WMUSER

WMUSER, commonly written as WM_USER, is a constant in the Windows API that marks the start of the range of window messages that applications can define themselves. Its value is 0x0400 in hexadecimal (1024 decimal).

Developers create custom messages by adding an offset to WM_USER, such as WM_USER + 1. These messages are sent to a window's procedure (WndProc) using SendMessage or PostMessage, enabling custom communication between windows or custom behavior in controls that is not provided by standard system messages.

The general range for these application-defined messages runs from WM_USER up to a higher limit (commonly involved in practice as WM_USER through 0x7FFF). Because other libraries or built-in controls may use messages in this range, developers are advised to exercise caution to avoid conflicts. For messages that need to be unique across modules or processes, alternatives such as WM_APP (0x8000) or RegisterWindowMessage can be used. WM_APP is intended for private messages within an application and helps reduce potential clashes with control-defined messages.

In use, WMUSER-derived messages are handled inside the window procedure alongside standard messages, typically using a switch statement to implement custom behavior when the message value matches WM_USER + N. They are part of the broader Win32 messaging model that enables inter-window communication without requiring separate IPC mechanisms.

Summary: WMUSER refers to the Windows API starting point for application-defined window messages, defined as 0x0400. It enables custom messaging via SendMessage or PostMessage, with considerations about conflicts guiding the choice between WM_USER, WM_APP, or RegisterWindowMessage depending on the scope and needs of the application.