Critical sections and scheduler locks

The code

Embedded C for Cortex-M. It compiles. There are 4 real bugs in it: the kind that pass a bench test and fail in the field.

void update_config(const config_t *c)
{
    taskENTER_CRITICAL();
    memcpy(&g_cfg, c, sizeof g_cfg);
    flash_save(&g_cfg);
    taskEXIT_CRITICAL();
}

void stats_task(void *p)
{
    for (;;) {
        vTaskSuspendAll();
        uint32_t n   = g_stats.count;
        float    avg = g_stats.sum / n;
        xTaskResumeAll();
        printf("avg %.2f\n", avg);
        vTaskDelay(1000);
    }
}

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

Matched on the skills each company's embedded roles emphasise.

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