Книга: Writing Windows WDM Device Drivers

Common IRP Parameters

Common IRP Parameters

This section lists the parameters that are set for the common IRPs. In the following discussion, "the stack" means the current I/O stack location.

The main parameter of interest to the Create IRP handler is the FileObject field in the stack. This is a pointer to a _FILE_OBJECT structure. The FileName field in here is a UNICODE_STRING with any characters after the basic device name. If you appended file to the symbolic link name found in the GetDeviceViaInterface routine in Wdm1Test, file would appear in FileName. If no characters are appended, FileName has a length of zero.

Other parameters to the Create IRP are given in the Parameters.Create structure in the stack, such as the FileAttributes and ShareAccess.

If need be, you can double-check that the FileObject in the IRP header matches the one you were sent in the create request.

If you have queued up Read or Write IRPs for this file, the I/O Manager will have cancelled them before the close request is received. It does this by calling an IRP's Cancel routine and issuing a Cleanup IRP, as described in Chapter 16.

The Parameters.Read structure in the IRP stack has Length and ByteOffset fields that say how many bytes are requested and the file pointer. ByteOffset is a 64-bit integer stored in a LARGE_INTEGER structure. The Microsoft compiler can handle this type directly (i.e., Parameters.Read.ByteOffset.Quad Part is an __int64). If you need to specify 64-bit constant values in your code, append i64 to the constant (e.g., 100i64).

The user buffer can be specified in one of two ways, depending on whether your driver uses Buffered I/O or Direct I/O. See the following text for details of these terms. If using Buffered I/O, a pointer to the user buffer is in the IRP header at AssociatedIrp.SystemBuffer. For Direct I/O, a Memory Descriptor List (MDL) is in the IRP header in the MdlAd-dress field.

The Key field in the IRP stack Parameters.Read structure does not seem to be used for anything, and so could be used by a driver for any purpose.

The parameters for Write IRPs are identical to Read IRPs, except that the relevant parameters are in the stack Parameters.Write structure.

The Parameters.DeviceIoControl structure in the IRP stack has IoControlCode, InputBufferLength, and OutputBufferLength parameters.

The user buffer is specified using one or more of the AssociatedIrp.SystemBuffer, MdlAddress, or stack Parameters.DeviceIoControl.Type3InputBuffer fields. See the next section for details.

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