SAMPLE: Demo of a Keyboard Hook Function Keyboard Filter

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

KBHOOK is a file in the Software Library that contains sample code to demonstrate the installation and use of a system-wide keyboard filter function (otherwise known as a keyboard hook function). This code monitors the status of the CAPS LOCK key. Whenever the CAPS LOCK key is pressed, its status is displayed on the application's icon. Terminating the application removes the keyboard filter from the system.

Download KBHOOK.EXE, a self-extracting file, from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download KBHOOK.EXE (size: 22361 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get KBHOOK.EXE (size: 22361 bytes) 
  

MORE INFORMATION

KBHOOK contains an application program, KEYAPP, and a dynamic-link library (DLL), KEYHOOK, that implements the keyboard filter function.

KEYAPP calls KEYHOOK to install the keyboard filter function and then makes itself iconic. Windows calls KEYHOOK each time a key is pressed. When the CAPS LOCK key is pressed, KEYHOOK posts a message to KEYAPP. KEYAPP processes the message by painting its icon to display the current state of CAPS LOCK. When KEYAPP is terminated, the KEYHOOK filter is removed from the system.

Because the keyboard filter function is called regardless of the application that is currently active, the code must be in memory at all times. When Windows 3.0 is running in real mode with extended memory, code for inactive applications can be placed into extended memory banks that are switched out of memory. Windows's memory management scheme is defined so that code in fixed DLLs will remain in memory at all times and will remain available for execution.

For more information on the keyboard filter function, see the documentation for the SetWindowsHook() function on pages 4-419 through 4-427 in the "Microsoft Windows Software Development Kit Reference Volume 1" for the Windows SDK version 3.0. For additional information on the SetWindowsHookEx() function see pages 896-899 of "Programmer's Reference, Volume 2: Functions" from the Windows SDK version 3.1. The "Microsoft Windows Software Development Kit Guide to Programming" provides additional information on DLLs.