A Single DispatchCreateClose Routine

Many drivers, particularly lower-level drivers in a chain of layered drivers, merely need to establish their existence on receipt of a create request and merely need to acknowledge the receipt of a close request.

For example, a port driver for a device controller with one or more closely coupled class drivers that call IoGetDeviceObjectPointer might have a minimal DispatchCreateClose routine. The routine might do nothing more than complete the IRP as follows:

    :    : 
    Irp->IoStatus.Status = STATUS_SUCCESS; 
 Irp->IoStatus.Information = 0; 
    IoCompleteRequest(Irp, IO_NO_INCREMENT); 

This minimal DispatchCreateClose routine sets the Information member of the I/O status block to zero, indicating the file object is opened for a create request; Information has no meaning for a close request. The routine sets the Status member to STATUS_SUCCESS and also returns this status value, indicating that the driver is ready to accept I/O requests.

This minimal DispatchCreateClose routine completes the create IRP without boosting the priority of the originator of the IRP (IO_NO_INCREMENT), because the originator is assumed to wait for an indeterminate but very small interval for the request to complete.

How much work a DispatchCreateClose routine does depends partly on the nature of the driver's device or the underlying device and partly on the design of the driver. If a driver performs very different operations for create and close requests, it should handle these requests in separate DispatchCreate and DispatchClose routines.

To handle a create request to open a file object representing a logical or physical device, a highest-level driver should do the following:

  1. Call IoGetCurrentIrpStackLocation to get a pointer to its I/O stack location in the IRP.

  2. Check FileObject.FileName in the I/O stack location and complete the IRP with STATUS_SUCCESS if the Unicode string at FileName has a zero length; otherwise, complete the IRP with STATUS_INVALID_PARAMETER.

Following the preceding steps ensures that no attempt to open a pseudofile on a device can cause problems later. For example, this prevents attempts to open a nonexistent \\device\parallel0\temp.dat.



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