# append(length:allocator:deallocator:range:flags:) Adds a memory block to a block buffer using a custom allocator and deallocator. ``` func append(length: Int, allocator: @escaping CMBlockBuffer.CustomBlockAllocator, deallocator: @escaping CMBlockBuffer.CustomBlockDeallocator, range: Range? = nil, flags: CMBlockBuffer.Flags = []) throws ``` ## Parameters `length` The length of the memory block in bytes. Must not be zero. This is the size to allocate when you call the [`assureBlockMemory()`](/documentation/CoreMedia/CMBlockBuffer/assureBlockMemory()) function. `allocator` An object that allocates a memory block. `deallocator` An object that deallocates a memory block. `range` The range within the memory block to which the block buffer should refer to data. If this value is `nil`, the block buffer uses the whole memory block. `flags` Feature and control flags. --- 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)