typedef struct _IO_STACK_LOCATION
{
     UCHAR MajorFunction;
     UCHAR MinorFunction;
     UCHAR Flags;
     UCHAR Control;
     BYTE Parameters[16];
     PDEVICE_OBJECT DeviceObject;
     PFILE_OBJECT FileObject;
     LONG * CompletionRoutine;
     PVOID Context;
} IO_STACK_LOCATION, *PIO_STACK_LOCATION;

Windows Vista Kernel Structures