evdev

evdev, short for event device, is a Linux kernel interface that exposes input devices to user space in a uniform, event-driven manner. It is part of the Linux input subsystem and provides a standard way for applications to receive events from keyboards, mice, touchscreens, joysticks, and other input hardware.

Each physical input device is represented by a character device under /dev/input, typically /dev/input/eventX. Userspace programs read a stream of input_event records, each containing a timestamp, a type (such as EV_KEY, EV_REL, EV_ABS, EV_SYN), a code, and a value. EV_KEY carries key or button events; EV_REL covers relative motion (e.g., mouse movement); EV_ABS covers absolute coordinates (e.g., touchscreens); EV_SYN synchronizes event packets. Codes (like KEY_A, BTN_LEFT, ABS_X, REL_X) identify the specific control within the event type.

Applications can query device capabilities using ioctls to list supported event types and codes and can grab devices to prevent other processes from receiving events. Common user-space libraries, such as libevdev and libinput, provide higher-level access to evdev devices and are used by graphical environments and input stacks on Linux.

History and usage: The evdev interface has been part of the kernel's input subsystem for many years and underpins modern input handling in Linux. It is used by desktop environments, Wayland compositors, and many multimedia applications. It complements other input subsystems and tools, including uinput for injecting synthetic events, and serves as a foundational API for both system-level and user-space input processing.