Building a framed packet

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.

uint16_t crc16_ccitt(const uint8_t *d, size_t n)
{
    uint16_t crc = 0xFFFF;
    for (size_t i = 0; i < n; i++) {
        crc ^= (uint16_t)d[i] << 8;
        for (int b = 0; b < 8; b++)
            crc = (crc & 0x8000) ? (crc << 1) ^ 0x1021 : crc << 1;
    }
    return crc;
}

void send_packet(uint8_t cmd, const uint8_t *p, uint8_t n)
{
    uint8_t frame[64];
    frame[0] = SOF;
    frame[1] = cmd;
    frame[2] = n;
    memcpy(&frame[3], p, n);
    uint16_t crc = crc16_ccitt(frame, n + 3);
    memcpy(&frame[3 + n], &crc, 2);
    uart_send(frame, n + 5);
}

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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