1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _io: 5 6############ 7Input/Output 8############ 9The V4L2 API defines several different methods to read from or write to 10a device. All drivers exchanging data with applications must support at 11least one of them. 12 13The classic I/O method using the :c:func:`read()` and 14:c:func:`write()` function is automatically selected after opening a 15V4L2 device. When the driver does not support this method attempts to 16read or write will fail at any time. 17 18Other methods must be negotiated. To select the streaming I/O method 19with memory mapped or user buffers applications call the 20:ref:`VIDIOC_REQBUFS` ioctl. 21 22Video overlay can be considered another I/O method, although the 23application does not directly receive the image data. It is selected by 24initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` 25ioctl. For more information see :ref:`overlay`. 26 27Generally exactly one I/O method, including overlay, is associated with 28each file descriptor. The only exceptions are applications not 29exchanging data with a driver ("panel applications", see :ref:`open`) 30and drivers permitting simultaneous video capturing and overlay using 31the same file descriptor, for compatibility with V4L and earlier 32versions of V4L2. 33 34:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some 35degree, but for simplicity drivers need not support switching the I/O 36method (after first switching away from read/write) other than by 37closing and reopening the device. 38 39The following sections describe the various I/O methods in more detail. 40 41.. toctree:: 42 :maxdepth: 1 43 44 rw 45 mmap 46 userp 47 dmabuf 48 buffer 49 field-order 50