MessageProc

The MessageProc hook procedure is an application-defined or 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 hook procedure can monitor messages for a dialog box, message box, menu, or scroll bar created by a particular application or all applications.

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

LRESULT CALLBACK MessageProc(
  int code,       // hook code
  WPARAM wParam,  // undefined
  LPARAM lParam   // address of structure with message data
);
 

Parameters

code

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

Value	Meaning
MSGF_DDEMGR	The input event occurred while the Dynamic Data Exchange Management Library (DDEML) was waiting for a synchronous transaction to finish. For more information about DDEML, see Dynamic Data Exchange Management Library.
MSGF_DIALOGBOX	The input event occurred in a message box or dialog box.
MSGF_MENU	The input event occurred in a menu.
MSGF_NEXTWINDOW	The input event occurred as a result of the user's pressing the alt+tab key combination to activate a different window.
MSGF_SCROLLBAR	The input event occurred in a scroll bar.

If code is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and return the value returned by CallNextHookEx.

wParam

Specifies a NULL value.

lParam

Pointer to an MSG 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_MSGFILTER 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_MSGFILTER hook type and the address of the hook procedure in a call to the SetWindowsHookEx function.

If an application that uses the DDEML and performs synchronous transactions must process messages before they are dispatched, it must use the WH_MSGFILTER hook.

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, SetWindowsHookEx, MSG