请参考msdn,第三个参数如下:
SNMPAPI_CALLBACK
The Microsoft WinSNMP implementation calls the SNMPAPI_CALLBACK function to notify a WinSNMP session that an SNMP message or asynchronous event is available.
SNMPAPI_CALLBACK is a placeholder for an application- or library-defined callback function name.
SNMPAPI_STATUS CALLBACK SNMPAPI_CALLBACK(
HSNMP_SESSION hSession, // handle to the WinSNMP session
HWND hWnd, // handle to the notification window
UINT wMsg, // window notification message number
WPARAM wParam, // type of notification
LPARAM lParam, // request identifier of PDU
LPVOID lpClientData // optional application-defined data
);