# AURenderCallback Called by the system when an audio unit requires input samples, or before and after a render operation. ``` typealias AURenderCallback = (UnsafeMutableRawPointer, UnsafeMutablePointer, UnsafePointer, UInt32, UInt32, UnsafeMutablePointer?) -> OSStatus ``` ## Parameters `inRefCon` Custom data that you provided when registering your callback with the audio unit. `ioActionFlags` Flags used to describe more about the context of this call (pre or post in the notify case for instance). `inTimeStamp` The timestamp associated with this call of audio unit render. `inBusNumber` The bus number associated with this call of audio unit render. `inNumberFrames` The number of sample frames that will be represented in the audio data in the provided ioData parameter. `ioData` The AudioBufferList that will be used to contain the rendered or provided audio data. ## Return Value A result code. ## Discussion If you named your callback function `MyAURenderCallback`, you would declare it like this: ### Discussion You can use this callback function with both the audio unit render notification API (see the [`AudioUnitAddRenderNotify(_:_:_:)`](/documentation/AudioToolbox/AudioUnitAddRenderNotify(_:_:_:)) function) and the render input callback (see the `kAudioUnitProperty_SetRenderCallback` property). As a notification listener, the system invokes this callback before and after an audio unit’s render operations. As a render operation input callback, it is invoked when an audio unit requires input samples for the input bus that the callback is attached to. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)