Task check-in watchdog

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

#define ALIVE_SENSOR (1u << 0)
#define ALIVE_COMMS  (1u << 1)
#define ALIVE_UI     (1u << 2)
#define ALIVE_ALL    (ALIVE_SENSOR | ALIVE_COMMS | ALIVE_UI)

static volatile uint32_t alive;

void task_checkin(uint32_t bit)
{
    alive |= bit;
}

void supervisor_task(void *p)
{
    for (;;) {
        vTaskDelay(pdMS_TO_TICKS(500));
        if ((alive & ALIVE_ALL) == ALIVE_ALL) {
            IWDG->KR = 0xAAAA;
        }
        alive = 0;
    }
}

void USART3_IRQHandler(void)
{
    comms_rx_byte((uint8_t)USART3->RDR);
    task_checkin(ALIVE_COMMS);
}

void ui_task(void *p)
{
    for (;;) {
        ui_event_t e;
        xQueueReceive(ui_q, &e, portMAX_DELAY);
        task_checkin(ALIVE_UI);
        ui_handle(&e);
    }
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

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