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Updated notes on Mach-O
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Eduard Urbach 2025-07-10 15:26:32 +02:00
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## Notes ## Notes
- Headers must be included in the __TEXT segment. - The start of the file must be loaded in some segment.
- The start of the file must be marked as readable + executable.
- Load command size must be divisible by 8. - Load command size must be divisible by 8.
- Segments must be page-aligned in the file[^1].
Due to the page alignment requirement on disk[^1] instead of just memory,
MacOS is the only OS that forces you to write bloated executables on disk.
Until this check is removed, we cannot produce smaller executables.
[^1]: https://github.com/apple-oss-distributions/xnu/blob/main/bsd/kern/mach_loader.c#L2021-L2027
## Links ## Links
- https://github.com/apple-oss-distributions/xnu/blob/main/bsd/kern/mach_loader.c
- https://github.com/apple-oss-distributions/xnu/blob/main/EXTERNAL_HEADERS/mach-o/loader.h - https://github.com/apple-oss-distributions/xnu/blob/main/EXTERNAL_HEADERS/mach-o/loader.h
- https://en.wikipedia.org/wiki/Mach-O - https://en.wikipedia.org/wiki/Mach-O
- https://github.com/aidansteele/osx-abi-macho-file-format-reference - https://github.com/aidansteele/osx-abi-macho-file-format-reference