Storage Classes --1

1)
#include
int main()
{
extern int a;
a=20;
printf("%d",sizeof(a));
return 0; }

2)
#include
int main()
{
extern int a;
printf("%d",sizeof(a));
return 0;
} 


In case 1 we will get linker error and in cae 2 we get 4 a o/p.please explain the behaviour.