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

You can use minimalist uml-state-machine framework implemented in c. It supports both finite and hierarchical state machine. The framework is very minimalist. It has only 3 API's, 2 structures and 1 enumeration.

The State machine is represented by state_machine_t structure. It is an abstract structure that can be inherited to create a state machine.

//! Abstract state machine structure
struct state_machine_t
{
   uint32_t Event;          //!< Pending Event for state machine
   const state_t* State;    //!< State of state machine.
};

State is represented by pointer to state_t structure in the framework.

If framework is configured for finite state machine then state_t contains,

typedef struct finite_state_t state_t;

// finite state structure
typedef struct finite_state_t{
  state_handler Handler;        //!< State handler function (function pointer)
  state_handler Entry;          //!< Entry action for state (function pointer)
  state_handler Exit;           //!< Exit action for state (function pointer)
}finite_state_t;

If framework is configured to support hierarchical state machine. It contains additional three members to represent the hierarchical relation between the states.

typedef struct hierarchical_state_t state_t;

//! Hierarchical state structure
typedef struct hierarchical_state_t
{
  state_handler Handler;      //!< State handler function
  state_handler Entry;        //!< Entry action for state
  state_handler Exit;         //!< Exit action for state.

  const state_t* const Parent;    //!< Parent state of the current state.
  const state_t* const Node;      //!< Child states of the current state.
  uint32_t Level;                 //!< Hierarchy level from the top state.
}hierarchical_state_t;

The framework provides an API dispatch_event to dispatch the event to the state machine and two API's for the state traversal.

state_machine_result_t dispatch_event(state_machine_t* const pState_Machine[], uint32_t quantity);

state_machine_result_t switch_state(state_machine_t* const pState_Machine, const state_t* pTarget_State);

state_machine_result_t traverse_state(state_machine_t* const pState_Machine, const state_t* pTarget_State);

For more details refer to GitHub project.