How do I get rid of -Wimplicit-function-declaration

Question

I've splitted up my C source code into 2 files: main.c and secondary.c respectively.

/* 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);
}

I use the following command to compile them:

$gcc -Wall main.c secondary.c

But in this case I receive the following warning:

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();
      |         ^~~~~~~~~~~

Then I decide to include secondary.c in main.c. To do so, I type #include "secondary.c" in main.c.

In this case, I get the following error:

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

What do I do to get rid of -Wimplicit-function-declaration?

Answer 1

According to the first compiler message the compiler does not know how the function print_value() used in the module main.c is declared when it compiles the module.

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

int main ()
{
         extern int my_var;

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

Create a header with the function declaration (prototype) like

void print_value( void );

and include the header in the both modules.

As for the second message then after including secondary.c in main.c you have two definitions of the same function in the both modules main.c and secondary.c. Do not include c midules in each other.

Pay attention to that according to the C Standard the function main without parameters shall be declared like

int main( void )

Also this statement

exit (0);

is redundant. Remove it.

Also bear in mind that according to the C23 Standard you may not use functions without function prototypes as it was allowed before the C23 Standard.