Event-driven Finite-state Machine - Example in C

Example in C

This code describes the state machine for a very basic car radio system. It is basically an infinite loop that reads incoming events. The state machine is only 2 states: radio mode, or CD mode. The event is either a mode change from radio to cd back and forth, or a go to next (next preset for radio or next track for CD).

/********************************************************************/ #include /********************************************************************/ typedef enum { ST_RADIO, ST_CD } STATES; typedef enum { EVT_MODE, EVT_NEXT } EVENTS; EVENTS readEventFromMessageQueue(void); /********************************************************************/ int main(void) { /* Default state is radio */ STATES state = ST_RADIO; int stationNumber = 0; int trackNumber = 0; /* Infinite loop */ while(1) { /* Read the next incoming event. Usually this is a blocking function. */ EVENTS event = readEventFromMessageQueue; /* Switch the state and the event to execute the right transition. */ switch(state) { case ST_RADIO: switch(event) { case EVT_MODE: /* Change the state */ state = ST_CD; break; case EVT_NEXT: /* Increase the station number */ stationNumber++; break; } break; case ST_CD: switch(event) { case EVT_MODE: /* Change the state */ state = ST_RADIO; break; case EVT_NEXT: /* Go to the next track */ trackNumber++; break; } break; } } }

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