# CFStringSetExternalCharactersNoCopy(_:_:_:_:) Notifies a CFMutableString object that its external backing store of Unicode characters has changed. ``` func CFStringSetExternalCharactersNoCopy(_ theString: CFMutableString!, _ chars: UnsafeMutablePointer!, _ length: CFIndex, _ capacity: CFIndex) ``` ## Parameters `theString` The string to act as a “wrapper” for the external backing store (`chars`). If this value is not a CFMutableString object, an assertion is raised. `chars` The external (client-owned) Unicode buffer acting as the backing store for `theString`. `length` The current length of the contents of `chars` (in Unicode characters). `capacity` The capacity of the Unicode buffer—that is, the total number of Unicode characters that can be stored in it before the buffer has to be grown. ## Discussion You use this function to reallocate memory for a string, if necessary, and change its references to the data in the buffer. The object must have been created with the [`CFStringCreateMutableWithExternalCharactersNoCopy(_:_:_:_:_:)`](/documentation/CoreFoundation/CFStringCreateMutableWithExternalCharactersNoCopy(_:_:_:_:_:)) function; see the discussion of this function for more information. --- 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)