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).
/********************************************************************/
#include <stdio.h>
/********************************************************************/
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;
}
}
}