Linking to iOS simulator binaries on OSX

Question

I was curious what would happen if I linked to an iOS simulator framework in a Mac app. So I copied UIKit to it's own folder (so the Framework search path wouldn't include all the iOS simulator framework, as like CoreFoundation is both on Mac and iOS but has different headers), and dragged it into the link section in Xcode. The error Xcode gave me was:

building for MacOSX, but linking against dylib built for iOS Simulator file '/Users/jonathan/Desktop/macuikit/UIKit.framework/UIKit' for architecture x86_64

Both architectures are x86_64, so how can it tell the framework is specifically for the iOS simulator, I removed all references to iOS in things like Info.plist, even tried deleted everything but the UIKit binary, but the same error came up. Is there something in the binary it self that tells the linker which platform it can run on, rather than just the architecture? I looked at the Mach-O header but there is only fields for CPU type and subtype, and neither has a value for the simulator as expected.

Answer 1

After a little bit of digging, it turns out that the platform on which the library can run on is indeed specified in the binary. In fact, you can edit the binary in your favorite Hex editor and make the linker skip this check entirely.

This information is not specified in the Mach-O header (as you have already realized). Instead, it is specified as a load command type. You can see the available types by digging through the LLVM sources . Specifically, the enum values LC_VERSION_MIN_MACOSX and LC_VERSION_MIN_IPHONEOS look interesting.

Now, find the offset for this in our binary. Open the same in MachOView (or any other editor/viewer or your choice) and note the offset:

Once the offset is noted, jump to the same in a Hex editor and update it. I modified LC_VERSION_MIN_IPHONEOS (25) to LC_VERSION_MIN_MACOSX (24)

Save the updates and try linking again. The error should go away. Of course, you will hit other issues when you try to actually run your example. Have fun with LLDB then :)