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1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later2 3.. _app-pri:4 5********************6Application Priority7********************8 9When multiple applications share a device it may be desirable to assign10them different priorities. Contrary to the traditional "rm -rf /" school11of thought, a video recording application could for example block other12applications from changing video controls or switching the current TV13channel. Another objective is to permit low priority applications14working in background, which can be preempted by user controlled15applications and automatically regain control of the device at a later16time.17 18Since these features cannot be implemented entirely in user space V4L219defines the :ref:`VIDIOC_G_PRIORITY <VIDIOC_G_PRIORITY>` and20:ref:`VIDIOC_S_PRIORITY <VIDIOC_G_PRIORITY>` ioctls to request and21query the access priority associate with a file descriptor. Opening a22device assigns a medium priority, compatible with earlier versions of23V4L2 and drivers not supporting these ioctls. Applications requiring a24different priority will usually call :ref:`VIDIOC_S_PRIORITY25<VIDIOC_G_PRIORITY>` after verifying the device with the26:ref:`VIDIOC_QUERYCAP` ioctl.27 28Ioctls changing driver properties, such as29:ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>`, return an ``EBUSY`` error code30after another application obtained higher priority.31