C++ 如何摆脱WIMPLICITFunction声明

我已经将我的C源代码拆分成两个文件:分别是main.c和secondary.c.

/* main.c */

#include <stdlib.h>
//#include "secondary.c"

int main ()
{
         extern int my_var;

         my_var = 123;
         print_value ();
         exit (0);
}

/* secondary.c */

#include <stdio.h>
int my_var;

void print_value ()
{
        printf("my_var = %d\n", my_var);
}

我使用以下命令来编译它们:

$gcc -Wall main.c secondary.c

但在这种情况下，我收到以下警告:

snowgold@iskhakov-r:~/clang$ gcc -Wall main.c secondary.c
main.c: In function ‘main’:
main.c:9:9: warning: implicit declaration of function ‘print_value’ [-Wimplicit-function-declaration]
    9 |         print_value();
      |         ^~~~~~~~~~~

然后我决定在main.c中加入secondary.c.为此，我在main.c中键入#include "secondary.c".

在本例中，我得到以下错误:

snowgold@iskhakov-r:~/clang$ gcc -Wall main.c secondary.c
/usr/bin/ld: /tmp/ccKKRtFc.o:(.bss+0x0): multiple definition of `my_var'; /tmp/ccx4HrNG.o:(.bss+0x0): first defined here/usr/bin/ld: /tmp/ccKKRtFc.o: in function `print_value':
secondary.c:(.text+0x0): multiple definition of `print_value'; /tmp/ccx4HrNG.o:main.c:(.text+0x0): first defined here
collect2: error: ld returned 1 exit status

我该怎么做才能摆脱-Wimplicit-function-declaration人呢？