UART receive ring buffer

The code

Embedded C for Cortex-M. It compiles. There are 4 real bugs in it: the kind that pass a bench test and fail in the field.

#define RX_BUF_SIZE 64

static uint8_t rx_buf[RX_BUF_SIZE];
static uint8_t head = 0;
static uint8_t tail = 0;
static bool data_ready = false;

void USART2_IRQHandler(void)
{
    if (USART2->ISR & USART_ISR_RXNE) {
        uint8_t byte = USART2->RDR;
        rx_buf[head] = byte;
        head = (head + 1) % RX_BUF_SIZE;
        data_ready = true;
        printf("RX: %c\n", byte);
    }
}

/* Called from the main loop */
int uart_read(uint8_t *out)
{
    while (!data_ready) { }
    *out = rx_buf[tail];
    tail = (tail + 1) % RX_BUF_SIZE;
    if (tail == head) {
        data_ready = false;
    }
    return 1;
}

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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