1gen_server(3) Erlang Module Definition gen_server(3)
2
6 gen_server - Generic server behavior.
7
9 This behavior module provides the server of a client-server relation. A
10 generic server process (gen_server) implemented using this module has a
11 standard set of interface functions and includes functionality for
12 tracing and error reporting. It also fits into an OTP supervision tree.
13 For more information, see section gen_server Behaviour in OTP Design
14 Principles.
15 A gen_server process assumes all specific parts to be located in a
17 callback module exporting a predefined set of functions. The relation‐
18 ship between the behavior functions and the callback functions is as
19 follows:
20 gen_server module Callback module
22 ----------------- ---------------
23 gen_server:start
24 gen_server:start_monitor
25 gen_server:start_link -----> Module:init/1
26 gen_server:stop -----> Module:terminate/2
28 gen_server:call
30 gen_server:send_request
31 gen_server:multi_call -----> Module:handle_call/3
32 gen_server:cast
34 gen_server:abcast -----> Module:handle_cast/2
35 - -----> Module:handle_info/2
37 - -----> Module:handle_continue/2
39 - -----> Module:terminate/2
41 - -----> Module:code_change/3
43 If a callback function fails or returns a bad value, the gen_server
45 process terminates.
46 A gen_server process handles system messages as described in sys(3).
48 The sys module can be used for debugging a gen_server process.
49 Notice that a gen_server process does not trap exit signals automati‐
51 cally, this must be explicitly initiated in the callback module.
52 Unless otherwise stated, all functions in this module fail if the spec‐
54 ified gen_server process does not exist or if bad arguments are speci‐
55 fied.
56 The gen_server process can go into hibernation (see erlang:hibernate/3)
58 if a callback function specifies 'hibernate' instead of a time-out
59 value. This can be useful if the server is expected to be idle for a
60 long time. However, use this feature with care, as hibernation implies
61 at least two garbage collections (when hibernating and shortly after
62 waking up) and is not something you want to do between each call to a
63 busy server.
64 If the gen_server process needs to perform an action immediately after
66 initialization or to break the execution of a callback into multiple
67 steps, it can return {continue,Continue} in place of the time-out or
68 hibernation value, which will immediately invoke the handle_continue/2
69 callback.
70 If the gen_server process terminates, e.g. as a result of a function in
72 the callback module returning {stop,Reason,NewState}, an exit signal
73 with this Reason is sent to linked processes and ports. See Processes
74 in the Reference Manual for details regarding error handling using exit
75 signals.
76
78 abcast(Name, Request) -> abcast
79 abcast(Nodes, Name, Request) -> abcast
80 Types:
82 Nodes = [Node]
84 Node = atom()
85 Name = atom()
86 Request = term()
87 Sends an asynchronous request to the gen_server processes lo‐
89 cally registered as Name at the specified nodes. The function
90 returns immediately and ignores nodes that do not exist, or
91 where the gen_server Name does not exist. The gen_server pro‐
92 cesses call Module:handle_cast/2 to handle the request.
93 For a description of the arguments, see multi_call/2,3,4.
95 call(ServerRef, Request) -> Reply
97 call(ServerRef, Request, Timeout) -> Reply
98 Types:
100 ServerRef = Name | {Name,Node} | {global,GlobalName}
102 | {via,Module,ViaName} | pid()
103 Node = atom()
104 GlobalName = ViaName = term()
105 Request = term()
106 Timeout = int()>0 | infinity
107 Reply = term()
108 Makes a synchronous call to the ServerRef of the gen_server
110 process by sending a request and waiting until a reply arrives
111 or a time-out occurs. The gen_server process calls Module:han‐
112 dle_call/3 to handle the request.
113 ServerRef can be any of the following:
115 * The pid
117 * Name, if the gen_server process is locally registered
119 * {Name,Node}, if the gen_server process is locally registered
121 at another node
122 * {global,GlobalName}, if the gen_server process is globally
124 registered
125 * {via,Module,ViaName}, if the gen_server process is regis‐
127 tered through an alternative process registry
128 Request is any term that is passed as the first argument to Mod‐
130 ule:handle_call/3.
131 Timeout is an integer greater than zero that specifies how many
133 milliseconds to wait for a reply, or the atom infinity to wait
134 indefinitely. Defaults to 5000. If no reply is received within
135 the specified time, the function call fails. If the caller
136 catches the failure and continues running, and the server is
137 just late with the reply, it can arrive at any time later into
138 the message queue of the caller. The caller must in this case be
139 prepared for this and discard any such garbage messages that are
140 two element tuples with a reference as the first element.
141 The return value Reply is defined in the return value of Mod‐
143 ule:handle_call/3.
144 The call can fail for many reasons, including time-out and the
146 called gen_server process dying before or during the call.
147 cast(ServerRef, Request) -> ok
149 Types:
151 ServerRef = Name | {Name,Node} | {global,GlobalName}
153 | {via,Module,ViaName} | pid()
154 Node = atom()
155 GlobalName = ViaName = term()
156 Request = term()
157 Sends an asynchronous request to the ServerRef of the gen_server
159 process and returns ok immediately, ignoring if the destination
160 node or gen_server process does not exist. The gen_server
161 process calls Module:handle_cast/2 to handle the request.
162 For a description of ServerRef, see call/2,3.
164 Request is any term that is passed as one of the arguments to
166 Module:handle_cast/2.
167 check_response(Msg, RequestId) -> Result
169 Types:
171 RequestId = term()
173 Result = {reply, Reply} | no_reply | {error, {Reason, Server‐
174 Ref}}
175 Msg = Reply = term()
176 Timeout = timeout()
177 Reason = term()
178 ServerRef = Name | {Name,Node} | {global,GlobalName}
179 | {via,Module,ViaName} | pid()
180 Node = atom()
181 GlobalName = ViaName = term()
182 This function is used to check if a previously received message,
184 for example by receive or handle_info/2, is a result of a re‐
185 quest made with send_request/2. If Msg is a reply to the handle
186 RequestId the result of the request is returned in Reply. Other‐
187 wise returns no_reply and no cleanup is done, and thus the func‐
188 tion must be invoked repeatedly until a reply is returned.
189 The return value Reply is defined in the return value of Mod‐
191 ule:handle_call/3.
192 The function returns an error if the gen_server dies before or
194 during this request.
195 enter_loop(Module, Options, State)
197 enter_loop(Module, Options, State, ServerName)
198 enter_loop(Module, Options, State, Timeout)
199 enter_loop(Module, Options, State, ServerName, Timeout)
200 Types:
202 Module = atom()
204 Options = [Option]
205 Option = {debug,Dbgs} | {hibernate_after,HibernateAfterTime‐
206 out}
207 Dbgs = [Dbg]
208 Dbg = trace | log | statistics
209 | {log_to_file,FileName} | {install,{Func,FuncState}}
210 State = term()
211 ServerName = {local,Name} | {global,GlobalName}
212 | {via,Module,ViaName}
213 Name = atom()
214 GlobalName = ViaName = term()
215 Timeout = int() | infinity
216 Makes an existing process into a gen_server process. Does not
218 return, instead the calling process enters the gen_server
219 process receive loop and becomes a gen_server process. The
220 process must have been started using one of the start functions
221 in proc_lib(3). The user is responsible for any initialization
222 of the process, including registering a name for it.
223 This function is useful when a more complex initialization pro‐
225 cedure is needed than the gen_server process behavior provides.
226 Module, Options, and ServerName have the same meanings as when
228 calling start[_link|_monitor]/3,4. However, if ServerName is
229 specified, the process must have been registered accordingly be‐
230 fore this function is called.
231 State and Timeout have the same meanings as in the return value
233 of Module:init/1. The callback module Module does not need to
234 export an init/1 function.
235 The function fails if the calling process was not started by a
237 proc_lib start function, or if it is not registered according to
238 ServerName.
239 multi_call(Name, Request) -> Result
241 multi_call(Nodes, Name, Request) -> Result
242 multi_call(Nodes, Name, Request, Timeout) -> Result
243 Types:
245 Nodes = [Node]
247 Node = atom()
248 Name = atom()
249 Request = term()
250 Timeout = int()>=0 | infinity
251 Result = {Replies,BadNodes}
252 Replies = [{Node,Reply}]
253 Reply = term()
254 BadNodes = [Node]
255 Makes a synchronous call to all gen_server processes locally
257 registered as Name at the specified nodes by first sending a re‐
258 quest to every node and then waits for the replies. The
259 gen_server process calls Module:handle_call/3 to handle the re‐
260 quest.
261 The function returns a tuple {Replies,BadNodes}, where Replies
263 is a list of {Node,Reply} and BadNodes is a list of node that
264 either did not exist, or where the gen_server Name did not exist
265 or did not reply.
266 Nodes is a list of node names to which the request is to be
268 sent. Default value is the list of all known nodes
269 [node()|nodes()].
270 Name is the locally registered name of each gen_server process.
272 Request is any term that is passed as the first argument to Mod‐
274 ule:handle_call/3.
275 Timeout is an integer greater than zero that specifies how many
277 milliseconds to wait for each reply, or the atom infinity to
278 wait indefinitely. Defaults to infinity. If no reply is received
279 from a node within the specified time, the node is added to
280 BadNodes.
281 When a reply Reply is received from the gen_server process at a
283 node Node, {Node,Reply} is added to Replies. Reply is defined in
284 the return value of Module:handle_call/3.
285 Warning:
287 If one of the nodes cannot process monitors, for example, C or
288 Java nodes, and the gen_server process is not started when the
289 requests are sent, but starts within 2 seconds, this function
290 waits the whole Timeout, which may be infinity.
291 This problem does not exist if all nodes are Erlang nodes.
293 To prevent late answers (after the time-out) from polluting the
296 message queue of the caller, a middleman process is used to do
297 the calls. Late answers are then discarded when they arrive to a
298 terminated process.
299 receive_response(RequestId, Timeout) -> Result
301 Types:
303 RequestId = term()
305 Result = {reply, Reply} | timeout | {error, {Reason, Server‐
306 Ref}}
307 Reply = term()
308 Timeout = timeout()
309 Reason = term()
310 ServerRef = Name | {Name,Node} | {global,GlobalName}
311 | {via,Module,ViaName} | pid()
312 Node = atom()
313 GlobalName = ViaName = term()
314 This function is used to receive a reply of a request made with
316 send_request/2 to a gen_server process. This function must be
317 called from the same process from which send_request/2 was made.
318 Timeout is an integer greater then or equal to zero that speci‐
320 fies how many milliseconds to wait for an reply, or the atom in‐
321 finity to wait indefinitely. If no reply is received within the
322 specified time, the function returns timeout. Assuming that the
323 server executes on a node supporting aliases (introduced in OTP
324 24) no response will be received after a timeout. Otherwise, a
325 garbage response might be received at a later time.
326 The return value Reply is defined in the return value of Mod‐
328 ule:handle_call/3.
329 The function returns an error if the gen_server dies before or
331 during this request.
332 The difference between wait_response() and receive_response() is
334 that receive_response() abandons the request at timeout so that
335 a potential future response is ignored, while wait_response()
336 does not.
337 reply(Client, Reply) -> ok
339 Types:
341 Client - see below
343 Reply = term()
344 This function can be used by a gen_server process to explicitly
346 send a reply to a client that called call/2,3 or
347 multi_call/2,3,4, when the reply cannot be defined in the return
348 value of Module:handle_call/3.
349 Client must be the From argument provided to the callback func‐
351 tion. Reply is any term given back to the client as the return
352 value of call/2,3 or multi_call/2,3,4.
353 send_request(ServerRef, Request) -> RequestId
355 Types:
357 ServerRef = Name | {Name,Node} | {global,GlobalName}
359 | {via,Module,ViaName} | pid()
360 Node = atom()
361 GlobalName = ViaName = term()
362 RequestId = term()
363 Timeout = int()>0 | infinity
364 Request = term()
365 Sends a request to the ServerRef of the gen_server process and
367 returns a handle RequestId. The return value RequestId shall
368 later be used with receive_response/2, wait_response/2, or
369 check_response/2 to fetch the actual result of the request.
370 The call gen_server:wait_response(gen_server:send_re‐
372 quest(ServerRef,Request), Timeout) can be seen as equivalent to
373 gen_server:call(Server,Request,Timeout), ignoring the error han‐
374 dling.
375 The gen_server process calls Module:handle_call/3 to handle the
377 request.
378 ServerRef can be any of the following:
380 * The pid
382 * Name, if the gen_server process is locally registered
384 * {Name,Node}, if the gen_server process is locally registered
386 at another node
387 * {global,GlobalName}, if the gen_server process is globally
389 registered
390 * {via,Module,ViaName}, if the gen_server process is regis‐
392 tered through an alternative process registry
393 Request is any term that is passed as the first argument to Mod‐
395 ule:handle_call/3.
396 start(Module, Args, Options) -> Result
398 start(ServerName, Module, Args, Options) -> Result
399 Types:
401 ServerName = {local,Name} | {global,GlobalName}
403 | {via,Module,ViaName}
404 Name = atom()
405 GlobalName = ViaName = term()
406 Module = atom()
407 Args = term()
408 Options = [Option]
409 Option = {debug,Dbgs} | {timeout,Time} | {hibernate_af‐
410 ter,HibernateAfterTimeout} | {spawn_opt,SOpts}
411 Dbgs = [Dbg]
412 Dbg = trace | log | statistics | {log_to_file,FileName} |
413 {install,{Func,FuncState}}
414 SOpts = [term()]
415 Result = {ok,Pid} | ignore | {error,Error}
416 Pid = pid()
417 Error = {already_started,Pid} | term()
418 Creates a standalone gen_server process, that is, a gen_server
420 process that is not part of a supervision tree and thus has no
421 supervisor.
422 For a description of arguments and return values, see
424 start_link/3,4.
425 start_link(Module, Args, Options) -> Result
427 start_link(ServerName, Module, Args, Options) -> Result
428 Types:
430 ServerName = {local,Name} | {global,GlobalName}
432 | {via,Module,ViaName}
433 Name = atom()
434 GlobalName = ViaName = term()
435 Module = atom()
436 Args = term()
437 Options = [Option]
438 Option = {debug,Dbgs} | {timeout,Time} | {hibernate_af‐
439 ter,HibernateAfterTimeout} | {spawn_opt,SOpts}
440 Dbgs = [Dbg]
441 Dbg = trace | log | statistics | {log_to_file,FileName} |
442 {install,{Func,FuncState}}
443 SOpts = [term()]
444 Result = {ok,Pid} | ignore | {error,Error}
445 Pid = pid()
446 Error = {already_started,Pid} | term()
447 Creates a gen_server process as part of a supervision tree. This
449 function is to be called, directly or indirectly, by the super‐
450 visor. For example, it ensures that the gen_server process is
451 linked to the supervisor.
452 The gen_server process calls Module:init/1 to initialize. To en‐
454 sure a synchronized startup procedure, start_link/3,4 does not
455 return until Module:init/1 has returned.
456 * If ServerName={local,Name}, the gen_server process is regis‐
458 tered locally as Name using register/2.
459 * If ServerName={global,GlobalName}, the gen_server process id
461 registered globally as GlobalName using global:regis‐
462 ter_name/2 If no name is provided, the gen_server process is
463 not registered.
464 * If ServerName={via,Module,ViaName}, the gen_server process
466 registers with the registry represented by Module. The Mod‐
467 ule callback is to export the functions register_name/2, un‐
468 register_name/1, whereis_name/1, and send/2, which are to
469 behave like the corresponding functions in global. Thus,
470 {via,global,GlobalName} is a valid reference.
471 Module is the name of the callback module.
473 Args is any term that is passed as the argument to Mod‐
475 ule:init/1.
476 * If option {timeout,Time} is present, the gen_server process
478 is allowed to spend Time milliseconds initializing or it is
479 terminated and the start function returns {error,timeout}.
480 * If option {hibernate_after,HibernateAfterTimeout} is
482 present, the gen_server process awaits any message for Hi‐
483 bernateAfterTimeout milliseconds and if no message is re‐
484 ceived, the process goes into hibernation automatically (by
485 calling proc_lib:hibernate/3).
486 * If option {debug,Dbgs} is present, the corresponding sys
488 function is called for each item in Dbgs; see sys(3).
489 * If option {spawn_opt,SOpts} is present, SOpts is passed as
491 option list to the spawn_opt BIF, which is used to spawn the
492 gen_server process; see spawn_opt/2.
493 Note:
495 Using spawn option monitor is not allowed, it causes the func‐
496 tion to fail with reason badarg.
497 If the gen_server process is successfully created and initial‐
500 ized, the function returns {ok,Pid}, where Pid is the pid of the
501 gen_server process. If a process with the specified ServerName
502 exists already, the function returns {error,{al‐
503 ready_started,Pid}}, where Pid is the pid of that process.
504 If Module:init/1 fails with Reason, the function returns {er‐
506 ror,Reason}. If Module:init/1 returns {stop,Reason} or ignore,
507 the process is terminated and the function returns {error,Rea‐
508 son} or ignore, respectively. An exit signal with the same Rea‐
509 son (or normal if Module:init/1 returns ignore) is sent to
510 linked processes and ports.
511 start_monitor(Module, Args, Options) -> Result
513 start_monitor(ServerName, Module, Args, Options) -> Result
514 Types:
516 ServerName = {local,Name} | {global,GlobalName}
518 | {via,Module,ViaName}
519 Name = atom()
520 GlobalName = ViaName = term()
521 Module = atom()
522 Args = term()
523 Options = [Option]
524 Option = {debug,Dbgs} | {timeout,Time} | {hibernate_af‐
525 ter,HibernateAfterTimeout} | {spawn_opt,SOpts}
526 Dbgs = [Dbg]
527 Dbg = trace | log | statistics | {log_to_file,FileName} |
528 {install,{Func,FuncState}}
529 SOpts = [term()]
530 Result = {ok,{Pid,Mon}} | ignore | {error,Error}
531 Pid = pid()
532 Error = {already_started,Pid} | term()
533 Creates a standalone gen_server process, that is, a gen_server
535 process that is not part of a supervision tree (and thus has no
536 supervisor) and atomically sets up a monitor to the newly cre‐
537 ated server.
538 For a description of arguments and return values, see
540 start_link/3,4. Note that the return value on successful start
541 differs from start_link/3,4. start_monitor/3,4 will return
542 {ok,{Pid,Mon}} where Pid is the process identifier of the
543 server, and Mon is a reference to the monitor set up to monitor
544 the server. If the start is not successful, the caller will be
545 blocked until the DOWN message has been received and removed
546 from the message queue.
547 stop(ServerRef) -> ok
549 stop(ServerRef, Reason, Timeout) -> ok
550 Types:
552 ServerRef = Name | {Name,Node} | {global,GlobalName}
554 | {via,Module,ViaName} | pid()
555 Node = atom()
556 GlobalName = ViaName = term()
557 Reason = term()
558 Timeout = int()>0 | infinity
559 Orders a generic server to exit with the specified Reason and
561 waits for it to terminate. The gen_server process calls Mod‐
562 ule:terminate/2 before exiting.
563 The function returns ok if the server terminates with the ex‐
565 pected reason. Any other reason than normal, shutdown, or {shut‐
566 down,Term} causes an error report to be issued using logger(3).
567 An exit signal with the same reason is sent to linked processes
568 and ports. The default Reason is normal.
569 Timeout is an integer greater than zero that specifies how many
571 milliseconds to wait for the server to terminate, or the atom
572 infinity to wait indefinitely. Defaults to infinity. If the
573 server has not terminated within the specified time, a timeout
574 exception is raised.
575 If the process does not exist, a noproc exception is raised.
577 wait_response(RequestId, Timeout) -> Result
579 Types:
581 RequestId = term()
583 Result = {reply, Reply} | timeout | {error, {Reason, Server‐
584 Ref}}
585 Reply = term()
586 Timeout = timeout()
587 Reason = term()
588 ServerRef = Name | {Name,Node} | {global,GlobalName}
589 | {via,Module,ViaName} | pid()
590 Node = atom()
591 GlobalName = ViaName = term()
592 This function is used to wait for a reply of a request made with
594 send_request/2 from the gen_server process. This function must
595 be called from the same process from which send_request/2 was
596 made.
597 Timeout is an integer greater then or equal to zero that speci‐
599 fies how many milliseconds to wait for an reply, or the atom in‐
600 finity to wait indefinitely. If no reply is received within the
601 specified time, the function returns timeout and no cleanup is
602 done, and thus the function can be invoked repeatedly until a
603 reply is returned.
604 The return value Reply is defined in the return value of Mod‐
606 ule:handle_call/3.
607 The function returns an error if the gen_server dies before or
609 during this request.
610 The difference between receive_response() and wait_response() is
612 that receive_response() abandons the request at timeout so that
613 a potential future response is ignored, while wait_response()
614 does not.
615
617 The following functions are to be exported from a gen_server callback
618 module.
619
621 Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error,
622 Reason}
623 Types:
625 OldVsn = Vsn | {down, Vsn}
627 Vsn = term()
628 State = NewState = term()
629 Extra = term()
630 Reason = term()
631 Note:
633 This callback is optional, so callback modules need not export
634 it. If a release upgrade/downgrade with Change={advanced,Extra}
635 specified in the appup file is made when code_change/3 isn't im‐
636 plemented the process will crash with an undef exit reason.
637 This function is called by a gen_server process when it is to
640 update its internal state during a release upgrade/downgrade,
641 that is, when the instruction {update,Module,Change,...}, where
642 Change={advanced,Extra}, is specifed in the appup file. For more
643 information, see section Release Handling Instructions in OTP
644 Design Principles.
645 For an upgrade, OldVsn is Vsn, and for a downgrade, OldVsn is
647 {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old
648 version of the callback module Module. If no such attribute is
649 defined, the version is the checksum of the Beam file.
650 State is the internal state of the gen_server process.
652 Extra is passed "as is" from the {advanced,Extra} part of the
654 update instruction.
655 If successful, the function must return the updated internal
657 state.
658 If the function returns {error,Reason}, the ongoing upgrade
660 fails and rolls back to the old release.
661 Module:format_status(Opt, [PDict, State]) -> Status
663 Types:
665 Opt = normal | terminate
667 PDict = [{Key, Value}]
668 State = term()
669 Status = term()
670 Note:
672 This callback is optional, so callback modules need not export
673 it. The gen_server module provides a default implementation of
674 this function that returns the callback module state.
675 This function is called by a gen_server process in the following
678 situations:
679 * One of sys:get_status/1,2 is invoked to get the gen_server
681 status. Opt is set to the atom normal.
682 * The gen_server process terminates abnormally and logs an er‐
684 ror. Opt is set to the atom terminate.
685 This function is useful for changing the form and appearance of
687 the gen_server status for these cases. A callback module wishing
688 to change the sys:get_status/1,2 return value, as well as how
689 its status appears in termination error logs, exports an in‐
690 stance of format_status/2 that returns a term describing the
691 current status of the gen_server process.
692 PDict is the current value of the process dictionary of the
694 gen_server process..
695 State is the internal state of the gen_server process.
697 The function is to return Status, a term that changes the de‐
699 tails of the current state and status of the gen_server process.
700 There are no restrictions on the form Status can take, but for
701 the sys:get_status/1,2 case (when Opt is normal), the recom‐
702 mended form for the Status value is [{data, [{"State", Term}]}],
703 where Term provides relevant details of the gen_server state.
704 Following this recommendation is not required, but it makes the
705 callback module status consistent with the rest of the
706 sys:get_status/1,2 return value.
707 One use for this function is to return compact alternative state
709 representations to avoid that large state terms are printed in
710 log files.
711 Module:handle_call(Request, From, State) -> Result
713 Types:
715 Request = term()
717 From = {pid(),Tag}
718 State = term()
719 Result = {reply,Reply,NewState} | {reply,Reply,NewState,Time‐
720 out}
721 | {reply,Reply,NewState,hibernate}
722 | {reply,Reply,NewState,{continue,Continue}}
723 | {noreply,NewState} | {noreply,NewState,Timeout}
724 | {noreply,NewState,hibernate}
725 | {noreply,NewState,{continue,Continue}}
726 | {stop,Reason,Reply,NewState} | {stop,Reason,NewState}
727 Reply = term()
728 NewState = term()
729 Timeout = int()>=0 | infinity
730 Continue = term()
731 Reason = term()
732 Whenever a gen_server process receives a request sent using
734 call/2,3 or multi_call/2,3,4, this function is called to handle
735 the request.
736 Request is the Request argument provided to call or multi_call.
738 From is a tuple {Pid,Tag}, where Pid is the pid of the client
740 and Tag is a unique tag.
741 State is the internal state of the gen_server process.
743 * If {reply,Reply,NewState} is returned, {reply,Reply,New‐
745 State,Timeout} or {reply,Reply,NewState,hibernate}, Reply is
746 given back to From as the return value of call/2,3 or in‐
747 cluded in the return value of multi_call/2,3,4. The
748 gen_server process then continues executing with the possi‐
749 bly updated internal state NewState.
750 For a description of Timeout and hibernate, see Mod‐
752 ule:init/1.
753 * If {noreply,NewState} is returned, {noreply,NewState,Time‐
755 out}, or {noreply,NewState,hibernate}, the gen_server
756 process continues executing with NewState. Any reply to From
757 must be specified explicitly using reply/2.
758 * If {stop,Reason,Reply,NewState} is returned, Reply is given
760 back to From.
761 * If {stop,Reason,NewState} is returned, any reply to From
763 must be specified explicitly using reply/2. The gen_server
764 process then calls Module:terminate(Reason,NewState) and
765 terminates.
766 Module:handle_cast(Request, State) -> Result
768 Types:
770 Request = term()
772 State = term()
773 Result = {noreply,NewState} | {noreply,NewState,Timeout}
774 | {noreply,NewState,hibernate}
775 | {noreply,NewState,{continue,Continue}}
776 | {stop,Reason,NewState}
777 NewState = term()
778 Timeout = int()>=0 | infinity
779 Continue = term()
780 Reason = term()
781 Whenever a gen_server process receives a request sent using
783 cast/2 or abcast/2,3, this function is called to handle the re‐
784 quest.
785 For a description of the arguments and possible return values,
787 see Module:handle_call/3.
788 Module:handle_continue(Continue, State) -> Result
790 Types:
792 Continue = term()
794 State = term()
795 Result = {noreply,NewState} | {noreply,NewState,Timeout}
796 | {noreply,NewState,hibernate}
797 | {noreply,NewState,{continue,Continue}}
798 | {stop,Reason,NewState}
799 NewState = term()
800 Timeout = int()>=0 | infinity
801 Continue = term()
802 Reason = normal | term()
803 Note:
805 This callback is optional, so callback modules need to export it
806 only if they return {continue,Continue} from another callback.
807 If continue is used and the callback is not implemented, the
808 process will exit with undef error.
809 This function is called by a gen_server process whenever a pre‐
812 vious callback returns {continue, Continue}. handle_continue/2
813 is invoked immediately after the previous callback, which makes
814 it useful for performing work after initialization or for split‐
815 ting the work in a callback in multiple steps, updating the
816 process state along the way.
817 For a description of the other arguments and possible return
819 values, see Module:handle_call/3.
820 Module:handle_info(Info, State) -> Result
822 Types:
824 Info = timeout | term()
826 State = term()
827 Result = {noreply,NewState} | {noreply,NewState,Timeout}
828 | {noreply,NewState,hibernate}
829 | {noreply,NewState,{continue,Continue}}
830 | {stop,Reason,NewState}
831 NewState = term()
832 Timeout = int()>=0 | infinity
833 Reason = normal | term()
834 Note:
836 This callback is optional, so callback modules need not export
837 it. The gen_server module provides a default implementation of
838 this function that logs about the unexpected Info message, drops
839 it and returns {noreply, State}.
840 This function is called by a gen_server process when a time-out
843 occurs or when it receives any other message than a synchronous
844 or asynchronous request (or a system message).
845 Info is either the atom timeout, if a time-out has occurred, or
847 the received message.
848 For a description of the other arguments and possible return
850 values, see Module:handle_call/3.
851 Module:init(Args) -> Result
853 Types:
855 Args = term()
857 Result = {ok,State} | {ok,State,Timeout} | {ok,State,hiber‐
858 nate}
859 | {ok,State,{continue,Continue}} | {stop,Reason} | ignore
860 State = term()
861 Timeout = int()>=0 | infinity
862 Reason = term()
863 Whenever a gen_server process is started using start/3,4,
865 start_monitor/3,4, or start_link/3,4, this function is called by
866 the new process to initialize.
867 Args is the Args argument provided to the start function.
869 If the initialization is successful, the function is to return
871 {ok,State}, {ok,State,Timeout}, {ok,State,hibernate}, or
872 {ok,State,{continue,Continue}} where State is the internal state
873 of the gen_server process.
874 If an integer time-out value is provided, a time-out occurs un‐
876 less a request or a message is received within Timeout millisec‐
877 onds. A time-out is represented by the atom timeout, which is to
878 be handled by the Module:handle_info/2 callback function. The
879 atom infinity can be used to wait indefinitely, this is the de‐
880 fault value.
881 If hibernate is specified instead of a time-out value, the
883 process goes into hibernation when waiting for the next message
884 to arrive (by calling proc_lib:hibernate/3).
885 If {continue,Continue} is specified, the process will execute
887 the Module:handle_continue/2 callback function, with Continue as
888 the first argument.
889 If the initialization fails, the function is to return
891 {stop,Reason}, where Reason is any term, or ignore. An exit sig‐
892 nal with this Reason (or with reason normal if ignore is re‐
893 turned) is sent to linked processes and ports, notably to the
894 process starting the gen_server when start_link/3,4 is used.
895 Module:terminate(Reason, State)
897 Types:
899 Reason = normal | shutdown | {shutdown,term()} | term()
901 State = term()
902 Note:
904 This callback is optional, so callback modules need not export
905 it. The gen_server module provides a default implementation
906 without cleanup.
907 This function is called by a gen_server process when it is about
910 to terminate. It is to be the opposite of Module:init/1 and do
911 any necessary cleaning up. When it returns, the gen_server
912 process terminates with Reason. The return value is ignored.
913 Reason is a term denoting the stop reason and State is the in‐
915 ternal state of the gen_server process.
916 Reason depends on why the gen_server process is terminating. If
918 it is because another callback function has returned a stop tu‐
919 ple {stop,..}, Reason has the value specified in that tuple. If
920 it is because of a failure, Reason is the error reason.
921 If the gen_server process is part of a supervision tree and is
923 ordered by its supervisor to terminate, this function is called
924 with Reason=shutdown if the following conditions apply:
925 * The gen_server process has been set to trap exit signals.
927 * The shutdown strategy as defined in the child specification
929 of the supervisor is an integer time-out value, not bru‐
930 tal_kill.
931 Even if the gen_server process is not part of a supervision
933 tree, this function is called if it receives an 'EXIT' message
934 from its parent. Reason is the same as in the 'EXIT' message.
935 Otherwise, the gen_server process terminates immediately.
937 Notice that for any other reason than normal, shutdown, or
939 {shutdown,Term}, the gen_server process is assumed to terminate
940 because of an error and an error report is issued using log‐
941 ger(3).
942 When the gen_server process exits, an exit signal with the same
944 reason is sent to linked processes and ports.
945
947 gen_event(3), gen_statem(3), proc_lib(3), supervisor(3), sys(3)
948Ericsson AB stdlib 3.16.1 gen_server(3)