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MessageProc info  Overview  Group

The MessageProc hook procedure is an application-defined callback function the system calls 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.


    int code,

// hook code

    WPARAM wParam,

// undefined

    LPARAM lParam 

// address of structure with message data



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




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.


The input event occurred in a message box or dialog box.


The input event occurred in a menu.


The input event occurred as a result of the userís pressing the ALT+TAB key combination to activate a different window.


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.

Specifies a NULL value.
Points to an MSG structure.

Return Values

If the hook procedure processes the message, the return value must be a nonzero value. Otherwise, it must be zero.


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.

MessageProc is a placeholder for the application-defined function name.

See Also

CallNextHookEx, SetWindowsHookEx, MSG 

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