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Glossary

IOSurface

The IOKit object that represents a chunk of memory mappable by both CPU and GPU. Zero-copy on Apple Silicon's unified memory.

An IOSurface is the IOKit-level abstraction for a piece of memory that can be mapped by both the CPU and GPU simultaneously. Foundation of zero-copy media and graphics on macOS.

What an IOSurface provides:

  • A CPU mapping (read/write via IOSurfaceGetBaseAddress).
  • A GPU mapping referenced by Metal MTLBuffer / MTLTexture objects.
  • Hardware coherency between the two — writes from one side are visible to the other without manual flush calls.
  • A reference-counted handle that can be passed between processes via XPC.
apple-oss-distributions/xnuiokit/Kernel/IOMemoryDescriptor.cppIOMemoryDescriptor — the kernel-side representation IOSurface builds on.View on GitHub(line —)

On Apple Silicon's unified memory, there is no "upload to GPU" step — an IOSurface is literally just DRAM both processors can address. On older Macs with discrete GPUs, the IOSurface infrastructure managed the upload, but the API stayed the same.

Common uses:

  • Video frames (AVFoundation produces IOSurface-backed pixel buffers).
  • Render targets (Metal automatically uses IOSurface under the hood for MTLStorageModeShared).
  • Cross-process image sharing (Quick Look thumbnails, screenshot APIs).

See also: the Apple GPU article, and IOKit.