# AudioFile_WriteProc A callback for writing file data when used in conjunction with the [`AudioFileOpenWithCallbacks(_:_:_:_:_:_:_:)`](/documentation/AudioToolbox/AudioFileOpenWithCallbacks(_:_:_:_:_:_:_:)) or [`AudioFileCreateWithURL(_:_:_:_:_:)`](/documentation/AudioToolbox/AudioFileCreateWithURL(_:_:_:_:_:)) functions. ``` typealias AudioFile_WriteProc = (UnsafeMutableRawPointer, Int64, UInt32, UnsafeRawPointer, UnsafeMutablePointer) -> OSStatus ``` ## Parameters `inClientData` A pointer to the client data as set in the `inClientData` parameter to [`AudioFileOpenWithCallbacks(_:_:_:_:_:_:_:)`](/documentation/AudioToolbox/AudioFileOpenWithCallbacks(_:_:_:_:_:_:_:)) or[`AudioFileInitializeWithCallbacks(_:_:_:_:_:_:_:_:_:)`](/documentation/AudioToolbox/AudioFileInitializeWithCallbacks(_:_:_:_:_:_:_:_:_:)). `inPosition` An offset into the data from which to read. `requestCount` The number of bytes to write. `buffer` A pointer to the buffer containing the data to write. `actualCount` Upon completion, the callback should set this to a pointer to the number of bytes successfully written. ## Return Value A result code. See Result Codes. ## Discussion If you named your function `MyAudioFile_WriteProc`, you would declare it like this: ### Discussion This callback function is invoked when Audio File Services needs to write data. --- 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)