IOService

IOService is a core class in Apple’s I/O Kit framework, used to implement device drivers within macOS and iOS. It serves as the base class for most driver objects and represents nodes in the I/O Registry, a dynamic database that models hardware devices, buses, and the software drivers that manage them.

The IOService class provides the fundamental lifecycle and service-management facilities that drivers rely on. Driver authors typically subclass IOService and implement methods such as start and stop to initialize and shut down hardware resources. IOService objects participate in an attach/detach process with providers (devices or buses that supply resources) and use a matching system to determine which driver is suitable for a given device. Matching relies on dictionaries that describe device properties and driver capabilities, enabling the I/O Kit to bind a driver to hardware at runtime.

In addition to lifecycle and matching, IOService handles properties and communication with other kernel components. Drivers can publish and query properties in the I/O Registry, enabling configuration, status reporting, and resource negotiation. IOService also provides mechanisms for publishing services, so that clients—often user-space processes or other kernel components—can locate and interact with a driver’s services through established interfaces such as IOUserClient.

IOService objects exist as nodes in the I/O Registry hierarchy, where parent-child relationships reflect device topology. This structure supports organized discovery, power management, and resource sharing across drivers. While IOService is a low-level, platform-specific facility, it provides the standard abstractions that enable modular, extensible driver development within Apple’s kernel environment. See also IORegistryEntry and IOUserClient for related interfaces.