MachO

Mach-O, short for Mach Object, is a binary file format used for executables, object code, shared libraries, and core dumps on Apple operating systems including macOS, iOS, watchOS, and tvOS. It originated from the Mach kernel and NeXTSTEP, and was adopted by Apple with Mac OS X to replace earlier formats such as a.out. Mach-O supports both 32-bit and 64-bit variants and multiple architectures in a single file (fat or universal binaries) through a fat header.

A Mach-O file begins with a header that identifies the architecture, file type (executable, dylib, bundle, object), and the number of load commands. After the header are a sequence of load commands, which describe segments and sections, as well as dynamic libraries to load, symbol tables, and entry points. Segments contain sections that hold code, data, or other resources. The dynamic linker (dyld) uses the load commands to map the image into memory and to resolve symbols at load time or run time. Mach-O supports position-independent code, code signing, and address space layout randomization.

The format supports universal binaries containing multiple architectures in a single file. Apple toolchain tools such as lipo, otool, nm, install_name_tool, and codesign operate on Mach-O files. The format remains the primary binary for macOS and iOS applications and libraries, with dyld as the dynamic linker.