1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2 3.. _app-pri: 4 5******************** 6Application Priority 7******************** 8 9When multiple applications share a device it may be desirable to assign 10them different priorities. Contrary to the traditional "rm -rf /" school 11of thought, a video recording application could for example block other 12applications from changing video controls or switching the current TV 13channel. Another objective is to permit low priority applications 14working in background, which can be preempted by user controlled 15applications and automatically regain control of the device at a later 16time. 17 18Since these features cannot be implemented entirely in user space V4L2 19defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and 20:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and 21query the access priority associate with a file descriptor. Opening a 22device assigns a medium priority, compatible with earlier versions of 23V4L2 and drivers not supporting these ioctls. Applications requiring a 24different priority will usually call :ref:`VIDIOC_S_PRIORITY 25<VIDIOC_G_PRIORITY>` after verifying the device with the 26:ref:`VIDIOC_QUERYCAP` ioctl. 27 28Ioctls changing driver properties, such as 29:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code 30after another application obtained higher priority. 31