# CFErrorCopyFailureReason(_:) Returns a human-presentable failure reason for a given error. ``` func CFErrorCopyFailureReason(_ err: CFError!) -> CFString! ``` ## Parameters `err` The CFError to examine. If this is not a valid CFError, the behavior is undefined. ## Return Value A localized, human-presentable failure reason for `err`, or `NULL` if no user-presentable string is available. Ownership follows the [The Create Rule](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-103029). ## Discussion The failure reason is a complete sentence which describes why the operation failed. In many cases this will be just the “because” part of the description (but as a complete sentence, which makes localization easier). For example, an error description “Could not save file ‘Letter’ in folder ‘Documents’ because the volume ‘MyDisk’ doesn’t have enough space.” might have a corresponding failure reason, “The volume ‘MyDisk’ doesn’t have enough space.” By default, this function looks for a value for the [`kCFErrorLocalizedFailureReasonKey`](/documentation/CoreFoundation/kCFErrorLocalizedFailureReasonKey) key in the user info dictionary. Toll-free bridged instances of `NSError` might provide additional behaviors for manufacturing this value. When you create a CFError, you should try to make sure the return value is human-presentable and localized by providing a value for [`kCFErrorLocalizedFailureReasonKey`](/documentation/CoreFoundation/kCFErrorLocalizedFailureReasonKey) in the user info dictionary. --- 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)