# CFReadStreamOpen(_:) Opens a stream for reading. ``` func CFReadStreamOpen(_ stream: CFReadStream!) -> Bool ``` ## Parameters `stream` The stream to open. ## Return Value `TRUE` if `stream` was successfully opened, `FALSE` otherwise. If `stream` is not in the [`CFStreamStatus.notOpen`](/documentation/CoreFoundation/CFStreamStatus/notOpen) state, this function returns `FALSE`. ## Discussion Opening a stream causes it to reserve all the system resources it requires. If the stream can open in the background without blocking, this function always returns `true`. To learn when a background open operation completes, you can either schedule the stream into a run loop with [`CFReadStreamScheduleWithRunLoop(_:_:_:)`](/documentation/CoreFoundation/CFReadStreamScheduleWithRunLoop(_:_:_:)) and wait for the stream’s client (set with [`CFReadStreamSetClient(_:_:_:_:)`](/documentation/CoreFoundation/CFReadStreamSetClient(_:_:_:_:))) to be notified or you can poll the stream using [`CFReadStreamGetStatus(_:)`](/documentation/CoreFoundation/CFReadStreamGetStatus(_:)), waiting for a status of [`CFStreamStatus.open`](/documentation/CoreFoundation/CFStreamStatus/open) or [`CFStreamStatus.error`](/documentation/CoreFoundation/CFStreamStatus/error). You do not need to wait until a stream has finished opening in the background before calling the [`CFReadStreamRead(_:_:_:)`](/documentation/CoreFoundation/CFReadStreamRead(_:_:_:)) function. The read operation will simply block until the open has completed. --- 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)