CBTProc

The CBTProc hook procedure is an application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function before activating, creating, destroying, minimizing, maximizing, moving, or sizing a window; before completing a system command; before removing a mouse or keyboard event from the system message queue; before setting the keyboard focus; or before synchronizing with the system message queue. A computer-based training (CBT) application uses this hook procedure to receive useful notifications from the system.

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

LRESULT CALLBACK CBTProc(
  int nCode,      // hook code
  WPARAM wParam,  // depends on hook code
  LPARAM lParam   // depends on hook code
);

Parameters

nCode

[in] Specifies a code that the hook procedure uses to determine how to process the message. This parameter can be one of the following values.

Value	Meaning
HCBT_ACTIVATE	The system is about to activate a window.
HCBT_CLICKSKIPPED	The system has removed a mouse message from the system message queue. Upon receiving this hook code, a CBT application must install a WH_JOURNALPLAYBACK hook procedure in response to the mouse message.
HCBT_CREATEWND	A window is about to be created. The system calls the hook procedure before sending the WM_CREATE or WM_NCCREATE message to the window. If the hook procedure returns a nonzero value, the system destroys the window; the CreateWindow function returns NULL, but the WM_DESTROY message is not sent to the window. If the hook procedure returns zero, the window is created normally.
	At the time of the HCBT_CREATEWND notification, the window has been created, but its final size and position may not have been determined and its parent window may not have been established. It is possible to send messages to the newly created window, although it has not yet received WM_NCCREATE or WM_CREATE messages. It is also possible to change the position in the Z order of the newly created window by modifying the hwndInsertAfter member of the CBT_CREATEWND structure.
HCBT_DESTROYWND	A window is about to be destroyed.
HCBT_KEYSKIPPED	The system has removed a keyboard message from the system message queue. Upon receiving this hook code, a CBT application must install a WH_JOURNALPLAYBACK_hook hook procedure in response to the keyboard message.
HCBT_MINMAX	A window is about to be minimized or maximized.
HCBT_MOVESIZE	A window is about to be moved or sized.
HCBT_QS	The system has retrieved a WM_QUEUESYNC message from the system message queue.
HCBT_SETFOCUS	A window is about to receive the keyboard focus.
HCBT_SYSCOMMAND	A system command is about to be carried out. This allows a CBT application to prevent task switching by means of hot keys.

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

[in] Depends on the nCode parameter. For details, see the following Remarks section.

lParam

[in] Depends on the nCode parameter. For details, see the following Remarks section.

Return Values

The value returned by the hook procedure determines whether the system allows or prevents one of these operations. For operations corresponding to the following CBT hook codes, the return value must be 0 to allow the operation, or 1 to prevent it:

HCBT_ACTIVATE
HCBT_CREATEWND
HCBT_DESTROYWND
HCBT_MINMAX
HCBT_MOVESIZE
HCBT_SETFOCUS
HCBT_SYSCOMMAND

For operations corresponding to the following CBT hook codes, the return value is ignored:

HCBT_CLICKSKIPPED
HCBT_KEYSKIPPED
HCBT_QS

Remarks

The hook procedure should not install a WH_JOURNALPLAYBACK_hook hook procedure except in the situations described in the preceding list of hook codes.

This hook procedure must be in a dynamic-link library (DLL). An application installs the hook procedure by specifying the WH_CBT hook type and a pointer to the hook procedure in a call to the SetWindowsHookEx function.

The following table describes the wParam and lParam parameters for each HCBT_ hook code.

For information, see WinEvents.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winuser.h; include Windows.h.

See Also

Hooks Overview, Hook Functions, CallNextHookEx, CreateWindow, SetWindowsHookEx, WM_SYSCOMMAND