typedef struct _LPCP_MESSAGE
{
     union
     {
          LIST_ENTRY Entry;
          struct
          {
               SINGLE_LIST_ENTRY FreeEntry;
               ULONG Reserved0;
          };
     };
     PVOID SenderPort;
     PETHREAD RepliedToThread;
     PVOID PortContext;
     PORT_MESSAGE Request;
} LPCP_MESSAGE, *PLPCP_MESSAGE;

Windows Vista Kernel Structures