VfatReadDiskPartial函数主要用来构造一个IRP给底层驱动程序去读取数据返回。具体实现代码如下：

#001 NTSTATUS

#002 VfatReadDiskPartial (IN PVFAT_IRP_CONTEXT IrpContext,

#003 IN PLARGE_INTEGER ReadOffset,

#004 IN ULONG ReadLength,

#005 ULONG BufferOffset,

#006 IN BOOLEAN Wait)

#007 {

#008 PIRP Irp;

#009 PIO_STACK_LOCATION StackPtr;

#010 NTSTATUS Status;

#011 PVOID Buffer;

#012

#013 DPRINT ("VfatReadDiskPartial(IrpContext %p,ReadOffset %I64x,ReadLength %d,BufferOffset %x,Wait %d)/n",

#014 IrpContext,ReadOffset->QuadPart,ReadLength,BufferOffset,Wait);

#015

#016 DPRINT ("Building asynchronous FSD Request.../n");

#017

从IRP的MDL地址获取内存缓冲区。

#018 Buffer = (PCHAR)MmGetMdlVirtualAddress(IrpContext->Irp->MdlAddress) + BufferOffset;

#019

创建一个新的IRP。

#020 Irp = IoAllocateIrp(IrpContext->DeviceExt->StorageDevice->StackSize,TRUE);

#021 if (Irp == NULL)

#022 {

#023 DPRINT("IoAllocateIrp Failed/n");

#024 return(STATUS_UNSUCCESSFUL);

#025 }

#026

设置IRP属性。

#027 Irp->UserIosb = NULL;

#028 Irp->Tail.Overlay.Thread = PsGetCurrentThread();

#029

#030 StackPtr = IoGetNextIrpStackLocation(Irp);

#031 StackPtr->MajorFunction = IRP_MJ_READ;

#032 StackPtr->MinorFunction = 0;

#033 StackPtr->Flags = 0;

#034 StackPtr->Control = 0;

设置访问设备对象。

#035 StackPtr->DeviceObject = IrpContext->DeviceExt->StorageDevice;

#036 StackPtr->FileObject = NULL;

#037 StackPtr->CompletionRoutine = NULL;

#038 StackPtr->Parameters.Read.Length = ReadLength;

#039 StackPtr->Parameters.Read.ByteOffset = *ReadOffset;

#040

创建一个直接内存访问MDL。

#041 if (!IoAllocateMdl(Buffer,FALSE,Irp))

#042 {

#043 DPRINT("IoAllocateMdl Failed/n");

#044 IoFreeIrp(Irp);

#045 return STATUS_UNSUCCESSFUL;

#046 }

#047

#048 IoBuildPartialMdl(IrpContext->Irp->MdlAddress,Irp->MdlAddress,Buffer,ReadLength);

#049

设置I/O完成端口函数。

#050 IoSetCompletionRoutine(Irp,

#051 VfatReadWritePartialCompletion,

#052 IrpContext,

#053 TRUE,

#054 TRUE,

#055 TRUE);

#056

#057 if (Wait)

#058 {

#059 KeInitializeEvent(&IrpContext->Event,NotificationEvent,FALSE);

#060 IrpContext->RefCount = 1;

#061 }

#062 else

#063 {

#064 InterlockedIncrement((PLONG)&IrpContext->RefCount);

#065 }

#066

调储存驱动程序来读取文件数据。

#067 DPRINT ("Calling IO Driver... with irp %p/n",Irp);

#068 Status = IoCallDriver (IrpContext->DeviceExt->StorageDevice,Irp);

#069

如果读取数据在阻塞状态，就等底层驱动程序完成。

#070 if (Wait && Status == STATUS_PENDING)

#071 {

#072 KeWaitForSingleObject(&IrpContext->Event,Executive,KernelMode,NULL);

#073 Status = IrpContext->Irp->IoStatus.Status;

#074 }

#075

#076 DPRINT("%x/n",Status);

#077 return Status;

#078}