# reallocate A prototype for a function callback that reallocates memory of a requested size for an existing block of memory. ``` var reallocate: CFAllocatorReallocateCallBack! ``` ## Discussion When implementing this function, change the size of the block of memory pointed to by `ptr` to the size specified by `newsize` and return the pointer to the larger block of memory. Return `NULL` on any reallocation failure, leaving the old block of memory untouched. Also return `NULL` immediately if any of the following conditions apply: The `ptr` parameter is `NULL`. The `newsize` parameter is not greater than 0. Leave the contents of the old block of memory unchanged up to the lesser of the new or old sizes. If the `ptr` parameter is not a block of memory that has been previously allocated by the allocator, the results are undefined; abnormal program termination can occur. The hint argument is a bitfield that you should currently not use (that is, assign 0). If you set this callback to `NULL` the `CFAllocatorReallocate(_:_:_:_:)` function returns `NULL` in most cases when it attempts to use this allocator. --- 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)