# CMAudioFormatDescriptionGetMagicCookie(_:sizeOut:) Returns a read-only pointer to, and size of, the magic cookie in an audio format description. ``` func CMAudioFormatDescriptionGetMagicCookie(_ desc: CMAudioFormatDescription, sizeOut: UnsafeMutablePointer?) -> UnsafeRawPointer? ``` ## Parameters `desc` `CMFormatDescription` being interrogated. `sizeOut` Pointer to variable that will be written with the size of the cookie. Can be `NULL`. ## Return Value A read-only pointer to the magic cookie inside the audio format description. ## Discussion The magic cookie is a completely opaque piece of data, written and read only by the codec itself. A magic cookie is only present for codecs that require it; this API will return `NULL` if one does not exist. This API is specific to audio format descriptions, and will return `NULL` if called with a non-audio format description. --- 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)