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<!DOCTYPE erlref SYSTEM "erlref.dtd">
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<header>
<copyright>
<year>1996</year><year>2016</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.
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<title>gen_fsm</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>gen_fsm</module>
<modulesummary>Generic finite state machine behavior.</modulesummary>
<description>
<note>
<p>
There is a new behaviour
<seealso marker="gen_statem"><c>gen_statem</c></seealso>
that is intended to replace <c>gen_fsm</c> for new code.
<c>gen_fsm</c> will not be removed for the foreseeable future
to keep old state machine implementations running.
</p>
</note>
<p>This behavior module provides a finite state machine.
A generic finite state machine process (<c>gen_fsm</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
<seealso marker="doc/design_principles:fsm">OTP Design Principles</seealso>.
</p>
<p>A <c>gen_fsm</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_fsm module Callback module
-------------- ---------------
gen_fsm:start
gen_fsm:start_link -----> Module:init/1
gen_fsm:stop -----> Module:terminate/3
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 <c>gen_fsm</c>
process terminates.</p>
<p>A <c>gen_fsm</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_fsm</c> process.</p>
<p>Notice that a <c>gen_fsm</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_fsm</c> process does not exist or if bad arguments
are specified.</p>
<p>The <c>gen_fsm</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 state machine.</p>
</description>
<funcs>
<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
<c>gen_fsm</c> process that calls this function.</p>
<p><c>Ref</c> is a reference returned from
<seealso marker="#send_event_after/2">
<c>send_event_after/2</c></seealso> or
<seealso marker="#start_timer/2"><c>start_timer/2</c></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 is no false timer event after
returning from this function.</p>
<p>Returns the remaining time in milliseconds until the timer would
have expired if <c>Ref</c> referred to an active timer, otherwise
<c>false</c>.</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 <c>gen_fsm</c> receive loop.</fsummary>
<type>
<v>Module = atom()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics</v>
<v> | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v>StateName = atom()</v>
<v>StateData = term()</v>
<v>FsmName = {local,Name} | {global,GlobalName}</v>
<v> | {via,Module,ViaName}</v>
<v> Name = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Timeout = int() | infinity</v>
</type>
<desc>
<p>Makes an existing process into a <c>gen_fsm</c> process.
Does not return,
instead the calling process enters the <c>gen_fsm</c> receive
loop and becomes a <c>gen_fsm</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_fsm</c> behavior 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"><c>start[_link]/3,4</c></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"><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>FsmName</c>.</p>
</desc>
</func>
<func>
<name>reply(Caller, Reply) -> Result</name>
<fsummary>Send a reply to a caller.</fsummary>
<type>
<v>Caller - see below</v>
<v>Reply = term()</v>
<v>Result = term()</v>
</type>
<desc>
<p>This function can be used by a <c>gen_fsm</c> process to
explicitly send a reply to a client process that called
<seealso marker="#sync_send_event/2">
<c>sync_send_event/2,3</c></seealso> or
<seealso marker="#sync_send_all_state_event/2">
<c>sync_send_all_state_event/2,3</c></seealso>
when the reply cannot be defined in the return value of
<seealso marker="#Module:StateName/3">
<c>Module:StateName/3</c></seealso> or
<seealso marker="#Module:handle_sync_event/4">
<c>Module:handle_sync_event/4</c></seealso>.</p>
<p><c>Caller</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>sync_send_event/2,3</c> or
<c>sync_send_all_state_event/2,3</c>.</p>
<p>Return value <c>Result</c> is not further defined, and
is always to be ignored.</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}</v>
<v> | {via,Module,ViaName} | pid()</v>
<v> Name = Node = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Event = term()</v>
</type>
<desc>
<p>Sends an event asynchronously to the <c>FsmRef</c> of the
<c>gen_fsm</c> process and returns <c>ok</c> immediately.
The <c>gen_fsm</c> process calls
<seealso marker="#Module:handle_event/3">
<c>Module:handle_event/3</c></seealso> to handle the event.</p>
<p>For a description of the arguments, see
<seealso marker="#send_event/2"><c>send_event/2</c></seealso>.</p>
<p>The difference between <c>send_event/2</c> and
<c>send_all_state_event/2</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>send_event(FsmRef, Event) -> ok</name>
<fsummary>Send an event asynchronously to a generic FSM.</fsummary>
<type>
<v>FsmRef = Name | {Name,Node} | {global,GlobalName}</v>
<v> | {via,Module,ViaName} | pid()</v>
<v> Name = Node = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Event = term()</v>
</type>
<desc>
<p>Sends an event asynchronously to the <c>FsmRef</c> of the
<c>gen_fsm</c> process
and returns <c>ok</c> immediately. The <c>gen_fsm</c> process calls
<seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso> to handle the event, where
<c>StateName</c> is the name of the current state of
the <c>gen_fsm</c> process.</p>
<p><c>FsmRef</c> can be any of the following:</p>
<list type="bulleted">
<item>The pid</item>
<item><c>Name</c>, if the <c>gen_fsm</c> process is locally
registered</item>
<item><c>{Name,Node}</c>, if the <c>gen_fsm</c> process is locally
registered at another node</item>
<item><c>{global,GlobalName}</c>, if the <c>gen_fsm</c> process is
globally registered</item>
<item><c>{via,Module,ViaName}</c>, if the <c>gen_fsm</c> process is
registered through an alternative process registry</item>
</list>
<p><c>Event</c> is any term that is passed as one of
the arguments to <c>Module:StateName/2</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 <c>gen_fsm</c> process
that calls this function after <c>Time</c> milliseconds.
Returns immediately a
reference that can be used to cancel the delayed send using
<seealso marker="#cancel_timer/1"><c>cancel_timer/1</c></seealso>.</p>
<p>The <c>gen_fsm</c> process calls
<seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso> to handle
the event, where <c>StateName</c> is the name of the current
state of the <c>gen_fsm</c> process at the time the delayed event is
delivered.</p>
<p><c>Event</c> is any term that is passed as one of
the arguments to <c>Module:StateName/2</c>.</p>
</desc>
</func>
<func>
<name>start(Module, Args, Options) -> Result</name>
<name>start(FsmName, Module, Args, Options) -> Result</name>
<fsummary>Create a standalone <c>gen_fsm</c> process.</fsummary>
<type>
<v>FsmName = {local,Name} | {global,GlobalName}</v>
<v> | {via,Module,ViaName}</v>
<v> Name = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics</v>
<v> | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v> SOpts = [term()]</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v> Pid = pid()</v>
<v> Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a standalone <c>gen_fsm</c> process, that is, a 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(FsmName, Module, Args, Options) -> Result</name>
<fsummary>Create a <c>gen_fsm</c> process in a supervision tree.
</fsummary>
<type>
<v>FsmName = {local,Name} | {global,GlobalName}</v>
<v> | {via,Module,ViaName}</v>
<v> Name = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Module = atom()</v>
<v>Args = term()</v>
<v>Options = [Option]</v>
<v> Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
<v> Dbgs = [Dbg]</v>
<v> Dbg = trace | log | statistics</v>
<v> | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
<v> SOpts = [SOpt]</v>
<v> SOpt - see erlang:spawn_opt/2,3,4,5</v>
<v>Result = {ok,Pid} | ignore | {error,Error}</v>
<v> Pid = pid()</v>
<v> Error = {already_started,Pid} | term()</v>
</type>
<desc>
<p>Creates a <c>gen_fsm</c> process as part of a supervision tree.
The function is to be called, directly or indirectly, by
the supervisor. For example, it ensures that
the <c>gen_fsm</c> process is linked to the supervisor.</p>
<p>The <c>gen_fsm</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>FsmName={local,Name}</c>, the <c>gen_fsm</c> process is
registered locally as <c>Name</c> using <c>register/2</c>.</p>
</item>
<item>
<p>If <c>FsmName={global,GlobalName}</c>, the <c>gen_fsm</c> process
is registered globally as <c>GlobalName</c> using
<seealso marker="kernel:global#register_name/2">
<c>global:register_name/2</c></seealso>.</p>
</item>
<item>
<p>If <c>FsmName={via,Module,ViaName}</c>, the <c>gen_fsm</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>If no name is provided, the <c>gen_fsm</c> process is not
registered.</p>
<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 <c>Module:init/1</c>.</p>
<p>If option <c>{timeout,Time}</c> is present, the <c>gen_fsm</c>
process is allowed to spend <c>Time</c> milliseconds initializing
or it terminates and the start function returns
<c>{error,timeout}</c>.</p>
<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>
<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 that is used to
spawn the <c>gen_fsm</c> process; see
<seealso marker="erts:erlang#spawn_opt/2">
<c>spawn_opt/2</c></seealso>.</p>
<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_fsm</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_fsm</c> process. If a process with the
specified <c>FsmName</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>start_timer(Time, Msg) -> Ref</name>
<fsummary>Send a time-out 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 time-out event internally in the <c>gen_fsm</c>
process that calls this function after <c>Time</c> milliseconds.
Returns immediately a
reference that can be used to cancel the timer using
<seealso marker="#cancel_timer/1"><c>cancel_timer/1</c></seealso>.</p>
<p>The <c>gen_fsm</c> process calls
<seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso> to handle
the event, where <c>StateName</c> is the name of the current
state of the <c>gen_fsm</c> process at the time the time-out
message is delivered.</p>
<p><c>Msg</c> is any term that is passed in the
time-out message, <c>{timeout, Ref, Msg}</c>, as one of
the arguments to <c>Module:StateName/2</c>.</p>
</desc>
</func>
<func>
<name>stop(FsmRef) -> ok</name>
<name>stop(FsmRef, Reason, Timeout) -> ok</name>
<fsummary>Synchronously stop a generic FSM.</fsummary>
<type>
<v>FsmRef = Name | {Name,Node} | {global,GlobalName}</v>
<v> | {via,Module,ViaName} | pid()</v>
<v> Node = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Reason = term()</v>
<v>Timeout = int()>0 | infinity</v>
</type>
<desc>
<p>Orders a generic finite state machine to exit with the specified
<c>Reason</c> and waits for it to terminate. The <c>gen_fsm</c>
process calls <seealso marker="#Module:terminate/3">
<c>Module:terminate/3</c></seealso> before exiting.</p>
<p>The function returns <c>ok</c> if the generic finite state machine
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:error_logger#format/2">
<c>error_logger:format/2</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 generic FSM
to terminate, or the atom <c>infinity</c> to wait
indefinitely. The default value is <c>infinity</c>. If the
generic finite state machine 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>
<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}</v>
<v> | {via,Module,ViaName} | pid()</v>
<v> Name = Node = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Event = term()</v>
<v>Timeout = int()>0 | infinity</v>
<v>Reply = term()</v>
</type>
<desc>
<p>Sends an event to the <c>FsmRef</c> of the <c>gen_fsm</c>
process and waits until a reply arrives or a time-out occurs.
The <c>gen_fsm</c> process calls
<seealso marker="#Module:handle_sync_event/4">
<c>Module:handle_sync_event/4</c></seealso> to handle the event.</p>
<p>For a description of <c>FsmRef</c> and <c>Event</c>, see
<seealso marker="#send_event/2">send_event/2</seealso>.
For a description of <c>Timeout</c> and <c>Reply</c>, see
<seealso marker="#sync_send_event/3">
<c>sync_send_event/3</c></seealso>.</p>
<p>For a discussion about the difference between
<c>sync_send_event</c> and <c>sync_send_all_state_event</c>, see
<seealso marker="#send_all_state_event/2">
<c>send_all_state_event/2</c></seealso>.</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}</v>
<v> | {via,Module,ViaName} | pid()</v>
<v> Name = Node = atom()</v>
<v> GlobalName = ViaName = term()</v>
<v>Event = term()</v>
<v>Timeout = int()>0 | infinity</v>
<v>Reply = term()</v>
</type>
<desc>
<p>Sends an event to the <c>FsmRef</c> of the <c>gen_fsm</c>
process and waits until a reply arrives or a time-out occurs.
<c>The gen_fsm</c> process calls
<seealso marker="#Module:StateName/3">
<c>Module:StateName/3</c></seealso> to handle the event, where
<c>StateName</c> is the name of the current state of
the <c>gen_fsm</c> process.</p>
<p>For a description of <c>FsmRef</c> and <c>Event</c>, see
<seealso marker="#send_event/2"><c>send_event/2</c></seealso>.</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.</p>
<p>Return value <c>Reply</c> is defined in the return value
of <c>Module:StateName/3</c>.</p>
</desc>
</func>
</funcs>
<section>
<title>Callback Functions</title>
<p>The following functions are to be exported from a <c>gen_fsm</c>
callback module.</p>
<p><em>state name</em> denotes a state of the state machine.</p>
<p><em>state data</em> denotes the internal state of the Erlang process
that implements the state machine.</p>
</section>
<funcs>
<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> Vsn = term()</v>
<v>StateName = NextStateName = atom()</v>
<v>StateData = NewStateData = term()</v>
<v>Extra = 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/4</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_fsm</c> process when it is to
update its internal state data during a release upgrade/downgrade,
that is, when instruction <c>{update,Module,Change,...}</c>,
where <c>Change={advanced,Extra}</c>, is given in
the <c>appup</c> file; 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>StateName</c> is the current state name and <c>StateData</c> the
internal state data of the <c>gen_fsm</c> process.</p>
<p><c>Extra</c> is passed "as is" from the <c>{advanced,Extra}</c>
part of the update instruction.</p>
<p>The function is to return the new current state name and
updated internal data.</p>
</desc>
</func>
<func>
<name>Module:format_status(Opt, [PDict, StateData]) -> Status</name>
<fsummary>Optional function for providing a term describing the
current <c>gen_fsm</c> process status.</fsummary>
<type>
<v>Opt = normal | terminate</v>
<v>PDict = [{Key, Value}]</v>
<v>StateData = term()</v>
<v>Status = term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_fsm</c> module provides a default
implementation of this function that returns the callback
module state data.</p>
</note>
<p>This function is called by a <c>gen_fsm</c> process in the
following situations:</p>
<list type="bulleted">
<item>One of <seealso marker="sys#get_status/1">
<c>sys:get_status/1,2</c></seealso>
is invoked to get the <c>gen_fsm</c> status. <c>Opt</c> is set to
the atom <c>normal</c> for this case.</item>
<item>The <c>gen_fsm</c> process terminates abnormally and logs an
error. <c>Opt</c> is set to the atom <c>terminate</c> for
this case.</item>
</list>
<p>This function is useful for changing the form and
appearance of the <c>gen_fsm</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_fsm</c> process.</p>
<p><c>PDict</c> is the current value of the process dictionary of the
<c>gen_fsm</c> process.</p>
<p><c>StateData</c> is the internal state data of the
<c>gen_fsm</c> process.</p>
<p>The function is to return <c>Status</c>, a term that
change the details of the current state and status of
the <c>gen_fsm</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, [{"StateData",
Term}]}]</c>, where <c>Term</c> provides relevant details of
the <c>gen_fsm</c> state data. 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 data representations to avoid that large state terms
are printed in log files.</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> | {next_state,NextStateName,NewStateData,Timeout}</v>
<v> | {next_state,NextStateName,NewStateData,hibernate}</v>
<v> | {stop,Reason,NewStateData}</v>
<v> NextStateName = atom()</v>
<v> NewStateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Whenever a <c>gen_fsm</c> process receives an event sent using
<seealso marker="#send_all_state_event/2">
<c>send_all_state_event/2</c></seealso>,
this function is called to handle the event.</p>
<p><c>StateName</c> is the current state name of the <c>gen_fsm</c>
process.</p>
<p>For a description of the other arguments and possible return values,
see <seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso>.</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> | {next_state,NextStateName,NewStateData,Timeout}</v>
<v> | {next_state,NextStateName,NewStateData,hibernate}</v>
<v> | {stop,Reason,NewStateData}</v>
<v> NextStateName = atom()</v>
<v> NewStateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = normal | term()</v>
</type>
<desc>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_fsm</c> module provides a default
implementation of this function that logs about the unexpected
<c>Info</c> message, drops it and returns
<c>{next_state, StateName, StateData}</c>.</p>
</note>
<p>This function is called by a <c>gen_fsm</c> process 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>For a description of the other arguments and possible return values,
see <seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso>.</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> | {reply,Reply,NextStateName,NewStateData,Timeout}</v>
<v> | {reply,Reply,NextStateName,NewStateData,hibernate}</v>
<v> | {next_state,NextStateName,NewStateData}</v>
<v> | {next_state,NextStateName,NewStateData,Timeout}</v>
<v> | {next_state,NextStateName,NewStateData,hibernate}</v>
<v> | {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}</v>
<v> Reply = term()</v>
<v> NextStateName = atom()</v>
<v> NewStateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<p>Whenever a <c>gen_fsm</c> process receives an event sent using
<seealso marker="#sync_send_all_state_event/2">
<c>sync_send_all_state_event/2,3</c></seealso>,
this function is called to handle the event.</p>
<p><c>StateName</c> is the current state name of the <c>gen_fsm</c>
process.</p>
<p>For a description of the other arguments and possible return values,
see <seealso marker="#Module:StateName/3">
<c>Module:StateName/3</c></seealso>.</p>
</desc>
</func>
<func>
<name>Module:init(Args) -> Result</name>
<fsummary>Initialize process and internal state name and state data.
</fsummary>
<type>
<v>Args = term()</v>
<v>Result = {ok,StateName,StateData} | {ok,StateName,StateData,Timeout}</v>
<v> | {ok,StateName,StateData,hibernate}</v>
<v> | {stop,Reason} | ignore</v>
<v> StateName = atom()</v>
<v> StateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<marker id="Moduleinit"></marker>
<p>Whenever a <c>gen_fsm</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 initialization is successful, the function is to 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 <c>gen_fsm</c> process.</p>
<p>If an integer time-out value is provided, a time-out occurs
unless an event or a message is received within <c>Timeout</c>
milliseconds. A time-out is represented by the atom
<c>timeout</c> and is to be handled by the
<seealso marker="#Module:StateName/2">
<c>Module:StateName/2</c></seealso> 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 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 returns
<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> | {next_state,NextStateName,NewStateData,Timeout}</v>
<v> | {next_state,NextStateName,NewStateData,hibernate}</v>
<v> | {stop,Reason,NewStateData}</v>
<v> NextStateName = atom()</v>
<v> NewStateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = term()</v>
</type>
<desc>
<p>There is to be one instance of this function for each
possible state name. Whenever a <c>gen_fsm</c> process receives
an event sent using
<seealso marker="#send_event/2"><c>send_event/2</c></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 time-out occurs.</p>
<p><c>Event</c> is either the atom <c>timeout</c>, if a time-out
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 <c>gen_fsm</c> process.</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
<c>gen_fsm</c> process continues executing with the current state
name set to <c>NextStateName</c> and with the possibly
updated state data <c>NewStateData</c>. For a description of
<c>Timeout</c> and <c>hibernate</c>, see
<seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
<p>If the function returns <c>{stop,Reason,NewStateData}</c>,
the <c>gen_fsm</c> process calls
<c>Module:terminate(Reason,StateName,NewStateData)</c> and
terminates.</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> | {reply,Reply,NextStateName,NewStateData,Timeout}</v>
<v> | {reply,Reply,NextStateName,NewStateData,hibernate}</v>
<v> | {next_state,NextStateName,NewStateData}</v>
<v> | {next_state,NextStateName,NewStateData,Timeout}</v>
<v> | {next_state,NextStateName,NewStateData,hibernate}</v>
<v> | {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}</v>
<v> Reply = term()</v>
<v> NextStateName = atom()</v>
<v> NewStateData = term()</v>
<v> Timeout = int()>0 | infinity</v>
<v> Reason = normal | term()</v>
</type>
<desc>
<p>There is to be one instance of this function for each
possible state name. Whenever a <c>gen_fsm</c> process receives an
event sent using <seealso marker="#sync_send_event/2">
<c>sync_send_event/2,3</c></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/2,3</c>.</p>
<p><c>From</c> is a tuple <c>{Pid,Tag}</c> where <c>Pid</c> is
the pid of the process that 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 <c>gen_fsm</c> process.</p>
<list type="bulleted">
<item>
<p>If <c>{reply,Reply,NextStateName,NewStateData}</c>,
<c>{reply,Reply,NextStateName,NewStateData,Timeout}</c>, or
<c>{reply,Reply,NextStateName,NewStateData,hibernate}</c> is
returned, <c>Reply</c> is given back to <c>From</c> as the return
value of <c>sync_send_event/2,3</c>. The <c>gen_fsm</c> process
then continues executing with the current state name set to
<c>NextStateName</c> and with the possibly updated state data
<c>NewStateData</c>. 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>{next_state,NextStateName,NewStateData}</c>,
<c>{next_state,NextStateName,NewStateData,Timeout}</c>, or
<c>{next_state,NextStateName,NewStateData,hibernate}</c> is
returned, the <c>gen_fsm</c> process continues executing in
<c>NextStateName</c> with <c>NewStateData</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 the function returns
<c>{stop,Reason,Reply,NewStateData}</c>, <c>Reply</c> is
given back to <c>From</c>. If the function returns
<c>{stop,Reason,NewStateData}</c>, any reply to <c>From</c>
must be specified explicitly using <c>reply/2</c>.
The <c>gen_fsm</c> process then calls
<c>Module:terminate(Reason,StateName,NewStateData)</c> and
terminates.</p>
</item>
</list>
</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>
<note>
<p>This callback is optional, so callback modules need not
export it. The <c>gen_fsm</c> module provides a default
implementation without cleanup.</p>
</note>
<p>This function is called by a <c>gen_fsm</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_fsm</c>
process 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 <c>gen_fsm</c> process.</p>
<p><c>Reason</c> depends on why the <c>gen_fsm</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_fsm</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_fsm</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_fsm</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_fsm</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_fsm</c> process
is assumed to terminate because of an error and an error report is
issued using <seealso marker="kernel:error_logger#format/2">
<c>error_logger:format/2</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_server"><c>gen_server(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>
