warning: control reaches end of non-void function 定义了一个有返回值的函数，却没有写return.
warning: suggest parentheses around assignment used as truth value 告诉我们检查if 中的 '=='
warning:(前面是in function xxx) incompatible types in assignment 参数和调用的参数类型对不上
warning:return makes pointer from integer without a cast 函数定义的返回类型跟实际返回类型不匹配。
warning: no newline at the end of file 在文件的结尾加个回车