Startup flash CRC self-test

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.

extern uint32_t __app_start, __app_end;    /* linker-script symbols */
extern const uint32_t __app_crc;           /* CRC word stored right after the image */

static uint32_t crc32(const uint8_t *p, uint32_t n)
{
    uint32_t c = 0;
    while (n--) {
        c ^= *p++;
        for (int k = 0; k < 8; k++) c = (c >> 1) ^ (0xEDB88320u & (0u - (c & 1u)));
    }
    return c;
}

bool flash_selftest(void)
{
    uint32_t len = __app_end - __app_start;
    uint32_t crc = crc32((const uint8_t *)__app_start, len);
    if (crc != __app_crc) {
        log_warn("flash CRC mismatch");
    }
    return true;
}

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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