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Blocking and Non-blocking I/O

A Linux driver should define what happens when user space tries to read data while no event or data is available.

The usual behavior is:

  • blocking mode: sleep until data becomes available
  • non-blocking mode: return -EAGAIN

Read Path Model

read()
    ↓
check event queue
    ↓
if data available: copy_to_user()
    ↓
if no data and O_NONBLOCK: return -EAGAIN
    ↓
if no data and blocking: wait_event_interruptible()

Why This Matters

This behavior allows the same driver to support different user-space styles:

User-space Style Driver Behavior
Simple blocking read() Sleep until data is ready
O_NONBLOCK loop Return -EAGAIN if empty
poll / epoll Report readiness only when data is available

Driver-Side Checks

if (queue_empty(dev)) {
    if (file->f_flags & O_NONBLOCK)
        return -EAGAIN;

    ret = wait_event_interruptible(dev->wq, !queue_empty(dev));
    if (ret)
        return ret;
}

Common Pitfalls

Warning

Do not return 0 when no data is available unless the driver intentionally reports EOF. For non-blocking device reads, -EAGAIN is usually the correct result.

Warning

Always re-check the condition after waking up. Wakeups can be interrupted or spurious.