When Sent

The IRP_MJ_READ request is sent by the I/O Manager or by a file system driver. This request can be sent, for example, when a user-mode application has called a Microsoft Win32 function such as ReadFile, or when a kernel-mode component has called ZwReadFile.

Operation: File System Drivers

The file system driver should extract and decode the file object to determine the parameters and minor function code.

For memory descriptor list (MDL) read requests, the file system should check the minor function code to determine which operation is requested. The following are the valid minor function codes, which can be used only for cached file I/O:

For more information about handling this IRP, study the CDFS and FASTFAT samples that are included in the Windows Driver Kit (WDK).

Operation: File System Filter Drivers

The filter driver should perform any needed processing and, depending on the nature of the filter, either complete or fail the IRP or pass it down to the next-lower driver on the stack.

Parameters

A file system or filter driver calls IoGetCurrentIrpStackLocation with the given IRP to get a pointer to its own stack location in the IRP, shown in the following list as IrpSp. (The IRP is shown as Irp.) The driver can use the information contained in the following members of the IRP and the IRP stack location in processing a read request:

DeviceObject

Pointer to the target device object.

Irp->AssociatedIrp.SystemBuffer

Pointer to a system-supplied buffer to be used as an intermediate system buffer, if the DO_BUFFERED_IO flag is set in DeviceObject->Flags. Otherwise, this member is set to NULL.

Irp->IoStatus

Pointer to an IO_STATUS_BLOCK structure that receives the final completion status and information about the requested operation. For more information, see the description of the IoStatusBlock parameter to ZwReadFile.

Irp->MdlAddress

Address of a memory descriptor list (MDL) describing the pages that contain the data to be read.

Irp->UserBuffer

Pointer to a caller-supplied output buffer that receives the data that is read from the file.

IrpSp->FileObject

Pointer to the file object that is associated with DeviceObject. If the FO_SYNCHRONOUS_IO flag is set in IrpSp->FileObject->Flags, the file object was opened for synchronous I/O.

The IrpSp->FileObject parameter contains a pointer to the RelatedFileObject field, which is also a FILE_OBECT structure. The RelatedFileObject field of the FILE_OBJECT structure is not valid during the processing of IRP_MJ_READ and should not be used.

IrpSp->MajorFunction

Specifies IRP_MJ_READ.

IrpSp->MinorFunction

One of the following:

IRP_MN_COMPLETE

IRP_MN_COMPLETE_MDL

IRP_MN_COMPLETE_MDL_DPC

IRP_MN_COMPRESSED

IRP_MN_DPC

IRP_MN_MDL

IRP_MN_MDL_DPC

IRP_MN_NORMAL

IrpSp->Parameters.Read.ByteOffset

Pointer to a LARGE_INTEGER variable that specifies the starting byte offset within the file of the data to be read.

Under certain circumstances, this parameter might validly contain a NULL or negative value. For example, if the file object was opened for synchronous I/O, and one of the following conditions is true, this indicates that the current file position should be used instead of an explicit file offset value:

IrpSp->Parameters.Read.ByteOffset.QuadPart == 0

IrpSp->Parameters.Read.ByteOffset.LowPart == FILE_USE_FILE_POINTER_POSITION and IrpSp->Parameters.Read.ByteOffset.HighPart == -1.

IrpSp->Parameters.Read.Key

Key value associated with a byte-range lock on the target file.

IrpSp->Parameters.Read.Length

Length in bytes of the data to be read. If the read operation is successful, the number of bytes read is returned in the Information member of the IO_STATUS_BLOCK structure pointed to by Irp->IoStatus.

See Also

CcMdlRead, CcMdlReadComplete, IO_STACK_LOCATION, IO_STATUS_BLOCK, IoGetCurrentIrpStackLocation, IRP, IRP_MJ_READ (WDK Kernel Reference), IRP_MJ_WRITE, ZwReadFile