Character driver read()

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.

static DEFINE_SPINLOCK(frame_lock);
static u8 frame[64];
static size_t frame_len;

static irqreturn_t sensor_irq(int irq, void *dev)
{
    spin_lock(&frame_lock);
    frame_len = sensor_fifo_read(frame, sizeof(frame));
    spin_unlock(&frame_lock);
    return IRQ_HANDLED;
}

static ssize_t sensor_read(struct file *f, char __user *buf,
                           size_t count, loff_t *off)
{
    spin_lock(&frame_lock);
    memcpy(buf, frame, frame_len);
    spin_unlock(&frame_lock);
    return count;
}

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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