KeyboardProc

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

The HOOKPROC type defines a pointer to this callback function. KeyboardProc is a placeholder for the application-defined or library-defined function name.

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

Parameters

code

[in] 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

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

lParam

[in] Specifies the repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag. This parameter can be one or more 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

If nCode is less than zero, the hook procedure must return the value returned by CallNextHookEx.

If nCode is greater than or equal to zero, and the hook procedure did not process the message, it is highly recommended that you call CallNextHookEx and return the value it returns; otherwise, other applications that have installed WH_KEYBOARD hooks will not receive hook notifications and may behave incorrectly as a result. If the hook procedure processed the message, it may return a nonzero value to prevent the system from passing the message to the rest of the hook chain or the target window procedure.

Remarks

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

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winuser.h; include Windows.h.

See Also

Hooks Overview, Hook Functions, CallNextHookEx, GetMessage, PeekMessage, SetWindowsHookEx, WM_KEYUP, WM_KEYDOWN