# write(from:startingAt:length:completionHandler:) Writes data from from a buffer to the resource and executes a closure afterwards. ``` func write(from buffer: UnsafeRawBufferPointer, startingAt offset: off_t, length: Int, completionHandler: @escaping (Int, (any Error)?) -> Void) ``` ## Parameters `buffer` A buffer to provide the data. `offset` The offset into the resource from which to start writing. `length` A maximum number of bytes to write. The completion handler receives a parameter with the actual number of bytes write. `completionHandler` A closure that executes after the write operation completes. If successful, the first parameter contains the number of bytes actually written. In the case of an error, the second parameter contains a non-`nil` error. This value is `EFAULT` if `buffer` is `NULL`, or `errno` if writing to the resource failed. ## Discussion For the read to succeed, requests must conform to any transfer requirements of the underlying resource. Disk drives typically require sector (`physicalBlockSize`) addressed operations of one or more sector-aligned offsets. --- 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)