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Create an FDO in the battery miniclass driver

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To create a functional device object (FDO) in the battery miniclass driver and attach it to the device stack, follow these steps:

  1. Call IoCreateDevice to create an FDO for the current device:

    Status = IoCreateDevice(
         DriverObject,
         sizeof (DeviceExtension),
         NULL,
         FILE_DEVICE_BATTERY,
         0,
         FALSE,
         &Fdo
         );

    The input parameters to IoCreateDevice are a pointer to the driver object that was passed to the AddDevice routine, the size of the device extension, NULL in place of a device name, and the system-defined device type (FILE_DEVICE_BATTERY). Battery miniclass drivers can specify zero for the DeviceCharacteristics parameter, which is irrelevant to these drivers. More than one thread can send I/O requests to the battery, so the miniclass driver passes FALSE as the Exclusive parameter. IoCreateDevice returns a pointer to the created FDO.

  2. Set flags and the stack size in the returned FDO:

    Fdo->Flags |= DO_BUFFERED_IO;
Fdo->Flags |= DO_POWER_PAGABLE;
Fdo->StackSize = Pdo->StackSize + 2;

    Setting the DO_BUFFERED_IO flag allows the miniclass driver to use buffered I/O for IRPs. Setting the DO_POWER_PAGABLE flag indicates that the driver is pageable and prevents it from getting power IRPs at IRQL >= DISPATCH_LEVEL. Finally, because battery IRPs require an additional stack location, miniclass drivers should set StackSize to the PDO stack size plus two, so that the driver can pass the IRP down to the PDO.

  3. Store the pointer to the device's PDO, the pointer to the FDO, the device type, the device name, and any other necessary state in the device extension:

    NewBatt = (PNEW_BATT) Fdo->DeviceExtension;
NewBatt->Type = NEW_BATTERY_TYPE;
NewBatt->Fdo = Fdo;
NewBatt->Pdo = Pdo;
NewBatt->IsCacheValid = FALSE;

    The example stores the pointers to the FDO and PDO in the device extension. (The PnP Manager supplied a pointer to the PDO as the PhysicalDeviceObject pointer input to AddDevice.) In addition, the example above keeps track of its own battery type (NEW_BATTERY_TYPE, defined elsewhere in this hypothetical miniclass driver) and whether any cached information is valid.

    You determine the information stored in the device extension. For example, a smart battery driver might retain the number of batteries, a Boolean value indicating whether a battery selector is present, and, optionally, information about that battery selector.

  4. Call IoAttachDeviceToDeviceStack to attach the FDO to the device stack and store the returned pointer:

    NewBatt->LowerDO = IoAttachDeviceToDeviceStack(Fdo,Pdo);

    The call returns a pointer to the next-lower device object, which this example stores in the device extension.

  5. Clear the DO_DEVICE_INITIALIZING flag in the FDO:

    Fdo->Flags &= ~DO_DEVICE_INITIALIZING;

Clearing the DO_DEVICE_INITIALIZING flag allows the device object to be opened subsequently by components higher in the device stack.