Editing Mach-O (section)

== Multi-architecture binaries ==
Multiple Mach-O files can be combined in a [[Fat binary#NeXT's/Apple's multi-architecture binaries|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<ref>{{cite web |work=OS X ABI Mach-O File Format Reference |title=Universal Binaries and 32-bit/64-bit PowerPC Binaries |date=2009-02-04 |orig-date=2003 |publisher=[[Apple Inc.]] |url=https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/doc/uid/20001298-154889 |url-status=dead |archive-url=https://web.archive.org/web/20140904004108/https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/doc/uid/20001298-154889 |archive-date=2014-09-04}}</ref> such as [[ARM64]] and [[x86-64]].<ref>{{cite web|url=https://developer.apple.com/documentation/apple-silicon/building-a-universal-macos-binary|title=Building a Universal macOS Binary|website=Apple Developer}}</ref>

All fields in the universal header are big-endian.<ref name="mach-o-ref-data-types" />

The universal header is in the following form:<ref>{{cite web |work=OS X ABI Mach-O File Format Reference |title=fat_header |url=https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/c/tag/fat_header |date=2009-02-04 |orig-date=2003 |publisher=[[Apple Inc.]] |archive-url=https://web.archive.org/web/20140904004108/https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/c/tag/fat_header |archive-date=2014-09-04 |url-status=dead}}</ref>

{|class="wikitable"
|+ Mach-O universal header
|-
! Offset !! Bytes !! Description
|-
|  0 || 4 || Magic number
|-
|  4 || 4 || Number of binaries
|}

The magic number in a multi-architecture binary is {{code|0xcafebabe}} in big-endian byte order, so the first 4 bytes of the header will always be {{code|0xca 0xfe 0xba 0xbe}}, in that order.

The number of binaries is the number of entries that follow the header.

The header is followed by a sequence of entries in the following form:<ref>{{cite web |work=OS X ABI Mach-O File Format Reference |title=fat_arch |url=https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/c/tag/fat_arch |date=2009-02-04 |orig-date=2003 |publisher=[[Apple Inc.]] |archive-url=https://web.archive.org/web/20140904004108/https://developer.apple.com/library/mac/documentation/developertools/conceptual/MachORuntime/Reference/reference.html#//apple_ref/c/tag/fat_arch |archive-date=2014-09-04 |url-status=dead}}</ref>

{|class="wikitable"
|+ Mach-O universal file entries
|-
! Offset !! Bytes !! Description
|-
|  0 || 4 || CPU type
|-
|  4 || 4 || CPU subtype
|-
| 8 || 4 || File offset
|-
| 12 || 4 || Size
|-
| 16 || 4 || Section alignment (Power of 2)
|}

The sequence of entries is followed by a sequence of Mach-O images. Each entry refers to a Mach-O image.

The CPU type and subtype for an entry must be the same as the CPU type and subtype for the Mach-O image to which the entry refers.

The file offset and size are the offset in the file of the beginning of the Mach-O image, and the size of the Mach-O image, to which the entry refers.

The section alignment is the logarithm, base 2, of the byte alignment in the file required for the Mach-O image to which the entry refers; for example, a value of 14 means that the image must be aligned on a 2<sup>14</sup>-byte boundary, i.e. a 16384-byte boundary. This is required by tools that modify the multi-architecture binary, in order for them to keep the image properly aligned.