# writeFrom:startingAt:length:completionHandler: Writes data from from a buffer to the resource and executes a block afterwards. ``` - (void) writeFrom:(void *) buffer startingAt:(off_t) offset length:(size_t) length completionHandler:(void (^)(size_t actuallyWritten, NSError *error)) completionHandler; ``` ## 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 block 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 write 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)