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KeyboardProc callback function

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Description

An application-defined or library-defined callback function used with the SetWindowsHookExA/SetWindowsHookExW function.

The system calls this function whenever an application calls the GetMessage or PeekMessageA/PeekMessageW 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(
  _In_ int    code,
  _In_ WPARAM wParam,
  _In_ LPARAM lParam
);

Parameters

code [in]

Type: int

A code the hook procedure uses to determine how to process the message.

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.

This parameter can be one of the following values.

Value Meaning
HC_ACTION 0 The wParam and lParam parameters contain information about a keystroke message.
HC_NOREMOVE 3 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.)

wParam [in]

Type: WPARAM

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

lParam [in]

Type: LPARAM

The repeat count, scan code, extended-key flag, context code, previous key-state flag, and transition-state flag. For more information about The lParam parameter, see Keystroke Message Flags. The following table describes the bits of this value.

Bits Description
0-15 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 The scan code. The value depends on the OEM.
24 Indicates 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 The context code. The value is 1 if the ALT key is down; otherwise, it is 0.
30 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 The transition state. The value is 0 if the key is being pressed and 1 if it is being released.

Returns

Type: LRESULT

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

If code 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 SetWindowsHookExA/SetWindowsHookExW function.

This hook may be 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.

See also