The LowLevelKeyboardProc hook procedure is 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. 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". However, the WH_KEYBOARD_LL hook is not injected into another process. Instead, the context switches back to the process that installed the hook and it is called in its original context. Then the context switches back to the application that generated the event.
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(
int nCode,
WPARAM wParam,
LPARAM lParam
);
Parameters
- nCode
-
[in] Specifies 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.
HC_ACTION- The wParam and lParam parameters contain information about a keyboard message.
- wParam
-
[in] 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
-
[in] Pointer to a KBDLLHOOKSTRUCT structure.
Return Value
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.
Remarks
An application installs the hook procedure by specifying the WH_KEYBOARD_LL hook type and a pointer to 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.
Function Information
| Header | Declared in Winuser.h, include Windows.h |
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| Import library | None |
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| Minimum operating systems |
Windows NT 4.0 SP3 |
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See Also