在Xcode 9.0b5中，你可能会遇到这种情况，因为Xcode 9.0b5有一个bug，当你添加源代码时，它不遵守目标设置。你必须在后面手动设置每个文件的目标:

对我来说，这是在合并冲突之后开始发生的。

我试图清理和删除构建文件夹，但没有帮助。不管怎样，这个问题一直在发生。然后，我通过删除有问题的组来重新链接引用，并重新添加到项目中，它起作用了。

这个问题我已经遇到过很多次了。这通常发生在您删除构建文件夹时。

简单的解决方案是解集成并重新安装pod文件。

pod deintegrate
pod install

If you're getting this error when trying to link to a C file, first double check the function names for typos. Next double check that you are not trying to call a C function from a C++ / Objective-C++ environment without using the extern C {} construct. I was tearing my hair out because I had a class that was in a .mm file which was trying to call C functions. It doesn't work because in C++, the symbols are mangled. You can actually see the concrete symbols generated using the nm tool. Terminal to the path of the .o files, and run nm -g on the file that is calling the symbol and the one that should have the symbol, and you should see if they match up or not, which can provide clues for the error.

nm -g file.o

你可以这样检查c++符号:

nm -gC file.o

在Xcode 9.0b5中，你可能会遇到这种情况，因为Xcode 9.0b5有一个bug，当你添加源代码时，它不遵守目标设置。你必须在后面手动设置每个文件的目标:

当你在你当前的项目中使用arm6、arm7架构时，任何你试图在你的项目中使用的第三方框架都是基于x86_64构建的。

例如，如果你在你的项目中使用Cocoa Pods，那么你需要添加以下脚本，以确保所有第三方框架都能确保arm6, arm7。即

示例podfile与脚本添加在结束

target 'XYZ_ProjectTarget' do
  # Comment the next line if you don't want to use dynamic frameworks
  use_frameworks!
  
  # Pods for XYZ_ProjectTarget
  pod 'pod_name'
  
  target 'XYZ_TargetTests' do
    inherit! :search_paths
    # Pods for testing
  end
  
  target 'XYZ_TargetUITests' do
    # Pods for testing
  end
  
end

post_install do |installer_representation|
  installer_representation.project.targets.each do |target|
    target.build_configurations.each do |config|
      config.build_settings['ARCHS'] = 'armv7 armv7s'
    end
  end
end