c></seealso>
              is invoked to get the <c>gen_server</c> status. <c>Opt</c> is set
              to the atom <c>normal</c>.</p>
          </item>
          <item>
            <p>The <c>gen_server</c> process terminates abnormally and logs an
              error. <c>Opt</c> is set to the atom <c>terminate</c>.</p>
          </item>
        </list>
        <p>This function is useful for changing the form and
          appearance of the <c>gen_server</c> status for these cases. A
          callback module wishing to change
          the <c>sys:get_status/1,2</c> return value, as well as how
          its status appears in termination error logs, exports an
          instance of <c>format_status/2</c> that returns a term
          describing the current status of the <c>gen_server</c> process.</p>
        <p><c>PDict</c> is the current value of the process dictionary of
          the <c>gen_server</c> process..</p>
        <p><c>State</c> is the internal state of the <c>gen_server</c>
          process.</p>
        <p>The function is to return <c>Status</c>, a term that
          changes the details of the current state and status of
          the <c>gen_server</c> process. There are no restrictions on the
          form <c>Status</c> can take, but for
          the <c>sys:get_status/1,2</c> case (when <c>Opt</c>
          is <c>normal</c>), the recommended form for
          the <c>Status</c> value is <c>[{data, [{"State",
          Term}]}]</c>, where <c>Term</c> provides relevant details of
          the <c>gen_server</c> state. Following this recommendation is not
          required, but it makes the callback module status
          consistent with the rest of the <c>sys:get_status/1,2</c>
          return value.</p>
        <p>One use for this function is to return compact alternative
          state representations to avoid that large state terms are
          printed in log files.</p>
      </desc>
    </func>

    <func>
      <name>Module:handle_call(Request, From, State) -> Result</name>
      <fsummary>Handle a synchronous request.</fsummary>
      <type>
        <v>Request = term()</v>
        <v>From = {pid(),Tag}</v>
        <v>State = term()</v>
        <v>Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}</v> 
        <v>&nbsp;&nbsp;| {reply,Reply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {reply,Reply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState} | {noreply,NewState,Timeout}</v> 
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,Reply,NewState} | {stop,Reason,NewState}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Continue = term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process receives a request sent using
          <seealso marker="#call/2"><c>call/2,3</c></seealso> or
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>,
          this function is called to handle the request.</p>
        <p><c>Request</c> is the <c>Request</c> argument provided
          to <c>call</c> or <c>multi_call</c>.</p>
        <p><c>From</c> is a tuple <c>{Pid,Tag}</c>, where <c>Pid</c> is
          the pid of the client and <c>Tag</c> is a unique tag.</p>
        <p><c>State</c> is the internal state of the <c>gen_server</c>
          process.</p>
        <list type="bulleted">
          <item>
            <p>If <c>{reply,Reply,NewState}</c> is returned, 
              <c>{reply,Reply,NewState,Timeout}</c> or 
              <c>{reply,Reply,NewState,hibernate}</c>, <c>Reply</c> is
              given back to <c>From</c> as the return value of
              <c>call/2,3</c> or included in the return value of
              <c>multi_call/2,3,4</c>. The <c>gen_server</c> process then
              continues executing with the possibly updated internal state
              <c>NewState</c>.</p>
            <p>For a description of <c>Timeout</c> and <c>hibernate</c>, see
              <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
          </item>
          <item>
            <p>If <c>{noreply,NewState}</c> is returned,
              <c>{noreply,NewState,Timeout}</c>, or
              <c>{noreply,NewState,hibernate}</c>, the <c>gen_server</c>
              process continues executing with <c>NewState</c>. Any reply to
              <c>From</c> must be specified explicitly using
              <seealso marker="#reply/2"><c>reply/2</c></seealso>.</p>
          </item>
          <item>
            <p>If <c>{stop,Reason,Reply,NewState}</c> is returned,
              <c>Reply</c> is given back to <c>From</c>.</p>
          </item>
          <item>
            <p>If <c>{stop,Reason,NewState}</c> is returned, any reply
              to <c>From</c> must be specified explicitly using
              <seealso marker="#reply/2"><c>reply/2</c></seealso>.
              The <c>gen_server</c> process then calls
              <c>Module:terminate(Reason,NewState)</c> and terminates.</p>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name>Module:handle_cast(Request, State) -> Result</name>
      <fsummary>Handle an asynchronous request.</fsummary>
      <type>
        <v>Request = term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Continue = term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process receives a request sent using
          <seealso marker="#cast/2"><c>cast/2</c></seealso> or
          <seealso marker="#abcast/2"><c>abcast/2,3</c></seealso>,
          this function is called to handle the request.</p>
        <p>For a description of the arguments and possible return values, see
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:handle_continue(Continue, State) -> Result</name>
      <fsummary>Handle a continue instruction.</fsummary>
      <type>
        <v>Continue = term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Continue = term()</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need to
            export it only if they return <c>{continue,Continue}</c>
            from another callback. If continue is used and the callback
            is not implemented, the process will exit with <c>undef</c>
            error.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process whenever
          a previous callback returns <c>{continue, Continue}</c>.
          <c>handle_continue/2</c> is invoked immediately after the previous
          callback, which makes it useful for performing work after
          initialization or for splitting the work in a callback in
          multiple steps, updating the process state along the way.</p>
        <p>For a description of the other arguments and possible return values,
          see <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:handle_info(Info, State) -> Result</name>
      <fsummary>Handle an incoming message.</fsummary>
      <type>
        <v>Info = timeout | term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_server</c> module provides a default
            implementation of this function that logs about the unexpected
            <c>Info</c> message, drops it and returns <c>{noreply, State}</c>.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when a
          time-out occurs or when it receives any other message than a
          synchronous or asynchronous request (or a system message).</p>
        <p><c>Info</c> is either the atom <c>timeout</c>, if a time-out
          has occurred, or the received message.</p>
        <p>For a description of the other arguments and possible return values,
          see <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>Initialize process and internal state.</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result =  {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}</v>
        <v>&nbsp;| {ok,State,{continue,Continue}} | {stop,Reason} | ignore</v>
        <v>&nbsp;State = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process is started using
          <seealso marker="#start/3"><c>start/3,4</c></seealso> or
          <seealso marker="#start_link/3"><c>start_link/3,4</c></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 the initialization is successful, the function is to
          return <c>{ok,State}</c>, <c>{ok,State,Timeout}</c>, or
          <c>{ok,State,hibernate}</c>, where <c>State</c> is the internal
          state of the <c>gen_server</c> process.</p>
        <p>If an integer time-out value is provided, a time-out occurs
          unless a request or a message is received within
          <c>Timeout</c> milliseconds. A time-out is represented by
          the atom <c>timeout</c>, which is to be handled by the
          <seealso marker="#Module:handle_info/2">
          <c>Module:handle_info/2</c></seealso> callback function. 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 time-out value,
          the process goes into
          hibernation when waiting for the next message to arrive (by calling 
          <seealso marker="proc_lib#hibernate/3">
          <c>proc_lib:hibernate/3</c></seealso>).</p>
        <p>If the initialization fails, the function is to return
          <c>{stop,Reason}</c>, where <c>Reason</c> is any term, or
          <c>ignore</c>.</p>
      </desc>
    </func>

    <func>
      <name>Module:terminate(Reason, State)</name>
      <fsummary>Clean up before termination.</fsummary>
      <type>
        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
        <v>State = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_server</c> module provides a default
            implementation without cleanup.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when it is
          about to terminate. It is to be the opposite of
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          and do any necessary cleaning up. When it returns,
          the <c>gen_server</c> process terminates with <c>Reason</c>.
          The return value is ignored.</p>
        <p><c>Reason</c> is a term denoting the stop reason and <c>State</c>
          is the internal state of the <c>gen_server</c> process.</p>
        <p><c>Reason</c> depends on why the <c>gen_server</c> process is
          terminating. If it is because another callback function has returned
          a stop tuple <c>{stop,..}</c>, <c>Reason</c> has
          the value specified in that tuple. If it is because of a failure,
          <c>Reason</c> is the error reason.</p>
        <p>If the <c>gen_server</c> process is part of a supervision tree and
          is ordered by its supervisor to terminate, this function is
          called with <c>Reason=shutdown</c> if the following
          conditions apply:</p>
        <list type="bulleted">
          <item>
            <p>The <c>gen_server</c> process has been set to trap exit
              signals.</p>
          </item>
          <item>
            <p>The shutdown strategy as defined in the child specification
              of the supervisor is an integer time-out value, not
              <c>brutal_kill</c>.</p>
           </item>
        </list>
        <p>Even if the <c>gen_server</c> process is <em>not</em> part of a
          supervision tree, this function is called if it receives an
          <c>'EXIT'</c> message from its parent. <c>Reason</c> is the same
          as in the <c>'EXIT'</c> message.</p>
        <p>Otherwise, the <c>gen_server</c> process terminates immediately.</p>
        <p>Notice that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c>, the <c>gen_server</c>
          process is assumed to terminate because of an error and
          an error report is issued using
          <seealso marker="kernel:logger"><c>logger(3)</c></seealso>.</p>
      </desc>
    </func>
  </funcs>

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