C and C++ Finite State Machine Framework

Author: Thomas Block
$Date: 2016-11-11 19:16:10 +0100 (Fr, 11 Nov 2016) $

This is an event driven Finite State Machine (FSM) framework in C++ and C including a PERL script for generating c++ code from a transition table.


The goals are:

New since last release

The generated C++ file (options -sm and -sw) contains now doxygen conform comments.
  • It is now possible to add an event within a state.
  • VECTORCAST FSM test case generation, which allows to make compound tests to test the state transitions.
  • New transition table token FSM_TT_SS to generate superstate transitions.

    Provided files:

    • For C++
      • The C++ framework h file fsm.h.
      • The example Fsm using the C++ framework fsm.cpp.
    • For C
      • The C framework h file fsmc.h.
      • The example Fsm using the C framework fsm.c.
    • A PERL script with following powerfull features:
      • Converting the transition table in a PlantUML file so you can have a graphical documentation of the states and transitions.
        As an example see Figure 2 generated from the fsm.cpp file.
      • Creating an FSM C++ template from the transition table using the base class in fsm.h or without the base class.
      • Creating an FSM C template from the transition table using functions of module fsmc.h or without the module.


    You can send an EMAIL to Tom. I cannot promise to answer, but will check my emails and may implement suggestions, if I have time.


    In UML a simple state is defined as a box with a name and some more description e.g. the boxes with the names State1 and State 2 in Figure 1.
    To get a state machine the boxes are connected with arrows telling when the state machine has a transition from one state to the other or to itself (see Figure 1 from State1 to State2).
    The transition to State 2 in the state machine of Figure 1 happens when EVENT occurs and when the condition defined by [guard] is true. Before entering the new state the action defined by /action is executed.
    In our framework the action is optional and [guard] also when it is set to true.

    simple UML state machine

    Figure 1: simple UML state machine

    As an example of the C++ FSM framework the States and the transitions are defined in the transition table as follows:
    // state type, event type
    FSM_TT(tState, tEvent)    // start of transition table
      //   current state, next state, event, guard action
      FSM_TT_BEG( State1, State2, EVENT, guard1, action() )
      FSM_TT_CNT(         State1, EVENT, guard2, action() )
    FSM_TT_END                // end of transition table
    // < FSM_ST>< entry> State1
    // < FSM_ST>
    // < FSM_ST>< do> State1
    if( EVENT1.1 == getEvent() ) { 
    // < /FSM_ST>
    // < FSM_ST>< exit> State1
    // < /FSM_ST>
    // < FSM_ST>< entry> State2
    // < /FSM_ST>
    // < FSM_ST>< do> State2
    if( EVENT2.1 == getEvent() ) { 
    // < /FSM_ST>
    // < FSM_ST>< exit> State2
    // < /FSM_ST>

    Table 1: Transition Table

    with tState and tEvent just being enums defined as:
    // available states 
    typedef enum { eStateVoid=0, State1, State2 } tState;
    // available events
    typedef enum { eEventVoid=0, EVENT, EVENT1.1, EVENT2.1 } tEvent;
    The initialization and exit of the FSM is done in the part controlling the FSM (see the main function below).

    As seen above a state is represented in the transition table as an enum value. The actions executed by the state machine are defined as methods. In C++ the methods of the class (here FsmMy) implementing the state machine will have methods as follows:
    void FsmMy::doState1 (void)
      if( isEntry() ) {
        // do entry action1.1
      if( EVENT1.1 == getEvent() ) {
        // if we consume an event in a state, 
        //   we have to tell that to the base class
        // do normal action1.3
      if( isExit() ) {
        // do exit action1.2
    void FsmMy::doState2 (void)
      if( isEntry() ) {
        // do entry action2.1
      if( EVENT2.1 == getEvent() ) {
        // if we consume an event in a state, 
        //   we have to tell that to the base class
        // do normal action2.3
      if( isExit() ) {
        // do exit action2.2
    As you can see the entry and exit parts are just if clauses.
    Note: In this framework it is a must to use the isEntry and isExit methods, even if there are no actions to do.
    The reason is that internal variables are set and the state scheduler is called.

    The last part is to connect the enum for the State with the method. This is done as follows:
      FSM_ST_TO_MTHD( State1, doState1() )
      FSM_ST_TO_MTHD( State2, doState2() )
    That is quite easy, isn't it?
    For a test you need a main function which gets the events and dispatches it. So something like:
    int main (void)
      FsmMy fsm;
      // default event
      tEvent evnt = eEvntVoid;
      // initialize the state machine with state 1
      // so process will start with state1 execution.
      // when getEvent returns eEvntVoid the program is stopped
      fsm.init( State1 );
      do   {
        evnt = getEvent();
        fsm.dispatch( evnt );  
      } while( evnt != eEvntVoid );
      return 0;

    C++/C Framework

    The framework consists of only one h file (one for C and one for C++). So you just need to include it in your state machine implementation and derive from the base class Fsm or use the functions provided. This distribution is available under the 3-clause-license, providing a wide flexibility in usage.

    Compiling / Generating diagrams:

    • Perl interpreter for running Perl scripts, e.g. Strawberry Perl , which also contains the GCC tool chain.
    • E.g. GCC toolchain for compiling a runnable executable from the example files
    • PlantUML tool for generating the diagrams (this also requires a specific version of Graphviz software).

    Creating a state chart

    Okay so far that is not really much new. But with the perl script we get a bit spice on it.
    First you can create very easy a state chart. When you have installed all the necessary stuff, just type:

    perl fsm2plantuml_pl -tt transition_table_file > state_chart.uml.

    This creates the file state_chart.uml which is converted to a state chart uml graphic automatically, e.g. by the tool PlantUML File Watcher.

    Creating the state machine C++/C file from the transition table

    Not hot enough? Okay some more spice.
    How would it be just having a transition table and generating the code framework from this? That would save some time of coding and prevent some errors. Therefore e.g. save Table 1 in file transition_table_file.
    Call the perl script just with an additional option:
    C++ C
    perl fsm2plantuml_pl -tt transition_table_file -cpp MyFsm -base > MyFsm.cpp perl fsm2plantuml_pl -tt transition_table_file -c MyFsm -base > MyFsm.c

    The lines above create the C++/C template having all the necessary declarations and definitions for the framework in the MyFsm.cpp file (including a small test part).
    You just need to add the parts which make the specific behaviour of your state machine (the bold parts in doState1 and doState2 from above).
    Also you need to have a look at the TODO parts.
    When filled correctly compile and execute on a e.g. Windows command shell with.
    C++ C
    cls && g++ MyFsm.cpp -o fsm.exe && fsm cls && gcc MyFsm.c -o fsm.exe && fsm

    For a complete example please see the fsm.cpp/fsm.c files from this web page.
    Note: The parameter for FSM_TT(... macro vary between the C and C++ framework (please have a look in the specific files). The script takes care about that and creates the correct FSM_TT macro included in the C template.

    More Code generator (script) options / Examples

    The PERL script is much more powerful.
    It can create the template for use of the base class.
    If you do not like a base class with virtual functions at all, the script offers you a way to create the C++ template without any base class from the transition table. The result is that you will just have one C++ file containing everything.
    Note: In case of C we do not have classes, but in that case you will have a set of functions provided in the fsmc.h module. So when I write class think of a module in C. When I write method think of a function in C.

    C++ C
    Single file without use of the base class using methods
    perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm > fsm.cpp perl fsm2plantuml_pl -tt fsm.tt -c MyFsm > fsm.c
    Single file without use of the base class using switch/case
    perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm -sw > fsm.cpp perl fsm2plantuml_pl -tt fsm.tt -c MyFsm -sw > fsm.c
    Without an entry/exit in each state method
    perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm -nee > fsm.cpp perl fsm2plantuml_pl -tt fsm.tt -c MyFsm -nee > fsm.c
    Without an entry/exit in each case when using switch
    perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm -sw -nee > fsm.cpp perl fsm2plantuml_pl -tt fsm.tt -c MyFsm -sw -nee > fsm.c
    Expanding the transition table to switch/case code (less use of macros).
    perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm -sw -extt > fsm.cpp perl fsm2plantuml_pl -tt fsm.tt -c MyFsm -sw -extt > fsm.c

    General Signature

    perl fsm2plantuml_pl -tt transition_table [-{cpp|c|vctst} MyFsm [-nee] [-sw] [-base] [-extt]]

    [] means optional
    {a|b} means chose a or b.
    -extt can only be used without -base option.


    Transition Table Signature

    //  Start of transition table giving
    //  the type of state and event.
    FSM_TT(type_of_state, type_of_event)    
      //   Transition to go from state1 
      //   to state2 when event is received and 
      //   guard1 is true. 
      //   Do action1 when transition to next state.
      FSM_TT_BEG( state1, state2, event, guard1, action1() )
      //   Additional transition to go from state1 to 
      //   state3 when event2 is received and guard2 is true. 
      //   Do action2 when transition to state3.
      FSM_TT_CNT(         state3, event2, guard2, action2() )
      FSM_TT_CNT(         ...
      FSM_TT_CNT(         ...
    // end of transition table
    //    The start state when entering the state machine for the first time
    //    and initialization actions                            
    // < FSM_ST>< entry> state1
    //    Entry actions for state1 in table
    // < /FSM_ST>
    // < FSM_ST>< do> State1
    //    Do actions for state1 in table
    // < /FSM_ST>
    // < FSM_ST>< exit> State1
    //    Exit actions for state1 in table
    // < /FSM_ST>
    // < FSM_ST>< attr>
    // defines FSM attributes
    // < /FSM_ST>
    /* The following macro adds a superstate condition
       the method is called when an event 
         could not be processed in the states.
       When event and guard are valid action is performed and 
         then switched to next state
       If the next state shall be proccessed set bDoTransition = true;
       If a new event shall be proccessed directly add 
         addNewEvent( your_new_event );
       This macro is automatically placed to doSuperstate
    FSM_TT_SS( nextState, event, guard, action )
    Note ... stands for not shown but similar parts as written before.
    Note the space after the <, e.g. < FSM_ST, shall be omitted. This is inserted since the HTML interpreter erases otherwise the parts between <>.

    Configuring the Fsm behaviour

    The behaviour of the Fsm can be configured in the fsm.h file. The part is marked with user settings and user settings end. What the macros do is described in the file.

    Step by step example

    Let's generate the state machine described in Figure 2.

    fsm.cpp plantuml figure
    Figure 2: Generated UML Diagram

    The transition table belonging to this figure is:
    FSM_TT(tState, tEvent)
      FSM_TT_BEG( State1, State2, eEvntStart,    mData==1, TRACE("\n%S  ", "Goto state 2\n") )
      FSM_TT_CNT(         State1, eEvntStart,    mData==0, TRACE("\n%s  ", "Self Transition to state 1\n"); mData=1  )
      FSM_TT_CNT(         State3, eEvntContinue, mData==1, TRACE("\n%s  ", "Goto state 3\n"); ++mData  )
      FSM_TT_BEG( State2, State3, eEvntStart,    true, TRACE("\n%s  ", "Goto state 3 \n") )
      FSM_TT_BEG( State3, State1, eEvntContinue, true,  )
    FSM_TT_INIT( State1, mData=0 )
    // < FSM_ST>< attr>
    int mData;
    // < /FSM_ST>
    This table is stored in the file fsm.tt.
    Note: The init tells the script that State 1 shall be set at startup and mData shall be set to 0.
    Now call perl fsm2plantuml_pl -tt fsm.tt -cpp MyFsm > fsm.cpp

    Thus the fsm.cpp template is created. Adapt the getEvent function for a test:
    tEvent getEvent(void)
      // TODO: adapt to your needs for a test
      static int i = 0;
      static MyFsm::tEvent event[] = { 
    Save the work, compile and execute in a windows command shell with
    cls && g++ -s fsm.cpp -o fsm.exe && fsm

    Some notes on the class attribute mData.
    The class attribute mData is added by the script via // < FSM_ST>< attr> in MyFsm in the file fsm.cpp. Look at TODO in the code at the class declaration:
      tState mState;
      // TODO: enter other parts as necessary
      // < FSM_ST>< attr>
      int mData;
      // < /FSM_ST>
    The variable becomes initialized via FSM_TT_INIT in the init method:
    void MyFsm::init( tState StStart )
      TRACE_FUNC_ENTRY( StStart );
      mState = StStart;
      bDoTransition = FSM_ENTRY_INIT;
      mEventNew = eEventVoid;
      // TODO: initialization

    Design Decisions

    Why Using Macros?

    Many Programmes are lazy or in other words want to program very efficiently. If you need to switch tools all the time for the work you may start avoiding to use the tool and then code and documentation becomes different.
    You probably use the perl script as a starting point for generating your state machine code. When you have that rough idea done you may continue on coding and not switching the tool.
    This has many reasons like you put different other parts into the code which will be lost or you would need to merge the differences before it works again as intended. Therefore generating the code again from the tool is more effort thus you may prefer just to work on the code.
    If you now use the macros for the state table, the documentation and code are one. You easily can create the state machine figures when you use the state table macros with nearly no additional effort.
    Of course if you have a modelling tool which can do a round trip engineering or a tool chain which supports your design/implementation phases very smoothly you probably will prefer that.

    The constrain you get using macros is that the code may be not the most efficient one. E.g. in case you set the option FSM_TR_ACTION_AFT_EXIT to true the evaluation if a transition is valid and to execute the action of the transition needs the evaluation of the transition conditions two times.
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