Handling I/O Requests in KMDF

After initializing the KMDF driver and creating the device object, the next critical step is to make your driver interact with user-mode applications. This happens through I/O requests – Create, Close, Read, Write, and sometimes Device Control (IOCTL).

1 I/O Request Lifecycle in KMDF

When a user-mode application interacts with your driver (e.g., via CreateFile(), ReadFile(), or WriteFile(), the Windows I/O Manager generates IRPs (I/O Requests Packets) and delivers them to your driver.

KMDF queues receive these IRPs and route them to callback functions your register.

2 Registering Create and Close Handlers

These two handlers are used to manage file open and close events, such as when an app opens your device using CreateFile().

Registering the Callbacks:

WDF_FILEOBJECT_CONFIG fileConfig;
WDF_FILEOBJECT_CONFIG_INIT(
    &fileConfig,
    EvtDeviceFileCreate,
    EvtFileClose,
    WDF_NO_EVENT_CALLBACK
);
WdfDeviceInitSetFileObjectConfig(DeviceInit, &fileConfig, WDF_NO_OBJECT_ATTRIBUTES);

Example Callbacks:

VOID EvtDeviceFileCreate(WDFDEVICE Device, WDFREQUEST Request, WDFFILEOBJECT FileObject) {
    KdPrint(("Device opened\n"));
    WdfRequestComplete(Request, STATUS_SUCCESS);
}

VOID EvtFileClose(WDFFILEOBJECT FileObject) {
    KdPrint(("Device closed\n"));
}

3 Setting Up the I/O Queue

KMDF uses I/O queues to manage Read, Write, and DeviceIoControl operations. You create a default queue and register callbacks for the I/O operations you want to support.

Queue Setup:

WDF_IO_QUEUE_CONFIG queueConfig;
WDF_IO_QUEUE_CONFIG_INIT_DEFAULT_QUEUE(&queueConfig, WdfIoQueueDispatchSequential);

queueConfig.EvtIoRead = EvtIoRead;
queueConfig.EvtIoWrite = EvtIoWrite;

WdfIoQueueCreate(device, &queueConfig, WDF_NO_OBJECT_ATTRIBUTES, &queue);

4 Handling Read and Write Requests

Once your queue is set up, you define how your driver responds to user-mode ReadFile(), or WriteFile() calls.

EvtIoWrite Example:

VOID EvtIoWrite(WDFQUEUE Queue, WDFREQUEST Request, size_t Length) {
    PVOID buffer;
    WdfRequestRetrieveInputBuffer(Request, Length, &buffer, NULL);
    KdPrint(("Write received: %s\n", (char*)buffer));
    WdfRequestCompleteWithInformation(Request, STATUS_SUCCESS, Length);
}

EvtIoRead Example:

VOID EvtIoRead(WDFQUEUE Queue, WDFREQUEST Request, size_t Length) {
    PVOID buffer;
    const char* message = "Hello from kernel!";
    size_t msgLen = strlen(message) + 1;

    WdfRequestRetrieveOutputBuffer(Request, msgLen, &buffer, NULL);
    RtlCopyMemory(buffer, message, msgLen);

    WdfRequestCompleteWithInformation(Request, STATUS_SUCCESS, msgLen);
}

I/O Dispatch Modes

KMDF queues support three main dispatch modes:

Sequential is safest and simplest for beginners.