Using ruby ffi on Os X with mixed architectures, universal binaries

On OS X Snow Leopard, you may have libs of different architectures installed and many binaries are universal binaries, meaning that they are cross compiled for several architectures, and the best architectures is selected at run time.

This may cause issues when you go to run your FFI code using 64bit ruby against a lib that is 32 bit for example you may get an error similar to this :

/Library/Ruby/Gems/1.8/gems/ffi-0.6.2/lib/ffi/library.rb:61:in 

`ffi_lib’: Could not open library ‘libmylib.dylib’: 

dlopen(libmylib.dylib, 9): no suitable image found.  Did find: 

(LoadError)  /usr/local/lib/libmylib.dylib: mach-o, but wrong architecture. Could 

not open library ‘libmylib’: dlopen(libmylib, 9): image not found   from /Library/Ruby/Gems/1.8/gems/ffi-0.6.2/lib/ffi/library.rb: 

43:in `map’  from /Library/Ruby/Gems/1.8/gems/ffi-0.6.2/lib/ffi/library.rb: 

43:in `ffi_lib’   from xxx.rb:14

Using lipo (man page for lipo) to examine the lib an find out what architecture it is, we can see that it is is i386)

$ lipo -info /usr/local/lib/libmylib.dylib 

Non-fat file: /usr/local/lib/libmylib.dylib is architecture: i386 

Running lipo on my stock install of Snow Leopard ruby, we can see that it is fact a fat binary(universal binary)

$ lipo -detailed_info /usr/bin/ruby 

Fat header in: /usr/bin/ruby 

fat_magic 0xcafebabe 

nfat_arch 3 

architecture x86_64 

    cputype CPU_TYPE_X86_64 

    cpusubtype CPU_SUBTYPE_X86_64_ALL 

    offset 4096 

    size 14176 

    align 2^12 (4096) 

architecture i386 

    cputype CPU_TYPE_I386 

    cpusubtype CPU_SUBTYPE_I386_ALL 

    offset 20480 

    size 14112 

    align 2^12 (4096) 

architecture ppc7400 

    cputype CPU_TYPE_POWERPC 

    cpusubtype CPU_SUBTYPE_POWERPC_7400 

    offset 36864 

    size 13904 

    align 2^12 (4096) 

To force ruby to use the i386 arch we can use the arch command (man page for arch)

$ arch -arch i386 /usr/bin/ruby libmylib.rb 

and everything shall work correctly.