1.9. Use common header file¶

To fix the compiler warning as shown in Compiler Flags — Warning we curdely modified main.c. But that is not the correct way to fix the problem for missing declaration of greeting().

incorrect main.c¶

extern void greeting(void);

int main() {
    greeting();
    return 0;
}

The correct way would be to use a common header file between the consumer, main.c and the provider greeting.c

main.c with header inclusion¶

#include "greeting.h"

int main() {
    greeting();
    return 0;
}

greeting.h¶

#ifndef GREETING_H
#define GREETING_H

void greeting(void);

#endif /* GREETING_H */

greeting.c (With header include)¶

#include <stdio.h>
#include "greeting.h"

void greeting() {
    printf ("Hello World!\n");
}

To compile that file with GCC, the command would be the same as used earlier:

gcc main.c greeting.c -Wall -Werror -o greeting

As usual, we should not see any compiler warnings during this compilation step.

1.9.1. Anatomy of the C header file