# CFWriteStreamCreateWithBuffer(_:_:_:) Creates a writable stream for a fixed-size block of memory. ``` func CFWriteStreamCreateWithBuffer(_ alloc: CFAllocator!, _ buffer: UnsafeMutablePointer!, _ bufferCapacity: CFIndex) -> CFWriteStream! ``` ## Parameters `alloc` The allocator to use to allocate memory for the new object. Pass `NULL` or kCFAllocatorDefault to use the current default allocator. `buffer` The memory buffer into which to write data. This buffer must exist for the lifetime of the stream. `bufferCapacity` The size of `buffer` and the maximum number of bytes that can be written. ## Return Value A new write stream, or `NULL` on failure. Ownership follows the [The Create Rule](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-103029). ## Discussion When `buffer` is filled after writing `bufferCapacity` bytes, the stream is exhausted and its status becomes [`CFStreamStatus.atEnd`](/documentation/CoreFoundation/CFStreamStatus/atEnd). You must open the stream, using [`CFWriteStreamOpen(_:)`](/documentation/CoreFoundation/CFWriteStreamOpen(_:)), before writing to it. --- 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)