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Creating a single static or shared library (dylib) that supports multiple arches

OSX has a nice tool that takes static, dylibs, or shared libraries for different system architectures and can pack them into a single library that can be loaded on different hardware called `lipo`.

Not sure where the name comes from (I see people call the libraries it creates `fat` libraries but lipo is supposed to remove fat? Idk). Anyways it's simple to use:

lipo -output <file> -create <lib/arch1.dylib> <lib/arch2.dylib> # etc...

Use man lipo for more info

LIPO(1)                                                                LIPO(1)

       lipo - create or operate on universal files

       lipo  [-info]  [-detailed_info]  [-arch  arch_type  input_file]  ...  [
       input_file] ...  [-arch_blank arch_type]  [-create]  [-thin  arch_type]
       [-replace  arch_type  filename] ...  [-remove arch_type] ...  [-extract
       arch_type]  ...    [-extract_family   arch_type]   ...    [-verify_arch
       arch_type ...]  [-output output_file] [-segalign arch_type value] ...

       The  lipo command creates or operates on ``universal'' (multi-architec-
       ture) files.  It only ever produces one output file, and  never  alters
       the  input  file.   The  operations that lipo performs are: listing the
       architecture types in a universal file;  creating  a  single  universal
       file from one or more input files; thinning out a single universal file
       to one specified architecture type; and extracting,  replacing,  and/or
       removing architectures types from the input file to create a single new
       universal output file.

So for Python for iOS, the toolchain builds libraries for x86_64, i386, armv7, and arm64. To make this run on any device (iPhone or simulator) just run lipo and use that as the library. The os will load either one.

You can test which arches are packed in a file using the `file` command. Without it:

mbp:armv7 jrm$ file libffi.dylib
libffi.dylib: Mach-O dynamically linked shared library arm_v7

After running lipo:

mbp:Libraries jrm$ file libffi.dylib
libffi.dylib: Mach-O universal binary with 4 architectures: [i386: Mach-O dynamically linked shared library i386] [x86_64: Mach-O 64-bit dynamically linked shared library x86_64] [arm_v7: Mach-O dynamically linked shared library arm_v7] [arm64: Mach-O 64-bit dynamically linked shared library 
libffi.dylib (for architecture i386): Mach-O dynamically linked shared library i386
libffi.dylib (for architecture x86_64): Mach-O 64-bit dynamically linked shared library x86_64
libffi.dylib (for architecture armv7): Mach-O dynamically linked shared library arm_v7
libffi.dylib (for architecture arm64): Mach-O 64-bit dynamically linked shared library arm64

You can also add and remove arches with the tool (for release you probably don't need the IAs). Cool, one step closer to a complete Python for iOS build system!

Happy coding!


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