typedef struct _PORT_MESSAGE
{
     ULONG u1;
     ULONG u2;
     union
     {
          CLIENT_ID ClientId;
          Float DoNotUseThisField;
     };
     ULONG MessageId;
     union
     {
          ULONG ClientViewSize;
          ULONG CallbackId;
     };
} PORT_MESSAGE, *PPORT_MESSAGE;

Windows Vista Kernel Structures