SysMsgProc function

Article
06/30/2023

Description

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 function can monitor messages for any dialog box, message box, menu, or scroll bar in the system.

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

LRESULT CALLBACK SysMsgProc(
  _In_ int    nCode,
       WPARAM wParam,
  _In_ LPARAM lParam
);

Parameters

nCode [in]

Type: int

The type of input event that generated the message.

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

This parameter can be one of the following values.

Value	Meaning
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.

wParam

Type: WPARAM

This parameter is not used.

lParam [in]

Type: LPARAM

A pointer to an MSG message structure.

Returns

Type: LRESULT

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_SYSMSGFILTER 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 target window procedure.

Remarks

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