WMKEYUP

WM_KEYUP is a Windows message sent to the window that currently has keyboard focus when a non-system key is released. Its numeric constant is 0x0101, and it serves as the counterpart to WM_KEYDOWN. For keys that involve the system (such as when ALT is used), the system key result is delivered as WM_SYSKEYUP instead.

Parameters and meaning:

The wParam parameter contains the virtual-key code of the key that was released (for example, VK_A, VK_LEFT, etc.). The lParam parameter packs additional information about the key event, including details such as the repeat count, the hardware scan code, whether the key is an extended key, and flags related to the key state and transition. This information can be used to distinguish between an initial key press, a repeat, or a key release, and to identify the exact key involved.

Usage:

WM_KEYUP is typically handled in a window procedure to react when a user finishes pressing a key, such as finalizing an action started on keydown, stopping continuous input, or triggering behaviors that should only occur on release. It is commonly processed in conjunction with WM_KEYDOWN and, when relevant, WM_SYSKEYUP for system-key interactions.

Notes:

WM_KEYUP is delivered to the window with focus unless input processing is redirected (for example, by certain modal dialogs or keyboard hooks). Many applications use this message to implement key-release semantics separate from key-press semantics, or to support input handling that avoids repeated actions on held keys.