# CMBlockBuffer An object the system uses to move blocks of memory through a processing system. ## Discussion A block buffer is a `CFType` object that represents a contiguous range of data offsets (from zero to [`CMBlockBufferGetDataLength(_:)`](/documentation/CoreMedia/CMBlockBufferGetDataLength(_:))) across a possibly noncontiguous memory region. The memory region contains memory blocks and buffer references. The buffer references can in turn refer to additional regions. `CMBlockBuffer` uses [`CMAttachmentBearerProtocol`](/documentation/CoreMedia/CMAttachmentBearerProtocol) to propagate attachments. ## Topics ### Creating a Block Buffer [`func CMBlockBufferCreateEmpty(allocator: CFAllocator?, capacity: UInt32, flags: CMBlockBufferFlags, blockBufferOut: UnsafeMutablePointer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferCreateEmpty(allocator:capacity:flags:blockBufferOut:)) Creates an empty block buffer. [`func CMBlockBufferCreateWithMemoryBlock(allocator: CFAllocator?, memoryBlock: UnsafeMutableRawPointer?, blockLength: Int, blockAllocator: CFAllocator?, customBlockSource: UnsafePointer?, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags, blockBufferOut: UnsafeMutablePointer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferCreateWithMemoryBlock(allocator:memoryBlock:blockLength:blockAllocator:customBlockSource:offsetToData:dataLength:flags:blockBufferOut:)) Creates a block buffer that’s backed by a memory block. [`func CMBlockBufferCreateWithBufferReference(allocator: CFAllocator?, referenceBuffer: CMBlockBuffer, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags, blockBufferOut: UnsafeMutablePointer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferCreateWithBufferReference(allocator:referenceBuffer:offsetToData:dataLength:flags:blockBufferOut:)) Creates a block buffer that refers to another block buffer object. [`func CMBlockBufferCreateContiguous(allocator: CFAllocator?, sourceBuffer: CMBlockBuffer, blockAllocator: CFAllocator?, customBlockSource: UnsafePointer?, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags, blockBufferOut: UnsafeMutablePointer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferCreateContiguous(allocator:sourceBuffer:blockAllocator:customBlockSource:offsetToData:dataLength:flags:blockBufferOut:)) Creates a block buffer that contains a contiguous copy of, or reference to, the data specified by the parameters. [`typealias CMBlockBufferFlags`](/documentation/CoreMedia/CMBlockBufferFlags) A type for flags that control behaviors and features of block buffer APIs. [Block Buffer Flags](/documentation/CoreMedia/block-buffer-flags) An enumeration of flags that control behaviors and features of block buffer APIs. [`struct CMBlockBufferCustomBlockSource`](/documentation/CoreMedia/CMBlockBufferCustomBlockSource) A structure to support custom memory allocation and deallocation for a block used in a block buffer. [Custom Block Source Version](/documentation/CoreMedia/custom-block-source-version) A custom block source version identifier. ### Modifying a Block Buffer [`func CMBlockBufferAppendMemoryBlock(CMBlockBuffer, memoryBlock: UnsafeMutableRawPointer?, length: Int, blockAllocator: CFAllocator?, customBlockSource: UnsafePointer?, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferAppendMemoryBlock(_:memoryBlock:length:blockAllocator:customBlockSource:offsetToData:dataLength:flags:)) Adds a memory block to an existing block buffer. [`func CMBlockBufferAppendBufferReference(CMBlockBuffer, targetBBuf: CMBlockBuffer, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferAppendBufferReference(_:targetBBuf:offsetToData:dataLength:flags:)) Adds a reference to an existing block buffer. [`func CMBlockBufferAssureBlockMemory(CMBlockBuffer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferAssureBlockMemory(_:)) Assures that the system allocates memory for all memory blocks in a block buffer. [`func CMBlockBufferAccessDataBytes(CMBlockBuffer, atOffset: Int, length: Int, temporaryBlock: UnsafeMutableRawPointer, returnedPointerOut: UnsafeMutablePointer?>) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferAccessDataBytes(_:atOffset:length:temporaryBlock:returnedPointerOut:)) Accesses potentially noncontiguous data in a block buffer. [`func CMBlockBufferCopyDataBytes(CMBlockBuffer, atOffset: Int, dataLength: Int, destination: UnsafeMutableRawPointer) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferCopyDataBytes(_:atOffset:dataLength:destination:)) Copies bytes from a block buffer into a provided memory area. [`func CMBlockBufferReplaceDataBytes(with: UnsafeRawPointer, blockBuffer: CMBlockBuffer, offsetIntoDestination: Int, dataLength: Int) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferReplaceDataBytes(with:blockBuffer:offsetIntoDestination:dataLength:)) Copies bytes from a given memory block into a block buffer replacing bytes in the underlying data blocks. [`func CMBlockBufferFillDataBytes(with: CChar, blockBuffer: CMBlockBuffer, offsetIntoDestination: Int, dataLength: Int) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferFillDataBytes(with:blockBuffer:offsetIntoDestination:dataLength:)) Fills the destination buffer with the specified data byte. ### Inspecting a Block Buffer [`func CMBlockBufferGetDataPointer(CMBlockBuffer, atOffset: Int, lengthAtOffsetOut: UnsafeMutablePointer?, totalLengthOut: UnsafeMutablePointer?, dataPointerOut: UnsafeMutablePointer?>?) -> OSStatus`](/documentation/CoreMedia/CMBlockBufferGetDataPointer(_:atOffset:lengthAtOffsetOut:totalLengthOut:dataPointerOut:)) Gains access to the data represented by a block buffer. [`func CMBlockBufferGetDataLength(CMBlockBuffer) -> Int`](/documentation/CoreMedia/CMBlockBufferGetDataLength(_:)) Returns the total length of data that’s accessible by a block buffer. [`func CMBlockBufferIsRangeContiguous(CMBlockBuffer, atOffset: Int, length: Int) -> Bool`](/documentation/CoreMedia/CMBlockBufferIsRangeContiguous(_:atOffset:length:)) Returns a Boolean value that indicates whether the specified range within a block buffer is contiguous. [`func CMBlockBufferIsEmpty(CMBlockBuffer) -> Bool`](/documentation/CoreMedia/CMBlockBufferIsEmpty(_:)) Returns a Boolean value that indicates whether the buffer is empty. ### Accessing the Type Identifier [`func CMBlockBufferGetTypeID() -> CFTypeID`](/documentation/CoreMedia/CMBlockBufferGetTypeID()) Returns the type identifier for block buffer objects. ### Data Types [`class CMBlockBuffer`](/documentation/CoreMedia/CMBlockBuffer) A reference to a block buffer instance. [`protocol CMBlockBufferProtocol`](/documentation/CoreMedia/CMBlockBufferProtocol) A protocol for objects that operate on a range of a block buffer. ### Errors [Block Buffer Error Codes](/documentation/CoreMedia/block-buffer-error-codes) Error codes that framework operations produce. --- 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)