# CMBlockBufferCreateWithMemoryBlock(allocator:memoryBlock:blockLength:blockAllocator:customBlockSource:offsetToData:dataLength:flags:blockBufferOut:) Creates a block buffer that’s backed by a memory block. ``` func CMBlockBufferCreateWithMemoryBlock(allocator structureAllocator: CFAllocator?, memoryBlock: UnsafeMutableRawPointer?, blockLength: Int, blockAllocator: CFAllocator?, customBlockSource: UnsafePointer?, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags, blockBufferOut: UnsafeMutablePointer) -> OSStatus ``` ## Parameters `structureAllocator` Allocator to use for allocating the `CMBlockBuffer` object. Pass `NULL` to use the default allocator. `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 new `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 memory block 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` Offset within the `memoryBlock` at which the `CMBlockBuffer` should refer to data. `dataLength` Number of relevant data bytes, starting at `offsetToData`, within the memory block. `flags` Feature and control flags. `blockBufferOut` Receives newly-created `CMBlockBuffer` object with a retain count of 1. Must not be `NULL`. ## Return Value Returns `kCMBlockBufferNoErr` if successful. ## Discussion Creates a new `CMBlockBuffer` backed by a memory block. The memory block may be statically allocated, dynamically allocated using the given allocator (or `customBlockSource`) or not yet allocated. The returned `CMBlockBuffer` may be further expanded using `CMBlockBufferAppendMemoryBlock` and/or `CMBlockBufferAppendBufferReference`. --- 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)