BufferSize起作用了吗？

在Charis Cant的书中的WDM1例子中，Dispatch.cpp中的BufferSize有用吗？
// Wdm1Create Handle Create/Open file IRP
// Wdm1Close Handle Close file IRPs
// Wdm1Read Handle Read IRPs
// Wdm1Write Handle Write IRPs
// Wdm1DeviceControl Handle DeviceIoControl IRPs
// Wdm1SystemControl Handle WMI IRPs
/////////////////////////////////////////////////////////////////////////////
// Version history
// 27-Apr-99 1.0.0 CC creation
/////////////////////////////////////////////////////////////////////////////

#include \"wdm1.h\"
#include \"Ioctl.h\"

/////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////
// Buffer and BufferSize and guarding spin lock globals (in unpaged memory)

KSPIN_LOCK BufferLock;
PUCHAR Buffer = NULL;
ULONG BufferSize = 0;//这里的BufferSize

/////////////////////////////////////////////////////////////////////////////
// Wdm1Create:
//
// Description:
// Handle IRP_MJ_CREATE requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// IrpStack->Parameters.Create.xxx has create parameters
// IrpStack->FileObject->FileName has file name of device
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1Create( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
{
PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);
DebugPrint( \"Create File is %T\", &(IrpStack->FileObject->FileName));

// Complete successfully
return CompleteIrp(Irp,STATUS_SUCCESS,0);
}

/////////////////////////////////////////////////////////////////////////////
// Wdm1Close:
//
// Description:
// Handle IRP_MJ_CLOSE requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1Close( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
{
DebugPrintMsg(\"Close\");

// Complete successfully
return CompleteIrp(Irp,STATUS_SUCCESS,0);
}
 
/////////////////////////////////////////////////////////////////////////////
// Wdm1Read:
//
// Description:
// Handle IRP_MJ_READ requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// IrpStack->Parameters.Read.xxx has read parameters
// User buffer at: AssociatedIrp.SystemBuffer (buffered I/O)
// MdlAddress (direct I/O)
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1Read(IN PDEVICE_OBJECT fdo,
 IN PIRP Irp)
{
PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);
NTSTATUS status = STATUS_SUCCESS;
LONG BytesTxd = 0;

// Get call parameters
LONGLONG FilePointer = IrpStack->Parameters.Read.ByteOffset.QuadPart;
ULONG ReadLen = IrpStack->Parameters.Read.Length;
DebugPrint(\"Read %d bytes from file pointer %d\",(int)ReadLen,(int)FilePointer);

// Get access to the shared buffer
KIRQL irql;
KeAcquireSpinLock(&BufferLock,&irql);

// Check file pointer
if( FilePointer<0)
status = STATUS_INVALID_PARAMETER;
if( FilePointer>=(LONGLONG)BufferSize)//这里的BufferSize
status = STATUS_END_OF_FILE;

if( status==STATUS_SUCCESS)
{
// Get transfer count
if( ((ULONG)FilePointer)+ReadLen>BufferSize)
{
BytesTxd = BufferSize - (ULONG)FilePointer;
if( BytesTxd<0) BytesTxd = 0;
}
else
BytesTxd = ReadLen;

// Read from shared buffer
if( BytesTxd>0 && Buffer!=NULL)
RtlCopyMemory( Irp->AssociatedIrp.SystemBuffer, Buffer+FilePointer, BytesTxd);
}

// Release shared buffer
KeReleaseSpinLock(&BufferLock,irql);

DebugPrint(\"Read: %d bytes returned\",(int)BytesTxd);

// Complete IRP
return CompleteIrp(Irp,status,BytesTxd);
}

/////////////////////////////////////////////////////////////////////////////
// Wdm1Write:
//
// Description:
// Handle IRP_MJ_WRITE requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// IrpStack->Parameters.Write.xxx has write parameters
// User buffer at: AssociatedIrp.SystemBuffer (buffered I/O)
// MdlAddress (direct I/O)
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1Write( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
{
PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);
NTSTATUS status = STATUS_SUCCESS;
LONG BytesTxd = 0;

// Get call parameters
LONGLONG FilePointer = IrpStack->Parameters.Write.ByteOffset.QuadPart;
ULONG WriteLen = IrpStack->Parameters.Write.Length;
DebugPrint(\"Write %d bytes from file pointer %d\",(int)WriteLen,(int)FilePointer);

if( FilePointer<0)
status = STATUS_INVALID_PARAMETER;
else
{
// Get access to the shared buffer
KIRQL irql;
KeAcquireSpinLock(&BufferLock,&irql);

BytesTxd = WriteLen;

// (Re)allocate buffer if necessary
if( ((ULONG)FilePointer)+WriteLen>BufferSize)
{
ULONG NewBufferSize = ((ULONG)FilePointer)+WriteLen;
PVOID NewBuffer = ExAllocatePool(NonPagedPool,NewBufferSize);
if( NewBuffer==NULL)
{
BytesTxd = BufferSize - (ULONG)FilePointer;
if( BytesTxd<0) BytesTxd = 0;
}
else
{
RtlZeroMemory(NewBuffer,NewBufferSize);
if( Buffer!=NULL)
{
RtlCopyMemory(NewBuffer,Buffer,BufferSize);
ExFreePool(Buffer);
}
Buffer = (PUCHAR)NewBuffer;
BufferSize = NewBufferSize;
}
}

// Write to shared memory
if( BytesTxd>0 && Buffer!=NULL)
RtlCopyMemory( Buffer+FilePointer, Irp->AssociatedIrp.SystemBuffer, BytesTxd);

// Release shared buffer
KeReleaseSpinLock(&BufferLock,irql);
}

DebugPrint(\"Write: %d bytes written\",(int)BytesTxd);

// Complete IRP
return CompleteIrp(Irp,status,BytesTxd);
}

/////////////////////////////////////////////////////////////////////////////
// Wdm1DeviceControl:
//
// Description:
// Handle IRP_MJ_DEVICE_CONTROL requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// Buffered: AssociatedIrp.SystemBuffer (and IrpStack->Parameters.DeviceIoControl.Type3InputBuffer)
// Direct: MdlAddress
//
// IrpStack->Parameters.DeviceIoControl.InputBufferLength
// IrpStack->Parameters.DeviceIoControl.OutputBufferLength
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1DeviceControl( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
{
PIO_STACK_LOCATION IrpStack = IoGetCurrentIrpStackLocation(Irp);
NTSTATUS status = STATUS_SUCCESS;
ULONG BytesTxd = 0;

ULONG ControlCode = IrpStack->Parameters.DeviceIoControl.IoControlCode;
ULONG InputLength = IrpStack->Parameters.DeviceIoControl.InputBufferLength;
ULONG OutputLength = IrpStack->Parameters.DeviceIoControl.OutputBufferLength;

DebugPrint(\"DeviceIoControl: Control code %x InputLength %d OutputLength %d\",
ControlCode, InputLength, OutputLength);

// Get access to the shared buffer
KIRQL irql;
KeAcquireSpinLock(&BufferLock,&irql);
switch( ControlCode)
{
/////// Zero Buffer
case IOCTL_WDM1_ZERO_BUFFER:
// Zero the buffer
if( Buffer!=NULL && BufferSize>0)
RtlZeroMemory(Buffer,BufferSize);
break;

/////// Remove Buffer
case IOCTL_WDM1_REMOVE_BUFFER:
if( Buffer!=NULL)
{
ExFreePool(Buffer);
Buffer = NULL;
BufferSize = 0;
}
break;

/////// Get Buffer Size as ULONG
case IOCTL_WDM1_GET_BUFFER_SIZE:
if( OutputLength<sizeof(ULONG))
status = STATUS_INVALID_PARAMETER;
else
{
BytesTxd = sizeof(ULONG);
RtlCopyMemory(Irp->AssociatedIrp.SystemBuffer,&BufferSize,sizeof(ULONG));
}
break;

/////// Get Buffer
case IOCTL_WDM1_GET_BUFFER:
if( OutputLength>BufferSize)
status = STATUS_INVALID_PARAMETER;
else
{
BytesTxd = OutputLength;
RtlCopyMemory(Irp->AssociatedIrp.SystemBuffer,Buffer,BytesTxd);
}
break;

/////// Invalid request
default:
status = STATUS_INVALID_DEVICE_REQUEST;
}
// Release shared buffer
KeReleaseSpinLock(&BufferLock,irql);

DebugPrint(\"DeviceIoControl: %d bytes written\",(int)BytesTxd);

// Complete IRP
return CompleteIrp(Irp,status,BytesTxd);
}

/////////////////////////////////////////////////////////////////////////////
// Wdm1SystemControl:
//
// Description:
// Handle IRP_MJ_SYSTEM_CONTROL requests
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// Various minor parameters
// IrpStack->Parameters.WMI.xxx has WMI parameters
//
// Return Value:
// This function returns STATUS_XXX

NTSTATUS Wdm1SystemControl( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
{
DebugPrintMsg(\"SystemControl\");

// Just pass to lower driver
IoSkipCurrentIrpStackLocation(Irp);
PWDM1_DEVICE_EXTENSION dx = (PWDM1_DEVICE_EXTENSION)fdo->DeviceExtension;
return IoCallDriver( dx->NextStackDevice, Irp);
}

/////////////////////////////////////////////////////////////////////////////
// Wdm1Cleanup:
//
// Description:
// Handle IRP_MJ_CLEANUP requests
// Cancel queued IRPs which match given FileObject
//
// Arguments:
// Pointer to our FDO
// Pointer to the IRP
// IrpStack->FileObject has handle to file
//
// Return Value:
// This function returns STATUS_XXX

// Not needed for Wdm1

/////////////////////////////////////////////////////////////////////////////
// CompleteIrp: Sets IoStatus and completes the IRP

NTSTATUS CompleteIrp( PIRP Irp, NTSTATUS status, ULONG info)
{
Irp->IoStatus.Status = status;
Irp->IoStatus.Information = info;
IoCompleteRequest(Irp,IO_NO_INCREMENT);
return status;
}

当然起作用了,虽然BufferSize=0在开始,但是它将在读之前,在NTSTATUS Wdm1Write( IN PDEVICE_OBJECT fdo,
IN PIRP Irp)
写将内容写进去,而且BufferSize = NewBufferSize;
或者会在其他地方给他赋值得!

是这样啊！谢了