# withUnsafeMutableBytes(atOffset:_:) Accesses the data that a block buffer represents. ``` func withUnsafeMutableBytes(atOffset offset: Int = 0, _ body: (UnsafeMutableRawBufferPointer) throws -> R) throws -> R ``` ## Parameters `offset` An offset within the buffer’s offset range. `body` A closure the system calls with an parameter that points to contiguous storage in the block buffer. ## Return Value The value, if any, that the body parameter returns --- 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)