gen_fsm
Generic finite state machine behavior.
Note!
There is a new behaviour
gen_statem
that is intended to replace gen_fsm
for new code.
gen_fsm
will not be removed for the foreseeable future
to keep old state machine implementations running.
This behavior module provides a finite state machine.
A generic finite state machine process (gen_fsm
) 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
OTP Design Principles.
A gen_fsm
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:
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
If a callback function fails or returns a bad value, the gen_fsm
process terminates.
A gen_fsm
process handles system messages as described in
sys(3)
. The sys
module
can be used for debugging a gen_fsm
process.
Notice that a gen_fsm
process does not trap exit signals
automatically, this must be explicitly initiated in the callback
module.
Unless otherwise stated, all functions in this module fail if
the specified gen_fsm
process does not exist or if bad arguments
are specified.
The gen_fsm
process can go into hibernation
(see
erlang:hibernate/3
) if a callback function
specifies 'hibernate'
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.
Functions
cancel_timer(Ref) -> RemainingTime | false
Ref = reference()
RemainingTime = integer()
Cancels an internal timer referred by Ref
in the
gen_fsm
process that calls this function.
Ref
is a reference returned from
send_event_after/2
or
start_timer/2
.
If the timer has already timed out, but the event not yet been delivered, it is cancelled as if it had not timed out, so there is no false timer event after returning from this function.
Returns the remaining time in milliseconds until the timer would
have expired if Ref
referred to an active timer, otherwise
false
.
enter_loop(Module, Options, StateName, StateData)
enter_loop(Module, Options, StateName, StateData, FsmName)
enter_loop(Module, Options, StateName, StateData, Timeout)
enter_loop(Module, Options, StateName, StateData, FsmName, Timeout)
Module = atom()
Options = [Option]
Option = {debug,Dbgs}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
StateName = atom()
StateData = term()
FsmName = {local,Name} | {global,GlobalName}
| {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Timeout = int() | infinity
Makes an existing process into a gen_fsm
process.
Does not return,
instead the calling process enters the gen_fsm
receive
loop and becomes a gen_fsm
process. The process must
have been started using one of the start functions in
proc_lib(3)
. The user is
responsible for any initialization of the process, including
registering a name for it.
This function is useful when a more complex initialization
procedure is needed than the gen_fsm
behavior provides.
Module
, Options
, and FsmName
have
the same meanings as when calling
start[_link]/3,4
.
However, if FsmName
is specified, the process must have
been registered accordingly before this function is
called.
StateName
, StateData
, and Timeout
have
the same meanings as in the return value of
Module:init/1
.
The callback module Module
does not need to
export an init/1
function.
The function fails if the calling process was not started by a
proc_lib
start function, or if it is not registered
according to FsmName
.
reply(Caller, Reply) -> Result
Caller - see below
Reply = term()
Result = term()
This function can be used by a gen_fsm
process to
explicitly send a reply to a client process that called
sync_send_event/2,3
or
sync_send_all_state_event/2,3
when the reply cannot be defined in the return value of
Module:StateName/3
or
Module:handle_sync_event/4
.
Caller
must be the From
argument provided to
the callback function. Reply
is any term
given back to the client as the return value of
sync_send_event/2,3
or
sync_send_all_state_event/2,3
.
Return value Result
is not further defined, and
is always to be ignored.
send_all_state_event(FsmRef, Event) -> ok
FsmRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Name = Node = atom()
GlobalName = ViaName = term()
Event = term()
Sends an event asynchronously to the FsmRef
of the
gen_fsm
process and returns ok
immediately.
The gen_fsm
process calls
Module:handle_event/3
to handle the event.
For a description of the arguments, see
send_event/2
.
The difference between send_event/2
and
send_all_state_event/2
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 handle_event
clause is needed to handle
the event instead of one clause in each state name function.
send_event(FsmRef, Event) -> ok
FsmRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Name = Node = atom()
GlobalName = ViaName = term()
Event = term()
Sends an event asynchronously to the FsmRef
of the
gen_fsm
process
and returns ok
immediately. The gen_fsm
process calls
Module:StateName/2
to handle the event, where
StateName
is the name of the current state of
the gen_fsm
process.
FsmRef
can be any of the following:
- The pid
Name
, if thegen_fsm
process is locally registered{Name,Node}
, if thegen_fsm
process is locally registered at another node{global,GlobalName}
, if thegen_fsm
process is globally registered{via,Module,ViaName}
, if thegen_fsm
process is registered through an alternative process registry
Event
is any term that is passed as one of
the arguments to Module:StateName/2
.
send_event_after(Time, Event) -> Ref
Time = integer()
Event = term()
Ref = reference()
Sends a delayed event internally in the gen_fsm
process
that calls this function after Time
milliseconds.
Returns immediately a
reference that can be used to cancel the delayed send using
cancel_timer/1
.
The gen_fsm
process calls
Module:StateName/2
to handle
the event, where StateName
is the name of the current
state of the gen_fsm
process at the time the delayed event is
delivered.
Event
is any term that is passed as one of
the arguments to Module:StateName/2
.
start(Module, Args, Options) -> Result
start(FsmName, Module, Args, Options) -> Result
FsmName = {local,Name} | {global,GlobalName}
| {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Module = atom()
Args = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [term()]
Result = {ok,Pid} | ignore | {error,Error}
Pid = pid()
Error = {already_started,Pid} | term()
Creates a standalone gen_fsm
process, that is, a process that
is not part of a supervision tree and thus has no supervisor.
For a description of arguments and return values, see
start_link/3,4
.
start_link(Module, Args, Options) -> Result
start_link(FsmName, Module, Args, Options) -> Result
FsmName = {local,Name} | {global,GlobalName}
| {via,Module,ViaName}
Name = atom()
GlobalName = ViaName = term()
Module = atom()
Args = term()
Options = [Option]
Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}
Dbgs = [Dbg]
Dbg = trace | log | statistics
| {log_to_file,FileName} | {install,{Func,FuncState}}
SOpts = [SOpt]
SOpt - see erlang:spawn_opt/2,3,4,5
Result = {ok,Pid} | ignore | {error,Error}
Pid = pid()
Error = {already_started,Pid} | term()
Creates a gen_fsm
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 gen_fsm
process is linked to the supervisor.
The gen_fsm
process calls
Module:init/1
to
initialize. To ensure a synchronized startup procedure,
start_link/3,4
does not return until
Module:init/1
has returned.
-
If
FsmName={local,Name}
, thegen_fsm
process is registered locally asName
usingregister/2
. -
If
FsmName={global,GlobalName}
, thegen_fsm
process is registered globally asGlobalName
usingglobal:register_name/2
. -
If
FsmName={via,Module,ViaName}
, thegen_fsm
process registers with the registry represented byModule
. TheModule
callback is to export the functionsregister_name/2
,unregister_name/1
,whereis_name/1
, andsend/2
, which are to behave like the corresponding functions inglobal
. Thus,{via,global,GlobalName}
is a valid reference.
If no name is provided, the gen_fsm
process is not
registered.
Module
is the name of the callback module.
Args
is any term that is passed as
the argument to Module:init/1
.
If option {timeout,Time}
is present, the gen_fsm
process is allowed to spend Time
milliseconds initializing
or it terminates and the start function returns
{error,timeout}
.
If option {debug,Dbgs}
is present, the corresponding
sys
function is called for each item in Dbgs
; see
sys(3)
.
If option {spawn_opt,SOpts}
is present, SOpts
is
passed as option list to the spawn_opt
BIF that is used to
spawn the gen_fsm
process; see
spawn_opt/2
.
Note!
Using spawn option monitor
is not
allowed, it causes the function to fail with reason
badarg
.
If the gen_fsm
process is successfully created and
initialized, the function returns {ok,Pid}
, where Pid
is the pid of the gen_fsm
process. If a process with the
specified FsmName
exists already, the function returns
{error,{already_started,Pid}}
, where Pid
is
the pid of that process.
If Module:init/1
fails with Reason
,
the function returns {error,Reason}
. If
Module:init/1
returns {stop,Reason}
or
ignore
, the process is terminated and the function
returns {error,Reason}
or ignore
, respectively.
start_timer(Time, Msg) -> Ref
Time = integer()
Msg = term()
Ref = reference()
Sends a time-out event internally in the gen_fsm
process that calls this function after Time
milliseconds.
Returns immediately a
reference that can be used to cancel the timer using
cancel_timer/1
.
The gen_fsm
process calls
Module:StateName/2
to handle
the event, where StateName
is the name of the current
state of the gen_fsm
process at the time the time-out
message is delivered.
Msg
is any term that is passed in the
time-out message, {timeout, Ref, Msg}
, as one of
the arguments to Module:StateName/2
.
stop(FsmRef) -> ok
stop(FsmRef, Reason, Timeout) -> ok
FsmRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Node = atom()
GlobalName = ViaName = term()
Reason = term()
Timeout = int()>0 | infinity
Orders a generic finite state machine to exit with the specified
Reason
and waits for it to terminate. The gen_fsm
process calls
Module:terminate/3
before exiting.
The function returns ok
if the generic finite state machine
terminates with the expected reason. Any other reason than
normal
, shutdown
, or {shutdown,Term}
causes an
error report to be issued using
error_logger:format/2
.
The default Reason
is normal
.
Timeout
is an integer greater than zero that
specifies how many milliseconds to wait for the generic FSM
to terminate, or the atom infinity
to wait
indefinitely. The default value is infinity
. If the
generic finite state machine has not terminated within the specified
time, a timeout
exception is raised.
If the process does not exist, a noproc
exception
is raised.
sync_send_all_state_event(FsmRef, Event) -> Reply
sync_send_all_state_event(FsmRef, Event, Timeout) -> Reply
FsmRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Name = Node = atom()
GlobalName = ViaName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the FsmRef
of the gen_fsm
process and waits until a reply arrives or a time-out occurs.
The gen_fsm
process calls
Module:handle_sync_event/4
to handle the event.
For a description of FsmRef
and Event
, see
send_event/2.
For a description of Timeout
and Reply
, see
sync_send_event/3
.
For a discussion about the difference between
sync_send_event
and sync_send_all_state_event
, see
send_all_state_event/2
.
sync_send_event(FsmRef, Event) -> Reply
sync_send_event(FsmRef, Event, Timeout) -> Reply
FsmRef = Name | {Name,Node} | {global,GlobalName}
| {via,Module,ViaName} | pid()
Name = Node = atom()
GlobalName = ViaName = term()
Event = term()
Timeout = int()>0 | infinity
Reply = term()
Sends an event to the FsmRef
of the gen_fsm
process and waits until a reply arrives or a time-out occurs.
The gen_fsm
process calls
Module:StateName/3
to handle the event, where
StateName
is the name of the current state of
the gen_fsm
process.
For a description of FsmRef
and Event
, see
send_event/2
.
Timeout
is an integer greater than zero that
specifies how many milliseconds to wait for a reply, or
the atom infinity
to wait indefinitely. Defaults
to 5000. If no reply is received within the specified time,
the function call fails.
Return value Reply
is defined in the return value
of Module:StateName/3
.
Note!
The ancient behavior of sometimes consuming the server exit message if the server died during the call while linked to the client was removed in Erlang 5.6/OTP R12B.
Callback Functions
The following functions are to be exported from a gen_fsm
callback module.
state name denotes a state of the state machine.
state data denotes the internal state of the Erlang process that implements the state machine.
Functions
Module:code_change(OldVsn, StateName, StateData, Extra) -> {ok, NextStateName, NewStateData}
OldVsn = Vsn | {down, Vsn}
Vsn = term()
StateName = NextStateName = atom()
StateData = NewStateData = term()
Extra = term()
This function is called by a gen_fsm
process when it is to
update its internal state data during a release upgrade/downgrade,
that is, when instruction {update,Module,Change,...}
,
where Change={advanced,Extra}
, is given in
the appup
file; see section
Release Handling Instructions in OTP Design Principles.
For an upgrade, OldVsn
is Vsn
, and for a downgrade,
OldVsn
is {down,Vsn}
. Vsn
is defined by the
vsn
attribute(s) of the old version of the callback module
Module
. If no such attribute is defined, the version is
the checksum of the Beam file.
StateName
is the current state name and StateData
the
internal state data of the gen_fsm
process.
Extra
is passed "as is" from the {advanced,Extra}
part of the update instruction.
The function is to return the new current state name and updated internal data.
Module:format_status(Opt, [PDict, StateData]) -> Status
Opt = normal | terminate
PDict = [{Key, Value}]
StateData = term()
Status = term()
Note!
This callback is optional, so callback modules need not
export it. The gen_fsm
module provides a default
implementation of this function that returns the callback
module state data.
This function is called by a gen_fsm
process in the
following situations:
- One of
sys:get_status/1,2
is invoked to get thegen_fsm
status.Opt
is set to the atomnormal
for this case. - The
gen_fsm
process terminates abnormally and logs an error.Opt
is set to the atomterminate
for this case.
This function is useful for changing the form and
appearance of the gen_fsm
status for these cases. A callback
module wishing to change the sys:get_status/1,2
return value as well as how its status appears in
termination error logs, exports an instance
of format_status/2
that returns a term describing the
current status of the gen_fsm
process.
PDict
is the current value of the process dictionary of the
gen_fsm
process.
StateData
is the internal state data of the
gen_fsm
process.
The function is to return Status
, a term that
change the details of the current state and status of
the gen_fsm
process. There are no restrictions on the
form Status
can take, but for
the sys:get_status/1,2
case (when Opt
is normal
), the recommended form for
the Status
value is [{data, [{"StateData",
Term}]}]
, where Term
provides relevant details of
the gen_fsm
state data. Following this recommendation is not
required, but it makes the callback module status
consistent with the rest of the sys:get_status/1,2
return value.
One use for this function is to return compact alternative state data representations to avoid that large state terms are printed in log files.
Module:handle_event(Event, StateName, StateData) -> Result
Event = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm
process receives an event sent using
send_all_state_event/2
,
this function is called to handle the event.
StateName
is the current state name of the gen_fsm
process.
For a description of the other arguments and possible return values,
see
Module:StateName/2
.
Module:handle_info(Info, StateName, StateData) -> Result
Info = term()
StateName = atom()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
This function is called by a gen_fsm
process when it receives
any other message than a synchronous or asynchronous event (or a
system message).
Info
is the received message.
For a description of the other arguments and possible return values,
see
Module:StateName/2
.
Module:handle_sync_event(Event, From, StateName, StateData) -> Result
Event = term()
From = {pid(),Tag}
StateName = atom()
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm
process receives an event sent using
sync_send_all_state_event/2,3
,
this function is called to handle the event.
StateName
is the current state name of the gen_fsm
process.
For a description of the other arguments and possible return values,
see
Module:StateName/3
.
Module:init(Args) -> Result
Args = term()
Result = {ok,StateName,StateData} | {ok,StateName,StateData,Timeout}
| {ok,StateName,StateData,hibernate}
| {stop,Reason} | ignore
StateName = atom()
StateData = term()
Timeout = int()>0 | infinity
Reason = term()
Whenever a gen_fsm
process is started using
start/3,4
or
start_link/3,4
,
this function is called by the new process to initialize.
Args
is the Args
argument provided to the start
function.
If initialization is successful, the function is to return
{ok,StateName,StateData}
,
{ok,StateName,StateData,Timeout}
, or
{ok,StateName,StateData,hibernate}
, where StateName
is the initial state name and StateData
the initial
state data of the gen_fsm
process.
If an integer time-out value is provided, a time-out occurs
unless an event or a message is received within Timeout
milliseconds. A time-out is represented by the atom
timeout
and is to be handled by the
Module:StateName/2
callback functions. The atom
infinity
can be used to wait indefinitely, this is
the default value.
If hibernate
is specified instead of a time-out value, the
process goes into hibernation when waiting for the next message
to arrive (by calling
proc_lib:hibernate/3
).
If the initialization fails, the function returns
{stop,Reason}
, where Reason
is any term,
or ignore
.
Module:StateName(Event, StateData) -> Result
Event = timeout | term()
StateData = term()
Result = {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,NewStateData}
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = term()
There is to be one instance of this function for each
possible state name. Whenever a gen_fsm
process receives
an event sent using
send_event/2
,
the instance of this function with the same name as
the current state name StateName
is called to handle
the event. It is also called if a time-out occurs.
Event
is either the atom timeout
, if a time-out
has occurred, or the Event
argument provided to
send_event/2
.
StateData
is the state data of the gen_fsm
process.
If the function returns
{next_state,NextStateName,NewStateData}
,
{next_state,NextStateName,NewStateData,Timeout}
, or
{next_state,NextStateName,NewStateData,hibernate}
, the
gen_fsm
process continues executing with the current state
name set to NextStateName
and with the possibly
updated state data NewStateData
. For a description of
Timeout
and hibernate
, see
Module:init/1
.
If the function returns {stop,Reason,NewStateData}
,
the gen_fsm
process calls
Module:terminate(Reason,StateName,NewStateData)
and
terminates.
Module:StateName(Event, From, StateData) -> Result
Event = term()
From = {pid(),Tag}
StateData = term()
Result = {reply,Reply,NextStateName,NewStateData}
| {reply,Reply,NextStateName,NewStateData,Timeout}
| {reply,Reply,NextStateName,NewStateData,hibernate}
| {next_state,NextStateName,NewStateData}
| {next_state,NextStateName,NewStateData,Timeout}
| {next_state,NextStateName,NewStateData,hibernate}
| {stop,Reason,Reply,NewStateData} | {stop,Reason,NewStateData}
Reply = term()
NextStateName = atom()
NewStateData = term()
Timeout = int()>0 | infinity
Reason = normal | term()
There is to be one instance of this function for each
possible state name. Whenever a gen_fsm
process receives an
event sent using
sync_send_event/2,3
,
the instance of this function with the same name as
the current state name StateName
is called to handle
the event.
Event
is the Event
argument provided to
sync_send_event/2,3
.
From
is a tuple {Pid,Tag}
where Pid
is
the pid of the process that called sync_send_event/2,3
and Tag
is a unique tag.
StateData
is the state data of the gen_fsm
process.
-
If
{reply,Reply,NextStateName,NewStateData}
,{reply,Reply,NextStateName,NewStateData,Timeout}
, or{reply,Reply,NextStateName,NewStateData,hibernate}
is returned,Reply
is given back toFrom
as the return value ofsync_send_event/2,3
. Thegen_fsm
process then continues executing with the current state name set toNextStateName
and with the possibly updated state dataNewStateData
. For a description ofTimeout
andhibernate
, seeModule:init/1
. -
If
{next_state,NextStateName,NewStateData}
,{next_state,NextStateName,NewStateData,Timeout}
, or{next_state,NextStateName,NewStateData,hibernate}
is returned, thegen_fsm
process continues executing inNextStateName
withNewStateData
. Any reply toFrom
must be specified explicitly usingreply/2
. -
If the function returns
{stop,Reason,Reply,NewStateData}
,Reply
is given back toFrom
. If the function returns{stop,Reason,NewStateData}
, any reply toFrom
must be specified explicitly usingreply/2
. Thegen_fsm
process then callsModule:terminate(Reason,StateName,NewStateData)
and terminates.
Module:terminate(Reason, StateName, StateData)
Reason = normal | shutdown | {shutdown,term()} | term()
StateName = atom()
StateData = term()
This function is called by a gen_fsm
process when it is about
to terminate. It is to be the opposite of
Module:init/1
and do any necessary cleaning up. When it returns, the gen_fsm
process terminates with Reason
. The return value is ignored.
Reason
is a term denoting the stop reason,
StateName
is the current state name, and
StateData
is the state data of the gen_fsm
process.
Reason
depends on why the gen_fsm
process is
terminating. If
it is because another callback function has returned a stop
tuple {stop,..}
, Reason
has the value
specified in that tuple. If it is because of a failure,
Reason
is the error reason.
If the gen_fsm
process is part of a supervision tree and is
ordered by its supervisor to terminate, this function is called
with Reason=shutdown
if the following conditions apply:
-
The
gen_fsm
process has been set to trap exit signals. -
The shutdown strategy as defined in the child specification of the supervisor is an integer time-out value, not
brutal_kill
.
Even if the gen_fsm
process is not part of a
supervision tree,
this function is called if it receives an 'EXIT'
message from its parent. Reason
is the same as in
the 'EXIT'
message.
Otherwise, the gen_fsm
process terminates immediately.
Notice that for any other reason than normal
,
shutdown
, or {shutdown,Term}
the gen_fsm
process
is assumed to terminate because of an error and an error report is
issued using
error_logger:format/2
.
See Also
gen_event(3)
,
gen_server(3)
,
gen_statem(3)
,
proc_lib(3)
,
supervisor(3)
,
sys(3)