# gen_fsm

### MODULE

gen_fsm

### MODULE SUMMARY

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_event/3` 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`.

### 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)

### AUTHORS

Gunilla Hugosson - support@erlang.ericsson.se

stdlib 1.10.1
Copyright © 1991-2002 Ericsson Utvecklings AB