Mach-O

Introduction

Mach-O, short for Mach object, is the format of programs, libraries and object files on Apple’s macOS, iOS, iPadOS, watchOS and tvOS. It came from the Mach kernel through NeXTSTEP, which became Mac OS X.

Mach-O programs have no extension: the program inside an app bundle, such as Safari.app/Contents/MacOS/Safari, is a Mach-O file. The extensions that do appear are .dylib for dynamic libraries, .bundle for plug-ins and .o for object files. A universal binary wraps several Mach-O files, one per processor, such as one for Intel and one for Apple silicon, in a single file.

Structure

A Mach-O file starts with a header: a magic number that gives the word size and byte order (0xFEEDFACE for 32-bit, 0xFEEDFACF for 64-bit), the processor type, the file type (1 object, 2 executable, 6 dynamic library, 8 bundle and others), and the number and total size of the load commands that follow. The header is 28 bytes for 32-bit files and 32 bytes for 64-bit ones.

Load commands describe everything else: the segments to map into memory and the sections inside them, the symbol table, the libraries to link, the entry point, the code signature and more. Each command starts with its type and size, so a reader can skip the ones it does not know.

A universal binary starts instead with the big-endian magic CA FE BA BE, a count of architectures, and one 20-byte entry per architecture giving the processor type and the offset and size of that architecture’s Mach-O file inside it.

Also used for

• Executable and Linkable Format .o

Links