Compile errors after adding a new target to Xcode project

I added a new target to my project开发者_如何学Python, however when I compile, I get thousands of errors pointing to appkit.h. These are precompile errors. I have .c, .h and .m files in the project. What can be the cause?

Thanks for the suggestions.

You are including AppKit.h in a C file. This is impossible as AppKit.h is an Objective-C header file.

Change all c files to m files (or force the compiler to use Objective-C).