MessageProc function


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

  _In_ int    code,
       WPARAM wParam,
  _In_ LPARAM lParam


code [in]

Type: int

The type of input event that generated the message.

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.

This parameter can be one of the following values.

Value Meaning
MSGF_DDEMGR 0x8001 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 0 The input event occurred in a message box or dialog box.
MSGF_MENU 2 The input event occurred in a menu.
MSGF_SCROLLBAR 5 The input event occurred in a scroll bar.



This parameter is not used.

lParam [in]


A pointer to an MSG structure.



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_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.


An application installs the hook procedure by specifying the WH_MSGFILTER hook type and a pointer to the hook procedure in a call to the SetWindowsHookExA/SetWindowsHookExW 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.

See also