main.c

#include "mylib.h"

int main(int __attribute__((__unused__)) argc, char **argv)
{
	print_message(argv[argc == 1? 0 : argc - 1]);
    return 0;
}
  

mylib.c

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

void print_message(char *message)
{
	printf("The message is \"%s\"\n", message);
}

mylib.h

#ifndef MYLIB_H
#define MYLIB_H

void print_message(char *);

#endif