Today I saw a command line option in the output of otool (this is a MacOS X program, offering similar functionality as objdump) that is named:

-f -- print the fat headers

So, what are the fat headers ?

I tried to Google 'fat headers' and 'fat headers elf' but didn't find anything useful.

1 Answer 1


A fat header is the header of a fat binary.

See pages 67-68 in Mac OS X Internals:

Note that a fat binary is essentially a wrapper—a simple archive that concatenates Mach-O files for multiple architectures. A fat binary begins with a fat header (struct fat_header) that contains a magic number followed by an integral value representing the number of architectures whose binaries reside in the fat binary.


fat header

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