| .. -*- coding: utf-8; mode: rst -*- |
| |
| .. _app-pri: |
| |
| ******************** |
| Application Priority |
| ******************** |
| |
| When multiple applications share a device it may be desirable to assign |
| them different priorities. Contrary to the traditional "rm -rf /" school |
| of thought a video recording application could for example block other |
| applications from changing video controls or switching the current TV |
| channel. Another objective is to permit low priority applications |
| working in background, which can be preempted by user controlled |
| applications and automatically regain control of the device at a later |
| time. |
| |
| Since these features cannot be implemented entirely in user space V4L2 |
| defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and |
| :ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and |
| query the access priority associate with a file descriptor. Opening a |
| device assigns a medium priority, compatible with earlier versions of |
| V4L2 and drivers not supporting these ioctls. Applications requiring a |
| different priority will usually call :ref:`VIDIOC_S_PRIORITY |
| <VIDIOC_G_PRIORITY>` after verifying the device with the |
| :ref:`VIDIOC_QUERYCAP` ioctl. |
| |
| Ioctls changing driver properties, such as |
| :ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code |
| after another application obtained higher priority. |
