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LowLevelKeyboardProc info  Overview  Group

[New for Windows NT 4.0 Service Pack 3.]

The LowLevelKeyboardProc hook procedure is an application-defined or library-defined callback function the system calls every time a new keyboard input event is about to be posted into a thread input queue. The system calls this hook before the event is processed.

The keyboard input can come from the local keyboard driver or from calls to the keybd_event function. If the input comes from a call to keybd_event, the input was “injected”.

LRESULT CALLBACK LowLevelKeyboardProc(

    int nCode,

// hook code

    WPARAM wParam,

// message identifier

    LPARAM lParam

// pointer to structure with message data

   );

Parameters

nCode
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 keyboard message.

If nCode 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 identifier of the keyboard message. This parameter can be one of the following messages: WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, or WM_SYSKEYUP.
lParam
Pointer to the KBDLLHOOKSTRUCT structure.

Return Value

To prevent the system 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 the system 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_LL hook type and the address of the hook procedure in a call to the SetWindowsHookEx function.

This hook is called in the context of the thread that installed it. The call is made by sending a message to the thread that installed the hook. Therefore, the thread that installed the hook must have a message loop.

The hook procedure should process a message in less time than the data entry specified in the LowLevelHooksTimeout value in the following registry key:

HKEY_CURRENT_USER\Control Panel\Desktop

The value is in milliseconds. If the hook procedure does not return during this interval, the system will pass the message to the next hook.

Note that debug hooks cannot track this type of hook.

See Also

CallNextHookEx, KBDLLHOOKSTRUCT, keybd_event, SetWindowsHookEx, WM_KEYDOWN, WM_KEYUP, WM_SYSKEYDOWN, WM_SYSKEYUP

Questions:

Comments:

See also:


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