v4L2

Video4Linux2 (v4l2) is the second version of the Linux kernel API and user-space interface for video capture and output devices. It provides a standardized way to access webcams, TV tuners, capture cards, and other video hardware across different drivers and devices.

v4l2 defines both kernel-side and user-space interfaces. Device drivers implement the kernel API and expose devices under node names such as /dev/videoN. User-space applications interact with these devices through ioctl calls defined in videodev2.h, using libraries and wrappers such as libv4l2. The API supports multiple I/O models, including memory-mapped buffers, user pointers, and direct read/write access, with streaming I/O being the common mode for smooth capture and playback.

A core concept in v4l2 is buffer management and streaming. Applications request buffers (VIDIOC_REQBUFS), queue buffers to the device (VIDIOC_QBUF), and dequeue filled buffers (VIDIOC_DQBUF). They start and stop data capture with VIDIOC_STREAMON and VIDIOC_STREAMOFF. The API also provides mechanisms to discover device capabilities (V4L2_CAP_* flags), enumerate supported formats and pixel formats (e.g., YUYV, UYVY, MJPEG, H.264), and negotiate frame sizes and frame rates. Pixel formats are identified by fourcc codes and can be queried or set via standard ioctls (VIDIOC_ENUM_FMT, VIDIOC_S_FMT, VIDIOC_G_FMT).

v4l2 is widely used by multimedia frameworks and applications, including GStreamer, FFmpeg, VLC, OpenCV, and desktop tools like Cheese. It is designed to be hardware-agnostic, enabling compatibility with USB video class (UVC) devices and other capture hardware through drivers in the Linux kernel.

Historically, v4l2 superseded the original Video4Linux (v4l1) and became the standard API in the Linux ecosystem, with ongoing enhancements for formats, performance, and device support.