How Can I Efficiently Implement State Machines in C Using Proven Techniques?

C State Machine Design: Implementing Proven Techniques

State machines play a crucial role in software development, offering a structured approach to managing complex sequences of events and transitions. This article aims to provide proven state-machine design techniques in C, drawing insights from the expertise of developers on Stack Overflow.

Implementation Considerations

One common approach involves using a struct array to represent the state machine. Each element contains the current state, an event identifier, and a function pointer to handle the transition. For instance:

typedef struct {
    int st;
    int ev;
    int (*fn)(void);
} tTransition;

Complementing the struct, define states and events using macros:

#define ST_ANY              -1
#define ST_INIT              0
#define ST_ERROR             1
#define EV_ANY              -1
#define EV_KEYPRESS       5000
#define EV_MOUSEMOVE      5001

Next, define functions associated with each transition and return the next state:

static int GotKey (void) { ... };
static int FsmError (void) { ... };

Creating an array of transitions then defines the state-event relationships:

tTransition trans[] = {
    { ST_INIT, EV_KEYPRESS, &GotKey},
    { ST_ANY, EV_ANY, &FsmError}
};

The core of the state machine is a simple loop:

state = ST_INIT;
while (state != ST_TERM) {
    event = GetNextEvent();
    for (i = 0; i < TRANS_COUNT; i++) {
        if ((state == trans[i].st) || (ST_ANY == trans[i].st)) {
            if ((event == trans[i].ev) || (EV_ANY == trans[i].ev)) {
                state = (trans[i].fn)();
                break;
            }
        }
    }
}

Avoiding Global Variables

To enhance maintainability, globals can be replaced by passing a structure pointer to transition functions. This allows multiple state machines to run concurrently without interference:

typedef struct {
    int state;
    // Additional machine-specific data
} StateMachine;

Flexibility and Extensibility

This approach provides flexibility in handling events and transitions, and allows for easy configuration changes by modifying the transitions array. Higher-level abstractions may exist, but the underlying concept remains similar.

By adopting these proven techniques, developers can create robust and efficient state machines in C, ensuring efficient event handling and smooth state transitions.

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