gen_fsm

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.

A gen_fsm handles system messages as documented in sys(3). 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.

The gen_fsm process can go into hibernation (see erlang(3)) if a callback function specifies 'hibernate' instead of a timeout value. This might be useful if the server is expected to be idle for a long time. However this feature should be used with care as hibernation implies at least two garbage collections (when hibernating and shortly after waking up) and is not something you'd want to do between each call to a busy state machine.

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Creates a gen_fsm process as part of a supervision tree. The function should be called, directly or indirectly, by the supervisor. It will, among other things, ensure that the gen_fsm is linked to the supervisor.

The gen_fsm process calls Module:init/1 to initialize. To ensure a synchronized start-up procedure, start_link/3,4 does not return until Module:init/1 has returned.

If FsmName={local,Name}, the gen_fsm is registered locally as Name using register/2. If FsmName={global,GlobalName}, the gen_fsm is registered globally as GlobalName 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. See sys(3).

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. See erlang(3).

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.

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Creates a stand-alone gen_fsm process, i.e. a gen_fsm which is not part of a supervision tree and thus has no supervisor.

See start_link/3,4 for a description of arguments and return values.

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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,GlobalName}, if the gen_fsm is globally registered.

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

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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.

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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.

The ancient behaviour of sometimes consuming the server exit message if the server died during the call while linked to the client has been removed in OTP R12B/Erlang 5.6.

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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.

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This function can be used by a gen_fsm 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: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/2,3 or sync_send_all_state_event/2,3.

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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.

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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.

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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.

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Makes an existing process into a gen_fsm. Does not return, instead the calling process will enter the gen_fsm receive loop and become a gen_fsm process. The process must have been started using one of the start functions in proc_lib, see 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 behaviour 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. Also, the callback module Module does not need to export an init/1 function.

Failure: If the calling process was not started by a proc_lib start function, or if it is not registered according to FsmName.

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.

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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}, {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.

If an integer timeout value is provided, a timeout will occur unless an event or a message is received within Timeout milliseconds. A timeout is represented by the atom timeout and should 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 timeout value, the process will go into hibernation when waiting for the next message to arrive (by calling proc_lib:hibernate/3).

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

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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 occurred, or the Event argument provided to send_event/2.

StateData is the state data of the gen_fsm.

If the function returns {next_state,NextStateName,NewStateData}, {next_state,NextStateName,NewStateData,Timeout} or {next_state,NextStateName,NewStateData,hibernate}, 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 and hibernate.

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

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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.

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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/2,3 and Tag is a unique tag.

StateData is the state data of the gen_fsm.

If the function returns {reply,Reply,NextStateName,NewStateData}, {reply,Reply,NextStateName,NewStateData,Timeout} or {reply,Reply,NextStateName,NewStateData,hibernate}, Reply will be given back to From as the return value of sync_send_event/2,3. 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 and hibernate.

If the function returns {next_state,NextStateName,NewStateData}, {next_state,NextStateName,NewStateData,Timeout} or {next_state,NextStateName,NewStateData,hibernate}, 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.

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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.

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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.

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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 supervisor 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.

Even if the gen_fsm is not part of a supervision tree, this function will be called if it receives an 'EXIT' message from its parent. Reason will be the same as in the 'EXIT' message.

Otherwise, the gen_fsm will be immediately terminated.

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

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This function is called by a gen_fsm when it should update its internal state data during a release upgrade/downgrade, i.e. when the instruction {update,Module,Change,...} where Change={advanced,Extra} is given in the appup file. See OTP Design Principles.

In the case of an upgrade, OldVsn is Vsn, and in the case of 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.

Extra is passed as-is from the {advanced,Extra} part of the update instruction.

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

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This function is called by a gen_fsm process when:

• One of sys:get_status/1,2 is invoked to get the gen_fsm status. Opt is set to the atom normal for this case.
• The gen_fsm terminates abnormally and logs an error. Opt is set to the atom terminate for this case.

This function is useful for customising the form and appearance of the gen_fsm status for these cases. A callback module wishing to customise 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.

PDict is the current value of the gen_fsm's process dictionary.

StateData is the internal state data of the gen_fsm.

The function should return Status, a term that customises the details of the current state and status of the gen_fsm. 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 isn't required, but doing so will make 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 having large state terms printed in logfiles.

gen_event(3), gen_server(3), supervisor(3), proc_lib(3), sys(3)