# CMBlockBufferAppendBufferReference(_:targetBBuf:offsetToData:dataLength:flags:) Adds a reference to an existing block buffer. ``` func CMBlockBufferAppendBufferReference(_ theBuffer: CMBlockBuffer, targetBBuf: CMBlockBuffer, offsetToData: Int, dataLength: Int, flags: CMBlockBufferFlags) -> OSStatus ``` ## Parameters `theBuffer` The existing `CMBlockBuffer`. The target `CMBlockBuffer` will be added to the memory being managed by `theBuffer` (the existing `CMBlockBuffer`). Must not be `NULL`. `targetBBuf` The target `CMBlockBuffer`. The target `CMBlockBuffer` will be added to the memory managed by the `theBuffer` (the existing`CMBlockBuffer`).This parameter must not be `NULL`. Unless the `kCMBlockBufferPermitEmptyReferenceFlag` is passed, the target `CMBlockBuffer` must not be empty and it must have a data length at least large enough to supply the data subset specified (i.e. `offsetToData` + `dataLength` bytes). `offsetToData` The reference maintained by the existing `CMBlockBuffer` will begin after this offset within the target `CMBlockBuffer`. `dataLength` Number of relevant data bytes, starting at `offsetToData`, within the target `CMBlockBuffer`. If zero, the target buffer’s total available `dataLength` (starting at offsetToData) will be referenced. `flags` Feature and control flags. ## Return Value Returns `kCMBlockBufferNoErr` if successful. ## Discussion Adds a buffer reference of (a possibly subset portion of) another `CMBlockBuffer`, the target `CMBlockBuffer`, to an existing `CMBlockBuffer`. The existing `CMBlockBuffer's` total data length will be increased by the specified `dataLength`. 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)