gen_fsm(3)

MODULE
gen_fsm - Generic Finite State Machine Behaviour

DESCRIPTION

A behaviour module for implementing a finite state machine. A generic finite state machine process (gen_fsm) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.

A gen_fsm assumes all specific parts to be located in a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:
gen_fsm module                    Callback module
--------------                    ---------------
gen_fsm:start_link                -----> Module:init/1
gen_fsm:send_event                -----> Module:StateName/2
gen_fsm:send_all_state_event      -----> Module:handle_event/3
gen_fsm:sync_send_event           -----> Module:StateName/3
gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
-                                 -----> Module:handle_info/3
-                                 -----> Module:terminate/3
-                                 -----> Module:code_change/4

If a callback function fails or returns a bad value, the gen_fsm will terminate.

The sys module can be used for debugging a gen_fsm.

Note that a gen_fsm 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 does not exist or if bad arguments are given.

EXPORTS

start(Module, Args, Options) -> Result
start(FsmName, Module, Args, Options) -> Result
start_link(Module, Args, Options) -> Result
start_link(FsmName, Module, Args, Options) -> Result

    Types
        FsmName = {local, Name} | {global, Name}
         Name = atom()
        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 gen_fsm process which calls Module:init/1 to initialize. To ensure a synchronized start-up procedure, this function does not return until Module:init/1 has returned.

    A gen_fsm started using start_link is linked to the calling process, this function must be used if the gen_fsm is included in a supervision tree. A gen_fsm started using start is not linked to the calling process.

    If FsmName={local, Name}, the gen_fsm is registered locally as Name using register/2. If FsmName={global, Name}, the gen_fsm is registered globally as Name using global:register_name/2. If no name is provided, the gen_fsm is not registered.

    Module is the name of the callback module.

    Args is an arbitrary term which is passed as the argument to Module:init/1.

    If the option {timeout, Time} is present, the gen_fsm is allowed to spend Time milliseconds initializing or it will be terminated and the start function will return {error, timeout}.

    If the option {debug, Dbgs} is present, the corresponding sys function will be called for each item in Dbgs. Refer to sys(3) for more information.

    If the option {spawn_opt, SOpts} is present, SOpts will be passed as option list to the spawn_opt BIF which is used to spawn the gen_fsm process. Refer to erlang(3) for information about the spawn_opt options.

    If the gen_fsm is successfully created and initialized the function returns {ok, Pid}, where Pid is the pid of the gen_fsm. If there already exists a process with the specified FsmName, 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.

send_event(FsmRef, Event) -> ok

    Types
        FsmRef = Name | {Name, Node} | {global, Name} | pid()
         Name = Node = atom()
        Event = term()

    Sends an event asynchronously to the gen_fsm FsmRef and returns ok immediately. The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm.

    FsmRef can be:

        *   the pid,

        *   Name, if the gen_fsm is locally registered,

        *   {Name, Node}, if the gen_fsm is locally registered at another node, or

        *   {global, Name}, if the gen_fsm is globally registered.

    Event is an arbitrary term which is passed as one of the arguments to Module:StateName/2.

send_all_state_event(FsmRef, Event) -> ok

    Types
        FsmRef = Name | {Name, Node} | {global, Name} | pid()
         Name = Node = atom()
        Event = term()

    Sends an event asynchronously to the gen_fsm FsmRef and returns ok immediately. The gen_fsm will call Module:handle_event/3 to handle the event.

    See send_event/2 for a description of the arguments.

    The difference between send_event and send_all_state_event 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.

sync_send_event(FsmRef, Event) -> Reply
sync_send_event(FsmRef, Event, Timeout) -> Reply

    Types
        FsmRef = Name | {Name, Node} | {global, Name} | pid()
         Name = Node = atom()
        Event = term()
        Timeout = int()>0 | infinity
        Reply = term()

    Sends an event to the gen_fsm FsmRef and waits until a reply arrives or a timeout occurs. The gen_fsm will call Module:StateName/3 to handle the event, where StateName is the name of the current state of the gen_fsm.

    See send_event/2 for a description of FsmRef and Event.

    Timeout is an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely. Default value is 5000. If no reply is received within the specified time, the function call fails.

    The return value Reply is defined in the return value of Module:StateName/3.

    In the case where the gen_fsm terminates during the handling of the event and the caller is linked to the gen_fsm and trapping exits, the exit message is removed from the caller's receive queue before the function call fails.
     This behaviour is retained for backwards compatibility only and may change in the future. Note that if the gen_fsm crashes in between calls, a linked process must take care of the exit message anyway.
     Warning: Under certain circumstances (e.g. FsmRef = {Name, Node}, and Node goes down) the exit message cannot be removed.

sync_send_all_state_event(FsmRef, Event) -> Reply
sync_send_all_state_event(FsmRef, Event, Timeout) -> Reply

    Types
        FsmRef = Name | {Name, Node} | {global, Name} | pid()
         Name = Node = atom()
        Event = term()
        Timeout = int()>0 | infinity
        Reply = term()

    Sends an event to the gen_fsm FsmRef and waits until a reply arrives or a timeout occurs. The gen_fsm will call Module:handle_sync_event/4 to handle the event.

    See send_event/2 for a description of FsmRef and Event. See sync_send_event/3 for a description of Timeout and Reply.

    See send_all_state_event/2 for a discussion about the difference between sync_send_event and sync_send_all_state_event.

reply(Caller, Reply) -> true

    Types
        Caller - see below
        Reply = term()

    This function can be used by a gen_fsm to explicitly send a reply to a client process that called sync_send_event or sync_send_all_state_event, when the reply cannot be defined in the return value of Module:State/3 or Module:handle_sync_event/4.

    Caller must be the From argument provided to the callback function. Reply is an arbitrary term, which will be given back to the client as the return value of sync_send_event or sync_send_all_state_event.

send_event_after(Time, Event) -> Ref

    Types
        Time = integer()
        Event = term()
        Ref = reference()

    Sends a delayed event internally in the gen_fsm that calls this function after Time ms. Returns immediately a reference that can be used to cancel the delayed send using cancel_timer/1.

    The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm at the time the delayed event is delivered.

    Event is an arbitrary term which is passed as one of the arguments to Module:StateName/2.

start_timer(Time, Msg) -> Ref

    Types
        Time = integer()
        Msg = term()
        Ref = reference()

    Sends a timeout event internally in the gen_fsm that calls this function after Time ms. Returns immediately a reference that can be used to cancel the timer using cancel_timer/1.

    The gen_fsm will call Module:StateName/2 to handle the event, where StateName is the name of the current state of the gen_fsm at the time the timeout message is delivered.

    Msg is an arbitrary term which is passed in the timeout message, {timeout, Ref, Msg}, as one of the arguments to Module:StateName/2.

cancel_timer(Ref) -> RemainingTime | false

    Types
        Ref = reference()
        RemainingTime = integer()

    Cancels an internal timer referred by Ref in the gen_fsm 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 will be no false timer event after returning from this function.

    Returns the remaining time in ms until the timer would have expired if Ref referred to an active timer, false otherwise.

CALLBACK FUNCTIONS

The following functions should be exported from a gen_fsm callback module.

In the description, the expression state name is used to denote a state of the state machine. state data is used to denote the internal state of the Erlang process which implements the state machine.

EXPORTS

Module:init(Args) -> Result

    Types
        Args = term()
        Return = {ok, StateName, StateData} | {ok, StateName, StateData, Timeout}
         | {stop, Reason} | ignore
         StateName = atom()
         StateData = term()
         Timeout = int()>0 | infinity
         Reason = term()

    Whenever a gen_fsm is started using gen_fsm:start/3, 4 or gen_fsm: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 should return {ok, StateName, StateData} or {ok, StateName, StateData, Timout}, where StateName is the initial state name and StateData the initial state data of the gen_fsm.

    If an integer timout value is provided, a timout will occur unless an event or a message is received within Timeout milliseconds. A timout is represented by the atom timeout and should be handled by the Module:StateName/2 callback functions. The atom inifinity can be used to wait indefinitely, this is the default value.

    If something goes wrong during the initialization the function should return {stop, Reason}, where Reason is any term, or ignore.

Module:StateName(Event, StateData) -> Result

    Types
        Event = timeout | term()
        StateData = term()
        Result = {next_state, NextStateName, NewStateData} | {next_state, NextStateName, NewStateData, Timeout}
          | {stop, Reason, NewStateData}
         NextStateName = atom()
         NewStateData = term()
         Timeout = int()>0 | infinity
         Reason = term()

    There should be one instance of this function for each possible state name. Whenever a gen_fsm receives an event sent using gen_fsm: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 timeout occurs.

    Event is either the atom timeout, if a timeout has occured, or the Event argument provided to send_event.

    StateData is the state data of the gen_fsm.

    If the function returns {next_state, NextStateName, NewStateData} or {next_state, NextStateName, NewStateData, Timeout}, the gen_fsm will continue executing with the current state name set to NextStateName and with the possibly updated state data NewStateData. See Module:init/1 for a description of Timeout.

    If the function returns {stop, Reason, NewStateData}, the gen_fsm will call Module:terminate(Reason, NewStateData) and terminate.

Module:handle_event(Event, StateName, StateData) -> Result

    Types
        Event = term()
        StateName = atom()
        StateData = term()
        Result = {next_state, NextStateName, NewStateData} | {next_state, NextStateName, NewStateData, Timeout}
          | {stop, Reason, NewStateData}
         NextStateName = atom()
         NewStateData = term()
         Timeout = int()>0 | infinity
         Reason = term()

    Whenever a gen_fsm receives an event sent using gen_fsm:send_all_state_event/2, this function is called to handle the event.

    StateName is the current state name of the gen_fsm.

    See Module:StateName/2 for a description of the other arguments and possible return values.

Module:StateName(Event, From, StateData) -> Result

    Types
        Event = term()
        From = {pid(), Tag}
        StateData = term()
        Result = {reply, Reply, NextStateName, NewStateData} | {reply, Reply, NextStateName, NewStateData, Timeout}
          | {next_state, NextStateName, NewStateData} | {next_state, NextStateName, NewStateData, Timeout}
          | {stop, Reason, Reply, NewStateData} | {stop, Reason, NewStateData}
         Reply = term()
         NextStateName = atom()
         NewStateData = term()
         Timeout = int()>0 | infinity
         Reason = normal | term()

    There should be one instance of this function for each possible state name. Whenever a gen_fsm receives an event sent using gen_fsm: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.

    From is a tuple {Pid, Tag} where Pid is the pid of the process which called sync_send_event and Tag is a unique tag.

    StateData is the state data of the gen_fsm.

    If the function returns {reply, Reply, NextStateName, NewStateData} or {reply, Reply, NextStateName, NewStateData, Timeout}, Reply will be given back to From as the return value of sync_send_event. The gen_fsm then continues executing with the current state name set to NextStateName and with the possibly updated state data NewStateData. See Module:init/1 for a description of Timeout.

    If the function returns {next_state, NextStateName, NewStateData} or {next_state, NextStateName, NewStateData, Timeout}, the gen_fsm will continue executing in NextStateName with NewStateData. Any reply to From must be given explicitly using gen_fsm:reply/2.

    If the function returns {stop, Reason, Reply, NewStateData}, Reply will be given back to From. If the function returns {stop, Reason, NewStateData}, any reply to From must be given explicitly using gen_fsm:reply/2. The gen_fsm will then call Module:terminate(Reason, NewStateData) and terminate.

Module:handle_sync_event(Event, From, StateName, StateData) -> Result

    Types
        Event = term()
        From = {pid(), Tag}
        StateName = atom()
        StateData = term()
        Result = {reply, Reply, NextStateName, NewStateData} | {reply, Reply, NextStateName, NewStateData, Timeout}
          | {next_state, NextStateName, NewStateData} | {next_state, NextStateName, NewStateData, Timeout}
          | {stop, Reason, Reply, NewStateData} | {stop, Reason, NewStateData}
         Reply = term()
         NextStateName = atom()
         NewStateData = term()
         Timeout = int()>0 | infinity
         Reason = term()

    Whenever a gen_fsm receives an event sent using gen_fsm: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.

    See Module:StateName/3 for a description of the other arguments and possible return values.

Module:handle_info(Info, StateName, StateData) -> Result

    Types
        Info = term()
        StateName = atom()
        StateData = term()
        Result = {next_state, NextStateName, NewStateData} | {next_state, NextStateName, NewStateData, Timeout}
          | {stop, Reason, NewStateData}
         NextStateName = atom()
         NewStateData = term()
         Timeout = int()>0 | infinity
         Reason = normal | term()

    This function is called by a gen_fsm when it receives any other message than a synchronous or asynchronous event (or a system message).

    Info is the received message.

    See Module:StateName/2 for a description of the other arguments and possible return values.

Module:terminate(Reason, StateName, StateData)

    Types
        Reason = normal | shutdown | term()
        StateName = atom()
        StateData = term()

    This function is called by a gen_fsm when it is about to terminate. It should be the opposite of Module:init/1 and do any necessary cleaning up. When it returns, the gen_fsm 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.

    Reason depends on why the gen_fsm is terminating. If it is because another callback function has returned a stop tuple {stop, ..}, Reason will have the value specified in that tuple. If it is due to a failure, Reason is the error reason.

    If the gen_fsm is part of a supervision tree and is ordered by its superviser to terminate, this function will be called with Reason=shutdown if the following conditions apply:

        *   the gen_fsm has been set to trap exit signals, and

        *   the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill.

    Otherwise, the gen_fsm will be immediately terminated.

    Note that for any other reason than normal or shutdown, the gen_fsm is assumed to terminate due to an error and an error report is issued using error_logger:format/2.

Module:code_change(OldVsn, StateName, StateData, Extra) -> {ok, NextStateName, NewStateData}

    Types
        OldVsn = undefined | term()
        StateName = NextStateName = atom()
        StateData = NewStateData = term()
        Extra = term()

    This function is called by a gen_fsm when it should update its state data due to a code replacement, i.e. when the instruction {update, Module, Change, PrePurge, PostPurge, Modules} where Change={advanced, Extra} has been given to the release handler. See SASL User's Guide for more information.

    OldVsn is the vsn attribute of the old version of the callback module Module, or undefined if no such attribute is defined.

    StateName is the current state name and StateData the state data of the gen_fsm.

    Extra is the same as in the {advanced, Extra} part of the update instruction.

    The function should return the new current state name and updated state data.

SEE ALSO

supervisor(3), sys(3)

AUTHOR
Gunilla Hugosson - support@erlang.ericsson.se