GPIO through /dev/mem

The code

Embedded Linux C. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

#define GPIO_BASE  0x30200400u     /* GPIO bank 2 */
#define DR   0x00
#define PSR  0x08

static uint32_t *gpio;

int gpio_map(void)
{
    int fd = open("/dev/mem", O_RDWR);
    gpio = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, GPIO_BASE);
    return gpio ? 0 : -1;
}

void led_set(bool on)
{
    if (on) gpio[DR / 4] |= (1u << 9);
    else    gpio[DR / 4] &= ~(1u << 9);
}

bool fault_active(void)
{
    return gpio[PSR / 4] & (1u << 3);
}

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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