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Embedded Linux Learning

fasync_helper

`fasync_helper()` and `kill_fasync()`¶

Purpose¶

The fasync API allows a driver to notify user space through SIGIO.

Driver Callback¶

static int mydev_fasync(int fd, struct file *file, int on)
{
    struct mydev_data *dev = file->private_data;

    return fasync_helper(fd, file, on, &dev->async_queue);
}

Event Notification¶

kill_fasync(&dev->async_queue, SIGIO, POLL_IN);

User-space Setup¶

User space typically needs to:

Set the owner with F_SETOWN
Enable O_ASYNC
Install a SIGIO handler

Common Pitfalls¶

Warning

Signals are not a structured event queue. For complex event loops, bridging SIGIO into eventfd may be easier to integrate with epoll.