macOS Mach-O multiarch ("fat") binary: GraphViz block diagram (.dot) source

This is a simple container format that encapsulates multiple Mach-O files, each generally for a different architecture. XNU can execute these files just like single-arch Mach-Os and will pick the appropriate entry.

KS implementation details

License: CC0-1.0

This page hosts a formal specification of macOS Mach-O multiarch ("fat") binary using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

GraphViz block diagram source

mach_o_fat.dot

digraph {
	rankdir=LR;
	node [shape=plaintext];
	subgraph cluster__mach_o_fat {
		label="MachOFat";
		graph[style=dotted];

		mach_o_fat__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
			<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
			<TR><TD PORT="magic_pos">0</TD><TD PORT="magic_size">4</TD><TD></TD><TD PORT="magic_type">magic</TD></TR>
			<TR><TD PORT="num_fat_arch_pos">4</TD><TD PORT="num_fat_arch_size">4</TD><TD>u4be</TD><TD PORT="num_fat_arch_type">num_fat_arch</TD></TR>
			<TR><TD PORT="fat_archs_pos">8</TD><TD PORT="fat_archs_size">20</TD><TD>FatArch</TD><TD PORT="fat_archs_type">fat_archs</TD></TR>
			<TR><TD COLSPAN="4" PORT="fat_archs__repeat">repeat num_fat_arch times</TD></TR>
		</TABLE>>];
		subgraph cluster__fat_arch {
			label="MachOFat::FatArch";
			graph[style=dotted];

			fat_arch__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
				<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
				<TR><TD PORT="cpu_type_pos">0</TD><TD PORT="cpu_type_size">4</TD><TD>u4be‚ÜíMachO::CpuType</TD><TD PORT="cpu_type_type">cpu_type</TD></TR>
				<TR><TD PORT="cpu_subtype_pos">4</TD><TD PORT="cpu_subtype_size">4</TD><TD>u4be</TD><TD PORT="cpu_subtype_type">cpu_subtype</TD></TR>
				<TR><TD PORT="ofs_object_pos">8</TD><TD PORT="ofs_object_size">4</TD><TD>u4be</TD><TD PORT="ofs_object_type">ofs_object</TD></TR>
				<TR><TD PORT="len_object_pos">12</TD><TD PORT="len_object_size">4</TD><TD>u4be</TD><TD PORT="len_object_type">len_object</TD></TR>
				<TR><TD PORT="align_pos">16</TD><TD PORT="align_size">4</TD><TD>u4be</TD><TD PORT="align_type">align</TD></TR>
			</TABLE>>];
			fat_arch__inst__object [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
				<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
				<TR><TD PORT="object_pos">ofs_object</TD><TD PORT="object_size">len_object</TD><TD>MachO</TD><TD PORT="object_type">object</TD></TR>
			</TABLE>>];
		}
	}
	mach_o_fat__seq:fat_archs_type -> fat_arch__seq [style=bold];
	mach_o_fat__seq:num_fat_arch_type -> mach_o_fat__seq:fat_archs__repeat [color="#404040"];
	fat_arch__seq:ofs_object_type -> fat_arch__inst__object:object_pos [color="#404040"];
	fat_arch__seq:len_object_type -> fat_arch__inst__object:object_size [color="#404040"];
	fat_arch__inst__object:object_type -> mach_o__seq [style=bold];
}