KeyboardProc info  Overview  Group

The KeyboardProc hook procedure is an application-defined or library-defined callback function the system calls whenever an application calls the GetMessage or PeekMessage function and there is a keyboard message (WM_KEYUP or WM_KEYDOWN) to be processed.

LRESULT CALLBACK KeyboardProc(

int code,	// hook code
WPARAM wParam,	// virtual-key code
LPARAM lParam	// keystroke-message information
);

Parameters

code

Specifies a code the hook procedure uses to determine how to process the message. This parameter can be one of the following values:

Value	Meaning
HC_ACTION	The wParam and lParam parameters contain information about a keystroke message.
HC_NOREMOVE	The wParam and lParam parameters contain information about a keystroke message, and the keystroke message has not been removed from the message queue. (An application called the PeekMessage function, specifying the PM_NOREMOVE flag.)

If code is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and should return the value returned by CallNextHookEx.

wParam

Specifies the virtual-key code of the key that generated the keystroke message.

lParam

Specifies the repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag. This parameter can be a combination of the following values:

Value	Description
0-15	Specifies the repeat count. The value is the number of times the keystroke is repeated as a result of the user’s holding down the key.
16-23	Specifies the scan code. The value depends on the original equipment manufacturer (OEM).
24	Specifies whether the key is an extended key, such as a function key or a key on the numeric keypad. The value is 1 if the key is an extended key; otherwise, it is 0.
25-28	Reserved.
29	Specifies the context code. The value is 1 if the ALT key is down; otherwise, it is 0.
30	Specifies the previous key state. The value is 1 if the key is down before the message is sent; it is 0 if the key is up.
31	Specifies the transition state. The value is 0 if the key is being pressed and 1 if it is being released.

For more information about the lParam parameter, see Keystroke Message Flags. 

Return Values

To prevent Windows from passing the message to the rest of the hook chain or to the target window procedure, the return value must be a nonzero value. To allow Windows to pass the message to the target window procedure, bypassing the remaining procedures in the chain, the return value must be zero.

Remarks

An application installs the hook procedure by specifying the WH_KEYBOARD hook type and the address of the hook procedure in a call to the SetWindowsHookEx function.

KeyboardProc is a placeholder for the application-defined or library-defined function name.

See Also

CallNextHookEx, GetMessage, PeekMessage, SetWindowsHookEx, WM_KEYUP, WM_KEYDOWN