Xcode build failure "Undefined symbols for architecture x86_64"

Undefined symbols for architecture x86_64: "_OBJC_CLASS_$_xxx", referenced from: objc-class-ref in yyy.o

This generally means, you are calling "xxx" (it may be a framework or class) from the class "yyy". The compiler can not locate the "xxx" so this error occurs.

You need to add the missing files(in this case "xxx") by right click on your project folder in navigator window and tap on "Add files to "YourProjectName"" option.

A popup window will open your project files in Finder. There, you can see the missing files and just add them to your project. Don't forget to check the "Copy items if needed" box. Good luck!!

It looks like you are missing including the IOBluetooth.framework in your project. You can add it by:

• Clicking on your project in the upper left of the left pane (the blue icon).

• In the middle pane, click on the Build Phases tab.

• Under "Link Binary With Libraries", click on the plus button.

• Find the IOBluetooth.framework from the list and hit Add.

This will make sure that the IOBluetooth.framework definitions are found by the linker. You can see that the framework is a member of your target by clicking on the framework in the left pane and seeing the framework's target membership in the right pane (note I've moved the framework under the Frameworks group for organization purposes):

UPD

Apple requires to use arm64 architecture. Do not use x32 libraries in your project

So the answer below is not correct anymore!

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Old answer

The new Xcode 5.1 sets the architecture armv7,armv7s,and arm64 as default.

And sometimes the error "build failure “Undefined symbols for architecture x86_64”" may be caused by this. Because, some libs (not Apple's) were compiled for x32 originally and doesn't support x64.

So what you need, is to change the "Architectures" for your project target like this

NB. If you're using Cocoapods - you should do the same for "Pods" target.