3. 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 read() and 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 ioctl VIDIOC_REQBUFS ioctl. The asynchronous I/O method is not defined yet.

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 VIDIOC_S_FMT ioctl. For more information see Video Overlay Interface.

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 open) and drivers permitting simultaneous video capturing and overlay using the same file descriptor, for compatibility with V4L and earlier versions of V4L2.

VIDIOC_S_FMT and ioctl 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.

• 3.1. Read/Write
• 3.2. Streaming I/O (Memory Mapping)
• 3.3. Streaming I/O (User Pointers)
• 3.4. Streaming I/O (DMA buffer importing)
• 3.5. Asynchronous I/O
• 3.6. Buffers
• 3.7. Field Order