# writeFrom:startingAt:length:error: Synchronously writes data from from a buffer to the resource and executes a block afterwards. ``` - (size_t) writeFrom:(void *) buffer startingAt:(off_t) offset length:(size_t) length error:(NSError **) error; ``` ## 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. `error` On return, any error encountered while writing data, or `nil` if no error occurred. ## Return Value The actual number of bytes written. ## Discussion This is a synchronous version of [`writeFrom:startingAt:length:completionHandler:`](/documentation/FSKit/FSBlockDeviceResource/writeFrom:startingAt:length:completionHandler:). > Note: In some cases, this method performs a partial write. In this case, the return value is shorter than the requested length, and the `error` is set to `nil`. --- 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)