See ReceiveRtDataMessage, GetRtLongDataMessage
typedef struct longMessageDesc {
LOCATION location;
DWORD msgId;
DWORD reserved1;
DWORD dwUserParam;
DWORD length;
DWORD reserved;
DWORD reserved2;
} MSG_DESCRIPTOR, *LPMSG_DESCRIPTOR;
location
dwUserParam
length
All other fields are for internal use only. Do not change any of the fields in this structure.