#include <stdio.h>

int main(void){
    printf("%s\n", a);
    return 0;
};
-------main.c---------
#include <stdio.h>

extern char a[] = "hello world";
------a.c-------
gcc main.c a.c
-->
segment.c: In function ‘main’:
segment.c:4:17: error: ‘a’ undeclared (first use in this function)
  printf("%s\n", a);
                 ^
segment.c:4:17: note: each undeclared identifier is reported only once for each function it appears in
a.c:3:13: warning: ‘a’ initialized and declared ‘extern’
 extern char a[] = "hello world";
             ^