Mach-O

Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. It was developed to replace the a.out format.

This article is about the file format. For the airport, see Mach-O Acres Airport. For other uses, see Mach O.

Mach-O
Filename extension
none, .o, .dylib, .bundle
Uniform Type Identifier (UTI)com.apple.mach-o-binary
Developed byCarnegie Mellon University, Apple Inc.
Type of formatBinary, executable, object, shared libraries, core dump
Container forARM, SPARC, PA-RISC, PowerPC and x86 executable code, memory image dumps

Mach-O is used by most systems based on the Mach kernel. NeXTSTEP, macOS, and iOS are examples of systems that use this format for native executables, libraries and object code.

Mach-O file layout

Each Mach-O file is made up of one Mach-O header, followed by a series of load commands, followed by one or more segments, each of which contains between 0 and 255 sections. Mach-O uses the REL relocation format to handle references to symbols. When looking up symbols Mach-O uses a two-level namespace that encodes each symbol into an 'object/symbol name' pair that is then linearly searched for, first by the object and then the symbol name.[1]

The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file[2]—was also used in the executable file format for Accent. The Accent file format was in turn, based on an idea from Spice Lisp.

Multi-architecture binaries

Multiple Mach-O files can be combined in a multi-architecture binary. This allows a single binary file to contain code to support multiple instruction set architectures, for example for different generations and types of Apple devices, including different processor architectures like ARM and x86.

Minimum OS version

With the introduction of Mac OS X 10.6 platform the Mach-O file underwent a significant modification that causes binaries compiled on a computer running 10.6 or later to be (by default) executable only on computers running Mac OS X 10.6 or later. The difference stems from load commands that the dynamic linker, in previous Mac OS X versions, does not understand. Another significant change to the Mach-O format is the change in how the Link Edit tables (found in the __LINKEDIT section) function. In 10.6 these new Link Edit tables are compressed by removing unused and unneeded bits of information, however Mac OS X 10.5 and earlier cannot read this new Link Edit table format. To make backwards-compatible executables, the linker flag "-mmacosx-version-min=" can be used.

Other implementations

Some versions of NetBSD have had Mach-O support added as part of an implementation of binary compatibility, which allowed some Mac OS 10.3 binaries to be executed.[3][4]

For Linux, a Mach-O loader was written by Shinichiro Hamaji[5] that can load 10.6 binaries. As a more extensive solution based on this loader, the Darling Project aims at providing a complete environment allowing macOS applications to run on Linux.

For the Ruby programming language, the ruby-macho[6] library provides an implementation of a Mach-O binary parser and editor.

See also

References
  1. "OS X ABI Mach-O File Format Reference". Apple Inc. February 4, 2009. Archived from the original on August 19, 2009. Retrieved April 27, 2016.
  2. Avadis Tevanian, Jr.; Richard F. Rashid; Michael W. Young; David B. Golub; Mary R. Thompson; William Bolosky; Richard Sanzi (1987). "A Unix Interface for Shared Memory and Memory Mapped Files Under Mach": 53–67. {{cite journal}}: Cite journal requires |journal= (help)
  3. Emmanuel Dreyfus (June 20, 2006). "Mach and Darwin binary compatiblity [sic] for NetBSD/powerpc and NetBSD/i386". Retrieved October 18, 2013.
  4. Emmanuel Dreyfus (September 2004), Mac OS X binary compatibility on NetBSD: challenges and implementation (PDF)
  5. Shinichiro Hamaji, Mach-O loader for Linux - I wrote...
  6. William Woodruff (November 15, 2021), A pure-Ruby library for parsing Mach-O files.
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