v4L

Video4Linux (v4L) is the Linux kernel subsystem and user-space interface for video capture and output devices, including webcams, analog and digital capture cards, and TV tuners. It provides drivers, hardware abstraction, and APIs that allow applications to acquire video frames and drive display devices.

The original Video4Linux API (v4L) emerged in the late 1990s and evolved into Video4Linux2 (v4L2), the current kernel API. V4L2 standardizes device nodes under /dev/videoN, adds a streaming I/O model, a richer control framework, and a more flexible media topology. While v4L is still referenced for compatibility, most modern drivers implement v4L2.

Architecture and operation: A video device is exposed as a character device. The kernel side implements a driver and a v4l2 core that handles formats, buffers, and event notification. Userspace interacts through ioctl calls (via libraries such as libv4l2 or through application APIs). Buffers are exchanged between driver and user space either by memory mapping (mmap), user pointers, or, for legacy setups, read calls. Formats can include raw video as well as any supported compressed variants.

Usage and tooling: Common tools include v4l-utils (v4l2-ctl, v4l2-req-capabilities) to query and configure devices, and libraries such as libv4l2. Applications such as FFmpeg, GStreamer, and OBS use V4L2 for capture and input. The Video4Linux framework also integrates with the Linux media controller API to support complex devices with multiple subdevices.

Status and scope: V4L2 remains the standard interface for Linux video devices. The term v4L is sometimes used generically to refer to Video4Linux concepts, but modern development relies on v4L2.