<Programming the Windows Driver Model>一书中一个范例代码的疑问

extern "C" NTSTATUS DriverEntry(IN PDRIVER_OBJECT DriverObject,
  IN PUNICODE_STRING RegistryPath)
  {
  DriverObject->DriverUnload = DriverUnload;
  DriverObject->DriverExtension->AddDevice = AddDevice;
  DriverObject->MajorFunction[IRP_MJ_PNP] = DispatchPnp;
  DriverObject->MajorFunction[IRP_MJ_POWER] = DispatchPower;
  DriverObject->MajorFunction[IRP_MJ_SYSTEM_CONTROL] =
    DispatchWmi;

  servkey.Buffer = (PWSTR) ExAllocatePool(PagedPool,
    RegistryPath->Length + sizeof(WCHAR));
  if (!servkey.Buffer)
    return STATUS_INSUFFICIENT_RESOURCES;
  servkey.MaximumLength = RegistryPath->Length + sizeof(WCHAR);
  RtlCopyUnicodeString(&servkey, RegistryPath);
  servkey.Buffer[RegistryPath->Length/sizeof(WCHAR)] = 0;
  return STATUS_SUCCESS;
  }
其中ExAllocatePool函数第二个参数为什么是RegistryPath->Length + sizeof(WCHAR)而不是RegistryPath->Length？
作者考虑了对齐后可能pool不够用？还是其他因素？  
本人初学驱动，各位帮帮忙，谢谢！

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谢谢楼上的牛人！
windows编程的数据类型可真是蛮搞的~ 我都把Length的类型当成WCHAR了~