# CFWriteStream ``` class CFWriteStream ``` ## Overview `CFWriteStream` provides an interface for writing a byte stream either synchronously or asynchronously. You can create streams that write bytes to a block of memory, a file, or a generic socket. All streams need to be opened, using [`CFWriteStreamOpen(_:)`](/documentation/CoreFoundation/CFWriteStreamOpen(_:)), before writing. Use [`CFReadStream`](/documentation/CoreFoundation/CFReadStream) for reading byte streams, and for the functions, such as [`CFStreamCreatePairWithSocketToHost(_:_:_:_:_:)`](/documentation/CoreFoundation/CFStreamCreatePairWithSocketToHost(_:_:_:_:_:)), that create socket streams). `CFWriteStream` is “toll-free bridged” with its Cocoa Foundation counterpart, . This means that the Core Foundation type is interchangeable in function or method calls with the bridged Foundation object. Therefore, in a method where you see an `NSOutputStream *` parameter, you can pass in a `CFWriteStreamRef`, and in a function where you see a `CFWriteStreamRef` parameter, you can pass in an `NSOutputStream` instance. Note, however, that you may have either a delegate or callbacks but not both. See [Toll-Free Bridged Types](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFDesignConcepts/Articles/tollFreeBridgedTypes.html#//apple_ref/doc/uid/TP40010677) for more information on toll-free bridging. ## Topics ### Creating a Write Stream [`CFWriteStreamCreateWithAllocatedBuffers(_:_:)`](/documentation/CoreFoundation/CFWriteStreamCreateWithAllocatedBuffers(_:_:)) Creates a writable stream for a growable block of memory. [`CFWriteStreamCreateWithBuffer(_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamCreateWithBuffer(_:_:_:)) Creates a writable stream for a fixed-size block of memory. [`CFWriteStreamCreateWithFile(_:_:)`](/documentation/CoreFoundation/CFWriteStreamCreateWithFile(_:_:)) Creates a writable stream for a file. ### Opening and Closing a Stream [`CFWriteStreamClose(_:)`](/documentation/CoreFoundation/CFWriteStreamClose(_:)) Closes a writable stream. [`CFWriteStreamOpen(_:)`](/documentation/CoreFoundation/CFWriteStreamOpen(_:)) Opens a stream for writing. ### Writing to a Stream [`CFWriteStreamWrite(_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamWrite(_:_:_:)) Writes data to a writable stream. ### Scheduling a Write Stream [`CFWriteStreamScheduleWithRunLoop(_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamScheduleWithRunLoop(_:_:_:)) Schedules a stream into a run loop. [`CFWriteStreamUnscheduleFromRunLoop(_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamUnscheduleFromRunLoop(_:_:_:)) Removes a stream from a particular run loop. ### Examining Stream Properties [`CFWriteStreamCanAcceptBytes(_:)`](/documentation/CoreFoundation/CFWriteStreamCanAcceptBytes(_:)) Returns whether a writable stream can accept new data without blocking. [`CFWriteStreamCopyProperty(_:_:)`](/documentation/CoreFoundation/CFWriteStreamCopyProperty(_:_:)) Returns the value of a property for a stream. [`CFWriteStreamCopyError(_:)`](/documentation/CoreFoundation/CFWriteStreamCopyError(_:)) Returns the error associated with a stream. [`CFWriteStreamGetError(_:)`](/documentation/CoreFoundation/CFWriteStreamGetError(_:)) Returns the error status of a stream. [`CFWriteStreamGetStatus(_:)`](/documentation/CoreFoundation/CFWriteStreamGetStatus(_:)) Returns the current state of a stream. ### Setting Stream Properties [`CFWriteStreamSetClient(_:_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamSetClient(_:_:_:_:)) Assigns a client to a stream, which receives callbacks when certain events occur. [`CFWriteStreamSetProperty(_:_:_:)`](/documentation/CoreFoundation/CFWriteStreamSetProperty(_:_:_:)) Sets the value of a property for a stream. ### Getting the CFWriteStream Type ID [`CFWriteStreamGetTypeID()`](/documentation/CoreFoundation/CFWriteStreamGetTypeID()) Returns the type identifier of all CFWriteStream objects. ### Callbacks [`CFWriteStreamClientCallBack`](/documentation/CoreFoundation/CFWriteStreamClientCallBack) Callback invoked when certain types of activity takes place on a writable stream. --- 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)