I am not clear about the life time and scope of global extern variable and local extern variable
extern may be local or global. What is difference between local extern and global extern variable? . I think the difference is only with the life time not in scope
The difference is purely in the scope of the declaration. Either way, it declares a variable as having external linkage (although if there is a prior declaration of that variable visible that has internal linkage, this declaration will refer to that variable instead).