# ExtAudioFileWriteAsync(_:_:_:) Perform an asynchronous, sequential write operation on an audio file. ``` func ExtAudioFileWriteAsync(_ inExtAudioFile: ExtAudioFileRef, _ inNumberFrames: UInt32, _ ioData: UnsafePointer?) -> OSStatus ``` ## Parameters `inExtAudioFile` The extended audio file object that represents the file you want to write to. `inNumberFrames` The number of frames to write. `ioData` The buffer(s) from which audio data is written to the file. ## Return Value A result code. ## Discussion Writes the provided buffer list to an internal ring buffer and notifies an internal thread to perform the write at a later time. The first time this function is called, allocations may be performed. You can call this function with `0` frames and a `NULL` buffer in a non-time-critical context to initialize the asynchronous mechanism. Once initialized, subsequent calls are very efficient and do not take locks. This technique may be used to write to a file from a realtime thread. Your application must not mix synchronous and asynchronous writes to the same file. Pending writes are not guaranteed to be flushed to disk until the [`ExtAudioFileDispose(_:)`](/documentation/AudioToolbox/ExtAudioFileDispose(_:)) function is called. Errors may occur after this call has returned. Such errors may be returned from subsequent calls to this function. --- 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)