gen_fsm

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.

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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.

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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.

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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.

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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.

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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 the gen_fsm process is locally registered
• {Name,Node}, if the gen_fsm process is locally registered at another node
• {global,GlobalName}, if the gen_fsm process is globally registered
• {via,Module,ViaName}, if the gen_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.

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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.

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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.

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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}, the gen_fsm process is registered locally as Name using register/2.

• If FsmName={global,GlobalName}, the gen_fsm process is registered globally as GlobalName using global:register_name/2.

• If FsmName={via,Module,ViaName}, the gen_fsm process registers with the registry represented by Module. The Module callback is to export the functions register_name/2, unregister_name/1, whereis_name/1, and send/2, which are to behave like the corresponding functions in global. 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.

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.

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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.

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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.

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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.

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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.

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.

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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.

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This function is called by a gen_fsm process in the following situations:

• 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 process terminates abnormally and logs an error. Opt is set to the atom terminate 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.

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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.

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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.

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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.

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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.

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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.

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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 to From as the return value of sync_send_event/2,3. The gen_fsm process then 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 {next_state,NextStateName,NewStateData}, {next_state,NextStateName,NewStateData,Timeout}, or {next_state,NextStateName,NewStateData,hibernate} is returned, the gen_fsm process continues executing in NextStateName with NewStateData. Any reply to From must be specified explicitly using reply/2.

• If the function returns {stop,Reason,Reply,NewStateData}, Reply is given back to From. If the function returns {stop,Reason,NewStateData}, any reply to From must be specified explicitly using reply/2. The gen_fsm process then calls Module:terminate(Reason,StateName,NewStateData) and terminates.

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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.

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