Undefined reference to global variable and relocation truncated to fit R_X86_64_PC32 against symbol

Question

What can this error be due to and how can I fix it?

Undefined reference to 'flag'
relocation truncated to fit R_X86_64_PC32 against symbol 'flag'

header.h

extern bool flag;       //declaration
extern int shm[100];    //declaration

static inline function() {
   if (flag) {
      shm[0] = 10;
   }
}

source.c

#include "header.h"
bool flag;      //definition
int shm[100];   //definition

flag = true;
function();
printf("value: %d", shm[0]);