Mach-O, short for

Mach Mach may refer to Mach number, the speed of sound in local conditions. It may also refer to: Computing * Mach (kernel), an operating systems kernel technology * ATI Mach, a 2D GPU chip by ATI * GNU Mach, the microkernel upon which GNU Hurd is bas ...

object file format, is a

file format A file format is a Computer standard, standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary format, pr ...

for

executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data fil ...

s, object code,

shared libraries In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and su ...

, dynamically-loaded code, and

core dump In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working Computer storage, memory of a computer program at a specific time, generally when the program has crash (comp ...

s. It was developed to replace the

a.out a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries. This is an abbreviated form of "assembler output", the filename of the output of Ken Th ...

format. Mach-O is used by some systems based on the Mach kernel.

NeXTSTEP NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT Computer in the late 1980s and early 1990s and was initially used for its range of proprieta ...

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...

, 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 In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...

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. The basic structure—a list of variable-length "load commands" that reference pages of data elsewhere in the file—was also used in the executable file format for

Accent Accent may refer to: Speech and language * Accent (sociolinguistics), way of pronunciation particular to a speaker or group of speakers * Accent (phonetics), prominence given to a particular syllable in a word, or a word in a phrase ** Pitch ac ...

. 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 such as

ARM64 AArch64 or ARM64 is the 64-bit extension of the ARM architecture family. It was first introduced with the Armv8-A architecture. Arm releases a new extension every year. ARMv8.x and ARMv9.x extensions and features Announced in October 2011, ...

and

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...

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 computing, a dynamic linker is the part of an operating system that loads and links the shared libraries needed by an executable when it is executed (at " run time"), by copying the content of libraries from persistent storage to RAM, filli ...

, 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 NetBSD is a free and open-source Unix operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was forked. It continues to be actively developed and is a ...

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. For Linux, a Mach-O loader was written by Shinichiro Hamaji 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 A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapp ...

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

References

External links