# withUnsafeMutableBytes(of:_:) Invokes the given closure with a mutable buffer pointer covering the raw bytes of the given argument. ``` func withUnsafeMutableBytes(of value: inout T, _ body: (UnsafeMutableRawBufferPointer) throws(E) -> Result) throws(E) -> Result where E : Error, T : ~Copyable, T : ~Escapable, Result : ~Copyable ``` ## Parameters `value` An instance to temporarily access through a mutable raw buffer pointer. Note that the `inout` exclusivity rules mean that, like any other `inout` argument, `value` cannot be directly accessed by other code for the duration of `body`. Access must only occur through the pointer argument to `body` until `body` returns. `body` A closure that takes a raw buffer pointer to the bytes of `value` as its sole argument. If the closure has a return value, that value is also used as the return value of the `withUnsafeMutableBytes(of:_:)` function. The buffer pointer argument is valid only for the duration of the closure’s execution. ## Return Value The return value, if any, of the `body` closure. ## Discussion The buffer pointer argument to the `body` closure provides a collection interface to the raw bytes of `value`. The buffer is the size of the instance passed as `value` and does not include any remote storage. --- 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)