Integer promotion traps

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.

bool frame_valid(const uint8_t *p, uint8_t len, uint8_t expected)
{
    uint8_t sum = 0;
    for (uint8_t i = 0; i < len; i++) {
        sum += p[i];
    }
    return ~sum == expected;
}

void irq_enable(volatile uint32_t *iser, int irqn)
{
    *iser = (1 << irqn);
}

void send_reversed(const uint8_t *buf, uint8_t len)
{
    for (uint8_t i = len - 1; i >= 0; i--) {
        uart_putc(buf[i]);
    }
}

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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