State Machine Framework

Overview

The State Machine Framework (SMF) is an application agnostic framework that provides an easy way for developers to integrate state machines into their application. The framework can be added to any project by enabling the CONFIG_SMF option.

State Creation

A state is represented by three functions, where one function implements the Entry actions, another function implements the Run actions, and the last function implements the Exit actions. The prototype for these functions is as follows: void funct(void *obj), where the obj parameter is a user defined structure that has the state machine context, smf_ctx, as its first member. For example:

struct user_object {
   struct smf_ctx ctx;
   /* All User Defined Data Follows */
};

The smf_ctx member must be first because the state machine framework’s functions casts the user defined object to the smf_ctx type with the SMF_CTX macro.

For example instead of doing this (struct smf_ctx *)&user_obj, you could use SMF_CTX(&user_obj).

By default, a state can have no ancestor states, resulting in a flat state machine. But to enable the creation of a hierarchical state machine, the CONFIG_SMF_ANCESTOR_SUPPORT option must be enabled.

By default, the hierarchical state machines do not support initial transitions to child states on entering a superstate. To enable them the CONFIG_SMF_INITIAL_TRANSITION option must be enabled.

The following macro can be used for easy state creation:

State Machine Creation

A state machine is created by defining a table of states that’s indexed by an enum. For example, the following creates three flat states:

enum demo_state { S0, S1, S2 };

const struct smf_state demo_states[] = {
   [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit, NULL, NULL),
   [S1] = SMF_CREATE_STATE(s1_entry, s1_run, s1_exit, NULL, NULL),
   [S2] = SMF_CREATE_STATE(s2_entry, s2_run, s2_exit, NULL, NULL)
};

And this example creates three hierarchical states:

enum demo_state { S0, S1, S2 };

const struct smf_state demo_states[] = {
   [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit, parent_s0, NULL),
   [S1] = SMF_CREATE_STATE(s1_entry, s1_run, s1_exit, parent_s12, NULL),
   [S2] = SMF_CREATE_STATE(s2_entry, s2_run, s2_exit, parent_s12, NULL)
};

This example creates three hierarchical states with an initial transition from parent state S0 to child state S2:

enum demo_state { S0, S1, S2 };

/* Forward declaration of state table */
const struct smf_state demo_states[];

const struct smf_state demo_states[] = {
   [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit, NULL, demo_states[S2]),
   [S1] = SMF_CREATE_STATE(s1_entry, s1_run, s1_exit, demo_states[S0], NULL),
   [S2] = SMF_CREATE_STATE(s2_entry, s2_run, s2_exit, demo_states[S0], NULL)
};

To set the initial state, the smf_set_initial() function should be called.

To transition from one state to another, the smf_set_state() function is used.

Note

If CONFIG_SMF_INITIAL_TRANSITION is not set, smf_set_initial() and smf_set_state() function should not be passed a parent state as the parent state does not know which child state to transition to. Transitioning to a parent state is OK if an initial transition to a child state is defined. A well-formed HSM should have initial transitions defined for all parent states.

Note

While the state machine is running, smf_set_state() should only be called from the Entry or Run function. Calling smf_set_state() from Exit functions will generate a warning in the log and no transition will occur.

State Machine Execution

To run the state machine, the smf_run_state() function should be called in some application dependent way. An application should cease calling smf_run_state if it returns a non-zero value.

Preventing Parent Run Actions

Calling smf_set_handled() prevents calling the run action of parent states. It is not required to call smf_set_handled() if the state calls smf_set_state().

State Machine Termination

To terminate the state machine, the smf_set_terminate() function should be called. It can be called from the entry, run, or exit actions. The function takes a non-zero user defined value that will be returned by the smf_run_state() function.

UML State Machines

SMF follows UML hierarchical state machine rules for transitions i.e., the entry and exit actions of the least common ancestor are not executed on transition, unless said transition is a transition to self.

The UML Specification for StateMachines may be found in chapter 14 of the UML specification available here: https://www.omg.org/spec/UML/

SMF breaks from UML rules in:

  1. Executing the actions associated with the transition within the context of the source state, rather than after the exit actions are performed.

  2. Only allowing external transitions to self, not to sub-states. A transition from a superstate to a child state is treated as a local transition.

  3. Prohibiting transitions using smf_set_state() in exit actions.

SMF also does not provide any pseudostates except the Initial Pseudostate. Terminate pseudostates can be modelled by calling smf_set_terminate() from the entry action of a ‘terminate’ state. Orthogonal regions are modelled by calling smf_run_state() for each region.

State Machine Examples

Flat State Machine Example

This example turns the following state diagram into code using the SMF, where the initial state is S0.

digraph smf_flat { node [style=rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; STATE_S2 [shape = box]; init -> STATE_S0; STATE_S0 -> STATE_S1; STATE_S1 -> STATE_S2; STATE_S2 -> STATE_S0; }

Flat state machine diagram

Code:

#include <zephyr/smf.h>

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { S0, S1, S2 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Other state specific data add here */
} s_obj;

/* State S0 */
static void s0_entry(void *o)
{
        /* Do something */
}
static void s0_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
}
static void s0_exit(void *o)
{
        /* Do something */
}

/* State S1 */
static void s1_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S2]);
}
static void s1_exit(void *o)
{
        /* Do something */
}

/* State S2 */
static void s2_entry(void *o)
{
        /* Do something */
}
static void s2_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        [S0] = SMF_CREATE_STATE(s0_entry, s0_run, s0_exit, NULL, NULL),
        /* State S1 does not have an entry action */
        [S1] = SMF_CREATE_STATE(NULL, s1_run, s1_exit, NULL, NULL),
        /* State S2 does not have an exit action */
        [S2] = SMF_CREATE_STATE(s2_entry, s2_run, NULL, NULL, NULL),
};

int main(void)
{
        int32_t ret;

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
                k_msleep(1000);
        }
}

Hierarchical State Machine Example

This example turns the following state diagram into code using the SMF, where S0 and S1 share a parent state and S0 is the initial state.

digraph smf_hierarchical { node [style = rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; STATE_S2 [shape = box]; subgraph cluster_0 { label = "PARENT"; style = rounded; STATE_S0 -> STATE_S1; } init -> STATE_S0; STATE_S1 -> STATE_S2; STATE_S2 -> STATE_S0; }

Hierarchical state machine diagram

Code:

#include <zephyr/smf.h>

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { PARENT, S0, S1, S2 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Other state specific data add here */
} s_obj;

/* Parent State */
static void parent_entry(void *o)
{
        /* Do something */
}
static void parent_exit(void *o)
{
        /* Do something */
}

/* State S0 */
static void s0_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
}

/* State S1 */
static void s1_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S2]);
}

/* State S2 */
static void s2_run(void *o)
{
        smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        /* Parent state does not have a run action */
        [PARENT] = SMF_CREATE_STATE(parent_entry, NULL, parent_exit, NULL, NULL),
        /* Child states do not have entry or exit actions */
        [S0] = SMF_CREATE_STATE(NULL, s0_run, NULL, &demo_states[PARENT], NULL),
        [S1] = SMF_CREATE_STATE(NULL, s1_run, NULL, &demo_states[PARENT], NULL),
        /* State S2 do not have entry or exit actions and no parent */
        [S2] = SMF_CREATE_STATE(NULL, s2_run, NULL, NULL, NULL),
};

int main(void)
{
        int32_t ret;

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
                k_msleep(1000);
        }
}
When designing hierarchical state machines, the following should be considered:

  • Ancestor entry actions are executed before the sibling entry actions. For example, the parent_entry function is called before the s0_entry function.

  • Transitioning from one sibling to another with a shared ancestry does not re-execute the ancestor's entry action or execute the exit action. For example, the parent_entry function is not called when transitioning from S0 to S1, nor is the parent_exit function called.

  • Ancestor exit actions are executed after the exit action of the current state. For example, the s1_exit function is called before the parent_exit function is called.

  • The parent_run function only executes if the child_run function does not call either smf_set_state() or smf_set_handled().

Event Driven State Machine Example

Events are not explicitly part of the State Machine Framework but an event driven state machine can be implemented using Zephyr Events.

digraph smf_flat { node [style=rounded]; init [shape = point]; STATE_S0 [shape = box]; STATE_S1 [shape = box]; init -> STATE_S0; STATE_S0 -> STATE_S1 [label = "BTN EVENT"]; STATE_S1 -> STATE_S0 [label = "BTN EVENT"]; }

Event driven state machine diagram

Code:

#include <zephyr/kernel.h>
#include <zephyr/drivers/gpio.h>
#include <zephyr/smf.h>

#define SW0_NODE        DT_ALIAS(sw0)

/* List of events */
#define EVENT_BTN_PRESS BIT(0)

static const struct gpio_dt_spec button =
        GPIO_DT_SPEC_GET_OR(SW0_NODE, gpios, {0});

static struct gpio_callback button_cb_data;

/* Forward declaration of state table */
static const struct smf_state demo_states[];

/* List of demo states */
enum demo_state { S0, S1 };

/* User defined object */
struct s_object {
        /* This must be first */
        struct smf_ctx ctx;

        /* Events */
        struct k_event smf_event;
        int32_t events;

        /* Other state specific data add here */
} s_obj;

/* State S0 */
static void s0_entry(void *o)
{
        printk("STATE0\n");
}

static void s0_run(void *o)
{
        struct s_object *s = (struct s_object *)o;

        /* Change states on Button Press Event */
        if (s->events & EVENT_BTN_PRESS) {
                smf_set_state(SMF_CTX(&s_obj), &demo_states[S1]);
        }
}

/* State S1 */
static void s1_entry(void *o)
{
        printk("STATE1\n");
}

static void s1_run(void *o)
{
        struct s_object *s = (struct s_object *)o;

        /* Change states on Button Press Event */
        if (s->events & EVENT_BTN_PRESS) {
                smf_set_state(SMF_CTX(&s_obj), &demo_states[S0]);
        }
}

/* Populate state table */
static const struct smf_state demo_states[] = {
        [S0] = SMF_CREATE_STATE(s0_entry, s0_run, NULL, NULL, NULL),
        [S1] = SMF_CREATE_STATE(s1_entry, s1_run, NULL, NULL, NULL),
};

void button_pressed(const struct device *dev,
                struct gpio_callback *cb, uint32_t pins)
{
        /* Generate Button Press Event */
        k_event_post(&s_obj.smf_event, EVENT_BTN_PRESS);
}

int main(void)
{
        int ret;

        if (!gpio_is_ready_dt(&button)) {
                printk("Error: button device %s is not ready\n",
                        button.port->name);
                return;
        }

        ret = gpio_pin_configure_dt(&button, GPIO_INPUT);
        if (ret != 0) {
                printk("Error %d: failed to configure %s pin %d\n",
                        ret, button.port->name, button.pin);
                return;
        }

        ret = gpio_pin_interrupt_configure_dt(&button,
                GPIO_INT_EDGE_TO_ACTIVE);
        if (ret != 0) {
                printk("Error %d: failed to configure interrupt on %s pin %d\n",
                        ret, button.port->name, button.pin);
                return;
        }

        gpio_init_callback(&button_cb_data, button_pressed, BIT(button.pin));
        gpio_add_callback(button.port, &button_cb_data);

        /* Initialize the event */
        k_event_init(&s_obj.smf_event);

        /* Set initial state */
        smf_set_initial(SMF_CTX(&s_obj), &demo_states[S0]);

        /* Run the state machine */
        while(1) {
                /* Block until an event is detected */
                s_obj.events = k_event_wait(&s_obj.smf_event,
                                EVENT_BTN_PRESS, true, K_FOREVER);

                /* State machine terminates if a non-zero value is returned */
                ret = smf_run_state(SMF_CTX(&s_obj));
                if (ret) {
                        /* handle return code and terminate state machine */
                        break;
                }
        }
}

State Machine Example With Initial Transitions And Transition To Self

tests/lib/smf/src/test_lib_self_transition_smf.c defines a state machine for testing the initial transitions and transitions to self in a parent state. The statechart for this test is below.

digraph smf_hierarchical_initial { compound=true; node [style = rounded]; "smf_set_initial()" [shape=plaintext fontname=Courier]; ab_init_state [shape = point]; STATE_A [shape = box]; STATE_B [shape = box]; STATE_C [shape = box]; STATE_D [shape = box]; DC[shape=point height=0 width=0 label=<>] subgraph cluster_root { label = "ROOT"; style = rounded; subgraph cluster_ab { label = "PARENT_AB"; style = rounded; ab_init_state -> STATE_A; STATE_A -> STATE_B; } subgraph cluster_c { label = "PARENT_C"; style = rounded; STATE_B -> STATE_C [ltail=cluster_ab] } STATE_C -> DC [ltail=cluster_c, dir=none]; DC -> STATE_C [lhead=cluster_c]; STATE_C -> STATE_D } "smf_set_initial()" -> STATE_A [lhead=cluster_ab] }

Test state machine for UML State Transitions

API Reference

State Machine Framework API

Related code samples