Flags shared with an ISR

The code

Embedded C for Cortex-M. It compiles. There are 2 real bugs in it: the kind that pass a bench test and fail in the field.

typedef struct {
    uint8_t rx_ready : 1;
    uint8_t tx_busy  : 1;
    uint8_t error    : 1;
} uart_flags_t;

static volatile uart_flags_t flags;
static volatile uint32_t     pending;

void USART1_IRQHandler(void)
{
    if (USART1->ISR & USART_ISR_RXNE) {
        rx_push(USART1->RDR);
        flags.rx_ready = 1;
        pending++;
    }
}

void main_loop(void)
{
    flags.tx_busy = 1;
    start_tx();
    if (pending) {
        handle_one();
        pending--;
    }
}

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.

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