# CMBlockBufferAppendMemoryBlock(_:memoryBlock:length:blockAllocator:customBlockSource:offsetToData:dataLength:flags:) Adds a memory block to an existing block buffer. ``` func CMBlockBufferAppendMemoryBlock(_ theBuffer: CMBlockBuffer, memoryBlock: UnsafeMutableRawPointer?, length blockLength: Int, blockAllocator: CFAllocator?, customBlockSource: UnsafePointer?, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags) -> OSStatus ``` ## Parameters `theBuffer` The existing `CMBlockBuffer` to which the new `memoryBlock` will be added. Must not be `NULL` `memoryBlock` Block of memory to hold buffered data. If `NULL`, a memory block will be allocated when needed (via a call to `CMBlockBufferAssureBlockMemory`) using the provided `blockAllocator` or `customBlockSource`. If non-`NULL`, the block will be used and will be deallocated when the `CMBlockBuffer` is finalized (i.e. released for the last time). `blockLength` Overall length of the memory block in bytes. Must not be zero. This is the size of the supplied `memoryBlock` or the size to allocate if `memoryBlock` is `NULL`. `blockAllocator` Allocator to be used for allocating the `memoryBlock`, if `memoryBlock` is `NULL`. If `memoryBlock` is non-`NULL`, this allocator will be used to deallocate it, if provided. Passing `NULL` will cause the default allocator (as set at the time of the call) to be used. Pass `kCFAllocatorNull` if no deallocation is desired. `customBlockSource` If non-NULL, it will be used for the allocation and freeing of the memory block (the `blockAllocator` parameter is ignored). If provided, and the `memoryBlock` parameter is `NULL`, its `AllocateBlock())` routine must be non-`NULL`. Allocate will be called once, if successful, when the `memoryBlock` is allocated. `FreeBlock()` will be called once when the `CMBlockBuffer` is disposed. `offsetToData` The reference maintained by the existing `CMBlockBuffer` will begin after this offset within the `memoryBlock` . `dataLength` Number of relevant data bytes, starting at `offsetToData`, within the memory block. `flags` Feature and control flags ## Return Value Returns `kCMBlockBufferNoErr` if successful. ## Discussion Adds a `memoryBlock` to an existing `CMBlockBuffer`. The memory block may be statically allocated, dynamically allocated using the given allocator or not yet allocated. The `CMBlockBuffer's` total data length will be increased by the specified `dataLength`. If the `kCMBlockBufferAssureMemoryNowFlag` is set in the flags parameter, the memory block is allocated immediately using the `blockAllocator` or `customBlockSource`. Note that append operations are not thread safe, so care must be taken when appending to block buffers that are used by multiple threads. --- 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)