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Understanding the distinction between device setup classes and device interface classes is crucial for successful Windows driver development and device management. While these classes might appear similar, they serve fundamentally different purposes in the Windows device ecosystem. Setup classes govern how the operating system installs and configures devices. Interface classes enable runtime communication and functionality between drivers, applications, and devices. Understanding this distinction helps you choose the correct class type for device installation, implement proper device notifications, and avoid common development pitfalls.
A device often belongs to a setup class and exposes several device interfaces in different interface classes at the same time. Nevertheless, the two types of classes serve different purposes and are not interchangeable.
Comparison
The following table summarizes the key differences and relationships between setup classes and interface classes:
| Aspect | Setup classes | Interface classes |
|---|---|---|
| Primary Purpose | Grouping devices with similar installation and configuration | Grouping device interfaces that provide similar functionality and communication |
| Usage | Used by Windows during device installation | Used by drivers and applications for device interaction |
| GUID Definitions | Usually defined in Devguid.h | Defined in device-specific header files (for example, Ntddmou.h, Ntddpar.h) |
| Grouping Criteria | Devices installed and configured similarly | Generally, device interfaces that implement a particular IO contract |
| Notification | Not used for device arrival or removal notifications | Used for registering device interface arrival and removal notifications |
| Examples | Sensors, Firmware, Bluetooth | Audio Capture, Audio Render, Ambient Light Sensor |
| Relationship | A device belongs to one setup class | A device can expose zero or more device interfaces in different interface classes |
| Lifetime | Relevant during installation process | Relevant during device operation |
It's important to distinguish between the two types of device classes: device interface classes and device setup classes. The two can be easily confused. In user-mode code, both classes use the same set of device installation functions. They also use the same set of data structures (device information sets).
Same setup class, different interface classes:
- Devices: Two different sensor devices
- Setup Class: Both devices are in the Sensors class.
- Interface Classes: One sensor exposes an ambient light sensor device interface, and the other exposes an accelerometer device interface.
Consider a USB mouse device, and a USB keyboard device. Both use the same interface class (Human Interface Device). But they belong to different setup classes, Mouse and Keyboard, respectively.
Same interface class, different setup classes:
- Devices: A USB mouse and a USB keyboard
- Interface Class: Both devices utilize the same interface class (Human Interface Device) to communicate with the system.
- Setup Classes: The mouse device belongs to the Mouse setup class, while the keyboard device belongs to the Keyboard setup class.
Device setup classes
Device setup classes are predefined categories used solely for the installation and configuration of devices in Windows. They help the operating system understand how to manage the device during the installation process. Setup classes provide a mechanism for grouping devices that are installed and configured in the same way. For more information about setup classes, see System-defined device setup classes available to vendors.
Windows device setup classes are defined in the system file Devguid.h. This file defines a series of GUIDs for setup classes. However, the device setup classes represented in Devguid.h must not be confused with device interface classes. The Devguid.h file only contains GUIDs for setup classes.
Device interface classes
Device interface classes provide a mechanism for grouping device interfaces according to shared characteristics or functionality. Drivers and user applications can register for notifications about device interfaces in a particular interface class. This means they get notified when any device interface belonging to that class is enabled (arrives) or is disabled (removed). They don't need to track individual devices. Device interface classes are grouping device interfaces exposed by devices. You register for notifications of arrival and removal of device interfaces.
Definitions of interface classes aren't provided in a single file. A device interface class is always defined in a header file that belongs exclusively to a particular class of devices. For example, Ntddmou.h contains the definition of GUID_DEVINTERFACE_MOUSE, the GUID representing the mouse interface class. Ntddpar.h defines the interface class GUID for parallel devices. Ntddpcm.h defines the standard interface class GUID for PCMCIA devices. Ntddstor.h defines the interface class GUID for storage devices.
To register for device interface notifications, use the GUIDs found in the header files that are specific to the device interface class. These GUIDs allow you to be notified when a device interface instance arrives. If a driver registers for notification using a setup class GUID instead of an interface class GUID, it isn't notified when an interface arrives.
When defining a new interface class, don't use a single GUID to identify both a setup class and an interface class.