请教一段WDM代码的作用

NTSTATUS
FilemonReadDispatch(
    PDEVICE_OBJECT HookDevice,
    IN PIRP Irp)
{
  PIO_STACK_LOCATION currentIrpStack = IoGetCurrentIrpStackLocation(Irp);
  PIO_STACK_LOCATION nextIrpStack = IoGetNextIrpStackLocation(Irp);
  PFILE_OBJECT fileObject;
  PHOOK_EXTENSION hookExt;
  PCHAR fullPathName;
  CHAR  name[PROCNAMELEN];

  PUCHAR SysBuffer = NULL; //IRP中原有的MDL缓冲区

  *nextIrpStack = *currentIrpStack;
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  这里为什么要将这样赋值？？这个2个PIO_STACK_LOCATION
  数据究竟干什么用的？
  ......
}