# CFWriteStreamWrite(_:_:_:) Writes data to a writable stream. ``` func CFWriteStreamWrite(_ stream: CFWriteStream!, _ buffer: UnsafePointer!, _ bufferLength: CFIndex) -> CFIndex ``` ## Parameters `stream` The stream to which to write. `buffer` The buffer holding the data to write. `bufferLength` The number of bytes from `buffer` to write. ## Return Value The number of bytes successfully written, `0` if the stream has been filled to capacity (for fixed-length streams), or `-1` if either the stream is not open or an error occurs. ## Discussion If `stream` is in the process of opening, this function waits until it has completed. If the stream is not full, this call blocks until at least one byte is written; it does not block until all the bytes in `buffer` is written. To avoid blocking, call this function only if [`CFWriteStreamCanAcceptBytes(_:)`](/documentation/CoreFoundation/CFWriteStreamCanAcceptBytes(_:)) returns `true` or after the stream’s client (set with [`CFWriteStreamSetClient(_:_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamSetClient(_:_:_:_:))) is notified of a [`canAcceptBytes`](/documentation/CoreFoundation/CFStreamEventType/canAcceptBytes) event. --- 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)