Universal Binaries
Overview
Universal binaries are executables or application bundles that run natively on PowerPC or 32/64 bit Macintosh computers. They are larger in size (hence the name fat binaries) since they contain code compiled for multiple architectures. However, they contain library code shared between architectures so the total size is still less than the sum of individual binaries for each arch.
How to work with fat binaries
Compilation
To create a fat binary, simply specify the architectures to compile for:
$ gcc -o hello -arch i386 -arch x86_64 hello.c
$ file hello
hello: Mach-O universal binary with 2 architectures: [i386:Mach-O executable i386] [x86_64:Mach-O 64-bit executable x86_64]
hello (for architecture i386): Mach-O executable i386
hello (for architecture x86_64): Mach-O 64-bit executable x86_64
View
To view the header containing the information for all architectures, use otool (1):
$ otool -f hello
Fat headers
fat_magic 0xcafebabe
nfat_arch 2
architecture 0
cputype 7
cpusubtype 3
capabilities 0x0
offset 4096
size 8416
align 2^12 (4096)
architecture 1
cputype 16777223
cpusubtype 3
capabilities 0x80
offset 16384
size 8432
align 2^12 (4096)
lipo tool is more friendly and displays human readable information for the fields above:
$ lipo -detailed_info hello
Fat header in: hello
fat_magic 0xcafebabe
nfat_arch 2
architecture i386
cputype CPU_TYPE_I386
cpusubtype CPU_SUBTYPE_I386_ALL
offset 4096
size 8416
align 2^12 (4096)
architecture x86_64
cputype CPU_TYPE_X86_64
cpusubtype CPU_SUBTYPE_X86_64_ALL
offset 16384
size 8432
align 2^12 (4096)
Force a specific architecture
To force a specific arch to be loaded when launching the binary, use arch (1) tool:
$ arch -i386 ./hello
hello world!
$ arch -x86_64 ./hello
hello world!