HOWTO: Define Private Messages for Application Use

• Microsoft Windows Software Development Kit (SDK) version 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5, 3.51
      - Microsoft Windows 95
  

SUMMARY

In the Microsoft Windows environment, an application can define a private message for its own use without calling the RegisterWindowMessage API. Message numbers between 0x8000 and 0xBFFF are reserved for this purpose.

For Windows NT and Windows 95, the system defines a new message WM_APP (value 0x8000). Applications can make use of the range WM_APP through 0xBFFF for private messages without conflict. The only requirement is that the .EXE file must be marked version 4.0 (use the linker switch /subsystem:windows,4.0). Windows NT 3.5 and 3.51 and Windows 95 will run applications marked version 4.0.

MORE INFORMATION

The documentation for the WM_USER message lists four ranges of message numbers as follows:

   Message Number           Description
   --------------           -----------

   0 through WM_USER-1      Messages reserved for use by Windows.

   WM_USER through 0x7FFF   Integer messages for use by private window
                            classes.

   0x8000 through 0xBFFF    Messages available for use by applications.

   0xC000 through 0xFFFF    String messages for use by applications.

   Greater than 0xFFFF      Reserved by Windows for future use.

To avoid this inconvenience, messages between 0x8000 and 0xBFFF were redefined to make them available to an application. Messages in this range do not conflict with any other messages in the system.

The distinction here is that WM_USER messages are specific to a window class (whether it is a control or a user-defined class), while WM_APP messages are not. While WM_USER messages can be treated as application messages, the danger in doing so is that if any are already pre-defined for a particular window class, they could be misinterpreted upon receipt.