一 派遣函數
驅動程序的主要功能是負責處理I/O請求。
其中大部分I/O請求是在派遣函數中處理的。
用戶模式下所有對驅動程序的I/O請求，全部由操作系統轉化爲一個叫做IRP的數據結構，不同的IRP數據會被“派遣”到不同的派遣函數中。
二 IRP與派遣函數
IRP的全稱是輸入輸出請求包。
其部分結構如下：
typedef struct DECLSPEC_ALIGN(MEMORY_ALLOCATION_ALIGNMENT) _IRP {
CSHORT Type;
USHORT Size;
//
// Define the common fields used to control the IRP.
//
//
// Define a pointer to the Memory Descriptor List (MDL) for this I/O
// request. This field is only used if the I/O is "direct I/O".
//
PMDL MdlAddress;
//
// Flags word - used to remember various flags.
//
ULONG Flags;
//
// The following union is used for one of three purposes:
//
// 1. This IRP is an associated IRP. The field is a pointer to a master
// IRP.
//
// 2. This is the master IRP. The field is the count of the number of
// IRPs which must complete (associated IRPs) before the master can
// complete.
//
// 3. This operation is being buffered and the field is the address of
// the system space buffer.
//
union {
struct _IRP *MasterIrp;
__volatile LONG IrpCount;
PVOID SystemBuffer;
} AssociatedIrp;
//
// Thread list entry - allows queueing the IRP to the thread pending I/O
// request packet list.
//
LIST_ENTRY ThreadListEntry;
//
// I/O status - final status of operation.
//
IO_STATUS_BLOCK IoStatus;
//
// Requestor mode - mode of the original requestor of this operation.
//
KPROCESSOR_MODE RequestorMode;
//
// Pending returned - TRUE if pending was initially returned as the
// status for this packet.
//
BOOLEAN PendingReturned;
//
// Stack state information.
//
CHAR StackCount;
CHAR CurrentLocation;
//
// Cancel - packet has been canceled.
//
BOOLEAN Cancel;
//
// Cancel Irql - Irql at which the cancel spinlock was acquired.
//
KIRQL CancelIrql;
//
// ApcEnvironment - Used to save the APC environment at the time that the
// packet was initialized.
//
CCHAR ApcEnvironment;
//
// Allocation control flags.
//
UCHAR AllocationFlags;
//
// User parameters.
//
PIO_STATUS_BLOCK UserIosb;
PKEVENT UserEvent;
union {
struct {
union {
PIO_APC_ROUTINE UserApcRoutine;
PVOID IssuingProcess;
};
PVOID UserApcContext;
} AsynchronousParameters;
LARGE_INTEGER AllocationSize;
} Overlay;
//
// CancelRoutine - Used to contain the address of a cancel routine supplied
// by a device driver when the IRP is in a cancelable state.
//
__volatile PDRIVER_CANCEL CancelRoutine;
//
// Note that the UserBuffer parameter is outside of the stack so that I/O
// completion can copy data back into the user's address space without
// having to know exactly which service was being invoked. The length
// of the copy is stored in the second half of the I/O status block. If
// the UserBuffer field is NULL, then no copy is performed.
//
PVOID UserBuffer;
//
// Kernel structures
//
// The following section contains kernel structures which the IRP needs
// in order to place various work information in kernel controller system
// queues. Because the size and alignment cannot be controlled, they are
// placed here at the end so they just hang off and do not affect the
// alignment of other fields in the IRP.
//
union {
struct {
union {
//
// DeviceQueueEntry - The device queue entry field is used to
// queue the IRP to the device driver device queue.
//
KDEVICE_QUEUE_ENTRY DeviceQueueEntry;
struct {
//
// The following are available to the driver to use in
// whatever manner is desired, while the driver owns the
// packet.
//
PVOID DriverContext[4];
} ;
} ;
//
// Thread - pointer to caller's Thread Control Block.
//
PETHREAD Thread;
//
// Auxiliary buffer - pointer to any auxiliary buffer that is
// required to pass information to a driver that is not contained
// in a normal buffer.
//
PCHAR AuxiliaryBuffer;
//
// The following unnamed structure must be exactly identical
// to the unnamed structure used in the minipacket header used
// for completion queue entries.
//
struct {
//
// List entry - used to queue the packet to completion queue, among
// others.
//
LIST_ENTRY ListEntry;
union {
//
// Current stack location - contains a pointer to the current
// IO_STACK_LOCATION structure in the IRP stack. This field
// should never be directly accessed by drivers. They should
// use the standard functions.
//
struct _IO_STACK_LOCATION *CurrentStackLocation;
//
// Minipacket type.
//
ULONG PacketType;
};
};
//
// Original file object - pointer to the original file object
// that was used to open the file. This field is owned by the
// I/O system and should not be used by any other drivers.
//
PFILE_OBJECT OriginalFileObject;
} Overlay;
//
// APC - This APC control block is used for the special kernel APC as
// well as for the caller's APC, if one was specified in the original
// argument list. If so, then the APC is reused for the normal APC for
// whatever mode the caller was in and the "special" routine that is
// invoked before the APC gets control simply deallocates the IRP.
//
KAPC Apc;
//
// CompletionKey - This is the key that is used to distinguish
// individual I/O operations initiated on a single file handle.
//
PVOID CompletionKey;
} Tail;
} IRP;
當然，此結構是微軟定義的；字段含義參見驅動開發相關資料；
三 簡單示例代碼
在驅動入口函數 DriverEntry 中註冊派遣函數的代碼如下：
//設置派遣函數
pDriverObject->MajorFunction[IRP_MJ_CREATE] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_CLOSE] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_WRITE] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_READ] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_CLEANUP] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_DEVICE_CONTROL] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_SET_INFORMATION] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_SHUTDOWN] = HelloDDKDispatchRoutin;
pDriverObject->MajorFunction[IRP_MJ_SYSTEM_CONTROL] = HelloDDKDispatchRoutin;
對派遣函數做簡單的處理
NTSTATUS HelloDDKDispatchRoutin(IN PDEVICE_OBJECT pDevObj, IN PIRP pIrp)
{
KdPrint(("Enter HelloDDKDispatchRoutin\n"));
PIO_STACK_LOCATION stack = IoGetCurrentIrpStackLocation(pIrp);
UCHAR type = stack->MajorFunction;
......
NTSTATUS status = STATUS_SUCCESS;
// 完成IRP
pIrp->IoStatus.Status = status;
pIrp->IoStatus.Information = 0; // bytes xfered
IoCompleteRequest( pIrp, IO_NO_INCREMENT );
KdPrint(("Leave HelloDDKDispatchRoutin\n"));
return status;
}
這個派遣函數僅僅是設置了IRP的完成狀態，並通過IoCompleteReques結束請求。