# \[question\] v4l read() operation

- From: Dave Hylands \<hidden\>
- Date: 2012-02-25 23:14:37 
- List: kernelnewbies
- Page: https://ratatoskr.run/kernelnewbies/2012/02/8351108
- Thread: https://ratatoskr.run/kernelnewbies/2012/02/8351177/t.md

```text
Hi Ezequiel

2012/2/25 Ezequiel Garc?a [ref]:
> Hi,
>
> If I register a video device with this fops:
>
> static const struct v4l2_file_operations v4l2_fops = {
> ? ? ? ?.owner ? ? ?= THIS_MODULE,
> ? ? ? ?.open ? ? ? ?= xxx_open,
> ? ? ? ?.unlocked_ioctl = xxx_unlocked_ioctl,
> ? ? ? ?.poll ? ? ? ? ? = xxx_poll,
> ? ? ? ?.mmap ? ? ?= xxx_mmap,
> };
>
> then if I "cat" the device
>
> $ cat /dev/video0
>
> Who is supporting read() ?
>
> I thought it could be v4l2_read(),
> however this function seems to return EINVAL:
>
> static ssize_t v4l2_read(struct file *filp, char __user *buf,
> ? ? ? ? ? ? ? ?size_t sz, loff_t *off)
> {
> ? ? ? ?struct video_device *vdev = video_devdata(filp);
> ? ? ? ?int ret = -ENODEV;
>
> ? ? ? ?if (!vdev->fops->read)
> ? ? ? ? ? ? ? ?return -EINVAL;
> ? ? ? ?if (vdev->lock && mutex_lock_interruptible(vdev->lock))
> ? ? ? ? ? ? ? ?return -ERESTARTSYS;
> ? ? ? ?if (video_is_registered(vdev))
> ? ? ? ? ? ? ? ?ret = vdev->fops->read(filp, buf, sz, off);
> ? ? ? ?if (vdev->lock)
> ? ? ? ? ? ? ? ?mutex_unlock(vdev->lock);
> ? ? ? ?return ret;
> }

I'm not all that familiar with v4l, but based on what you've posted,
you need to populate the read routine in your v4l2_fops structure to
support read.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com
```