没有声明与定义不同的功能的GCC警告

Question

C file definition:

#include <stdint.h>
void foo(int i)
{
    if(i>0) bar();
}

H file declaration:

#ifndef FILE_H
#define FILE_H

void foo(void);

#endif FILE_H

main:

#include "file.h"

int main(void)
{ 
    foo();
}

The file.h is not included in file.c for some abstraction reasons. Why isn't GCC generating any warning in this particular case?

Answer 1

^{As noted in comments…}

In general terms, you must include the header that declares a function both where the function is defined and where it is used. You didn't, so the compiler couldn't warn. Headers are the glue that allow the compiler to do the cross-checking necessary to ensure basic sanity in your code.

The question claims:

• The file.h is not included in file.c for some abstraction reasons.

It isn't clear what those abstraction reasons are, but whatever the reasoning, it is faulty. Since the question does not (yet) specify in any more detail what the problem is, we can't help you fix the issues. However, they're spurious — you are doing something wrong.

Specifically, you must include file.h in file.c as well as in main.c . Otherwise, you don't get the cross-checking you need — or the warning you desire. You also need a prototype for bar() in file.c — you shouldn't be calling a function without a prototype (strictly, a non-prototype declaration is sufficient, but you really want a prototype) in scope, and there's no prototype for bar() in either <stdint.h> or file.h .

I compile using GCC and options:

gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes …

I don't bother to run the code until that's clean — indeed, the -Werror means I can't run the code until that's clean because the compilation fails if there is any warning or error. You can afford to go more stringent if you prefer; you shouldn't risk going much less stringent than that.

Note that both C99 and C11 require that a function is declared or defined before it is used. Only the ancient C90 standard permits calling functions that have not been pre-declared or pre-defined.