[C51]Problem "Illegal pointer conversion"!

hi all, i have two unit : unit1.c and unit2.c

in unit1.c :
xdata char str1 [8];


in unit 2.c :

#include <stdlib.h>

extern char str1;

void func (){
int a;
a = atoi(str1);
}

why i get "Illegal pointer conversion" ?
why when i move str1 define to unit2 problem reolve ?