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# Low-Level Information

Understand the underlying structures that support HID drivers.

## Topics

### Internal Structures

[`typedef void (*)(void *, void *, int, unsigned int) IOHIDCompletionAction;`](/documentation/HIDDriverKit/IOHIDCompletionAction)

A function to call when a report operation completes.

[`typedef struct IOHIDCompletion { ... } IOHIDCompletion;`](/documentation/HIDDriverKit/IOHIDCompletion)

A structure specifying the action to perform when a set/get report completes.

[`typedef enum { ... } HIDReportCommandType;`](/documentation/HIDDriverKit/HIDReportCommandType)

The type of the report command for a DriverKit driver.

[`typedef enum { ... } HIDActionType;`](/documentation/HIDDriverKit/HIDActionType)

### Version Information

[`extern double HIDDriverKitVersionNumber;`](/documentation/HIDDriverKit/HIDDriverKitVersionNumber)

The project version number for HIDDriverKit.

[`extern const unsigned char HIDDriverKitVersionString[];`](/documentation/HIDDriverKit/HIDDriverKitVersionString)

The project version string for HIDDriverKit.


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