Mach-O
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| Filename extension | none, .o, .dylib, .bundle |
|---|---|
| Uniform Type Identifier (UTI) | com.apple.mach-o-binary |
| Developed by | Carnegie Mellon University Apple Inc. |
| Type of format | Binary, executable, object, shared libraries, core dump |
| Container for | ARM, SPARC, PA-RISC, PowerPC and x86 executable code, memory image dumps |
Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically-loaded code, and core dumps. A replacement for the a.out format, Mach-O offers more extensibility and faster access to information in the symbol table.
Mach-O is used by most systems based on the Mach kernel. NeXTSTEP, macOS, and iOS are examples of systems that have used this format for native executables, libraries and object code.
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 by first 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. The Accent file format was in turn, based on an idea from Spice Lisp.
Under NeXTSTEP, OPENSTEP, macOS, and iOS, 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, a multi-architecture binary for iOS can have 6 instruction set architectures, namely ARMv6 (for iPhone, 3G and 1st / 2nd generation iPod touch), ARMv7 (for iPhone 3GS, 4, 4S, iPad, 2, 3rd generation and 3rd - 5th generation iPod touch), ARMv7s (for iPhone 5 and iPad (4th generation)), ARMv8 (for iPhone 5S), x86 (for iPhone simulator on 32-bit machines) and x86_64 (64-bit simulator).
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