---
title: "withUnsafeMutableBytes(atOffset:_:)"
framework: coremedia
role: symbol
role_heading: Instance Method
path: "coremedia/cmblockbuffer/withunsafemutablebytes(atoffset:_:)"
---

# withUnsafeMutableBytes(atOffset:_:)

Accesses the data that a block buffer represents.

## Declaration

```swift
func withUnsafeMutableBytes<R>(atOffset offset: Int = 0, _ body: (UnsafeMutableRawBufferPointer) throws -> R) throws -> R
```

## Parameters

- `offset`: An offset within the buffer’s offset range.
- `body`: A closure the system calls with an doc://com.apple.documentation/documentation/Swift/UnsafeMutableRawBufferPointer parameter that points to contiguous storage in the block buffer.

## Return Value

Return Value The value, if any, that the body parameter returns

## See Also

### Modifying a Block Buffer

- [append(length:allocator:range:flags:)](coremedia/cmblockbuffer/append(length:allocator:range:flags:).md)
- [append(buffer:allocator:flags:)](coremedia/cmblockbuffer/append(buffer:allocator:flags:)-28keu.md)
- [append(buffer:allocator:flags:)](coremedia/cmblockbuffer/append(buffer:allocator:flags:)-8fws8.md)
- [append(length:allocator:deallocator:range:flags:)](coremedia/cmblockbuffer/append(length:allocator:deallocator:range:flags:).md)
- [append(buffer:deallocator:flags:)](coremedia/cmblockbuffer/append(buffer:deallocator:flags:)-3bfef.md)
- [append(buffer:deallocator:flags:)](coremedia/cmblockbuffer/append(buffer:deallocator:flags:)-1ibzz.md)
- [CMBlockBuffer.CustomBlockAllocator](coremedia/cmblockbuffer/customblockallocator.md)
- [CMBlockBuffer.CustomBlockDeallocator](coremedia/cmblockbuffer/customblockdeallocator.md)
- [append(bufferReference:flags:)](coremedia/cmblockbuffer/append(bufferreference:flags:).md)
- [assureBlockMemory()](coremedia/cmblockbuffer/assureblockmemory().md)