LowLevelKeyboardProc

Description

An application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function every time a new keyboard input event is about to be posted into a thread input queue.

Note When this callback function is called in response to a change in the state of a key, the callback function is called before the asynchronous state of the key is updated. Consequently, the asynchronous state of the key cannot be determined by calling GetAsyncKeyState from within the callback function.

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

Syntax

LRESULT CALLBACK LowLevelKeyboardProc(
  _In_  int nCode,
  _In_  WPARAM wParam,
  _In_  LPARAM lParam
);

Parameters

nCode [in]

Type: int

A code the hook procedure uses to determine how to process the 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. This parameter can be one of the following values.

Value	Meaning
HC_ACTION 0	The wParam and lParam parameters contain information about a keyboard message.

wParam [in]

Type: WPARAM

The identifier of the keyboard message. This parameter can be one of the following messages

• WM_KEYDOWN (0x100)
• WM_KEYUP (0x101)
• WM_SYSKEYDOWN (0x104)
• WM_SYSKEYUP (0x105)

lParam [in]

Type: LPARAM

A pointer to a KBDLLHOOKSTRUCT structure.

Return value

Type:

Type: LRESULT

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_LL 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.