Ebook: Apple File System Reference

Apple File System is the default file format used on Apple platforms. Apple File System is the successor to HFS Plus, so some aspects of its design intentionally follow HFS Plus to enable data migration from HFS Plus to Apple File System. Other aspects of its design address limitations with HFS Plus and enable features such as cloning files, snapshots, encryption, and sharing free space between volumes.
Most apps interact with the file system using high-level interfaces provided by Foundation, which means most de- velopers donʼt need to read this document. This document is for developers of software that interacts with the file system directly, without using any frameworks or the operating system—for example, a disk recovery utility or an im- plementation of Apple File System on another platform. The on-disk data structures described in this document make up the file system; software that interacts with them defines corresponding in-memory data structures.