SysMsgProc

The SysMsgProc hook procedure is a library-defined callback function used with the SetWindowsHookEx function. The system calls this function after an input event occurs in a dialog box, message box, menu, or scroll bar, but before the message generated by the input event is processed. The function can monitor messages for any dialog box, message box, menu, or scroll bar in the system.

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

LRESULT CALLBACK SysMsgProc(
  int nCode,      // message flag
  WPARAM wParam,  // undefined
  LPARAM lParam   // address of structure with message data
);
 

Parameters

nCode

Specifies the type of input event that generated the message. This parameter can be one of the following values:

Value	Meaning
MSGF_DIALOGBOX	The input event occurred in a message box or dialog box.
MSGF_MENU	The input event occurred in a menu.
MSGF_SCROLLBAR	The input event occurred in a scroll bar.
MSGF_NEXTWINDOW	The input event occurred as a result of the user's pressing the alt+tab key combination to activate a different window.

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

Specifies a NULL value.

lParam

Pointer to an MSG message structure.

Return Values

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_SYSMSGFILTER 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 target window procedure.

Remarks

An application installs the hook procedure by specifying the WH_SYSMSGFILTER hook type and the address of the hook procedure in a call to the SetWindowsHookEx function.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in winuser.h.
  Import Library: User-defined.

See Also

Hooks Overview, Hook Functions, CallNextHookEx, MSG, SetWindowsHookEx