这些警告对C意味着什么？

Question

 final code2.c:9:1: warning: implicit declaration of function 'choice' is invalid in C99 [-Wimplicit-function-declaration]
choice();
^
final code2.c:12:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
choice()
^~~~~~
final code2.c:23:1: warning: implicit declaration of function 'wrong' is invalid in C99 [-     Wimplicit-function-declaration]
wrong();
^
final code2.c:25:1: warning: implicit declaration of function 'formula1' is invalid in C99 [-Wimplicit-function-declaration]
formula1();
^
final code2.c:27:1: warning: implicit declaration of function 'formula2' is invalid in C99 [-Wimplicit-function-declaration]
formula2();
^
final code2.c:29:1: warning: implicit declaration of function 'formula3' is invalid in C99 [-Wimplicit-function-declaration]
formula3();
^
final code2.c:30:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
final code2.c:32:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
wrong()
^~~~~
final code2.c:35:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
final code2.c:37:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
formula1()
^~~~~~~~
final code2.c:47:1: warning: implicit declaration of function 'question' is invalid in C99 [-Wimplicit-function-declaration]
question();
^
final code2.c:50:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
final code2.c:52:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
formula2()
^~~~~~~~
 final code2.c:63:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
final code2.c:65:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
formula3()
^~~~~~~~
final code2.c:85:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
final code2.c:87:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
question()
^~~~~~~~
final code2.c:99:1: warning: control reaches end of non-void function [-Wreturn-type]
 } 

我有这些警告,我一直试图摆脱我的代码.每个警告意味着什么,最好的方法是摆脱它们？

Answer 1

它们意味着您没有为函数声明返回类型,并且return在编译器int为这些函数添加了默认返回类型之后您没有使用语句.宣布它们全部void,警告将消失.

编辑:并在使用它们之前声明您的函数,也可以在头文件中,或者在它们被调用的代码上方.