Packing a log record

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.

static uint8_t rec[32];

static void put_u32(uint8_t *p, uint32_t v)
{
    *(uint32_t *)p = v;
}

size_t pack_record(uint8_t type, uint32_t ts, uint64_t id, float val)
{
    rec[0] = type;
    put_u32(&rec[1], ts);
    *(uint64_t *)&rec[5] = id;
    *(float *)&rec[13] = val;
    return 17;
}

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.

Related challenges