Provided by: erlang-manpages_16.b.3-dfsg-1ubuntu2_all bug

NAME

       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.

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

EXPORTS

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

              Types:

                 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  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 EventMgrName={via,Module,ViaName},  the
              event  manager  will  register  with the registry represented by Module. The Module
              callback  should   export   the   functions   register_name/2,   unregister_name/1,
              whereis_name/1  and send/2, which should behave like the corresponding functions in
              global. Thus, {via,global,GlobalName} is a valid reference.

              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(3erl).

              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(3erl).

          Note:
              Using the spawn option monitor  is  currently  not  allowed,  but  will  cause  the
              function to fail with reason badarg.

              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.

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

              Types:

                 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 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.

       send_event(FsmRef, Event) -> ok

              Types:

                 FsmRef = Name | {Name,Node} | {global,GlobalName} | {via,Module,ViaName} | pid()
                  Name = Node = atom()
                  GlobalName = ViaName = term()
                 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,GlobalName}, if the gen_fsm is globally registered.

                * {via,Module,ViaName}, if the event manager is registered through an alternative
                  process registry.

              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,GlobalName} | {via,Module,ViaName} | pid()
                  Name = Node = atom()
                  GlobalName = ViaName = term()
                 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,GlobalName} | {via,Module,ViaName} | pid()
                  Name = Node = atom()
                  GlobalName = ViaName = term()
                 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.

              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.

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

              Types:

                 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 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/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.

       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.

       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)

              Types:

                 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. 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(3erl).  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.

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()
                 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 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.

       Module:StateName(Event, StateData) -> Result

              Types:

                 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 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.

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

              Types:

                 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 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}
                  | {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 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.

       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}
                  | {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      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}
                  | {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 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 | {shutdown,term()} | 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  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.

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

              Types:

                 OldVsn = Vsn | {down, Vsn}
                  Vsn = term()
                 StateName = NextStateName = atom()
                 StateData = NewStateData = term()
                 Extra = term()

              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.

       Module:format_status(Opt, [PDict, StateData]) -> Status

              Types:

                 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 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.

SEE ALSO

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