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# I/O Audio Unit Function Selectors

Audio unit component selectors, specific to I/O audio units, that correspond to functions in the audio unit API.

## Topics

### Constants

[`var kAudioOutputUnitRange: Int`](/documentation/AudioToolbox/kAudioOutputUnitRange)

The start of the numerical range for I/O audio unit function selectors.

[`var kAudioOutputUnitStartSelect: Int`](/documentation/AudioToolbox/kAudioOutputUnitStartSelect)

Used by the system to start an I/O audio unit when you call the [`AudioOutputUnitStart(_:)`](/documentation/AudioToolbox/AudioOutputUnitStart(_:)) function.

[`var kAudioOutputUnitStopSelect: Int`](/documentation/AudioToolbox/kAudioOutputUnitStopSelect)

Used by the system to stop an I/O audio unit when you call the [`AudioOutputUnitStop(_:)`](/documentation/AudioToolbox/AudioOutputUnitStop(_:)) function.


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