How do I parse a Mach-O file header in C?

Use the mach_header or mach_header_64 structs defined in the reference, reading from the file offset. Check the magic field for endianness and filetype for the format, as detailed in the header structure section.

What's the difference between Mach-O and ELF file formats?

Mach-O is used by macOS/iOS with segments like __TEXT and __DATA, while ELF is common on Linux/Unix with different section and program header layouts. Mach-O supports features like two-level namespaces, as explained in the dynamic linking details.

How does dynamic linking work in Mach-O files?

It uses load commands such as LC_LOAD_DYLIB to list dependent libraries and sections like __la_symbol_ptr for lazy binding. The dynamic linker processes these at runtime, as covered in the segment and load command documentation.

What tools can I use to inspect Mach-O files on macOS?

Command-line tools like otool and objdump are standard; the reference helps interpret their output by explaining structures like sections and symbols, but it doesn't provide tool usage instructions.

How to create a simple Mach-O executable from assembly code?

Assemble the code into an object file using as, then link with ld to produce a Mach-O executable, ensuring the header and segment commands align with the spec, though the reference lacks step-by-step guides.

What are the common sections in the __DATA segment?

Includes __data for initialized mutable variables, __bss for uninitialized statics, and __la_symbol_ptr for lazy symbol pointers, as listed in Table 2 of the README with explanations of each.

Mach-O file format reference — Apple Mach-O Documentation

What is Mach-O file format reference?

OS X ABI Mach-O File Format Reference is a mirror of Apple's official documentation that describes the Mach-O (Mach object) file format. This format is the standard for storing executable programs, libraries, and object files on disk in macOS and iOS. It details the binary structure, including headers, load commands, segments, and sections, which is essential for understanding how the operating system loads and runs software.

Target Audience

Low-level systems programmers, compiler and linker developers, security researchers performing binary analysis, and macOS/iOS developers needing to understand executable file internals for debugging or tool creation.

Value Proposition

It provides the authoritative, detailed specification for the Mach-O format that is otherwise difficult to find on Apple's current developer site. Developers choose this reference for accurate, comprehensive information needed to build analysis tools, debug complex linking issues, or understand the macOS runtime environment at a fundamental level.

Mirror of OS X ABI Mach-O File Format Reference

Use Cases

Best For

Understanding the internal structure of macOS and iOS executables and libraries
Writing tools for binary analysis, patching, or inspection of Mach-O files
Debugging low-level linking errors or runtime crashes related to binary format
Learning about the application binary interface (ABI) for Apple platforms
Developing compilers, linkers, or packagers that output Mach-O files
Researching macOS security and reverse engineering

Not Ideal For

Developers seeking a hands-on library or tool to programmatically manipulate Mach-O files
Teams needing documentation updated for the latest macOS/iOS versions and new Mach-O extensions
Individuals looking for step-by-step tutorials or code examples to apply the specification practically
Projects focused on cross-platform binary analysis where ELF or PE format knowledge is equally important

Pros & Cons

Pros

Comprehensive Format Specification

Covers all major regions including header, load commands, and data segments with detailed tables for standard sections like __TEXT and __DATA, as outlined in the README's segment and section references.

Authoritative Reference Source

Mirrors Apple's official documentation, providing hard-to-find technical details essential for tool developers and low-level debugging, as noted in the preamble about its scarcity.

Detailed Data Structures

Includes exact C struct definitions for mach_header, segment_command, and others with field-by-field explanations, enabling precise implementation or analysis.

Architecture and Linking Coverage

Explains support for 32-bit and 64-bit architectures, universal binaries, and dynamic linking mechanisms like LC_LOAD_DYLIB, based on the header and load command documentation.

Cons

Static and Potentially Outdated

As a mirror of older Apple documentation, it may not include recent macOS changes or new Mach-O features, with no updates indicated in the repository.

No Practical Implementation Guidance

Purely a specification document with no code examples, tutorials, or tools, making it difficult for developers to apply the knowledge without additional resources.

Dense and Assumes Expertise

Written in complex, low-level language that presupposes prior knowledge of binary formats, which can be inaccessible for those new to systems programming.

Frequently Asked Questions

What is Mach-O file format reference?

Target Audience

Value Proposition

Use Cases

Best For

Understanding the internal structure of macOS and iOS executables and libraries
Writing tools for binary analysis, patching, or inspection of Mach-O files
Debugging low-level linking errors or runtime crashes related to binary format
Learning about the application binary interface (ABI) for Apple platforms
Developing compilers, linkers, or packagers that output Mach-O files
Researching macOS security and reverse engineering

Not Ideal For

Developers seeking a hands-on library or tool to programmatically manipulate Mach-O files
Teams needing documentation updated for the latest macOS/iOS versions and new Mach-O extensions
Individuals looking for step-by-step tutorials or code examples to apply the specification practically
Projects focused on cross-platform binary analysis where ELF or PE format knowledge is equally important

Pros & Cons

Pros

Comprehensive Format Specification

Authoritative Reference Source

Mirrors Apple's official documentation, providing hard-to-find technical details essential for tool developers and low-level debugging, as noted in the preamble about its scarcity.

Detailed Data Structures

Includes exact C struct definitions for mach_header, segment_command, and others with field-by-field explanations, enabling precise implementation or analysis.

Architecture and Linking Coverage

Explains support for 32-bit and 64-bit architectures, universal binaries, and dynamic linking mechanisms like LC_LOAD_DYLIB, based on the header and load command documentation.

Cons

Static and Potentially Outdated

As a mirror of older Apple documentation, it may not include recent macOS changes or new Mach-O features, with no updates indicated in the repository.

No Practical Implementation Guidance

Purely a specification document with no code examples, tutorials, or tools, making it difficult for developers to apply the knowledge without additional resources.

Dense and Assumes Expertise

Written in complex, low-level language that presupposes prior knowledge of binary formats, which can be inaccessible for those new to systems programming.

Frequently Asked Questions

Mach-O file format reference

What is Mach-O file format reference?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?

Mach-O file format reference

What is Mach-O file format reference?

Overview

Use Cases

Best For

Not Ideal For

Pros & Cons

Pros

Cons

Frequently Asked Questions

Related Projects

Found a gem we're missing?