Logging through a queue

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 {
    char    text[48];
    uint8_t level;
} log_msg_t;

QueueHandle_t log_q;   /* item size: sizeof(log_msg_t *) */

void log_info(const char *s)
{
    log_msg_t m;
    strncpy(m.text, s, sizeof m.text);
    m.level = 1;
    log_msg_t *pm = &m;
    xQueueSend(log_q, &pm, 0);
}

void log_task(void *p)
{
    log_msg_t *pm;
    for (;;) {
        xQueueReceive(log_q, &pm, portMAX_DELAY);
        uart_puts(pm->text);
    }
}

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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