Synchronous request and reply

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.

typedef struct { uint8_t sensor; int32_t *result; } req_t;
QueueHandle_t req_q, reply_q;

bool sensor_get(uint8_t sensor, int32_t *out)
{
    int32_t value;
    req_t r = { sensor, &value };
    xQueueSend(req_q, &r, portMAX_DELAY);
    uint8_t done;
    if (xQueueReceive(reply_q, &done, pdMS_TO_TICKS(100)) != pdTRUE)
        return false;
    *out = value;
    return true;
}

void hub_task(void *p)
{
    for (;;) {
        req_t r;
        xQueueReceive(req_q, &r, portMAX_DELAY);
        *r.result = sensor_read_slow(r.sensor);
        uint8_t done = 1;
        xQueueSend(reply_q, &done, 0);
    }
}

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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