Event-driven finite-state machine

 
In computation, a finite-state machine (FSM) is event driven if the transition from one state to another is triggered by an event or a message. This is in contrast to the parsing-theory origins of the term finite-state machine where the machine is described as consuming characters or tokens. 
Often these machines are implemented as threads or processes communicating with one another as part of a larger application. For example, a telecommunication protocol is most of the time implemented as an event-driven finite-state machine.
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). 
/********************************************************************/
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;