Export (0) Print
Expand All
8 out of 23 rated this helpful - Rate this topic

KeyboardProc callback function

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.

Syntax


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.

ValueMeaning
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.

BitsDescription
0-15The repeat count. The value is the number of times the keystroke is repeated as a result of the user's holding down the key.
16-23The scan code. The value depends on the OEM.
24Indicates 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.
25-28Reserved.
29The context code. The value is 1 if the ALT key is down; otherwise, it is 0.
30The 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.
31The transition state. The value is 0 if the key is being pressed and 1 if it is being released.

 

Return value

Type:

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 SetWindowsHookEx 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.

Requirements

Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]

Header

Winuser.h (include Windows.h)

See also

Reference
CallNextHookEx
GetMessage
PeekMessage
SetWindowsHookEx
WM_KEYUP
WM_KEYDOWN
Conceptual
Hooks

 

 

Did you find this helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

ADD
Show:
© 2014 Microsoft. All rights reserved.