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(
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int code, |
// hook code |
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WPARAM wParam, |
// undefined |
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LPARAM lParam |
// address of structure with message data |
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); |
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Value |
Meaning |
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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. |
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MSGF_DIALOGBOX |
The input event occurred in a message box or dialog box. |
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MSGF_MENU |
The input event occurred in a menu. |
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MSGF_NEXTWINDOW |
The input event occurred as a result of the user’s pressing the ALT+TAB key combination to activate a different window. |
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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.
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.