项目中添加C语言文件后编译出现大量错误，因为工程中有pch文件，作为预编译头文件，Objective-C和C都会引用到它，而里面都是Objective-C代码，C编译器是编译不了的，所以就出现了大量错误。
the pch is included as a prefix into both C and ObjC files. So my C files were getting Foundation.h which of course would never work :)
所以要在pch文件中加上：
#ifdef __OBJC__
#import ...
#endif
屏蔽C对它的引用。
参考链接
