What a binary format actually promises
A Mach-O file is not “machine code with a header.” It is a structured agreement between producers and consumers of executable code.
The producer may be a compiler, assembler, linker, packager, or code-signing tool. Consumers include the kernel, dyld, debuggers, symbolication tools, static analyzers, crash reporters, and your own parser.
That distinction matters because the file contains several different kinds of truth:
- identity — architecture, file type, byte order, and format width;
- mapping instructions — which bytes become which virtual-memory ranges and with what protections;
- linking instructions — what needs to be bound, rebased, or resolved against other images;
- metadata — symbols, strings, runtime structures, build information, and platform requirements;
- trust material — hashes, code-signing structures, entitlements, and related blobs.
The first habit for this course is simple: do not memorize a screenshot of a file layout. Learn to ask which consumer needs each field and what that consumer can legitimately infer from it.
The four magic values you should recognize
Classic Mach-O uses distinct magic values for 32-bit and 64-bit files and for native versus byte-swapped representations. Universal (“fat”) containers have their own magic values.
When you inspect the first four bytes, you are not merely identifying “an Apple binary.” You are determining which parser path is legal.
For 64-bit little-endian Mach-O on modern Apple platforms, the bytes commonly appear as:
cf fa ed fe
Interpreted as a little-endian 32-bit integer, that is MH_MAGIC_64.
Do not let the visual byte order confuse you: the bytes on disk and the integer value printed in a header file are two views of the same value under an endian convention.
Verification habit
For every tool-produced claim in this course, practice finding the bytes or structure that justify it. Tools are useful; they are not oracles.