| .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later |
| .. c:namespace:: V4L |
| |
| .. _io: |
| |
| ############ |
| Input/Output |
| ############ |
| The V4L2 API defines several different methods to read from or write to |
| a device. All drivers exchanging data with applications must support at |
| least one of them. |
| |
| The classic I/O method using the :c:func:`read()` and |
| :c:func:`write()` function is automatically selected after opening a |
| V4L2 device. When the driver does not support this method attempts to |
| read or write will fail at any time. |
| |
| Other methods must be negotiated. To select the streaming I/O method |
| with memory mapped or user buffers applications call the |
| :ref:`VIDIOC_REQBUFS` ioctl. |
| |
| Video overlay can be considered another I/O method, although the |
| application does not directly receive the image data. It is selected by |
| initiating video overlay with the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` |
| ioctl. For more information see :ref:`overlay`. |
| |
| Generally exactly one I/O method, including overlay, is associated with |
| each file descriptor. The only exceptions are applications not |
| exchanging data with a driver ("panel applications", see :ref:`open`) |
| and drivers permitting simultaneous video capturing and overlay using |
| the same file descriptor, for compatibility with V4L and earlier |
| versions of V4L2. |
| |
| :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_REQBUFS` would permit this to some |
| degree, but for simplicity drivers need not support switching the I/O |
| method (after first switching away from read/write) other than by |
| closing and reopening the device. |
| |
| The following sections describe the various I/O methods in more detail. |
| |
| .. toctree:: |
| :maxdepth: 1 |
| |
| rw |
| mmap |
| userp |
| dmabuf |
| buffer |
| field-order |
