MessageProc

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.

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

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.

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.

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

See Also

CallNextHookEx, SetWindowsHookEx, MSG

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