State machines in C

Question

What is the best way to write a state machine in C?
I usually write a big switch-case statement in a for(;;), with callbacks to re-enter the state machine when an external

Answer 1

Switch statements are a good way to get started, but they tend to get unwieldy when the FSM gets larger.

A couple related (or duplicate) SO questions with great information and ideas:

• state machines tutorials
• C state-machine design

Answer 2

That's pretty much the standard approach. If you're interested in studying a well considered library and comparing specifics, take a look at Ragel:

Ragel compiles executable finite state machines from regular languages. Ragel targets C, C++, Objective-C, D, Java and Ruby. Ragel state machines can not only recognize byte sequences as regular expression machines do, but can also execute code at arbitrary points in the recognition of a regular language. Code embedding is done using inline operators that do not disrupt the regular language syntax.

Answer 3

I use function pointers and a 2d look-up table where I use the state for one parameter and the event as the other.

I use excel (or any spreadsheet tool) to map a function to every state/event combination.

When an event occurs, I que it up, so then I have something that looks like this

int main(void)
{
    StateList currentState = start_up;
    EventList currentEvent;

    uint8_t stateArray[STATE_COUNT][EVENT_COUNT];

    InitializeStateArray(stateArray);
    InitializeEventQue();

    while(1)
    {
        currentEvent = GetPriorityEvent();
        currentState = (StateList)(*(stateArray[currentState][currentEvent]))();
    }
    return 1;  //should never get here
}

This method essentially forces the developer to consider all possible events in each state, and in my experience makes debugging a little easier.