# CMBlockBufferGetDataPointer(_:atOffset:lengthAtOffsetOut:totalLengthOut:dataPointerOut:) Gains access to the data represented by a block buffer. ``` func CMBlockBufferGetDataPointer(_ theBuffer: CMBlockBuffer, atOffset offset: Int, lengthAtOffsetOut: UnsafeMutablePointer?, totalLengthOut: UnsafeMutablePointer?, dataPointerOut: UnsafeMutablePointer?>?) -> OSStatus ``` ## Parameters `theBuffer` `CMBlockBuffer` to operate on. Must not be `NULL` `offset` Offset within the buffer’s offset range. `lengthAtOffsetOut` On return, contains the amount of data available at the specified offset. May be `NULL`. `totalLengthOut` On return, contains the block buffer’s total data length (from offset 0). May be `NULL`. `dataPointerOut` On return, contains a pointer to the data byte at the specified offset; `lengthAtOffset` bytes are available at this address. May be `NULL`. ## Return Value Returns `kCMBlockBufferNoErr` if data was accessible at the specified offset within the given `CMBlockBuffer`. Returns an error otherwise. ## Discussion Gains access to the data represented by a [`CMBlockBuffer`](/documentation/CoreMedia/CMBlockBuffer). A pointer into a memory block is returned which corresponds to the offset within the `CMBlockBuffer`. If `lengthAtOffset` is non `NULL`, the number of bytes addressable at the pointer is returned. This length-at-offset may be smaller than the number of bytes actually available starting at the offset if the `dataLength` of the `CMBlockBuffer` is covered by multiple memory blocks (a noncontiguous `CMBlockBuffer`). The caller can compare (`offset`+`lengthAtOffset`) with `totalLength` to determine whether the entire `CMBlockBuffer` has been referenced and whether it is possible to access the `CMBlockBuffer`’s data with a contiguous reference. The data pointer returned will remain valid as long as the original `CMBlockBuffer` is referenced - once the `CMBlockBuffer` is released for the last time, any pointers into it will be invalid. --- 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)