Forwarding I/O Requests
When a driver receives an I/O request that it cannot process, it typically does one of the following:
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It forwards the received request to another driver.
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It creates additional requests and sends them to another driver.
Framework-based drivers forward requests by using I/O targets, which represent other drivers on the system. Drivers can use any of the following techniques to forward a request to an I/O target:
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A driver can forward I/O requests to the next-lower driver by calling WdfDeviceGetIoTarget, followed by WdfRequestFormatRequestUsingCurrentType, and finally WdfRequestSend.
This technique is useful only if the driver receives a request that it does not have to modify before forwarding.
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A driver can call WdfFdoInitSetFilter to register itself as a filter driver.
If a filter driver does not provide an I/O queue for a particular type of I/O request, the framework automatically forwards requests of that type to the next-lower driver.
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Typically, a function driver examines each I/O request's contents. If a function driver cannot process a request, it might modify the request and forward it to an I/O target. Or, it might create one or more new requests and send them to an I/O target.
The framework's I/O target object defines several methods for sending I/O requests to other drivers. For example, a driver can call WdfIoTargetFormatRequestForRead, followed by WdfRequestSend, to send a read request to an I/O target. For more information about I/O targets, see Using I/O Targets.
Rarely, a driver writer might want to specify the contents of a request's underlying WDM I/O stack location before sending a request to an I/O target. For those cases, the driver can call WdfRequestWdmFormatUsingStackLocation before it calls WdfRequestSend.
Sometimes, a driver must send the same request to several I/O targets, typically because the driver must send a single command to all of its devices. Before sending a request to an I/O target, the driver can call WdfRequestChangeTarget to verify that the I/O target is accessible.
The driver must eventually complete every request that it forwards to an I/O target, unless it sets the WDF_REQUEST_SEND_OPTION_SEND_AND_FORGET flag when calling WdfRequestSend.
Note that when a driver forwards a request, the framework does not literally transfer the framework request object from the sending driver to the receiving driver. Instead, the framework creates a new request object in the driver that receives the request. Only the request's underlying I/O request packet (IRP) is transferred from one driver to another.
